912 



SPERM, OVA, AND PREGNANCY 



evaluation of their ultrastructure. The 

 transported substance next traverses the 

 amorphous perivascular ground substance 

 which varies in amount regionally, being 

 abundant, moderate, or entirely absent. In 

 areas in which this ground substance is lack- 

 ing, the syncytium abuts on maternal endo- 

 thelium. The endothelial margin of the 

 ground substance is regular, but its opposite 

 border, which is in contact with the syncy- 

 tium, is irregular and seems to be sculptured 

 by the syncytium. The lipid-rich syncytium 

 sends branched processes between the 

 cytotrophoblast to end in foot-like expan- 

 sions on the trophoblastic basement mem- 

 brane. Later in gestation, as the cytotropho- 

 blasts diminish in number, these processes 

 extend through large extracellular spaces. 

 After crossing the two trophoblastic layers 

 and the chorionic basement membrane, a 

 substance then passes through a thin connec- 

 tive tissue space containing fibroblasts and 

 collagen fibers. From there it next passes 

 through the basement membrane and endo- 

 thelium of ordinary fetal capillaries. Thus, 

 despite the loss of two uterine layers, the 

 placental barrier in the cat is structurally 

 complex. 



The paraplacental or brown border of the 

 cat's chorion is a specialized region com- 

 posed of cells rich in iron and caj^able of 

 phagocytosing red blood cells, trypan blue, 

 and other substances (Figs. 15.62 and 

 15.63). The absorptive surface of these cells j 

 bears a striking resemblance to that of the 

 visceral endoderm of the rodent yolk sac. 

 The surface plasma membrane of these co- 

 lumnar cells evaginates to form elaborate 

 microvilli and invaginates to form a system 

 of canals immediately beneath the cell sur- 

 face. It is assumed that pinocytotic vesicles 

 are farmed as ingested substances are seg- 

 regated in the canalicular- system. In the 

 paraplaQehtal cells, ingested erythrocytes in 

 various stages of breakdown were frequently 

 observed. 



The hemochorial type of placenta, in 

 which extensive erosion of the uterine wall 

 occurs, has been studied in the human and 

 rodents by several investigators (Boyd and 

 Hughes, 1954; Wislocki and Dempsey, 

 1955a, b; Wislocki, Weiss, Burgos and Ellis, 

 1957; Bargmann and Knoop, 1959; Schieb- 



ler and Knoop, 1959). In the "definitive" 

 hemochorial placenta the trophoblast is 

 bathed directly by circulating maternal 

 blood. Observations on the ultrastructure 

 of the human placenta (Wislocki and Demp- 

 sey, 1955a) have been incorporated with the 

 histochemical findings in Section III of this 

 review. The syncytium of the human pla- 

 centa resembles a pinocytotic epithelium 

 having numerous pleomorphic microvilli and 

 containing many large vesicles which prob- 

 ably represent engulfed material (Wislocki 

 and Dempsey, 1955a, Plates 1-4). Although 

 the Langhans cells diminish in number as 

 gestation proceeds, some flattened cyto- 

 trophoblasts remain at term interposed be- 

 tween the syncytium and the trophoblastic 

 basement membrane. At term, when the pla- 

 cental barrier is thinnest, a maternal sub- 

 stance encounters the following successive 

 layers in reaching the fetal blood: syncy- 

 tium, trophoblastic basement membrane, 

 fetal connective tissue, basement membrane, 

 and endothelium of the fetal sinusoidal cap- 

 illary. 



Many significant observations have been 

 made on the fine structure of the chorio- 

 allantoic placenta of the rat. To aid in ori- 

 enting the reader, a brief description of the 

 histology of the rat placenta follows. In the 

 established chorio-allantoic disc of this spe- 

 cies, three general zones are recognizable in 

 the fetal portion: (1) a trophoblastic lab- 

 yrinth which is served by both maternal 

 and fetal blood vessels and is considered to 

 be the principal area of transport; (2) a 

 spongiotrophoblastic zone which partially 

 surrounds the labyrinth and which, al- 

 though it is not penetrated by fetal blood 

 vessels, is perfused by maternal blood; and 

 (3) a meshwork of giant cells which caps 

 the spongy zone, is permeated by maternal 

 blood, and forms the frontier of the fetal 

 tissue. 



The first description of the ultrastructure 

 of the labyrinth of the rat at 15, 17, and 21 

 days of gestation quickly established two 

 important points (Wislocki and Dempsey, 

 1955b ) . It was demonstrated that the rat 

 and rabbit placentas are hemochorial rather 

 than hemoendothelial as had been proposed 

 by Mossman (1926, 1937). Furthermore, it 

 was shown that there is no syncytial tropho- 



