297 EFFECT OF CUMULUS CELL REMOVAL DURING IN VITRO MATURATION OF PORCINE CUMULUS–OOCYTE COMPLEXES ON THE APOPTOTIC STATUS AND MEIOTIC PROGRESSION OF THE OOCYTES

Abstract

This study was undertaken to examine the apoptotic status and meiotic progression of oocytes from small (SF) and medium follicles (MF) when the oocytes were denuded from cumulus cells (CC) before, during and after culture for in vitro maturation (IVM). Cumulus-oocyte complexes (COC) were aspirated from SF (0.5–2 mm in diameter) or MF (3–6 mm in diameter) of slaughtered prepubertal gilt ovaries. Only COC with a good morphology of the surrounding cumulus cells were cultured for IVM in modified porcine oocyte medium supplemented with 50 µM β-mercaptoethanol, 1 mM dibutyryl c-AMP, 10 IU mL^–1 of eCG, and 10 IU mL^–1 of hCG for 20 h at 39°C and 5% CO₂ in air and then continued culture in the absence of dibutyryl c-AMP, eCG, and hCG in the same medium for another 24 h. Before and 20 h after the start of IVM culture, some of the oocytes were denuded of CC and the oocytes continued the IVM culture. After IVM culture, oocyte viability and meiotic progression were examined by the annexin V/PI viability assay and DAPI staining. Statistical analyses of 5 replicate data were performed with a 2-way ANOVA and a Tukey's multiple comparisons test. Before IVM culture, there was no significant difference between the viability of SF and MF oocytes, but the incidence of oocytes at the GV₀ stage was higher in specimens from SF than MF (24.8 v. 3.3%), and that of oocytes at the GV_I stage was the opposite (57.8 in MF v. 22.7% in SF). After IVM culture, apoptotic status of oocytes was only affected by the decumulation timing. The percentage of normal live oocytes was significantly higher when CC were removed after 20 and 44 h of IVM in both SF (39.7 and 39.3 v. 17.7%) and MF (45.4 and 37 v. 22.2%). The incidence of early and late apoptotic oocytes was significantly higher when the CC were removed before IVM culture in both SF (74.3 and 7.4%) and MF (69.4 and 6.7%). The incidence of mature live oocytes was significantly affected by both the origin of COC and the decumulation timing. Although the percentage of mature oocytes was higher in MF, maturation rates were significantly higher when oocytes were denuded at 20 h of IVM culture (SF 65.4%, MF 83.1%) as compared at 0 (SF 27.9%, MF 32.3%) and 44 h (SF 41%, MF 68.5%). However, the percentage of oocytes with normal spindle morphology was significantly higher when oocytes were denuded at 44 h of IVM culture (SF 70.6%, MF 91.5%) than 20 h (SF 66.8%, MF 73%). In summary, regardless of COC from SF and MF, removal of CC at 20 h of IVM culture seems to promote meiotic progression of the oocytes to the MII stage, but factor(s) from or communication with CC during the latter half of IVM culture may be needed to obtain a normal spindle morphology in mature oocytes.