The first ovulation induced by male effect in sheep during seasonal anoestrus usually results in the development of a short cycle that can be avoided by progesterone priming before ram introduction. In elucidating the involvement of the hypothalamic-pituitary-gonadal axis on the occurrence of short cycles, the effects of progesterone and the time of anoestrus on the development of male-induced preovulatory follicles were investigated in anoestrus ewes using morphological, endocrine and molecular approaches. Ewes were primed with progesterone for 2 (CIDR-2) or 12 days (CIDR-12) and untreated ewes used as controls during early (April) and late (June) anoestrus. The duration of follicular growth and the lifespan of the male-induced preovulatory follicles were prolonged by about 1.6 days in CIDR-12 ewes compared to the controls. These changes were accompanied by a delay in the preovulatory LH and FSH surges and ovulation. Intrafollicular oestradiol concentration and mRNA levels of LHCGR and STAR in the granulosa and theca cells of the preovulatory follicles were higher in CIDR-12 than the control ewes. The expression of mRNA levels of CYP11A1 and CYP17A1 also increased in theca cells of CIDR-12 ewes. CIDR-2 ewes gave intermediate results. Moreover, ewes ovulated earlier in June than April, without changes in the duration of follicular growth, but these effects were unrelated to the lifespan of corpus luteum. Our results give the first evidence supporting the positive effect of progesterone priming on the completion of growth and maturation of preovulatory follicles induced by male effect in seasonal anoestrus ewes, thereby preventing short cycles.
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