In the transition from a Graafian follicle to a functional corpus luteum, which of the following cellular events occurs?

The secretion of estrogen by the developing follicle can best be explained using the "two cell" hypothesis. Theca cells are stimulated by LH (theca cells express LH receptors prior to formation of the corpus luteum, choice D) to secrete the androgens androstenedione and testosterone. The androgens then diffuse into the granulosa cells, where they are aromatized to estrogens. Hence, theca cells express side-chain cleavage enzyme (first step in steroidogenesis) prior to the formation of the corpus luteum . FSH stimulates aromatase activity in the granulosa cells (receptors for FSH and aromatase enzyme are present prior to the formation of the corpus luteum, choices A and B). The granulosa cells apparently have the ability to produce steroids (progesterone), but lack 17a-hydroxylase activity and cannot synthesize estrogen themselves. Only as the follicle approaches ovulation do LH receptors begin to be expressed by the granulosa cells. Estrogen and FSH probably are responsible for the change. After ovulation, the scar of the follicle undergoes luteinization. The theca cells decrease 17a-hydroxylase activity and secrete more progesterone. The granulosa cells decrease aromatase activity and also secrete more progesterone.