The methodology used in diagnostic pathology has for a long time been rather conventional. Macroscopical pathology is mostly a matter of skill and experience and for microscopical pathology standard tissue preparation techniques are still very important. Electronmicroscopy allowed further detail in the morphological analysis and morphometry allowed the quantitation of morphological characteristics.
That situation has changed dramatically with the introduction of a large variety of histochemical tools which have provided possibilities for detailed biological and chemical analysis at the tissue, cell and subcellular level. Enzyme activities can be visualized; carbohydrate moieties can be analyzed using lectins; proteins can be specifically identified and localized by immunocytochemistry; nucleic acid content can be assessed by specific staining methods and the genome as well as the complex of cytoplasmic mRNA's can be probed by in situ hybridization. DNA quantitation (by flow or static cytometry) has gained enormously in popularity due to the fact that in many tumor types aneuploid cell populations occur and predict aggressive tumor behavior. Enzymehistochemistry is used for a limited number of specific applications (e.g. analysis of muscle diseases). In the last two decades immunocytochemistry has firmly rooted itself in the diagnostic pathology laboratory. Many diagnoses can no longer be made without support from immunocytochemical studies. In (auto)immune diseases specific patterns of immunoglobulin and complement deposition may reveal the pathogenesis of disease processes (as e.g. in glomerulonephritis). Inflammatory cells can be characterized by cell surface (CD) markers. Cancer cells can be classified using marker proteins. Proteins expressed in a cell cycle dependent manner allow analysis of proliferation at the cell and tissue level. Viral and bacterial antigens' can be detected in order to establish the cause of infectious disease. New possibilities have been provided by in situ hybridization techniques for the localization of DNA and RNA. DNA in situ hybridization has allowed the analysis of cytogenetic abnormalities in interphase nuclei. This is no longer limited to numerical aberrations but also includes complex structural abnormalities. Gene expression can be analyzed by probing for specific mRNA's. The latter techniques have carried diagnostic pathology into the molecular field.