Morphology Embryonic Development

The early differentiation of trophoblast cells at the periphery of the blastocyst occurs within a week after fertilization in most domestic mammals. These cells facilitate nutrient absorption before uterine attachment and formation of the extraembryonic membranes, around the time of gastrulation.[4] Shortly before implantation in ruminants, binucleate cells appear in the predominantly uninucleate trophoblast. These cells form 15 20% of this layer throughout gestation and when mature, migrate out of the trophectoderm and fuse with uterine epithelium to produce a persistent fetomaternal syncytium. This facil itates the delivery to maternal tissue of granules containing placental lactogen and other bioactive factors synthesized in the binucleate giant cells, thereby enabling important endocrine functions of the placenta.[5]

Shortly after the blastocyst enters the uterus, implantation is achieved by attachment of the trophoblast to the endometrium, usually before the end of the second week of gestation in domestic mammals. In large domestic mammals, the functional placenta is formed from separable trophoblastic and maternal tissues, each with distinct, albeit intimately associated, vasculature. After initial attachment, the closely apposed chorionic and endometrial epithelia progressively fold and interdigitate to form microvilli. Simultaneously, rapid development of blood vessels on both sides of the newly formed placenta allows completion of the transition from histotrophic to hemotrophic nutrition of the implanted embryo.[4]

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