Expression of steroid hormone receptors, proliferation and apoptotic markers in primate endometrium

In humans, circulating levels of steroid hormones vary during the menstrual cycle, with oestrogen elevated in the proliferative phase and progesterone increased during the secretory phase. In humans, oestrogen levels also increased during the mid-secretory phase, but this is not observed in non-human primates. In marmosets, the New World monkeys, plasma oestrogen and progesterone levels exhibit similar profiles as those found in Old World monkeys, however, these animals do not menstruate. Proliferative and apoptotic changes occur at specific phases of the menstrual cycle, suggesting regulation by steroid hormones. It was, therefore, of interest to compare expression of steroid hormone receptors, proliferation and apoptotic markers in the uterine endometrium of marmosets and humans. Localization of oestrogen receptor-α (ER-α) was observed in glandular epithelial cells during the proliferative phase in marmosets and in human endometria. A decrease in ER-α and progesterone receptor (PR-A) was observed during the mid luteal phase in both species although a postovulatory oestradiol peak is observed in human but not in marmoset. A decrease in PR-A in the late secretory phase was seen in marmoset as well as in human endometria. The proliferation marker PCNA was enhanced in the proliferative phase in both species, but it was increased in late secretory phase only in marmosets. Apoptosis as revealed by a TUNEL assay was moderate in early stage in both species. Surprisingly, apoptosis, as well as the localization of the pro-apoptotic Bcl-2 family member, Bax, was optimal in marmosets during the mid-secretory phase when plasma progesterone levels were high. On the other hand, in the human, apoptosis was maximal by TUNEL assay in late secretory phase, but Bax protein was highest in the mid-secretory phase. Thus, Bax may be initiating apoptosis in the endometrial glands as well as in the stroma. Although the pattern of steroid receptor expression in endometria of marmosets and humans are similar, proliferation and apoptosis markers appear to be regulated by other factors along with steroid hormones.