CULTIVATION PERIOD FOR MATURATION OF RECIPIENT OOCYTES AND THE DEVELOPMENTAL ABILITY OF CLONED PORCINE EMBRYOS

The duration of the in vitro maturation (IVM) of recipient oocytes appears to have prominent effects on the development of cloned porcine embryos; however, little is known about the reason for the change in efficiency of recipient oocytes during IVM. Here we aimed to clarify if the age of oocytes affects a maturation promoting factor (MPF) activity level of recipient oocytes during the nuclear transfer (NT) procedure and morphological remodeling in the transferred donor nuclei, including premature chromosome condensation (PCC) and its spindle-like structure. The optimum window of the IVM period in terms of the developmental ability of NT embryos in vitro was determined to be 36–40 h. The use of recipient oocytes matured for 36 and 40 h resulted in a high level of MPF activity before and after NT, and increased the occurrence of PCC in transferred nuclei compared to the use of oocytes matured for 44 and 52 h. The proportion of abnormal spindle-like structures increased as the IVM period was prolonged. In addition, NT embryos constructed from recipient cytoplasts obtained after 40 h of maturation and fetal fibroblasts of miniature pigs were transferred to surrogate recipients, and developed to full term. These results suggest that recipient oocytes in the early metaphase II stage retain a high level of MPF activity during the NT procedure and thus effectively induced PCC with a normal cytoskeletal structure, followed by a resultant improvement in the subsequent development. (platform)