920 



SPERM, OVA, AND PREGNANCY 



tion as phagocytes. The absorption of large 

 molecules is believed to occur by the process 

 of pinocytosis. The fine structure of these 

 cells has been described in the guinea pig 

 (Dempsey, 1953) and rat (Wislocki and 

 Dempsey, 1955b). Some recent electron mi- 

 crographs of the rat yolk sac (Figs. 15.80- 

 15.83) are presented in this review by Pady- 

 kula. The free surface of these cells typifies 

 that of a membrane engaged in pinocytosis. 

 There are numerous surface projections or 

 microvilli which are pleomorphic and 

 branch frequently (Figs. 15.81 and 15.82). 

 These projections form the brush border 

 which has long been recognized with the 

 light microscope, and which is rich in gly- 

 coprotein and, at certain times in gestation, 

 in alkaline phosphatase. In both the rat and 

 guinea pig these surface projections l)c- 

 come simpler and shorter near term. In ad- 

 dition to these evaginations, the surface 

 plasma membrane is invaginated in the 

 form of minute anastomosing tubules which 

 have a denser thicker wall than tiie micro- 

 villi (Fig. 15.82). It seems fairly certain 

 that during pinocytosis a local enlargement 

 of such a tubule is produced and a pinocy- 

 totic vesicle is formed. Vesicles fill much of 

 the supranuclear cytoplasm, and there is 

 considerable heterogeneity in the size and 

 content of the supranuclear vesicles (Fig. 

 15.80). Filamentous mitochondria occur 

 throughout the cytoplasm. The endoplasmic 

 reticulum is most concentrated around tiio 

 nuclei, although it is also diffusely distrib- 

 uted throughout the cytoplasm. The typical 

 agranular membranes and vesicles of the 

 Golgi apparatus can be recognized near the 

 nucleus (Fig. 15.83). Glycogen is stored in 

 the lower half of the cell, especially in the 

 infranuclear region. Lipid droplets occur 



throughout the cytoplasm, but the larger 

 ones are usually infranuclear where they 

 are often in close association with the basal 

 surface of the nucleus (Figs. 15.81 and 

 15.83). The supranuclear lipid is often in 

 the form of aggregations in complicated as- 

 sociations with membranes (Figs. 15.79 and 

 15.81). Minute lipid droplets are also found 

 within the nucleus (Figs. 15.78 and 15.80). 

 The lateral cell boundaries of these cells are 

 closely apposed early in gestation, whereas 

 near term large lateral intercellular dilata- 

 tions occur. Between these dilations, where 

 the plasma membranes are closely apposed, 

 desmosomes are evident. The bases of the 

 cells rest on a narrow basement membrane. 



With the electron microscope, experimen- 

 tal cytologic analyses of absorption by the 

 ^•isceral cndoderm of the rabbit and mouse 

 have been made by Luse and her associates 

 (Luse, 1957; Luse, Davies and Smith, 1959; 

 Luse, Davies and Clark, 1959). The follow- 

 ing materials were injected into the uterus 

 of the rabbit and mouse: colloidal gold, egg 

 albumin, lii)ids, saccharated iron, bovine 

 y-globulin, and salivary gland virus. All of 

 these materials entered cytoplasmic pino- 

 cytotic vesicles. However, more interest- 

 ingly, iron, colloidal carbon, and salivary 

 gland virus penetrated the nuclei. Further 

 work suggests that pinocytosis by the nu- 

 clear membrane is the method of nuclear 

 ]ienetration. Similar nuclear inclusions arise 

 in the nuclei of newborn rat duodenum after 

 suckling. This amazing intracellular path- 

 way of absorption i)robably occurs natu- 

 rally as witnessed by the occurrence of lipid 

 in the nuclei of normal visceral endoderm, 

 duodenum, and liver. 



The inverted yolk sac placenta of the rat 

 undergoes a striking differentiation during 



Plate 15. XI 



Fig. 15.46. A portion of a placental septum showing cytotroplioblasts surrounded by 

 ground substance. Human placenta at 3% months of gestation. Masson's triacid stain. X 240. 



Fig. 15.47. A cell island containing a group of cytotioi)liol)last8 surrounded by ground sub- 

 stance. Human placenta at 2 months of gestation. Azan stain. X 240. 



Fig. 15.48. Cytotrophobla.sts of the trophoblastic slioU stained by Baker's acid hematein 

 method for phospholipids. Human placenta at 3V2 months of gestation. Observe the intense 

 reaction in the cytoplasm of the tiophoblasts. Compare with Figure 15.25. X 160. 



Fig. 15.49. A portion of a placental septum of a human placenta at full term, stained by 

 the periodic acid-Schiff method after exposure of the section to saliva. The cyto])l;ism of the 

 cytotrophoblasts exhibits a faint reaction which should be compared with Figuio 15.55 which 

 illustrates the cells at higher magnification. The clumps of cells are surrounded by masses of 

 intensely stained fibrin. Compare with Figures 15.32 and 15.46 which illustrate the tropho- 

 blastic shell at 3 to 3M montlis of gestation before a great deal of fibiin has appeared. X 300. 



