Nitrogen compounds are of fundamental importance. Amino groups are present in tissues in amino acids and as reactive side groups on proteins. Likewise, basic dyes invariably have one or more reactive amino groups. Positively charged amino groups both on dyes and proteins are involved in numerous histological reactions with corresponding negatively charged carboxyl or hydroxyl groups also found on dyes and proteins or other tissue components.
A major class of dyes is the azo class. These are very commonly used histologically. They are based on the azo (-N=N-) chromophore. Nitrogen is also found in the chromophores of other dye classes as well.