Pure preovulatory follicular fluid promotes in vitro maturation of in vivo aspirated equine oocytes.

This research investigates the possibility of improving the successful maturation and development of horse oocytes (the cells which develop into eggs) in an ‘in vitro’ environment, by closely mimicking their natural, ‘in vivo’ environment. The study evaluates the use of equine preovulatory follicular fluid (fluid from the follicle which surrounds the ovary prior to ovulation) and the influence of equine gonadotrophins and meiosis activating sterols in this process.

Study goals

• The study was designed to explore whether equine oocytes can mature in vitro in pure equine preovulatory follicular fluid. This examination seeks to discover if mimicking the oocyte’s natural environment can increase maturation rates.
• The researchers aimed to determine if ‘priming’ (pre-treating) the follicle fluid donor with crude equine gonadotrophins (hormones that stimulate the growth and maturation of the oocytes) before aspiration could enhance the in vitro maturation process.
• The study also tested if the in vitro maturation rate varied depending on whether the oocytes were aspirated during estrus (the sexually receptive stage of the mare’s reproductive cycle) or aspirated again 8 days after the initial follicular aspiration.
• The investigation additionally analyzed if high follicular concentrations of meiosis activating sterols (chemicals promoting the maturation of the oocyte) were beneficial for the in vitro maturation of equine oocytes.

Methods and results

• The study involved 19 pony mares treated with equine gonadotrophins during estrus. After 24 hours and again after eight days, all follicles above a certain size were aspirated and cultivated in various media for 30 hours, including a standard culture medium and three different preovulatory follicular fluid media.
• Maturity rates significantly varied, both according to the type of media used and whether the aspiration (removal of the follicle) was conducted at the first or later point.
• The results demonstrated that equine oocytes can mature in vitro in pure equine preovulatory follicular fluid. Moreover, fluid collected after hormone-priming is more effective at supporting in vitro maturation than a standard culture medium.
• Oocytes aspirated 8 days after a previous aspiration were less effective candidates for successful in vitro maturation.
• The occurrence of meiotic resumption (spontaneous continuation of cell division which was halted earlier) during in vitro culture seems related to the presence of a 200-kDa protein, indicating a biochemical determinant of oocyte maturation.