Pericytes in the stroma of the rat uterus.

After ovariectomy and 3 daily injections of progesterone plus one of oestradiol-17 beta, 69% of stromal capillaries of the rat uterus became sheathed with pericytes. On Day 5 of pregnancy the value was 74% but at other times (Days 4 and 6) and with progesterone-only or not treatment less than 15% of capillaries were associated with pericytes. This period of pericyte presence coincides with the time when implantation is possible.