Electron-Microscope Observations on the Organization of Hetero-Chromatin in Certain Cells

Abstract

Electron micrographs of sections through the heterochromatin (condensed chromosomes) of erythrocytes from chicken and lamprey reveal alternate equispaced electron-dense and relatively less dense bands lying adjacent and parallel to, and extending considerable distances along, the nuclear envelope. Frequently a triple-structured band (average half-width 183 Å), consisting of a less dense band, a dense band (width 130-170 Å), and a second less dense band is encountered; sometimes there are many (10-12) bands. The dense bands have a variable structure; they may appear continuous or consist of equispaced granules about 220 Å apart, which sometimes have a less dense centre. Similar but shorter dark bands and granules appear throughout the chromatin. Tilting the section in the microscope causes the micrographs to change, bands and granules disappearing and reappearing elsewhere, demonstrating that the images are produced by superposition of structures small in dimensions compared with the thickness of the section. Tilting about an axis normal to the band may resolve it into granules. These data indicate that the heterochromatin contains a well-defined structural unit which can form ordered regions, namely layers in contact with the nuclear envelope. The possibility of errors in interpreting the substructure of the bands due to complexity of superposition effects is stressed. The simplest hypothesis to account for the images is that the units are microtubes of outside diameter 130-170 Å, perhaps microhelices formed by coiling DNA with protein, spaced apart equally, perhaps by spacing elements. Microtubes parallel to the optic axis would appear as hollow granules, several one above another in a plane parallel to the axis appearing as a dark band. A similar triple-structured band of average half-width 224 Å is found in cells from newt spleen, lymphocytes, polychromatophil erythroblasts, mature erythrocytes, basophilic granulocytes, reticular cells and macrophages. A survey of published micrographs shows a similar triple-structured band of average half-width 212 Å to be a common feature of many cell types. A triple-structured band probably gives rise to the sheets of chromatin, now shown to have a similar triple-layered structure, limited on both sides by nuclear envelope, previously found attached to interphase nuclei and mitotic chromosomes in certain polychromatic erythroblasts from the newt. The effects of tilting the section were studied on objects of known geometry, membranes and cytoplasmic microtubules.