Abstract:
Knowledge about the developmental pattern of small follicles (<4�mm) will be crucial to understanding the endogenous control of folliculogenesis and to developing methods to control it for clinical purposes. This study was designed to characterize the developmental pattern of 1-3�mm follicles and to determine, retrospectively, the stage at which the future dominant follicle first attains a size advantage among follicles in the cohort. In Experiment 1, the ovaries of Hereford-cross heifers (n�=�18) were examined daily by high resolution transrectal ultrasonography for one natural interovulatory interval to evaluate changes in the number of 1-3-mm follicles in relation to the wave pattern of follicles e4�mm. Interovulatory interval was divided into 2- and 3-wave based on number of waves exhibited by heifers. In Experiment 2, the ovaries of Hereford-cross cows (n�=�9) were examined every 6�h from Day 5 to Day 13 (Day 0�=�ovulation) to monitor precisely the diameter changes of individual follicles e1�mm during emergence of the second follicular wave. Data were analyzed by Proc. Mixed procedure for repeated measures (Littell RC et al., 2000 Stat in med 19, 1793-1819) in the Statistical Analysis System software package (SAS version 8.2 for MS Windows;; 2002 SAS Institute Inc. Cary, NC, USA). Results of Experiment 1 revealed a day effect (P�<�0.05) on the number of small (1-3�mm) follicles, with a peak (P�<�0.05) 1 or 2 days before wave emergence (defined as the day the dominant follicle was first detected at 4�mm), followed 3 to 4 days later by a peak (P�<�0.05) in the number of large follicles (e4�mm). There was an inverse relationship between the number of small and large follicles during Wave 1 (r�=�-0.66; P�=�0.05) and Wave 2 (r�=�-0.62; P�=�0.04) in 2-wave interovulatory intervals. Similarly, an inverse relation was detected between the number of small and large follicles for Wave 1 (r�=�-0.79; P�=�0.01) and Wave 3 (r�=�-0.90; P�<�0.01) but not for Wave 2 (r�=�-0.57; P�=�0.14) in 3-wave interovulatory intervals. The number of 1-3-mm follicles detected in anovulatory waves did not differ (P�=�0.53) between 2- versus 3-wave interovulatory intervals;; however, a difference (P�<�0.05) was observed between anovulatory and ovulatory waves in 3-wave interovulatory intervals but not (P�=�0.63) in 2-wave interovulatory intervals. Experiment 2 permitted the identification of the future dominant follicle at a diameter of 1�mm and its emergence at 6-12�h earlier than the largest subordinate follicle (P�<�0.01). Emergence of the future dominant (r�=�0.71; P�=�0.05) and 1st subordinate (r�=�0.78; P�=�0.02) follicles was temporally associated with a wave-eliciting rise in circulating concentrations of FSH. Growth rate of the dominant and the 1st subordinate follicle differed (P�<�0.01) from 2nd subordinate follicle at 84�h after their detection at 1�mm. It was concluded that small antral follicles (1-3�mm) develop in a wave-like manner, and apparent selection of the dominant follicle was manifest much earlier than previously documented.