Mitochondrial distribution and adenosine triphosphate content of bovine oocytes before and after in vitro maturation: Correlation with morphological criteria and developmental capacity after in vitro fertilization and culture

In this study we evaluated mitochondrial distribution and ATP content of in dividual bovine oocytes before and after in vitro maturation (IVM). Cumulus -oocyte complexes were classified according to morphological criteria: cate gory 1, homogeneous oocyte cytoplasm, compact multilayered cumulus oophorus ; category 2, cytoplasm with small inhomogeneous areas, more than five laye rs of compact cumulus; category 3, heterogeneous/vacuolated cytoplasm, thre e to five layers of cumulus including small areas of denuded zona pellucida ; category 4, heterogeneous cytoplasm, completely or in great part denuded. In immature oocytes, staining with MitoTracker green revealed mitochondria l clumps in the periphery of the cytoplasm, with a strong homogenous signal in category 1 oocytes, a weaker staining in category 2 oocytes,allocation of mitochondria around vacuoles in category 3 oocytes, and poor staining of mitochondria in category 4 oocytes. After IVM, mitochondrial clumps were a llocated more toward the center, became larger, and stained more intensive in category land 2 oocytes. This was also true for category 3 oocytes; howe ver, mitochondria maintained their perivacuolar distribution. No mitochondr ial reorganization was seen for category 4 oocytes. Before IVM, the average ATP content of category 1 oocytes (1.8 pmol) tended to be higher than that of category 2 oocytes (1.6 pmol) and was significantly (P < 0.01) higher t han in category 3 (1.4 pmol) and 4 oocytes (0.9 pmol). The IVM resulted in a significant (P < 0.01) increase in the average ATP content of all oocyte categories, with no difference between oocytes extruding versus non-extrudi ng a polar body. After in vitro fertilization (IVF) and culture, significan tly (P < 0.05) more category 1 and 2 than category 3 and 4 oocytes develope d to the morula or blastocyst stage (determined 168 h after IVF). Total cel l numbers of expanded blastocysts derived from category 1 and 2 oocytes wer e significantly (P < 0.05) higher than of those originating from category 3 and 4 oocytes. These data indicate that mitochondrial reorganization and A TP levels are different between morphologically good and poor oocytes and m ay be responsible for their different developmental capacity after IVF.