PLACENTA. 



305 



PLACENTA. 



The chorionic villi, the interlacing branches of which form the fetal 

 portion of the placenta, are shaped as shown in Fig. 349. The finding 

 of such structures in a uterine discharge or curetting is diagnostic of 

 pregnancy. The villi in the earliest stages are composed entirely of epi- 

 thelium, but they soon acquire a core of the chorionic mesenchymal tissue 

 in which are the terminal branches of the umbilical vessels. The epithe- 

 lium is very early divisible into two layers. The outer layer consists of 

 densely staining protoplasm containing dark round or flattened nuclei. 

 Since cell boundaries are lacking, this is called the syncytial layer. Mitoses 

 are seldom seen in it. Generally its nuclei are in a single layer but they 



Xl9 



FIG. 349. ISOLATED TERMINAL BRANCHES OP CHORIONIC VILLI; THAT ON THE LEFT is FROM AN EMBRYO 

 OF TWELVE WEEKS; ON THE RIGHT AT FULL TERM. (Minot.) 



may accumulate in "knots" or "proliferation islands," especially in late 

 stages. The knots project from the surface of the villi so that in certain 

 planes of section they appear completely detached and suggest multinu- 

 cleate giant cells. The syncytial layer perhaps completely invests the 

 villi at first, but later it is interrupted in many places. 



The deeper layer of the chorionic epithelium consists of distinct cells 

 with round nuclei and clear protoplasm. Although this is a single layer 

 at the base of young villi, it produces great masses of cells at their tips. 

 These columns or caps of cells in which the villi terminate, fuse with one 

 another next the decidua, and the uterine tissue seems to be dissolved as 

 this mass of epithelium proliferated. All the superficial epithelium of 



