Endocrinology

In the absence of fertilization, the corpus luteum degenerates and progesterone and estrogen levels subsequently fall. At day 28 of the menstrual cycle, estrogen and progesterone are at their lowest levels. Negative feedback on FSH is removed, and FSH secretion can again increase to initiate the beginning of the next cycle.

Follicular Development

When fertilization does not occur, the corpus luteum undergoes atresia. Towards the end of the cycle, the corpus luteum devolves into the corpus albicans, or white scar.

Uterine Endometrial Cycle

As progesterone and estrogen levels decrease with the degeneration of the corpus luteum, the endometrium begins to undergo involution. At days 25-26, the spiral arteries begin to vasoconstrict. By day 28, marked ischemia causes apoptosis of the functionalis.