The effects of progesterone and progestins on endometrial proliferation.

A total of 99 premenopausal and 27 postmenopausal women were evaluated to determine the quantity of glandular proliferation resulting from progestin inhibition of estrogen-primed subjects and of subjects without hormonal stimulation. Endometrial glandular proliferation rates were determined by using mitosis counts, proliferating-cell nuclear antigen (PCNA), and nuclear cyclin (MIB1) immunocytological staining. The endometria of normally cycling premenopausal women, of women who received a synthetic progestin, and of untreated postmenopausal women were studied. In untreated normally cycling premenopausal women, the proliferation of the glandular epithelium was increased during the follicular phase and decreased during the luteal phase. Premenopausal women receiving a synthetic progestin and untreated postmenopausal women who were not estrogen-primed showed minimal epithelial proliferation. Endometrial glandular proliferation is inhibited by endogenous progesterone in premenopausal women. Endometrial proliferation is markedly reduced in premenopausal women receiving a synthetic progestin and in untreated postmenopausal women.