The placenta of the pig. I. Finestructural changes of the placental barrier during pregnancy.

The finestructural changes of the interareolar porcine placenta during pregnancy are described. After perfusion fixation of the placenta the change in the thickness of the placental barrier from day 30 to day 110 of gestation is much more evident than after immerson fixation as has been used by all former authors. The alterations are due to the indentation of both the trophoblast and uterine epithelium by their corresponding capillary-network. This indentation is limited to the lateral wall and the summit of the chorionic ridges, while at the base the trophoblast as well as the uterine epithelium remains high columnar. This indicates that in the interareolar porcine placenta, which is represented by the chorionic ridges and the corresponding endometrial folds, at least two different areas with different structure and function may be discerned. 1) The lateral side and the top of the chorionic ridges seem to be predestinated for gaseous exchange. The placental barrier in this area is often less than 2 micrometers. 2) The transport of blood-borne nutrients takes place at the base of the chorionic ridges. This transport seems to be facilitated by an intercellular channel system between the uterine epithelial cells.