Structure of the endometrium
The uterine wall consists of the serosa, the muscular layer (myometrium), and the inner mucous membrane — the endometrium. It is the endometrium that serves as the main target for sex hormones.
Morphologically, it is divided into two layers:
- Basal layer: adjoins the myometrium, remains practically unchanged throughout the cycle, and is not shed during menstruation. It serves as a reservoir for mucosal regeneration.
- Functional (superficial) layer: consists of compact epithelial cells. It undergoes constant structural remodeling and is completely shed during bleeding.
Main phases of the uterine cycle
Cyclic changes are biphasic in nature (on average, the cycle lasts 28 days, ovulation occurs on the 14th day). There are four consecutive periods:
- Desquamation (menstruation): shedding of the necrotic functional layer.
- Regeneration: epithelialization of the wound surface (completed by the 4th day of the cycle).
- Proliferative phase (days 5–14): occurs under the influence of estrogens and coincides with the follicular phase in the ovaries. A new superficial layer is formed, and elongated tubular glands with high mitotic activity develop.
- Secretory phase (14±1 days): controlled by progesterone and estrogens, corresponding to the luteal phase of the ovary. The glands begin to produce secretion (glycogen, glycoproteins, acidic glycosaminoglycans). Peak activity is on days 20–21: the stroma undergoes decidual transformation, and the spiral arteries coil into "glomeruli", creating ideal conditions for implantation.
Mechanism of menstruation
If fertilization does not occur, regression of the corpus luteum (corpus luteum) begins on days 24–27. Progesterone levels drop, and endometrial trophism is impaired. Granulosa cells secrete relaxin, preparing tissues for shedding.
Pathogenesis of desquamation:
- A sharp decrease in sex steroids causes prolonged arterial spasm.
- Blood stasis and thrombosis occur, accompanied by increasing hypoxia and tissue acidosis.
- Leukocytic infiltration leads to the release of proteolytic enzymes that dissolve the tissue.
- Spasm is replaced by paretic dilation of vessels. Hydrostatic pressure rises, and fragile capillary walls rupture.
Against the background of these changes, the functional layer is shed. By the end of the first day, about 2/3 of the layer is gone, and complete desquamation is completed by the 3rd day.
Hormonal regulation and extragenital targets
Changes in the uterus depend on the level of steroids in the blood and the state of tissue receptors. The hormone penetrates the cell, binds to a specific cytoplasmic receptor, after which the complex moves to the nucleus, binds to chromatin, and triggers mRNA transcription for protein synthesis.
Sex hormones affect not only the uterus but also other organs:
- Fallopian tubes: in the luteal phase, ciliated epithelium and muscle contractions are activated for gamete transport.
- Skin: estradiol stimulates collagen synthesis, while an excess of androgens causes acne and increased sebum production.
- Bone tissue: steroids prevent bone resorption.
- CNS: action on the hippocampus alters the emotional background and autonomic reactions.
To remember the order of the phases of the uterine cycle, use the mnemonic: «Girls Regularly Process Secrets» (Desquamation, Regeneration, Proliferation, Secretion).
Class and exam questions
Which layer of the endometrium is shed during menstruation?
Only the superficial (functional) layer is shed. The basal layer remains unchanged and serves as the source for subsequent regeneration of the mucous membrane.
What is the main trigger for the onset of menstrual bleeding?
Regression of the corpus luteum in the ovary, which leads to a sharp drop in progesterone and estrogen levels. This causes vascular spasm, hypoxia, and endometrial tissue necrosis.
How does the number of estradiol receptors change during the cycle?
Their concentration rises in the first half of the cycle, peaking in the late proliferative phase. After ovulation and in the late secretory phase, receptor expression drops sharply.
What else to study on this topic
- Dynamics of pituitary hormones (LH and FSH) and their relationship with ovulation
- Histological picture of the endometrium at different stages of the cycle
- Mechanism of action of relaxin in the premenstrual period
- Effect of sex steroids on adipose tissue metabolism
- Changes in the contractile activity of the fallopian tubes
Master the entire topic in the "Obstetrics and Gynecology" course
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- Complete study guide on the «Uterine cycle» and 973 more pages of the subject
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