The blood vessels of the cat girdle placenta. Observations on corrosion casts, scanning electron microscopical and histological studies. II. Fetal vasculature.

The fetal microvascular architecture of the feline near-term placenta was investigated using scanning electron micrographs of partially fractured corrosion casts from plastic-filled vessels. The findings were compared with those on corresponding semithin histological sections. The branches of both umbilical arteries and veins roughly follow a course parallel to the zonary girdle on the allantochorionic side of the feline placenta in an acute-angled pattern of ramifications. They join the double-layered capillary networks in the chorionic lamellae of the labyrinth, which generally exhibit a chorio-uterine orientation and are partially twirled. On the allantochorionic side of the labyrinth, these fetal capillary networks are "suspended" on the maternal stem-artery-system of the placenta; on the uterine side, they have peduncular or tuft-like endings of capillary loops and are flattened by the uterine septa, which at this level converge into the maternal veins. The chorionic capillary lamellae have a variable breadth and length and therefore need shorter or longer arterioles and venules from the allantochorionic side to become irrigated at any level of the labyrinth. As a result, the feline placenta is characterized by a generally one-way crosscurrent type of materno-fetal blood flow.