[Cytological study of the nucleolus organizer regions in the chromosomes of CBA and C57BL mice and their hybrids].

Transcriptionally active NORs of chromosomes visualized by AgNO3 staining were studied in bone marrow and embryos (day 10 of gestation) of CBA and C57BL mice, as well as of (CBA x C57BL)F1 hybrids. These mouse strains were shown to differ by the average number of Ag-positive NORs in marrow cells; in hybrids, the number of NORs is greater than in the parent strains. During embryogenesis, the number of chromosomes carrying silver-stained NORs increases; however, no significant differences by this parameter was detected between hybrid and C57BL embryos. The average number of silver-stained NORs was the smallest in embryos of CBA mice.