Ultrastructural and autoradiographic studies of nucleolar development and rDNA transcription in preimplantation mouse embryos.

The development of the nucleoli and the sites of rDNA transcription have been studied by high-resolution autoradiography during the cleavage stages of mouse embryos. The appearance of fibrillar centres at the periphery of the fibrillar primary nucleoli has been observed at the 4-cell stage. Several fibrillar centres, interconnected by electron-dense fibrillar strands, form a reticulated region around the fibrillar mass at the 6-to 8-cell stage. After a 10 min pulse with [3H]uridine, only this peripheral network is labelled. At the late morula and at the blastocyst stage, the fibrillar component (nucleolonema) of the reticulated nucleoli is labelled after 10 min [3H]uridine incorporation. When the embryos are reincubated for 2 h in cold medium, the label is localized mainly in the granular component. Fibrillar centres are not labelled. Autoradiograms of in vitro developed embryos pulsed for 2 h with [3H]uridine confirm that the central fibrillar core of the nucleoli of 6-to 8-cell embryos is never labelled. Thus, the fibrillar constituent of this core is not homologous to the fibrillar component of the nucleoli of later stage embryos, which is the site of active rDNA transcription. An interpretation of nucleologenesis during early mouse embryogenesis is proposed.