Functions of the Epithelium

Epithelia tissue forms boundaries between different environments, and nearly all substances must pass through the epithelium. In its role as an interface tissue, epithelium accomplishes many functions, including:

1. Protection for the underlying tissues from radiation, desiccation, toxins, and physical trauma.
2. Absorption of substances in the digestive tract lining with distinct modifications. 
3. Regulation and excretion of chemicals between the underlying tissues and the body cavity.  
4. The secretion of hormones into the blood vascular system. The secretion of sweat, mucus, enzymes, and other products that are delivered by ducts come from the glandular epithelium.
5. The detection of sensation.

Characteristics of Epithelial Layers

Epithelial tissue is composed of cells laid out in sheets with strong cell-to-cell attachments. These protein connections hold the cells together to form a tightly connected layer that is avascular but innervated in nature. 

The epithelial cells are nourished by substances diffusing from blood vessels in the underlying connective tissue. One side of the epithelial cell is oriented towards the surface of the tissue, body cavity, or external environment and the other surface is joined to a basement membrane. The basement layer is non-cellular in nature and helps to cement the epithelial tissue to the underlying structures. 

Types of Epithelial Tissue

Epithelial tissues are identified by both the number of layers and the shape of the cells in the upper layers. There are eight basic types of epithelium: six of them are identified based on both the number of cells and their shape; two of them are named by the type of cell (squamous) found in them.

Classification of Epithelial Tissue

Epithelial tissue is classified based on the number of cells, the shape of those cells, and the types of those cells.

This is a chart of the eight basic types of epithelium. It contains three columns, named cells, location, and function. Cell shapes and their arrangement of layers are used to classify epithelial tissues. The cells and their descriptions are: Simple squamous epithelium, located in the lungs and the lining of the heart, blood vessels, and lymphatic vessels are air sacs. They allow materials to pass through by diffusion and filtration, and secrete a lubricating substance. Simple cuboidal epithelium, located in the ducts and secretory portions of small glands and in kidney tubules, secrete and absorb substances. Simple columnar epithelium is ciliated tissue located in the bronchi, uterine tubes, and uterus. In the digestive tract and bladder, it is a smooth (nonciliated) tissue. Pseudostratified columnar epithelium is ciliated tissue that lines the trachea and much of the upper respiratory tract. It secretes and moves mucus. Stratified squamous epithelium lines the esophagus, mouth, and vagina and protects against abrasion. Stratified cuboidal epithelium, located in the sweat glands, salivary glands, and the mammary glands, is a protective tissue. Stratified columnar epithelium, located in the male urethra and the ducts of some glands, secretes and protects those areas. Transitional epithelium lines the bladder, urethra, and the ureters. It allows the urinary organs to expand and stretch.