Dynamics of Human Follicle Development

In 1979 the first report appeared on follicular growth during the normal menstrual cycle as determined by ultrasonography (1). Only cystic struc­ tures above 10 mm in size were regarded as follicles. During the days be­ fore the luteinizing hormone (LH) peak, follicle size and serum estradiol levels were strongly correlated. It was also noticed that growth of the follicle was linear, progressing 1.5-3 mm each day (2). Subsequently, numerous studies have focused on follicular growth and the diameter of the graafian follicle before disappearance (i.e., ovulation). Accuracy of sonographic estimates of follicle size was documented by comparison with the diameter derived from follicular volumes obtained during laparoscopic oocyte collection (3). Introduction of the transvaginal route was a major step toward better resolution (4, 5). Using this technique, follicles as small as 2 mm in diameter could be quantified reliably (6), which allows careful monitoring of the dynamics of development of dominant and nondominant follicles. Data are accumulating underlining the significance of local (intra­ ovarian) modification of gonadotropin action for regulation of gonadal function under normal, pathological, and pharmacological conditions. Awareness is growing that endocrine estimates in peripheral blood alone cannot explain all the conditions that clinicians are faced with. It appears that the classical concept of endocrine regulation of follicular develop­ ment should be revised. In general, the stage of development of the follicle is represented by its size, which can be estimated by ultrasound. Follicle development is believed to be primarily regulated by follicle stimulating hormone (FSH) -induced granulosa cell aromatase activity. Regulation and function of these cells could be indicated by estimates in follicular fluid. It should therefore be possible to obtain additional information on the regulation of follicle development in the human by careful monitoring of intra ovarian changes using transvaginal sonography