Placental Vasculature and Circulation in Primates

When the fertilized primate ovum, whether of human, monkey, baboon, or mangabey, comes to rest on the lining epithelium of the uterus, the trophoblast of its wall promptly begins to invade the endometrium over an ever widening base (Heuser and Streeter, 1941; Enders, 1976; Houston, 1969, 1975; Enders et al., 1983). This usually occurs in the immediate vicinity of a capillary (Figure 1). In the human the whole blastocyst sinks into the endometrial stroma while in nonhuman primates the implantation remains superficial (compare Figures 2 and 3). As the trophoblast encounters maternal capillaries in the course of its penetration of the endometrium it surrounds them, and they thus become one source of the lacunae which appear in the trophoblastic shell. The tissue spaces which develop between trophoblastic cells constitute the second source (Figures 3 and 4). These lacunae enlarge and intercommunicate. Further communication between them and the maternal vascular system is established as progressive trophoblastic invasion erodes maternal vessel walls. Maternal blood in the lacunae, propelled at first only by capillary pressure, moves sluggishly. With deeper penetration of the trophoblast endometrial spiral arteries are tapped. Blood from them enters the lacunae under higher pressure and thus the first stage of placental circulation is established.