DISTRIBUTION OF MITOCHONDRIA IN THE PLACENTA. Ill 



The vitelline membrane covering the placenta likewise presents a single layer 

 of columnar cells. The cells here are considerably larger than those just described 

 and in gross appearance resemble those of the chorionic membrane of the cat, in 

 that they are crowded together with their nuclei at different levels and their cell- 

 outlines are distorted by pressure from neighboring cells. Also, the older cells are 

 constantly being constricted at their bases and pinched off by new cells arising 

 next to the basement membrane. 



Each cell possesses a single large oval nucleus which is usually found in the 

 central zone of the cell unless influenced by pressure, when it may be found any- 

 where from the base to the apex. It stains deep green and contains one to three 

 basophilic nucleoli. 



The protoplasm stains faintly and in nearly all cells is extensively vacuolated, 

 the vacuoles being largest and most numerous in the cells which are constricted at 

 the base. Occasionally large vacuoles in the apical zone contain fragments of 

 phagocytosed red blood-cells. The mitochondria, present as small granules and 

 short rods, are fairly numerous, and especially so in the young cells next to the 

 basement membrane. They are not concentrated in any particular part of the cells 

 but are distributed uniformly through the protoplasm, the rods usually lying with 

 their long axes parallel to the long axis of the cell. In the senile cells with irregular 

 pycnotic nuclei the mitochondria are less numerous and many bulb-like swellings 

 and mitochondrial vesicles are present. However, the mitochondria persist as long 

 as the cell remains attached to the basement membrane. 



Just beneath the thin basement membrane supporting these cells is a layer, one 

 to four cells in thickness, of large round or oval giant cells. In the mature placenta 

 most of these cells are senile and have vacuolated protoplasm and pycnotic nuclei, 

 although a few can be found which appear to be quite healthy and normal. As a 

 rule they are mononuclear, though cells containing two to four nuclei are occasionally 

 seen. The nuclei are large, round to oval in shape, stain slightly with green, and 

 contain a single large, deep-green nucleolus from which strands of chromatin 

 radiate to the nuclear membrane. The protoplasm stains a faint green and is 

 quite rich in minute granular and rod-like mitochondria. These may be con- 

 centrated in a zone around the nucleus or distributed uniformly through the proto- 

 plasm. In the degenerating cells few or no mitochondria can be seen. No other 

 granules are visible in these cells. 



Beneath the layer of giant cells is a well-defined layer of connective tissue 

 which separates it from the cortical syncytium covering the labyrinth. From 

 the inner surface of the cortical syncytium heavy bands of interlobular syncytium 

 project into the labyrinth and divide it up into lobules. The structure of the cortical 

 and interlobular syncytium is the same and they will be described together. 



The syncytium is a coarse reticulum of heavy protoplasmic bands in which no 

 cell boundaries can be distinguished. The interspaces of the reticulum are the 

 sinusoids in which the maternal blood circulates and, as they are not lined by endo- 

 thelium, the blood comes in direct contact with the syncytium. The small round 

 to oval nuclei are very numerous and are distributed quite uniformly through the 



