IN-VIVO AGING OF OOCYTES INFLUENCES THE BEHAVIOR OF NUCLEI TRANSFERRED TO ENUCLEATED RABBIT OOCYTES

The present study examined nuclear remodeling in rabbit nuclear transf er (NT) embryos formed from metaphase II (MII) oocytes aged in vivo un til 19 hr postcoitum (hpc), enucleated, and fused at 22-26 hpc with 32 -cell morula blastomeres by means of electric fields, which also induc ed recipient oocyte activation. Post-activation events observed during the first hour following the fusion/activation pulse were studied in terms of chromatin, lamins, and microtubules, and revealed that transf erred nuclei underwent premature chromosomes condensation (PCC) in onl y one-third of NT embryos and remained in interphase in others. Recipi ent oocytes were mostly not activated by manipulations performed befor e the fusion/activation pulse. The persistance of transferred nuclei i n interphase resulted from the rapid progression of recipient oocytes to interphase after activation, suggesting that the cytoplasmic state of MII oocytes aged in vivo was poised for the approach to interphase. Studying microtubular organization in MII oocytes before nuclear tran sfer manipulations, we found that 19 hpc MII oocytes aged in vivo diff ered from 14 hpc MII oocytes (freshly ovulated) and from 19-hpc MII oo cytes aged in vitro (collected at 14 hpc and cultured for 5 hr), notab ly by the presence of microtubule asters and tubulin foci or only tubu lin foci dispersed throughout the cytoplasm. When PCC was avoided, rem odeling of the transferred nucleus was well advanced 1 hr after nuclea r transfer, and NT embryos developed better to the blastocyst stage.