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 which2-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 oftenassumed that the block of early development is due to the failure of zygotic gene activation(ZGA)in culturedembryos.In this study we examined protein synthesis patterns by two-dimensional gel electrophoresis of[S]methionine radiolabeled 2-cell embryos.Embryos from the blocking strain and the nonblocking strainwere compared in their development both in vitro and in vivo.The detection of TRC expression,a markerof ZGA,at 42 h post hCG in KM embryos developed in vitro suggested that ZGA was also initiated even inthe 2-cell arrested embryos.Nevertheless,a significant delay of ZGA was observed in KM strain as comparedwith normally developed B6C3F1 embryos.At the very beginning of major ZGA as early as 36 h post