Identity of human endometrial tissue: Potent source of stem cells

Abstract

The uterus is by far the largest female organ of the body, playing an integral role in the reproductive life of every woman. It plays a pivotal role in implantation and in absence of pregnancy, menstruation. The innermost layer of the uterus is known as tunica mucosa, popularly termed as endometrium, opposed to the outer perimetrium and median myometrium. The uterus is the only organ whose lining is almost entirely expelled and reconstructed periodically, both phenomena taking place at each ovarian cycle. With the purpose of facilitating the periodic elimination of the endometrium that undergoes regression, shrinkage, and necrosis at end of each cycle, the uterus also exhibits the unique peculiarity of physiological bleeding. The endometrial histophysiology is entirely controlled by the ovarian hormones along the cycle. Of all tissues of the human body, the endometrium is the one that, throughout the ovarian cycle, most accurately reflects the levels of estrogen and progesterone. Estradiol, produced by the ovaries on approximately day 4 or 5 of the cycle, induces growth and proliferation of the endometrium. The levels of estrogen are normally elevated during the proliferative phase of the menstrual cycle as it serves to promote proliferation of the luminal and glandular epithelial cells associated with the thickening of the endometrial lining as well as vascularization. The cessation of endometrial growth occurs before estradiol levels reach their peak and prior to the onset of progesterone production, thereby indicating that nonsteroidal factors limit the growth of endometrium. Progesterone is responsible for the secretory phase of the ovulatory cycle, and its action upon the endometrium serves two purposes. The first can be regarded as “medical.” it greatly reduces the proliferative activity of the endometrial glands, thereby preventing the appearance of endometrial hyperplasic alterations. The second is essentially “reproductive,” that is vital to create an ideal condition in the endometrium for the implantation and development of the egg.