The Ag-NOR proteins present a crescent-shaped distribution at the secondary constrictions of metaphase PtK1 chromosomes.

The ultrastructural distribution of the proteins responsible for the silver staining of NORs was analyzed on serial thin sections of preembedded silver-stained rat kangaroo PtK1 metaphase chromosomes. Our results show that the Ag-NOR proteins present a crescent-shaped distribution at the secondary constriction of each chromatid. Moreover, in some cases the crescent-shaped structures of both chromatids are not symmetrically arranged but show different orientations. These observations, together with our own and previously reported light microscopy results obtained on silver-stained mammalian chromosomes, lead us to suggest a mechanism for the formation of apparent secondary constrictions at NORs.