Follicle selection in sheep with breed differences in ovulation rate

Abstract

Genetic variations in ovulation rates which occur in different breeds of sheep provide useful models to explore the mechanisms underlying the antral of follicle development. Two principal strategies have evolved which enable these high fecundity breeds to over-ride the mechanisms which drastically restrict the number of ovulatory follicles. Firstly, Finnish Landrace involves reducing the sensitivity of the hypothalamic–pituitary unit to the feedback effects of ovarian hormones (inhibin and/or oestradiol) which normally suppress FSH. Treatment with oestrogens or immunisation against oestradiol or inhibin are pharmacological methods of manipulating the feedback system. The second mechanism as exemplified by the Fec B gene results in an increased ovulation rate associated with the ‘precocious’ development of a large number of small antral follicles. The gene mutation responsible for this effect has not yet been identified. It is likely that the product will be a factor which enhances follicular development as well as affecting gonadotrophic function. Identification of the gene product should permit further research into the factors which regulate the orderly development of follicles within the ovary.