Abstract
The role and interaction of microtubules and microfilaments, which are important for progressing the events during oocyte maturation and activation, are not well understood. This study was designed to examine the cytoskeletal changes of the porcine oocyte activated electrically or by sperm with relation to the effects of oocyte aging and the paternal and maternal contributions. During electric activation, fusion of the first polar body (PBI) into the oocyte was attempted to evaluate changes in the cytoskeleton induced by incorporation of maternal chromatin in comparison with penetrated sperm (paternal) chromatin. Aged oocytes matured for 50-60 h displayed an elongated spindle and a less dense distribution of microfilaments compared to young oocytes matured for 44 h. Oocytes were effectively activated with double electric pulses regardless of aging (93-100%). Fusion of PBI into the oocyte declined with oocyte aging (from 52% to 22%). When fusion occurred, PBI chromatin was incorporated into the microtubule networks of the ooplasm and was frequently transformed into one "extra" nuclear-like structure. Young parthenotes possessed one microtubule-rich domain including one or more pronuclei. In aged parthenotes, however, the cortical and cytoplasmic microfilaments decreased in density, resulting in frequent fragmentation of eggs. In zygotes, male and female pronuclei were included in separate domains of microtubules, respectively, anchored by microfilaments. The present results suggest that the instability of cytoskeleton of the oocyte induced by aging may increase egg fragmentation and that there may be a difference between the paternal and maternal contributions to the cytoskeletal reorganization during pronuclear formation and migration.
