Mitosis in the Rat Uterus during the Estrous Cycle, Early Pregnancy, and Early Pseudopregnancy

Mitotic activity was assessed in the endometrium and myometrium of the rat uterus during the estrous cycle, early pregnancy, and electrically induced pseudopregnancy. At proestrus there was pronounced proliferation in the luminal epithelium, the stroma, and in the myometrium. Glandular proliferation occurred only during diestrus. During progestation, epithelial proliferation occurred on the first 2 days of diestrus and then ceased, while stromal mitosis began about 70 h after ovulation, reached maximal intensity during the 4th day of pregnancy or pseudopregnancy, and subsided gradually thereafter. A resurgence of mitotic activity began only in the vicinity of the implanting blastocyst late on Day 5 of pregnancy. Myometrial proliferation also occurred during progestation, on Days 4 and 5. The occurrence of mitosis in the stroma during the estrous cycle indicates that the cyclic corpus luteum secretes functionally significant amounts of progesterone.