Pronucleus formation in bovine oocytes activated by a single electric pulse

Bovine oocytes matured in vitro were stimulated by a single pulse of direct current (DC), then cultured for 0.5-6 h and evaluated by light (LM) and transmission electron microscopy (TEM). At the light microscopic level, the beginning of oocyte chromatin decondensation was first observed 3 h post-activation (14%). A well-developed pronucleus with evenly granulated nucleoplasm surrounded by nuclear membrane was found in 12, 61 and 81% oocytes at 4, 5 and 6 h post-activation, respectively. The TEM evaluation revealed that nuclear membrane vesicules were first 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 membrane starts to form immediately after oocyte activation, while a well-developed pronucleus appears at 4-6 h.