Fact-checked by Grok 2 weeks ago

Egg cell

The egg cell, or ovum (in animals), is the female reproductive cell or gamete in anisogamous species, including animals and plants. In animals, it is a large haploid cell produced in the ovaries through oogenesis. Unlike typical somatic cells, egg cells are often the largest cells in the organism—measuring up to 100–150 micrometers in diameter in mammals—and contain stockpiles of nutrients, proteins, RNA, and organelles essential for early embryonic development following fertilization. The structure varies by species but generally includes a central nucleus with haploid genetic material and abundant cytoplasm rich in yolk or nutrient reserves. In mammals, protective layers include the zona pellucida—a glycoprotein shell surrounding the plasma membrane—and an outer cumulus oophorus layer of follicular cells that aids in transport and recognition during ovulation. In animals, egg cell maturation () involves , with asymmetric divisions ensuring the ovum retains most and resources while polar bodies degenerate. In mammals, oogonia in the fetal form primary oocytes that in I until ; each cycle, one resumes meiosis I to yield a secondary , with meiosis II completing upon fertilization. In plants, the egg cell forms within the ovule's embryo sac through and awaits fertilization by from . Functionally, the egg cell's primary role is : it fuses with a to form a diploid , contributing half the genetic material and providing resources for early development. In animals, upon , it enters the ( in mammals), where fertilization activates completion of and initiates embryogenesis. Egg cells express surface proteins for species-specific binding and prevent via mechanisms like the . Across species, variations exist—such as nutrient reserves in yolky eggs of enabling meroblastic versus holoblastic in mammals—but the core purpose is providing a protected for genetic transmission and early embryogenesis.

General Biology

Definition and Terminology

The egg cell, known as the in , is the in anisogamous organisms, a specialized reproductive cell produced in female reproductive structures that is capable of fusing with a male (sperm) during fertilization to form a . Unlike the smaller, more mobile sperm, the egg cell is typically much larger, often containing substantial cytoplasmic reserves such as or nutrients to support early embryonic development following fertilization. In animals, the terms "egg cell," "ovum," and "oocyte" refer to distinct stages in female gamete development, with the oocyte denoting an immature female germ cell arrested in prophase of meiosis I within the ovarian follicle. Upon hormonal stimulation, the secondary oocyte completes meiosis I, extruding a small first polar body—a non-functional byproduct containing chromosomes but minimal cytoplasm—while the mature ovum results from the completion of meiosis II after fertilization, rendering it haploid with a single set of chromosomes. Polar bodies, which typically degenerate, ensure that only one functional haploid ovum receives the bulk of the cytoplasm. The term "ovum" derives from the Latin word ovum, meaning "," reflecting its historical association with bird eggs as a model for reproductive cells. Similarly, "" originates from the Greek ōon (egg) and kytos (cell or hollow vessel), emphasizing its cellular nature as an egg precursor. Across taxa, egg cells exemplify , the evolutionary divergence of gametes into unequal sizes, where larger female gametes evolved to provision offspring while smaller male gametes prioritize quantity and mobility for fertilization success; this dimorphism arose from disruptive selection on ancestral isogamous populations, as modeled in seminal game-theoretic analyses.

Structure of the Egg Cell

The egg cell, or ovum, is a specialized gamete characterized by its large size relative to other cells and its role in providing genetic and nutritional support for embryonic development. At its core, the egg cell is bounded by a plasma membrane that regulates the exchange of materials and facilitates recognition during fertilization. Enclosed within this membrane is the , known as ooplasm, a viscous matrix rich in , proteins, and other molecules essential for early embryonic processes. The ooplasm houses the , which contains a haploid consisting of 23 chromosomes in humans and equivalent reduced sets in other organisms, ensuring upon fertilization. Key organelles within the ooplasm include mitochondria, which provide through ATP critical for metabolic demands, and ribosomes, which support protein synthesis for cellular maintenance and . These components are universally present across egg cells in , , and fungi, adapting to the organism's reproductive strategy while maintaining eukaryotic . In plant egg cells, located within the embryo sac of the , the ooplasm similarly contains mitochondria and ribosomes, though nutrient storage differs from animal counterparts./06:_Unit_VI-_Plant_Structure_and_Function/6.03:_Plant_Reproduction/6.3.02:_Reproductive_Development_and_Structure) Egg cells are enveloped by protective layers that shield the internal components from environmental stresses and mechanical damage. In animals, the vitelline membrane (or in mammals) is a layer surrounding the outer surface of the plasma membrane, providing protection and aiding in species-specific sperm binding. In and fungi, equivalent protection comes from cell walls made of or , respectively, with plant egg cells further safeguarded by the ovule's nucellus and integuments. These layers collectively prevent , infection, and premature activation./06:_Unit_VI-_Plant_Structure_and_Function/6.03:_Plant_Reproduction/6.3.02:_Reproductive_Development_and_Structure) Nutrient reserves, primarily in the form of yolk or deutoplasm, are stored in the ooplasm to nourish the developing embryo until independent feeding begins. Yolk consists mainly of lipids, proteins, and polysaccharides, distributed variably depending on the organism's developmental needs. Egg cells are classified by yolk quantity into oligolecithal (minimal yolk, as in mammals and echinoderms), mesolecithal (moderate yolk, typical in amphibians and some fish), and macrolecithal (abundant yolk, seen in birds, reptiles, and most fish), with the yolk often concentrated at the vegetal pole in larger eggs to support uneven cleavage patterns. In plants, the egg cell has scant yolk, relying instead on surrounding endosperm or nucellar tissue for nourishment post-fertilization./06:_Unit_VI-_Plant_Structure_and_Function/6.03:_Plant_Reproduction/6.3.02:_Reproductive_Development_and_Structure) Size variations among egg cells reflect their yolk content and developmental strategy, with diameters ranging from about 0.1 mm in and eggs to 1–2 mm in eggs, and up to several centimeters in and eggs due to extensive yolk reserves. These dimensions ensure sufficient resources for embryogenesis, with larger eggs supporting more complex early development in oviparous species. Unique structural features enhance the egg cell's functionality, such as cortical granules located in the peripheral ooplasm, which release enzymes upon sperm entry to modify the protective layers and block in . In immature eggs, the often appears as a prominent germinal vesicle, a large, intact structure before completion, visible under and essential for genetic integrity. These adaptations underscore the egg cell's role in safeguarding across diverse organisms.

Function in Reproduction

The egg cell, or ovum, serves as the female gamete essential for in eukaryotes, primarily functioning to unite with a cell during fertilization to form a diploid . Fertilization begins with attachment to the egg's plasma , often mediated by species-specific receptors, triggering a calcium release within the egg that facilitates between the gametes. This allows the to enter the egg , initiating syngamy where the haploid male and female pronuclei fuse to restore the diploid number. To ensure monospermy, the egg activates rapid defensive mechanisms, including a fast electrical of the and a slower cortical granule that modifies the egg's —such as the in mammals—preventing additional penetration. These processes, conserved across many , protect the genetic integrity of the and are critical for successful reproduction. Post-fertilization, the egg undergoes activation that completes the second meiotic division ( II), extruding chromosomes into a second and yielding a mature haploid ready for pronuclear fusion. This resumption of , arrested in II prior to fertilization, is driven by the calcium signals from entry, which also initiate metabolic rearrangements like increased protein synthesis and mobilization to support formation. The resulting then begins , a series of rapid mitotic divisions that partition the into blastomeres, marking the onset of embryogenesis. These events transform the egg from a static into an active developmental unit, with the reserves briefly referenced here as providing initial energy for until further metabolic shifts occur. Beyond genetic contribution, the egg cell supplies vital nutritional support to the early through maternally deposited mRNAs, proteins, and organelles, which orchestrate prior to zygotic activation (ZGA). These maternal factors, accumulated during , drive the first few cycles by regulating translation and progression, compensating for the embryo's transcriptional silence until ZGA—typically at the 4- to 8-cell stage in mammals. This provisioning ensures totipotency and viability during the vulnerable pre-implantation phase, with disruptions leading to developmental arrest. Evolutionarily, the egg cell's meiosis promotes crucial for , as recombination during I exchanges genetic material between homologous , while independent assortment randomizes distribution into gametes. This shuffling generates novel combinations in eggs, which, upon fertilization, enhance offspring variability and evolutionary potential far beyond . Such mechanisms underscore the egg's role in maintaining population resilience against environmental pressures. However, the asymmetric meiosis in egg production—where the oocyte retains nearly all cytoplasm and discards chromosomes into diminutive polar bodies—increases susceptibility to segregation errors, elevating aneuploidy risks. These errors, often resulting from premature separation of sister chromatids or merotelic attachments, become more prevalent with advanced maternal age due to prolonged meiotic arrest, contributing to over 90% of aneuploidies in human embryos and associated infertility or miscarriage.

Formation and Development

Oogenesis in Animals

, the formation and development of female gametes in animals, originates from cells (PGCs) that migrate to the developing gonads during embryogenesis. These PGCs proliferate mitotically to form oogonia, which are diploid cells that serve as precursors to oocytes. Oogonia then initiate meiosis I, entering prophase I and undergoing and recombination, before arresting at the diplotene stage as primary oocytes. This arrest, known as the dictyate stage, persists for extended periods, often until or specific reproductive cues in various species. The resumption of meiosis in primary oocytes is triggered by hormonal signals, leading to the completion of I and the production of a haploid secondary and the first through asymmetric . This division ensures that the secondary retains the majority of the , nutrients, and organelles necessary for early embryonic development, while the , containing minimal , typically degenerates. The secondary then arrests at II until fertilization, at which point II proceeds, extruding the second and forming the mature ovum. In mammals, primary oocytes arrest at I from fetal stages until , with only a subset resuming development each cycle. Hormonal regulation is critical for coordinating follicle growth and oocyte maturation. Follicle-stimulating hormone (FSH) from the promotes the recruitment and development of primordial follicles containing primary s, stimulating proliferation and theca cell differentiation. (LH) surges, induced by rising levels, trigger final oocyte maturation, , and luteinization of the ruptured follicle. , produced by granulosa cells under FSH influence, provides to amplify the LH surge while also exerting earlier to fine-tune follicle selection. Genetically, oogenesis involves key mechanisms that ensure genetic diversity and proper inheritance. During prophase I of meiosis I, homologous chromosomes pair and undergo crossing over, facilitated by proteins like SPO11, which introduces double-strand breaks repaired as crossovers to promote and chiasma formation for accurate chromosome segregation. , an epigenetic modification, is established primarily in the oocyte, where maternal-specific patterns silence or activate alleles on imprinted genes, influencing embryonic development and parent-of-origin effects. Additionally, (mtDNA) inheritance is strictly maternal, as the oocyte supplies nearly all mitochondria to the ; during oogenesis, mtDNA replicates extensively to amass up to 100,000–500,000 copies per mature oocyte, supporting the high energy demands of embryogenesis. Variations in oogenesis occur across animal classes, reflecting adaptations to reproductive strategies. In mammals, is largely continuous post-puberty but limited by a fixed pool of oocytes formed during fetal life, with cyclic . In contrast, many and amphibians exhibit seasonal oogenesis synchronized with environmental cues like temperature and photoperiod, allowing multiple spawning events annually. Invertebrates such as often display continuous oogenesis in adults, with nurse cells providing nutrients directly to oocytes, differing from the follicular support in vertebrates. Birds, as seasonal breeders, undergo tied to photoperiod, accumulating seasonally rather than continuously.

Gametogenesis in Plants and Other Organisms

In plants, particularly angiosperms, the formation of the egg cell occurs through within the of the flower. This process begins with the diploid megaspore mother cell undergoing to produce four haploid megaspores, of which typically only one—the chalazal functional megaspore—survives and develops further. The surviving megaspore then undergoes three rounds of mitotic divisions to form a seven-celled, eight-nucleate embryo sac, known as the type, which is the most common pattern observed in about 70% of angiosperm species. Within this embryo sac, one haploid nucleus differentiates into the egg cell at the micropylar end, accompanied by two synergids and three antipodal cells, establishing the female gametophyte structure essential for fertilization. Unlike animal , which results in a single large often arrested in , plant involves post-meiotic mitotic divisions to create a multicellular , with the egg cell being a small, haploid component lacking significant reserves; nutrients for the are instead provided by the formed after . Environmental factors such as availability and light quality influence the development of the surrounding tissues in lower like ferns, and indirectly affect in angiosperms by modulating maturation and floral induction. In fungi, egg cell formation differs markedly, as these organisms lack true eggs but produce specialized female structures for . In ascomycetes, sexual development initiates with the formation of the ascogonium, a coiled hyphal structure containing multiple haploid nuclei that serves as the female organ, which receives nuclei from a compatible male via without immediate . occurs later in ascogenous hyphae derived from the ascogonium, leading to diploid zygote-like cells that undergo to form ascospores. In basidiomycetes, involves hyphal fusion to create a dikaryotic state, with zygote-like structures forming transiently after in basidia, where produces basidiospores; this process emphasizes and dikaryosis over distinct egg cells. Among and protists, exhibits a spectrum from to oogamy, with egg cell marking a transition to differentiated sexes. In the green alga , reproduction is primarily isogamous, producing similar-sized motile gametes, but related species show with slight size differences; environmental cues like nitrogen starvation trigger by inducing vegetative cells to differentiate into gametes. In contrast, the colonial alga Volvox carteri displays full oogamy, where large, immotile egg cells form in specialized gonidia within colonies, while small, motile packets develop in males; this differentiation is controlled by an expanded mating-type locus and triggered by species-specific glycoproteins rather than nutrient depletion. These transitions highlight how oogamy in involves the of non-motile gametes adapted for , differing from embryo sacs by lacking multicellular complexity but sharing the absence of , with resources drawn from the parental .

Egg Cells in Animals

Mammalian Egg Cells

Mammalian egg cells, or , are characterized by their within ovarian follicles, a process that ensures maturation and preparation for fertilization. In humans, begins in fetal life with the formation of primordial follicles, each containing a primary arrested in prophase I of . These follicles progress through primary, secondary, and antral stages, driven by (FSH) and (LH), culminating in the dominant preovulatory follicle that reaches approximately 20-25 mm in diameter. The mature itself measures about 100-120 μm in diameter, excluding the , and is classified as alecithal due to its minimal content, relying instead on maternal transfer post-fertilization for embryonic . is triggered by a mid-cycle LH surge, which induces resumption of , leading to the release of a metaphase II-arrested secondary from the follicle into the . Unique to mammalian oocytes are specialized extracellular structures that facilitate sperm interaction and protection. The cumulus oophorus consists of granulosa cells surrounding the , providing nutritional support and signaling during maturation, while the forms the innermost layer of these cells directly adjacent to the . The , a matrix approximately 10-20 μm thick, encases the and is composed primarily of four glycoproteins—ZP1, ZP2, ZP3, and ZP4 in humans—which mediate species-specific binding, induction, and prevention of . These structures are conserved across eutherian mammals, enabling and implantation, though monotremes like the exhibit more yolk-rich (telolecithal) oocytes adapted to brief external incubation before hatching. Clinically, mammalian handling has advanced through techniques like maturation (IVM), which culture immature germinal vesicle-stage oocytes to metaphase II outside the body, achieving maturation rates of 30-50% in humans, particularly beneficial for patients with (PCOS) to avoid ovarian hyperstimulation risks. , primarily via , preserves mature oocytes with survival rates exceeding 90%, enabling preservation in cancer patients or those delaying , though post-thaw fertilization rates can be 10-20% lower than fresh oocytes. PCOS, affecting 5-10% of reproductive-age women, impairs oocyte quality through and disrupted , resulting in higher rates and reduced development potential compared to non-PCOS counterparts. Ongoing research highlights epigenetic in mammalian as a critical yet incomplete process, where and modifications erase parental imprints to establish totipotency, but vulnerabilities persist, leading to developmental disorders. Recent / studies have targeted oocyte epigenomes to model and correct imprinting defects, revealing roles of enzymes in active demethylation during maturation, with applications in enhancing IVM efficiency and understanding linked to epigenetic dysregulation.

Egg Cells in Oviparous Animals

In oviparous animals, egg cells are specialized for external embryonic development, relying heavily on reserves to nourish the until without maternal input post-laying. These eggs typically feature substantial provisions, enabling independent growth in varied environments. Macrolecithal eggs, characterized by a large mass, predominate in birds and reptiles, where the forms early to facilitate nutrient absorption during embryogenesis. The in these eggs exhibits telolecithal distribution, concentrated toward the vegetal pole to support uneven patterns limited to the animal pole region. Protective envelopes around the egg cell vary by and evolutionary , adapting to prevent and predation while permitting . In amphibians, such as frogs, eggs acquire multiple jelly coats during oviduct passage, forming concentric layers that provide hydration and species-specific fertilization barriers. In contrast, amniotes like birds and reptiles develop a more complex shell: the , an outer membrane derived from follicle cells, encloses the and , while albumen layers secreted in the oviduct supply water and proteins. These structures mark a key for terrestrial , retaining without dependence. Ovulation and egg-laying in oviparous are tightly regulated by environmental cues to optimize . In , photoperiod serves as the primary trigger, synchronizing daily cycles via circadian rhythms in luteinizing hormone surges, often resulting in clutches of 4–12 eggs laid sequentially over days. Clutch sizes vary by and conditions; for instance, reptiles like produce smaller clutches of 2–20 eggs, influenced by resource availability. Frogs exemplify high , with such as the coqui (Eleutherodactylus coqui) releasing clutches of 34–75 eggs multiple times per breeding season in response to seasonal moisture cues. In , eggs feature multilayered chorions for protection, with micropyles—specialized openings—enabling entry and aeration; for example, in hemipterans like Rhodnius prolixus, these structures consist of aeropyles and pore canals integrated into the shell. The of ovipary reflects a transition from aquatic to terrestrial , driven by innovations in egg . Early vertebrates laid yolky eggs in , but the lineage developed cleidoic eggs with impermeable shells around 312 million years ago, allowing full terrestrial oviposition by reducing loss. In amphibians, flexible oviposition strategies—such as laying jelly-coated eggs in moist terrestrial sites—bridged this shift, with species like terrestrial breeders producing larger, fewer eggs compared to fully aquatic ones. This progression enhanced reproductive independence, enabling colonization of diverse habitats while maintaining external development.

Egg Cells in Viviparous and Ovoviviparous Species

In viviparous species, egg cells undergo significant modifications to support embryonic within the maternal reproductive tract, enabling direct and between mother and rather than reliance on yolk reserves alone. Unlike oviparous systems where eggs are laid externally with substantial yolk for independent , viviparous eggs often feature reduced yolk mass and adaptations for placental or pseudo-placental interfaces. This internal retention enhances survival in challenging environments, such as cold climates or aquatic habitats, by leveraging maternal provisioning. Yolk-sac viviparity, observed in certain like the (Squalus acanthias), represents a transitional form where the egg's initially nourishes the but later fuses with uterine tissues to form a , facilitating uptake from maternal secretions. In these , the egg capsule is thinner and more permeable than in oviparous , allowing selective exchange of ions, oxygen, and organic compounds while protecting against predators. This adaptation underscores the evolutionary shift from yolk-dependent to maternal-supported embryogenesis in elasmobranchs. Ovoviviparity involves internal egg retention until , with embryos deriving primarily from but benefiting from maternal protection and limited supplemental secretions; this mode is prevalent in some reptiles, such as viviparous (Zootoca vivipara), and like certain paraneopterans. In these systems, eggs exhibit meroblastic , where division is incomplete and confined to the yolk-rich blastodisc, conserving resources for internal . Eggshells are notably thinned due to reduced uterine gland activity, permitting gas and preventing premature . Uterine secretions, rich in and , provide limited supplemental to the embryos in addition to reserves, as seen in ovoviviparous reptiles. Specific adaptations include the development of pseudoplacentae in seahorses (Hippocampus spp.), where male brood pouches form vascularized compartments around eggs, mimicking placental function by supplying oxygen and nutrients via direct contact with paternal tissues. In viviparous lizards like the common lizard (Zootoca vivipara), maternal uterine provisioning of calcium and steroids via a simple chorioallantoic placenta ensures skeletal development and stress response modulation. Similarly, ovoviviparous fish, such as black rockfish (Sebastes schlegelii), retain eggs in ovarian compartments with thin internal cases, allowing oxygen regulation and yolk utilization until live birth. These modifications highlight convergent evolution for internal gestation across taxa. Recent genomic studies reveal epigenetic differences in viviparous eggs, including altered patterns that regulate for nutrient transport and at the maternal-fetal interface. For instance, transcriptomic analyses of squamate reptiles show upregulated genes for placental adhesion and in viviparous lineages, with hypomethylation in promoter regions facilitating adaptive responses absent in oviparous eggs. These molecular shifts, identified in transitional species like the three-toed skink (Saiphos equalis), suggest epigenetic mechanisms drive the evolution of by enabling flexible embryonic nutrition.

Egg Cells in Plants

Structure and Location in Ovules

In angiosperms, the egg cell is a haploid, thin-walled structure typically measuring 10-20 μm in diameter, located at the micropylar end of the mature within the . The , derived from the functional megaspore through mitotic divisions, consists of seven cells and eight nuclei: the egg is flanked by two synergid cells that aid in pollen tube guidance, while three antipodal cells occupy the opposite chalazal end, and a central cell with two polar nuclei provides for development post-fertilization. Unlike animal eggs, the plant egg lacks reserves and relies on surrounding maternal tissues and future for nourishment. In gymnosperms, the egg cell's location and structure vary by group. In , the develops through a free-nuclear stage within the , followed by cellularization to form at the apical end, each containing a large, haploid egg cell surrounded by a neck canal and ventral canal cells. In cycads, multiple form within the mature in the , with each archegonium housing a single, prominent egg cell that is larger and more vacuolated than in angiosperms, adapted for motile fertilization. The itself is protected by integuments, one or two layers of that enclose the nucellus and develop into the coat after fertilization, providing mechanical protection and facilitating . The nucellus, a diploid maternal surrounding the gametophyte, serves as a source for the developing sac and egg cell during early stages. A notable variation occurs in , an reproductive mode in some , where unreduced (diploid) egg cells form within the embryo sac without , enabling production without fertilization and resulting in clonal .

Role in Plant Fertilization

In angiosperms, the egg cell plays a central role in , a unique to flowering plants where one cell from the fuses with the egg to form the diploid , while the second cell fuses with the central cell to produce the triploid that nourishes the developing . This coordinated ensures the genetic contributions from both parents are balanced, with the initiating embryogenesis and the providing essential nutrients for development. The is guided to the by chemical signals from the female gametophyte, particularly from the synergid cells adjacent to the , which secrete attractant peptides like LUREs to direct the tube precisely toward the . Upon arrival, the bursts within one synergid cell, releasing the two cells; of one with the triggers calcium oscillations that coordinate the second with the central cell and initiate downstream developmental signaling. This also induces synergid cell degeneration, preventing additional entries and ensuring monospermy. Following fertilization, the undergoes asymmetric division to form the apical , which develops into the proper, while the basal contributes to the suspensor that anchors and nourishes the ; in the egg , mediated by epigenetic marks such as , silences certain paternal alleles to regulate seed viability and prevent developmental conflicts. For instance, imprinted genes like in are expressed maternally from the egg-derived genome, influencing patterning and growth. In non-angiosperm plants, fertilization processes differ markedly. Gymnosperms exhibit fertilization, where a sperm fuses with the egg cell in the of the female to form the , without an equivalent endosperm-forming fusion, resulting in direct nutrient provisioning from maternal tissue to the . Similarly, in ferns and mosses, motile swim to the and fertilize the stationary egg cell to produce a that develops into the generation, relying on water for sperm delivery and lacking specialized mechanisms. Agriculturally, manipulating egg cell fertilization enhances production; for example, inducing unreduced (2n) eggs through chemical treatments or genetic modifications allows fixation of hybrid vigor without segregation in subsequent generations, as demonstrated in crops like where 2n eggs fertilized by haploid sperm yield partial hybrids with desirable traits. This approach reduces the labor-intensive crossing required for F1 hybrids and supports sustainable seed systems in major cereals.

Egg Cells in Other Organisms

In Fungi

In fungi, egg cell equivalents exist primarily in basal lineages such as Chytridiomycota, where sexual reproduction often involves oogamy with large, non-motile female gametes (eggs) and smaller, motile male gametes. In Chytridiomycota, eggs develop within specialized oogonia—swollen hyphal structures—while antheridia produce flagellated sperm that swim to fertilize the stationary eggs, leading to zygote formation and resting spore development. This represents an early evolutionary form of oogamy in fungi. In contrast, higher fungi like Ascomycota, Basidiomycota, and Mucoromycota (formerly Zygomycota) lack distinct motile or large non-motile gametes typical of oogamy; instead, sexual reproduction involves specialized structures such as the ascogonium in Ascomycota or progametangia in Mucoromycota, which function as female-like components in compatible mating interactions. These structures arise from hyphal cells of opposite mating types, where compatibility is determined by genetic loci controlling mating type (e.g., MAT genes). The formation of these structures begins with hyphal fusion in compatible strains. In , a "" hypha develops the ascogonium, a coiled or , while the "male" hypha forms the , a smaller structure that contacts and fertilizes the ascogonium through , the fusion of without immediate . This leads to the development of ascogenous hyphae, which are dikaryotic (containing paired nuclei from each parent) and branch out to form asci, the sac-like cells where () occurs, followed by to produce ascospores. In Mucoromycota, compatible hyphae of (+) and minus (-) strains grow toward each other to form progametangia, elongated branches that touch and develop , separating gametangia (true gamete-producing cells) from suspensor cells; then fuses the gametangia, creating a that undergoes to form a resistant . Fertilization in Basidiomycota similarly emphasizes plasmogamy preceding karyogamy, but without distinct ascogonium-like structures; instead, hyphae from compatible mating types fuse to establish a prolonged dikaryotic phase, where unpaired nuclei coexist in each cell. This dikaryon is maintained during hyphal growth by clamp connections, specialized septal structures that ensure each daughter cell receives one nucleus from each parent during mitosis. Karyogamy occurs later in the basidium, a terminal cell on the fruiting body (basidiocarp), leading to meiosis and the production of four basidiospores. For example, in the ascomycete Neurospora crassa, ascus formation follows crozier development from the ascogonium, ensuring linear arrangement of meiotic products for genetic analysis. In basidiomycetes like rust fungi, clamp connections stabilize the dikaryon across extensive hyphal networks in infected plant tissues. Ecologically, fungal reproductive structures contribute indirectly to plant reproduction through mycorrhizal associations, where hyphae (including those involved in sexual phases) form symbiotic networks with , enhancing and water uptake to support host vigor and . This , prevalent in many ascomycete and basidiomycete , boosts reproductive success under nutrient-limited conditions without direct involvement of fungal gametes in .

In Algae and Protists

In algae and protists, egg cells represent a key evolutionary innovation in , particularly through the transition from —where gametes are morphologically similar and motile—to oogamy, characterized by large, non-motile eggs and smaller, motile . This shift is evident in lineages, such as the volvocine algae, where isogamous species like produce equal-sized gametes that fuse randomly, while more derived forms exhibit leading to full oogamy. In advanced like , oogamy is pronounced, with immobile eggs retained within protective oogonia and actively swimming to fertilize them, enhancing reproductive efficiency in aquatic environments. This evolutionary progression likely arose multiple times independently, driven by selection for gamete size dimorphism to optimize and dispersal. Structurally, algal egg cells, or oogonia, are typically large and sessile, lacking flagella to conserve energy for nutrient storage and embryogenesis. In green algae such as Oedogonium, oogonia develop as swollen, cylindrical cells within unbranched filaments, featuring thick cell walls of cellulose and chitin that protect the single egg inside. Brown algae (Phaeophyceae) exhibit similar non-motile eggs embedded in gelatinous matrices, which provide structural support and hydration in marine habitats; these matrices, composed of alginates and fucoidans, encase oogonia in conceptacles or sori on the thallus. Protist egg-like structures vary but often share this non-motile, provisioned form, contrasting with motile gametes in ancestral isogamous relatives. Reproductive processes involving egg cells in these groups highlight diverse mechanisms for fertilization and dispersal. In colonial like Volvox, eggs form within specialized reproductive cells in the posterior hemisphere of the spherical colony; upon maturation, the parent colony disintegrates to release daughter colonies or zygotes, with eggs fertilized internally by packets before dispersal. Conjugation in filamentous such as Spirogyra involves transfer between adjacent cells via conjugation tubes, though these are not true eggs but passive protoplasts that fuse to form zygospores, representing an intermediate step toward oogamy. In (Rhodophyta), fertilized eggs develop into carposporophytes within the female , producing diploid carpospores that are released to germinate into tetrasporophytes, completing a triphasic without flagellated cells. Some protists, including certain , exhibit where unfertilized egg-like cells develop into viable offspring, bypassing sexual fusion under favorable conditions. Recent phylogenomic studies have illuminated the genetic underpinnings of egg cell evolution in , revealing conserved developmental s across lineages. For instance, analyses of and genomes in like Coleochaetophyceae show that oogamy-related traits, such as modifications and mating-type loci, emerged recently through duplications and horizontal transfers, predating land plant embryogenesis. In , genomic comparisons highlight the diversification of families linked to phenotypic changes in reproductive strategies and motility, supporting the independent evolution of complex reproductive structures from ancient ancestors. These findings underscore the modular evolution of egg cells, with shared toolkits enabling transitions from simple s to advanced algal systems.

Historical Perspectives

Early Discoveries

In , proposed that animal embryos developed from menstrual blood provided by the female, which served as the material basis shaped by the male's semen into a , a theory outlined in his work On the Generation of Animals. This view dominated early understandings of reproduction, portraying the female contribution as passive matter rather than an active . For , ancient observers like noted seed formation and germination through macroscopic examinations in works such as Enquiry into Plants, but lacked recognition of cellular structures like eggs due to the absence of magnification tools. The advent of in the 17th century enabled the first direct observations of reproductive structures. Dutch Regnier de Graaf described ovarian follicles in mammals during the 1660s, identifying them as fluid-filled vesicles in the ovaries of animals like rabbits and dogs, which he believed contained the eggs; these structures later became known as Graafian follicles. In 1677, Antonie van Leeuwenhoek, using improved single-lens microscopes, observed spermatozoa in from humans, dogs, and , describing their motile "animalcules" and implying the existence of complementary female elements in , though he did not visualize eggs directly. By the , refined microscopes allowed identification of actual egg cells. In 1827, discovered the mammalian ovum while examining ovaries, describing it as a distinct, transparent vesicle within the follicle and publishing his findings in De Ovi Mammalium et Hominis Genesi, establishing the ovum as the female across mammals including humans. In plants, Wilhelm Hofmeister elucidated the embryo sac in the 1850s through detailed microscopic studies of ovules in angiosperms, demonstrating that the egg cell resides within this sac and is fertilized by pollen tube-delivered , as detailed in his 1849 monograph on angiosperm embryology. These discoveries fueled key debates in , particularly versus . Preformationists, influential from the late , argued that miniature organisms were preformed within eggs or and simply enlarged during , a view supported by early microscopic glimpses of gametes. Epigenesis proponents, echoing but bolstered by von Baer's observations of gradual embryonic formation, contended that organisms arose through progressive from unformed material in the egg, resolving the debate toward epigenesis by the mid-19th century. The development of compound and simple microscopes profoundly impacted egg visualization, transitioning from macroscopic speculations to cellular resolution; Hooke's 1665 Micrographia popularized the instrument, while van Leeuwenhoek's lenses magnified up to 270 times, revealing gametes, and 19th-century achromatic lenses enabled von Baer and Hofmeister to discern egg structures within tissues.

Modern Advances in Egg Cell Research

In the 20th century, significant strides were made in understanding meiosis within egg cells, building on earlier observations to elucidate its role in gamete formation and genetic diversity. By the early 1900s, researchers like Walter Sutton had demonstrated that chromosomes in meiotic cells, including those in oocytes, segregate to produce haploid gametes, laying the groundwork for chromosome theory. A landmark achievement came in 1978 with the first successful in vitro fertilization (IVF) of a human egg, leading to the birth of Louise Brown; this breakthrough, pioneered by Robert Edwards and Patrick Steptoe, involved extracting, fertilizing, and implanting an egg outside the body, revolutionizing assisted reproduction. Molecular insights into egg cell biology advanced rapidly in the late , particularly through the identification of genes that control early embryonic patterning via egg cytoplasm. In the 1980s, and colleagues discovered the bicoid gene in Drosophila eggs, whose mRNA gradient establishes anterior-posterior polarity, earning her the in 1995 for revealing how maternal contributions dictate development. Entering the 2000s, research uncovered epigenetic reprogramming in egg cells, where patterns are erased and re-established during to ensure totipotency in the ; studies showed that this process, involving enzymes, is critical for imprinting and preventing developmental disorders. Technological innovations further transformed egg cell research, with (SCNT) demonstrating the egg's reprogramming capacity. In 1996, Ian Wilmut's team cloned the sheep by transferring a somatic nucleus into enucleated egg cytoplasm, which provided factors to reset the donor to an embryonic state, proving eggs' totipotency-inducing environment. The 2010s introduced CRISPR-Cas9 editing in oocytes, enabling precise genome modifications; early applications in human tripronuclear zygotes in 2015 confirmed high editing efficiency (up to 89%) for modeling genetic diseases, though off-target effects prompted refinements. Post-2020 developments have expanded egg cell research frontiers, including the generation of functional oocytes from stem cells. Induced pluripotent stem cells (iPSCs) have been differentiated into primordial germ cell-like cells and matured into oocytes , achieving fertilization and viable embryos in mice by 2023, offering potential for treatments without donor eggs. In 2025, researchers at reported success in generating functional oocytes from cells using induced pluripotent stem cells, marking a potential step toward treating in humans. (AI) models now predict oocyte quality and oogenesis outcomes by analyzing morphokinetics, with 2025 studies showing AI outperforming experts in assessing bovine oocyte viability based on imaging, enhancing IVF selection rates. Concurrently, investigations into impacts reveal that rising temperatures reduce egg viability in ; for instance, 2022 experiments on embryos demonstrated that projected 2050 conditions (elevated temperature and acidity) halve hatch success, underscoring threats to . These advances have sparked ethical debates, particularly around and germline editing. Egg donation raises concerns over donor health risks, , and long-term effects on donor-conceived children, with reviews emphasizing the need for equitable compensation without commodifying gametes. Discussions on "designer babies" via in oocytes highlight risks of , , and unintended heritable changes, as seen in the 2018 controversy over edited human embryos, prompting calls for international regulations.

References

  1. [1]
    Gamete - National Human Genome Research Institute
    A gamete is a reproductive cell of an animal or plant. In animals, female gametes are called ova or egg cells, and male gametes are called sperm.
  2. [2]
    Oogenesis - Developmental Biology - NCBI Bookshelf - NIH
    Most of the cytoplasm is retained by the mature egg (ovum), and a second polar body receives little more than a haploid nucleus. Thus, oogenic meiosis ...
  3. [3]
    Eggs - Molecular Biology of the Cell - NCBI Bookshelf - NIH
    The eggs of most animals are giant single cells, containing stockpiles of all the materials needed for initial development of the embryo through to the stage at ...
  4. [4]
    Structure of the Gametes - Developmental Biology - NCBI Bookshelf
    The mammalian egg is also surrounded by a layer of cells called the cumulus (Figure 7.7), which is made up of the ovarian follicular cells that were nurturing ...
  5. [5]
    Fertilization - Molecular Biology of the Cell - NCBI Bookshelf - NIH
    Through fertilization, the egg and sperm are saved: the egg is activated to begin its developmental program, and the haploid nuclei of the two gametes come ...
  6. [6]
    The cell biology of fertilization: Gamete attachment and fusion - PMC
    Fertilization is defined as the union of two gametes. During fertilization, sperm and egg fuse to form a diploid zygote to initiate prenatal development.
  7. [7]
    Animal Reproductive Structures and Functions | Organismal Biology
    Reproductive structures produce gametes (eggs and sperm) and facilitate the meeting of gametes to produce a zygote (fertilized egg).
  8. [8]
    Unscrambling the oocyte and the egg: clarifying terminology of the ...
    The mammalian oocyte is the 'founder cell' and refers to a female germ cell arrested in prophase of meiosis I. In the ovary, follicle-enclosed oocytes are found ...
  9. [9]
    Polar Bodies – more a lack of understanding than a lack of respect
    A polar body is the byproduct of an oocyte meiotic division. It is the small cell that normally apoptoses, and in textbook figures, it usually just disappears.
  10. [10]
    Oocyte - English Definition & Meaning - WordZo
    Etymology. The word "oocyte" comes from the Greek words "ōon" (egg) and "kytos" (cell). It literally means "egg cell." The term accurately describes the oocyte ...
  11. [11]
    The origin and evolution of gamete dimorphism and the male-female ...
    As high anisogamy is approached, the disadvantageous dominant homozygote is lost leaving two sexes (sperm producers and ovum producers) in a stable 1 : 1 ratio.<|separator|>
  12. [12]
    Ovum (Egg Cell): Structure, Function & Fertilization - Cleveland Clinic
    Nucleus: The central core of the ovum containing half of the genetic material for reproduction (23 chromosomes). Cytoplasm (cell plasma or yolk): A jelly-like ...
  13. [13]
    Egg Cell: Structure, Types, Functions and Oogenesis - Microbe Notes
    Mar 27, 2024 · Cortical granules are found in the outer cortex of the egg cytoplasm and play an important role in the activation of the egg upon sperm entry.
  14. [14]
    Classification of Eggs in Animals - Vetscraft
    Microlecithal: The eggs containing small amount of yolk and they can also be called as oligolecithal eggs. E.g. Amphioxus. Mesolecithal: Moderate amount of yolk ...
  15. [15]
    Five fascinating egg cell facts - Extend Fertility
    Nov 20, 2017 · Egg are larger than any other cell in the human body, at about 100 microns (or millionths of a meter) in diameter, about the same as a strand of hair.
  16. [16]
    The Fertilization Enigma: How Sperm and Egg Fuse - Annual Reviews
    Oct 6, 2021 · Fertilization is a multistep process that culminates in the fusion of sperm and egg, thus marking the beginning of a new organism in sexually ...
  17. [17]
    The molecular basis of fertilization (Review) - Spandidos Publications
    Aug 31, 2016 · The process involves the fusion of an oocyte with a sperm, creating a single diploid cell, the zygote, from which a new individual organism will ...
  18. [18]
    Meiosis and Fertilization - The Cell - NCBI Bookshelf
    Following completion of oocyte meiosis, the fertilized egg (now called a zygote) contains two haploid nuclei (called pronuclei), one derived from each parent.Missing: post- | Show results with:post-
  19. [19]
    The maternal-to-zygotic transition revisited | Development
    Jun 12, 2019 · Early animal development is controlled by maternal mRNAs and proteins that are loaded into the egg during oogenesis. These implement fundamental ...
  20. [20]
    https://journals.biologists.com/dev/article/146/11/dev161471/48907/The-maternal-to-zygotic-transition-revisited
  21. [21]
    Evolutionary mysteries in meiosis | Philosophical Transactions of the ...
    Oct 19, 2016 · Meiosis reduces a cell's chromosome number by half, while also creating new allele combinations distributed across daughter cells through ...
  22. [22]
    Meiosis and Maternal Aging: Insights from Aneuploid Oocytes and ...
    Extending this period beyond ∼35 years greatly increases the risk of aneuploidy in human oocytes, resulting in a dramatic increase in infertility, miscarriage, ...
  23. [23]
    Oocyte aneuploidy—more tools to tackle an old problem - PNAS
    Failure to transmit exactly one copy of each chromosome during fertilization gives rise to aneuploid embryos resulting in infertility and congenital ...
  24. [24]
    Oogenesis in Women: From Molecular Regulatory Pathways and ...
    Apr 6, 2023 · Oogenesis begins in the fetal ovaries when oogonia are developed from primordial germ cells (PGC), as soon as the development of the embryo ...Oogenesis In Women: From... · 2. Oogenesis, Oocyte Growth... · 3. Oocyte Release, Relevant...
  25. [25]
    A Review of Oocyte Development and Its Regulation
    Mar 31, 2020 · The formation of primordial follicle marks the beginning of the process of oogenesis, the production of oocytes. After activation, they are ...
  26. [26]
    Hormonal Regulation of Female Reproduction - PubMed Central - NIH
    This review examines the regulation and dysregulation of the LH surge that triggers ovulation, hypothalamic control of reproductive behavior and how these ...
  27. [27]
    Hormonal regulation of ovarian follicle growth in humans
    Aug 21, 2022 · In the ovaries, LH and FSH regulate folliculogenesis. Each growing follicle faces one of two fates: either apoptosis or becoming the dominant ...
  28. [28]
    Genomic imprinting in mammals: its life cycle, molecular ...
    Genomic imprinting is an epigenetic gene-marking in the germline, causing parental-origin-specific gene expression in mammals, impacting development and ...
  29. [29]
    The role of mtDNA in oocyte quality and embryo development - PMC
    Aug 20, 2022 · This review focuses on the role of the mitochondrial genome from the perspective of the oocyte and how it can influence fertilization outcome and development.Missing: aspects | Show results with:aspects
  30. [30]
    Comparative Aspects of Animal Oogenesis - ScienceDirect.com
    This review discusses the comparative aspects of animal oogenesis.
  31. [31]
    A Comparative Analysis of Oocyte Development in Mammals - PMC
    In animals, oogenesis has been studied in organisms as diverse as insects, worms, amphibians and mammals.
  32. [32]
    An introduction to the embryology of angiosperms - Internet Archive
    Apr 30, 2008 · An introduction to the embryology of angiosperms. by: Maheshwari, P. ... PDF download · download 1 file · SINGLE PAGE ORIGINAL JP2 TAR download.
  33. [33]
    [PDF] Megagametogenesis in Arabidopsis wild type and the Gf mutant
    The most common form, exhibited by 70% of the species ex- amined, is monosporic-type megasporogenesis combined with Polygonum-type megagametogenesis (Maheshwari.
  34. [34]
    Full article: Megasporogenesis and megagametogenesis in species ...
    The Polygonum type is common among angiosperms, having been found in 81% of the species studied so far (Davis Citation1966). In all seven species, the ...
  35. [35]
    Megasporogenesis, Megagametogenesis and Ontogeny of the Aril ...
    Key Results A monosporic Polygonum-type of megagametogenesis is observed in both species but with megasporogenesis characterized by formation of a triad of ...
  36. [36]
    Effects of light, macronutrients, and sucrose on germination and ...
    Jun 28, 2010 · Some of these showed that light, nutrients and sucrose greatly influenced spore germination, gametophyte development and sex determination ( ...Missing: triggers | Show results with:triggers
  37. [37]
    Sex in Fungi - PMC - PubMed Central
    (a) Sexual development begins with the germination of ascospores, followed by the growth of vegetative mycelium with the formation of an ascogonium, which ...
  38. [38]
    Evolution of sex and mating loci: An expanded view from Volvocine ...
    Oct 27, 2011 · Unlike Chlamydomonas where –N induces gametogenesis, sexual reproduction in Volvox is triggered by a species-specific diffusible glycoprotein ...
  39. [39]
    Three genomes in the algal genus Volvox reveal the fate of ... - PNAS
    May 19, 2021 · The genus Volvox is an oogamous (with large, immotile female gametes and small, motile male gametes) and includes both heterothallic species ( ...
  40. [40]
    Embryology, Ovarian Follicle Development - StatPearls - NCBI - NIH
    Aug 3, 2025 · After puberty, primordial follicles undergo a multi-step process to transform into mature, pre-ovulatory follicles. Go to: Development. Ovarian ...Missing: mammalian alecithal yolk
  41. [41]
    Follicle Growth and Development | GLOWM
    All primordial follicles are composed of a small primary oocyte (about 25 μm in diameter) arrested in the diplotene (or dictyate) stage of meiosis, a single ...Missing: alecithal yolk
  42. [42]
    Maturation capacity, morphology and morphometric assessment of ...
    When oocytes are matured in-vivo, the size of oocyte normality is approximately 110-120 µm, excluding the ZP, also whole size of oocyte was almost 150 µm (23).
  43. [43]
    Yolkless Egg and the Evolution of Eutherian Viviparity
    It is the only form of reproduction in eutherians (“placental mammals”). Their mature follicles are vesicular, and their oocytes are yolkless.
  44. [44]
    Cumulus Oophorus - an overview | ScienceDirect Topics
    The zona pellucida is primarily composed of three glycoproteins; ZP1, ZP2 and ZP3.
  45. [45]
    Mammalian zona pellucida glycoproteins: structure and function ...
    In humans, it is composed of 4 glycoproteins designated as ZP1, ZP2, ZP3 and ZP4, whereas mouse ZP is composed of ZP1, ZP2 and ZP3 (Zp4 being a pseudogene). In ...Missing: cumulus oophorus corona radiata
  46. [46]
    Human Zona Pellucida Glycoproteins: Binding Characteristics With ...
    Human zona pellucida (ZP) matrix is composed of four glycoproteins designated as ZP glycoprotein -1 (ZP1), -2 (ZP2), -3 (ZP3), and -4 (ZP4).
  47. [47]
    Mammalian oocytes and early development. Monotreme oocytes ...
    Monotreme oocytes contain large amounts of yolk (telocithal), the marsupial eggs are less yolky, and yolk is largely absent in eutherian mammals. Development in ...
  48. [48]
    Advances in human oocyte in vitro maturation - PubMed Central - NIH
    Oct 29, 2025 · Oocyte in vitro maturation (IVM) is a technique used to retrieve immature oocytes from the ovary at either the germinal vesicle (GV) or ...
  49. [49]
    In vitro maturation: a committee opinion (2021) - ASRM
    The authors concluded that the reproductive potential of vitrified IVM oocytes is impaired owing to the vitrification-warming procedure. Similarly, another ...
  50. [50]
    Twenty-first century oocyte cryopreservation—in vitro maturation of ...
    This review found that OTO-IVM could improve FP treatment outcome among cancer population. Following IVM, the OMR affects the number of mature oocytes that can ...
  51. [51]
    Oocyte quality in adult polycystic ovary syndrome - PubMed
    Folliculogenesis in the PCOS ovary is often disrupted, leading to suboptimal oocyte competence for fertilization.
  52. [52]
    PCOS and Role of Cumulus Gene Expression in Assessing Oocytes ...
    There is a reduction in the development potential of oocytes and high-quality embryos in PCOS patients compared to non-PCOS patients. A critical step in IVF ...
  53. [53]
    Epigenetic Reprogramming in Early Animal Development - PMC
    In this review, we summarize the latest findings on epigenetic reprogramming in gametogenesis and embryogenesis, and how it contributes to gamete maturation.
  54. [54]
    The Mammalian Oocyte: A Central Hub for Cellular Reprogramming ...
    Feb 18, 2025 · This review aims to provide an in-depth analysis of the critical components of the oocyte and their roles in reprogramming and early ...The Mammalian Oocyte: A... · Reprogramming Mechanisms · Epigenetic Reprogramming
  55. [55]
    CRISPR/Cas9 technology: applications in oocytes and early embryos
    Oct 24, 2023 · Collectively, the CRISPR/Cas9 system has been successfully applied in mammalian oocytes and embryos across various animal models including mice ...
  56. [56]
    [PDF] The global diversity and distribution of lizard clutch sizes
    May 5, 2020 · In amniotes (birds, mammals, and reptiles) laying large, yolk-laden macrolecithal eggs, clutch (and litter) size varia- tion is narrower but ...
  57. [57]
    [PDF] Maternal transfer and embryonic assimilation of trace elements in ...
    In oviparous amniote vertebrates (birds and most reptiles), trace el- ements are primarily transferred to offspring during egg formation, via vitellogenesis ( ...
  58. [58]
    [PDF] Classification of Egg
    Alecithal Egg: When the egg contains no yolk, it is called alecithal egg. Eg ... Most animal eggs contain both kinds of yolk. Since the yolk is heavier ...Missing: yolkless | Show results with:yolkless
  59. [59]
    [PDF] Phylogeny and Evolutionary History of the Amniote Egg
    Jan 7, 2021 · In extant amphibians, the yolk is surrounded by a vitelline membrane and externally covered by a jelly coat (= egg capsule). Egg size (diame-.
  60. [60]
    The physicochemical features of eggshell, thick albumen, amniotic ...
    The role of the albumen is to form embryonic fluid. Albumen contains approx. 10.5% proteins. The proteins in the albumen are transported to the amniotic cavity, ...
  61. [61]
    Grand and Less Grand Challenges in Avian Physiology - PMC
    The 24-h periodicity of ovulation or LH surge is a circadian rhythm synchronizing the hen's ovulation cycle with environmental cues (e.g., photoperiod) (Fraps, ...
  62. [62]
    Life Cycle - Control of Coqui Frog in Hawai'i
    In its native Puerto Rico, the female coqui frog usually lays a cluster or clutch of 34-75 eggs four to six times a year; however, under laboratory conditions ...Missing: multiple | Show results with:multiple
  63. [63]
    The ecological function of insect egg micropyles - Iossa - 2022
    Feb 11, 2022 · Insect egg micropyles are openings through the chorion allowing sperm entry for fertilisation. Micropyles are diverse structures showing ...
  64. [64]
    Flexible oviposition behavior enabled the evolution of terrestrial ...
    Jul 30, 2024 · Species with obligate terrestrial reproduction have larger eggs than species with aquatic reproduction, and species with flexible reproduction ...
  65. [65]
    The evolution of reproductive modes and life cycles in amphibians
    Nov 17, 2022 · We categorized each species into one of six modes (Fig. 1): aquatic (aquatic eggs and larva), semi-terrestrial (terrestrial eggs, aquatic larva) ...
  66. [66]
    Understanding the evolution of viviparity using intraspecific variation ...
    Apr 14, 2025 · Here we explore the biology and evolution of transitional forms of pregnancy that are midway between the ancestral state of oviparity (egg‐laying) and the ...
  67. [67]
    Structure and permeability of the egg capsule of the placental ... - NIH
    Feb 4, 2022 · In oviparous sharks, the egg capsule is relatively thick, providing mechanical support for the egg, and protects the developing embryos from ...
  68. [68]
    Structure of the paraplacenta and the yolk sac placenta of the ...
    While most transitions from oviparity to viviparity have resulted in non-placental forms of viviparity, some sharks develop a yolk sac placenta during pregnancy ...
  69. [69]
    Egg retention, viviparity and ovoviviparity in Paraneoptera - PMC - NIH
    Jul 31, 2021 · The egg can start developing only after fertilization or when fertilization is not needed. If the egg is fertilized inside the vitellarium, it ...
  70. [70]
    Ex utero culture of viviparous embryos of the lizard, Zootoca vivipara ...
    Viviparous Z. vivipara eggs are surrounded by a thin proteinaceous eggshell positioned between the uterine epithelium and the chorioallantoic membrane ...
  71. [71]
    Uterine and eggshell structure and histochemistry in a lizard with ...
    One mechanism leading to thinning of the eggshell is reduction in size of uterine shell glands. Saiphos equalis is an Australian scincid lizard with an unusual ...
  72. [72]
    Morphology of brood pouch formation in the pot-bellied seahorse ...
    Oct 17, 2017 · Embryos are finally embedded within individual compartments formed in pseudoplacenta [11], and there, the number of blood vessels increases. The ...
  73. [73]
    Genomic and transcriptomic investigations of the evolutionary ...
    Feb 11, 2019 · (E) Differences between the oviparous and viviparous species related to egg retention at S3 and S4. Red text represents up-regulated ...
  74. [74]
    Comparing the potential for maternal–fetal signalling in oviparous ...
    Oct 17, 2022 · In egg-laying species, the embryo receives nutrients from the yolk sac, but live-bearing mothers can provide nutrients prior to fertilization ...
  75. [75]
    Genomic Signatures Associated with Transitions to Viviparity in ...
    Oct 4, 2023 · Here, we investigate the potential role of molecular convergence during independent transitions to viviparity across an order of ray-finned ...
  76. [76]
    Gamete Production in Angiosperms - Developmental Biology - NCBI
    ... embryo sac with eight nuclei (Figure 20.8). One of these cells is the egg. The two synergid cells surrounding the egg may be evolutionary remnants of the ...
  77. [77]
    Angiosperm ovules: diversity, development, evolution - PMC
    The inner integument always forms a tubular sheath around the nucellus. The outer integument is more variable. In orthotropous ovules it also forms a tubular ...
  78. [78]
    https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/General_Biology_(Boundless)/32:_Plant_Reproductive_Development_and_Structure/32.02:_Plant_Reproductive_Development_and_Structure_-_Sexual_Reproduction_in_Gymnosperms
  79. [79]
    Plant reproductive system - Gymnosperms, Pollination, Seeds
    Archegonia are present in the female gametophytes of Ephedra, but only eggs occur in those of Gnetum and Welwitschia. The sperm, like those of the conifers, ...
  80. [80]
    Natural Variation in Ovule Morphology Is Influenced by Multiple ...
    Oct 31, 2019 · ... integuments that surround the nucellus. The nucellus is located at the distal tip of the ovule and gives rise to the megasporocyte and ...
  81. [81]
    The Rise of Apomixis in Natural Plant Populations - PMC - NIH
    However, fertilization of unreduced 3x egg cells with well-developed haploid pollen from surrounding diploid sexuals can result in tetraploid plants in the next ...
  82. [82]
    The beginning of a seed: regulatory mechanisms of double fertilization
    In angiosperms, the haploid gametophytic generations produce the male and female gametes required to execute double fertilization. Both gametophytes are reduced ...
  83. [83]
    Fertilization Mechanisms in Flowering Plants - PMC - PubMed Central
    Following release of the two sperm cells they interact and fuse with two dimorphic female gametes (egg and central cell) forming the major seed components ...
  84. [84]
    Gametophytic Pollen Tube Guidance: Attractant Peptides, Gametic ...
    Recent studies have elucidated a molecular framework for the attraction of the pollen tube by synergid cells and the control of attraction by female gametes.
  85. [85]
    Male–female communication triggers calcium signatures during ...
    Aug 22, 2014 · Fertilization in flowering plants is more complex, involving interaction of a pollen tube with egg adjoining synergid cells, culminating in ...
  86. [86]
    Elucidating the Function of Synergid Cells: A Regulatory Role for ...
    The synergid cells are necessary for pollen tube guidance in the ovule, as shown by Higashiyama et al. (2001) using a laser ablation technique in Tourenia to ...
  87. [87]
    Evolution and function of genomic imprinting in plants - PMC
    At the time of fertilization, the diploid female central cell and a haploid male sperm cell fuse to give rise to the endosperm, while the haploid female egg ...
  88. [88]
    [PDF] Imprinting of the MEDEA Polycomb Gene in the Arabidopsis ... - UCLA
    In flowering plants, two cells are fertilized in the haploid female gametophyte. Egg and sperm nuclei fuse to form the embryo. A second sperm nucleus fuses ...Missing: post- | Show results with:post-
  89. [89]
    Plant Life Cycles - Developmental Biology - NCBI Bookshelf - NIH
    Sperm travel to a neighboring plant via a water droplet, are chemically attracted to the entrance of the archegonium, and fertilization results. The embryonic ...
  90. [90]
    Lab 8 - Primitive Plants - Bryophytes, Ferns and Fern Allies
    Once the sperm enters the archegonia, it fuses with the egg. The 2N zygote develops into a diploid sporophyte plant, a small stalk that grows directly out of ...
  91. [91]
    Evidence That 2n Eggs Explain Partial Hybrids between Medicago ...
    May 23, 2022 · It was proposed in 2008 that the partial hybrids could be explained by fertilization of 2n eggs in alfalfa by normal pollen from M. arborea, ...
  92. [92]
    Plants cloned as seeds - UC Davis
    Feb 17, 2011 · In about one-third of the seeds produced, the diploid eggs were successfully fertilized, then the chromosomes from one parent were eliminated, ...Missing: manipulating | Show results with:manipulating
  93. [93]
    Ascomycota (Ascomycetes, Ascomycotina) -- Sac Fungi
    During sexual reproduction antheridia and ascogonia fuse. Ascogenous hyphae branch out from the ascogonium and the ascus forms as a result of crozier formation ...
  94. [94]
    24.3B: Zygomycota - The Conjugated Fungi - Biology LibreTexts
    Nov 22, 2024 · Zygomycota, a small group in the fungi kingdom, can reproduce asexually or sexually, in a process called conjugation.
  95. [95]
    Chapter 12: Fungal sex and fungal groups - Milne Publishing
    The ascogonium develops extensions and two nuclei (one from each mating type) associate and migrate into them. A cross wall is formed, producing the first cell ...
  96. [96]
    Classifications of Fungi - OpenEd CUNY
    The “male” strain produces an antheridium and the “female” strain develops an ascogonium. At fertilization, the antheridium and the ascogonium combine in ...
  97. [97]
    Ascomycota - Soil Ecology Wiki
    May 6, 2022 · During sexual reproduction, thousands of asci (ascus, plural) fill a fruiting body called the ascocarp. The ascospores are then released, ...
  98. [98]
    3.5 The traditional zygomycetes - David Moore's World of Fungi
    So zygomycetes reproduce through the fusion or conjugation of two hyphal branches (= gametangia) to form a zygote which eventually gives rise to the ...
  99. [99]
    [PDF] Kingdom Fungi - PLB Lab Websites
    To make genetically diverse spores, most fungi also have sexual reproduction. When compatible cells or hyphae meet, they fuse and bring their nuclei together.
  100. [100]
    Basidiomycota - microbewiki - Kenyon College
    Aug 7, 2010 · They form when dikaryotic hyphae divide. However, not all Basidiomycota produce them. It is believed that clamp connections developed early on ...
  101. [101]
    Mycorrhizal Fungi | Oklahoma State University - OSU Extension
    Mycorrhiza, which means “fungus-root,” is defined as a beneficial, or symbiotic relationship between a fungus and the roots of its host plant. This relationship ...
  102. [102]
    Mechanisms underlying beneficial plant–fungus interactions in ...
    Jul 27, 2010 · Many of the worlds' plants and trees have a symbiotic relationship with micorrhizal fungi, which associate with their roots.<|control11|><|separator|>
  103. [103]
    New “missing link” genus of the colonial volvocine green algae ...
    Mar 3, 2014 · Considering the external obligate anisogamy, oogamy evolution may have been preceded by the transition from external to internal fertilization ...
  104. [104]
    Oogamy: inventing the sexes - PubMed
    The male-female dichotomy has evolved independently in nearly all lineages of multicellular organisms. Why this should be the case is still uncertain, ...
  105. [105]
    Green algae and the origin of land plants - American Journal of Botany
    Oct 1, 2004 · The familiar Oedogonium is often used as an example of oogamous sexual reproduction in green algae. All genera produce unusual motile cells ...
  106. [106]
    Groups of Protists – Introductory Biology: Evolutionary and ...
    Volvox colonies contain 500 to 60,000 cells, each with two flagella, contained within a hollow, spherical matrix composed of a gelatinous glycoprotein ...
  107. [107]
    Volvox: a simple algal model for embryogenesis, morphogenesis ...
    The asexual life cycle of Volvox follows a recursive pattern of reproduction with each gonidial cell undergoing embryogenesis to produce a new miniature ...<|separator|>
  108. [108]
    Carposporophyte - Marine Botany at FHL
    A carposporophyte is formed by the union of haploid gametes in red algae, is the diploid stage carried by the female gametophyte, and produces carpospores.
  109. [109]
    THE PROTOZOAN LIFE CYCLE
    It is conceivable that parthenogenesis is only a means of offsetting physiological death or in other words, of stimulating physiological activities in ...
  110. [110]
    Phylogenomics unveil a recent origin of morphological complexity in ...
    Oct 20, 2025 · In our new phylogenomic framework for Coleochaetophyceae, we find that all Coleochaete species with complex bodies formed by perpendicular cell ...
  111. [111]
    Phylotranscriptomic relationships of the Oophila clade of green ...
    We study the phylogenetic relationships of these algae using a phylotranscriptomic data set of 76 protein-coding single-copy nuclear genes. Our data set ...
  112. [112]
    On the Generation of Animals, by Aristotle
    Oct 2, 2010 · The material cause is the stuff a thing is made of, which in Aristotle's theory is the female menstrual blood. The efficient cause is the ...
  113. [113]
    https://www.journals.uchicago.edu/doi/pdf/10.2307/1535745
  114. [114]
    Regnier de Graaf (1641-1673) - Embryo Project Encyclopedia
    Sep 30, 2008 · Though he published papers on both pancreatic and male reproductive anatomy, he is best known for his discovery of the mature ovarian follicles ...
  115. [115]
    An Amazing 10 Years: The Discovery of Egg and Sperm in the 17th ...
    Jul 25, 2012 · The 17th century discovery of the role of egg and sperm in reproduction can be traced to two letters, written 7 years apart, each by a remarkable man who is ...Missing: sources | Show results with:sources
  116. [116]
    De ovi mammalium et hominis genesi (1827), by Karl Ernst von Baer
    Feb 9, 2017 · He writes about the first stages of development, and he describes the embryo's shape, color, and position of the anatomical structures.
  117. [117]
    The Genius of Wilhelm Hofmeister: The Origin of Causal-Analytical ...
    pre- and postfertilization ovule development, Hofmeister showed that the egg cell and/or the synergids, comprising the egg apparatus, were present in the ovule ...
  118. [118]
    Watching the embryo: Evolution of the microscope for the study of ...
    Apr 10, 2021 · Beginning of the 17th century, the use of the microscope revolutionized our understanding of how embryos develop. Marcello Malpighi, regarded ...Missing: Impact | Show results with:Impact
  119. [119]
    Developing the Chromosome Theory | Learn Science at Scitable
    Sutton, a Kansas farm boy, had made the serendipitous discovery that it was possible to distinguish individual chromosomes in cells undergoing meiosis in the ...
  120. [120]
    Epigenetic reprogramming of the zygote in mice and men
    It is the aim of this review to delineate the cellular and molecular events involved in maternal, paternal and zygotic epigenetic reprogramming.
  121. [121]
    25th ANNIVERSARY OF CLONING BY SOMATIC-CELL NUCLEAR ...
    The birth and adult development of 'Dolly' the sheep, the first mammal produced by the transfer of a terminally differentiated cell nucleus into an egg, ...
  122. [122]
    CRISPR/Cas9-mediated gene editing in human tripronuclear zygotes
    In this report, we used tripronuclear (3PN) zygotes to further investigate CRISPR/Cas9-mediated gene editing in human cells. We found that CRISPR/Cas9 could ...
  123. [123]
    Advances in in vitro oocyte generation from pluripotent stem cells ...
    Aug 11, 2025 · Reconstructed ovaries can facilitate the maturation of mPGCLCs into oocytes, leading to the production of fertile offspring through in vitro ...
  124. [124]
    Multistressor global change drivers reduce hatch and viability of ...
    Dec 20, 2022 · Exposure to temperature, pH, and dissolved oxygen concentrations projected by the year 2050 halved the successful hatch of Lingcod embryos and significantly ...
  125. [125]
    Egg donation compensation: ethical and legal challenges | MB
    Sep 26, 2014 · This paper examines international debates about the legality and morality of the use of donor eggs to assist in family development and just recompense of ...
  126. [126]
    Ethical Challenges of Germline Genetic Enhancement - PMC - NIH
    We identify procreative beneficence, genetic disassociation, gender selection, the value of disability, embryo chimerization, and the psychosocial inequality.