The effects of cooling mouse oocytes

Area of Science:

• Reproductive Biology
• Cell Biology
• Cryobiology

Background:

• Meiotic spindle integrity is crucial for oocyte viability and successful fertilization.
• Understanding the effects of temperature stress on oocytes is vital for improving cryopreservation techniques.

Purpose of the Study:

• To investigate the impact of various cooling and rewarming temperatures on the meiotic spindle structure and cytoplasmic organization of mouse oocytes.
• To compare the response of mouse oocytes to cooling with previously reported data for human oocytes.

Main Methods:

• Transmission electron microscopy was used to examine intact cumulus-oocyte complexes.
• Oocytes were cooled to 15°C, 4°C, 0°C, and -7°C, with some subsequently rewarmed to 37°C.
• Spindle microtubules, chromosomes, centrosomal material, kinetochores, and cytoplasmic organelles were analyzed.

Main Results:

• Complete disappearance of spindle microtubules occurred at 0°C and -7°C; depolymerization was observed at 4°C.
• Cooling to 15°C caused minimal spindle disruption, while chromosome clumping occurred at lower temperatures.
• Centrosomal material fragmented at 4°C and 15°C, and was absent at 0°C and -7°C.
• Ooplasmic changes included endoplasmic reticulum disruption and fibrillar inclusion disappearance, largely reversible upon rewarming.
• Spindle structure was mostly restored after rewarming to 37°C.

Conclusions:

• Mouse oocyte meiotic spindles are sensitive to cold temperatures, with significant microtubule depolymerization below 4°C.
• While cooling induces cytoplasmic alterations, rewarming can largely reverse these effects, preserving spindle structure in many cases.
• These findings provide insights into the thermal sensitivity of oocytes and inform strategies for assisted reproductive technologies.