The silver-stained NOR and argentophilic nuclear proteins in early mouse embryogenesis: a cytological study.

Silver staining (Howell and Black, 1980) was used in light and electron microscopic studies for detecting the localization of argentophilic nuclear proteins in fertilized ova and cleaving mouse embryos. No silver-stained nucleolus organizing regions (NORs) (Ag-NORs) were visualized in the metaphase chromosomes of the first cleavage mitosis. From the 2-cell stage on, metaphase chromosomes contained Ag-NORs. Argentophilic proteins were detected in the pronuclei of the 1-cell embryos, i.e. before transcription of the ribosomal genes started. After fertilization these proteins accumulated on the decondensing sperm chromatin and telophase maternal chromosomes, then migrated into the pronuclei to be stored in pronucleoli, and, during mitosis, were transferred into the cytoplasm. In the metaphase chromosomes of the cleaving embryos Ag-NORs adequately reflected the transcriptional activity of the ribosomal genes, whereas in pronuclei of the 1-cell stage embryos argentophilic proteins were not involved in this process, but are likely to play a part in the formation and maturation of pronucleoli, and in the cell cycle regulation.