HISTOCHEMISTRY OF PLACENTA 



m: 



placental barrier is the fact that the troph- 

 oblast of the human placenta forms a 

 syncytial sheet completely devoid of inter- 

 cellular spaces or cement. Unlike capillar- 

 ies, in which diffusion and filtration of 

 water-soluble substances are believed by 

 some to occur solely or mainly through the 

 intercellular spaces, this continuous sheet 

 of cytoplasm affords the only possible route 

 of placental transfer. In the sow, to take 

 another example, it is interesting that the 

 thinnest epithelium covering the chorionic 

 rugae is syncytial in character; it is par- 

 ticularly through these rugae that the 

 transmission of gases, water, and salts is 

 believed to occur. The relative inability of 

 leukocytes to traverse the placental barrier 

 may possibly be related to the absence of 

 intercellular spaces. In contrast to many 

 chorio-allantoic placentas, yolk sac placen- 

 tas are composed of discrete epithelial cells 

 with well defined intercellular spaces which 

 appear with great prominence under the 

 electron microscope (Dempsey, 1953). 



XI. Suiiiinarizing Reflections on 



Comparative Placentation and 



Placental Permeability 



The chorion, chorio-allantois, and yolk 

 sac of mammals become variously apposed 

 to the uterine mucosa to give rise to the "pla- 

 cental barrier" which mediates the physio- 

 logic exchange between the mother and the 

 fetus. The chorio-allantoic placenta of eu- 

 therian mammals undergoes changes in the 

 course of gestation. This aging process is 

 characterized by a gradual diminution in 

 width and cytologic simplification of the var- 

 ious layers of the placental barrier, and by a 

 progressive elimination of one or more of 

 the maternal layers in most groups of mam- 

 mals. This gradual diminution in width of 

 the chorio-allantoic placental barrier is be- 

 lieved to account for the fact that placental 

 transmission of some readily diffusible sub- 

 stances, such as oxygen, water, and various 

 salts, increases as gestation proceeds. 



In those groups of mammals possessing 

 an inverted yolk sac placenta, an elimina- 

 tion of several fetal, instead of maternal, 

 layers occurs in the course of gestation. The 

 principal remaining layer, the visceral endo- 

 dermal epithelium, undergoes some degree 



of cytologic aging but does not diminish in 

 width. Some mammals possess still other 

 structures which mediate exchange between 

 the mother and fetus. The principal of 

 these, the various central and paraplacental 

 hematomas of carnivores, persist through- 

 out gestation, without apparent cytologic 

 changes or any reduction in width or num- 

 ber of layers. 



In mammals possessing only a yolk sac 

 placenta, such as some marsupials, or solely 

 a chorio-allantoic placenta, such as man, 

 physiologic exchange must be mediated en- 

 tirely through one type of placenta. In 

 many groups of mammals both types of 

 placentas develop and exist concurrently for 

 varying periods of time, in which event pla- 

 cental exchange seems to be divided between 

 them. However, the respective functional 

 roles of each of these very different pla- 

 cental structures in the transmission of var- 

 ious substances has not been extensively 

 investigated. 



Until recently it was generally assumed 

 that nearly all substances which traverse 

 the placenta do so by diffusion through the 

 thinnest parts of the chorio-allantoic pla- 

 centa. However, it now seems more likely 

 that most substances are regulated in their 

 passage through the placental barrier and 

 that possibly only some readily diffusible 

 substances, such as oxygen, water, and some 

 salts whose rates of placental exchange in- 

 crease during the course of gestation, are 

 transmitted mainly through the thinner 

 parts of the chorio-allantoic placenta. Pro- 

 teins, lipids, and carbohydrates which seem 

 to be regulated in their passage are prob- 

 ably transmitted in many different ways 

 and are acted on variously by the enzymes 

 and complex organelles present in the cyto- 

 plasm of the cells of the barrier. Recent 

 experimental observations indicate that in 

 the visceral endoderm of the rodent yolk 

 sac the nucleus may be involved in some 

 transfers. In man and monkeys, which pos- 

 sess only a chorio-allantoic placenta, this 

 regulation must take place solely in the 

 trophoblast, but in other groups of animals 

 which possess, in addition, either a yolk sac 

 placenta or placental hematomas, transfer 

 of some substances evidently occurs through 

 the latter structures. Recent investigations 



