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Deep trophoblast invasion in the placental bed has been considered the hallmark of human pregnancy. It occurs by two routes, interstitial and endovascular, and results in transformation of the walls of the spiral arteries as they traverse the decidua and the inner third of the myometrium. Disturbances in this process are associated with reproductive disorders such preeclampsia. In contrast, trophoblast invasion in Old World monkeys occurs only by the endovascular route and seldom reaches the myometrium. Recently, it was shown that this pattern is maintained in gibbons, but that the human arrangement also occurs in chimpanzee and gorilla. There is an interesting parallel with results from placental immunology regarding the evolution of the major histocompatability complex class I antigen HLA-C and its cognate receptors. HLA-C is not present in Old World monkeys or gibbons. It emerged in the orangutan and became polymorphic in the lineage leading to gorilla, bonobo, chimpanzee, and human. Interaction between HLA-C1 and HLA-C2 on the surface of trophoblast and killer immunoglobulin-like receptors (KIRs) expressed by uterine natural killer cells are important regulators of trophoblast invasion. Evolution of this system in great apes may have been one prerequisite for deep trophoblast invasion but seems to have come at a price. The evidence now suggests that certain combinations of maternal genotype for KIRs and fetal genotype for HLA-C imply an increased risk of preeclampsia, fetal growth restriction, and recurrent abortion. The fetal genotype is in part derived from the father providing an explanation for the paternal contribution to reproductive disorders.

The mouse is often criticized as a model for pregnancy research as gestation is short, with much of organ development completed postnatally. There are also differences in the structure and physiology of the placenta between mouse and human. This review considers eight alternative models that recently have been proposed and two established ones that seem underutilized. A promising newcomer among rodents is the spiny mouse, which has a longer gestation than the mouse with organogenesis complete at birth. The guinea pig is also recommended both because it has well-developed neonates and because there is a wealth of information on pregnancy and placentation in the literature. Several smaller primates are considered. The mouse lemur has its advocates yet is less suited as a model for human pregnancy as its young are altricial, placentation very different from that of humans, and husbandry requirements not fully assessed. In contrast, the common marmoset, a New World monkey, has well-developed neonates and is kept at many primate centres. Marmoset placenta has some features that closely resemble human placentation, such as the interhaemal barrier, although it is uncertain if invasion of the uterine arteries occurs in this species. In conclusion, pregnancy research would benefit greatly from increased use of alternative models such as the spiny mouse and common marmoset.

Summary.

The effect of vasopressin on the flow of blood through the uterus and placenta in the rabbit has been investigated by an angio-graphic technique, with contrast injection through a catheter inserted in the external iliac artery or selectively in the urogenital artery. It was found that the drug markedly reduced contrast filling of the vessels in the placentae and uterine wall including the placental sinuses. It is suggested that this effect was produced by constriction of the venules in the uterine vascular bed. The possible operation of a shunt mechanism in the uterus is discussed.