Fact-checked by Grok 2 weeks ago

Embryo

An embryo constitutes the early developmental phase of a multicellular organism in sexually reproducing species, initiating upon the fusion of gametes to form a zygote and progressing through mitotic cleavages that establish foundational cellular organization and body plan prior to fetal maturation.^[1]^[2] In animals, this phase encompasses rapid cell division yielding a blastula, gastrulation to delineate germ layers—ectoderm, mesoderm, and endoderm—and the onset of organogenesis, whereby rudimentary structures like the neural tube and somites emerge under genetic and environmental cues.^[3]^[4] These processes reflect inherent, directed morphogenesis, with the embryo functioning as a cohesive entity capable of self-organization toward organismal complexity, as evidenced by its intrinsic developmental potential from the outset.^[5]^[6] In vertebrates, including humans, the embryonic stage persists until basic organ systems are outlined, spanning approximately eight weeks post-fertilization in Homo sapiens, during which vulnerabilities to teratogens underscore the period's criticality for viability.^[7] Plant embryos, conversely, develop within seeds from fertilized ovules, featuring apical-basal polarity and tissue layering that enable dormancy and subsequent germination, highlighting conserved principles of pattern formation across kingdoms despite divergent reproductive strategies.^[2] Defining characteristics include axis establishment—anterior-posterior, dorsal-ventral, and left-right—and regulative capacity, allowing compensatory adjustments to perturbations, as demonstrated in classic amphibian and mammalian models.^[8] Controversies in embryology often intersect with ethical considerations in research, such as embryonic stem cell derivation, where biological continuity from zygote to adult challenges reductive characterizations, prioritizing empirical ontogeny over ideological framings.^[9] Developmental biology's elucidation of these stages has yielded milestones like the identification of Hox genes governing segmentation and the role of signaling pathways (e.g., Wnt, BMP) in patterning, informing applications from regenerative medicine to evolutionary homology inferences across taxa.^[2] Empirical data affirm the embryo's status as the progenitor organism, with no substantive discontinuity in causal agency throughout ontogenesis, countering claims minimizing its integrated functionality based on size or dependency.^[5]^[10]

Definition and Fundamentals

Biological Definition and Characteristics

An embryo constitutes the early multicellular phase of development in sexually reproducing animals, commencing at fertilization when the sperm nucleus fuses with the oocyte to form a diploid zygote possessing a unique genome derived from both parents.^[11] This zygote initiates cleavage divisions, yielding a blastula or equivalent structure, and progresses through organized growth until major organ primordia emerge, typically spanning the period of highest morphological transformation prior to fetal designation—approximately the first 8 weeks post-fertilization in humans.00807-3) Biologically, the embryo qualifies as a distinct organism due to its integrated, self-regulating behavior, evidenced by coordinated cellular processes that direct tissue formation without external imposition, as observed in time-lapse imaging of early cleavage stages.^[12] Central characteristics include the initial totipotency of the zygote and early blastomeres, enabling differentiation into all embryonic and extra-embryonic lineages, which transitions to pluripotency in the inner cell mass or epiblast by the blastocyst stage.^[13] Totipotent cells demonstrate this capacity experimentally through their ability to generate complete organisms when isolated and cultured, as seen in mammalian blastomere transfers yielding viable offspring.00039-0) Pluripotent cells, conversely, form all somatic lineages but exclude trophoblast, reflecting progressive restriction driven by epigenetic modifications and gene regulatory networks intrinsic to the embryo's genome.^[13] Genetic analyses, such as whole-genome sequencing of single-cell embryos, confirm the embryo's DNA as a novel, stable entity with half the parental chromosomes recombined, underscoring its independence from maternal tissues beyond nutritional support.^[11] Rapid mitotic proliferation characterizes embryonic growth, with cell cycles shortening to under 10 hours in early human stages, facilitating exponential increase in cell number while preserving developmental potency through asymmetric divisions and spatial patterning.30562-6) This self-directed differentiation arises from causal interactions within the embryo, including morphogen gradients and cell-cell signaling, as demonstrated by microsurgical perturbations in model organisms like amphibians that disrupt but do not abolish intrinsic axial organization.^[11] Empirical microscopy reveals hallmarks such as compaction at the morula stage and cavitation forming the blastocoel, evidencing coordinated morphogenesis absent in disorganized cell aggregates.^[12] These traits collectively affirm the embryo's status as an autonomous developmental entity governed by its foundational genetic program.

Distinction from Zygote, Blastocyst, and Fetus

The zygote forms immediately upon fertilization, consisting of a single diploid cell resulting from the fusion of a sperm pronucleus with an oocyte pronucleus, marking the commencement of development without prior multicellularity.^[14] This unicellular stage persists briefly before undergoing rapid mitotic cleavages, producing a solid mass of cells known as the morula by day 3-4 post-fertilization, followed by cavitation into the blastocyst by day 5, which features a fluid-filled cavity, an outer trophoblast layer, and an inner cell mass destined to form the embryo proper.^[15] ^[16] The blastocyst implants into the uterine endometrium around days 6-10, initiating organized differentiation; at this juncture, the developing entity transitions terminologically to an embryo due to its multicellular architecture and the emergence of developmental patterning, contrasting the zygote's totipotent singularity.^[17] Proposals of a "pre-embryo" phase up to implantation or 14 days, intended to denote undifferentiated totipotency, falter empirically, as cellular organization, lineage commitment in the inner cell mass, and continuous causal progression toward organismal form are evident from fertilization, rendering such distinctions artificial conveniences without substantive biological discontinuity.^[18] ^[19] The embryonic phase spans from implantation through the eighth week post-fertilization (equivalent to the end of the tenth gestational week), encompassing gastrulation, germ layer formation, and organogenesis, by which point the foundational body plan—including axial structures, limb buds, and visceral primordia—is established.^[17] ^[14] Key physiological milestones, such as the heart's initial contractions around day 22 (week 3) and detectable electrical neural activity by weeks 6-7, underscore the embryo's integrated functionality, though these do not precipitate the fetal transition.^[17] This stage concludes at week 9 post-fertilization, yielding to the fetal period, wherein major organs refine histologically, sensory systems mature, and proportional growth predominates over primary morphogenesis, with the entity now termed a fetus based on morphological completion rather than novel genesis.^[20] ^[21] The shift reflects observable criteria like relative head-to-body proportions and organ presence, yet preserves underlying causal continuity from zygotic origins, as developmental mechanisms—cellular proliferation, migration, and induction—persist without rupture.^[14]

Comparative Embryology Across Organisms

Comparative embryology reveals conserved developmental principles across diverse organisms, particularly in the establishment of body axes and polarity, despite fundamental mechanistic differences between animals and plants. In animals, embryonic development typically begins with rapid cleavage divisions of the zygote, forming a multicellular blastula that precedes gastrulation, where cells invaginate to establish the three germ layers and define the primary body axes. This process is evident in vertebrates, where the anterior-posterior axis is patterned by Hox gene clusters, which exhibit remarkable evolutionary conservation across bilaterian phyla, directing segmental identity and regional specification in structures like the vertebral column.^[22]^[23] Hox genes maintain colinear expression along the chromosome, mirroring their spatial deployment in the embryo, a pattern preserved from insects to mammals through over 500 million years of evolution.^[24] In contrast, plant embryos lack true cleavage, instead undergoing patterned mitotic divisions from the zygote to form a proembryo with apical-basal polarity established by the first asymmetric division. Dicotyledonous plants, such as Arabidopsis thaliana, progress through globular, heart-shaped, and torpedo stages, during which bilateral symmetry emerges via cotyledon outgrowth, contrasting with the radial symmetry in many monocots.^[25] Monocot embryos share early developmental similarities with dicots up to the octant stage but diverge by forming a single cotyledon without a distinct heart phase, resulting in a more elongate scutellum structure.^[26] Axis formation in plants relies on auxin gradients and PIN-FORMED transporters for apical-basal patterning, independent of animal-like Hox systems.^[27] Evolutionary developmental biology underscores similarities in the logic of axis specification—both kingdoms prioritize early polarity to organize the body plan—yet highlights kingdom-specific divergences: animals employ transcription factor networks like Hox for anteroposterior coordination, while plants use hormone-mediated signaling for meristem establishment. Empirical studies in vertebrates demonstrate Hox paralog groups (e.g., Hox4, Hox7) expressed in precise rhombomeres and somites, conserved from fish to humans, enabling adaptive modifications to conserved morphologies.^[28] In plants, transcriptomic analyses reveal distinct gene activation patterns post-zygotic genome activation, with maternal contributions dominating early dicot stages, differing from the biparental balance in many animal models.^[29] These comparisons illuminate how independent multicellular evolutions converged on hierarchical patterning while adapting to sessile versus motile lifestyles.^[30]^[31]

Etymology and Historical Context

Linguistic Origins

The word embryo originates from the Ancient Greek émbryon (ἔμβρυον), denoting "a young one" or an organism in its early formative state, derived from the prefix en- ("in") and the verb bryein ("to swell" or "be full," referring to growth or burgeoning).^[32] This term, attested in Homeric texts as applying to young animals, entered Medieval Latin as embryo or embryon, and by the mid-14th century, it appeared in Middle English to describe a fetus in utero during initial development stages.^[33] The Greek roots emphasized enclosure and expansion within the womb, aligning with observations of early biological swelling and differentiation rather than later maturation.^[32] Aristotle, in works like On the Generation of Animals (circa 350 BCE), applied concepts akin to émbryon to describe the progressive formation of animal structures from undifferentiated material, particularly in chick dissections that revealed sequential organ emergence, laying groundwork for the term's association with pre-formed developmental phases.^[34] This usage distinguished early, plastic stages from more defined offspring, influencing subsequent Greco-Roman biology where embryo connoted the nascent, womb-bound entity prior to viability. In contrast, the Latin fetus—from fetu ("offspring" or "bearing"), rooted in fē- ("to produce" or "bring forth")—originally signified the brought-forth young without strict temporal delimitation, though by late antiquity, it increasingly denoted later gestational phases. The term's scientific precision crystallized in the 19th century through Ernst Haeckel's comparative embryology, where embryo standardized nomenclature for homologous early vertebrate stages, as in his 1866 biogenetic law positing ontogeny recapitulating phylogeny, thereby embedding the word in evolutionary and developmental contexts distinct from fetus, which retained connotations of advanced, offspring-like form.^[35] This evolution prioritized empirical staging over vague "young" descriptors, shaping modern biological taxonomy.^[36]

Early Scientific Observations and Theories

Aristotle, in the 4th century BCE, proposed the theory of epigenesis, positing that embryos develop through the successive formation of parts from undifferentiated material rather than from pre-existing structures, based on dissections of chick embryos revealing progressive organ appearance.^[34] This contrasted with earlier speculative ideas but lacked microscopic evidence, relying on macroscopic observations that suggested gradual differentiation driven by formative forces within the semen and menstrual blood.^[37] In the 17th and 18th centuries, preformationism gained prominence, asserting that organisms existed as miniature adults (homunculi) fully formed within gametes, unfolding rather than developing anew; proponents like Jan Swammerdam and Antonie van Leeuwenhoek interpreted early embryonic stages seen through primitive microscopes as evidence of preformed entities.^[38] Marcello Malpighi's 1672 microscopic examinations of chick embryos, however, demonstrated sequential stages of blood vessel and organ formation without visible preformed miniatures, providing empirical support for epigenesis by revealing development as a constructive process rather than mere enlargement.^[39] These observations, combined with improved microscopy disproving infinite regress of nested homunculi, shifted consensus toward epigenesis by the early 19th century, emphasizing observable gradualism over preformationist claims unsubstantiated by direct evidence.^[40] The 19th century integrated cell theory into embryology, with Wilhelm Roux's 1888 experiments on frog embryos—killing one blastomere to yield a partial larva—demonstrating mosaic development where cell fate is determined early, applying physiological mechanisms to refute purely vitalistic views.^[41] August Weismann's 1893 germ plasm theory further clarified heredity's role, arguing that developmental instructions reside exclusively in immutable germ cells, separate from modifiable somatic cells, thus framing embryogenesis as directed by stable hereditary determinants rather than Lamarckian acquisition.^[42] Hans Spemann and Hilde Mangold's 1924 transplantation experiments identified the "organizer" in the dorsal lip of amphibian gastrulae, which induces neural tissue formation in host embryos, revealing regulative interactions and signaling centers as causal drivers of pattern formation; this earned Spemann the 1935 Nobel Prize and marked experimental embryology's peak in elucidating causal mechanisms through intervention.^[43] Post-1953, the elucidation of DNA's double-helix structure by Watson and Crick positioned molecular genetics as central to embryogenesis, shifting focus from cellular mechanics to gene expression regulating differentiation, as evidenced by subsequent discoveries of regulatory genes influencing developmental trajectories.^[44]

Developmental Processes

Pre-Embryonic Cleavage and Implantation

Following fertilization, the zygote undergoes cleavage, a series of rapid mitotic divisions that partition the cytoplasm into progressively smaller blastomeres without significant overall increase in embryo size.^[16] These divisions establish initial cellular polarity through asymmetric distribution of maternal factors and cytoskeletal rearrangements, enabling totipotency in early blastomeres while setting the stage for differential fates.^[45] In humans, the first cleavage typically occurs 24 to 30 hours post-fertilization, yielding a 2-cell embryo; subsequent divisions produce 4-cell (day 2), 8-cell (day 3), and 16- to 32-cell stages.^[16] Blastomeres remain loosely associated within the zona pellucida, the acellular glycoprotein shell, during this phase.^[46] By days 3 to 4, the embryo compacts into a morula, a solid ball of cells where blastomeres adhere tightly via E-cadherin-mediated junctions and actin-myosin contractility, reducing intercellular spaces and initiating epithelial-like polarity.^[45] This compaction, driven by cell contraction and adhesion molecule upregulation, marks the transition from totipotent to pluripotent inner cells and differentiating outer cells.^[47] Compaction failures correlate with developmental arrest, as disrupted contractility impairs the structural integrity needed for cavity formation.^[48] The morula then cavitates to form the blastocyst by day 5 to 6, featuring a fluid-filled blastocoel, an inner cell mass (ICM) of pluripotent cells destined for the embryo proper, and an outer trophectoderm layer that secretes fluid via Na+/K+-ATPase and aquaporins while expressing adhesion molecules for uterine interaction.^[46] The blastocyst hatches from the zona pellucida through trophoblast-derived proteases, enabling direct endometrial contact.^[16] Implantation commences around day 6 to 7, with trophectoderm invasion of the endometrial epithelium, facilitated by integrins and proteases; syncytiotrophoblast formation follows, producing human chorionic gonadotropin (hCG) to signal maternal recognition of pregnancy by sustaining the corpus luteum and progesterone production.^[49] hCG modulates endometrial receptivity by upregulating adhesion factors and immune tolerance mechanisms, such as shifting toward Th2 responses.^[50] Pre-implantation development exhibits high attrition, with natural loss rates estimated at 30 to 50% due to chromosomal errors, metabolic inefficiencies, and implantation failures, though precise quantification remains challenging from ethical constraints on direct observation.^[51] In vitro data indicate 50 to 70% of embryos arrest before blastocyst stage, often from aberrant cleavage patterns like irregular blastomere size or fragmentation exceeding 15%, which reduce compaction success and predict demise.^[46] Despite this, surviving embryos demonstrate inherent organization from fertilization, with polarity and adhesion cues ensuring causal progression toward implantation competence absent external pathology.^[52]

Gastrulation and Germ Layer Formation

Gastrulation represents a critical phase in embryonic development, transforming the bilaminar disc of the epiblast and hypoblast into a trilaminar structure comprising the three primary germ layers: ectoderm, mesoderm, and endoderm.^[53] In human embryos, this process initiates around the third week post-fertilization, coinciding with the establishment of the body axes and the onset of cell fate specification through coordinated morphogenetic movements such as invagination, involution, and migration.^[53] These movements reorganize the epiblast cells, with approximately 20-30% undergoing epithelial-to-mesenchymal transition to ingress and displace the hypoblast, forming definitive endoderm while generating intraembryonic mesoderm.^[54] The primitive streak emerges as a midline thickening on the posterior epiblast, serving as the site of cell ingression and defining the craniocaudal axis.^[55] At its anterior terminus lies the primitive node (Hensen's node in some vertebrates), which acts as the primary organizer, regulating gastrulation by secreting signaling molecules that induce mesendoderm formation and pattern the anterior-posterior axis.^[56] Cells migrating through the streak differentiate into mesoderm laterally and posteriorly, while those passing anteriorly contribute to the notochord, a structure essential for neural tube induction.^[55] The ectoderm remains as the non-ingressing epiblast layer, fated to form neural and epidermal tissues.^[53] Genetic regulation of gastrulation involves conserved signaling pathways, with Wnt/β-catenin signaling indispensable for primitive streak induction and mesoderm specification; ablation of Wnt3 in mouse models arrests development at the pre-streak stage.^[57] Notch signaling refines germ layer boundaries by modulating cell-cell interactions and inhibiting premature differentiation, as evidenced by disrupted mesendoderm patterning in Notch pathway mutants across bilaterian embryos.^[58] These pathways interact dynamically, with Wnt upstream of Notch in presomitic mesoderm segmentation, ensuring precise spatiotemporal control verified through lineage tracing and gene expression analyses in mammalian models.^[59] Left-right asymmetry originates during gastrulation at the node, where monociliated cells generate a directional fluid flow via posterior tilt and rotation, activating asymmetric Nodal signaling preferentially on the left periphery.^[60] This ciliary mechanism, conserved in vertebrates, breaks bilateral symmetry independently of earlier dorsoventral cues, with disruptions in ciliogenesis leading to situs inversus in up to 50% of affected mouse embryos.^[61] Empirical observations from high-speed imaging confirm flow speeds of 10-20 μm/s sufficient to initiate calcium transients and gene cascades dictating organ lateralization.^[62]

Organogenesis and Major Milestones

Organogenesis encompasses the differentiation of the three primary germ layers—ectoderm, mesoderm, and endoderm—into foundational organ systems during embryonic weeks 3 through 8 post-fertilization. This phase establishes the basic body plan, with rapid cellular proliferation and migration driven by genetic programs and signaling gradients. By the end of week 8, the embryo measures approximately 30 mm in length, and major organs have formed, transitioning to the fetal period focused on growth and refinement.^[63]^[17] In week 3, the neural groove appears on day 18, initiating central nervous system development from ectoderm, while paired heart tubes fuse by day 21 and begin primitive beating on day 22–23. Limb buds emerge in week 4, with upper buds on day 26 and lower on day 28; the heart undergoes looping to establish chambers, and the neural tube closes progressively—rostral neuropore on day 24 and caudal on day 26–28—marking a critical vulnerability period, as failure leads to defects like anencephaly or spina bifida. Sensory primordia form concurrently, including optic placodes on day 22 and otic vesicles by day 23.^[17]^[64]^[17] Weeks 5–6 feature elongation of limb buds with paddle-like hand plates by day 37, brain vesicle division into prosencephalon, mesencephalon, and rhombencephalon, and initial retinal pigmentation on day 35. Electrical brain activity becomes detectable around week 6, with primitive EEG patterns recorded as early as 6 weeks and 2 days. Nociceptors, specialized sensory receptors for potential tissue damage, first appear around the mouth by week 7, enabling reflexive withdrawal responses to peri-oral touch by weeks 7–8, though conscious pain perception requires later thalamocortical integration.^[17]^[65]^[66] By week 8, facial features distinguish, eyelids begin forming, and organ systems like the outflow tract of the heart complete septation, rendering the embryo recognizably human-like. Disruptions during this integrated phase reveal causal dependencies; for instance, thalidomide exposure between days 20–36 post-fertilization (weeks 3–5) inhibits angiogenesis and cereblon-mediated signaling, primarily causing phocomelia and other limb reductions due to halted mesenchymal development. Such teratogenic windows underscore the embryo's coordinated, directive complexity, where external agents exploit precise temporal vulnerabilities.^[63]^[67]^[68]

Plant Embryos

Zygotic Embryogenesis in Angiosperms

Zygotic embryogenesis in angiosperms begins with double fertilization, a defining reproductive event unique to flowering plants, in which one sperm nucleus fuses with the egg cell to form the diploid zygote, while the second sperm nucleus fuses with the two polar nuclei to produce the triploid endosperm, which serves as nutritive tissue for the developing embryo.^[69] This process occurs within the embryo sac of the ovule following pollination and pollen tube growth. The zygote remains quiescent for a period before undergoing its first division, which is asymmetric and establishes the foundational apical-basal polarity: the upper terminal cell gives rise to the embryo proper, while the lower basal cell differentiates into the suspensor.^[70] The suspensor, a transient filamentous structure, anchors the embryo proper to the endosperm and facilitates nutrient and hormone transfer, including the production of gibberellins and auxins that promote embryo growth; it typically degenerates as embryogenesis progresses.^[71] Early embryonic stages commence with the proembryo phase, characterized by rapid transverse divisions of the terminal cell, forming a tiered structure of 4 to 16 cells depending on the species. This transitions to the globular stage, where periclinal divisions establish the dermatogen layer (future epidermis) and internal cell layers, accompanied by the onset of radial symmetry.^[72] Subsequent heart-shaped and torpedo stages mark bilateral symmetry and organogenesis, with cotyledons, hypocotyl, and radicle forming along the apical-basal axis; in the torpedo phase, the embryo elongates, and the procambium differentiates into vascular tissues.^[73] In the model angiosperm Arabidopsis thaliana, auxin gradients, mediated by polar auxin transport via PIN-FORMED (PIN) proteins, direct these patterns: an auxin maximum at the apical region of the proembryo promotes hypophysis specification for root development, while basipetal flow maintains polarity and cell fate decisions.^[74] Mutations disrupting auxin biosynthesis or transport, such as in YUCCA genes, arrest development at early globular stages, underscoring auxin's causal role in axis establishment.^[75] These conserved processes across angiosperms yield a mature embryo encapsulated in the seed, poised for post-germinative growth.^[76]

Apomictic and Somatic Embryos

Apomictic embryos form via apomixis, an asexual reproductive process in angiosperms that generates viable seeds without meiosis or fertilization, yielding clonal offspring genetically identical to the maternal plant. This mechanism encompasses types such as diplosporous and aposporous development, where unreduced embryo sacs produce embryos parthenogenetically. Apomixis occurs in over 300 species across about 30 of the 460 angiosperm families, with notable examples in dandelions (Taraxacum spp.), hawkweeds (Hieracium spp.), and certain citrus varieties, facilitating persistent clonal propagation in natural populations.^[77]^[78] In agricultural contexts, apomixis enables the perpetual reproduction of hybrid vigor through seeds, bypassing the instability of F1 hybrids in sexual systems and accelerating the deployment of elite cultivars without repeated crossing. Efforts to engineer apomixis into major crops like rice and maize aim to enhance yield stability and reduce breeding costs, as demonstrated in experimental models where apomictic traits fixed desirable alleles across generations. However, unlike sexual reproduction, apomixis eliminates genetic recombination and segregation, resulting in uniform progeny that may exhibit diminished adaptability to evolving pathogens, climate shifts, or soil variations due to the absence of novel allelic combinations.^[77]^[79] Somatic embryos develop from differentiated somatic (non-gametic) cells, typically induced in vitro through hormonal treatments that reprogram cells to totipotent states mimicking zygotic embryogeny, progressing through globular, heart, and torpedo stages. The process was first reported in 1958 using carrot (Daucus carota) suspension cultures, with independent confirmations by Steward et al. and Reinert, establishing carrots as a model system for subsequent studies across species. Somatic embryogenesis has since been applied to over 100 plant species, including recalcitrant woody plants like conifers and tropical fruits, enabling high-throughput clonal multiplication.^[80]^[81] Biotechnological uses of somatic embryogenesis include synthetic seed production for uniform planting material, integration with genetic engineering for trait enhancement (e.g., herbicide resistance), and conservation of endangered germplasm via scalable propagation. In forestry and horticulture, it supports elite tree clonal forestry, as seen in protocols for species like Picea spp. yielding thousands of embryos per gram of tissue. Yet, reliance on somatic routes forgoes the heterozygosity generated by sexual fusion, constraining population-level variability and potentially amplifying vulnerabilities to abiotic stresses or genetic drift in monocultures.^[82]^[83]

Research and Technological Advances

In Vitro Culture and IVF Techniques

In vitro fertilization (IVF) involves the fertilization of oocytes outside the body followed by culture of the resulting embryos in controlled laboratory conditions to mimic aspects of the uterine environment. Developed in the 1970s, the technique achieved its first live birth on July 25, 1978, with the delivery of Louise Brown in the United Kingdom following procedures by Patrick Steptoe and Robert Edwards.^[84]30261-9/fulltext) Early protocols focused on cleavage-stage embryos transferred on day 3 post-fertilization, but advancements shifted toward extended culture to the blastocyst stage (day 5-6), which allows selection of more viable embryos capable of self-sustaining development.^[85]^[86] Embryo culture media consist of nutrient-rich solutions supplemented with amino acids, vitamins, growth factors, and energy substrates like glucose and lactate to replicate tubal and uterine fluids, often using sequential formulations that change composition over development stages.^[87] Incubators maintain precise conditions of 37°C, 5-6% CO2, and stable humidity to support cleavage, morula formation, and blastocyst expansion without ex vivo stressors disrupting epigenetic or metabolic processes. Time-lapse imaging systems, such as EmbryoScope, enable continuous, non-invasive observation of morphokinetic parameters like division timing and fragmentation, aiding viability assessment without repeated handling that could impair development.^[88]^[89] Live birth rates per single embryo transfer in IVF cycles vary by maternal age and embryo quality, reaching approximately 48% for women under 35 using euploid blastocysts, though overall rates per initiated cycle hover around 20-35% due to attrition from oocyte retrieval to implantation.^[90]^[91] Implantation success per transferred embryo typically ranges from 30-50% for high-quality blastocysts, reflecting inherent developmental inefficiencies where only a fraction achieve uterine attachment and gestation, akin to natural cycles but amplified by laboratory variables. Protocols now emphasize preimplantation genetic testing and single embryo transfer to optimize outcomes and minimize multiples.^[92] IVF routinely produces surplus embryos beyond those transferred, leading to cryopreservation of excess blastocysts for future use. In the United States, an estimated 1.2 million embryos remain in frozen storage, with many clinics reporting that 50-60% of patients eventually face disposition decisions involving thawing without transfer, donation, or disposal due to storage costs or changing circumstances.^[93] In the United Kingdom, over 500,000 embryos are currently stored, while at least 130,000 have been discarded since 1991, highlighting the scale of non-utilized material from multi-embryo production strategies.^[94] These practices underscore the trade-off in IVF: enhanced per-cycle success through surplus generation versus the downstream management of untransferred embryos.

Stem Cell-Based Embryo Models and Synthetics

Stem cell-based embryo models (SCBEMs) are three-dimensional structures derived from pluripotent stem cells, such as induced pluripotent stem cells (iPSCs) or embryonic stem cells, that recapitulate aspects of early embryonic development without requiring sperm or egg fertilization. These non-zygotic models, including blastoids (blastocyst-like structures) and gastruloids (gastrulation-stage mimics), enable the study of implantation, germ layer formation, and organ primordia in vitro, addressing ethical and practical limitations of natural embryos. Advances since 2023 have focused on integrating embryonic and extra-embryonic lineages to model post-implantation stages up to approximately day 14 of human development, demonstrating developmental dynamics like cavity formation and tissue patterning.^[95]^[96] Key progress includes 2023 reports on synthetic embryo models (SEMs) generated from mouse and human PSCs, which exhibit hallmarks of post-gastrulation growth, such as neural and cardiac progenitor development, though confined to ex utero culture for limited durations. In 2025, researchers at the University of California, Santa Cruz, used epigenetic editing of mouse ESCs to produce embryoids where approximately 80% of cells self-organize into pre-gastrulation structures mimicking natural embryo architecture, highlighting programmable self-organization without genetic alterations. The International Society for Stem Cell Research (ISSCR) issued targeted guideline updates in August 2025, endorsing SCBEMs for implantation research while recommending enhanced institutional oversight to monitor morphological and functional fidelity, as these models bypass gamete-based constraints and facilitate high-throughput perturbation studies.00981-3)^[97]^[98] Despite empirical gains, SCBEMs exhibit limitations in replicating full organismal integrity, including incomplete extra-embryonic support for prolonged development beyond early post-implantation phases and discrepancies in gene expression profiles compared to zygotic embryos. Critics argue that rapid advancements outpace ethical frameworks, with insufficient scrutiny on potential off-target effects or transitions to heritable germline modifications, as models could inadvertently normalize edits transmissible across generations without the regulatory hurdles of true embryos. The ISSCR's 2025 recommendations emphasize risk-benefit assessments and public reporting to mitigate these gaps, underscoring that while SCBEMs advance causal understanding of developmental mechanisms, their partial mimicry necessitates validation against natural systems.^[99]^[100]^[101]

Cryopreservation and Genomic Editing

Cryopreservation of embryos involves rapid cooling techniques, primarily vitrification, to prevent ice crystal formation and preserve cellular integrity. Developed in the 1980s for animal embryos, vitrification has since achieved high post-thaw survival rates exceeding 90% for human embryos in in vitro fertilization (IVF) procedures, surpassing traditional slow-freezing methods and enabling live birth rates comparable to fresh transfers.^[102]^[103] In livestock and wildlife species, vitrification protocols have yielded survival rates of 80-95% for early-stage embryos, facilitating genetic banking and population management.^[104] For plant embryos, cryopreservation via vitrification has preserved zygotic embryos from over 100 species since the late 1980s, supporting ex situ conservation of biodiversity by storing tissues at -196°C in liquid nitrogen.^[105] In conservation biology, embryo cryopreservation serves as a tool for endangered species banking, allowing long-term storage of genetic material to counteract population declines and restore heterozygosity. Successful applications include cryopreserved embryos from species like black-footed ferrets and rhinos, integrated with assisted reproductive technologies to bolster captive breeding programs and mitigate extinction risks.^[106]^[107] These biobanks enhance biodiversity preservation by enabling future reintroduction without ongoing live maintenance, though challenges persist in species-specific protocol optimization and post-thaw viability.^[108] Genomic editing of embryos, particularly via CRISPR-Cas9, enables targeted modifications to DNA sequences, with initial demonstrations in human embryos occurring in 2015-2017 for proof-of-concept studies. The 2018 case of He Jiankui involved CRISPR editing of the CCR5 gene in human embryos to confer HIV resistance, resulting in the birth of twin girls, though the edits produced mosaicism and incomplete protection, sparking global scrutiny over efficacy and safety.^[109]^[110] Recent advances by 2024-2025 include refined prime editing and base editing techniques achieving up to 100% allelic efficiency in mammalian embryos with reduced mosaicism, aimed at correcting disease-causing mutations like those in hypertrophic cardiomyopathy.^[111]^[112] Despite progress, genomic editing in embryos carries risks including off-target mutations, where unintended DNA alterations occur at non-target sites, potentially leading to oncogenic or functional disruptions, as documented in multiple CRISPR studies with mutation rates varying from 0.1-10% depending on guide RNA design.^[113]^[114] Mosaicism, arising from asynchronous editing across cell divisions, results in genetically heterogeneous embryos, complicating heritable outcomes and observed in up to 50% of edited founders in early experiments.^[115]^[101] These technical limitations underscore the need for enhanced delivery methods and verification protocols before broader applications in conservation or therapeutics.^[116]

Fossilized Embryos and Evolutionary Insights

Major Discoveries and Preservation Methods

Fossilized embryos from the Ediacaran Doushantuo Formation in South China, dating to approximately 600 million years ago (Ma), represent some of the earliest evidence of metazoan development, preserved as phosphatized microfossils exhibiting cleavage stages and gastrulation-like structures.^[117] These specimens, recovered from phosphate-rich deposits, display cellular divisions and embryonic envelopes consistent with early animal ontogeny, including morula and blastula forms up to 500 micrometers in diameter.^[118] Phosphatization, involving rapid replacement of organic material with apatite minerals in low-oxygen, phosphate-saturated environments, enabled three-dimensional preservation of soft tissues before bacterial decay, distinguishing these fossils from typical compressions.^[119] In the Cambrian Chengjiang biota, approximately 520 Ma, embryos such as those of the ctenophore Eoobdella have been identified, showing late-stage development with fine internal structures like ciliary combs and muscle fibers preserved in exquisite detail.^[120] Additional Cambrian finds, including diapause embryos from contemporaneous strata, reveal arrested cleavage and yolk-rich cells, indicating adaptive developmental strategies in early marine ecosystems.^[121] Preservation here often combines phosphatization with clay coatings, but advanced micro-computed tomography (CT) scans applied in the 2010s have non-destructively imaged internal cellular architectures, such as discoidal division patterns and subcellular details, previously obscured in two-dimensional views.^[121] These discoveries demonstrate that metazoan embryos possessed developmental complexity, including spiral cleavage and bilaterian-like traits, by the Ediacaran-Cambrian transition, contradicting models positing a gradual progression from simplistic to complex forms and instead supporting rapid emergence of sophisticated ontogenetic mechanisms in ancestral lineages.^[118] Empirical fossil data from such sites underscore continuity in core developmental processes across deep time, with phosphatization's chemical fidelity preserving causal sequences of cell fate determination unaltered by taphonomic bias.^[122]

Implications for Developmental Evolution

Fossilized embryos provide direct empirical evidence for the timing and sequence of developmental innovations across evolutionary history, enabling integration of paleontological data with modern phylogenetic and genetic analyses in evolutionary developmental biology (evo-devo). By preserving snapshots of ancient ontogenies, these fossils constrain the minimum ages for traits such as cleavage patterns, gastrulation modes, and organogenesis, revealing when shifts from direct to indirect development occurred in clades like echinoderms during the Cambrian explosion around 540 million years ago. For instance, exceptionally preserved bilaterian embryos from early Cambrian deposits in China demonstrate spiralian-like cleavage and early bilaterian body axis formation, indicating that core developmental modules were established prior to the diversification of major phyla.^[123]^[124] These fossils challenge and refine hypotheses on developmental constraint and evolvability, such as the hourglass model, which posits greater conservation in mid-embryonic stages (phylotypic period) flanked by more variable early and late phases. Dinosaur embryos, like those of Massospondylus from the Early Jurassic (approximately 183 million years ago), exhibit precocial features including head-first hatching postures and advanced skeletal ossification near term, suggesting evolutionary continuity in reproductive strategies from basal sauropodomorphs to modern birds without invoking unpreserved larval stages. Similarly, ecdysozoan embryos from the basal Cambrian (Fortunian stage, ~529 million years ago) reveal early cuticle formation and molting precursors, providing insights into the developmental origins of arthropod and nematode body plans before the Cambrian radiation.^[125]^[126]^[127] Fossils also highlight heterochrony—shifts in developmental timing—as a driver of macroevolution, with examples like oviraptorosaur embryos showing accelerated pelvic development akin to avian transitions from reptilian ancestors. However, interpretive challenges persist, including taphonomic biases favoring late-stage embryos and uncertainties in assigning developmental ages without modern analogs, necessitating cautious integration with extant data to avoid overextrapolation. Overall, these records underscore causal links between conserved genetic regulatory networks (e.g., Hox genes) and morphological stasis or change, affirming that embryonic development has been a labile yet phylogenetically informed substrate for evolutionary divergence since the Ediacaran-Cambrian transition.^[128]^[124]

Ethical, Philosophical, and Legal Dimensions

Biological Evidence for Moral Status

At fertilization, or syngamy, the human zygote forms through the fusion of sperm and oocyte, resulting in a single-celled entity with a unique diploid genome of 46 chromosomes, distinct from both parental gametes and the mother.^[129] This genetic individuality equips the zygote with the complete blueprint for human development, directing its intrinsic, self-organizing progression toward maturity without requiring additional genetic input.^[11] Biologically, the zygote constitutes a distinct organism of the species Homo sapiens, initiating a continuous trajectory of growth, differentiation, and maturation that persists unbroken from conception through fetal stages to adulthood.^[130] This organismal continuity manifests in sequential milestones, such as cleavage to blastocyst by day 5-6 post-fertilization, implantation around day 7, and gastrulation by week 3, each building on prior cellular directives without external imposition of new organismal identity.^[16] By week 6, neuron production commences around embryonic day 42, marking the onset of rudimentary neural structures capable of generating spontaneous electrical activity, foundational to later sensory and cognitive functions.^[131] Such activity precedes organized responses, underscoring the embryo's progressive realization of integrated physiological capacities inherent to its developmental program.^[132] Around weeks 7-8, the embryo exhibits reflexive responses to tactile stimuli, such as touch near the mouth or face, mediated by emerging neural circuits and peripheral nerves.^[133] These behaviors, including limb flexion, indicate functional integration of sensory input with motor output, emerging from the same causal developmental lineage established at fertilization.^[20] High rates of natural embryonic loss—estimated at 30-50% prior to clinical detection—reflect environmental vulnerabilities rather than ontological discontinuity, paralleling mortality risks in later human stages without impugning the organism's status throughout.^[46] Empirical embryology thus affirms the embryo's status as a unified, human organism whose moral relevance derives from its biological autonomy and directed teleology toward full human form.^[134]

Major Viewpoints in the Moral Status Debate

One prominent viewpoint asserts that human embryos acquire full moral status at fertilization, based on the biological fact that a distinct, self-directing human organism emerges at that moment, possessing a unique genome and the intrinsic capacity for organized development toward maturity absent external interference.^[135] This position emphasizes continuity in human development, rejecting arbitrary thresholds and arguing that membership in the species Homo sapiens confers equal moral consideration, as the embryo is not a mere potential human but an actual one in its earliest stage.^[136] Proponents, including bioethicists like Robert P. George, contend that denying this status undermines the equal value of all human life, with empirical support from developmental biology showing no qualitative break in organismal identity post-fertilization.^[137] The 2024 Alabama Supreme Court ruling in LePage v. Center for Reproductive Medicine exemplified this by classifying frozen IVF embryos as "unborn children" under state wrongful death law, affirming their legal equivalence to other humans.^[138] In contrast, gradualist perspectives hold that moral status accrues incrementally with developmental milestones, such as viability around 24 weeks gestation or the onset of sentience, rather than vesting fully at conception.^[139] Advocates argue this reflects the continuous nature of gestation, where early embryos lack capacities like independent viability or relational awareness that enhance ethical weight, as evidenced by clinicians prioritizing factors like potential survival outside the womb over mere genetic humanity.^[140] Critics of gradualism counter that such criteria are arbitrary and environmentally contingent—viability depends on medical technology, which has advanced from 28 weeks in the 1980s to under 24 weeks today—failing to ground status in the embryo's inherent biological organization.^[141] A third major stance denies embryos personhood, distinguishing potentiality from actuality by requiring traits like consciousness, self-awareness, or rationality, which pre-implantation embryos empirically lack.^[142] This view, often aligned with utilitarian frameworks, posits that embryos' high mortality rates (up to 70% naturally fail to implant) and absence of these attributes justify lower or no protections, prioritizing aggregate benefits like medical research.^[143] Detractors argue this conflates accidental properties with essential ones, ignoring the embryo's actual status as a goal-directed human entity whose development causally unfolds from fertilization, and that utilitarian trade-offs lack justification if basic organismal rights precede consequentialist calculations.^[144] Such critiques highlight that personhood criteria, when applied consistently, would exclude infants or severely disabled individuals, revealing inconsistencies in the framework.^[136]

Regulations, Legal Rulings, and Research Restrictions

The 14-day rule, established in the 1980s as an international ethical benchmark limiting human embryo culture in vitro to 14 days post-fertilization or until the primitive streak appears, has faced challenges from advances in embryo modeling and extended culture techniques.^[145] In its 2021 guidelines, the International Society for Stem Cell Research (ISSCR) withdrew blanket endorsement of the rule, recommending instead case-by-case ethical review for research beyond 14 days, citing potential scientific benefits while acknowledging moral sensitivities.^[146] The ISSCR's 2025 targeted update further refined oversight for stem cell-based embryo models (SCBEMs), prohibiting their use to initiate pregnancy and mandating enhanced institutional review, though it maintained flexibility for non-reproductive research under strict justification.^[98] These shifts reflect tensions between empirical progress in developmental biology and regulatory caution, with the rule remaining legally binding in jurisdictions like the United Kingdom (under the Human Fertilisation and Embryology Act 1990) despite scientific advocacy for revision.^[147] In the United States, federal policy prohibits taxpayer funding for human embryo research involving destruction or serious harm via the Dickey-Wicker Amendment, first enacted in 1996 and annually renewed in appropriations bills, which bars National Institutes of Health (NIH) support for such activities regardless of derivation method.^[148] This restriction, upheld through legal challenges including post-2010 court rulings affirming its scope over embryonic stem cell lines, limits public investment while permitting private or state-funded work, creating a bifurcated research landscape.^[149] State-level approaches diverge sharply: California and New York courts treat frozen embryos as contractual property in custody disputes, prioritizing agreements over personhood claims, whereas Alabama's Supreme Court ruled in February 2024 that IVF embryos constitute "extrauterine children" under wrongful death statutes, enabling lawsuits for their destruction and prompting clinics to pause services amid liability fears.^[150] ^[151] At least 13 states considered embryo personhood bills in 2024, though none passed, underscoring ongoing federalism-driven variability in embryo legal status.^[152] European Union frameworks emphasize embryo dignity, with the 1998 Biotech Directive (98/44/EC) excluding patents on processes using human embryos from the totipotent stage, interpreting "human dignity" to preclude commodification.^[153] Member states vary—Germany's Embryo Protection Act (1990) bans embryo creation solely for research, while the UK permits it under licensed conditions—but EU funding under Horizon Europe (Regulation 2021/695) restricts grants for destructive embryo research, aligning with the Oviedo Convention's requirement for adequate embryo protection where research is allowed.^[154] ^[155] This precautionary approach contrasts with scientific pressures, as seen in debates over stem cell patents invoking dignity clauses to block innovations.^[156] In China, regulations on embryo research and editing have tightened post-2018 He Jiankui scandal, where CRISPR-edited embryos led to births; a 2021 criminal code update made clinical germline editing punishable by imprisonment, and a 2024 Ministry of Science and Technology directive banned all clinical germline genome editing research as "irresponsible."^[157] ^[158] Pre-clinical embryo editing for research remains permissible with ethics committee approval, though frameworks exhibit gaps in enforcement and detail compared to Western standards, facilitating rapid advancements amid weaker prohibitions on heritable modifications.^[159] These policies highlight global disparities, with China's state-driven model enabling broader experimentation while post-scandal reforms aim to align with international norms without fully adopting restrictive dignity-based limits.^[160]

Controversies and Criticisms

Debates on Embryo Destruction in Research

The derivation of embryonic stem cells (ESCs) from human blastocysts, first achieved in 1998 by James Thomson at the University of Wisconsin, necessitates the destruction of the embryo to harvest the inner cell mass, sparking ongoing ethical debates about the justification of such practices in research.^[161] Proponents argue that surplus embryos generated during in vitro fertilization (IVF) procedures—estimated at 1.5 to 1.8 million in the U.S. alone that have never resulted in birth—would otherwise be discarded or stored indefinitely, rendering their use for research a utilitarian repurposing that advances medical knowledge without additional loss of life.^[162] This perspective emphasizes empirical outcomes, such as ESC-derived models for studying developmental disorders like Parkinson's disease and spinal cord injuries, which have enabled insights into cell differentiation and drug testing efficacy.^[163] However, critics counter that the "surplus" designation masks the intentional creation and subsequent destruction of human organisms at the blastocyst stage, which possess organized developmental potential akin to any early embryo, thereby introducing a novel moral culpability absent in routine IVF attrition where loss occurs without research intent.^[164] The advent of induced pluripotent stem cells (iPSCs) in 2006 by Shinya Yamanaka, which reprogram somatic cells to a pluripotent state without embryo involvement, has empirically diminished the necessity for ESC lines by providing comparable tools for disease modeling and personalized medicine, though some researchers maintain ESCs exhibit superior potency and lower genetic variability in certain differentiation protocols.^[165] Advocates for continued embryo use cite these potency differences to justify persistence, noting that ESC research has yielded foundational data on human gastrulation and organoid formation not fully replicable with iPSCs due to reprogramming artifacts.^[166] Opponents, however, highlight that iPSCs have driven parallel achievements, such as patient-specific models for amyotrophic lateral sclerosis and diabetes, underscoring how ethical alternatives mitigate the risks of normalizing embryo destruction, which could erode societal regard for nascent human life and invite commodification pressures.^[167] Critiques of embryo-destructive research extend to causal concerns about devaluing human organisms at their earliest stages, potentially fostering a precedent for selective elimination based on utility, reminiscent of historical eugenic rationales that prioritized aggregate benefits over individual rights.^[168] While IVF's high discard rates—approaching 50% of created embryos in some protocols—demonstrate baseline embryo loss, research adds deliberate intent, prompting arguments that true moral equivalence fails because IVF aims at gestation whereas research precludes it entirely.^[169] Empirical evidence from over two decades shows ESC contributions to therapeutic prototypes, yet the field's shift toward non-embryonic sources indicates that prohibitive ethical barriers may not substantially hinder progress, prioritizing causal realism in assessing whether destruction yields irreplaceable gains.^[170]

Synthetic Embryos and the 14-Day Rule

Synthetic embryos, including blastoids and gastruloids, are artificially generated structures derived from pluripotent stem cells that recapitulate early mammalian embryonic development, such as blastocyst formation and gastrulation, without originating from fertilized eggs or requiring uterine implantation.^[171] These models enable the study of lineage specification and tissue morphogenesis in vitro, with advancements in 2023 producing human blastoids capable of mimicking pre-implantation stages and gastruloids exhibiting rudimentary body axis formation.^[172] By May 2025, researchers at the Francis Crick Institute developed a stem cell-based model of the human amniotic sac that sustains fluid-filled cavities and extra-embryonic tissue development equivalent to 21-28 days post-fertilization, surpassing prior two-week limits through optimized signaling pathways.^[173]^[174] The 14-day rule, established in 1989 by the UK Warnock Report and adopted internationally, restricts research on human embryos to the period before primitive streak appearance, approximately 14 days after fertilization, to balance scientific inquiry with ethical concerns over potential sentience or individuality.^[175] Synthetic embryo models circumvent this limit, as they lack gamete-derived genetic material and are not classified as "embryos" under many regulations, permitting culture extensions to probe post-gastrulation events like amnion-embryo interactions without violating consent requirements for human donors.^[176] Proponents argue this evades barriers to understanding congenital defects, such as neural tube closure, which occur around days 21-28.^[177] However, the International Society for Stem Cell Research (ISSCR) 2021 guidelines categorize such entities with embryo-like features (SHEEFs) for case-by-case review, reflecting uncertainty over their regulatory status.^[178] Ethical critiques question the biological equivalence of these models to natural embryos, noting empirical shortcomings: blastoids often fail to fully integrate all cell lineages or achieve implantation competence, limiting their viability beyond gastrulation stages observed in mouse equivalents.^[172] While synthetics avoid direct embryo destruction, critics contend they risk normalizing the engineering of quasi-human forms, potentially eroding distinctions between research tools and entities deserving protection, especially if scalability improves toward organoid complexity.^[179] This has prompted calls for updated frameworks, such as public consultations on extending the rule to 28 days for viable models only, emphasizing causal developmental fidelity over morphological similarity.^[180] Sources advocating unrestricted extension, often from academic bioethics outlets, may underweight these viability gaps due to institutional pressures favoring research expansion, underscoring the need for first-principles evaluation of entity status based on integrated functionality rather than intent to bypass rules.

Impacts on Broader Reproductive and Abortion Policies

Advances in embryo science, particularly regarding the continuous developmental trajectory from fertilization, have intersected with reproductive policies by highlighting inconsistencies in treating early human organisms as disposable while restricting later-term interventions. In the United States, in vitro fertilization (IVF) has resulted in an estimated 1.2 to 1.5 million cryopreserved embryos in storage as of recent assessments, many of which are surplus to patients' family-building needs.^[93]^[181] Patients typically face disposition options including indefinite storage, donation for research, destruction, or transfer for adoption, with destruction often involving thawing without implantation, effectively ending the embryo's potential development.^[182] These practices fuel debates over whether embryos warrant protection akin to born children, paralleling abortion discussions by framing disposal as the termination of a distinct human entity with unique DNA from conception.^[183] Post the 2022 Dobbs v. Jackson Women's Health Organization decision overturning Roe v. Wade, embryo-related rulings have accelerated personhood recognitions, influencing broader abortion frameworks. The Alabama Supreme Court's February 2024 decision in LePage v. Center for Reproductive Medicine classified frozen embryos as "unborn children" under the state's Wrongful Death of a Minor Act, enabling lawsuits for their destruction and prompting temporary halts in IVF services statewide.^[151] This ruling, grounded in statutory interpretation extending protections to extrauterine embryos, spurred legislative responses like Alabama's March 2024 law shielding IVF providers from liability while affirming embryo status, and has inspired similar personhood initiatives in at least 14 states by mid-2024.^[184]^[185] Such developments challenge abortion policies by extending legal continuity from embryonic stages, where empirical evidence confirms organized cellular differentiation and genetic uniqueness immediately post-fertilization, to later gestation. Critiques of mainstream abortion policies emphasize developmental continuity, arguing that minimizing early human status overlooks data on fetal sensory capacities emerging in the second trimester, which undermines gestational limit rationales. Peer-reviewed analyses indicate thalamocortical connections requisite for pain perception may form by 20-24 weeks, with behavioral and physiological responses to stimuli observable earlier, contradicting claims of no capacity before 24 weeks propagated by some medical bodies.^[186]^[187] This evidence, combined with embryo science affirming seamless progression without qualitative leaps in moral ontology, informs policy arguments for uniform protections from conception, as partial recognitions post-Dobbs reveal causal inconsistencies in permitting late-term procedures while debating early embryo disposal.^[133] Sources advancing later pain thresholds, often from institutions with documented ideological alignments, have faced scrutiny for underweighting neuroanatomical milestones, thereby biasing policy toward delayed personhood thresholds.^[188]

References

[1]
Embryo - Definition and Examples - Biology Online Dictionary
Aug 13, 2023 · An embryo refers to the early developmental stage of a multicellular organism that follows fertilization.Embryo Definition · Plant Embryonic Development · Research And Technology
[2]
Embryonic Development - Developmental Biology - NCBI Bookshelf
In plants, the term embryogenesis covers development from the time of fertilization until dormancy occurs.
[3]
The Circle of Life: The Stages of Animal Development - NCBI
This series of extensive cell rearrangements is called gastrulation, and the embryo is said to be in the gastrula stage.
[4]
Animal Development I: Fertilization & Cleavage - Organismal Biology
Learning Objectives. List the sequence of early events in animal development: fertilization, cleavage, gastrulation, and organogenesis ...
[5]
Human embryo: a biological definition - PubMed
A human embryo is a discrete entity that has arisen from either: the first mitotic division when fertilization of a human oocyte by a human sperm is complete.
[6]
Human embryo: a biological definition - Oxford Academic
Definitions of a human embryo normally include those entities created by the fertilization of a human oocyte by a human sperm. However, there have been a number ...Abstract · Introduction · Discussion · The biological definition of...
[7]
Human embryonic development: from peri-implantation to gastrulation
The basic body plan of the mammalian embryo is established through gastrulation, a pivotal early postimplantation event.
[8]
An Introduction to Early Developmental Processes - NCBI - NIH
Embryos must develop three very important axes that are the foundations of the body: the anterior-posterior axis, the dorsal-ventral axis, and the right-left ...
[9]
Re-defining the human embryo: A legal perspective on the creation ...
Jan 2, 2024 · And if so, the definition of an embryo should be: a cell or group of cells that have the inherent capacity to develop into a human being, which ...
[10]
What is an embryo?1 - Fertility and Sterility
161), “embryo” is defined as, “In mammals, the stage between blastocyst and fetus, when organs and organ systems are coming into existence.” In the midst of ...
[11]
Embryonic human persons. Talking Point on morality and ... - NIH
This means that the embryo has the same nature—it is the same kind of entity, a whole human organism—from fertilization forward; there is only a difference in ...
[12]
A Scientific View of When Life Begins - Lozier Institute
Jun 11, 2014 · Human embryos from the one-cell (zygote) stage forward show uniquely integrated, organismal behavior that is unlike the behavior of mere human ...
[13]
Hallmarks of Totipotent and Pluripotent Stem Cell States - PMC - NIH
In a natural embryo, totipotency is a trait of the blastomeres, whereas pluripotency is characteristic of the epiblast cells.
[14]
Stages of Fetal Development - Women's Health Issues
A fertilized egg develops into a blastocyst, then an embryo, then a fetus. ... (zygote) divides and develops into a hollow ball of cells called the blastocyst.Fertilization · Development Of The... · Development Of The Embryo...
[15]
Blastocyst: Definition, Stage & Implantation - Cleveland Clinic
Apr 29, 2022 · A blastocyst is one step among many that lead to a pregnancy. A blastocyst forms about five to six days after a sperm fertilizes an egg.Overview · What Is A Blastocyst? · Conditions And Disorders
[16]
Embryology, Week 1 - StatPearls - NCBI Bookshelf
Apr 17, 2023 · During the first eight weeks of development, the conceptus shifts from a single-celled zygote into a multi-layered, multi-dimensional fetus ...
[17]
Timeline human development - UNSW Embryology
Week 1 to Week 8 (GA 10)are considered the embryonic period of development. Week 9 to week 37 (GA 11-39) or birth are considered the fetal period of development ...
[18]
The Pre-Embryo: An Illusory Category Of Convenience
Aug 1, 1999 · There are serious flaws with this stage of the argument. First, it relies on social construction as a valid method of defining an entity or ...
[19]
[PDF] The Big Lie in Human Embryology: The Case of the Preembryo
Nov 4, 1997 · As a consequence, more misinformation, misrepresentations, and outright lies about human embryology have found their way into the public.
[20]
Fetal Development: Week-by-Week Stages of Pregnancy
It's now called an embryo. Structures and organs like the neural tube (which later becomes the brain and spinal cord), head, eyes, mouth and limbs form. The ...Morning Sickness · Conception · Placenta
[21]
Fetal development: MedlinePlus Medical Encyclopedia
Aug 23, 2023 · Week 5 is the start of the "embryonic period." This is when all the baby's major systems and structures develop. The embryo's cells multiply and ...
[22]
Hox genes in development and beyond
Jan 16, 2023 · Hox genes encode evolutionarily conserved transcription factors that are essential for the proper development of bilaterian organisms.Hox protein structure and... · Regional specificity and... · Continuing roles for Hox...
[23]
Establishment of Hox vertebral identities in the embryonic spine ...
In vertebrates, Hox genes are arranged colinearly in intact clusters of ~100kb in length (Duboule, 2007). The striking conservation of the integrity of ...
[24]
Hox genes regulation in vertebrates - Soshnikova - 2014
Jul 5, 2013 · Hox genes transcription is tightly controlled at multiple levels, and involves an intricate combination of local and long-range cis-regulatory elements.
[25]
4.6.1: Embryogenesis - Biology LibreTexts
Jul 28, 2025 · The process of embryogenesis in monocots is similar to that of eudicots, but as there is only a single cotyledon, no heart stage occurs. ...
[26]
Monocot embryogenesis - PropG - University of Florida
Proembryo stage Monocots have a more complex embryo structure in the mature seed compared to dicots, but early embryo development is similar to dicots. The ...
[27]
Developmental and genomic architecture of plant embryogenesis
Jan 11, 2021 · In this review on embryogenesis, we compare the model eudicot Arabidopsis thaliana with monocot crops, focusing on genome activation, paternal and maternal ...Embryo Development In Plants · Arabidopsis Thaliana: A... · Transcriptome Studies In...
[28]
[PDF] For submission to Genome Research Evolutionary conservation of ...
The Hox4, 7 and 11 genes are expressed in different regions of the developing embryos (rhombomeres 6 and 7 in the hindbrain for Hox4 paralogous group, thoracic ...<|separator|>
[29]
Comparative Transcriptomics Reveals Distinct Patterns of Gene ...
Vertebrates, like arthropods or flowering plants, establish a conserved body plan during development from which species-specific elaborations develop. Yet ...
[30]
Of mice and plants: Comparative developmental systems biology
Apr 1, 2020 · Multicellular animals and plants represent independent evolutionary experiments with complex multicellular bodyplans.
[31]
Plant Development - Molecular Biology of the Cell - NCBI Bookshelf
The development of a flowering plant, like that of an animal, begins with division of a fertilized egg to form an embryo with a polarized organization.
[32]
Embryo - Etymology, Origin & Meaning
"fetus in utero at an early stage of development," mid-14c., from Medieval Latin embryo, properly embryon, from Greek embryon "a young one," in Homer, "young ...
[33]
https://www.merriam-webster.com/dictionary/embryo
[34]
Aristotle (384-322 BC): the beginnings of Embryology
May 10, 2022 · Embryology proper really began with Aristotle, who 'discovered' in the chick embryo the ideal object for embryological studies (Hamburger and ...
[35]
Haeckel's ABC of evolution and development - PubMed
We re-examine Haeckel's work and its significance for modern efforts to develop a rigorous comparative framework for developmental studies.Missing: nomenclature | Show results with:nomenclature
[36]
Embryology - Etymology, Origin & Meaning
"fetus in utero at an early stage of development," mid-14c., from Medieval Latin embryo, properly embryon, from Greek embryon "a young one," in Homer, "young ...
[37]
Aristotle (384-322 BCE) | Embryo Project Encyclopedia
Jul 7, 2016 · He originated the theory that an organism develops gradually from undifferentiated material, later called epigenesis. Aristotle was born in 384 ...
[38]
Preformationism in the Enlightenment | Embryo Project Encyclopedia
Aug 11, 2008 · Preformationism was a theory of embryological development used in the late seventeenth through the late eighteenth centuries.
[39]
(PDF) Watching the embryo: Evolution of the microscope for the ...
Apr 26, 2021 · Recent advances in bioimaging include light sheet, super resolution, and multiphoton microscopy for imaging embryonic processes at single cell ...
[40]
Epigenesis and Preformationism
Oct 11, 2005 · Epigenesis and Preformation are two persistent ways of describing and seeking to explain the development of individual organic form.
[41]
"Contributions to the Development of the Embryo. On the Artificial ...
Jul 20, 2009 · Wilhelm Roux was an influential figure in the early history of experimental embryology. Although he originally studied medicine, he was invited ...
[42]
The Germ-Plasm: a Theory of Heredity (1893), by August Weismann
Jan 26, 2015 · In The Germ Plasm, Weismann rejects the theory and argues that acquired characteristics are traits of the soma cells, and the hereditary ...
[43]
Spemann-Mangold Organizer | Embryo Project Encyclopedia
Jan 12, 2012 · Spemann and Mangold published their finding of the amphibious organizer in 1924 in the article, “Über Induktion von Embryonalanlagen durch ...
[44]
https://www.nature.com/scitable/topicpage/discovery-of-dna-structure-and-function-watson-397
[45]
Mechanisms of human embryo development: from cell fate to tissue ...
Compaction. At the 8- to 16-cell stage, human embryos undergo the process of compaction; blastomeres (see Glossary, Box 1) adhere tightly to each other, ...
[46]
Human pre-implantation embryo development - PMC - NIH
Implantation in humans occurs on approximately day 7 of development and depends on steroid hormones, such as estradiol-17β and progesterone, to induce changes ...
[47]
Mechanics of human embryo compaction - bioRxiv
Jan 11, 2022 · The shaping of the human embryo begins with compaction, during which cells come into close contact and form a tighter structure.
[48]
Cell contraction drive the initial shaping of human embryos
May 2, 2024 · Human embryo compaction, an essential step in the first days of an embryo's development, is driven by the contractility of its cells.
[49]
Human Chorionic Gonadotropin-Mediated Immune Responses That ...
hCG represents one of the first molecular messages send out by the pre-implanting embryo to modulate the implantation site and to ensure a timely initiation of ...
[50]
Enhancing endometrial receptivity: the roles of human chorionic ...
Apr 4, 2024 · Human chorionic gonadotropin (hCG) is a key signaling molecule secreted during early embryonic development, which regulates embryonic maternal ...
[51]
Early embryo mortality in natural human reproduction: What the data ...
Loss from implantation to birth is approximately one third. Natural pre-implantation embryo loss remains quantitatively undefined. In the ...
[52]
Association of early cleavage, morula compaction and blastocysts ...
Jan 7, 2024 · Another two points of interest are embryo compaction and morula formation. Those are complex processes occurring immediately before blastocyst ...
[53]
Gastrulation and Body Axes Formation: A Molecular Concept and Its ...
During the third week of human pregnancy, an embryo transforms from two germinal disc layers of hypoblast and epiblast to three germinal layers of endoderm, ...Introduction · Gastrulation Event · Dorsoventral Body Axis
[54]
Human gastrulation: The embryo and its models - ScienceDirect.com
The analysis revealed patterns of gene expression associated with the three germ layers, PGCs, yolk sac mesoderm and haematopoiesis (pre-dominantly in the yolk ...
[55]
The enigmatic primitive streak: prevailing notions and challenges ...
During gastrulation, the streak extends through the embryonic midline, creating the anterior-posterior (A-P) body axis, and transforming primitive ectoderm (“ ...
[56]
Induction and patterning of the primitive streak, an organizing center ...
Jan 28, 2004 · The primitive streak is the organizing center for amniote gastrulation. It defines the future embryonic midline and serves as a conduit of ...INTRODUCTION · MORPHOGENESIS OF THE... · PATTERNING OF THE INITIAL...
[57]
Genetics of Wnt Signaling During Early Mammalian Development
The Wnt/β-catenin pathway plays a major role in the formation of the PS during gastrulation. Both Wnt3 and Wnt8a are radially expressed in the proximal epiblast ...
[58]
Notch signaling in the division of germ layers in bilaterian embryos
In this review, we discuss the role of the Notch pathway in the establishment and refinement of boundaries between germ layers in bilaterians.
[59]
WNT signaling, in synergy with T/TBX6, controls Notch signaling by ...
WNT signals regulate Dll1 transcription, controlling Notch activity, somite formation, and patterning in the presomitic mesoderm.
[60]
Establishment of left–right asymmetry in vertebrate development
In this review article, we discuss the mechanisms by which the initial symmetry along the left–right axis is broken in the mouse embryo.
[61]
Roles of Motile and Immotile Cilia in Left-Right Symmetry Breaking
Jun 25, 2016 · The breaking of L-R symmetry takes place in the node, an embryonic midline structure located at the anterior tip of the primitive streak in ...
[62]
Nodal Flow and the Generation of Left-Right Asymmetry - Cell Press
Nodal flow is autonomously generated by the rotation of cilia that are tilted toward the posterior on cells of the ventral node.
[63]
Embryology, Weeks 6-8 - StatPearls - NCBI Bookshelf
Oct 10, 2022 · By week 8, organogenesis is complete. The fetus appears human-like and is prepared to undergo further growth and differentiation. Go to ...Missing: milestones | Show results with:milestones
[64]
Embryology, Neural Tube - StatPearls - NCBI Bookshelf - NIH
It starts during the 3rd and 4th week of gestation. This process is called primary neurulation, and it begins with an open neural plate, then ends with the ...
[65]
6 to 7 Weeks | Prenatal Overview
The embryo has brainwaves by 6 weeks, 2 days! From 6 to 6½ weeks, the cerebral vesicles will double in size.1 Individualized brainwaves recorded via ...
[66]
When Can a Fetus Feel Pain in the Womb? - WebMD
Apr 21, 2025 · Pain receptors start developing in the body by 7 weeks and are linked to the brain by 12-15 weeks. There are pathways for pain in a brain ...
[67]
Recent advances in the molecular mechanism of thalidomide ...
The most sensitive time frame for the teratogenic effects of thalidomide in humans is approximately 20–36 days post-fertilization [[22], [23], [24]].
[68]
Thalidomide - MotherToBaby
Feb 1, 2023 · When a woman who is pregnant takes thalidomide early in pregnancy (between the 20th and 36th day after conception or the 34-50th day after the ...Missing: timing | Show results with:timing
[69]
Cellular Dynamics of Double Fertilization and Early Embryogenesis ...
Jul 8, 2020 · Flowering plants (angiosperms) perform a unique double fertilization in which two sperm cells fuse with two female gamete cells in the embryo sac to develop a ...
[70]
MAB4-induced auxin sink generates local auxin gradients in ... - PNAS
Our results show that NONPHOTOTROPIC HYPOCOTYL 3-like proteins establish auxin sink, but not auxin convergence, through the control of PIN1 localization.
[71]
The suspensor: not just suspending the embryo - PubMed
The suspensor is a terminally differentiated embryonic region that connects the embryo to surrounding tissues during early seed development.
[72]
Embryogenesis in Angiosperms: Development of the Suspensor
Classically, the suspensor was thought to play a pas- sive role in embryo development by holding the embryo proper in a fixed position within the seed ...
[73]
Building a plant: cell fate specification in the early Arabidopsis embryo
Feb 1, 2015 · PAT and the establishment of auxin maxima control embryo development. Accordingly, auxin distribution changes dynamically at key steps of ...Introduction · Cell division patterns in the... · Formative vascular divisions in...
[74]
An Essential Function for Auxin in Embryo Development - PMC
The phytohormone auxin profoundly controls pattern formation during embryogenesis. Auxin functions in the embryo through its maxima/minima distribution.
[75]
Induction of Somatic Embryos in Arabidopsis Requires Local ...
In this study, we showed that YUCCAs (YUCs) encoding key enzymes in auxin biosynthesis are required for SE induction in Arabidopsis.
[76]
Experimental data and computational modeling link auxin gradient ...
The presence of an auxin gradient in the Arabidopsis root is crucial for proper root development and importantly, for stem cell niche (SCN) maintenance.
[77]
Apomixis in plant reproduction: a novel perspective on an old dilemma
As a reproductive strategy for cloning plants via seeds, apomixis is a highly desirable trait in modern agriculture.
[78]
[PDF] Apomixis: new horizons in plant breeding
Jul 8, 2015 · Apomixis is a mechanism of seed formation without fertilization and is observed in more than 300 species in. 30 out of 460 angiosperm families, ...<|separator|>
[79]
The Rise of Apomixis in Natural Plant Populations - Frontiers
Apomixis, the asexual reproduction via seed, has many potential applications for plant breeding by maintaining desirable genotypes over generations.
[80]
https://plantcelltechnology.com/blogs/blog/blog-somatic-embryogenesis
[81]
(PDF) Somatic Embryogenesis - ResearchGate
The earliest report of Somatic Embryo (SE) formation in cultured cells of Daucus carota is generally credited to Reinert (1958) and Steward et al. (1958).
[82]
Application of Somatic Embryogenesis in Woody Plants - PMC
In woody plants, somatic embryogenesis plays a critical role in clonal propagation and is a powerful tool for synthetic seed production, germplasm conservation ...
[83]
Signaling Overview of Plant Somatic Embryogenesis - PMC
Feb 7, 2019 · Somatic embryogenesis has several biological and scientific advantages. For instance, it has the potential for the improvement of plants of ...
[84]
World's first "test tube" baby born | July 25, 1978 - History.com
Mar 12, 2010 · On July 25, 1978, Louise Joy Brown, the world's first baby to be conceived via in-vitro fertilization (IVF) is born at Oldham and District General Hospital in ...
[85]
IVF Step-by-step | UR Medicine
Embryo transfer typically occurs on day 3 (cleavage stage) or day 5 (blastocyst stage). It is a simple, outpatient procedure that does not require anesthesia.
[86]
Embryo Transfer: Types, Process, Costs, FAQs, and More
Sep 29, 2025 · Most cleavage-stage transfers occur three days after retrieval, and most blastocyst-stage transfers occur five days after. However, embryos ...Single Embryo Transfer For IVF · Day 3 Vs Day 5 Embryo Transfer
[87]
Time-lapse technology for embryo culture and selection - PMC - NIH
If single culture media are used, the embryos can be kept inside the time-lapse system continuously from directly after intracytoplasmic sperm injection (ICSI) ...
[88]
EmbryoScope time-lapse system - Vitrolife
EmbryoScope™+ time-lapse systems allow embryologists to monitor your embryos while culturing them in an undisturbed stable environment for the entire culture ...
[89]
Time-lapse imaging: Morphokinetic analysis of in vitro fertilization ...
Time-lapse imaging (TLI) allows continuous monitoring of embryo development without disturbing culture conditions by removing embryos from the incubator.
[90]
Success Rates | RMA of New York - World Class Fertility Service
What is the success rate of IVF on the first try? · Under the age of 35: 48.0% live birth rate per first embryo transfer · Ages 35-37: 41% live birth rate per ...
[91]
SART: Success Rates - Society for Assisted Reproductive Technology
SART provides an online calculator to predict success. This is an excellent tool to estimate the cumulative live birth rate up to three cycles.<|separator|>
[92]
Fertility Success Rates - IVF, IUI, & More - CCRM Fertility
CCRM success rates by region ; Live Birth Per Single Embryo Transfer, 49.7%, 50.9%, 53.7%, 54.5% ; Singleton Delivery Per Live Birth Resulting from Single Embryo ...
[93]
Too many embryos: a critical perspective on a global challenge - PMC
Jun 5, 2024 · It has been estimated that 1.2 million frozen embryos are currently in storage in the United States alone [8]. To address the prospect of unused ...
[94]
A quantitative analysis of stored frozen surplus embryos in the UK
Jun 26, 2024 · Data analysis show that at least 130,000 stored embryos have been discarded in the UK since 1991, while another 500,000 embryos are currently ...
[95]
Complete human day 14 post-implantation embryo models ... - Nature
Sep 6, 2023 · These human complete SEMs demonstrated developmental growth dynamics that resemble key hallmarks of post-implantation stage embryogenesis up to 13–14 days ...
[96]
Stem cell-based models of early human development
Apr 17, 2025 · Stem cell-based embryo models (SCBEMs) are structures generated from three-dimensional (3D) culture of pluripotent stem cells and their derivatives.
[97]
Epigenetic Editing of Mouse Stem Cells Generates Embryoids for ...
Mar 20, 2025 · When they analyzed the embryoids, the scientists noted that 80% of the stem cells organize themselves into a structure that mimics a basic ...
[98]
The ISSCR Releases Targeted Update to the Guidelines for Stem ...
Aug 11, 2025 · The targeted update addresses significant advances in the development and application of human stem cell-based embryo models (SCBEMs) and ...Missing: implantation | Show results with:implantation
[99]
Three-Dimensional Models of Implantation Using Human Stem Cells
Jul 16, 2025 · While advances in stem cell science have enabled the possibility of using blastoids to gain insights into the intricacies of implantation, and ...
[100]
2. Laboratory-based Human Embryonic Stem Cell Research ...
Aug 11, 2025 · Stem cell and embryo research show great promise for advancing our understanding of human development and disease, including research to address ...
[101]
Risks and benefits of human germline genome editing - NIH
Correcting a pathogenic genetic mutation in the germ cells of an individual patient means that the genetic errors disappear from the germline.
[102]
A comprehensive review and update on human fertility ...
Apr 18, 2023 · Vitrification is currently recommended for freezing oocytes and embryos as it has shown remarkably increased live birth rates (LBRs) (44, 45).
[103]
Highly efficient vitrification for cryopreservation of human oocytes ...
Jan 1, 2007 · This technology alone has resulted in more healthy babies after cryopreservation of blastocysts than any other vitrification technique, and more ...
[104]
Cryopreservation of Oocyte in Livestock: Principles, Techniques and ...
Successful cryopreservation of livestock animals' oocytes and embryos by ultrarapid vitrification, resulting in the production of offspring. Oocytes and embryos ...2.1 Slow Freezing · 3.1 Cryoprotective Agents · 3.2 Cryopreservation...
[105]
Cryopreservation of biological materials: applications and economic ...
Apr 23, 2025 · Vitrification avoids damage associated with ice crystals and often results in higher post-thaw survival rates for embryos and oocytes.Cryoprotectants · Cryopreservation Procedure... · Future Research Directions<|separator|>
[106]
Resurrecting biodiversity: advanced assisted reproductive ... - NIH
Following cryopreservation, ART/aART can be used to reinstate lost genetics back into a population, resurrecting biodiversity. However, for this to be ...
[107]
The ART of bringing extinction to a freeze - ScienceDirect.com
Jul 15, 2021 · Cryopreservation of viable cells can keep otherwise extinct species suspended and buy time until the methodology and technology are in place to ...
[108]
The Role of Cryopreservation in Endangered Species Conservation
Aug 25, 2022 · Cryopreservation has reached a level where it now has an invaluable role to play in the conservation of endangered species.
[109]
CRISPR'd babies: human germline genome editing in the 'He ...
The world was shocked in Nov. 25, 2018 by the revelation that He Jiankui had used clustered regularly interspaced short palindromic repeats ('CRISPR') to edit ...
[110]
CRISPR bombshell: Chinese researcher claims to have created ...
HONG KONG, CHINA—On the eve of an international summit here on genome editing, a Chinese researcher has shocked many by claiming to have altered the genomes ...
[111]
Embryo and fetal gene editing: Technical challenges and progress ...
Mar 4, 2024 · Editing efficacy was 44%–100% with allelic frequencies of 100% and no evidence of mosaicism. These complementary experiments demonstrated site- ...<|control11|><|separator|>
[112]
The quest to create gene-edited babies gets a reboot - NPR
Aug 6, 2025 · There's a fresh push to edit the genes of human embryos to prevent diseases and enhance characteristics that parents value.
[113]
The technical risks of human gene editing - PMC - PubMed Central
Nov 14, 2019 · Numerous studies have reported unexpected genomic mutation and mosaicism following the use of CRISPR/Cas nucleases, and it is currently unclear ...
[114]
Evaluation of mutation rates, mosaicism and off target ... - Nature
Dec 18, 2020 · Although an efficient and targeted tool, the CRISPR/Cas9 system can have some drawbacks, including off-target mutations and mosaicism, ...
[115]
Mosaicism in CRISPR/Cas9-mediated genome editing - ScienceDirect
Jan 15, 2019 · However, one of the consequences of CRISPR-mediated gene editing in embryos is the existence of genetic mosaicism in founders. In particular, ...
[116]
Ethical Issues: Germline Gene Editing | ASGCT
Feb 3, 2025 · Mosaicism is a risk associated with clinical germline editing, in which multiple populations of cells that have different genetic makeups exist ...
[117]
Precambrian animal diversity: Putative phosphatized embryos from ...
This high-resolution fossil bed is about 30% phosphate, present as the mineral fluorapatite [Ca5(PO4)3F]. Phosphatic beds within this deposit are grainstones ...
[118]
Complex embryos displaying bilaterian characters from ... - PNAS
Here we report the discovery of two Doushantuo embryos that are three-dimensionally preserved and complex.
[119]
The role of amorphous silica coating on apatite nanocrystals in the ...
Aug 14, 2025 · The Ediacaran Doushantuo Formation (ca. 600 Ma) in South China is renowned for its exceptionally preserved phosphatized microfossils, ...
[120]
Raman spectra of a Lower Cambrian ctenophore embryo ... - PNAS
Apr 10, 2007 · We here report the finding in this assemblage of an exquisitely preserved late-stage embryo of a ctenophore (“comb jelly”), its fine structure ...<|separator|>
[121]
Early Cambrian animal diapause embryos revealed by X-ray ...
Feb 23, 2018 · However, fossil embryos are extremely rare, in terms of their diversity, abundance, and stratigraphic distribution (Donoghue et al., 2006).Missing: major | Show results with:major
[122]
Early metazoan life: divergence, environment and ecology - PMC
Yet the first fossil evidence for metazoans (the Doushantuo embryos) about 600 Ma is followed by the Ediacaran fossils after 580 Ma, the earliest undisputed ...
[123]
Exceptionally preserved early Cambrian bilaterian developmental ...
Feb 15, 2021 · Fossilized invertebrate embryonic and later developmental stages are rare and restricted largely to the Ediacaran-Cambrian, providing direct ...
[124]
Macroevolutionary developmental biology: Embryos, fossils, and ...
Aug 6, 2015 · Fossil embryos help constrain when developmental innovations first appeared. Fossil embryos help reveal the order in which related ...
[125]
Massospondylus embryos and hatchling provide new insights into ...
Aug 4, 2025 · Fossils representing the early life stages of dinosaurs reveal vital insights into their complex development, ecologies, reproduction, and ...
[126]
Researchers Discover Ecdysozoan Fossil Embryos from the ...
Dec 23, 2024 · Fossilized invertebrate embryos are rare, but when preserved, they offer key insights into the evolutionary developmental biology of extinct ...
[127]
New ecdysozoan fossil embryos from the basal Cambrian of China
Feb 1, 2025 · Here we report new exceptionally preserved fossil embryos from the early Fortunian (early Cambrian) of China, which are assigned to two new taxa.
[128]
Interpretation of fossil embryos requires reasonable assessment of ...
Jul 20, 2022 · Well-preserved, articulated oviraptorosaur embryos in eggs are usually interpreted as representing a stage of development close to hatching.
[129]
Personhood status of the human zygote, embryo, fetus - PMC
In human gestation, the zygote undergoes development into an embryo and fetus. Parturition usually occurs at a full-length term of about thirty-eight weeks. The ...
[130]
When Human Life Begins - American College of Pediatricians
The single-celled embryo is a very different kind of cell than that of sperm or oocyte, and contains a unique genome that will determine most future bodily ...
[131]
The Basics of Brain Development - PMC - PubMed Central
Neuron production in humans begins on embryonic day 42. E42, i.e. 42 days post conception (Bystron et al. 2008; Stiles 2008) and is largely complete by ...
[132]
Development of Brain Networks In Utero: Relevance for Common ...
Proliferation of neural precursor cells, neuroblasts, occurs between the 4th and 20th weeks of gestation, whereas the production of glioblasts, precursors of ...
[133]
Fact Sheet: A Timeline of the Development of Fetal Pain Sensation
... capacity to experience pain by at least 12 to 15 weeks' gestation ... The embryo reflexively responds to touch around the mouth at 7-8 weeks of gestation ...
[134]
[PDF] Early Human Embryonic Development: Individuation Before ...
We now know that at completed fertilization, the human embryo contains all the genetic material necessary for development. We also know that, genetically ...
[135]
Moral Status of the Human Embryo - Oxford Academic
Mar 13, 2018 · If human embryos have moral status as human beings, then they have the full panoply of human rights, including the right not to be killed in ...
[136]
The Human Embryo: Potential Person or Person with Great Potential?
Oct 12, 2018 · Human embryos are not potential persons; they are actual persons with potential. ... In order to demonstrate the personhood of an embryo, it is ...
[137]
Debating the Moral Status of the Embryo | Harvard Magazine
Jul 1, 2004 · George holds what is known among moral philosophers as the "equal moral status" view of the human embryo.
[138]
Embryos as unborn children: the Alabama Supreme Court's ruling ...
Apr 2, 2024 · On February 16, 2024, rhe Alabama Supreme Court issued a landmark ruling, defining frozen embryos as “unborn children located outside of a biological uterus.”
[139]
When does the fetus acquire a moral status of a human being? The ...
Jun 30, 2022 · Gradualism insists that the development of moral status is a gradual process that occurs continuously throughout gestation.
[140]
Clinicians' criteria for fetal moral status: viability and relationality, not ...
Aug 21, 2024 · Clinicians tended to take a gradualist approach to moral status during pregnancy as they developed and viewed viability, the ability to live outside of the ...
[141]
The ethics of foetal moral status - Calum Miller
Feb 10, 2022 · It is widely agreed among ethicists that viability does not change the inherent moral status of the foetus. It is hard to see how it could: ...<|separator|>
[142]
[PDF] The Moral Status of the Human Embryo According to Peter Singer
Peter Singer's arguments against the early human embryo being an individual human person are unconvincing. While admitting that he wants to free us from ...
[143]
[PDF] Moral Status, Moral Value, and Human Embryos - Scholars Archive
Another explanation for support for ESC research on the part of opponents of abortion is that abortion is morally charged in a way that embryo research is not.
[144]
Why we should not extend the 14-day rule | Journal of Medical Ethics
The committee provided two reasons for morally justifying research on early embryos—the utilitarian argument, based on the perceived benefits of such research, ...
[145]
Promises and rules: The implications of rethinking the 14‐day rule ...
Aug 16, 2021 · The “14‐day rule,” which stipulates that human embryos cannot be grown in vitro longer than 14 days after post‐fertilization or beyond the appearance of a ...
[146]
ISSCR Guidelines for Stem Cell Research and Clinical Translation
May 27, 2021 · Key advances that the new 2021 Guidelines cover include the following: the culture of human embryos and stem cell-derived models of embryo ...
[147]
Are we ready for the revision of the 14-day rule? Implications from ...
The 14-day limit on human embryo research has been removed by ISSCR, the first scientific society which has stated that the study should not be governed as ...
[148]
Dickey-Wicker Amendment, 1996 | Embryo Project Encyclopedia
Aug 27, 2010 · It is important to note that the Dickey-Wicker amendment only prohibited federal funding for experimentation using human embryos, not the ...
[149]
Federal funding for stem cell research: 15 years of indecision - NIH
The battle over whether to devote government funding to ES cell research has waged since the Dickey-Wicker amendment passed in 1996.
[150]
Who Gets Frozen Embryos in a Divorce? - DivorceNet
Feb 29, 2024 · Several states, including New York, California, and Oregon treat frozen embryos as property. California, for example, applies contract, not ...What Happens To Unused... · What Is The Legal Status Of... · How Judges Resolve Disputes...
[151]
The Alabama Supreme Court's Ruling on Frozen Embryos
Feb 27, 2024 · The Alabama Supreme Court issued a ruling on February 16 declaring that embryos created through in vitro fertilization (IVF) should be considered children.Missing: moral | Show results with:moral
[152]
The push for embryo rights in state legislatures worries IVF patients ...
Jun 4, 2024 · 13 states introduced bills this year that could give rights to embryos and fetuses associated with people. None passed yet but IVF patients ...
[153]
Human Dignity, Human Rights, and Article 6(1) of the EU Directive ...
Patents on human embryonic stem cells could potentially be excluded under the EU Directive on Biotechnological Inventions either because they fall under the ...
[154]
Report on Human Embryonic Stem Cell Research
Concerning the countries acceding to the EU, no specific regulations regarding human embryonic stem cell research have at present been implemented. Estonia, ...
[155]
Embryo/Fœtus - Human Rights and Biomedicine
It also stipulates that, where the law allows research on embryos, it shall ensure adequate protection of the embryo. Scientific, technological and medical ...
[156]
Brüstle to ISCC: Rethinking “Human Embryo” in EU Patent Law
Aug 22, 2025 · The European Union's legal treatment of human embryonic stem cell (hESC) technologies has long stood at the intersection of science, ethics, and ...Missing: regulations | Show results with:regulations
[157]
In wake of gene-edited baby scandal, China sets new ethics rules ...
Mar 7, 2023 · In 2021, China also made human germline editing for clinical use a crime. Last year, the government called on all institutions conducting ...
[158]
All clinical research using germline genome editing banned in China
Jul 19, 2024 · China's science and technology ministry has prohibited all clinical research involving germline genome editing, describing the approach as 'irresponsible'.
[159]
Regulatory framework of human germline and heritable genome ...
May 13, 2025 · China's existing regulatory framework for human genome editing has improved with several laws enacted and updated, but there are shortcomings.
[160]
Chinese scientist who produced genetically altered babies ... - Science
The Chinese government later tightened regulations covering human genome editing. He has not been seen in public since his presentation at the Hong Kong ...
[161]
Celebrating 25 years of embryonic stem cell research at UW–Madison
May 24, 2021 · Thomson then published his world-changing breakthrough on human embryonic stem cell derivation in Science on Nov. 6, 1998.
[162]
More human embryos destroyed through IVF than abortion every year
Mar 13, 2024 · Somewhere between 1.5 million and 1.8 million embryos created through IVF were never born.
[163]
Examining the ethics of embryonic stem cell research
MS: Proponents argue that embryonic stem cell research holds great promise for understanding and curing diabetes, Parkinson's disease, spinal cord injury, and ...
[164]
The Case Against Embryo Destruction | MCCL
Second, ESC research is unethical because it requires the killing of human embryos. As a matter of biological fact, a human embryo—whether a result of natural ...
[165]
Comparison of Human Induced Pluripotent and Embryonic Stem Cells
Human induced pluripotent stem cells (hiPSCs) have been hailed as an effective replacement for human embryonic stem cells (hESCs) and a prime candidate cell ...
[166]
Comparing ESC and iPSC—Based Models for Human Genetic ... - NIH
In this review we compare ESC-based models to iPSC-based models, and highlight the advantages and disadvantages of each system.
[167]
Harnessing the Potential of Stem Cells for Disease Modeling
In this review, we discuss important breakthroughs of stem cell-based models highlighting current directions, advantages, and limitations.
[168]
Keeping the Backdoor to Eugenics Ajar?: Disability and the Future of ...
Noninvasive prenatal testing arguably constitutes a form of eugenics in a social context in which certain reproductive outcomes are not valued.Missing: criticisms | Show results with:criticisms
[169]
1.7 Million Human Embryos Created for IVF Thrown Away
Official statistics show that almost half of embryos used to help a women conceive through in vitro fertilisation were thrown away during or after the process.
[170]
Advancements in Human Embryonic Stem Cell Research
Mar 19, 2024 · The development and use of human embryonic stem cells (hESCs) in regenerative medicine have been revolutionary, offering significant advancements in treating ...
[171]
A new era of stem cell and developmental biology: from blastoids to ...
Oct 2, 2023 · This review aims to explore developments in stem cell research, focusing on stem cell-based in vitro early embryonic developmental models.
[172]
Current Status of Synthetic Mammalian Embryo Models - MDPI
In this review, we summarize the recent advancements in embryo developmental research across different species, focusing on mice, humans, and non-human ...
[173]
New stem cell model sheds light on human amniotic sac development
May 15, 2025 · Researchers at the Francis Crick Institute have developed a new stem cell model of the mature human amniotic sac, which replicates development of the tissues ...Missing: synthetic | Show results with:synthetic
[174]
Stem cells coaxed into most advanced amniotic sacs ever ... - Nature
May 15, 2025 · Researchers have coaxed stem cells to grow into amniotic sacs filled with fluid. 1 . The model sacs, which grew to roughly the same size as a four-week-old sac.
[175]
Should the 14‐day rule for embryo research become the 28 ... - NIH
As research advances our knowledge and capacity to create SCDGs, it will also become possible to use these to create embryos and study them beyond 14 days.Missing: blastoids 2023-2025
[176]
Synthetic human embryos could allow for research beyond the 14 ...
Jul 25, 2023 · Synthetic human embryos could allow for research beyond the 14-day limit, but this raises ethical questions · Authors · Disclosure statement.
[177]
[PDF] The 14-day rule in the Dutch Embryo Act
Oct 31, 2023 · Research in the third and fourth weeks could improve understanding of how congenital cardiac abnormalities and neural tube defects. (anencephaly ...
[178]
Adapting the 14-day rule for embryo research to encompass ... - NIH
We consider the scientific evidence that research on in-vitro development of embryos beyond 14 days is necessary.Missing: gastruloids 2023-2025
[179]
Addressing the ethical issues raised by synthetic human entities with ...
Mar 21, 2017 · The 14-day rule for embryo research stipulates that experiments with intact human embryos must not allow them to develop beyond 14 days or the appearance of ...
[180]
If it's time to revisit the 14-day rule, it's also time to engage the public
Aug 13, 2025 · The 14-day rule limits human embryo research to 14 days, but scientific advances now allow for longer culture, raising ethical questions. ...
[181]
Frozen Embryos Are the New Orphan Crisis - Christianity Today
Since Hannah was born, the number of frozen embryos sitting in storage in the United States has risen from roughly 100,000 to an estimated 1.5 million. British ...
[182]
Disposition of unclaimed embryos: an Ethics Committee opinion
Accurate current numbers are difficult to ascertain, but estimates suggest that a large number of unclaimed embryos remain cryopreserved. There may be many ...
[183]
In vitro fertilization and the ethics of frozen embryos - PMC - NIH
May 20, 2024 · As some examples of the magnitude of the problem, it has been estimated that, in 2016, between 400,000 and 1.4 million frozen embryos were ...
[184]
Alabama Supreme Court's Ruling Regarding In Vitro Fertilization ...
May 15, 2024 · The court held that the Act applies to all born and unborn children regardless of their location, which includes embryos created through in vitro fertilization ...
[185]
An uptick in state personhood bills fuels growing fears over IVF ...
Feb 26, 2024 · As of Friday, fetal personhood bills had been introduced in at least 14 state legislatures during their ongoing 2024 sessions, according to the ...Missing: post- | Show results with:post-
[186]
The fetal pain paradox - PMC - PubMed Central
Mar 21, 2023 · Some medical groups state that below 24 weeks gestation, there is no pain capacity. Thus, a paradox exists in the disparate acknowledgment of pain capability.
[187]
Reconsidering fetal pain - Journal of Medical Ethics
Arguably, there never was a consensus that fetal pain is not possible before 24 weeks. Many papers discussing fetal pain have speculated a lower limit for fetal ...
[188]
The fetal pain paradox - Frontiers
Mar 20, 2023 · However, some medical groups state that below 24 weeks gestation, there is no pain capacity. Thus, a paradox exists in the disparate ...