Cytochemical properties of euchromatin and heterochromatin

Two classic cytochemical tests, the Feulgen-Schiff reaction and Toluidine Blue basophilia, have been employed for investigating the differential characteristics of heterochromatin and euchromatin. Differences have been detected in the Feulgen hydrolysis kinetics, the Feulgen absorption spectrum, the image analysis of Feulgen-stained material, and the binding of Toluidine Blue under ordinary and Mg2+ competitive staining conditions. The differences are assumed to be a function of the composition and stereo-arrangement of the DNA and DNA-protein complexes present in these chromatin types and are possibly associated with physiological activities whose whole meaning is far from being clear. Differences in optical retardations in Toluidine Blue-stained material were also found. These are interpreted as being due to chromatin packing state and selective removal of histones promoted by the acetic acid-ethanol fixative.