8 Characteristics of guinea pig (Cavia porcellus) oocytes at two stages of the estrous cycle on nuclear status and mitochondrial distribution

Functional characterisation of guinea pig oocytes is essential for the subsequent application of assisted reproduction techniques in this species. This study aimed to assess the effect of two stages of the oestrous cycle (ECS) on nuclear status, and on distribution (MD) and aggregation of mitochondria (MA). Guinea-pigs (n = 40) were synchronized with oral progesterone (0.22 mg/kg per day) for 15 days. Cumulus cell–oocyte complexes (COCs, n = 942) were collected on Days 2 (periovulatory period; n = 20) and 9 (diestrus; n = 20) after oral progesterone was suspended. According to the characteristics of the cytoplasm and cumulus cells, the COCs were classified into three categories: A, B, and C (Hawk and Wall 1994 Theriogenology 41, 1571–1583). To assess nuclear stage, oocytes were fixed in 2% glutaraldehyde and stained in bisbenzimide (1 mg/mL; Hoechst 33342). The fluorescent probe BioTracker 488 Green Mitochondria Dye (SCT136, MilliporeSigma) was used to evaluate the distribution and aggregation of mitochondria in the cytoplasm. Data were analysed by logistic regression of SAS. Nuclear status was not affected by ECS (P = 0.5003) nor COC type (P = 0.4029). An effect of ECS and COC type was observed in MD (P < 0.01) and MA (P < 0.01). Proportion of oocytes in MI, and oocytes with diffuse mitochondrial distribution and homogeneous mitochondrial aggregation are depicted in Table 1. Regardless of the ECS, the proportion of MI oocytes was similar among COC types. In both ECS there was a greater proportion of A than B and C oocytes with a diffuse distribution and homogeneous aggregation of mitochondria. In both ECS, type A and B COCs had better quality and seem to be competent for in vitro maturation.