Heterogeneity of coilin-containing nuclear domains in early mouse embryos

Abstract

The nucleus of two-cell mouse embryos contains coilin-containing bodies of two types: (i) one to three large spherical structures 1 μm in size and (ii) small foci, which vary in number in different blastomeres. The largest coilin-containing structures, unlike the smallest ones, contain RNA polymerase I, nucleic acid chaperon YB-1, and actin. Neither large nor small coilin-positive domains contain symplekin, one of the signature components of histone locus bodies. In nuclei of late two-cell embryos, symplekin localizes to one to two well-formed spherical bodies that are observed both in close proximity to the coilin-positive structures and apart from them. Large coilin-containing bodies were not observed in embryos at the morula stage, as well as in the nuclei of late two-cell embryos after artificial suppression of transcription activity. Thus, the population of coilin-containing bodies in the nuclei of late two-cell embryos of mice is heterogeneous in morphology and molecular composition. It can be suggested that the largest coilin-containing bodies are provisional nuclear domains that are formed against a background of significant changes in the nuclear metabolism at the final stages of embryonic genome activation and the initial stages of reactivation of nucleolar transcription.