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Ovarian and Menstrual Cycle

cyclus ovario-menstrualis

For medical students2 min readUpdated 2026-10-10

The ovarian and menstrual cycle is a rhythmically repeating complex of changes in the female reproductive system (ovaries, uterus, pituitary gland, and mammary glands). The name emphasizes the biological priority of the ovaries: their morphological and hormonal remodeling directs the cyclical reactions of the endometrium.

DurationNormally 28 days. The count always begins on the first day of bleeding.
Hormonal WithdrawalA sharp drop in progesterone and estrogens triggers a new cycle.
MenstruationBleeding caused by the shedding of the necrotic functional layer of the uterus.
RegenerationThe endometrial epithelium regenerates from the fundi of the basal layer glands.

Periodization and Phases

The standard duration of the ovarian and menstrual cycle is 28 days. The entire cycle is divided into three key periods, during each of which synchronous changes occur at the level of the pituitary gland, ovary, and uterus:

  1. Menstrual (days 1–4) — phase of endometrial desquamation.
  2. Postmenstrual (days 5–14) — follicular phase in the ovary and proliferative phase in the uterus (prior to ovulation).
  3. Premenstrual (days 15–28) — luteal phase of the ovary and secretory phase of the endometrium (following ovulation).

Menstrual Period: Shedding and New Start

The trigger for menstruation is a critical drop in sex hormone concentrations (primarily progesterone). This removes the inhibitory feedback on the pituitary gland, which begins actively secreting follicle-stimulating hormone (FSH) and, slightly slower, luteinizing hormone (LH).

In the ovaries, driven by FSH, secondary (antral) follicles awaken and begin synthesizing estrogens. The histological picture of the ovary at this moment is highly variegated: growing secondary follicles coexist with the regressing corpus luteum of the previous cycle, atretic and resting follicles, and atretic bodies producing androgens.

Simultaneously in the uterus, desquamation occurs—the shedding of the necrotic functional layer accompanied by bleeding. However, by the end of this phase, rising estrogen levels trigger the proliferation of epithelium from the bases of the uterine glands, sealing the wound surface and stopping the hemorrhage.

Postmenstrual Period: Preparation for Ovulation

During this period, LH levels rise steadily, while FSH begins to decline due to negative feedback from high estrogen levels.

In the ovary, a single dominant follicle emerges from the pool of secondary follicles. It becomes a tertiary (graafian) follicle and prepares for ovulation, which occurs around days 14–15. The remaining follicles of the current generation undergo atresia. Both growing and atretic follicles intensely produce estrogens, peaking right before ovulation.

Under the influence of estrogens, the endometrial stroma enters the proliferative phase. The functional layer actively regenerates, and the stroma thickens. Uterine glands elongate, though they remain straight, narrow, and non-secretory.

Premenstrual Period: Secretion and Anticipation

Following ovulation, the cycle transitions to the luteal phase. FSH remains low, LH drops precipitously (suppressed by progesterone), and luteotropic hormone (LTH, prolactin) reaches its maximum.

At the site of the ruptured follicle, driven by LH, a corpus luteum forms. Stimulated by LTH, it begins to produce progesterone.

In the uterus, progesterone dominates, driving the secretory phaseぞ:

If implantation does not occur, the function of the corpus luteum wanes. A sharp drop in progesterone triggers spasm of the spiral arteries in the myometrium. Blood flow slows, leading to thrombosis and ischemia of the functional endometrial layer. The tissue undergoes necrosis. When the arteries dilate again, the blood surge sloughs off the dead tissue, initiating menstruation and closing the cycle.

Mnemonic

Remember the hormonal linkage of the endometrial phases by their initial letters: Estrogens build (E — proliferative phase), while Progesterone nourishes and prepares (P — secretory phase).

Frequently asked questions

What layers are distinguished in the uterine endometrium?

The uterine endometrium features two main layers: the basal layer and the functional layer.

  • Basal layer — serves as the source for regeneration of the functional layer.
  • Functional layer — is shed during menstruation. It is subdivided into:
  • superficial layer (compact epithelial cells);
  • intermediate (spongy) layer.
What are the stages of corpus luteum development?

The life cycle of the corpus luteum includes four stages:

  • Proliferation and vascularization — collapse of the follicular cavity, ingrowth of connective tissue with blood vessels, and invasion by proliferating follicular cells.
  • Glandular metamorphosis — differentiation of cells into luteal cells (granulosa lutein and theca lutein cells).
  • Peak development (stage of maturity) — active secretion of progesterone (can be a menstrual corpus luteum or corpus luteum of pregnancy).
  • Regression (involution) — degeneration of luteal cells, replacement by connective tissue, and formation of a corpus albicans.
How does a primordial follicle morphologically differ from a secondary follicle?
FeaturePrimordial FollicleSecondary (Antral) Follicle
Oocyte / EggPrimary oocyte; arrested in the diplotene stage of prophase I of meiosisFollicular fluid pushes the oocyte to the periphery; granulosa cells form the cumulus oophorus
Follicular / Granulosa cellsA single layer of flattened follicular cells on a basement membraneContinued growth and proliferation of granulosa cells; granulosa cells produce follicular fluid
CavityContains only the oocyte and a single layer of follicular cellsA fluid-filled cavity (antrum) forms
ThecaConnective tissue theca cells are flattened/elongatedTheca is clearly differentiated into theca interna and theca externa
Why does the endometrium shed?

Shedding occurs due to a vascular reaction to hormonal withdrawal (a drop in progesterone). Spiral arteries spasm, causing ischemia and necrosis of the functional layer; subsequent arterial dilation and blood surge tear away the dead tissue.

Where does the cellular source for endometrial regeneration come from?

Regeneration of the functional layer is driven by the proliferation of epithelium from the fundi of the uterine glands, which remain intact in the basal layer.

Which follicles begin to grow in a new cycle under the influence of FSH?

Although it is traditionally taught that primordial follicles are initiated, it is more physiologically accurate to assume the stimulation of already "dormant" secondary (antral) follicles, as they can respond much faster to the stimulus by actively producing estrogens.

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