Electron microscopy of silver-stained core-like structures in metaphase chromosomes.

Chinese hamster metaphase chromosomes, stained with ammoniacal silver and examined by electron microscopy, were covered with fine silver grains of variable size. In addition, many chromosomes contained linear aggregates of silver grains running continuously from one end of each chromatid to the other, forming a core-like structure. Extensive deposits of silver were observed over the nucleolar organizers and the centromeric regions, and the silver precipitate in the latter region appeared to be a localized differentiation of the core-like structure. The silver cores ranged from thin to thick and continuous to discontinuous elements in different chromosomes. This extreme variability suggests that these cores are not true structural components of chromosomes. The amount of silver deposit over any given chromosome region may simply reflect the concentration of chromatin in that region of the chromosome. The silver-stained core-like structure probably reflects an underlying difference in the concentration of chromatin in the central and peripheral region of each chromatid. Such differences in chromatin concentration may be induced during the prolonged hypotonic treatment required for the subsequent visualization of cores.