How are epithelial tissues classified?

Epithelial tissues are classified by shape of the cells, number of cell layers and function of the cells. 

Classification of epithelial tissue based on cell shape

There are three types of epithelial tissue based on cell shape: 

• Squamous epithelial tissue: Squamous epithelial cells are flat, thin and scale-like in appearance. They are found in the alveoli of the lungs, esophagus, and the smooth lining of the mouth. 
• Cuboidal epithelial tissue: Cuboidal epithelial cells are short, cylindrical cells. They have a cube-like appearance with equal width, height, and depth. They appear hexagonal in cross-section. Cuboidal epithelial cells form the ducts of secretory organs and glands, and can be found in thyroid glands, kidney tubules, and sweat gland ducts. 
• Columnar epithelial tissue: Columnar epithelial cells are rectangular and taller than their width, giving them a column-like in appearance. These cells have a nucleus present at the base. There are two types of columnar epithelial tissue - ciliated columnar epithelium and glandular columnar epithelium. Columnar epithelium can be found in the ducts of glands, gallbladder, and intestinal lining. 

Classification of epithelial tissue based on arrangement of cells

There are three types of epithelial tissue based on how the cells are arranged. 

• Simple epithelial tissue: Simple epithelial tissue is formed by a single layer of cells. All cells are attached to the basement membrane. The thinness of the single cell layer allows the simple epithelium to perform functions such as secretion, absorption, and filtration. However, it is not thick enough to provide adequate protection. Simple epithelial tissue is found in the alveoli, uterus, bronchi, stomach, small intestine, and kidney tubules. 
• Stratified epithelial tissue: Stratified epithelial tissue is made up of two or more layers of cells stacked atop each other giving it a stratified appearance. The multilayered structure allows it to withstand mechanical or chemical forces, which plays a key role in protecting the underlying tissues. Even if the outermost cells get damaged or detached, the tissues lying underneath remain unexposed and undamaged. The cells in the lowermost layer of stratified epithelium are attached to the basal membrane. These cells are capable of dividing and generating new epithelial cells, which replenish the damaged or detached cells on the outer layer. Stratified epithelial tissue is found in the skin and the lining of the mouth, esophagus, and salivary glands. 
• Pseudostratified epithelial tissue: Pseudostratified epithelial tissue is made up of a single layer of cells of different sizes that appear to be arranged in multiple layers because the nuclei of the cells lie at different heights. All cells are attached to the basement membrane. Ciliated epithelial cells lining the trachea and upper respiratory tract are examples of pseudostratified epithelial tissues. 

Classification of epithelial tissue based on specialized functions

There are three types of epithelial tissue based on how their specialized functions: 

• Transitional epithelial tissue: Transitional epithelial tissue is composed of multiple layers of rounded cells that are capable of expanding and contracting depending on the distention of the organ. The cells become flattened when stretched. The extent of flattening depends on the extent of distention. Transitional epithelial tissue lines most of the urinary tract, which allows the bladder to expand when filled with urine and contract when empty.  
• Glandular epithelial tissue: Glandular epithelial tissue is composed of cuboidal or columnar cells that are specialized to produce and secrete substances such as hormones, milk, saliva, enzymes, mucus and body fluids. It lines the exocrine glands (salivary, mammary, and sweat glands) as well as the endocrine glands (ovaries, testes, thyroid, and pituitary, adrenal, and pineal glands). 
• Olfactory epithelial tissue: Olfactory epithelial tissue contains olfactory receptor cells that have specialized cilia extensions. Both of these features are responsible for the sense of smell. Odor molecules are captured by the cilia extensions when you inhale. The olfactory receptor cells transmit these captured odor molecules to the olfactory bulb in the brain, where they are then interpreted as smell. Olfactory epithelial tissue is found within the nasal cavity.