Ovary

Developmental Pattern of Small Antral Follicles in the Bovine Ovary

Abstract
The study was designed to characterize the developmental pattern of 1-3 mm follicles and to determine the stage at which the future dominant follicle first attains a size advantage among its cohorts. In Experiment 1, heifers (n = 18) were examined every 24 h by transrectal ultrasonography for one interovulatory interval (IOI). In Experiment 2, cows (n = 9) were examined every 6 h from Days 5 to 13 (Day 0 = ovulation) to monitor precisely the diameter changes of individual follicles 1 mm during emergence of Wave 2. Results revealed a change over days (P < 0.05) in the number of 1-3 mm follicles, with a peak (P < 0.05) 1 or 2 days before wave emergence (conventionally defined as the time when the dominant follicle is first detected at 4 mm), followed 3-4 days later by a peak (P < 0.05) in the number of 4 mm follicles. The profiles of small (1-3 mm) and large ( 4 mm) follicles were inversely proportional (r = -0.79; P = 0.01). The profile of the number of 1-3 mm follicles during wave emergence was similar (P = 0.63) between waves in 2-wave IOI, but differed (P < 0.01) among waves in 3-wave IOI as a result of a greater number of follicles in the ovulatory wave (P < 0.04). As well, the number of follicles in the ovulatory wave tended to be greater (P < 0.06) in 3-wave IOI than in 2-wave IOI. The future dominant follicle was first identified at a diameter of 1 mm and emerged 6-12 h earlier than the first subordinate follicle (P < 0.01). After detection of the dominant follicle at 1 mm (0 h), its diameter differed from that of the first and second subordinate follicles at 24 h (P = 0.04) and 12 h (P = 0.01), respectively, when the dominant follicle was 2.4 ± 0.17 mm and 1.7 ± 0.14 mm. The growth rate of the dominant follicle differed from that of the first and second subordinate follicles at 120 h (P = 0.03) and 108 h (P = 0.02), respectively, when the dominant follicle was 9.5 ± 0.30 mm and 8.8 ± 0.49 mm. Emergence of the future dominant (r = 0.71), first (r = 0.73), and second (r = 0.76) subordinate follicles was temporally associated (P < 0.01) with a rise in circulating concentrations of FSH. Transient, nocturnal elevations in plasma FSH concentration were followed within 6 h by an increase in the growth rate of 1-3 mm follicles. We conclude that: 1) 1-3 mm follicles develop in a wave-like manner in association with surges in plasma concentrations of FSH, 2) 1-3 mm follicles are exquisitely responsive to transient elevations in FSH, and 3) selection of the dominant follicle is manifest earlier than previously documented and is characterized by a hierarchical progression over a period encompassing the entire FSH surge (5 days).

Key words: Ovary • Follicle-stimulating hormone • Follicular development

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