A layer of secretory cells in a Graafian follicle that surround the oocyte. They have a function analogous to that of the Sertoli cells in the testis, supplying nutrients to the developing oocyte. After ovulation they form the corpus luteum. The granulosa cells, in conjunction with the outer layer of thecal cells, play a key role in determining which of the several developing follicles reaches maturity and undergoes ovulation in each ovarian cycle. Testosterone from the thecal cells diffuses into the granulosa cells, where it is converted to oestradiol. This steep rise in oestradiol, along with follicle-stimulating hormone (FSH), promotes the growth and maturation of the granulosa cells, and also their expression of receptors for FSH and for luteinizing hormone (LH). It is the follicle with the most FSH and LH receptors that matures and ovulates, while other follicles regress. The granulosa cells also secrete the hormone inhibin, which acts as negative feedback mechanism to regulate FSH levels.