The female reproductive cycle is a continuous, hormonally regulated process that prepares the body for potential pregnancy. This cycle culminates in ovulation, where a mature egg, or ovum, is released from the ovary. The release of the ovum sets in motion physiological changes intended to create the optimal environment for fertilization and implantation. If fertilization fails to occur, the body must systematically deconstruct these preparations to reset the cycle.
The Ovum’s Waiting Period and Degradation
Once the ovum is released, it enters the fallopian tube and begins its brief waiting period for fertilization. The human egg remains capable of being fertilized for approximately 12 to 24 hours after ovulation. After this short time frame, the ovum begins to undergo cellular breakdown.
The egg does not exit the body intact during menstruation. Instead, the non-viable cell disintegrates within the fallopian tube. The components of the broken-down ovum are then absorbed by surrounding cells and tissues through phagocytosis, ensuring the egg is recycled and cleared from the reproductive tract.
The Corpus Luteum’s Regression
The fate of the unfertilized egg is closely tied to the temporary endocrine structure left behind in the ovary: the corpus luteum. Following the rupture of the ovarian follicle during ovulation, the remaining cells transform into this structure. Its function is to secrete progesterone, the hormone responsible for maintaining the thick, nutrient-rich lining of the uterus.
The corpus luteum produces progesterone for about 10 to 14 days. If the ovum is fertilized and successfully implants, the developing embryo secretes human chorionic gonadotropin (hCG). This hCG acts as a rescue signal, ensuring the corpus luteum continues its function to support the early pregnancy.
In the absence of the hCG signal, the corpus luteum begins luteolysis. The lack of hormonal support triggers a decline in hormone production. The structure then undergoes structural regression, shrinking and degenerating into a non-functional scar tissue known as the corpus albicans. This degeneration systematically dismantles the temporary gland through programmed cell death pathways, such as apoptosis.
Hormonal Shift and Endometrial Shedding
The degeneration of the corpus luteum is the direct biological trigger for the next phase of the cycle. As the structure shrinks, the production of progesterone and estrogen plummets. This sudden withdrawal of hormonal support can no longer sustain the endometrium, the lining built up in preparation for a fertilized egg.
The drop in hormone levels initiates tissue death and detachment within the uterus. The spiral arteries that supply blood to the outer layers of the endometrium constrict, a process called vasoconstriction. This constriction cuts off the blood and oxygen supply to the upper endometrial layer, causing the tissue to die.
The dead tissue, mucus, and blood are then shed from the body through the cervix and vagina, which is known as menstruation. The shedding of the endometrium marks the end of one cycle and the beginning of the next. The body immediately starts recruiting new follicles for the follicular phase.