Microscopic cytochemistry as matrix chemistry

Microscopic cytochemical procedures, unlike reactions in test tubes, have to be undertaken while the compounds to be localized and quantified are present in the structural matrix of cells or tissues. Cytochemical reactions therefore differ from analogous staining reactions in homogeneous media because the diffusion of reagents into and out of the matrix and the molecular state around the compounds in the matrix to be stained, as well as the presence of many potentially interfering substances in the matrix, produce additional complications in terms of both specificity and quantitation. These complications can be studied quantitatively and in detail in artificially prepared matrices of defined geometry into which pure compounds or mixtures (of known composition) with other biological compounds can be incorporated. Since the composition of the matrix models is known, and the amount or activity of the compound to be stained can be analysed biochemically too, matrices are well suited to studies of the specificity of cytochemical reactions. They can also be used to study quantitative aspects of the influence of fixation procedures on the state of the incorporated compound. In addition, matrices can--by further biochemical analysis--be used to calibrate the intensity of cytophotometrically measured staining in terms of amounts of stained substance or in biochemical enzymic activity units. Finally, films and beads containing known amounts of known compounds can be used for objective quality control of commercially available cytochemical reagents.