In the endometrium, epithelial cells line the endometrial glands and are supported by stromal cells. These cells and supplying blood vessels replicate rapidly and cyclically in reproductiveaged women and are regenerated each ovarian-endometrial cycle. The superficial endometrium, termed the functionalis layer, is shed and reconstructed from the deeper basalis layer (Fig. 5-4). There is no other example in humans of such cyclical shedding and regrowth of an entire tissue.
Fluctuations in estrogen and progesterone levels produce striking efects on the endometrium. Follicular-phase estradiol production is the most important factor in endometrial recovery following menstruation, and both ERa and ER3 receptors are expressed here. Although up to rwo thirds of the functionalis endometrium is fragmented and shed with menses, reepithelialization begins even before menstrual bleeding has ceased. By the fifth day of the endometrial cycle-ifth day of menses-the
FIGURE 5-4 The endometrium consists of two layers, the functionalis layer and basalis layer. These are supplied by the spiral and basal arteries, respectively. Numerous glands also span these layers. As the menstrual cycle progresses, greater coiling of the spiral arteries and increased gland folding can be seen. Near the end of the menstrual cycle (day 27), the coiled arteries constrict, deprive blood supply to the functionalis layer, and lead to necrosis and sloughing of this layer.
epithelial surface of the endometrium has been restored, and revascularization has begun. he preovulatory endometrium is characterized by proliferation of glandular, stromal, and vascular endothelial cells. During the early part of the proliferative phase, the endometrium is usually less than 2 mm thick. he glands are narrow, tubular structures that pursue almost a straight and parallel course from the basalis layer toward the endometrial cavity. Mitotic igures, especially in the glandular epithelium, are identiied by the fifth cycle day. Mitotic activity in both epithelium and stroma persists until day 16 to 17, that is, 2 to 3 days after ovulation. Blood vessels are numerous and prominent.
Clearly, reepithelialization and angiogenesis are important to cessation of endometrial bleeding (Chennazhi, 2009; Rogers, 2009). hese are dependent on tissue regrowth, which is estrogen regulated. Epithelial cell growth also is regulated in part by epidermal growth factor and transforming growth factor a (TGPa). Stromal cells proliferate through paracrine and autocrine actions of estrogen and greater local levels of fibroblast growth factor-9 (Tsai, 2002). Estrogens also raise local production ofVEGF, which causes angiogenesis through vessel elongation in the basalis (Gargett, 2001; Sugino, 2002).
By the late proliferative phase, the endometrium thickens from both glandular hyperplasia and augmented stromal ground substance, which is edema and proteinaceous material. he loose stroma is especially prominent, and the glands in the functionalis layer are widely separated. his is compared with those of the basalis layer, in which the glands are more crowded and the stroma is denser.
At midcycle, as ovulation nears, glandular epithelium becomes taller and pseudostratiied. he surface epithelial cells acquire numerous microvilli, which increase epithelial surface area, and develop cilia, which move endometrial secretions during the secretory phase (Perenczy, 1976).