Histology of the human endometrium: from birth to senescence.

Embryologically, the human endometrium is of mesodermal origin, and constitutes the mucosal lining of the fused Mullerian ducts of the uterus. In the adult, premenopausal woman, the endometrium follows a precisely programmed series of morphologic and physiologic events, characterized by growth, secretory differentiation, and in the absence of conception, degeneration and regeneration. Proliferation, secretion and degeneration are confined to the upper functionalis layer, whereas the lower basalis layer is the seat of regenerative endometrium. In the postmenopausal years, absence of biologically significant estrogenic stimulation leads to progressive endometrial involution, from proliferative to inactive in the mid-50s to atrophy in the late-60s. Cyclic endometrial alterations are controlled by the ovarian estrogens and progesterone via their respective endometrial intranuclear receptors, and possibly other peptides and enzymes. They provide appropriate environment for the implanting conceptus. The intimate inter-relationship between endometrial structure/function and steroid hormones during the premenopausal period serves as an indicator of the hypothalamo-pituitary-ovarian axis as related to ovulatory states.