Apoptosis can occur by change in hormone levels in the ovarian cycle. When there is no fertilization of the ovum, the endometrial cells die because;

Ans. is 'a' i.e. The involution of corpus luteum causes estradiol and progesterone levels to fall dramaticallyRef. Shaws 13th/e page 47, (12th/e p 39& 38)After ovulation corpus luteum forms and starts secreting estrogen, progesterone and inhibin. (The increased LH at the time of ovulation is responsible for maintaining the corpus luteum).Continued increase in estrogen and progesterone exerts inhibitory effect on Hypothalamus and pituitary (Remember that increased estrogen plus moderate amount of progesterone exerts negative feedback on hypothalamus and pituitary whereas increased estrogen and small amount of progesterone exerts positive feedback on the pituitary and hypothalamus and cause LH surge).So due to negative feedback of increased estrogen and moderate amount of progesterone the LH and FSH secretion decreasesDue to decreased LH, the corpus luteum starts degenerating and degenerates completely by about 9 days after ovulation.Due to degeneration of corpus luteum estrogen and progesterone secretion decreases rapidly. Due to rapid decrease in estrogen and progesterone the endometrium is unable to sustain itself and sloughs off (this is the end of menstrual cycle)On the other hand if fertilization occurs placenta will be formed. The placenta starts secreting a hormone called b HCG which maintains the corpus luteum and thus corpus luteum continues secreting estrogen and progesterone. The estrogen and progesterone maintains endometrium and thus pregnancy is sustained.