Follicular and hormonal development in prepubertal heifers from 2 to 36 weeks of age

The aim of this study was to characterize changes in ovarian follicle dynamics in relation to changes in hormone secretion in heifer calves from birth to 8 months of age. The position and diameter of ovarian follicles > or = 4 mm in diameter were recorded, the number of ovarian follicles > or = 2 mm in diameter counted, and blood samples collected daily for periods of 18 days, starting at 2, 8, 14, 24 and 34 weeks of age in ten heifers. The mean age at first ovulation was 52.8 +/- 1.6 weeks. At all ages ovarian follicular development occurred in a wave-like manner, as in mature cattle. The maximum diameter of the dominant and the largest subordinate follicles increased between 2 and 34 weeks of age (P < 0.05); however, the greatest increase occurred between 2 and 8 weeks of age. There was a similar increase in the numbers of small and large ovarian follicles (P < 0.05). The duration of detection of dominant follicles (number of days visible at a diameter of > or = 4 mm) also increased between 2 and 34 weeks of age (P < 0.05). The emergence of waves of follicular development was preceded by peaks in plasma FSH concentrations (P < 0.05) at 2 weeks of age but this was less clear at other ages. There was a rise in circulating concentrations of gonadotrophins between 4 and 14 weeks of age. We concluded that in heifer calves as young as 2 weeks of age ovarian follicles grew in a wave-like fashion, similar to those of adult cattle.(ABSTRACT TRUNCATED AT 250 WORDS)