HISTOCHEMISTRY OF PLACENTA 



895 



the syncytium at 9 to 10 weeks typifies that 

 of a pinocytotic membrane (Wislocki and 

 Dempsey, 1955).^ A profusion of microvilli 

 of various shapes reach into the maternal 

 blood. Some microvilli are long and slender 

 with enlarged tips; others are short and 

 thick; and occasionally peninsulas of cyto- 

 plasm studded with microvilli extend into 

 the maternal blood space. Often in the 

 marginal zone immediately beneath the 

 microvilli there are large vesicles containing 

 finely stippled or flocculent material. They 

 are occasionally seen in the large tongues 

 •of cytoplasm which protrude from the free 

 surface. These vesicles are most likely 

 formed as a result of pinocytotic activity. 



Basal surface. The inner surface of the 

 syncytium is approximated to the surfaces 

 of the Ijanghans cells or the subjacent 

 stroma of the placental villus (Fig. 15.9). 

 As mentioned previously, the Langhans cells 

 gradually diminish in number so that the 

 syncytium eventually comes widely in con- 

 tact with the chorionic stroma. However, 

 even early in gestation, there are occasional 

 gaps between the Langhans cells where the 

 syncytium is in direct contact with the sub- 

 jacent mesenchyma. Through these gaps 

 metabolites traversing the placental bar- 

 rier can by-pass the Langhans cells. 



Cell organelles. Cytoplasmic basophilia 

 is very intense in the broad inner zone of 

 the syncytium, especially surrounding the 

 nuclei (Figs. 15.7 and 15.10). Since this 

 basophilia is abolished by ribonuclease, it 

 has been attributed to the presence of ribo- 

 nucleic acid (Dempsey and Wislocki, 1945). 

 Further evidence supporting this identifica- 

 tion is derived from: (1) the similarity of 

 this basophilia to that of Nissl substance 

 (Singer and Wislocki, 1948), (2) the meta- 

 ohromasia of this region in young placentas 

 (Wislocki and Dempsey, 1948), and (3) the 

 <'oncentration of the endoplasmic reticulum 

 (ergastoplasm) in the inner two-thirds of 

 the .syncytium (Wislocki and Dempsey, 

 1955). This rich cytoplasmic basophilia, 

 which has been shown to constitute the 



^Bargmann and Knoop (1959) have also de- 

 scribed the ultrastructure of the human placental 

 barrier. They emphasize the syncytial nature of the 

 outer trophoblastic layer, and offer further de- 

 scription of the ultrastructure of the Langhans cells, 

 Hofbauer cells, and stromal cells. 



microsomal fraction of the biochemist (Pa- 

 lade and Siekevitz, 1956) points toward an 

 active participation by these cells in pro- 

 tein synthesis. 



In contrast to the inner zone, the outer 

 zone of the syncytium is strongly acido- 

 philic, although the narrow outermost zone 

 corresponding to the brush border is less 

 acidophilic (Singer and Wislocki, 1948). 

 This acidophilia suggests the occurrence of 

 basic proteins in the outer zone. The ultra- 

 structure of this region shows that there 

 are relatively fewer ergastoplasmic elements 

 but there is a concentration of large vesicles 

 which most likely are the products of pino- 

 cytosis. Higher resolution electron micros- 

 copy of this region is needed. It should be 

 mentioned at this point that protein-bound 

 sulfhydryl groups are concentrated espe- 

 cially at the inner and outer borders of the 

 syncytial cytoplasm (Figs. 15.33 and 15.34). 



Mitochondria are abundant in the syncy- 

 tium, occurring as small granules and rods 

 (Figs. 15.26 and 15.30). There is the indica- 

 tion of high succinic dehydrogenase activity 

 here at 6 weeks of gestation, although this 

 histochemical determination was compli- 

 cated by the presence of lipid in the syncy- 

 tium (Figs. 15.37 and 15.38). With the 

 electron microscope, it was observed that 

 the mitochondria are smaller but more nu- 

 merous than those in the Langhans cells 

 (Wislocki and Dempsey, 1955). 



The Golgi apparatus forms a dispersed 

 network in the syncytium. This organelle 

 has been described in the various cells of 

 the placenta by Acconci (1912), Wislocki 

 and Bennett (1943), and Baker, Hook and 

 Severinghaus (1944). 



Glycogen and other pas positive ma- 

 terial. In the human placenta in the first 

 month of gestation a moderate amount of 

 glycogen is stored in the syncytial tropho- 

 blast. It disappears almost entirely by the 

 end of the second month. Similar early 

 storage and loss of glycogen occur in the 

 Langhans cells and stromal fibroblasts. 



PAS positive material which is resistant 

 to digestion by saliva is conspicuous in the 

 brush border and marginal cytoplasm of 

 the syncytium (Figs. 15.29 and 15.54). A 

 faint red stippling of reactive material is 

 also visible in the deeper cytoplasm (Fig. 

 15.54). A dark red reaction occurs also in 



