Histology

Histology

  • Definition of Tissue: A tissue is a group of cells that have a similar appearance and a common function.

  • Primary Tissue Types: There are four primary tissue types found in the body:

    • Epithelial
    • Connective
    • Muscle
    • Nerve

Epithelial Tissue (Epithelium)

  • Definition: Epithelial tissue is defined as a sheet of cells that covers a body surface, lines a body cavity, or forms glands.

  • Structure:

    • Apical Surface: This surface is exposed to the body exterior or a body cavity.
    • Basal Surface: This surface is attached to an underlying basement membrane.
    • Basement Membrane Composition:
    • Upper Basal Lamina
    • Lower Reticular Lamina

  • Vascularization: Epithelial layers do not contain blood vessels; however, they do have nerve fibers. They receive nutrients from blood vessels located below the basement membrane.

  • Classification of Epithelial Tissues:

    • Epithelial tissues are classified based on the number of cell layers and the shape of the epithelial cells.

    • Cell Layering:

    • Simple: Single layer of cells.

    • Stratified: More than one layer of cells.

    • Cell Shapes:

    • Squamous: Flat cells.

    • Cuboidal: Cells that are approximately the same height as they are wide.

    • Columnar: Cells that are taller than they are wide.

  • Stratification Naming: Stratified layers are named based on the shape of the cells at the apical surface.

Types of Epithelial Tissue

  1. Simple Squamous Epithelium:

    • Description: Composed of a single layer of squamous cells. Very smooth, which reduces friction, and allows small molecules to pass through easily.
    • Location: Found in the air sacs of the lungs, and it lines the heart and blood vessels.

  2. Simple Cuboidal Epithelium:

    • Description: Composed of a single layer of cuboidal cells.
    • Location: Found in the kidneys and in many glands.

  3. Simple Columnar Epithelium:

    • Description: Composed of a single layer of columnar cells.
    • Location: Lines most of the gastrointestinal tract.

  4. Pseudostratified Columnar Epithelium:

    • Description: Appears to be stratified but is actually columnar, with nuclei at different levels. Typically has cilia on the apical surface.
    • Location: Lines most of the upper respiratory tract.

  5. Stratified Squamous Epithelium:

    • Description: Composed of multiple layers, with squamous cells at the apical surface. Highly resistant to abrasion and injury.
    • Location: Forms the outer layer of the skin and lines the oral cavity and esophagus.

  6. Transitional Epithelium:

    • Description: Appears stratified cuboidal when relaxed and stratified squamous when stretched.
    • Location: Lines most of the urinary collecting system, including the bladder.