Insulin-like growth factors-I and -II and insulin-like growth factor-binding protein-2 in dominant equine follicles during spring transition and the ovulatory season.

This research observed the role of insulin-like growth factors (IGF) and their binding proteins in the onset of cyclicity in mares, proposing that fluctuating concentrations of these factors in dominant follicles could influence the transition from non-fertile to fertile cycles.

Background of the Study

• The study is based on the understanding that the insulin-like growth factor (IGF) system plays a crucial role in the growth and regression of follicles in mares.
• More specifically, the period bridging seasonal inactivity and cyclicity is marked by periodic growth and reduction of large dominant follicles in many mares.
• The team hypothesized that changes in the IGF system and its binding proteins could regulate the onset of cyclicity in mares.

Methodology

• The researchers collected ovaries from pony mares post-detection of an actively growing 30 mm transitional anovulatory follicle and also during the second or third oestrus of the breeding season.
• The latter collection happens when the preovulatory follicle touched the 30 mm mark.
• The size of dominant follicles at collection time was found to be similar in transition and at oestrus.

Findings

• IGF-I mRNA was found in granulosa cells with some expression in thecal cells, while IGF-II mRNA was specific to the theca layer.
• The expression of IGF-I and -II mRNAs and intrafollicular concentrations of oestradiol were lower in transitional anovulatory follicles than in preovulatory follicles.
• Messenger RNA encoding IGFBP-2 was present in both theca and granulosa layers, with steady-state concentrations of this mRNA found to be more in the preovulatory follicles’ theca.
• The concentrations of IGFBP-2 were higher in transitional follicles than in preovulatory follicles.
• The similarity in circulating concentrations of IGF-I in transitional and cyclic mares suggested that the somatotrophic axis might not play a role in the transition from anovulatory to ovulatory cycles.

Conclusion

• The results support the researchers’ initial hypothesis by suggesting that increased expression of IGF-I and -II mRNAs in preovulatory follicles, along with a decrease in IGFBP-2 concentrations, could increase the bioavailability of intrafollicular IGF in large follicles during the breeding season.
• This implies that intrafollicular IGF bioavailability must reach a certain level before ovulation can occur.