Classification of Epithelial Tissue
Classification of epithelial tissue is mainly based on the Shape, Layers and Specialized functions as follows:
Classification Based on Shape
- Epithelium Squamous: This type of tissue has cells that are longer than they are tall. They are mostly found in the mouth, oesophagus, alveoli of the lungs, and blood vessels.
- Epithelium Cuboidal: This type of tissue has a similar width and height, making it primarily cube-shaped.
- Epithelium Columnar: This type of tissue has a column-like shape and is taller than it is wide. There are two types of columnar epithelium: ciliated columnar epithelium and glandular columnar epithelium.
Classification Based on Layers
Epithelial tissue is classified as simple or stratified or compound epithelium based on the number of layers present.
Simple Epithelium
Simple epithelium is composed of a single layer of identical cells that are typically found on secretory and absorptive surfaces, where the single layer improves these processes. Simple epithelium is divided into three major types, which are named after the shape of the cells, which differ in function.
- Simple Squamous Epithelium: Their cells resemble scales and have a flattened or round shape. Capillary walls, pericardial, pleural, and peritoneal cavity linings, and lung alveolar linings are examples.
- Simple Cuboidal Epithelium: These cells can perform secretory, absorptive, or excretory functions. The collecting ducts of the kidney, pancreas and salivary gland are examples.
- Simple Columnar Epithelium: These cells can be secretory, absorbent, or excretory in nature. Some of these tissues, known as the simple glandular columnar epithelium, contain goblet cells. These are found in the stomach, colon, and rectum and secrete mucus.
- Pseudostratified: Some columnar epithelia become so crowded that some nuclei become displaced and several rows of nuclei appear in sections of the cell sheet, despite the fact that each cell remains attached to the basement membrane. It is difficult to trace the continuity of different cells from base to apex in these cases, and the epithelium may appear stratified despite containing only one layer of cells.
This type of epithelia is known as pseudostratified columnar epithelia. The most common location for pseudostratified columnar epithelia is in the respiratory airways. Cilia are found on the apical surface of these cells. Cilia are motile, meaning they beat in unison to move fluid in a straight line. Cilia in the trachea propel mucous and particles into the pharynx and out of the airways. Cilia are significantly longer than microvilli.
Stratified Epithelium
They may be ciliated or nonciliated. The respiratory epithelium is an example of the ciliated tissue. A stratified epithelium is made up of several layers of different-shaped cells, and basement membranes are usually absent. Daughter cells resulting from cell divisions push older cells upward toward the apical layer as basal cells divide. They become dehydrated and less metabolically active as they move toward the surface and away from blood supply in underlying connective tissue. Tough proteins predominate as cytoplasm decreases, and cells transform into tough, hard structures that eventually die.
The stratified epithelium is classified into two types: stratified squamous, stratified cuboidal, and stratified columnar.
- Stratified Squamous Epithelium: They primarily protect against abrasion and water loss. They are mostly found in the oral cavity, oesophagus, larynx, and other areas. Cuboidal epithelium stratified:
- Columnar Epithelium Stratified: Their primary functions are secretion and defence. They are found in the conjunctival covering of the eye.
- Cuboidal Stratified Epithelium: In nature, their primary function is to protect. They primarily line sweat gland excretory ducts and large excretory gland ducts.
Classification Based on Specialized Functions
Epithelial cells can also be classified based on their special functions, which include:
- Transitional Epithelium: (also known as urothelium) is made up of several layers of cells that flatten out when stretched. It lines the majority of the urinary tract and allows the bladder to expand.
- Glandular Epithelium: It specialised in the production and secretion (release) of substances. It is found in your glands, which are specialised organs capable of producing, storing, and/or releasing substances such as hormones, proteins, and water.
Epithelial Tissue
A structural organisation in animals begins with the smallest fundamental unit – the cell. Tissues are made up of a collection of cells that all serve the same purpose. When tissues join together to perform specific functions, organs form. All organisms in the animal kingdom are multicellular, but their cell organisation does not follow the same pattern.
Every cell in our bodies is trained to perform a specific function. A tissue is made up of similar types of cells that are classified based on their structure and functions. The human body is made up of four different types of tissue: epithelial, connective, neural, and muscular.