Anatomy

The closely related techniques of histochemistry and cytochemistry are concerned with the investigation of chemical activities of tissues and cells. For example, the presence of certain colors within cells indicates that particular chemical reactions have occurred. In addition, the density of the color reaction may serve as an index of the intensity of the reaction. Histochemical methods have been particularly successful in the study of enzymes, catalytic substances that control and direct many of the cell’s activities. Much knowledge of enzymes was gained in studies carried out after removal of the enzymes from their cells of origin, but not until the advent of histochemistry could the anatomist see through the microscope which cells carry specific enzymes or gauge how active these enzymes are in different cells under various conditions.

An important technique of histochemistry involves the use of radioactive isotopes of various chemical elements that are present in cells and tissues (see Isotope; Radioimmunoassay; Isotopic Tracer). Elements or compounds “tagged” or “labeled” with radioactive isotopes are administered to living materials, permitting the investigator to trace the pathways taken by these substances through the various tissues. The degree of concentration and dilution of elements within specific cellular constituents may be estimated by measuring the radiations emanating from these tissues. The technique of labeling compounds with radioactive isotopes makes it possible to study the distribution and concentration of isotopes in tissue slices similar to those studied routinely under the microscope. This study, called autoradiography, is accomplished by bringing the radioactive tissue slices into contact with photographic films and emulsions that are sensitive to radiation.

Another technique of localizing chemical compounds within tissue slices is microincineration: the heating of microscopic sections to the point at which the organic materials present are destroyed and only the mineral skeleton remains. The remaining minerals can then be identified by special chemical and microscopic procedures. Thus, microincineration provides still another way of locating specific chemical elements within particular cell or tissue components.

Another development in the field of histochemistry is microspectrophotometry, a precise method of color analysis. In this process the colors within a tissue slice are analyzed with a spectrophotometer, an instrument that measures the intensity of each color as a function of wavelength. Microspectrophotometry can be used to estimate the characteristics of unstained cells and tissues by measuring their absorption of particular wavelengths. Another application permits precise judgments to be made concerning the nature and intensity of color reactions. These judgments provide, in turn, accurate information about the location and intensity of chemical reactions in the components of living organisms.

See Physiology. For further information on individual anatomists, see biographies of those whose names are not followed by dates.