EPITHELIA:
GENERAL FEATURES & FUNCTIONS

There are two basic types of epithelial tissues: covering and lining epithelia and glandular epithelia


What are the histological characteristics common to ALL EPITHELIAL TISSUES?

  1. They are made of many cells close to each other (there is little extracellular material between epithelial cells).
  2. Several types of junctional specializations unite adjacent epithelial cells (tight junctions, desmosomes and gap junctions).
  3. With the exception of endocrine glands,
    • all epithelia have one free surface, called the apical surface, which is exposed at the body sur face or at the lumen (space) of the body cavity, duct, tube or vessel.
    • The lower surface of an epithelium (or basal surface) rests on a basement membrane: a non-living adhesive material secreted by the epithelium and the underlying connective tissue.
    • There are no blood vessels within the epithelial layer.
    • These cells are often characterized by frequent cell division because they are exposed to wear and tear and injury, necessitating replacement.


What are their functions?

Covering epithelia serve several important functions.
  1. Epithelia protect underlying tissues against physical damage, drying out, chemical injury and infection.
  2. Epithelia allow and regulate the passage of materials (diffusion, absorption, filtration, secretion, excretion) into and out of the deeper tissues of the body which they cover or line. Oxygen, water, food, and waste must pass through one or more epithelial layers.
  3. Specialized epithelia form sensory parts of organs such as the eye, ear, mouth (taste buds), and nose (olfactory epithelium).
Most glands are derived from epithelial cells specialized for producing secretions. The material secreted is usually a watery fluid containing substances such as salts, enzymes, hormones, mucus, fats, etc. The functions of glandular secretions are exceedingly diverse. Milk, insulin, sweat, saliva, calcitonin, tears and bile are all products of glands.