Maturation of oocytes from normal and atretic equine ovarian follicles as affected by steroid concentrations.

The research article evaluates the influence of steroid levels on the maturation of oocytes from normal and atretic (degenerating) horse ovarian follicles. It uses follicular histology and hormone concentration analysis to correlate with follicle size, and examines oocyte culture and fertilization outcomes.

Research Methodology

• The study involved the collection of ovaries from 23 horses during April-June. The obtained follicles, ranging from 4-40 mm in diameter, were dissected and punctured to retrieve oocytes for further analysis and culture.
• The follicles were classified based on histology into normal and those indicating primary and secondary atresia, essentially the normal and degenerating types.
• The antral fluid, the fluid within the follicular cavity, was analyzed for the presence of four different types of steroids: oestradiol, testosterone, progesterone, and androstenediol.
• Obtained oocytes were cultured post collection using two types of medium: Medium 199 (Difco OSI, France) or Medium B2.

Observations and Findings

• The researchers found that follicle size and histological state were closely associated with the concentrations of two steroids: oestradiol and testosterone. Notably, no significant correlation with progesterone or androstenediol was found.
• The most successful oocyte maturation, achieving Metaphase 2(an advanced stage of cell division), was observed 36-38 hours after being cultured. Around 53% of the oocytes reached this phase.
• Out of 52 cultured and inseminated oocytes, 1 resulted in a structure similar to a 4-blastomere (an early embryonic stage), 10 developed structures resembling pronuclei (early stages of fertilization), 9 were in Metaphase 1 (a phase of cell division), 14 were in Metaphase 2 and 18 were degenerated.

Overall Conclusions

• The study establishes that the maturation of oocytes in horses, from both normal and degenerating follicles, is influenced by the concentration of certain steroids – oestradiol and testosterone.
• These results highlight the potential importance of steroid hormone regulation in equine reproductive processes. The successful culture and maturation of oocytes provide useful insights into equine reproduction and could bear significant implications for enhancing equine fertility treatments.