After gentle erosion between epithelial cells of the surface endometrium, invading trophoblasts burrow deeper. At 9 days of development, the blastocyst wall facing the uterine lumen is a single layer of lattened cells. By the 10th day, the blastocyst becomes totally encased within the endometrium (Fig. 5-9). he blastocyst wall opposite the uterine lumen is thicker and comprises two zones-the trophoblasts and the embryo-forming inner cell mass. As early as 7Y2 days postfertilization, the inner cell mass or embryonic disc diferentiates into a thick plate of primitive ectoderm and an underlying layer of endoderm. Some small cells appear between the embryonic disc and the trophoblasts and enclose a space that will become the amnionic cavity.
FIGURE 5-9 Drawing of sections through implanted blastocysts. A. At 10 days. B. At 12 days ater fertilization. This stage is characterized by the intercommunication of the lacunae filled with maternal blood. Note in (B) that large cavities have appeared in the extraembryonic mesoderm, forming the beginning of the extraembryonic coelom. Also note that extraembryonic endodermal cells have begun to form on the inside of the primitive yolk sac. (Redrawn from Moore KL, Persaud, W, Torchia, MG (eds): The Developing Human. Clinically Oriented Embryology, 9th edition, Philadelphia, Saunders, 201o3.)
Extraembryonic mesenchyme first appears as groups of isolated cells within the blastocyst cavity, and later this mesoderm completely lines the cavity. Spaces form and then fuse within the extraembryonic mesoderm to form the chorionic cavity (extraembryonic coelom). The chorion is composed of trophoblasts and mesenchyme. Some mesenchymal cells eventually will condense to form the body stalk. his stalk joins the embryo to the nutrient chorion and later develops into the umbilical cord. The body stalk can be recognized at an early stage at the caudal end of the embryonic disc (Fig. 7-3, p. 126).
As the embryo enlarges, more maternal decidua basalis is invaded by syncytiotrophoblast. Beginning approximately 12 days after conception, the syncytiotrophoblast is permeated
FIGURE 5-10 Electron micrograph of term human placenta villus. A villus capillary filled with fetal red blood cells (asterisks) is seen in close proximity to the microvilli border. (Reproduced with permission from Boyd JD, Hamilton WJ: The Human Placenta. Cambridge, Heffer, 1970.) 90 Placentation, Embryogenesis, and Fetal Development by a system of intercommunicating channels called trophoblastic lacunae. After invasion of supericial decidual capillary walls, lacunae become filled with maternal blood. At the same time, the decidual reaction intensifies in the surrounding stroma. This is characterized by decidual stromal cell enlargement and glycogen storage.
With deeper blastocyst invasion into the decidua, solid primary villi arise from buds of cytotrophoblasts that protrude into the primitive syncytium before 12 days postfertilization. Primary villi are composed of a cytotrophoblast core covered by syncytiotrophoblast. s the lacunae join, a complicated labyrinth is formed that is partitioned by these solid cytotrophoblastic columns. The trophoblast-lined channels form the intervillous space, and the solid cellular columns form theprimay villous stalks.