Follicular dynamics and ovarian steroid secretion in sheep during anoestrus

The dynamics of ovarian follicular development and its relationship to ovarian and pituitary hormones during seasonal anoestrus were investigated for 10 days in nine ewes with autotransplanted ovaries in a longitudinal study. The size and position in the ovary of individual follicles over 2.5 mm in diameter were recorded by daily ultrasonography. Samples of ovarian and jugular venous blood were collected at intervals of 12 h, before and after a GnRH challenge (2.50 ng GnRH, i.v.) so that basal and LH-stimulated ovarian steroid secretion could be determined. Throughout the experimental period, all animals developed at least one large antral follicle > 5 mm, which secreted increased (P < 0.05) amounts of oestradiol and androstenedione in response to an LH challenge as the diameter of the follicle increased. However, a decrease (P < 0.05) in ovarian steroid secretion preceded any significant change in follicular diameter, indicating a dissociation between morphological and functional stages of dominance in sheep. We conclude that follicular growth and ovarian steroid secretion in sheep occur in wave-like forms, with the ascending and static part of both waves being synchronous but with a decline in steroid secretion preceding any changes in follicular diameter. Therefore, in sheep, follicular size alone is not an adequate parameter to assign dominance, and the secretory status of the follicle at any given time must be taken into account when studying the dynamics of follicular growth.