THE INCIDENCE OF BOVINE DIPLOID OOCYTES MATURED IN-VITRO

The present work describes a cytogenetic study of in vitro matured bov ine oocytes to determine the proportion of unreduced oocytes carrying the diploid number of chromosomes. Studied oocytes were derived either from a pool of oocytes collected from several different donors, or fr om oocytes collected separately from individual donors. In vitro matur ation was performed by culturing immature oocytes for 24 h in TCM199-m edium supplemented with estrous cow serum and hormones at 39 degrees C in 5% CO2. Chromosomal complement of in vitro matured oocytes was stu died by Giemsa-staining and produced analyzable results in approximate ly 60% of the cases. The results revealed that approximately 75% of oo cytes had matured to the MII stage in both groups of oocytes studied. Of these MII oocytes, 11 and 12.4% (from the oocyte pool or from indiv idual cows, respectively) contained the diploid set of chromosomes. Th e occurrence of diploid MII oocytes was not quite uniform among donors : 40.5% of all cows produced one, 18.9% produced two and 2.7% (one cow ) produced three diploid MII oocytes. However, a positive relationship between the number of MII oocytes in general and diploid MII oocytes among individual donors was not found. The possible factors that may l ead to the formation of diploid MII oocytes observed under IVM procedu res are discussed. The results of this study showed a higher incidence of diploid oocytes in cattle than previously reported.