Introduction to Histology

Study Notes

Histology is the study of tissues and cells in the body.
Basic elements of the body include cells (structural and functional units), tissues (made of cells and extracellular matrix), organs (made of different tissue types), and systems (groups of organs).
The four basic tissues of the body are epithelial, connective, muscular, and nervous tissue.

Microscopy is used to study histology.
- Light Microscopy (LM): Uses visible light to view stained tissue sections. The image is colored.
  - Components: light source, stage, objective lens, ocular lens.
- Electron Microscopy (EM): Uses a beam of electrons for higher resolution.
  - Transmission Electron Microscopy (TEM): Visualizes internal cell structure. The image is grayscale or black and white.
  - Scanning Electron Microscopy (SEM): Visualizes the surface ultrastructure. The image is grayscale or black and white.

Magnification is the ability to enlarge an image. LM magnification is up to 1500x, EM can reach 300,000x.
Resolution is the ability to distinguish fine details. LM resolving power is 0.2 µm, EM is 0.2 nm.
Units of measurement:
- Micrometer (µm) = 1/1000 of a millimeter
- Nanometer (nm) = 1/1000 of a micrometer

Two main steps are involved
1. Preparation of sections
2. Staining of sections
Paraffin sections: Process for preparing stained tissue sections.
Fixation: Preserves cells and tissue components, inhibiting autolysis and putrefaction (typical fixative solution is 10% neutral buffered formalin).
Dehydration: Gradually removes water from the tissue by increasing concentrations of alcohol (50%, 70%, 90%, 100%).
Clearing: Replaces alcohol with a substance like xylol, making the tissue transparent.
Impregnation: Replacing xylol with melted paraffin, soaking the tissue in it.
Embedding: Forming a hard paraffin block of the tissue.
Sectioning: Thin sections (5-8 µm) are cut from the paraffin block using a microtome.
Mounting: Sections are placed on glass slides and allowed to dry.