Histology Study Notes

Introduction to Histology

  • Focuses on the methods of studying tissues at the microscopic level.
  • Important for understanding the structure and function of cells and tissues.

Methods of Study in Histology

  1. Fixation

    • Purpose: Preserve cell and tissue structure.
    • Process: Small pieces of tissue are placed in solutions of chemicals.
    • Chemicals used: Cross-link proteins and inactivate degradative enzymes.

  2. Dehydration

    • Process: Tissue is transferred through a series of gradually concentrated alcohol solutions.
    • Final Solution: Ends with 100% alcohol, which removes all water from the tissue.

  3. Clearing

    • Purpose: Remove alcohol from the tissue.
    • Method: Alcohol is removed in organic solvents that are miscible with both alcohol and paraffin.

  4. Infiltration

    • Process: Tissue is placed in melted paraffin, allowing it to become completely infiltrated with the substance.

  5. Embedding

    • Process: Paraffin-infiltrated tissue is placed in a small mold containing melted paraffin and allowed to harden.

  6. Trimming

    • Purpose: Prepare the paraffin block for sectioning.
    • Method: The resulting paraffin block is trimmed to expose the tissue for slicing on a microtome.

Staining Techniques

  • Importance: Unstained tissue and cells are difficult to examine.
    • Example of unstained tissues:
    • Bone tissue, unstained
    • Blood smear, unstained
  • Staining enhances the visibility of cellular components, aiding in differentiation.

Staining Techniques Explained

  • Functions of Dyes:

    • Dyes make cells and tissues, and their components easier to distinguish visually.
    • Dyes react selectively with chemical components of the structures being stained.

  • Types of Dyes:

    1. Basic Dyes
    • Stain well components with net negative charges (anionic).
    • Examples include:
      • Hematoxylin
      • Toluidine blue
      • Alcian blue
      • Methylene blue
    • Basophilic: Refers to cell and tissue components that stain well with basic dyes, including nucleic acids.
    1. Acidic Dyes
    • Stain well components with net positive charges (cationic).
    • Examples include:
      • Eosin
      • Orange G
      • Acid fuchsin
    • Acidophilic: Refers to cell structures that stain well with acidic dyes, such as proteins with ionized amino groups.

Hematoxylin & Eosin (H&E)

  • Hematoxylin:
    • Most common acid-base dye used in histology.
    • Basic dye, which stains anionic components purple due to its affinity.
    • Key structures stained by Hematoxylin include:
    • Cell structures with acids in their composition
      • Nucleic acids (e.g., DNA in the nucleus)
      • Glycosaminoglycans
  • Eosin:
    • Acidic dye that stains cationic components pink, indicating its preference for basic components.
    • Key structures stained by Eosin include:
    • Mitochondria
    • Secretory granules
    • Collagen fibers

Special Staining Techniques

  1. Periodic Acid–Schiff (PAS) Reaction

    • Purpose: Stains carbohydrate-rich tissues and cellular components dark purple or magenta.
    • Example: Goblet cells, which contain secretory granules rich in carbohydrates, are PAS positive.

  2. Sudan Black

    • Type: Lipid-soluble dye.
    • Purpose: Stains cells and tissue structures rich in lipids black.
    • Example: Adipose (fat) cells.

Electron Microscope versus Light Microscope

  • Resolution:
    • Electron microscopes have a resolution around 3extnm3 ext{ nm}.
  • Magnification:
    • Up to 400,000imes400,000 imes in electron microscopy.
  • Detail: Electron microscopes provide more detail with respect to the ultrastructure of cells, including organelles.