68 Characteristics of guinea pig oocytes at two stages of the estrous cycle on lipid distribution and apoptosis rate

Functional characterisation of guinea pig oocytes is important for the future application of assisted reproduction techniques in this species. This study aimed to assess the effect of two oestrous cycle stages (ECS) on lipid distribution and apoptosis rate. Guinea-pigs (n = 40) were synchronized with oral progesterone (0.22 mg/kg per day) for 15 days. Cumulus cell–oocyte complexes (COCs, n = 274) were collected from the ovaries by slicing on Days 2 (periovulatory; n = 20) and 9 (diestrus; n = 20), after oral progesterone was suspended. According to the appearance of the cytoplasm and cumulus cells (Cu), the COCs were classified into A (homogeneous cytoplasm and more than four layers of Cu), B (homogeneous or slightly heterogeneous cytoplasm and less than four layers of Cu or partially denuded), and C (very heterogeneous cytoplasm, with no Cu or with expanded Cu). Lipid distribution was determined with LipidSpot™488 (Biotium, #70065), and the rate of apoptosis was assessed with Apoptotic, Necrotic, and Healthy Cells Quantification Kit (Biotium, #30018). Data were analysed by logistic regression of SAS. Lipid distribution was not affected by either ECS (P = 0.0676) or COC type (P = 0.0616). The apoptosis rate was affected by ECS (P = 0.0005) and COC type (P = 0.0390). No statistical effect of the interaction between ECS and COC type on mitochondrial distribution and apoptosis rate was observed. Lipid distribution and apoptosis rate according to ECS and COC type are shown in Table 1. Regardless of ESC, the homogeneous and central distribution of lipids was similar among COC categories. The percentage of apoptotic oocytes increased as oocyte quality decreased; however, the difference was statistically significant in diestrus only between oocytes B and C. In conclusion, in diestrus, there was a greater proportion of apoptotic oocytes and a greater homogeneous distribution of lipids in the cytoplasm.