cytochemistry

n.
The branch of biochemistry that deals with the study of the chemical composition and activity of cells.

cytochemical cy'to·chem'i·cal (-kĕm'ĭ-kəl) adj.

The science concerned with the chemistry of cells. Specifically, the macromolecules of subcellular structures can be treated chemically to form microscopically visible end products. For example, proteins, enzymes, lipids, carbohydrates, and nucleic acids can be directly visualized in cell nuclei, membranes, and organelles by cytochemical methods which generate images that can be viewed by either bright field or light, fluorescence, confocal, or electron microscopes. See also Electron microscope; Fluorescence microscope; Interference microscope; Phase-contrast microscope; Scanning electron microscope.

Enzyme cytochemical methods detect enzymes associated with subcellular structures. The method employs a substrate specifically cleaved by an enzyme in the cell to liberate a product which is transformed into a visible precipitate. For example, to localize the enzyme acid phosphatase in lysosomes, a subcellular organelle, a lead sulfide precipitate is produced by splitting a phosphate (product) from cytidine monophosphate (substrate) and transforming the phosphate product initially into lead phosphate and subsequently into lead sulfide; the latter is visible as a brown-black precipitate in the light microscope. Other methods employ azo dye or formazan precipitates rather than lead to produce a visible product. The number of enzymes that can be detected are limited by the availability and specificity of substrates. See also Enzyme.

Immunocytochemical methods are used to detect specific proteins associated with subcellular structures. The methods are based on the immunologic principle of the built-in specificity of antibody-antigen interactions. An antigen in a tissue or a cell is detected by employing either a direct or indirect method. The direct method requires a specific antibody linked to a molecule that produces a visible signal; the antibody-molecule complex is applied to a tissue or cells for antigen binding. The indirect method employs an unlabeled primary antibody which is applied to a tissue or cells. The antibody binds to the antigen. The bound primary antibody is then detected by application of a secondary antibody that yields a visible signal. Second antibodies can be labeled with fluorescent probes, enzymes, metals, and high-affinity complexes that are detectable with the appropriate microscope (such as bright-field light, fluorescence, confocal, and electron), in some cases, after application of other reagents to generate a visible product.

Double or triple label immunocytochemistry methods allow multiple antigens to be localized within a tissue or cells. Multiple antigens can be detected at the same time by using different antibodies labeled with probes that fluoresce at different wavelengths. Multiple antibodies can also be labeled with different enzymes or with different size gold particles. Fluorescent probes are detected with conventional fluorescence microscopy or with confocal microscopy using either appropriate filters or lasers. Enzyme probes and gold probes are detected with light and electron microscopy. See also Immunochemistry; Immunofluorescence; Protein.

Autoradiographic methods employ radioactive labeled molecules to identify sites within cells where synthesis of macromolecules occurs. This method is based on the property that an isotope emits ionizing particles during radioactive decay. Radioactive labeled precursor molecules (building blocks of molecules) become incorporated into specific cellular sites after injection into living organisms or cells, producing a radioactive product. To establish the location of the radioactive product in tissues or cells, a photographic emulsion is placed over tissues or cells in the dark for a period of time, followed by a photographic developer solution to reveal sites containing the radioactive product.

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(sī'tō-kĕm'ĭ-strē)
n.

The branch of biochemistry that deals with the study of the chemical composition and activity of cells.

The identification and localization of the different chemical compounds and their activities within the cell.