HISTOCHEMISTRY OF PLACENTA 



917 



pregnancy some of the human chorionic 

 villi lose their syncytial covering entirely; 

 the increase in placental permeability is at- 

 tributed to this structural alteration. How- 

 ever, it should be pointed out that the na- 

 ture and degree of degeneration and loss of 

 the syncytium in human villi have not been 

 carefully analyzed. Moreover, it is not 

 known whether such altered villi are func- 

 tionally active or dead and functionless. 

 However, the assumption that a fraction of 

 the villi becomes functionless would be con- 

 sonant with an observation of Flexner, 

 Cowic, Hellman, Wilde and Vosburgh 

 (1948) that there is a sharp terminal de- 

 cline in placental permeability after the 

 36th week of gestation. 



V. Yolk Sac Placentation 



In those lower vertebrates, such as some 

 fishes, amphibians, and reptiles which are 

 either ovoviviparous or viviparous, the yolk 

 sac plays the principal role as the fetal 

 membrane subserving the transfer of meta- 

 bolic materials (Amoroso, 1952) . An excep- 

 tion to this is encountered in some reptiles 

 (Weekes, 1935) in which chorio-allantoic 

 placentation occurs. In marsupials, the yolk 

 sac is very large, whereas the allantois is 

 always relatively small, and in only three 

 species does the latter vascularize a pla- 

 centa. In accordance with Grosser's termi- 

 nology, the chorio-allantoic placenta of 

 Perameles is "endothelio-endothelial" in 

 character, thus differing fundamentally 

 from the types he defined. "In all other 

 marsupials so far investigated the embryo 

 is nourished exclusively through the yolk- 

 sac and a definite yolk-sac placenta of 

 somewhat complex character is present" 

 (Amoroso, 1952). Thus it is apparent that 

 Grosser's theory does not apply to placen- 

 tation in the majority of lower placental 

 vertebrates or to the Metatheria (marsu- 

 pials). 



In eutherian mammals, on the other hand, 

 the most typical structure subserving physi- 

 ologic exchange between mother and fetus 

 and which is constantly present, is the 

 chorio-allantoic placenta (Hamilton, Boyd 

 and Mossman, 1952). The yolk sac in these 

 mammals is the most variable of the fetal 

 membranes. It may occur as a primitive bi- 



laminar yolk sac, or as a vascularized tri- 

 laminar yolk sac which develops early and 

 is temporary. In some orders of mammals 

 (rodents, bats, insectivores, armadillos), a 

 very different and more complex structure, 

 an "inverted" yolk sac placenta, develops. 

 This usually increases in extent during ges- 

 tation and in most species becomes covered 

 with elaborately branched, vascularized 

 villi which are in contact with the uterine 

 mucosa (Amoroso, 1952; Hamilton, Boyd 

 and Mossman, 1952). In ungulates, ceta- 

 ceans, lemurs, sloths, and the Simiae (mon- 

 keys, apes, man), it has been assumed that 

 the yolk sac, although present as a vesicle, 

 plays little or no role in the metabolic ex- 

 change between mother and fetus. However, 

 some histochemical findings on the human 

 yolk sac challenge this assumption. In a 

 histochemical study of 5-, 6-, and 7-mm. 

 human embryos, McKay, Adams, Hertig 

 and Danziger (1955a, b) localized the fol- 

 lowing substances in the yolk sac endoderm : 

 glycogen, glycoprotein, ribonucleoprotein, 

 acid and alkaline phosphatase, 5-nucleo- 

 tidase, and nonspecific esterase. These in- 

 vestigators suggested that the large amount 

 of glycogen in the yolk sac and its absence 

 from the fetal liver may indicate that the 

 yolk sac is supplying glucose to the embryo 

 during the first weeks of embryonic life. 

 McKay and his associates pointed out that 

 there is no iron in the human yolk sac en- 

 doderm, whereas the rodent yolk sac is rich 

 in this substance (Wislocki, Deane and 

 Dempsey, 1946). 



The inverted yolk sac placenta of rodents 

 and lagomorphs has received more atten- 

 tion since Brambell and his associates 

 (1948, 1949, 1951, 1957) demonstrated that 

 maternal antibodies are transferred exclu- 

 sively by this ancient membrane. A short 

 description of the histology of the inverted 

 yolk sac of the rat follows for the purpose 

 of general orientation. A good diagram of 

 the histology of the rat placenta was pub- 

 lished by Anderson (1959). The yolk sac 

 placenta of the rat is divided into two mor- 

 phologic zones. (1) An outer, nonvascular 

 parietal wall (bilaminar omphalopleure) 

 consists of scattered cuboidal endodermal 

 cells w^iich form an incomplete lining on the 

 interior surface of Reichert's membrane. 



