Functional changes in the uterus

Changes in the structure of the uterine walls occur both during pregnancy and in the ovarian-menstrual cycle. The ovarian-menstrual (sexual) cycle of a woman is characterized by the periodicity of changes in the mucous membrane of the uterus, interconnected with the process of maturation of the egg in the ovary and ovulation. In this cycle, which lasts approximately 28 days (from 21 to 30), menstrual, postmenstrual and premenstrual phases (periods) are distinguished.

The menstrual phase (the phase of desquamation, rejection of the endometrium) occurs if fertilization of the egg does not occur. During this phase, the superficial (functional) layer of the uterine mucosa is rejected and is released (menstruation) from the genital tract (from the vagina) together with blood. The menstrual phase lasts up to 3-5 days. Its 1st day corresponds to the time of death (reverse development) of the corpus luteum in the ovary and the beginning of maturation of a new follicle. Before the onset of the menstrual phase, the blood flow in the spiral arteries slows down, the muscles of their walls tonically contract - ischemia (insufficient blood supply) of various parts of the functional layer of the endometrium occurs. After a period of contraction, the muscles of the arteries relax, blood flows into the arteries, arterioles and capillaries. The spiral arteries contract again, and due to ischemia, their terminal sections become necrotic. In this case, sections of the functional layer of the uterine mucosa are rejected, their veins are simultaneously damaged, and bleeding increases. Necrosis of the functional layer progresses, and this layer is rejected completely, which is accompanied by bleeding. The described events are associated with a decrease in the level of progesterone in the blood. After the cessation of menstruation, the basal layer of the mucosa remains, in which sections of the uterine glands are preserved.

In the postmenstrual phase (proliferation phase), under the influence of estrogen, the functional layer of the endometrium regenerates, thickens, and the glands are restored. This phase lasts from the 5th day from the onset of menstruation to the 14th-15th day. Epithelialization of the wound surface of the uterine mucosa occurs due to the proliferation of the remaining epithelium of the basal layer, the remaining sections of the uterine glands. Within a few days, a new epithelial layer is formed. The epithelium of the glands proliferates. Newly formed epithelial cells cover the wound surface and hypertrophy. The epithelium becomes pseudo-multilayered due to an increase in the number of elongated nuclei.

In the premenstrual phase (secretion phase), which lasts from the 15th to the 28th day of the menstrual cycle, a short (2-3 days) period of relative rest can be distinguished, when the corpus luteum is just beginning to form in the ovary. Then, in the secretory phase, under the influence of the corpus luteum hormone progesterone, the mucous membrane of the uterus thickens to 8 mm, preparing for the implantation of the fertilized egg. At this time, the ovary experiences the blossoming (active period) of the corpus luteum. At this time, blood vessels grow in the endometrium. The mucous membrane of the uterus prepares to accept the fertilized egg. Progesterone slows down the development of follicles. In the secretory phase, the uterine glands become convoluted. Glycogen accumulates in the basal sections of the epithelial cells. The secretion of the uterine glands provides nutrition to the fertilized egg (if fertilization has occurred), which enters the uterine cavity 3 days after ovulation. In the late stages of the secretory phase, the dome-shaped apical part of the secretory cells increases and protrudes into the lumen of the glands.

At this time, extracellular fluid accumulates in the stroma of the uterine mucosa. Large polyhedral fibroblast-like cells form clusters around the spiral arteries and under the epithelium. They transform into decidual cells, which, if a fertilized egg is implanted, will develop into the decidual membrane of the placenta.

If the egg is not fertilized, the menstrual corpus luteum begins to develop rapidly, progesterone production decreases sharply, the functional layer of the endometrium begins to shrink, the spiral arteries twist more and more, the blood flow through them decreases and their spasm occurs. As a result, ischemia of the endometrium occurs and its degenerative changes occur. The walls of the blood vessels lose elasticity or become brittle, the functional layer is rejected, while the veins are damaged, bleeding begins. The next menstruation begins. The ovarian-menstrual cycle is repeated. The entire ovarian-menstrual cycle is under hormonal control.

A new follicle, which reaches maturity around day 14 from the onset of menstruation, grows in the ovary under the influence of follicle-stimulating hormone (FSH) from the pituitary gland. Around the middle of the menstrual cycle, the pituitary gland's production of luteinizing hormone (LH) increases sharply, which accelerates the maturation of one primary oocyte. The follicle matures and ruptures. By the time of ovulation, the uterus is capable of receiving a fertilized egg.

Ovulation occurs under the influence of luteinizing and follicle-stimulating hormones. It is the maximum increase in the level of luteinizing hormone that leads to ovulation and the formation of the corpus luteum. Between the onset of the peak secretion of this hormone and ovulation, 24-36 hours pass.

The content of follicle-stimulating hormone in the blood increases during the first days of the cycle. Estrogen produced by the cells of the maturing follicle also affects the maturation of primary follicles, the growth of the functional layer of the endometrium and uterine glands during the proliferative phase. Under the influence of progesterone and estrogen secreted by the corpus luteum, the secretory phase of endometrial transformation occurs. As a result, the mucous membrane of the uterus becomes capable of perceiving a fertilized egg. If the egg is fertilized and implanted in the endometrium under the influence of gonadotropin and lactogen produced by the placenta, the corpus luteum of pregnancy continues to function, progesterone secretion increases. If fertilization does not occur, the corpus luteum undergoes reverse development, the secretion of sex hormones ceases, and menstruation begins.

There are positive and negative feedbacks between sex hormones and gonadotropin-releasing hormone, which is produced by hypothalamic cells. Estrogen causes an increase in luteinizing hormone and ovulation (positive feedback). Increased synthesis of progesterone and estrogen in the secretory phase of the cycle inhibits the secretion of follicle-stimulating and luteinizing hormones (negative feedback). These connections are closed at the level of the hypophysotropic zone of the hypothalamus.

The fertilized egg is implanted into the uterine mucosa and pregnancy begins. During pregnancy, the uterus increases in size and its shape changes. Thus, in the 8th month of pregnancy, the longitudinal size of the uterus reaches 20 cm, the thickness of its wall is about 3 cm, and the shape of the uterus becomes round-ovoid. At this time, the size of muscle cells in the uterine wall increases (myometrial hypertrophy). After childbirth, the uterus acquires its characteristic shape and dimensions close to normal.

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