A silver-aldehyde reaction for studies of chromosome ultrastructure

A method is presented for the visualization of structures containing deoxyribonucleic acid at the electron microscope level. This is achieved by the substitution of an ammoniacal silver reagent for leucobasic fuchsin in a Feulgen-like procedure. Tissue aldehydes reduce this reagent to metallic silver, which is deposited at the reactive sites in the form of fine particles. The particles range in size from about 4 mfi downwards to below the resolving power of the microscope used. 3 fixatives have been investigated (buffered osmium tetroxide, formalin, glutaraldehyde) and the special problems encountered with each fixative are discussed. Detailed considerations regarding the preparation and handling of this somewhat unstable staining solution have been described previously (Bryan, 1964). Results obtained from an application of this procedure in an investigation of the ultrastructure of polytene chromosomes are also reported in detail. The fibrillar nature of these chromosomes is depicted more clearly following staining than in preparations resulting from the employment of the more conventional methods in current use. The fibrils are clearly delineated by silver particles and may be grouped into 2 classes. Thus, the finest fibrils are of about 4 mju diameter and they are usually associated in pairs to form units of about 10 m/x diameter. Although there is a predominant orientation of the fibrils composing individual bands, the nature of the association of the 10 m/i units which results in this is not clear. Evidence for the occurrence of similar fibrils in chromatin of interphase nuclei of mammalian origin is also presented. Taken together, the results of the various experiments suggest that this silver procedure may be used to advantage in a wide variety of investigations concerning the ultrastructure of chromosomes and chromatin.