Reproductive function in prepubertal lambs: ovulation, embryo development and ovarian steroidogenesis.

When 23 10-16 week-old Welsh Mountain lambs were treated with PMSG 19 ovulated, the number of eggs ovulated being directly correlated with the duration of progesterone pretreatment (0-5 +/- 0-29 (S.E.M.) after 3 days; 7-8 +/- 3-47 after 18 days). Injection of HCG at the time of the induced oestrus had no effect on ovulation. The eggs shed from immature ovaries became fertilized and developed normally when tested in the ligated rabbit oviduct for development to the morula stage and by transfer to adult ewes (1 live lamb). Luteal function in lambs with a single CL was similar to that in nonpregnant ewes; progesterone levels in entire lambs with multiple CL and in hysterectomized lambs remained elevated for at least 60 days. The capacity of ovarian follicles from PMSG-primed lambs to secrete oestrogen, testosterone and progesterone in vitro was similar to that of follicles from adult ewes. However, oestrogen production by lamb follicles immediately after explantation was higher than that of adult follicles and the administration of progesterone to lambs before PMSG treatment decreased subsequent follicular testosterone production.