Cytoskeletal alteration in aged porcine oocytes and parthenogenesis

The objective of this study was to examine the changes in microtubule and microfilament assembly in aged porcine oocytes and to determine their developmental pattern after parthenogenetic activation. Porcine oocytes were cultured in Whitten's medium containing 10% follicular fluid with hormonal supplements (eCG and hCG) for 22 hr and 40 hr additional culture without hormonal supplements. At 40, 50, and 60 hr of culture, the oocytes were fixed for immunocytochemistry or activated by electrical pulse. In metaphase II stage oocytes, microtubules were detected only in the meiotic spindle. Two microfilament domains existed in the egg cortex, a thick and a thin microfilament domain. In aged oocytes (50 and 60 hr of culture), the incidence of metaphase II plates observed outside of the thick microfilament domain was higher (P < 0.05) than in young oocytes (40 hr of culture). After activation, a polar body was usually emitted from the chromatin at the microfilament rich domain or two pronuclei were formed outside of the microfilament rich domain. The percentage of activated oocytes with one female pronucleus was higher (P < 0.05) in oocytes at 40 hr of maturation than at 50 and 60 hr of culture. At 24 and 30 hr after stimulation the incidence of cleavage to the 3- to 4-cell stage was higher (P < 0.05) in aged oocytes (50 and 60 hr of maturation) than that in oocytes at 40 hr of culture. These results suggested that a role of microfilaments is to retain the chromatin at the proper position in the oocyte cortex, and that aging results in a disruption of the microfilaments such that atypical development results after parthenogenetic activation.