How Does It Form?
The corpus luteum forms following ovulation, when the oocyte is released from the preovulatory follicle into the peritoneal cavity.
Formation involves the following stages:
- Vascularization: Capillaries rapidly grow into the remaining follicular cavity after oocyte release. Ovulation is accompanied by bleeding from ruptured capillaries surrounding the theca cells.
- Luteinization: Granulosa cells undergo luteinization—they increase in size and accumulate lipid droplets.
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 dominates the second (luteal) phase of the cycle.
- Effect on the endometrium: During the secretory phase, endometrial glands become coiled, and the endometrium thickens in preparation for implantation. Marked vascularization of the stroma occurs: the spiral arteries of the functional layer become heavily coiled and form 'glomeruli', and veins become dilated.
- Thermogenic effect: Progesterone acts on the thermoregulatory center in the hypothalamus. During the second phase of the cycle, the basal body temperature rises above 37.0 °C, typically exceeding the first-phase levels by 0.4–0.6 °C.
- During pregnancy: In the first 3 months of pregnancy, progesterone is primarily produced by the corpus luteum. Thereafter, synthesis is taken over by the placenta.
What Happens in the Absence of Pregnancy?
In the premenstrual period, around cycle days 24–27, 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.
- Basal body temperature decreases 1–2 days before menstruation.
- Endometrial trophic support is disrupted, leading to prolonged arterial spasm, blood stasis, and microthrombi formation.
- Hypoxia and tissue acidosis increase, while stromal granular cells release relaxin, preparing the mucosa for menstrual shedding.
- Desquamation, shedding, and subsequent regeneration of the functional layer of the endometrium occur.
Clinical Significance
- Anovulatory cycle: If ovulation fails to occur, the corpus luteum does not form and progesterone secretion is absent. On a 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 a background of relative or absolute hyperestrogenism.
- Evidence of external migration of the ovum: In tubal ectopic pregnancy, finding a corpus luteum in the ovary contralateral to the side of the ectopic pregnancy serves as clinical proof of external ovum migration.