Nucleolar changes in human embryo during the pre-implantation stage. Activation of ribosomal genes during the nucleologenesis.

We studied the structural and functional organization of human embryo nucleoli during the pre-implantation stage from spare embryos obtained by in vitro fertilization. In human embryo nucleoli, structural modifications occur during the first cleavages. They lead to the constitution of a reticulated nucleolus from an initial structure called primary nucleolus. They appear during the third cleavage and correspond to the nucleogenesis. Autoradiography shows no transcription in the primary nucleolus. Transcription of rDNA starts on the periphery of the initial structure after the 4-cell stage. It corresponds to the beginning of the embryonic gene expression. The entire nucleolus will be progressively concerned by this transcription during the reticulation. Silver staining at the ultrastructural level shows that Ag-NOR proteins are missing in the primary nucleolus. In the beginning of nucleogenesis, Ag-NOR proteins are first located on the nucleolar periphery. Their following distribution corresponds to the structures containing rDNA in the nucleolus. Nucleologenesis in the human embryo during the pre-implantation stage first requires an association of rDNA with a primary structure, then an activation of ribosomal genes.