914 



SPERM, OVA, AND PREGNANCY 



closely than the components of the pul- 

 monary alveolar lining or the renal glo- 

 merular membrane. The trophoblastic cells 

 are characterized by microvilli and other 

 surface projections which are pleomorphic 

 and often branched. Many observations 

 suggest that absorption by pinocytosis oc- 

 curs in many types of placental cells. 

 Whether or not erosion of the uterine wall 

 occurs, the placental barrier is structurally 

 complex. Along with the cellular layers, 

 basement membranes are regularly inter- 

 posed between the maternal and fetal blood- 

 streams. Further discussion is presented in 

 the section on yolk sac placentation where 

 some experimental cytologic observations 

 have been made during the process of ab- 

 sorption. 



C. REDUCTION IN NUMBER OF THE LAYERS 

 OF THE CHORIO-ALLANTOIC PLACENTA 



The successive elimination of the mater- 

 nal layers of chorio-allantoic placentas as 

 envisioned by Grosser's scheme has met 

 with general acceptance. However, Wislocki 

 and Dempscy (1946a I pointed out that the 

 endotheliochorial type of placenta is prob- 

 ably nonexistent, because in carnivores and 

 sloths the endothelial-lined maternal blood 

 vessels are surrounded by a basement mem- 

 brane and in some species, as in the cat, 

 large decidual cells are present in the laby- 

 rinth. Consequently, in some carnivores the 

 placenta is syndesmochorial rather than 

 endotheliochorial. 



The hemoendothelial type of placenta 

 postulated by Mossman finds no support in 

 recent observations by Wislocki and Demp- 

 sey (1955b) and Schiebler and Knoop 

 (1960) with the electron microscope. These 



investigators found in the chorio-allantoic 

 placental labyrinths of rat and rabbit late 

 in gestation a complete trophoblastic mem- 

 brane, consisting of 2 or 3 layers of flat- 

 tened, imbricated trophoblastic cells. The 

 presence of these layers is not detectable 

 with the light microscope. These findings 

 indicate that the placentas of these species 

 are hemochorial and not hemoendothelial. 



D. REDUCTION IN WIDTH OF THE LAYERS 



OF THE CHORIO-ALLANTOIC 



PLACENTA 



The diminution in width of the tissues 

 separating the maternal blood channels 

 from the fetal capillaries, as postulated in 

 Grosser's scheme of chorio-allantoic pla- 

 centas, seems to be borne out by histologic 

 observations of both his phylogenetic series 

 and successive ontogenetic stages. How- 

 ever, some have pictured the layers in a 

 schematic way (Huggett, 1944; Arey, 1946) 

 with no regard for their relative widths and 

 relationships. Actually, in progressing from 

 ei)itheliochorial to hemochorial placentas, 

 the gradual reduction in width of the thin- 

 nest areas is not nearly as striking as the 

 theoretic concept of the removal of succes- 

 sive layers implies. This is due partly to the 

 fact that in all species the connective tissue 

 layers at the sites of the thinnest places 

 consist only of basement membranes. Fur- 

 thermore, the capillaries in the thin areas 

 of all animals are pressed against the ad- 

 jacent epithelia and in some, for example 

 in the sow (Figs. 15.23, 15.28 and 15.64) 

 and many ungulates, the fetal capillaries 

 follow intra-epithelial courses in the tropho- 

 blast (Wislocki and Dempsey, 1946b: 

 Amoroso, 1947, 1952). In addition, in the 



Plate 15. IX 



Fig. 15.39. Human placental labyrinth at 4 months, showing acid phosphatase activity in 

 the syncytium and stroma of the chorionic villi. Gomori's method, using glycerophosphate as 

 substrate at pH 4.7. X 140. (Wislocki and Dempsey, 1948.) 



Fig. 15.40. The basal plate of a human placenta at full term, showing the presence of acid 

 phosphatase in the chorionic villi (above), its almost complete absence in the basal plate 

 (center), and a marked reaction at the line of junction of the basal plate with the decidua 

 (below). Gomori's method using glycerophosphate as substrate at pH 4.7. X 175. 



Fig. 15.41. Human placental labyrinth at full term, showing the activity of alkaline pho.s- 

 phatase in the syncytium clothing the chorionic villi. Gomori's method using glycerophos- 

 phatase as substrate at pH 9.4. X 220. 



Fig. 15.42. The basal plate of a human placenta at full term, showing an inten,se alkaline 

 phosphata,se reaction in the chorionic villi (above), its nearly complete absence in the basal 

 plate (center) and a slight reaction at the line of junction of the basal plate with the decidua 

 (below). Gomori's method using glycerophosphate as substrate at pH 9.4. 



