Fiber ultrastructure and dimensions in thin-sectioned chromatin

Cells of different tissues have been investigated to determine whether there are statistically significant variations in chromosome fiber diameter in thin sections of different cell types, and to evaluate the effects of chelating buffers in the fixing solutions on chromatin fiber diameter. Three significantly different fiber classes have been demonstrated in tissues fixed in phosphate-buffered glutaraldehyde and postfixed by phosphate-buffered osmium tetroxide. Fibers in metaphase chromosomes of milkweed bug spermatocytes average 80 Å. Significantly thicker fibers, of 118 Å, occur in the condensed chromatin of frog liver nuclei. Sectioned meiotic chromosomes from testes of the salmon and anthers of the lily, and condensed chromatin from frog connective cell nuclei show a third class of still thicker fibers, averaging 161 Å, 176 Å, and 177 Å, respectively. The presence of phosphate buffer in the fixing solution has no apparent effect on the size of thin-sectioned chromatin fibers. Milkweed bug chromosomes, frog liver chromatin, and lily chromosomes, when fixed in unbuffered glutaraldehyde and postfixed in unbuffered osmium tetroxide, show fibers averaging 71 Å, 121 Å, and 152 Å, respectively, about the same as those found for these tissues when fixed by the phosphate-buffered glutaraldehyde procedure. We conclude from these observations that various cells contain fibers with average diameters that are significantly thicker than the commonly reported 80-100 Å, and that the presence of a chelating buffer (phosphate) in the fixing solution has no detectable effect on the diameter of chromatin fibers.