In vitro oocyte maturation in a medium containing reduced sodium chloride improves the developmental competence of pig oocytes after parthenogenesis and somatic cell nuclear transfer.

The present study investigated the effects of IVM in hypotonic medium containing reduced (61.6 mM) NaCl compared with isotonic medium containing 108.0mM NaCl (designated L and N respectively) on oocyte maturation and embryonic development in pigs. IVM culture was divided into four periods at 11-h intervals. Oocytes cultured in N for 33 h and then in L for 11 h of IVM (N-N-N-L) showed significantly improved (P < 0.05) nuclear maturation of oocytes (75.4-79.0% vs 60.2-85.8%) and blastocyst formation (61.5-66.1% vs 45.2-67.5%) after parthenogenesis (PA) compared with other treatments (L-L-L-L, L-L-L-N, L-L-N-L, N-N-L-L, N-N-L-N, L-L-N-L, L-N-N-L and N-L-N-L). Oocytes matured in L-L-L-L and N-N-N-L had an increased (P < 0.05) perivitelline space (11.0-12.5 vs 5.5 mu m) and intraoocyte reduced glutathione (GSH) content (1.39-1.41 vs 1.00 pixels per oocyte) relative to oocytes matured in N-N-N-N. Somatic cell nuclear transfer (SCNT) embryos derived from the N-N-N-L treatment had significantly (P < 0.05) higher blastocyst formation (53.5%) than embryos derived from Medium-199 (37.4%) and N-N-N-N (41.8%) treatments. Overall, the results demonstrate that maturation of pig oocytes in hypotonic medium with reduced NaCl during the last 11 h of IVM increases the developmental competence of oocytes after PA and SCNT by improving the cytoplasmic microenvironment, including an increased GSH content in IVM oocytes.