This communiction reviews the current information on the developmental relationships between the various tissues of the growing human antral follicle. It also examines the various interrelationships between the hormone levels in antral fluid, the populations of granulosa cells, the steroidogenic capacities of thecal tissue and granulosa cells, and the status of the oocyte in antral follicles at different stages of growth or degeneration. Healthy antral follicles are considered to be these containing an oocyte which appears healthy at the level of the dissecting microscope and more than 50p. 100 of its full complement of ganulosa cells at each follicle diameter. In follicles with more than 75p. 100 of their full complement of granulosa cells, at a given follicle diameter, the endocrine microenvironment is one that is enriched with FSH and oestradiol. Follicles with between 51 and 75p. 100 of their full complement of granulosa cells at a given follicle diameter are invariably undergoing degenerative changes and their endocrine microenvironment is deficient in FSH and oestradiol but rich in androgen. However, it is believed that many of these follicles are still capable of being recruited back into the pool of healthy growing follicles. Atretic antral follicles are considered to be those undergoing irreversible degenerative changes: they usually contain <50p. 100 of their maximum number of granulosa cells and a degenerating oocyte. In atretic follicles, the endocrine microenvironment is deficient in FSH and oestradiol but contains levels of androstenedione comparable to those in healthy follicles. The functional capacities of granulosa cells and thecal tissue from healthy follicles differ from those of atretic follicles. In vitro, granulosa cells from healthy follicles have the capacity to produce large amounts of oestradiol and smaller amounts of androstenedione and these estrogen-secreting cells retain the capacity to undergo mitosis in culture. By contrast, granulosa cells from atretic follicles do not have the capacity to produce much oestradiol although they remain steroidogenically competent for some time since they continue to synthesize androgens. In vitro, these cells are incapable of mitotic activity or of maintaining their population. In vitro, thecal tissue from both healthy and atretic follicles produces large amounts of androstenedione. In addition, thecal tissue from healthy but not atretic follicles produces some oestradiol. The amount of degeneration of the oocyte in vivo and the subsequent maturational capabilities of the oocyte in vitro are correlated with the number of granulosa cells present and also with the levels of steroids in antral fluid. When healthy-looking oocytes were recovered from follicles with ≤50p. 100 of their granulosa cells and/or had been exposed to high levels of androgen compared to oestrogen in vivo, their potential for meiotic maturation in vitro was severely reduced compared to oocytes from follicles with > 50% of their full complement of granulosa cells and low androgen/oestrogen ratios. It is suggested that the uninterrupted development of a human antral follicle is dependent upon its granulosa cells sustaining an oestrogen-enriched micro-environment within the follicle and its thecal envelope retaining the capacity to produce oestradiol as well as androstenedione.
