Cyclic changes in the endometrium relate to its functional (surface) layer, consisting of compact epithelial cells, which are rejected during menstruation. The basal layer, which is not rejected during this period, ensures the restoration of the functional layer.
The following changes occur in the endometrium during the menstrual cycle: desquamation and rejection of the functional layer, regeneration, proliferation phase and secretion phase. The transformation of the endometrium occurs under the influence of steroid hormones: the proliferation phase – under the predominant action of estrogens, the secretion phase – under the influence of progesterone and estrogens.
The proliferation phase (corresponding to the follicular phase in the ovaries) lasts an average of 12-14 days, starting from the 5th day of the cycle. During this period, a new surface layer is formed with elongated tubular glands lined with cylindrical epithelium with increased mitotic activity. The thickness of the functional layer of the endometrium is 8 mm.
The secretion phase (luteal phase in the ovaries) is associated with activity of the corpus luteum, lasts 14 ± 1 day. During this period, the epithelium of the endometrial glands begins to produce a secret containing acid glycosaminoglycans, glycoproteins, glycogen.
The activity of secretion becomes the highest on the 20th day of the menstrual cycle. By this time, the maximum amount of proteolytic enzymes is detected in the endometrium, and decidual transformations occur in the stroma. There is a dramatic vascularization of the stroma – the spiral arteries of the functional layer are crimped, form “tangles”, veins are enlarged ovum.
By day 24, due to the onset of regression of the corpus luteum and a decrease in the concentration of progesterone produced by them, endometrial trophicity is disturbed, and degenerative changes are gradually increasing in it. From granular cells of the endometrial stroma are granules containing relaxin, preparing menstrual rejection of the mucous membrane. In the surface areas of the compact layer, lacunary dilations of the capillaries and hemorrhages in the stroma are noted, which can be detected 1 day before the onset of menstruation.
Menstruation includes desquamation, “rejection and” regeneration of the functional layer of the endometrium. Due to the regression of the corpus luteum and a sharp decrease in the content of sex steroids in the endometrium, hypoxia is increasing. The onset of menstruation is promoted by a prolonged spasm of the arteries, leading to blood stasis and the formation of blood clots. Tissue hypoxia (tissue acidosis) exacerbates the increased permeability of the endothelium, fragility of the vascular walls, numerous minor hemorrhages and massive leukocytic infiltration. Lysosomal proteolytic enzymes secreted from leukocytes enhance the melting of tissue elements.
Following a long spasm of the vessels, their paretic expansion with increased blood flow occurs. At the same time, the growth of hydrostatic pressure in the microvasculature and the rupture of the vessel walls, which by this time largely lose their mechanical strength, are noted. Against this background, active desquamation of necrotic areas of the endometrial functional layer occurs. By the end of the 1st day of menstruation, 2/3 of the functional layer is rejected, and its complete desquamation usually ends on the 3rd day of the menstrual cycle.
Regeneration of the endometrium begins immediately after the rejection of the necrotic functional layer. The basis for regeneration is the epithelial cells of the basal stump of the basal layer. Under physiological conditions, already on the 4th day of the cycle, the entire wound surface of the mucous membrane is epithelized. Then again cyclical changes of the endometrium follow – the phases of proliferation and secretion.
Sequential changes throughout the cycle in the endometrium — proliferation, secretion, and menstruation — depend not only on cyclic fluctuations in the level of sex steroids in the blood, but also on the state of the tissue receptors for these hormones.
The concentration of estradiol nuclear receptors increases to the middle of the cycle, reaching a peak by the late period of the endometrial proliferation phase. After ovulation, there is a rapid decrease in the concentration of the nuclear estradiol receptors, continuing to the late secretory phase, when their expression becomes significantly lower than at the beginning of the cycle. The functional state of the fallopian tubes varies depending on the phase of the menstrual cycle. So, in the luteal phase of the cycle, the ciliary apparatus of the ciliated epithelium and the contractile activity of the muscle layer are activated, aimed at optimal transport of sex gametes into the uterine cavity.
Changes in extragenital target organs
All sex hormones not only determine functional changes in the reproductive system itself, but also actively influence metabolic processes in other organs and tissues that have receptors for sex steroids.
In the skin under the influence of estradiol and testosterone, collagen synthesis is activated, which helps to maintain its elasticity. Increased greasiness, acne, folliculitis, skin porosity and excessive hair growth occur with increasing levels of androgens.
In bones, estrogen, progesterone, and androgens support normal remodeling, preventing bone resorption. The balance of sex steroids affects the metabolism and distribution of adipose tissue in the female body.
The effect of sex hormones on receptors in the central nervous system and hippocampal structures is associated with changes in the emotional sphere and autonomic reactions in women in the days preceding menstruation – the phenomenon of “menstrual waves”. This phenomenon is manifested by the imbalance of the processes of activation and inhibition in the cerebral cortex, fluctuations of the sympathetic and parasympathetic nervous system (especially affecting the cardiovascular system). External manifestations of these fluctuations are mood changes and irritability. In healthy women, these changes do not go beyond the physiological limits.