Abstract

Vitrification might be the best option to cryopreserve the immature cat oocyte because our recent studies have demonstrated that COCs are (1) highly sensitive to cryoprotectant (CPA) and require short-term exposure to prevent toxicity, and yet (2) resistant to extreme hyperosmotic conditions when pre-incubated in the cytoskeleton stabilizer cytochalasin B (CB). However, surrounding cumulus cells may inhibit the necessary rapid transport of water and CPA across the oocyte membrane during short-term exposure to vitrification solutions (VS) containing high CPA concentrations. The objective was to examine the influence of cumulus cells on oocyte survival during vitrification with or without pre-incubation in CB. Our metric of focus was the chromatin status after warming and IVM. Grade I immature oocytes (n = 420, 4 replicates) were equally allocated to one of four treatments: T1 (removal of cumulus cells in 0.2% hyaluronidase); T2 (pre-incubation in 7.5 µg mL^–1 CB for 20 min); T3 (removal of cumulus cells and pre-incubation in CB (as above)); and T4 (control, no removal of cumulus cells and no pre-incubation in CB). After each treatment, half of the oocytes was washed and immediately cultured for IVM (28 h; denuded oocytes being co-cultured with an equal number of fresh COCs to circumvent the absence of cumulus cells). The other half was exposed to 10% (v/v) ethylene glycol (EG) + 10% (v/v) DMSO for 30 s, followed by exposure to VS (20% EG + 20% DMSO + 0.5 m sucrose) for 20 s at room temperature. Oocytes then were vitrified in <1 µL of VS at the tip of a plastic gutter (2 oocytes/gutter) by direct plunge into liquid nitrogen. After 1 day of storage, vitrified oocytes were warmed in 0.25 m sucrose, extensively washed, and cultured for IVM as described above. All oocytes were then fixed and Hoechst-stained to assess their chromatin status. Oocytes subjected to T1, T2, T3, and T4 without vitrification exhibited no degenerated chromatin (clumped or dispersed) after IVM. After vitrification, the incidence of degenerated chromatin was different (P < 0.05; ANOVA) among treatments, with a lower percentage (P < 0.05) when oocytes were subjected to T3 (15.5 ± 3.9%; mean ± SD) compared to T1 (26.6 ± 3.1%), T2 (48.9 ± 3.8%), or T4 (71.1 ± 3.8%). The percentage of vitrified oocytes reaching the metaphase II stage (MII) also was different (P < 0.05) among treatments with a higher incidence (P < 0.05) in the T3 group (57.8 ± 3.9%) compared to T1 (44.6 ± 3.7%), T2 (31.1 ± 7.7%), or T4 (15.5 ± 3.9%) counterparts. Percentages of non-vitrified oocytes reaching the MII were not different (P > 0.05) among treatments (range, 84.7 to 89.0%) but were higher (P < 0.05) compared to vitrified oocytes. The combination of removal of cumulus cells and pre-incubation in CB prior to vitrification had a cumulative, beneficial influence on cat oocyte survival. Interestingly, removal of cumulus cells imparted a greater benefit on oocyte survival than CB pre-incubation alone.