Oocyte maturation and ovum quality in pigs.

Oocyte quality in pigs is defined as the potential of that oocyte to develop into a viable offspring. There is increasing evidence that the programming of the oocyte must be completed before leaving the ovarian follicle, including both nuclear and cytoplasmic maturation. Pig oocytes matured in vitro under basic conditions are deficient in some, as yet unidentified, cytoplasmic factors and thus developmentally incompetent. This developmental incompetence can be overcome to some extent by follicular supplementation (with follicular fluid or granulosa cells) of the culture medium, emphasizing the importance of somatic signals during oocyte maturation. Furthermore, evidence is accumulating that the status of the follicle has a critical impact on the competence of the oocyte in vitro and in vivo and this has been demonstrated by co-culture with follicles at different maturational stages or from breeds with enhanced embryo survival. It now also appears that manipulation of maternal nutrition before mating or oocyte collection can enhance embryo survival in vivo and oocyte developmental competence in vitro, presumably by altering follicular secretions and hence the environment in which the oocyte is nurtured. Identification of both the key follicular factors influencing pig oocyte quality and reliable markers of oocyte quality will undoubtedly yield major improvements in embryo survival in vivo and embryo production in vitro.