Developmental and Physiologic Anatomy of the Placenta
While the trophoblastic cords from the blastocyst are attaching to the uterus, blood capillaries grow into the cords from the vascular system of the newly forming embryo. By the 16th day after fertilization, blood also begins to be pumped by the heart of the embryo itself. Simultaneously, blood sinuses supplied with blood from the mother develop around the outsides of the tro-phoblastic cords. The trophoblast cells send out more and more projections, which become placental villi into
Implantation of the early human embryo, showing trophoblastic digestion and invasion of the endometrium. (Courtesy Dr. Arthur Hertig.)
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