Light- and electron-microscopic study of the near-term paraplacenta of the domestic cat. I. Polar zone and paraplacental junctional areas.

The entire chorioallantoic paraplacenta of the near-term domestic cat shows cytological evidence of maternal-fetal substance transfer, but there are probably qualitative and quantitative differences and increased transfer progressing from the poles to the placental border. In the interfetal polar zone, the cytotrophoblast remains proliferative, is squamous to columnar and shows only a small amount of pinocytotic activity. However, in the adjacent ring of the free polar zone, where the trophoblast is cuboidal or columnar and loosely apposed to the endometrium, material of uterine origin (histotroph) appears to be transferred from the columnar uterine surface epithelium to the trophoblast. The remaining paraplacenta extends from the polar zone to the girdle, forming an extravasate zone that is composed of hematomal areas containing extravasates of maternal blood, and interposed relatively unmodified paraplacental junctional areas. The trophoblast in this hematomal area is columnar and constitutes chorionic villi which face a surface formed by maternal symplasma, degenerative uterine tissue, and disintegrating blood, thus providing 'histolytic' and 'hemolytic' embryotroph to this hemophagous cytotrophoblast. When the junctional area is just forming, a cuboidal cytotrophoblast consisting of resting cells, opposite to the endometrial folds, appears to have both the capacity for digestion and transformation. However, during the recessive junctional stage, this cytotrophoblast is proliferative and absorptive only. Both sources of cuboidal trophoblast may give rise to syncytial trophoblast, which is presumably the invasive form that erodes the uterine tissue, including the interstitial layer and maternal endothelium. As long as there is firm junction of the fetal and maternal tissues, there appears to be a transfer of secretion products to form embryotroph. However, with the regression of the junctional area more products of cell breakdown (histolytic products) are absorbed by the cuboidal cytotrophoblast in the chorionic fossae.