Delay of ZGA initiation occurred in 2-cell blocked mouse embryos

One-cell mouse embryos from KM strain and B6C3F1 strain were cultured in M16 medium, in which 2-cell block generally occurs. Embryos of KM strain exhibited 2-cell block, whereas B6C3F1 embryos, which are regarded as a nonblocking strain, proceeded to the 4-cell stage in our culture condition. It is often assumed that the block of early development is due to the failure of zygotic gene activation (ZGA) in cultured embryos. In this study we examined protein synthesis patterns by two-dimensional gel electrophoresis of [S-35] methionine radiolabeled 2-cell embryos. Embryos from the blocking strain and the nonblocking strain were compared in their development both in vitro and in vivo. The detection of TRC expression, a marker of ZGA, at 42 h post hCG in KM embryos developed in vitro suggested that ZGA was also initiated even in the 2-cell arrested embryos. Nevertheless, a significant delay of ZGA was observed in KM strain as compared with normally developed B6C3F1 embryos. At the very beginning of major ZGA as early as 36 h post hCG, TRC has already been expressed in B6C3F1 embryos developed in vitro and KM embryos developed in vivo. But for 2-cell blocked KM embryos, TRC was still not detectable even at 38 h post hCG. These evidences suggest that 2-cell-blocked embryos do initiate ZGA, and that 2-cell block phenomenon is due not to the disability in initiating ZGA, but to a delay of ZGA.