HISTOCHEMISTRY OF PLACENTA 



913 



blast in the labyrinth, as had been described 

 by Grosser (1908, 1909) ; the fetal blood ves- 

 sels are clothed by two or three thin layers 

 of overlapping individual cytotrophoblasts 

 which together constitute a laminated mem- 

 brane (Wislocki and Dempsey, 1955b, Plate 

 2). These trophoblasts are held together by 

 small cytoplasmic pegs which fit into de- 

 pressions in adjacent cell surfaces. Wislocki 

 and Dempsey (1955b) observed that the 

 labyrinthine lipid droplets are located 

 mainly in the cytoplasm of the innermost 

 trophoblasts. Schiebler and Knoop ( 1959) 

 reported that there is a relatively wide 

 space between the outer and middle layers 

 which communicates with the maternal 

 blood but that the deeper cells are closely 

 apposed. The latter investigators also ob- 

 served that pinocytotic vesicles occur in 

 numerous trophoblasts and claimed that 

 two kinds of trophoblastic cells can be dif- 

 ferentiated in the labyrinth with the elec- 

 tron microscope. 



According to Wislocki and Dempsey, the 

 placental barrier in the labyrinth of the rat 

 is composed of : ( 1 ) two or three sheets of 

 laminated cytotrophoblast; (2) the base- 

 ment membrane supporting the trophoblast; 

 and (3j the basement membrane and endo- 

 thelium of the fetal capillary. Schiebler and 

 Knoop did not see two separate basement 

 membranes and report the occurrence of a 

 single basement membrane between the tro- 

 phoblast and fetal endothelium. 



Schiebler and Knoop (1959) also pre- 

 sented some interesting observations on the 

 fine structure and histochemistry of the 

 spongiotrophoblasts and giant cells. The 

 cytotrophoblasts of the spongy zone have 

 an intensely basophilic cytoplasm, and this 

 is consonant with the presence of an exten- 

 sive, highly oriented endoplasmic reticulum. 

 This luxuriant endoplasmic reticulum is 

 comparable in its arrangement and abun- 

 dance to that of the pancreatic acinar cells 

 and of the Nissl bodies of neurones. The 

 function of the spongiotrophoblast remains 

 unknown, but this observation points to- 

 ward a special role in protein synthesis. 

 Padykula (1958) reported a striking in- 

 crease in acid phosphatase and adenosine 

 triphosphatase activity in this zone during 

 the last week of gestation. 



In the same report, Schiebler and Knoop 

 offered much new information about the 

 fetal giant cells, and their observations 

 suggest a dynamic role for these strate- 

 gically placed cells. The giant cells are 

 contiguous with the spongiotrophoblasts, 

 and with the aid of the electron microscope 

 they can be differentiated into several types. 

 The nuclei of the giant cells are invaginated 

 in many places, and these recesses contain 

 cytoj^lasm. In some planes of section, this 

 morphologic arrangement gives the false 

 impression of intranuclear inclusions, es- 

 pecially when the invagination contains 

 lipid or glycogen. However, the cytoplasmic 

 mass enclosed by the nucleus maintains its 

 connection with the main body of cyto- 

 plasm. The cytoplasm proper is highly dif- 

 ferentiated. It contains a great complexity 

 and variety of vesicles and membranes, and 

 resembles the cytoplasm of phagocytic cells 

 in several aspects of fine structure. The 

 surface of the giant cells presents a compli- 

 cated interwoven array of microvillus-like 

 projections to the intercellular space. This 

 space is filled with a material which con- 

 tains mucopolysaccharide, is fibrous, and 

 appears to be continuous in some regions 

 with Reichert's membrane. In some regions 

 the maternal blood spaces among the giant 

 cells are lined by a thin layer of cytoplasm 

 which is judged to be endothelium by 

 Schiebler and Knoop. In this location a 

 subendothelial basement membrane seems 

 to be lacking. Elsewhere the surfaces of the 

 giant cells are in direct contact with the 

 maternal blood. 



The fine structure of the granular cells 

 of the metrial gland of the pregnant rat was 

 described by Wislocki, Weiss, Burgos and 

 Ellis (1957). The suggestion was offered 

 that the basic protein granules of these 

 cells contain relaxin. 



The fine structure of the hemochorial pla- 

 centa of the nine-banded armadillo (Dasy- 

 pus novemcinctus) was recently reported by 

 Enders (1960). 



Some generalizations may be made con- 

 cerning the fine structure of the placental 

 barriers. Certainly the absorptive tropho- 

 blasts resemble the cells of the proximal 

 convoluted tubule of the kidney and the 

 absorptive cells of the small intestine more 



