[The nucleolus organizer regions of the metaphase chromosomes in cambial cell populations of mouse and rat placenta].

The number of silver-stained nucleolus organizing regions (Ag-NORs) was counted in metaphase plates of the fetal part of placenta of mice and rats and in the tissues of their embryos. On day 9 of mouse gestation and on day 12 of rat gestation, up to 82% of metaphases in the fetal part of placenta have the highest possible number of chromosomes with Ag-NORs (9-10 for mice and 5-6 for rats). In the later embryogenesis (day 10 for mice and day 14 for rats), a great number of metaphases have either no Ag-NORs (34.9% for mice, 17.9% for rats), or only 1-2 Ag-NOR-chromosomes (17.4% for mice and 14.5% for rats). But in the tissues of embryos being on the same embryonic stages the changes in frequency of metaphases with different numbers of Ag-NOR-chromosomes have proved to be less obvious. In the differentiated polyploid placenta cells the number of chromosomes with Ag-NORs can be considerably lower than in diploid ones. It is proposed that the changes in frequency of metaphases with different numbers of Ag-NOR-chromosomes may reflect the regulation of ribosomal gene transcription in the embryogenesis, in particular, in differentiation of cambial cell populations of rodent placenta.