The suitability of X-ray microanalysis for histochemistry

The great majority of applications of X-ray microanalysis (XRMA) in biology may be regarded as histochemical, in the sense that they are studies of the chemical composition of cells or tissues in situ. Conventionally, however, the term histochemistry is used to denote the investigation of the chemistry of cells and tissues in situ using specific reactions that produce a distinctive reaction product that can be detected by some microscopical method. For light microscopy, a coloured reaction product is normally used. Although this approach has been extremely successful, not every cellular substance of interest can be identified in this way. In addition, coloured reaction products may diffuse, or dissolve in mountants, and can rarely be quantified satisfactorily. For electron microscopy, reliance is placed on reaction products which are electron dense. As well as the problems of diffusion and quantification noted above for light microscopical histochemistry, it may also be difficult at times to distinguish the reaction product from the normal staining of the section. Alternatively, if the section is left unstained, the histochemical reaction product may be clearly visible, but difficult to localise because of the low contrast of the rest of the material. X-ray microanalysis has the potential to help with all these problems.

Problems of diffusion of histochemical reaction products are likely to be greatest in multi-step reactions, such as those for many enzymes.