Hypothalamo-pituitary control
The hierarchy of reproductive function regulation begins in the hypothalamus, which secretes gonadotropin-releasing hormone (GnRH). This hormone is characterized by pulsatile secretion. The response of downstream structures depends on the current estrogen level: at low concentrations, GnRH stimulates the production of follicle-stimulating hormone (FSH), whereas at high concentrations, it switches to stimulating luteinizing hormone (LH).
Key regulators of the ovaries are synthesized in the adenohypophysis:
- FSH. Its targets are follicular cells. The hormone ensures their proliferation and the growth of the follicle itself. The most important function of FSH is the activation of the enzyme aromatase, which leads to enhanced estrogen secretion. In addition, FSH promotes the insertion of LH receptors into cell membranes and stimulates the release of inhibins. FSH is inhibited by estrogens and inhibins via a feedback loop.
- LH. This hormone acts on the theca cells (theca interna), triggering the synthesis of androgens in them, which are subsequently converted into estrogens in the granulosa cell layer (stratum granulosum). The peak surge of LH occurs 10–12 hours before follicular rupture — it stimulates ovulation. After the release of the oocyte, LH induces progesterone secretion, causes luteinization of the remaining cells, and ensures the growth of the corpus luteum (corpus luteum).
A distinct role is played by prolactin, which is stimulated by prolactin-releasing hormone and inhibited by dopamine. In combination with estrogens and growth hormone (GH), it ensures mammary gland growth. During lactation, prolactin triggers the synthesis of breast milk proteins, fats, and carbohydrates. It also maintains the functioning of the corpus luteum, and together with progesterone, effectively suppresses the development of new follicles. Additionally, prolactin is involved in water-salt metabolism (increases the reabsorption of sodium, chloride ions, and water), enhancing the effects of vasopressin and aldosterone.
Ovarian steroid hormones
The main peripheral hormones that form the female phenotype are estrogens and progesterone.
Estrogens (including estradiol, estrone, and estriol) are synthesized by granulosa cells. Their main physiological effects:
- Control of puberty and full development of female secondary sexual characteristics.
- Regulation of the ovarian-menstrual cycle with dominant control of its proliferative phase.
- Maintenance of a normal course of pregnancy and ensuring mammary gland growth.
- Participation in bone tissue development and stimulation of epiphyseal cartilage ossification.
- Metabolic effects: moderate anabolic action, retention of nitrogen, salts, and water, as well as female-pattern fat distribution. Estrogens also have a pronounced effect on psychoemotional status.
During pregnancy, total estrogen levels increase approximately 30-fold, with about 90% of this amount accounted for by estriol produced by the placenta.
Progesterone outside of gestation is produced predominantly by the corpus luteum, and starting from the second trimester of pregnancy, the placenta completely takes over this function. The hormone is responsible for regulating the secretory phase of the cycle, carefully preparing the endometrium for potential blastocyst implantation. It is necessary for maintaining pregnancy, and causes specific remodeling in the uterus and mammary glands. Its metabolic effect is also well known: progesterone increases the basal metabolic rate and basal body temperature.
Accessory hormones of the ovaries and adrenal glands
In addition to estrogens and progesterone, other important regulators are synthesized in the female body:
- Inhibins are peptide substances that, via a negative feedback loop, block the production of GnRH in the hypothalamus and FSH in the pituitary gland.
- Androgens are produced in women in the adrenal cortex and ovaries. Their main physiological tasks are to stimulate anabolic processes and enhance libido.
Endocrine function of the placenta
With the onset of pregnancy, the placenta becomes a potent endocrine organ. It synthesizes and secretes a wide range of hormones and biologically active substances that ensure gestation and fetal development:
- Human chorionic gonadotropin (hCG) is critical in early stages as it stimulates the continuous secretion of progesterone by the corpus luteum of pregnancy.
- Relaxins are specific hormones that relax the myometrium throughout pregnancy. Immediately before labor, they increase the elasticity of the pubic symphysis tissues.
- The placenta also produces progesterone, prolactin, and estrogens.
- A unique property of the placenta is its ability to synthesize virtually all known hypothalamic releasing and inhibiting factors, including corticotropin-releasing hormone (CRH), thyrotropin-releasing hormone (TRH), growth hormone-releasing hormone, somatostatin, and prolactin-releasing hormone.
Briefly: answers to topical questions
What are the phases of the ovarian-menstrual cycle and their duration?
The ovarian-menstrual cycle consists of two phases and normally lasts about 28 days.
- Phase 1 (follicular) — growth and maturation of the follicle in the ovary, regeneration and proliferation of the functional layer of the endometrium. Lasts until ovulation, which in a normal cycle occurs on day 14.
- Phase 2 (luteal) — formation of the corpus luteum in the ovary and secretory transformation of endometrial glands. Lasts from the moment of ovulation until day 28, ending with endometrial shedding in the absence of pregnancy.
How does hormone concentration change across the days of a normal menstrual cycle?
Hormone levels undergo pronounced changes depending on the cycle phase.
- Estradiol — rises smoothly in the first phase, reaching its maximum peak immediately before ovulation.
- LH — characterized by a sharp maximum surge before ovulation (ovulatory peak).
- FSH — has a moderate peak coinciding in time with the LH surge.
- Progesterone — remains at baseline levels until ovulation; in the second phase of the cycle, its concentration increases significantly (forming a broad peak), and drops toward the end of the cycle.
How to remember the action of pituitary gonadotropins: FSH forms the follicle, while LH liberates (ruptures) it (stimulates ovulation) and luteinizes the remaining cells.
Class and exam questions
What is the mechanism of estrogen synthesis in the ovaries?
In the theca cells, androgens are synthesized under the influence of LH. They are then transported to the granulosa cells, where they are converted into estrogens by the enzyme aromatase (whose activation is stimulated by FSH).
Why is the maturation of new follicles inhibited during lactation?
High prolactin levels, necessary for breast milk synthesis, work in concert with progesterone. This hormonal complex effectively suppresses the growth of new follicles in the ovaries.
Which hormone dominates in the secretory phase of the cycle?
Progesterone dominates in the secretory phase. It is produced by the corpus luteum and prepares the uterine endometrium for successful implantation of the fertilized egg.
What else to study on this topic
- The mechanism of aromatase activation in granulosa cells and details of androgen conversion to estrogens.
- The effect of prolactin on water-salt metabolism: mechanisms of synergism with vasopressin and aldosterone.
- Physiological mechanisms of the thermogenic action of progesterone.
- The role of hypothalamic releasing and inhibiting factors (CRH, TRH, somatostatin) synthesized by the placenta.
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