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Female Reproductive System

*Systema genitale femininum*

For medical students3 min readUpdated 2026-10-10

The female reproductive system performs the vital reproductive task of ensuring oocyte maturation and preparing the body for potential pregnancy. Its function relies on complex neurohumoral regulation, manifested as rhythmically repeating changes known as the ovarian and menstrual cycle.

Cycle durationAverages about 28 days, with normal physiological fluctuations ranging from 21 to 35 days.
LH surgeA sharp rise in luteinizing hormone occurs 10–12 hours before ovulation.
DesquamationShedding of the functional layer of the endometrium takes an average of 5±2 days.
CNS influenceSevere psychoemotional stress can trigger cycle disturbances up to amenorrhea.

Main Functions of the Ovaries

The ovaries are key paired glands of the female reproductive system, whose function is strictly subordinated to cyclic changes. Their activity is traditionally divided into two interrelated directions:

Both functions occur cyclically. The entire complex of regular physiological changes in the reproductive system is termed the ovarian and menstrual cycle.

Phases of the Ovarian Cycle

The ovarian cycle reflects processes occurring directly within the ovarian tissue. It consists of three sequential phases:

  1. Follicular phase. Begins on the 1st day of menstrual bleeding and continues until ovulation. At the start of the cycle, a decrease in estrogen and progesterone concentration is noted due to the involution of the corpus luteum from the previous cycle. Via negative feedback, this drop increases the secretion of gonadotropin-releasing hormone (GnRH) in the hypothalamus, which stimulates the production of follicle-stimulating hormone (FSH). Under the influence of FSH, a follicle matures, transforming into a vesicular ovarian follicle (Graafian follicle) in its final stage. As it grows, the follicle actively secretes increasing amounts of estrogens.
  2. Ovulatory phase. Occurs approximately on the 14th day of a classic 28-day cycle. A peak estrogen level is reached 24–36 hours before ovulation. This surge operates via a positive feedback mechanism, triggering a sharp rise in luteinizing hormone (LH). Maximum LH concentration is recorded 10–12 hours before follicular rupture. As a result, the follicular wall thins and ruptures, releasing the mature oocyte.
  3. Luteal phase. Lasts from days 15 to 28. A new temporary endocrine structure, the corpus luteum, forms at the site of the ruptured follicle. It actively secretes progesterone and estrogens. The peak progesterone concentration 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.

The Menstrual Cycle

Concurrently with ovarian changes, hormonal fluctuations directly affect target organs: the mucosa of the uterine tubes, the cervical canal, and primarily the uterine endometrium. The uterine cycle is also subdivided into three phases:

  1. Menstrual phase (desquamation). Characterized by the shedding of the old functional layer of the endometrium. This process is accompanied by bleeding and normally lasts 5±2 days.
  2. Proliferative phase. Covers the period from day 4 to day 14 (until ovulation). Under the increasing stimulatory influence of estrogens produced by the growing follicle, active proliferation of cells in the basal layer of the endometrium occurs, leading to the complete restoration of its functional layer.
  3. Secretory phase. Lasts from ovulation until the start of the next menstruation (days 15 to 28). High levels of progesterone secreted by the corpus luteum transform the endometrium, creating optimal conditions for potential embryo implantation. Additionally, progesterone causes an increase in basal body temperature. If implantation does not occur, the cycle closes, and the menstrual phase begins anew.

Regulatory Role of the CNS

The central nervous system acts as the master coordinator of all described processes. The CNS synchronizes ovarian and menstrual changes, ensuring the timely maturation of the oocyte, controlling ovulation, and coordinating the simultaneous preparation of the uterine endometrium.

The sensitivity of the hypothalamic-pituitary-ovarian axis to external influences is extremely high. Severe psychoemotional stress can disrupt these regulatory mechanisms, leading to various rhythm disturbances up to the complete absence of menstruation—amenorrhea.

Mnemonic

To remember the relationship between hormones and uterine cycle phases: "Estrogens Energetically build (proliferative phase), while Progesterone Prepares the 'pedestal' (secretory phase and implantation readiness)."

Frequently asked questions

What stages of development does the ovarian follicle go through from cycle onset 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 actively proliferate, and the connective tissue sheath (theca) forms.
  • Secondary follicle (antral) — granulosa cells begin secretory activity, producing follicular fluid and forming a cavity (antrum).
  • Preovulatory follicle (dominant, Graafian follicle) — the largest stage (up to 20 mm); the oocyte sits on the cumulus oophorus (cumulus oophorus), and meiosis resumes.
What structural changes occur in the endometrium during the secretory phase of the uterine cycle?

During the secretory phase, progesterone induces mucosal swelling and secretory transformation of the endometrium to prepare for implantation.

  • Epithelial changes — secretory vacuoles containing glycogen appear within cells, after which secretion (glycogen and acidic glycosaminoglycans) is released into the glandular lumens.
  • Stromal changes — spiral arteries undergo active growth, accompanied by decidua-like changes and leukocyte infiltration.
What happens to the corpus luteum when pregnancy occurs?

Upon pregnancy, the cyclic corpus luteum transforms into the corpus luteum of pregnancy and functions until the 4th month of gestation. Human chorionic gonadotropin stimulates progesterone secretion by the corpus luteum of pregnancy. Progesterone, together with estrogens, prepares the uterine mucosa for implantation; high progesterone levels block FSH release.

Why does a sharp LH surge occur before ovulation?

The LH surge is driven by a peak rise in estrogen levels 24–36 hours before ovulation. At this point, the hypothalamic-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 and then undergoes degeneration (involution). This leads to decreased progesterone and estrogen levels, triggering endometrial shedding.

Which hormone is responsible for the basal body temperature rise in the second half of the cycle?

The increase in basal body temperature during the secretory (luteal) phase is driven by high concentrations of progesterone produced by the corpus luteum.

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