How does it form?
The corpus luteum forms after ovulation, which is the release of the oocyte from the preovulatory follicle into the abdominal cavity.
Formation includes the following stages:
- Vascularization: newly formed capillaries rapidly sprout into the follicular cavity left after the release of the oocyte. Ovulation is accompanied by hemorrhage from damaged capillaries surrounding the theca cells.
- Luteinization: granulosa cells undergo luteinization: they increase in volume and develop lipid inclusions.
As a result of these processes, the corpus luteum is formed.
Functions of the corpus luteum
The cells of the corpus luteum secrete progesterone. The level of this hormone begins to rise after ovulation, and it dominates during the second, luteal phase of the cycle.
- Effect on the endometrium: during the secretory phase, endometrial glands become tortuous, the endometrium thickens, and prepares for implantation. Marked vascularization of the stroma occurs: spiral arteries of the functional layer are highly tortuous and form "glomeruli", and veins are dilated.
- Thermogenic effect: progesterone acts on the thermoregulation center in the hypothalamus. In phase II of the cycle, basal body temperature rises above 37.0 °C and is typically 0.4–0.6 °C higher than the values of the first phase.
- During pregnancy: in the first 3 months of pregnancy, progesterone is primarily produced by the corpus luteum. Subsequently, its synthesis is taken over by the placenta.
What happens in the absence of pregnancy?
In the premenstrual period, on days 24–27 of the cycle, regression of the corpus luteum begins, leading to a decrease in progesterone concentration.
Regression is accompanied by the following changes:
- Sex steroid levels drop sharply.
- 1–2 days before menstruation, basal body temperature decreases.
- Endometrial trophic function is impaired; prolonged arterial spasm, blood stasis, and thrombus formation occur.
- Hypoxia and tissue acidosis increase, and stromal granular cells release relaxin, preparing the mucous membrane for menstrual shedding.
- Desquamation, shedding, and subsequent regeneration of the functional layer of the endometrium take place.
Clinical significance
- Anovulatory cycle: if ovulation does not occur, the corpus luteum does not form, and progesterone secretion is absent. On the basal body temperature chart, this appears as a monophasic curve: temperature is usually below 37.0 °C, without a sustained rise in the second half of the cycle. The cycle occurs against the background of relative or absolute hyperestrogenism.
- Evidence of external egg migration: in tubal pregnancy, the detection of a corpus luteum in the ovary contralateral to the site of pregnancy serves as clinical evidence of external migration of the oocyte.
Class and exam questions
Why does basal body temperature drop before menstruation?
1–2 days before the onset of menstruation in the absence of pregnancy, progesterone levels drop, which is accompanied by a decrease in basal body temperature. The thermogenic effect of progesterone is due to its action on the thermoregulation center in the hypothalamus.
From which structures does the corpus luteum form?
After ovulation, granulosa cells undergo luteinization: they increase in volume, develop lipid inclusions, and newly formed capillaries rapidly sprout into the remaining follicular cavity.
How long does the corpus luteum produce progesterone during pregnancy?
In the first 3 months of pregnancy, progesterone is primarily produced by the corpus luteum. Subsequently, its synthesis is taken over by the placenta.
What else to study on this topic
- Ovary (ovarium) and folliculogenesis
- Menstrual cycle: ovarian and uterine
- Hormonal function of the placenta
- Endometrium: structure and phases of changes
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