Transcriptional activity of nuclei in 2-cell blocked mouse embryos.

Embryos of inbred mouse strains stop their cleavage at the late 2-cell stage and this phenomenon is known as the "2-cell block in vitro". In the present work we used microinjections of 5-bromouridine 5'-triphosphate (BrUTP) to examine the transcriptional status of blocked embryos. We also traced nuclear distribution of hyperphosphorylated form of RNA polymerase II in such embryos. We found that transcriptional activity is gradually decreased when the embryos stay in block. This is accompanied by prominent increasing of the size of SC35 nuclear speckles that begin to accumulate hyperphosphorylated RNA polymerase II.