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 fo llicles greater than or equal to 4 mm in diameter were recorded, the n umber of ovarian follicles greater than or equal to 2 mm in diameter c ounted, and blood samples collected daily for periods of 18 days, star ting 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 follicu lar development occurred in a wave-like manner, as in mature cattle. T he maximum diameter of the dominant and the largest subordinate follic les increased between 2 and 34 weeks of age (P < 0.05); however, the g reatest increase occurred between 2 and 8 weeks of age. There was a si milar increase in the numbers of small and large ovarian follicles (P < 0.05). The duration of detection of dominant follicles (number of da ys visible at a diameter of greater than or equal to 4 mm) also increa sed between 2 and 34 weeks of age (P < 0.05). The emergence of waves o f follicular development was preceded by peaks in plasma FSH concentra tions (P < 0.05) at 2 weeks of age but this was less clear at other ag es. There was a rise in circulating concentrations of gonadotrophins b etween 4 and 14 weeks of age. We concluded that in heifer calves as yo ung as 2 weeks of age ovarian follicles grew in a wave-like fashion, s imilar to those of adult cattle. We speculate that the early rise in g onadotrophin secretion stimulated the increase in numbers of follicles and follicle diameters observed, indicating an early critical step in reproductive development.