They are thin tissue that cover all the exposed surface of the body. They cover various surfaces of body parts like inner lining of mouth, digestive tract, secretory glands, lining of hollow parts of every organ such as heart, lungs, eyes, the urogenital tract, etc. The cells which form epithelial tissue are closely attached to each other through a structure called tight junctions. They don't have blood vessels and nerves and also they are supported by connective tissue called the basement membrane. They show polarity with distinct basal domains facing the basement membrane and other apical surfaces of the lumen of an organ or the external environment.
Classification of epithelial tissue is mainly based on their layers, shape and functioning.
1. Classification Based on Their Shape:
a. Squamous Epithelium: This type of tissue has cells with wider length than their height. They are mainly found as the lining of the mouth, oesophagus, alveoli of lungs including blood vessels.
b. Cuboidal Epithelium: This type of tissue has similar width and height i.e they are mainly cube shaped.
c. Columnar Epithelium: This type of tissues are taller in height than width i.e column like shape. They are further classified into two types i.e ciliated columnar epithelium and glandular columnar epithelium.
2. Based on Layer Epithelial Tissue is Classified into Two Types, They are:
Simple Epithelial/Unilayered Epithelial: Having a single layer. They are in direct contact with basement membranes that separate it from the underlying connective tissue. They are further divided into various types:
a. Simple Squamous Epithelium: Their cells appear like scale and flattened or round form. Example: walls of capillaries, linings of pericardial, pleural and peritoneal cavities, linings of alveoli of the lungs.
b. Simple Cuboidal Epithelium: These types of cells may have secretory, absorptive, or excretory functions. Examples: collecting ducts of the kidney, pancreas, and salivary gland.
c. Simple Columnar Epithelium: These types of cells can be secretive, absorptive, or excretory in nature.Some of these tissues contain goblet cells and are referred to as simple glandular columnar epithelium. These secrete mucus and are found in the stomach, colon, and rectum.
d. Pseudostratified Epithelium: They can be either ciliated or nonciliated.Example of ciliated are respiratory epithelium.
3. Stratified Epithelium: Having two or more cells i.e they are multilayered. Further they are of three types:
a. Stratified Squamous Epithelium:They mainly provide protection against abrasion and water loss. They are mainly found in the oral cavity, esophagus, larynx, etc. Stratified cuboidal epithelium:
b. Stratified Columnar Epithelium: Their main function is secretive and protective process. They are present in the covering of conjunctiva of the eye.
c. Stratified Cuboidal Epithelium: Their main function is protective in nature. They mainly line the excretory ducts of the sweat glands, large ducts of excretory glands.
They are formed from tightly fitted continuous layers of cells. They are either exposed to the external surface or the body fluid inside the body. There are specialised junctions present between the cells of epithelium. Some of these junction are given below:
Tight Junctions: They mainly prevent leakage across the tissue.
Adherens Junction: They help in keeping neighbouring tissues properly cemented.
Gap Junction: They help in movement of ions and molecules across the tissue.
Epithelial Cell Mainly have Two Types of Membrane:
Mucous Membrane: It is also known as mucosal cells. Goblet cells have mucosal membranes. They mainly perform lubrication, protection process, and easy movement of material across the cell. They are mainly present in digestive tracts.
Serous Membrane: They mainly line body cavities which mainly open inside not outsides. Example: pleural cavity, pericardial membranes.
Glands: They are mainly of two types i.e exocrine or endocrine in nature. Exocrine glands secrete their secretion into a duct, example: goblet cells, sweat gland. Whereas endocrine glands secretes their secretion directly into blood. Example: hormones.
Epithelial cell play major role in various function of body, some of the crucial function are given below:
Secretion Process: Most of the epithelial cells have capability to produce secretive macromolecules. One of the best examples is glandular epithelium. As endocrine gland is also secretive in nature i.e it secretes hormones directly into the bloodstream and some of these hormones are insulin, tyroxin, etc. Whereas exocrine glands which are also secretive in nature release their hormones into ducts, example: sebum of skin, digestive enzymes in the small intestine.
Absorption Process: Absorption process is mainly performed by surface epithelia with the help of microvilli as they increase surface area for absorption process. Another example is columnar epithelium in the small intestine. Their main function in absorption of nutrients in the digestive tract.
Transportation: There are various pumps which are operated by epithelial tissues. Example: Hydrogen ion pump of kidney( H+ ion pump) as they function during urine formation.
Protection Process: They act as selective barriers and protecting barriers. They produce intoxication, tearing and infections.
Receptor Function: Epithelial cells can receive receptor i.e sensory information and they convert them into neural signals. Example of such epithelial tissue is olfactory nasal mucosa, they detect the odors coming from outside and further transmit these information to the central nervous system.
1. What are Epithelial Tissues?
They are thin tissue that cover all the exposed surface of the body. They cover various surfaces of body parts like inner lining of mouth, digestive tract, secretory glands, lining of hollow parts of every organ such as heart, lungs, eyes, the urogenital tract, etc. The cells which form epithelial tissue are closely attached to each other through a structure called tight junctions.
2. Explain different Types of Epithelial Tissues?
Epithelial tissue is classified on the basis of shape, layer and function:
A. On the Basis of Shape:
B. On the Basis of Layer:
3. Write Three Major Functions of Epithelial Tissue?
Epithelial tissue perform crucial functions, some of their major functions are:
They help in the protection process.
They help in the secretion process.
They also act as the receptors during various sensational activities.