OVARIAN FOLLICULAR DYNAMICS DURING ANESTRUS IN EWES

The aim of the present study was to document ovarian antral follicle d ynamics throughout seasonal anoestrus in sheep. Daily transrectal ultr asonography was performed during four 17 day scanning periods from Mar ch to July in Western White-faced crossbred ewes. Blood samples were c ollected each day with ultrasonographic scanning for measurement of se rum concentrations of FSH, oestradioI and progesterone. Blood samples were also taken every 15 min for 6 h, mid-way through each period of u ltrasonographic examination, to determine the patterns of secretion of gonadotrophic hormones. Hormonal data were then related to observed c hanges in follicular populations and the patterns of antral ovarian fo llicle turnover. Ultrasonography showed that the ovaries of anoestrous ewes remained active and that the largest ovarian antral follicles gr ew to a periovulatory size (greater than or equal to 5 mm in diameter) at all stages of anoestrus. The total number of all ovarian follicles greater than or equal to 3 mm in diameter was lower during early anoe strus compared with at mid-anoestrus because of a significantly smalle r number of small (3 mm) and medium (4 mm) ovarian follicles. The larg est ovarian follicles (attaining greater than or equal to 5 mm in diam eter before regression) exhibited a wave-like pattern of growth; an av erage of three waves of follicular development were recorded in sheep during each of the four 17 day scanning periods in anoestrus, with fol licular waves emerging approximately every 5 days. This rhythmic patte rn of follicular emergence was found to be associated with the occurre nce of fluctuations in serum FSH concentrations. The growth rate of th e largest follicles of the wave increased significantly from early to late anoestrus in sheep. In addition, ovarian follicles not growing be yond 3 mm in diameter showed organized patterns of growth and regressi on; their numbers tended to be lower (P = 0.09) at 3 days before and o n the day of follicular wave emergence. Some ewes were seen to maintai n synthesis of progesterone throughout anoestrus. This submaximal prog esterone secretion tended to occur at irregular intervals and was not coupled with changes in concentrations or patterns of gonadotrophin re lease, ovulations or detectable morphological luteinization of ovarian antral follicles. It was concluded that the growth of ovarian antral follicles to an ovulatory size was maintained throughout anoestrus in ewes, with a transient shift in the number of small and medium-sized f ollicles during mid-anoestrus, and that the periodic emergence of wave s of large follicles (25 mm in diameter) occurred in synchrony with an endogenous rhythm of FSH secretion.