Ovarian activity in Booroola x Romney ewes which have a major gene influencing their ovulation rate

Abstract

A marked difference in both the function and composition of individual ovarian follicles was noted in Booroola x Romney ewes (6-7 years of age) which had previously been segregated on at least one ovulation rate record of 3-4 (F + ewes, N = 21) or < 3 (++ ewes, N = 21). Follicles in F + ewes produced oestradiol and reached maturity at a smaller diameter than in ++ ewes. In F + ewes (N = 3), the presumptive preovulatory follicles were 4.4 ± 0.5 (s.e.m.) mm in diameter and contained 2.1 ± 0.3 x 106 (s.e.m.) granulosa cells, whereas in ++ ewes (N = 3), such follicles were 7.3 ± 0.3 mm in diameter and contained 6.5 ± 0.8 x 106 cells. During a prostaglandin (PG)-induced follicular phase, the secretion rate of oestradiol from ovaries containing 3 presumptive preovulatory follicles in F + ewes was similar to that from ovaries with only one such follicle in ++ ewes. We suggest that the putative 'gene effect' in F + ewes is manifested during early follicular development and that it may be mediated via an enhanced sensitivity of granulosa cells to pituitary hormones. As a consequence, the development of 3 preovulatory follicles in F + ewes may be necessary to provide a cell mass capable of producing the same quantity of oestradiol as that from one preovulatory follicle in ++ ewes.