Developmental competence of IVM pig oocytes after SCNT in relation to the shrinkage pattern induced by hyperosmotic treatment

The objective of this study was to examine the developmental competence of IVM pig oocytes in relation to the pattern of morphologic changes after exposure to hyperosmotic medium to select oocytes of a higher quality. IVM oocytes were treated with a hyperosmotic (593 mOsm) medium containing NaCl, sorbitol, or sucrose. Oocytes that shrunk spherically (SSP oocytes) or in irregular shapes (SIR oocytes) were collected separately, and washed for 15 minutes in an isotonic (297 mOsm) medium for recovery. Irrespective of the chemicals used, hyperosmotic treatment of oocytes for 1 hour or 15 minutes did not alter embryonic development after parthenogenesis (PA) and SCNT. A significantly higher proportion of SSP oocytes developed to the blastocyst stage (34.0%) compared with SIR oocytes (15.8%) after PA. The intracellular glutathione content was significantly higher in SSP oocytes than in SIR oocytes. Conversely, the reactive oxygen species level was significantly higher in SIR oocytes than in SSP oocytes. The maturation promoting factor level as measured by p34cdc2 kinase activity was not influenced by hyperosmotic treatment itself but was 1.3-fold higher (P < 0.05) in SSP oocytes than in SIR oocytes. When IVM oocytes were divided into two groups according to their diameters (large and small), and treated separately in hyperosmotic medium, significantly more SSP oocytes (71.4%) were found in the large oocytes than in the small oocytes (51.4%). Moreover, the proportion of metaphase II oocytes was significantly higher in SSP oocytes than in SIR oocytes in both groups (98.5% vs. 73.1% in large oocytes, and 92.2% vs. 48.0% in small oocytes). After SCNT, a significantly higher proportion of SSP oocytes displayed blastocyst formation (36.4%) than untreated (29.0%) and SIR oocytes (22.1%). Our results demonstrated that SSP oocytes were of a higher quality than SIR oocytes, which was shown by higher intracellular glutathione and maturation promoting factor levels, lower reactive oxygen species levels, and improved embryonic development to the blastocyst stage after PA and SCNT.