Main functions of the ovaries
The ovaries are key paired glands of the female reproductive system, whose function is strictly subordinated to cyclical changes. Their activity is traditionally divided into two interrelated directions:
- Exocrine function. Consists in the formation and maturation of fully functional oocytes capable of subsequent fertilization.
- Endocrine function. Represents the synthesis of essential sex hormones, which include estrogens, progesterone, and inhibins.
Both of these functions are implemented cyclically. The entire complex of regular physiological changes in the reproductive system is collectively termed the ovarian-menstrual cycle.
Phases of the ovarian cycle
The ovarian cycle reflects the processes occurring directly within the ovarian tissue. It comprises three sequential phases:
- Follicular phase. Begins on the 1st day of menstrual bleeding and continues until ovulation occurs. At the beginning of the cycle, there is a decrease in the concentration of estrogens and progesterone, which is associated with the involution of the corpus luteum from the previous cycle. Via a feedback mechanism, this drop causes an increase in the secretion of gonadotropin-releasing hormone in the hypothalamus, which, in turn, stimulates the production of follicle-stimulating hormone (FSH). Under the influence of FSH, the follicle matures, transforming in its final stage into the so-called Graafian follicle. As it grows, the follicle begins to actively secrete increasing amounts of estrogens.
- Ovulatory phase. Occurs approximately on the 14th day of a classic 28-day cycle. The peak of estrogens is reached 24–36 hours before ovulation. This surge operates via a positive feedback mechanism, causing a sharp rise in the level of luteinizing hormone (LH). The maximum concentration of LH is recorded 10–12 hours before follicular rupture. As a result, the follicular wall thins and ruptures, leading to the release of a mature oocyte.
- Luteal phase. Lasts from the 15th to the 28th day. At the site of the ovulated follicle, a new temporary endocrine structure is formed — the corpus luteum. It begins to actively produce progesterone and estrogens. The peak concentration of progesterone is observed 8–9 days after ovulation. If implantation of the fertilized oocyte does not occur, the corpus luteum degenerates in about 2 weeks, hormone levels drop, and a new cycle begins.
Menstrual cycle
Concurrently with changes in the ovaries, hormonal fluctuations directly affect target organs: the mucosa of the fallopian tubes, the cervical canal, and, primarily, the uterine endometrium. The uterine cycle is also subdivided into three phases:
- Menstrual phase (desquamation phase). Characterized by the shedding of the old functional layer of the endometrium. This process is accompanied by bleeding and normally lasts 5±2 days.
- Proliferative phase. Covers the period from the 4th to the 14th day (until ovulation). Under the increasing stimulating influence of estrogens produced by the growing follicle, active proliferation of the cells of the basal layer of the endometrium occurs. This leads to the complete restoration of its functional layer.
- Secretory phase. Lasts from the moment of ovulation until the start of the next menstruation (from the 15th to the 28th day). High levels of progesterone secreted by the corpus luteum transform the endometrium, creating optimal conditions for potential embryo implantation. In addition, progesterone causes an increase in basal body temperature. If implantation does not take place, the cycle closes, and the menstrual phase begins anew.
Regulatory role of the CNS
The central nervous system plays the role of the supreme coordinator of all described processes. It is the CNS that synchronizes ovarian-menstrual changes: ensures the timely maturation of the oocyte, controls ovulation, and coordinates the parallel preparation of the uterine endometrium.
The sensitivity of the hypothalamic-pituitary-ovarian axis to external influences is extremely high. In cases of pronounced psycho-emotional stress, regulatory mechanisms can malfunction, leading to various cycle rhythm disorders up to the complete absence of menstruation — amenorrhea.
Briefly: answers to topical questions
What stages of development does the follicle undergo in the ovary from the beginning of the cycle to ovulation?
During folliculogenesis, the follicle sequentially passes through four main stages of development.
- Primordial follicle — consists of an immature oocyte surrounded by a single layer of granulosa cells.
- Primary follicle (preantral) — the oocyte enlarges, granulosa cells proliferate actively, and a connective tissue sheath (theca) is formed.
- Secondary follicle (antral) — secretory activity of granulosa cells begins with the formation of follicular fluid and the development of a cavity (antrum).
- Preovulatory follicle (dominant, Graafian follicle) — the largest stage (up to 20 mm), the oocyte is located on the cumulus oophorus (cumulus oophorus), and the meiotic process resumes.
What structural changes occur in the endometrium during the secretory phase of the uterine cycle?
During the secretory phase, under the influence of progesterone, mucosal swelling and secretory transformation of the endometrium occur in preparation for implantation.
- Epithelial changes — secretory vacuoles containing glycogen appear in the cells, after which secretion (glycogen and acidic glycosaminoglycans) is released into the glandular lumen.
- Stromal changes — active growth of spiral arteries occurs, along with decidua-like changes and leukocyte infiltration.
What happens to the corpus luteum upon the onset of pregnancy?
Upon the onset of pregnancy, the cyclic corpus luteum transforms into the corpus luteum of pregnancy and functions up to the 4th month of pregnancy. Human chorionic gonadotropin stimulates progesterone secretion by the corpus luteum of pregnancy. Progesterone from the corpus luteum, together with estrogens, prepares the uterine mucosa for implantation; the secretion of large amounts of progesterone blocks FSH release.
To memorize the relationship between hormones and phases of the uterine cycle: "Estrogens Energetically build (proliferative phase), while Progesterone Prepares the nest (secretory phase and readiness for implantation)".
Class and exam questions
Why does a sharp surge of LH occur before ovulation?
The LH surge is driven by a peak rise in estrogen levels 24–36 hours before ovulation. At this point, the hypothalamus-pituitary system responds to estrogens via positive rather than negative feedback.
What happens to the corpus luteum in the absence of pregnancy?
If implantation does not occur, the corpus luteum persists for about 2 weeks, after which it undergoes degeneration (involution). This leads to a decrease in progesterone and estrogen levels, which triggers the shedding of the endometrium.
Which hormone is responsible for raising basal temperature in the second half of the cycle?
The rise in basal temperature during the secretory (luteal) phase of the cycle occurs under the influence of high concentrations of progesterone produced by the corpus luteum.
What else to study on this topic
- Mechanism of positive feedback in gonadotropin secretion
- Histophysiology and stages of maturation of the Graafian follicle
- Role and mechanisms of action of inhibins in the ovarian cycle
- Cyclic changes in the mucosa of the fallopian tubes and cervical canal
- Mechanisms of stress-induced amenorrhea
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