4. Histochemistry. Methods of demonstration of carbohydrates, lipids, proteins and nucleic acids.

Histochemistry: Bridging Chemistry and Biology

• Definition: Histochemistry blends chemical and biological techniques to visually pinpoint and study the location and distribution of specific molecules within tissues. It's a powerful tool for understanding the structural and functional aspects of cells and tissues.
• Staining Techniques: Histochemistry relies on staining techniques, where specific dyes or reagents selectively bind to target molecules, making them visible under a microscope.

Demonstrating Carbohydrates

• Carbohydrates: These biomolecules consist of monosaccharides linked together to form polysaccharides like glycogen (energy storage) and cellulose (plant cell walls).
• Periodic Acid-Schiff (PAS) Reaction: This reaction specifically targets carbohydrates for visualisation.

• Mechanism: PAS relies on an oxidation process that converts adjacent hydroxyl groups in polysaccharides to aldehyde groups. Schiff reagent then reacts with these aldehydes, generating a distinctive purple-magenta colour, highlighting the presence of carbohydrates.
• Applications: The PAS reaction is valuable for visualising:
• Cells rich in polysaccharides: For example, goblet cells packed with mucin granules, which stain intensely due to their high carbohydrate content.
• Glycogen deposits in tissues: Such as in liver and muscle cells.
• Glycocalyx: The carbohydrate-rich coating on cell surfaces.

Demonstrating Lipids

• Lipids: Composed of fatty acids, lipids are hydrophobic molecules. Examples include triglycerides, phospholipids, and sterols. They play roles in energy storage, insulation, and cell structure.
• Lipid-Soluble Dyes: Visualising lipids requires special dyes that dissolve in these hydrophobic molecules, as water-based dyes won't bind.
• Examples:
• Sudan black: Stains lipids black.
• Sudan III: Imparts a reddish-brown colour to lipids.

Demonstrating Proteins

• Proteins: These complex molecules are constructed from chains of amino acids linked by peptide bonds. Proteins perform a wide array of functions:
• Enzymes: Catalyse (speed up) biochemical reactions.
• Antibodies: Key components of the immune system, binding to specific foreign substances.
• Structural components: Provide support and shape to cells and tissues.
• Transporters: Carry molecules across cell membranes.
• Signalling molecules: Transmit signals between cells.
• Protein Visualisation Techniques: The sources describe several methods for demonstrating proteins:
• Immunohistochemistry: A highly specific technique that utilises antibodies to target and visualise individual proteins.

• Two-Antibody System: It involves a primary antibody that binds to the target protein and a secondary antibody labelled with a visible marker (e.g., a fluorophore) that attaches to the primary antibody, allowing for visualisation.
• Enzyme Histochemistry: Employs substrates that react with specific enzymes in tissue sections. The location of the enzyme is revealed by the visible product of the enzymatic reaction.