Ovarian follicular fluid contains immunoreactive estriol: lack of correlation with estradiol concentrations

Estradiol and estrone concentrations in ovarian follicular fluid change according to the ovulatory cycle, but no studies on the possible presence and/or changes of estriol are available. The aim of the present study was to evaluate whether estriol is measurable in follicular fluid and how its concentration changes according to the volume of ovarian follicles and to the maturational stage of oocytes. A group of women (n = 39) undergoing a program of induction of ovulation was included in this study and divided into three groups according to the causes of infertility: those with unexplained infertility (n = 11); those with endocrine disturbances (n = 5); and normal ovulatory women (n = 23) (controls). The follicles recruited (n = 116) on the basis of morphology and the appearance of the oocyte cumulus–corona complex were divided into: mature (n = 22); intermediate (n = 75); immature (n = 11); and atretic (n = 8). Ovarian follicles were also divided according to the diameter of each: < 1.5 cm (n = 38); 1.6–2.4 cm (n = 66); and > 2.5 cm (n = 12). Ovarian follicular fluids were aspirated under ultrasound guidance and a blood specimen was collected from each subject. Estriol and estradiol concentrations were evaluated by radioimmunoassay in serum and follicular fluid following an ether extraction. Estriol was found in high concentration in each sample of follicular fluid, significantly higher than in the respective serum sample (p < 0.01). Although the estradiol concentration was significantly lower in follicles containing immature and atretic oocytes than in intermediate or mature follicles (p < 0.01), the estriol concentration did not depend upon the maturational stage. In addition, the follicular fluid estriol concentration did not differ according to the causes of infertility. Follicular fluid and serum estradiol concentrations showed significant correlation (p < 0.01), whereas no significant correlation was observed between serum and follicular estriol concentrations. The present data show that follicular fluid contains a high concentration of estriol and that its changes are independent of the ovulatory cycle and estradiol concentrations, supporting an independent origin and suggesting a different function for estriol.

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