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Conceptus

The conceptus is the entire product of conception in mammalian reproduction, comprising the developing embryo or fetus together with its extraembryonic membranes and placenta, originating from the fertilized zygote and persisting until birth.^[1] This entity represents the initial stages of prenatal development, during which rapid cellular division and differentiation occur, establishing the foundational structures for the offspring.^[2] In human embryology, the conceptus undergoes distinct developmental phases beginning with the pre-implantation stage, lasting approximately 7–9 days post-fertilization, when the zygote divides into a blastocyst while traveling through the fallopian tube and relying on maternal nutrients.^[1] Implantation follows around days 7–9, as the blastocyst adheres to and embeds in the uterine endometrium, initiating the formation of the placenta for nutrient and gas exchange.^[3] Post-implantation, gastrulation commences around day 14, generating the three primary germ layers—ectoderm, mesoderm, and endoderm—that give rise to all fetal tissues and organs.^[1] The embryonic period spans from fertilization to the end of week 8, marked by organogenesis, transitioning into the fetal stage from week 9 onward, characterized by growth and maturation until delivery.^[3] The conceptus plays a critical role in establishing and maintaining pregnancy through complex interactions with the maternal system, including immune tolerance mechanisms that prevent rejection of this semi-allogeneic entity.^[2] Trophoblast cells, derived from the conceptus, express immune checkpoints such as PD-L1 and lack major histocompatibility complex (MHC) antigens to evade maternal immune responses, while regulatory T cells further support implantation and placental development.^[3] Vulnerabilities during early stages contribute to high loss rates, with 50–70% of conceptuses failing in the first three weeks due to chromosomal anomalies or environmental factors, underscoring the precision required for successful gestation.^[3] Teratogenic risks, such as those from thalidomide during its critical window (days 20–36 post-fertilization), highlight the conceptus's sensitivity to xenobiotics, influencing clinical approaches to prenatal care.^[2]^[4]

Definition and Terminology

Etymology

The term conceptus originates from the Latin conceptus, the past participle of concipere, meaning "to take in, seize, comprehend, or conceive," thereby denoting "something conceived" or the direct result of conception.^[5]^[6] In biological contexts, the term entered English usage as early as 1745 to describe the product of fertilization, specifically a fertilized egg, embryo, or fetus.^[5]

Biological Definition

In embryology, the conceptus refers to the complete product of conception, comprising the developing embryo or fetus and all associated extraembryonic tissues, from the formation of the zygote at fertilization through the entirety of gestation until birth. This encompasses structures such as the placenta, chorion, amnion, and yolk sac, which provide nutritional, protective, and physiological support to the developing organism.^[7] The temporal scope of the conceptus typically spans the full prenatal period, but usage can vary by context; for instance, it may be restricted to the preimplantation phase (e.g., the blastocyst stage) or early post-implantation development in certain experimental or clinical discussions. In human development, this begins with the zygote—a single diploid cell resulting from gamete fusion—and extends through organogenesis and fetal maturation.^[8] Key distinctions set the conceptus apart from related terms: unlike the zygote, which is strictly the initial unicellular entity post-fertilization, the conceptus represents the multicellular entity thereafter; it differs from the embryo, defined as the developing human organism from implantation through the eighth week of gestation (when major organ systems form), and the fetus, the stage from the ninth week to birth (characterized by growth and refinement of structures). Notably, the conceptus uniquely includes extraembryonic membranes and tissues not destined to form the offspring's body.^[7]^[9] Across species, the term's application shows variations, particularly in veterinary contexts for mammals like pigs, where "conceptus" broadly denotes the pre- and post-implantation entity—including the elongating trophoblast and membranes—that signals maternal recognition of pregnancy to prevent luteolysis. This broader usage highlights species-specific developmental dynamics, such as rapid elongation in swine prior to attachment.

Early Developmental Stages

Fertilization and Zygote Formation

Fertilization in humans begins when a sperm cell meets the secondary oocyte in the ampulla of the uterine tube, typically within 12 to 24 hours following ovulation.^[10] The secondary oocyte, arrested in metaphase of the second meiotic division, is surrounded by the zona pellucida, an extracellular glycoprotein matrix, and cumulus cells.^[11] Sperm must first undergo capacitation in the female reproductive tract, a process involving membrane changes and increased motility that prepares them for the acrosome reaction.^[11] The acrosome reaction is initiated when capacitated sperm bind to the zona pellucida via interactions with ZP3 glycoproteins, triggering calcium influx and exocytosis of the acrosome—a cap-like structure containing hydrolytic enzymes such as acrosin and hyaluronidase.^[11] These enzymes disperse the cumulus cells and digest a path through the zona pellucida, allowing the sperm to reach the oocyte's plasma membrane.^[12] Only acrosome-reacted sperm can penetrate the zona, as the reaction exposes proteins necessary for fusion with the oocyte membrane.^[13] Upon contact with the oocyte plasma membrane, the sperm fuses via proteins like fertilin, leading to the release of sperm factors that induce a calcium wave across the oocyte.^[11] This triggers the cortical reaction, where cortical granules exocytose enzymes that modify the zona pellucida by cleaving ZP2 and altering ZP3, hardening it and blocking additional sperm binding to prevent polyspermy.^[11] A rapid membrane depolarization provides an initial fast block, while the cortical reaction establishes the slower, more enduring secondary block.^[12] The resulting cell is the zygote, a diploid entity formed by the fusion of the haploid sperm (contributing 23 chromosomes) and oocyte (contributing 23 chromosomes), restoring the full 46-chromosome complement.^[8] The sperm and oocyte nuclei decondense into male and female pronuclei, respectively, which migrate toward each other along microtubules and fuse in syngamy, typically around 12 hours post-fertilization.^[12] This event initiates embryonic genome activation, where maternal transcripts are degraded and zygotic transcription begins, marking the transition to autonomous embryonic development.^[14] The zygote thus represents the initial stage of the conceptus.^[10]

Cleavage and Blastocyst Development

Following fertilization, the zygote undergoes a series of mitotic divisions known as cleavage, which transform it into a multicellular structure without significant increase in overall size. In mammals, this process is characterized by holoblastic cleavage, where the entire zygote is divided into smaller cells called blastomeres. These divisions occur asynchronously, with the first cleavage typically happening around 24-30 hours post-fertilization in humans, producing two blastomeres, followed by subsequent divisions to form 4, 8, and 16 cells by days 2-3. Unlike cleavage in yolky eggs, mammalian cleavage features rotational holoblastic patterns, with the second division involving one meridional and one equatorial plane, ensuring even distribution of cellular components.^[15] As cleavage progresses, the embryo reaches the morula stage around days 3-4 post-fertilization, forming a solid ball of 16-32 tightly packed blastomeres. This transition is marked by compaction, a critical event where blastomeres flatten and adhere to one another, establishing cell polarity and intercellular junctions. Compaction is mediated primarily by the calcium-dependent cell adhesion molecule E-cadherin (also known as uvomorulin), which localizes to cell membranes and promotes tight adhesion between blastomeres. Seminal studies identified E-cadherin's role in this process, demonstrating that its inhibition prevents compaction and disrupts subsequent development. Additionally, pathways such as HIPPO-YAP and apical-basal polarity regulators (e.g., Par complex and ERM proteins) contribute to distinguishing outer polar cells from inner apolar ones during this stage.^[15]^[16] By days 5-6, the morula undergoes cavitation to form the blastocyst, a fluid-filled structure consisting of approximately 100-200 cells. The blastocoel, a blastocoel cavity, develops through active sodium transport by outer cells, creating an osmotic gradient that draws in fluid. This stage involves the first major cell lineage differentiation: the outer layer becomes the trophectoderm (TE), an epithelial sheet destined to form part of the placenta, while the inner cell mass (ICM) clusters eccentrically as the precursor to the embryo proper. TE specification is driven by positional cues and signaling, including inactivation of the Hippo pathway leading to CDX2 and GATA3 expression in outer cells, contrasting with OCT4 and NANOG in the ICM. Energy metabolism during these preimplantation stages relies predominantly on glycolysis, with the embryo utilizing maternal lactate and pyruvate initially, transitioning to glucose uptake for ATP production via anaerobic pathways, as oxidative phosphorylation is limited in the low-oxygen oviduct environment.^[17]^[16]^[18] The mature blastocyst then undergoes hatching, a process around days 6-7 where it partially or fully emerges from the protective zona pellucida glycoprotein shell. Hatching is facilitated by TE cell proliferation, blastocoel expansion, and localized enzymatic digestion (e.g., via trypsin-like proteases) of the zona, allowing the blastocyst to increase in volume and prepare for uterine interaction. This event is energy-intensive, sustained by heightened glycolytic flux to support cell motility and secretion without reliance on maternal mitochondrial substrates. Failure in hatching can impair developmental progression, as observed in assisted reproduction contexts.^[17]^[18]

Implantation and Establishment

Implantation Process

The implantation process begins when the blastocyst, having hatched from the zona pellucida, interacts with the uterine endometrium to establish pregnancy. This occurs approximately 6-10 days after fertilization in humans, typically at the upper posterior wall of the uterus in the midsagittal plane near the fundus.^[8]^[19] The process unfolds in three sequential phases: apposition, adhesion, and invasion. During apposition, the blastocyst loosely contacts the endometrial luminal epithelium, oriented with its inner cell mass directed toward the uterine wall. Adhesion follows, where the trophoblast cells firmly attach to the epithelium, and invasion ensues as trophoblast cells penetrate the endometrial stroma.^[20] Molecular mechanisms drive these phases, with trophoblast cells playing a central role in adhesion and penetration. Integrins, such as αVβ3, mediate attachment by recognizing endometrial ligands during the adhesion phase, peaking in expression during the mid-secretory phase of the menstrual cycle. Selectins, including L-selectin on trophoblast cells, facilitate initial loose interactions with endometrial pinopodes in apposition. For invasion, trophoblast cells secrete proteases like matrix metalloproteinases (MMPs), particularly MMP-3, which degrade the extracellular matrix to enable stromal penetration and vascular remodeling.^[20]^[21] Hormonal preparation is essential for creating a receptive endometrium. Progesterone, produced by the corpus luteum, induces decidualization of stromal cells around days 20-24 of the menstrual cycle, transforming them into decidual cells that express factors like α-smooth muscle actin and support implantation. This process establishes the window of implantation and promotes pinopode formation on the epithelium. Once implantation begins, the syncytiotrophoblast produces human chorionic gonadotropin (hCG), which sustains corpus luteum function, enhances progesterone secretion, and promotes angiogenesis and immune tolerance to prevent rejection.^[20] Species variations highlight differences in implantation invasiveness, reflecting placental types. In humans, implantation is highly invasive, leading to a hemochorial placenta where trophoblast directly contacts maternal blood after eroding uterine vasculature. This contrasts with non-invasive implantation in animals like pigs and ruminants, which form an epitheliochorial placenta; here, the conceptus remains in the uterine lumen with superficial trophoblast-epithelial contact, avoiding deep tissue invasion and relying initially on histotrophic nutrition.^[22]

Formation of Extraembryonic Structures

Following implantation, the trophoblast layer of the blastocyst undergoes differentiation into two distinct components: the inner cytotrophoblast, consisting of individual cuboidal cells, and the outer syncytiotrophoblast, a multinucleated layer formed by fusion of cytotrophoblast cells.^[23] This differentiation begins around day 9 post-fertilization and establishes the foundational structure for placental development.^[24] By the second week of development, small projections of cytotrophoblast cells, enveloped by syncytiotrophoblast, extend into surrounding spaces to form primary chorionic villi, which are essential precursors to the chorion.^[23] Concomitantly, lacunae—interconnected cavities—emerge within the syncytiotrophoblast around day 9, creating spaces that become filled with maternal blood as the trophoblast erodes adjacent uterine capillaries.^[24] These lacunae facilitate the initial nutrient and gas exchange between maternal and embryonic tissues by allowing diffusion across the thin syncytiotrophoblast barrier.^[25] The initiation of extraembryonic membranes occurs shortly after, during the second week. The amnion arises from epiblast cells that proliferate and line the amniotic cavity, forming a protective fluid-filled sac around the embryo.^[26] Simultaneously, the yolk sac develops from hypoblast cells, establishing a temporary structure that contributes to early hematopoiesis and nutrient absorption before being supplemented by the placenta.^[26] By the third week, the allantois emerges as an outpouching of endoderm from the hindgut, providing a conduit for waste excretion and vascular connections to the developing placenta.^[27] These formations integrate with gastrulation processes starting in week 3, where the primitive streak appears on the epiblast surface of the bilaminar disc, marking the site of cell ingress.^[28] Epiblast-derived cells migrate through the primitive streak to form the trilaminar embryonic disc, with a subset contributing to extraembryonic mesoderm that spreads between the cytotrophoblast and endoderm layers, supporting membrane expansion and vascularization.^[28]^[29]

Components and Organization

Embryonic Components

The inner cell mass (ICM) of the blastocyst, present at the time of implantation around day 7 post-fertilization, consists of pluripotent cells that give rise to the embryo proper.^[23] By the end of the first week, the ICM differentiates into a bilaminar embryonic disc, comprising two distinct layers: the epiblast on the dorsal surface and the hypoblast on the ventral surface.^[23] The epiblast, derived from the outer layer of the ICM, is a columnar epithelium that will primarily contribute to the fetal body, while the hypoblast forms a squamous layer that contributes to extraembryonic structures.^[23] This bilaminar configuration establishes the initial dorsal-ventral axis of the embryo.^[28] During the third week post-fertilization, gastrulation transforms the bilaminar disc into a trilaminar structure through the formation of the primitive streak in the caudal region of the epiblast.^[28] Epiblast cells ingress through the primitive streak, displacing the hypoblast to form the definitive endoderm as the innermost layer; these ingressing cells also migrate laterally and cranially to create the intraembryonic mesoderm between the endoderm and the remaining epiblast, which becomes the ectoderm.^[28] This process establishes the three primary germ layers—ectoderm, mesoderm, and endoderm—which serve as the foundational populations for all subsequent tissue and organ development.^[28] The primitive streak regresses by the end of week 4, completing gastrulation and setting the craniocaudal axis.^[28] Neurulation, occurring primarily during weeks 3 and 4, involves the ectoderm thickening to form the neural plate along the dorsal midline, which then folds and fuses to create the neural tube—the precursor to the central nervous system.^[30] Concurrently, the paraxial mesoderm adjacent to the neural tube segments into somites, paired blocks that appear sequentially from week 3 onward and provide the structural basis for vertebral column, skeletal muscle, and dermis formation.^[31] By the end of week 4, approximately 20–29 somite pairs have formed, marking progressive segmentation of the embryonic body plan.^[32] Early organogenesis begins in week 3 with the formation of the heart tube from lateral plate mesoderm in the cardiogenic region, which fuses midline and starts primitive contractions by the end of week 4, establishing the first functional circulatory system.^[33] In week 4, paired limb buds emerge as outgrowths from the lateral body wall, initially as mesenchymal cores covered by ectoderm, initiating the development of upper and lower extremities.^[34] The embryonic period, spanning weeks 3 through 8 post-fertilization, is characterized by rapid differentiation and organ primordia formation, during which the embryo transitions from a trilaminar disc to a recognizable vertebrate form with basic organ systems.^[35] Throughout this period, the embryo grows dramatically in size and complexity, measuring approximately 0.1 mm in diameter at implantation and reaching a crown-rump length of about 30 mm by the end of week 8, with key milestones including the closure of the neural tube by week 4 and the appearance of facial features and digit rays in limb buds by week 8.^[34]

Extraembryonic Membranes and Placenta

The extraembryonic membranes of the human conceptus consist of the amnion, chorion, yolk sac, and allantois, which collectively protect the embryonic components while facilitating nutrient uptake and waste elimination.^[36] These structures develop from the trophoblast and extraembryonic mesoderm, forming a supportive environment around the embryo. The placenta, arising from interactions between the chorion and maternal uterine tissues, serves as the primary interface for maternal-fetal exchange. The amnion is a thin, avascular membrane derived from the inner cell mass, forming a fluid-filled sac that encloses the embryo and amniotic fluid, which cushions against mechanical trauma and maintains a stable temperature. This membrane prevents adhesions between the embryo and surrounding tissues while allowing freedom of movement.^[36] The chorion, originating from the trophoblast, constitutes the outer fetal membrane and initially envelops the entire conceptus, later fusing with the amnion to form the chorioamnion. It contributes to the formation of chorionic villi essential for placental development and provides an initial barrier against maternal immune responses.^[36] The yolk sac, formed from hypoblast cells and extraembryonic mesoderm, serves as a site for early hematopoiesis, producing primitive blood cells and contributing to the development of the gastrointestinal tract as a gut precursor.^[36] Although vestigial for yolk storage in humans, it facilitates initial nutrient absorption from the trophoblast and establishes primitive circulation.^[36] The allantois, an endodermal outgrowth of the hindgut, is small in humans but plays a key role in forming the umbilical cord's blood vessels and contributing to the umbilical arteries and vein.^[36] It also aids in waste storage and gas exchange during early stages before placental dominance.^[37] Placental development in humans proceeds through the formation of chorionic villi, starting with primary villi composed of syncytiotrophoblast and cytotrophoblast around day 11 post-fertilization, progressing to secondary villi with mesenchymal cores by day 16, and tertiary villi incorporating fetal capillaries by the third week. The human placenta is discoid in shape, with villi branching extensively to maximize surface area at the fetal-maternal interface, enabling diffusion-based exchange of gases, nutrients, and wastes without direct blood mixing.^[38] Beyond transport, the placenta produces hormones such as progesterone to maintain uterine quiescence and human placental lactogen (hPL) to regulate maternal metabolism and promote fetal growth.^[39] The umbilical cord forms from the fusion of the allantoic and yolk sac stalks, connecting the fetus to the placenta and enclosing the vascular structures within a gelatinous matrix known as Wharton's jelly for protection.^[36] It contains two umbilical arteries, which carry deoxygenated fetal blood and wastes to the placenta, and one umbilical vein, which returns oxygenated, nutrient-rich blood to the fetus.^[40] This vascular arrangement ensures efficient circulatory support throughout gestation.^[41]

Biological and Clinical Significance

Role in Reproduction

The conceptus is essential for maintaining early pregnancy by secreting human chorionic gonadotropin (hCG) from its syncytiotrophoblast layer, which stimulates the corpus luteum to sustain progesterone production and prevent endometrial breakdown.^[42] This hormonal signaling ensures the uterine environment remains supportive until the placenta assumes progesterone production around weeks 8-10.^[43] Additionally, trophoblast cells in the conceptus promote immunological tolerance by evading maternal T-cell recognition through the absence of classical major histocompatibility complex (MHC) class I molecules and expression of non-classical MHC molecules like HLA-G, which inhibit natural killer cell activity.^[44] Throughout gestation, the conceptus achieves critical developmental milestones, progressing from the embryonic period—characterized by organogenesis and lasting until the end of week 8 post-fertilization—to the fetal stage starting at week 9, when the embryo measures about 3 cm and all major organ systems are established.^[8] This transition supports further growth and maturation, culminating in full-term development at approximately 40 weeks in humans, during which the conceptus increases in size from a few millimeters to around 50 cm.^[45] Evolutionarily, the conceptus represents a conserved structure across viviparous mammals, enabling internal development and species propagation through adaptations that optimize nutrient transfer via the placenta, such as specialized trophoblast invasion into maternal tissues for efficient exchange of oxygen, glucose, and waste products.^[46] These features have been maintained since the divergence of therian mammals, facilitating the shift from oviparity to viviparity and enhancing offspring survival rates.^[47] However, abnormalities in conceptus implantation can lead to significant risks, including ectopic pregnancy, where the conceptus attaches outside the uterine cavity—most commonly in the fallopian tube—resulting in potential tubal rupture, hemorrhage, and maternal mortality if untreated.^[48] Early loss is also prevalent, with approximately 40-50% of fertilized human ova failing to implant successfully under natural conditions, often due to chromosomal anomalies or suboptimal uterine receptivity.^[49]

Research and Medical Applications

In assisted reproductive technologies, the conceptus plays a central role in in vitro fertilization (IVF) and embryo transfer procedures, where monitoring and grading of blastocysts—typically at the morula to blastocyst stage—enable selection of viable embryos for transfer, thereby enhancing implantation and live birth rates. Blastocyst grading systems assess expansion, inner cell mass quality, and trophectoderm integrity, with higher-grade embryos demonstrating superior outcomes; for instance, day-5 blastocysts of optimal quality correlate with increased pregnancy success compared to lower grades or earlier-stage embryos. As of 2025, live birth rates per IVF cycle for women under 35 years old average approximately 40% in clinical settings, reflecting advancements in conceptus culture and non-invasive monitoring techniques that minimize embryo stress during handling.^[50]^[51]^[52] Embryonic stem cells derived from the inner cell mass of the conceptus have revolutionized regenerative medicine since their first isolation from human blastocysts in 1998, offering potential for treating degenerative diseases through differentiation into various cell types, such as neurons for Parkinson's or cardiomyocytes for heart repair. These pluripotent cells maintain indefinite self-renewal, with potential for immune matching in therapies, though typically requiring immunosuppression unless derived via methods like somatic cell nuclear transfer. Clinical trials have demonstrated safety in applications like macular degeneration treatment. However, ethical concerns arose immediately following the isolation, centering on the destruction of viable embryos to obtain the cells, prompting international guidelines that restrict derivation to surplus IVF conceptuses and prohibit reproductive cloning.^[53]^[54]^[55] Preimplantation genetic testing (PGT) targets the conceptus to screen for aneuploidy, a common chromosomal abnormality that contributes to implantation failure and miscarriage, by biopsying trophectoderm cells from blastocysts prior to transfer. PGT for aneuploidy (PGT-A) identifies euploid embryos, increasing live birth rates by up to 10-15% in women over 35 and reducing miscarriage risk, as evidenced by large-scale studies showing improved cumulative outcomes after multiple cycles. In animal models, CRISPR-Cas9 editing of pig conceptuses has advanced xenotransplantation research by knocking out immunogenic genes like alpha-gal and porcine endogenous retroviruses, enabling longer graft survival in primates and paving the way for human organ shortages; for example, multi-gene edited pig embryos produced via intracytoplasmic sperm injection have yielded viable xenogeneic kidneys functioning for over 170 days in non-human primates.^[56]^[57]^[58] In veterinary medicine, conceptus transfer techniques enhance breeding efficiency in livestock by allowing multiple offspring from elite females through superovulation and non-surgical embryo recovery, accelerating genetic selection for traits like milk yield in cattle and reducing generation intervals by up to 50% compared to natural breeding. These methods have disseminated superior genetics across herds, with pregnancy rates exceeding 50% for transferred embryos in commercial programs. Emerging research also explores conceptus vulnerabilities to zoonotic pathogens in livestock, such as Brucella species causing abortion, to mitigate disease transmission risks in intensive breeding systems, though comprehensive studies on edited conceptuses for zoonotic resistance remain limited.^[59]^[60]^[61]

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Nor is the more superficial form of placenta evolutionarily more ancient than the kinds that are invasive and make direct contact with maternal blood. On the ...
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Oct 25, 2019 · The evolution of viviparity in therian mammals, i.e. marsupials and “placental” mammals, occurred by retention of the conceptus in the female ...
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Tubal ectopic pregnancy - PMC - NIH
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Apr 16, 2020 · What proportion of fertilized human ova are lost before implantation? An estimated 40 to 50% of fertilized ova fail to implant.
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What Blastocyst Quality Means for IVF Success
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Embryonic Stem Cells - NCBI - NIH
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Embryonic stem cells: scientific possibilities, ethical considerations ...
The isolation of these cells involves the destruction of a five day old human embryo and is therefore ethically controversial. The aim of this review is to ...
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Success rates with preimplantation genetic testing for aneuploidy in ...
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Live Birth with or without Preimplantation Genetic Testing for ...
Nov 24, 2021 · Embryo selection with preimplantation genetic testing for aneuploidy (PGT-A) may improve pregnancy outcomes after initial embryo transfer.
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Comparison of ICSI, IVF, and in vivo derived embryos to produce ...
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Chapter 2 Applications of embryo transfer
The main use of embryo transfer in cattle has been to amplify reproductive rates of valuable females. Because of low reproductive rates and long generation ...Missing: veterinary | Show results with:veterinary
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Applications of embryo transfer and related technologies to cattle
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Economic burden of zoonotic and infectious diseases on livestock ...
May 15, 2025 · Zoonoses significantly impact human health and agricultural productivity, particularly affecting livestock farmers.