The growth, maturation of the follicles in the ovaries and the formation of the corpus luteum are accompanied by the production of sex hormones by both the granular cells of the follicle and the cells of the internal theca and to a lesser extent the external theca. Sexual steroid hormones include estrogens, progesterone and androgens. The source material for the formation of all steroid hormones is cholesterol. Up to 90% of steroid hormones are bound and only 10% of unbound hormones exert their biological effect.
Estrogens are divided into three fractions with different activity: estradiol, estriol, estrone. Estrone – the least active fraction, is secreted by the ovaries mainly in the period of aging – in postmenopause; the most active fraction is estradiol, it is significant in the onset and preservation of pregnancy.
The amount of sex hormones varies throughout the menstrual cycle. As the follicle grows, the synthesis of all sex hormones, but mostly estrogens, increases. In the period after ovulation and before the onset of menstruation, progesterone secreted by the cells of the corpus luteum is predominantly synthesized in the ovaries.
Androgens (androstenedione and testosterone) are produced by follicle tekkletkami and interstitial cells. Their level during the menstrual cycle does not change. Once in the cells of granulosa, androgens are actively undergoing aromatization, leading to their conversion to estrogens.
In addition to steroid hormones, the ovaries also secrete other biologically active compounds: prostaglandins, oxytocin, vasopressin, relaxin, epidermal growth factor (EGF), insulin-like growth factors (IPFR-1 and PFR-2). Growth factors are thought to contribute to the proliferation of granulosa cells, growth and maturation of the follicle, selection of the dominant follicle.
In the process of ovulation, prostaglandins (F2a and E2) play a definite role, as well as proteolytic enzymes contained in the follicular fluid, collagenase, oxytocin, relaxin.
The cyclical nature of the reproductive system is determined by the principles of direct and feedback, which is provided by specific hormone receptors in each of the links. A direct link is the stimulating effect of the hypothalamus on the pituitary gland and the subsequent formation of sex steroids in the ovary. Feedback is determined by the effect of elevated concentrations of sex steroids on the overlying levels, blocking their activity.
In the interaction of parts of the reproductive system, there are “long”, “short” and “ultrashort” loops. “Long” loop – the impact through the receptors of the hypothalamic-pituitary system on the production of sex hormones. The “short” loop defines the connection between the pituitary and the hypothalamus, the “ultrashort” – the connection between the hypothalamus and nerve cells, which, under the influence of electrical stimuli, perform local regulation using neurotransmitters, neuropeptides, neuromodulators.
Follicular phase
Pulsing secretion and secretion of GnRH leads to the release of FSH and LH from the anterior pituitary gland. LG contributes to the synthesis of androgens by the follicle hecklock. FSH affects the ovaries and leads to follicle growth and oocyte maturation. At the same time, an increasing level of FSH stimulates the production of estrogens in granulosa cells by flavoring androgens formed in the cells of the follicle, and also contributes to the secretion of inhibin and PFR-1-2. Before ovulation, the number of receptors for FSH and LH in the cells of theca and granulosis increases (Fig. 2.6).
Ovulation occurs in the middle of the menstrual cycle, 12-24 hours after reaching the peak of estradiol, which causes an increase in the frequency and amplitude of GnRH secretion and a sharp preovulatory increase in the secretion of LG according to the type of “positive feedback”. Against this background, proteolytic enzymes, collagenase and plasmin, are activated, destroying the collagen of the follicle wall and thus reducing its strength. At the same time, a marked increase in the concentration of prostaglandin F2a, as well as oxytocin, induces rupture of the follicle as a result of their stimulation of smooth muscle contraction and ejection of the oocyte with the oviform ax tubercle from the follicle cavity. The increase in the concentration of prostaglandin E2 and relaxin, which reduce the rigidity of its walls, also contributes to the rupture of the follicle.
Luteal phase
After ovulation, the LH level decreases with respect to the “ovulatory peak”. However, this amount of LH stimulates the luteinization process of granulosa cells remaining in the follicle, as well as the preferential secretion of progesterone formed by the corpus luteum. The maximum secretion of progesterone occurs on the 6-8th day of the corpus luteum, which corresponds to the 20-22nd day of the menstrual cycle. Gradually, by the 28-30th day of the menstrual cycle, the level of progesterone, estrogen, LH and FSH decreases, the corpus luteum regresses and is replaced by connective tissue (white body).
The fifth level of regulation of reproductive function consists of target organs sensitive to fluctuations in the level of sex steroids: uterus, fallopian tubes, vaginal mucosa, as well as mammary glands, hair follicles, bones, adipose tissue, and the central nervous system.
Steroid ovarian hormones affect metabolic processes in organs and tissues with specific receptors. These receptors can be both cytoplasmic and nuclear. Cytoplasmic receptors are strict for estrogen, progesterone and testosterone. Steroids penetrate into target cells by binding to specific receptors – respectively, to estrogen, progesterone, testosterone. The resulting complex enters the cell nucleus, where, when combined with chromatin, it provides for the synthesis of specific tissue proteins through the transcription of messenger RNA.
The uterus consists of the outer (serous) cover, myometrium and endometrium. The endometrium morphologically consists of two layers: the basal and functional. The basal layer during the menstrual cycle does not change significantly. The functional layer of the endometrium undergoes structural and morphological changes, manifested by a successive change in the stages of proliferation, secretion, desquamation, followed by regeneration. Cyclic secretion of sex hormones (estrogens, progesterone) leads to biphasic changes in the endometrium, aimed at the perception of a fertilized egg.