Epithelial Tissue Overview

Epithelial Tissue

• Epithelial tissue develops from three embryonic germ layers: ectoderm, mesoderm, and endoderm.

• Epithelial cells rest on a basement membrane, which can be clear or non-clear.

• Epithelial tissue is characterized by densely packed cells with minimal intercellular spaces.

• Epithelial tissue is avascular, meaning it lacks blood vessels. It receives nutrition through diffusion from the underlying connective tissue.

• Nerves can pass between epithelial cells.

• Epithelial tissue has a high regeneration capacity.

Types of Epithelium

• Surface epithelium: Forms continuous sheets that cover surfaces or line cavities.

• Glandular epithelium: Cells secrete substances.

• Neuroepithelium: Cells are specialized for receiving sensory stimuli.

• Myoepithelium: Cells have contractile function.

Surface Epithelium

• Classified by the number of cell layers:
  • Simple epithelium: Composed of a single layer of cells resting on the basement membrane.
  • Stratified epithelium: Composed of multiple layers of cells.

Simple Epithelium

• Simple squamous epithelium:

  • Single layer of flat, thin cells with flattened nuclei.
  • Functions:
    • Facilitates easy movement of fluids and gases.
    • Exchange of gases and fluids.
  • Locations:
    • Pleura, pericardium, and peritoneum (called mesothelium).
    • Blood vessels and heart (called endothelium).
    • Lung alveoli.
    • Bowman's capsule of the kidney (filtration of body fluids).

• Simple cuboidal epithelium:

  • Single layer of cube-shaped cells with round nuclei.
  • Functions:
    • Secretion.
    • Reabsorption.
  • Locations:
    • Thyroid follicles.
    • Acini and small ducts of all exocrine glands (e.g., salivary glands).
    • Convoluted tubules of the kidney.

• Simple columnar epithelium:

  • Single layer of tall, columnar cells with basal oval nuclei.
  • Functions:
    • Secretion.
    • Absorption.
  • Locations:
    • Stomach (secretion).
    • Intestine (absorption and secretion).
    • Female genital system.
    • Lower respiratory tract.

• Simple columnar epithelium, ciliated:

  • Single layer of tall, columnar cells with basal oval nuclei and cilia.
  • Functions:
    • Movement of substances along the surface.
  • Locations:
    • Intestine (may contain microvilli and goblet cells).

• Pseudo-stratified columnar epithelium:

  • Simple epithelium where cells are crowded, with some tall cells with central nuclei and others short cells with basal nuclei.

  • Nuclei are present at multiple levels, giving the appearance of being stratified.

  • Types:

    • Non-ciliated pseudo-stratified columnar epithelium:
      • Locations: Vas deferens.
    • Pseudo-stratified columnar epithelium with motile cilia and goblet cells:
      • Locations: Upper respiratory tract (nose, trachea, and bronchi).
    • Pseudo-stratified columnar epithelium with non-motile cilia (stereocilia):
      • Locations: Epididymis, male urethra.

Stratified Epithelium

• Stratified squamous epithelium:

  • Composed of multiple layers of cells, with the top layer being flat.
  • Functions: Protection.
  • Types:
    • Keratinized stratified squamous epithelium:
      • Locations: Skin, ear.
    • Non-keratinized stratified squamous epithelium:
      • Locations: Oral cavity, cornea, esophagus.

• Transitional epithelium (uroepithelium):

  • Multiple layers of cells with the ability to change shape.

  • Relaxed (empty): 6-8 layers of cells with cuboidal shape in the basal layer and dome-shaped cells at the surface.

  • Full (stretched): 3-4 layers of cells with flattened cells at the surface, with some cells potentially binucleated.

  • Functions: Protection.

  • Locations: Ureter, urinary bladder.

  • Unique characteristics:

    • May contain mucous on the surface.
    • Basement membrane is non-clear and non-wavy.

• Stratified columnar epithelium:

  • Less common type of epithelium.
  • Composed of a few layers with the top layer being columnar cells.
  • Locations:
    • Stratified columnar ciliated epithelium: Foetal esophagus.
    • Stratified columnar non-ciliated epithelium: Recto-anal junction, large ducts of glands, male urethra (penile part).

• Stratified cuboidal epithelium:

  • Rare type of epithelium.
  • Composed of two layers of cube-shaped cells.
  • Locations: Sweat glands.