Menstruation is considered as the beginning of the cycle, and it lasts for 1-5 days. Histological destruction in the stratum functionale gets ever more extensive, while contraction of the helical vessels comes to an end. Blood released from damaged vessels detaches the devitalized tissues from the intact part of stratum basale. Menstrual discharge, composed of blood and destroyed tissues is passed through the vagina.
IV./1.2.2.: Regeneration phase
On days 5-6, epithelial cells from remaining terminal portions of the glands of stratum basale are shifted to the surface of the damaged mucosa to restore its integrity.
IV./1.2.3.: Proliferation (follicular) phase
During days 7-15, the mucosa that had been destroyed after menstruation gets definitively restored. Thickness of the endometrium is increased 2 to 3-fold by the end of this phase. All components of the mucosa participate in the proliferation. Division of the remaining glandular epithelial cells of stratum basale restores the integrity of the epithelial covering of the surface. The glands grow in length, with an increasingly tortuous course. The cells accumulate glycogen and they discharge a small amount of dilute mucous secretion. Also dividing rapidly, the cells of the lamina propria restore the stroma. Helical arteries grow slowly until the end of proliferation phase. Regulated by follicular estrogen, this phase coincides with the growth of ovarian follicles.
IV./1.2.4.: Secretion phase
Days 16-27. Ovulation marks the border between the proliferation and secretion phases. Duration of this phase is quite stable; it lasts for about two weeks. Under the effect of progesteron secreted by the corpus luteum, the thickness of endometrium is further increased, amounting to 5-6 mm by the end of the phase. The main reason of this growth is tissue swelling, caused by the growth of interstitium in the stroma, and by enhanced secretion of the glands, rather than proliferation of cells. The glands that had become wavy by the end of proliferation phase get distended in the secretion phase and their cells display intense secretory activity.
The discharge product of these cells is rich in carbohydrates, thick and mucous. Secretory activity of the cells is enhanced by progesteron arising from the corpus luteum. The distended and wavy glands are responsible for the spongy appearance of the wide lower layer of stratum functionale. The stroma cells under the surface of stratum compactum become enlarged to form spurious decidual cells. These cells will turn into genuine decidual cells in case of pregnancy. The helical arteries get further elongated and twisted to form a network of lacunae near to the surface.
IV./1.2.5.: Ischemia
Day 28. One to two days prior to menstruation, an abrupt drop in the hormone production of corpus luteum evokes transient contraction of the wall of helical arteries. Due to vasoconstriction, vascular supply of tissues becomes poor, as a result of which the mucosa turns pale. The decreasing amount of glandular discharge and the smaller interstitium lead to a shrinkage of the mucosa. Ultimately, insufficient vascular supply of stratum functionale causes tissue necrosis. Vasocontriction occurs exclusively in the wall of helical vessels. The straight vessels supplying the stratum basale are insensitive to hormonal effects, thus this layer does not play a role in the cyclic changes.
