Visualization of DNA within mitochondria by osmium-ammine staining of mouse duodenal crypt cells.

Previous investigators have examined mitochondrial DNA (mtDNA) in the electron microscope (EM) after extraction from mitochondria and rotary shadowing. We have observed mtDNA in situ by the osmiumammine procedure for specific staining of DNA in the EM. The procedure was modified to improve the regularity of the staining and then applied to the rapidly dividing cells present in mouse duodenal crypts. In the stained sections of these cells, 25% of the mitochondria exhibited discrete reactive filaments. The filaments, whether observed directly or in stereopairs, appeared either irregular or arranged into distinct patterns, some of which were similar to those previously described after rotary shadowing of duplicating mtDNA: namely, simple and double circular figures, displacement loops and supercoiled forms. The filaments could be traced in serial sections of the same mitochondria and, therefore, were not artifacts. Moreover, their disappearance after DNase digestion demonstrated that they were composed of DNA. It is concluded that mtDNA can be visualized by the modified osmium-ammine technique and may show patterns that can be interpreted as phases in its replication.