Epithelial Tissues Overview

Study Notes

Epithelial tissue covers external and internal surfaces of the body.
It's found lining cavities like the digestive tract, airways, and blood vessels.
Most epithelial tissue has a free surface facing the cavity or outside environment.
The basal surface of epithelium is adjacent to a basement membrane.
Blood vessels do not extend into epithelial tissue, so nutrients and waste products must diffuse across the basement membrane.
The basement membrane is secreted partly by epithelial cells and partly by cells of the underlying tissue.

Protecting underlying structures: Skin protects underlying tissues from damage.
Acting as a barrier: Skin prevents the entry of harmful substances into the body.
Permitting the passage of substances: Lungs allow the passage of oxygen into the bloodstream.
Secreting substances: Sweat glands secrete sweat, mucous glands secrete mucus, and the pancreas secretes digestive enzymes.
Absorbing substances: The cell membrane of certain epithelial tissues contains carrier proteins that facilitate the absorption of substances.

Simple epithelium is composed of a single layer of cells.
Simple squamous epithelium: thin, flattened cells, found in areas where diffusion and filtration occur (e.g., lining of blood vessels, air sacs of the lungs).
Simple cuboidal epithelium: cube-shaped cells, involved in secretion and absorption (e.g., lining of kidney tubules, glands).
Simple columnar epithelium: tall, column-shaped cells, responsible for secretion and absorption, often have cilia or microvilli (e.g., lining of the digestive tract).

Stratified epithelium is composed of two or more layers of cells.
Stratified squamous epithelium: multiple layers of flattened cells, protects underlying tissues from abrasion (e.g., skin, lining of mouth).
Stratified cuboidal epithelium: rare tissue consisting of two or more layers of cube-shaped cells, found in sweat glands and ducts of certain glands.
Stratified columnar epithelium: multiple layers of column-shaped cells, also found in sweat glands and ducts of certain glands.

Pseudostratified columnar epithelium: appears stratified but all cells touch the basement membrane, often ciliated (e.g., lining of the trachea, nasal cavity).
Transitional epithelium: specialized stratified epithelium that can stretch and change shape, found in the urinary bladder and ureters, allows for expansion and contraction as the bladder fills and empties.

The free surface of epithelial tissues is not in contact with other cells.
It's the surface that faces away from underlying tissues.
Microvilli: cylindrical extensions of the cell membrane, increase surface area for absorption in the small intestine.
Cilia: hair-like projections that move fluids or particles across the epithelial surface (e.g., trachea, uterine tubes).

Desmosomes: mechanical links that bind cells together, providing structural support.
Tight junctions: bind adjacent cells and act as permeability barriers, preventing the passage of materials between cells.
Gap junctions: small channels that allow small molecules and ions to pass from one cell to another, facilitating communication.

Glands are secretory organs composed primarily of epithelium with supporting connective tissue.
Exocrine glands: have ducts that maintain open contact with the epithelium, secrete substances onto a surface.
Endocrine glands: lack ducts and secrete hormones directly into the bloodstream.

Draw all the different types of epithelial tissue.
Cite the organs or structures in the body where each type of epithelial tissue is found.
Describe the function of each type of epithelial tissue.