Periodization and phases
The standard duration of the ovarian-menstrual cycle is 28 days. The entire cycle is divided into three key periods, during each of which changes occur synchronously at the level of the pituitary gland, ovary, and uterus:
- Menstrual (days 1–4) — phase of endometrial desquamation.
- Postmenstrual (days 5–14) — follicular phase in the ovary and proliferative phase in the uterus (until ovulation).
- Premenstrual (days 15–28) — luteal phase of the ovary and secretory phase of the endometrium (after ovulation).
Menstrual period: shedding and new start
The trigger for the onset of menstruation is a critical drop in sex hormone concentrations (primarily progesterone). As a result, the inhibitory effect on the pituitary gland disappears, and it begins to actively produce follicle-stimulating hormone (FSH) and, slightly more slowly, luteinizing hormone (LH).
In the ovaries, under the influence of FSH, secondary (antral) follicles awaken and begin to actively synthesize estrogens. The histological picture of the ovary at this moment is extremely diverse: growing secondary follicles coexist with the regressing corpus luteum of the previous cycle, atretic and resting follicles, as well as atretic bodies producing androgens.
In the uterus, desquamation occurs at this time — the shedding of the necrotic functional layer, accompanied by bleeding. However, by the end of the phase, rising estrogen levels trigger the proliferation of the epithelium from the fundi of the uterine glands, the wound surface closes, and the bleeding stops.
Postmenstrual period: preparation for ovulation
During this period, LH levels rise smoothly, while FSH begins to decline due to a feedback mechanism from high estrogens.
In the ovary, one dominant follicle is selected from the pool of secondary follicles. It becomes a tertiary follicle and prepares for ovulation, which occurs on days 14–15. The remaining follicles of the current generation undergo atresia. At the same time, both growing and atretic follicles actively produce estrogens, the concentration of which peaks before ovulation.
Under the influence of estrogens, the uterine endometrium enters the proliferative phase. The functional layer actively regenerates, and the stroma thickens. The uterine glands lengthen, but for now remain straight, narrow, and non-secretory.
Premenstrual period: secretion and anticipation
After ovulation, the cycle enters the luteal phase. FSH levels remain low, LH drops sharply (suppressed by progesterone), and luteotropic hormone (LTH) reaches its maximum.
At the site of the ruptured follicle, under the influence of LH, a corpus luteum is formed, which is stimulated by LTH and begins to produce progesterone.
Progesterone dominates in the uterus, triggering the secretory phase:
- The uterine glands become convoluted and branched, secreting thick mucus.
- Ciliated cells appear in the surface epithelium.
- The endometrial stroma thickens to the maximum due to hyperemia (blood engorgement) and edema.
- Spiral arteries reach their peak development.
If implantation does not occur, the function of the corpus luteum fades. A sharp drop in progesterone causes spasm of the spiral arteries in the myometrium. Blood flow slows down, and thrombosis and ischemia of the functional layer of the endometrium develop. The tissue becomes necrotic. When the arteries dilate again, the surge of blood flushes away the necrotic layer — menstruation begins, and the cycle closes.
Briefly: answers to topical questions
What layers are distinguished in the uterine endometrium?
Two main layers are distinguished in the uterine endometrium: the basal layer and the functional layer.
- Basal layer — serves as the source of regeneration for the functional layer.
- Functional layer — is shed during menstruation. It is subdivided into:
- superficial layer (compact epithelial cells);
- intermediate (spongy) layer.
What stages does the formation of the corpus luteum consist of?
The life cycle of the corpus luteum includes four stages:
- Stage of proliferation and vascularization — collapse of the follicular cavity, ingrowth of connective tissue with vessels, and colonization by multiplying follicular cells.
- Stage of glandular metamorphosis — differentiation of cells into lutein cells (granulosa-lutein and theca-lutein cells).
- Stage of full development (flourishing) — active secretion of progesterone (may be a menstrual corpus luteum or corpus luteum of pregnancy).
- Stage of regression (involution) — degeneration of lutein cells, replacement by connective tissue, and formation of the corpus albicans (corpus albicans).
How does a primordial follicle morphologically differ from a secondary follicle?
| Sign | Primordial follicle | Secondary (antral) follicle |
|---|---|---|
| Oocyte / egg cell | Primary oocyte; located in the diplotene stage of prophase of the first meiotic division | Follicular fluid pushes the egg cell to the periphery; granulosa cells form the cumulus oophorus (cumulus oophorus) |
| Follicular / granulosa cells | A single layer of flat follicular cells on the basement membrane | Further growth and continued proliferation of granulosa cells; granulosa cells produce follicular fluid |
| Cavity | Sources describe an oocyte and a single layer of follicular cells for the primordial follicle | A cavity (antrum) with follicular fluid is formed |
| Theca | Connective tissue theca cells are elongated in shape | The theca is clearly differentiated into internal (theca interna) and external (theca externa) layers |
Remember the hormonal correlation of endometrial phases by their first letters: Estrogens build (E — proliferative phase), while Progesterone nourishes and prepares (P — secretory phase).
Class and exam questions
Why is the endometrium shed?
Shedding occurs due to a vascular reaction to hormone withdrawal (a drop in progesterone). Spiral arteries spasm, causing ischemia and necrosis of the functional layer, and upon their subsequent dilation, blood flow tears away the necrotic tissues.
What is the cellular source for endometrial regeneration?
Regeneration of the functional layer is ensured 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 the new cycle under the influence of FSH?
Although it is believed that the development of primordial follicles is initiated, it is more logical to assume the stimulation of already "dormant" secondary (antral) follicles, since they are able to respond faster to a stimulus with the active production of estrogens.
What else to study on this topic
- Morphology and stages of follicular development (from primordial to tertiary)
- Histological structure and stages of development of the corpus luteum
- Process of follicular atresia and hormonal function of atretic bodies
- Detailed structure of the wall of the spiral arteries of the endometrium
- Feedback mechanism in the hypothalamic-pituitary-ovarian axis
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