948 



SPERM, OVA, AND PREGNANCY 



indicate that in the rabbit and rat some 

 proteins and dyes, administered experimen- 

 tally, are transferred by way of the yolk 

 sac. In those marsupials which possess only 

 a yolk sac placenta nothing is known about 

 the mode of physiologic exchange, except 

 that it is sufficient to support fetal develop- 

 ment. 



Grosser arranged the chorio-allantoic pla- 

 centas of eutherian mammals in a phylo- 

 genic series comprising four placental types 

 dependent on a progressive decrease in the 

 number of layers intervening between the 

 maternal and fetal circulations. According 

 to his theory, the most primitive placental 

 barrier consists of six layers whereas the 

 most highly developed barriers have been 

 reduced to three layers. Although Grosser 

 recognized yolk sac placentas and hema- 

 tomas as supplemental means of transfer 

 of some nutritive materials from mother to 

 fetus, only his chorio-allantoic placental 

 types have been generally adopted to ex- 

 plain the passage of nearly all substances 

 from mother to fetus. By the removal of a 

 succession of three more or less functionally 

 equivalent, maternal layers in a phylogenic 

 sequence, the placental barrier has been en- 

 visioned as becoming progressively nar- 

 rower, or thinner, and increasingly more 

 permeable to the passage by diffusion of an 

 increasingly larger number of substances. 

 This scheme of the phylogenic simplifica- 

 tion of the placental barrier seemed to be 

 repeated in an ontogenic sense by the ob- 

 served thinning of the placental barrier in 

 the course of gestation in individual species. 

 According to this belief, substances of larger 

 molecular size, particularly proteins, are the 

 last which are enabled to diffuse across the 

 placental barrier in the postulated phylo- 

 genic and ontogenic series of stages. 



With the growing recognition that the 

 passage of a great many substances across 

 the placental barrier is chemically regulated 

 (Huggett and Hammond, 1952) and that 

 the cytoplasm of the barrier contains a host 

 of enzymes and numerous organelles, it 

 seems increasingly evident that Grosser's 

 doctrine must be reevaluated and modified. 

 It has become necessary to consider each 

 substance individually, with respect to its 

 place of passage and the fatcors regulating 

 its exchange. Thus, the regional cytologic 

 and histochemical organization of the bar- 

 rier, of which Grosser's doctrine takes little 

 cognizance, should assume much greater 

 importance; and, in addition to the relative 

 thickness and width of the barrier, the rela- 

 tive extent and nature of its absorbing sur- 

 faces will have to be more carefully ex- 

 plored. The electron microscope should be of 

 considerable value in ascertaining the struc- 

 ture of the absorbing surfaces and interior 

 of the barrier. Studies of the human pla- 

 centa in early months of gestation, recently 

 begun with the electron microscope, have 

 revealed a multitude of microvilli on the 

 surface of the trophoblast and vesicles in the 

 syncytium. This suggests that absorption 

 from the intervillous space takes place to 

 a considerable extent by the process of 

 pinocytosis. Indications of absorption by 

 pinocytosis have also been reported by 

 means of electron microscopy in the epi- 

 thelium of the yolk sac of the guinea pig 

 and rat. A major role in placental physio- 

 logic exchange may eventually have to 

 be assigned to pinocytosis. It seems ap- 

 parent that pinocytosis is a process which 

 predominates in the first part of human 

 gestation but subsequently diminishes as 

 the placental barrier becomes thinner and 

 more simplified. It seems probable that as 



Plate 15. XX 



Electronmicrographs oj the rat visceral endoderm at 13 days of gestation 



Fig. 15.82. Free surface of the visceral endoderm. The junction of two cells occurs at J 

 which is marked also by the dense cytoplasmic condensation of the terminal bar. Note that 

 the microvilli (mv) are penetrated by tiny tubules. At point x, the plasma membrane of the 

 microvilli is continuous with the denser membrane which forms the walls of the canaliculi. 

 Near the center of the photograph, the canaliculi are in the form of a figure 8, showing the 

 anastomosis of the system of superficial canals. X 28,000. 



Fig. 15.83. Basal surface of the visceral endoderm. The close association of a large lipid 

 droplet with the concave basal surface of the nucleus is illustrated here. Two typical clusters 

 of Golgi membranes (g) are located near the basal region of the nucleus. Mitochondria and 

 elements of the endoplasmic reticulum are abundant. X 13.200. 



