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Ultrastructural changes in guinea pig endometrial cells during the estrous cycle

In the guinea pig, the estrous cycle is characterized by constant measurable level of plasma progesterone with two peaks: the first one associated with the peak of plasma estradiol‐17β occurring at proestrus and the second, during diestrus, more pronounced at the time at which the level of estradiol‐17β is undetectable. The progesterone receptor content is the highest on day 1 and the lowest on day 10 of the estrous cycle, which lasts 16.3 · 1.5 days (n = 37; mean · SD). There is a positive correlation between the plasma level of estradiol‐17β and the progesterone receptors detected immunocytochemically in both endometrial epithelial and stromal cells. The general morphology of the endometrium during prcestrus and estrus is consistent with an estrogenic stimulation, i.e., a smooth and regular surface of the endometrium and the presence of numerous microvilli on the cell surface. However, a moderate secretory activity also occurs in proestrus and estrus. During postestrus, the glandular cells display an increase in characteristic secretory features which parallels the rise of progesterone in the plasma. © 1992 Wiley‐Liss, Inc.