What are the common types of epithelial tissues?

Epithelial tissues are classified on the basis of the shape and arrangement of cells. There is a standard format for naming the types of epithelial tissues. The arrangement of cells is mentioned first followed by the shape of the cells. Based on this classification, there are 8 common types of epithelial tissues: 

Simple squamous epithelium

Simple squamous epithelium consists of a single layer of flattened cells packed together closely. The thin, permeable nature of this layer of epithelium makes it ideal for facilitating the efficient exchange of nutrients and wastes between the bloodstream and the surrounding tissue cells. Simple squamous epithelial tissue lines blood vessels, the air sacs, and body cavities. Its main functions include diffusion, filtration, and regulation of the passage of substances.

Simple cuboidal epithelium

Simple cuboidal epithelium is made up of a single layer of cube-shaped cells and is found in kidney tissues and glandular tissues such as salivary and sweat glands. The main functions of the cuboidal epithelial tissue include absorption, secretion, and protection. 

Simple columnar epithelium

Simple columnar epithelium is composed of a single layer of tall cells that resemble columns or pillars. It is found lining the stomach and intestines and its main functions include absorption and secretion. Some simple columnar epithelium cells have apical cilia or microvilli, which increase the efficiency of absorption. 

Pseudostratified columnar epithelium

Pseudostratified columnar epithelium is composed of a single layer of differently-sized columnar cells packed together closely. Because the cells are of different sizes, the nuclei lie at varying heights, giving the appearance that the cells are arranged in multiple layers. The cells have a lot of cilia on the outer surface.  Pseudostratified columnar epithelial tissue is found linking the upper respiratory tract and the trachea. 

Stratified squamous epithelium

Stratified squamous epithelium is made up of two or more layers of squamous cells stacked one on top of the other. Because of the multiple layers of cells, the stratified squamous epithelial tissue is thick and tough. It plays a pivotal role in protecting the underlying tissue from microbes as well as water loss. Some stratified squamous epithelia are keratinized. These cover the outer surfaces of the body, forming the epidermis of the skin. Non-keratinized cells are found in the lining of the esophagus, mouth, and vagina. 

Stratified cuboidal epithelium 

Stratified cuboidal epithelium is generally made up two layers of cuboidal cells. It is relatively rare and can be found in the excretory ducts of some glands such as the salivary, sweat, and mammary glands.

Stratified columnar epithelium

Stratified columnar epithelium is made up of multiple layers of columnar cells. It is relatively rare and can be found in the large excretory ducts of the salivary glands. It is also found in small amounts in the mucous membrane lining the eyelids and in the male urethra. 

Transitional epithelium

Transitional epithelium is a type of stratified epithelial tissue composed of specialized cells that are capable of expanding or contracting depending on the distention in the underlying organ. It is found lining the insides of hollow structures in the urinary system, including the urinary bladder, urethra, and ureters. The transitional epithelial tissue is relaxed when the bladder is empty. As the bladder fills with urine, the apical cells unfold, which increases the surface area and allows the bladder to expand. The transitional epithelial tissue plays a critical role in forming an impermeable barrier that prevents urine from passing through the bladder wall into the body.