The Effect of Different Vitrification and Staining Protocols on the Visibility of the Nuclear Maturation Stage of Equine Oocytes.

This research investigates the best method for preserving horse eggs or oocytes for in vitro fertilization, focusing on different cryopreservation and staining techniques. The findings indicate that a specific staining protocol is more effective and that using certain protective solutions during freezing process results in more eggs reaching the necessary stage for fertilization.

Study Design and Methods

• The scientists sourced 155 oocytes from 32 mares from a slaughterhouse.
• They first matured the oocytes in a specific medium (M199) for 42 hours under controlled temperature and atmospheric conditions.
• For the first experiment, they tested two levels (10 μg/mL and 2.5 μg/mL) of a stain, Hoechst 33342 (HO), in combination with a 50 μg/mL concentration of a second stain, propidium iodide, to see how clearly the oocytes could be observed under a microscope.
• In the second experiment, they applied the staining protocol with the lower HO concentration to evaluate 111 oocytes. They observed the oocytes before and after vitrification (a rapid-freezing process) using two different protective solutions (cryoprotectants).

Key Findings

• The staining protocol with the lower concentration of HO (2.5 μg/mL) allowed the scientists to observe a higher percentage of matured oocytes.
• The results of the second experiment showed that none of the oocytes frozen without a permeable cryoprotectant reached the metaphase II maturation stage required for fertilization, a significantly poorer outcome than both the conventional vitrification method and the non-cryopreserved control group.

Conclusions

• The study concluded that freezing horse oocytes without the use of permeable cryoprotectants achieved unsatisfactory results and therefore cannot replace the traditional vitrification method that uses such protective solutions.
• The researchers also recommend using a lower concentration of the HO stain (2.5 μg/mL) to check the maturity of horse oocytes after in vitro maturation.