886 



SPERM, OVA. AND PREGNANCY 



the so-called definitive placenta. Through its 

 brief life history, the mammalian placenta 

 displays a pattern of differentiation which 

 is manifested by structural, physiologic, and 

 biochemical changes. Studies which define 

 these changes emphasize that the placenta 

 is different at various points in gestation. 

 Histochemical methods have aided in tag- 

 ging these changes in structure and function. 

 It should be emphasized that, despite the 

 kaleidoscopic changes which take place 

 while the fetal membranes are variously 

 differentiating into the so-called definitive 

 placenta, the structures involved are func- 

 tionally adequate at all times to meet the 

 metabolic demands of the developing em- 

 bryo and fetus. 



In various groups of manmials further 

 complications arise from the variety of 

 modes of placentation, involving several 

 kinds of nidation and subsequent intercres- 

 cence of the fetal and maternal membranes 

 to constitute the placenta. The walls of the 

 yolk sac and the allantois fuse in different 

 ways with the chorion of mammals so that 

 either a choriovitelline or chorio-allantoic 

 placenta, or both, may differentiate. The 

 former occurs in many orders of mammals 

 (but not in man and monkey), usually pre- 

 ceding the latter in time of appearance, and 

 disappearing after the allantoic placenta 

 has become established. However, in some 



groups {e.g., rodents) the two types of pla- 

 centas continue to function concurrently 

 throughout gestation. Until recently, the 

 significance of the yolk sac placenta has 

 been largely overlooked. The elegant physio- 

 logic experiments of Brambell and his as- 

 sociates (1948, 1950, 1951, 1956, 1957) have 

 high-lighted this membrane by establishing 

 it as the exclusive mediator of antibody 

 transport from mother to fetus in rodents 

 and lagomorphs. 



As a consequence of the many varieties of 

 placentas and numerous stages of differen- 

 tiation of the various types, much research 

 has been devoted to comparative placenta- 

 tion, seeking to elucidate the phylogenetic 

 relationships, the topographic structure, and 

 functional roles of the fetal membranes 

 and placenta as they manifest themselves in 

 different groups of mammals (Mossman, 

 1937; Amoroso, 1952). Despite extensive in- 

 vestigations of these questions, there are 

 still large areas of uncertainty and lack of 

 agreement in reference to the phylogeny of 

 the placenta and of the structural homolo- 

 gies and functional significance of its vari- 

 ous parts. As a result of the complexities 

 of placental structure and the attendant 

 preoccupation with comparative placental 

 topography, the placenta is less well known 

 histologically, cytologically, and histochem- 

 ically than most organs of the body. 



Plate 15.11 



Fig. 15.3. A section through the placenta of a rhesus monkey on the 29th day of gestation. 

 Secondary chorionic viUi are visible, each comprising a core of vascularized mesoderm sur- 

 rounded by a darkly stained mantle of cytotrophoblast and syncytium, bordering the inter- 

 villous space (i) in which maternal blood circulates. The tips of the definitive villi extend 

 downward as columns of cellular trophoblast (primary villi). The distal ends of these cell 

 columns (c) unite on the periphery of the growing placenta to form the trophoblastic shell 

 (.s). The latter merges indistinctly with the underlying decidually-transformed endometrium 

 (d); uterine gland (g). Iron hematoxylin stain. X 6'/2. 



Fig. 15.4. Localization of alkaline phosphatase in an anchoring villus from a human pla- 

 centa of 8 weeks. The enz.yme occurs only in the outer margin of the syncytium. X 235. 

 (Dempsey and Wislocki, 1945.) 



Fig. 15.5. Indophenol oxidase reaction in a human chorionic villus of the 6th week of preg- 

 nancy. The oxidase activity is confined to the syncytial trophoblast. X 100. (Dempsey and 

 Wislocki, 1944.) 



Fig. 15.6. The birefringent lipids in the syncytium of a monkey's placenta. A fresh spread 

 of the placenta was photographed between crossed prisms. X 80. (Dcmpsev and Wislocki. 

 1944.) 



Fig. 15.7. Localization of basophilic substance in a villus from a human placenta of 13 

 weeks. An outer, eosinophilic zone (e) and a deeper, basophilic region (b) may be seen. Zen- 

 ker formol fixative. Eosin methylene blue stain. X 1440. (Demp-sey and Wislocki, 1945.) 



Fig. 15.8. A chorionic villus of a normal himian placenta of 10 weeks' gestation stained for 

 iron by the TurnbuU blue method. The iron is concentrated in the outermost part of the 

 stroma just beneath the trophoblast. Some particles staining for iron extend into the interior 

 of the villus. X 250. 



