In the Preantral Follicle, Gonadotropin Receptors for Luteinizing Hormone Develop on the ThecarWhich Results in Androgen Synthesis; Follicle-Stimulating Hormone Directs the Granulosa to Transform the Androgens to Estrogens
In order for follicles to progress beyond the preantral stage, the granulosa and theca need to develop receptors for gonadotropins. FSH and LH receptors develop on the granulosa and theca, respectively.
The onset of the antral follicle is marked by the appearance of fluid that begins to divide the granulosa. The follicular fluid, a secretory product of the granulosa, coalesces to form an increasingly larger fluid cavity (antrum) within the granulosa. In later development of the antral follicle, the oocyte remains surrounded by a layer of granulosa cells called the cumulus oophorus, which are attached to the wall of the follicle by a small stalk of granulosa cells.The proximity of the granulosa and theca cells allows cooperative estrogen synthesis. The theca produces androgens (testosterone and androstenedione) under the influence of LH, which diffuses across the membrana propria into the granulosa, where the androgens are transformed into estrogen (estradiol-17). At this time of development, the granulosa is incapable of forming androgens, the precursors of estrogen biosynthesis, and the theca has limited capacity for producing estrogens. This concept of cooperative effort, called the twocell mechanism for estrogen secretion, is generally accepted as being the way most follicular estrogen is produced. These estrogens have a positive-feedback effect on the granulosa; they stimulate the cells to undergo mitotic division, and thus the follicle grows in size as the granulosa proliferates in response to its own secretory product (estrogen).
One effect of estrogen is the formation of additional receptors for FSH as follicle development proceeds. In this situation the antral follicle becomes increasingly sensitive to FSH as it develops and is able to grow under a relatively steady state of FSH secretion.