A serial sectioning study of the structure of human mitotic chromosomes.

The organization of chromosomes in mitotic HeLa cells was investigated by serial sectioning and electron microscopy. Mitotic HeLa cells were resuspended in a hypotonic buffer containing a magnesium ion concentration of 1.0 to 1.5 mM prior to fixation and sectioning. By treating the cells in this way, most of the material adjacent to the chromosomes were removed, and the chromosomes were slightly expanded, thereby separating the chromatin fibers and allowing their arrangement to be observed. Gazing sections that cut across the chromatids longitudinally and transversely are especially informative. The typical pattern of radially oriented fibers which is found for sections through the body of the chromatids is replaced by a dot pattern. The array of dots, which is almost exclusively found as the sections approach the chromosome boundary, must result from the sectioning knife cutting across chromatin strands. Longitudinal and transverse sections both show the transition from radial fibers to dots and demonstrate that chromosome organization is the same around the chromatid arms. As was previously found with random sections [1], consecutive, transverse sections through the body of the chromatid arms show the primary mode of organization to be a radial distribution of fibers. These consecutive sections firmly establish that this characteristic feature of chromosome organization extends through the particles and is not merely a feature of occasional, untypical sections. Serial sections that intersect the body of the particles approximately parallel to the chromatid arms further demonstrate that the fundamental orientation of the fibers is radial and extends uniformly through and along the chromatids.