Follistatin (FS) is an activin/inhibin binding protein which is believed to act in an autocrine/paracrine manner to regulate growth and differentiation. Although FS has been identified in human follicular fluid, it remains unclear how its concentration changes during selection and atresia, and what the concentrations of FS are in follicles of women with polycystic ovary syndrome (PCOS). Towards this goal, we have measured by radioimmunoassay the concentrations of FS in follicular fluid obtained from dominant and atretic cohort follicles of normal cycling women, preovulatory follicles of in-vitro fertilization (IVF) patients, and small Graafian follicles of patients with PCOS. In all cases, the follicular fluid concentration of FS was much higher (approximately 100-fold) than that reported in serum. The FS concentrations (ng/ml) were 203 +/- 42 (normal dominant), 185 +/- 17 (atretic cohort), 185 +/- 5 (IVF), and 250 +/- 14 (PCOS). There was no statistical difference between these mean values of FS. Further, there were no significant correlations between the follicular fluid concentrations of FS and the concentrations of oestradiol, progesterone, or androstenedione. These results indicate that human Graafian follicles, regardless of whether they are healthy or atretic, normal or PCOS, contain high steady-state concentrations of FS in the micro-environment. Collectively, these data fit with the hypothesis that major increases and decreases in the concentration of FS in the micro-environment may not play a key role in the mechanisms of selection, atresia, and PCOS in women. The possibility of regulation of intrinsic activin and inhibin activity through FS binding is discussed.

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