An ultrastructural study of implantation in the golden hamster. II. Trophoblastic invasion and removal of the uterine epithelium.

Sixty six implantation sites from 18 golden hamsters were examined with light and electron microscopy between 4 and 5 1/2 days of pregnancy (post-ovulation). At 4 days some blastocysts began to invade the uterine epithelium, with trophoblastic processes penetrating and engulfing portions of the uterine epithelium. The majority of epithelial cells appeared normal before invasion, although at two implantation sites three or four adjoining epithelial cells were necrotic before penetration by the trophoblast. In general the epithelial cells were degenerating at the time the trophoblast invaded the epithelium. Inclusions, representing portions of the engulfed epithelium, and varying in size and electron density, were present throughout the invading trophoblast cells at 4 1/2 and 5 days of pregnancy. At 5 1/2 days the uterine epithelium had disappeared and the embryo was now almost completely surrounded by blood lacunae.