Equine oocyte competence for nuclear and cytoplasmic in vitro maturation: effect of follicle size and hormonal environment.

The research explores the effectiveness of equine oocyte in vitro maturation influenced by the hormonal environment and follicle size, indicating a positive correlation between follicle size and the rate of nuclear and cytoplasmic maturation, with further enhanced maturation rate observed in certain stages of the estrous cycle.

Research Objective

• Investigating the competence of equine oocyte following in vitro maturation (IVM) was the key concern of this research. The scientists approached this by studying the effect of two major factors: the size of the follicle from where the oocyte originated and the stage of the estrous cycle.

Research Methodology

• The team used three criteria to evaluate the maturation: nuclear stage, meiotic spindle morphology, and cortical granule localization. These were assessed after DNA Hoechst staining, tubulin immunocytochemical staining, and lectin labeling, respectively.
• Carrying out seven sequential in vivo ultrasound-guided punctures on ten saddle mares, the team gathered data at the end of the follicular phase (after ovulation induction through gonadotropin injection) and during the mid-luteal phase (with or without a gonadotropin injection).
• The researchers stained the cumulus-oocyte complexes (COCs) that were expanded at collection, and the compact COCs after they had undergone in vitro culture, and observed them under a confocal microscope.

Research Findings

• Through the punctures, an average of 0.9 preovulatory COCs and 8 immature COCs were collected per mare every 22 days.
• Among the preovulatory oocytes, 55% had completed both nuclear and cytoplasmic maturation, with 86% of them displaying a regular meiotic spindle.
• Out of the 262 oocytes cultured in vitro, 37% completed nuclear maturation. Nuclear and cytoplasmic maturation rates were found to increase significantly with follicle diameter. There also seemed to be a higher IVM rate in the follicular phase and an increase in the luteal phase with the injection of gonadotropin.
• The research also found no significant difference in meiotic spindle morphology between different classes of follicular diameters.

Research Significance

• This study offers a way to increment the counts of characterized oocytes collected per cycle and per mare. Notably, it is the first research showing the progressive acquisition of meiotic competence in the equine oocyte during antral follicle growth and provides the only description of the meiotic spindle in horses.