Prenatal survival in relation to peri-ovulatory phenomena and the site of ovulation in sheep: A review

Abstract

Differences in ovarian follicular dynamics between ewe breeds and between lines within breeds were shown to result from a different follicle recruitment and selection process. Synchrony and asynchrony of embryos were shown to depend largely on luteinizing hormone (LH), progesterone and oestrogen levels in ewes before, during and post-mating. GnRH therapy suggested that the endocrine environment prior to ovulation has a fundamental influence on luteal function and hence on prenatal survival (PS). The decreasing effect of multiple ovulation on uterine efficiency was higher in androstenedione-immune Border Leicester×Merino (BL×M) than in immunized BL×Booroola Merino (BM) ewes. Exogenous progesterone increased lambing rate in Polypay (P) but not in Targhee ewes. The 200% of maintenance feeding level reduced plasma progesterone concentrations on day 12 near the threshold critical for embryo survival in M ewes but not in BL×Scottish Blackface ewes. The latter maintained pregnancy rate (PR) and PS at the same levels as the controls. In unilaterally twin ovulating ewes, foetal mortality was significantly lower in ewes in which distributive embryo migration occurred. Within this ovarian class, survival was lower in the Cheviot, probably because of a lower migration frequency, than in the Blackface breed. More embryos migrated in unilaterally twin ovulating M ewes in autumn than in summer mating, with a higher incidence of loss of one embryo in the latter season. A higher incidence of successful migration was observed in unilaterally twin ovulating BL×M than in M ewes. The higher mortality of a migrating embryo, compared with the nonmigrating, was attributed to an asynchronous environment in the contralateral horn, since ovine trophoblastine (oTP-1) might have preferentially altered the uterine environment of the ipsilateral horn. Studies on oTP-1 and protein in uterine flushings and on conceptus development at day 13 in ewes that displayed short or long oestrous cycles, as well as further studies on unilaterally ovariectomized ewes with two or three conceptuses, suggest that the ovine embryo stimulates the migratory process locally, in interaction with the dam. This review raises the question whether and to what degree pre-, peri-, as well as, post-ovulatory phenomena and the site of ovulation can be interacting factors for determining PS, especially in multiple ovulating sheep as a function of breed and breed–environment interactions.