PRONUCLEUS FORMATION IN BOVINE OOCYTES ACTIVATED BY A SINGLE ELECTRICPULSE

Bovine oocytes matured in vitro were stimulated by a single pulse of d irect current (DC), then cultured for 0.5-6 h and evaluated by light ( LM) and transmission electron microscopy (TEM). At the light microscop ic level, the beginning of oocyte chromatin decondensation was first o bserved 3 h post-activation (14%). A well-developed pronucleus with ev enly granulated nucleoplasm surrounded by nuclear membrane was found i n 12, 61 and 81% oocytes at 4, 5 and 6 h post-activation, respectively . The TEM evaluation revealed that nuclear membrane vesicules were fir st visible at 0.5 h post-activation and became even more prominent at 1 h. Based on these observations, it is concluded that a nuclear membr ane starts to form immediately after oocyte activation, while a well-d eveloped pronucleus appears at 4-6 h.