Inhibin and oestradiol in the control of FSH secretion in the sheep

In the sheep both FSH and LH are necessary for development of large antral follicles. The secretion of FSH is controlled by the negative feedback effect of two ovarian hormones, oestradiol and inhibin, acting at the level of the anterior pituitary. Both are derived from the granulosa cells of large antral follicles which are present in sheep ovaries throughout the oestrous cycle. FSH stimulates growth and mitosis and so the fully differentiated granulosa cells of the large preovulatory follicles acquire receptors for LH, have maximal aromatase activity and produce large amounts of inhibin. The number of these large antral follicles (oestrogenic) which have the potential for ovulation corresponds to the ovulation rate specific for each particular breed of sheep. Over 90% of the oestradiol secreted by the ovaries is derived from these chosen follicles. In contrast, inhibin (and androstenedione) is also secreted by large antral follicles which have lost or not yet acquired maximal aromatase activity.