Functional changes in the uterus
Last reviewed: 23.04.2024
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Changes in the structure of the walls of the uterus occur both during pregnancy and in the ovarian-menstrual cycle. The ovarian-menstrual cycle of a woman is characterized by the periodicity of changes in the mucous membrane of the uterus, which are interrelated with the process of egg maturation 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, endometrial rejection) occurs if the egg does not fertilize. In this phase, the superficial (functional) layer of the uterine mucosa is rejected and, together with blood, is secreted (menstruation) from the genital tract (from the vagina). The menstrual phase lasts for 3-5 days. Its 1st day corresponds to the time of death (reverse development) of the yellow body in the ovary and the beginning of the maturation of the new follicle. Before the beginning of the menstrual phase, the blood flow in the spiral arteries slows down, the musculature of their walls tonic decreases - ischemia (insufficiency of blood supply) of various sites of the functional layer of the endometrium occurs. After a period of contraction, the musculature of the arteries relaxes, blood enters the arteries, arterioles and capillaries. Spiral arteries are again reduced, and in connection with ischemia their terminal departments are necrotic. In this case, the sections of the functional layer of the mucous membrane of the uterus are rejected, at the same time their veins are damaged, the bleeding is intensified. Necrosis of the functional layer progresses, and this layer is completely rejected, which is accompanied by bleeding. The described events are associated with a decrease in the blood level of progesterone. After the termination of menstruation, the basal layer of the mucous membrane remains, in which the parts of the uterine gland are retained.
In the post-menstrual phase (proliferation phase) under the influence of estrogen, the functional layer of the endometrium regenerates, thickens, and the glands recovers. This phase lasts from the 5th day of the beginning of menstruation on the 14-15th day. Epithelization of the wound surface of the uterine mucosa occurs due to the proliferation of the preserved epithelium of the basal layer, the remaining sections of the uterine glands. Within a few days a new epithelial layer is formed. Epithelium of glands proliferates. Newly formed epithelial cells cover the wound surface, hypertrophy. The epithelium becomes pseudo-layered due to the increase in the number of elongated nuclei.
In the premenstrual phase (the phase of secretion) , which lasts from the 15th to the 28th day of the menstrual cycle, you can identify a short (2-3 days) period of relative dormancy, when the ovary is just beginning to form a yellow body. Then in the phase of secretion under the influence of the hormone of the yellow body of progesterone, the mucous membrane of the uterus thickens to 8 mm, is prepared for insertion into it of a fertilized egg. In the ovary at this time, the flowering (active period) of the yellow body is observed. In the endometrium at this time, blood vessels grow. The mucous membrane of the uterus is preparing to receive a fertilized egg. Progesterone slows the development of follicles. In the secretory phase, the uterine glands become convoluted. In the basal sections of epithelial cells, glycogen is accumulated. Secretion of uterine glands provides nutrition to a fertilized egg (if fertilization has taken place), which enters the uterine cavity 3 days after ovulation. In the late stages of the secretory phase, the domed apical part of the secretory cells grows and protrudes into the lumen of the glands.
At this time, the extracellular fluid accumulates in the stroma of the uterine mucosa. Large polyhedral fibroblast-like cells form accumulations around the spiral arteries and under the epithelium. They are transformed into decidual cells, from which, in the case of implantation of a fertilized egg, the decidual membrane of the placenta will develop.
If the egg is not fertilized, the rapid development of the menstrual yellow body begins, the progesterone production decreases sharply, the functional layer of the endometrium begins to wrinkle, the spiral arteries become more and more twisted, the blood flow through them decreases and their spasm sets in. As a result, endometrial ischemia occurs and 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. There comes another menstruation. The ovarian-menstrual cycle is repeated. The entire ovarian-menstrual cycle is under hormonal control.
The new follicle, which reaches maturity around the 14th day from the onset of menstruation, grows in the ovary under the influence of the follicle-stimulating hormone (FSH) of the pituitary gland. Approximately in the middle of the menstrual cycle, pituitary production of luteinizing hormone (LH) sharply increases, which leads to an acceleration of the maturation of one primary oocyte. The follicle ripens and bursts. By the time of ovulation the uterus becomes 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 yellow body. Between the onset of a peak in the secretion of this hormone and ovulation is 24-36 hours.
The content of follicle-stimulating hormone in the blood increases during the first days of the cycle. The estrogen produced by the cells of the maturing follicle also influences the maturation of the 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 yellow body, the secretory phase of the transformation of the endometrium proceeds. As a result, the mucous membrane of the uterus becomes capable of absorbing a fertilized egg. If the egg is fertilized and implanted in the endometrium under the influence of gonadotropin and lactogen produced by the placenta, then the yellow body of pregnancy continues to function, the secretion of progesterone increases. If fertilization does not occur, then the yellow body undergoes reverse development, the secretion of sex hormones ceases, menstruation occurs.
Between sex hormones and gonadotropin-releasing hormone, which is produced by the cells of the hypothalamus, there are positive and negative feedbacks. Estrogen causes an increase in the content of luteinizing hormone and ovulation (positive feedback). The 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 hypothalamic zone of the hypothalamus.
The fertilized egg is implanted into the uterine mucosa and pregnancy begins. During pregnancy, the size of the uterus increases, its shape changes. So, in the VIII month of pregnancy, the longitudinal size of the uterus reaches 20 cm, the thickness of its wall - about 3 cm, and the shape of the uterus becomes round-ovoid. In the wall of the uterus at this time, the dimensions of muscle cells increase (myometrium hypertrophy). After birth, the uterus acquires its characteristic shape and is close to the usual size.
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