Factors affecting folliculogenesis in ruminants

This review addresses the reasons for the lack of progress in the control o f superovulation and highlights the importance of understanding the mechani sms underlying follicular development. The present inability to provide lar ge numbers of viable embryos from selected females still restricts genetic improvement, whilst variability in ovarian response to hormones limit the p resent capacity for increasing reproductive efficiency. Females are born wi th a large store of eggs which rapidly declines as puberty approaches. If t hese oocytes are normal then there is scope for increasing the reproductive potential of selected females. Oocytes must reach a certain size before th ey can complete all stages of development and the final changes that occur late in follicular development. It is likely that oocytes that do not produ ce specific factors at precise stages of development will not be viable. He nce, it is important to characterize oocyte secreted factors since there ar e potential indicators of oocyte quality. The mechanisms that determine ovu lation rate have still not been fully elucidated. Indeed follicular atresia , the process whereby follicles regress, is still not known. A better under standing of these processes should prove pivotal for the synchronization of follicular growth, for move precise oestrous synchronization and improved superovulatory response. Nutrition can influence a whole range of reproductive parameters however, t he pathways through which nutrition acts have not been fully elucidated. Me tabolic hormones, particularly insulin and IGFs, appear to interact with go nadotrophins at the level of the gonads. Certainly gonadotropins provide th e primary drive for the growth of follicles in the later stages of developm ent and both insulin and IGF-1, possibly IGF-2, synergize with gonadotrophi ns to stimulate cell proliferation and hormone production. More research is required to determine the effects of other growth factors and their intera ction with gonadotropins. There is evidence, particularly from studies with rodents, that steroids can also modulate follicular growth and development , although information is very limited for ruminants. There may be a role f or oestrogens in synchronizing follicular waves, to aid in oestrous synchro nization regimes and for removing the dominant follicle to achieve improved superovulatory responses. However more information is required to determin e whether these are feasible approaches. Heritability for litter size is higher in sheep than in cattle. Exogenous g onadotropins are a commercially ineffective means of inducing twinning in s heep and cattle. Although there are differences in circulating gonadotropin concentrations, the mechanism(s) responsible for the high ovulation appear to reside essentially within the ovaries. The locus of the Booroola gene, a major gene for ovulation rate, has been established but not specifically identified. However sheep possessing major genes do provide extremely valua ble models for investigating the mechanisms controlling ovulation rate, inc luding a direct contrast to mono-ovulatory species such as cattle. In conclusion, the relationship between oocyte quality, in both healthy fol licles and those follicles destined for atresia, must be resolved before th e future potential for increasing embryo yield can be predicted. In additio n, a greater understanding of the factors affecting folliculogenesis in rum inants should ensure that the full benefits ensuing from the precise contro l of ovarian function are achieved. The improved use of artificial insemina tion and embryo transfer that would ensue from a greater understanding of t he processes of folliculogenesis, coupled with the new technologies of geno me and linkage mapping, should ensure a more rapid rate of genetic gain.