In humans, Old World monkeys, rodents, and rabbits, the growing placenta deeply invades the uterine wall. This invasion involves an extensive remodeling of the endometrial vasculature to create an interface with the placenta, which is itself primarily a large vascular bed (roughly the size of the liver in humans). By the 10th day of human pregnancy, the blastocyst is completely embedded in the endometrial stroma and cytotrophoblast cells, specialized stem cells that cover the surface of the conceptus, invade the endometrium, eventually penetrating it entirely, and even entering the outer third of the myometrium. During the initial stages of development, the growth of subepithelial capillaries, stimulated by factors from the embryo, are sufficient to support its growth. Further development, however, requires even greater access to maternal blood. This is accomplished when cytotrophoblasts invade the walls of the endometrial arteri-oles and veins, replacing endothelial and smooth muscle cells, to create new large-bore, low-resistance hybrid vessels. The arterial channels deliver maternal blood directly onto the surface of the placenta, which consists of finger-like-villi composed of a stromal core surrounded by the cytotrophoblast cells. Within the stromal core are fetal capillaries. In essence, then, the endometrial microvasculature is largely replaced by, and its function taken over by, the microvasculature of the fetal placenta. This condition persists until parturition when the placenta is shed, the endometrium is restored to the nonpregnant state, and cycling resumes as a result of the resumption of cyclic ovarian function and steroid hormone production.
Studies have shown that oxygen levels play an important role in the invasion of the cytotrophoblast cells . When levels are low, due to increasing oxygen demand by the embryo, the cytotrophoblasts are highly proliferative and invasive. When they encounter and invade the blood vessel wall, however, they are exposed to the higher oxygen levels in maternal blood. This inhibits further proliferation and invasion, and rather induces the merger of the cytotro-phoblasts into the multinucleate syncytiotrophoblast layer, which synthesizes large amounts of progesterone important for maintenance of pregnancy. This transformation involves the expression of endothelial cell-type adhesion molecules and other proteins typical of endothelial cells. Consistent with a vascular cell phenotype, their differentiation and survival appears to be dependent on members of the VEGF family .
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.