Epithelial Tissue Overview

Study Notes

Epithelial tissue is one of the four basic tissue types in the body, along with connective, muscular, and nervous tissue.
It is derived from all three germ layers: ectoderm, mesoderm, and endoderm.
Epithelial tissue is characterized by closely packed cells with minimal intercellular substance.
It rests upon a basement membrane, which separates it from underlying connective tissue.
It is avascular, meaning it lacks blood and lymph vessels, and receives nourishment via diffusion from the underlying connective tissue.
Epithelial tissue is highly innervated.
It has a remarkable ability to regenerate.

Surface epithelium forms continuous sheets of cells that cover external surfaces or line internal cavities.
It is classified based on:
- Number of layers: Simple (one layer) or Stratified (multiple layers).
- Shape of cells in the top layer: Squamous (flat), Cubical (cube-like), or Columnar (tall).
- Surface specializations: Keratinized or non-keratinized (for stratified squamous), Ciliated or non-ciliated (for columnar).

Forms thin sheets of flattened cells with flat nuclei.
Functions:
- Facilitates easy movement of fluids: Due to its smooth surface.
- Gas exchange: In alveoli of lungs.
- Filtration and diffusion: In Bowman's capsule of kidneys.
Locations:
- Endothelium: Lining of heart, blood vessels, and lymphatic vessels.
- Mesothelium: Lining of serous membranes.
- Alveoli of lungs (Pneumocytes I).
- Bowman's capsule of kidneys.

Composed of one layer of cube-like cells with centrally located, rounded nuclei.
Function: Secretion.
Location: Thyroid follicles.

Though appearing stratified, this epithelium has all cells resting on the basement membrane.
It is characterized by crowded cells, all attached to the basement membrane.
Function: Protection of the underlying tissue.
Location: Membranous urethra.

Protection: Physical barrier against external factors.
Secretion: Producing and releasing substances, such as hormones or digestive enzymes.
Absorption: Taking in substances, such as nutrients from the digestive tract.
Excretion: Removing waste products from the body.
Reproduction: Contributing to the formation of gametes (sperm and egg).
Sensation: Detecting stimuli, such as touch, taste, or smell.
Contraction: Some specialized epithelial cells can contract, like myoepithelial cells.