Increase in cholesterol in the apical plasma membrane of uterine epithelial cells during early pregnancy in the rat.

Freeze-fracture cytochemistry with the cholesterol-binding antibiotic filipin has been used to examine the plasma membrane of uterine epithelial cells at different stages of pregnancy in the rat. We find many more filipin-induced lesions on day 6 of pregnancy than on day 1 and suggest that this indicates a higher cholesterol content at this time. Since day 6 of pregnancy is the time at which blastocysts implant in the rat uterus, we consider the possible significance of an increased cholesterol content for implantation.