Epithelial tissue also known as the epithelium is one of the basic tissues which is the part of the whole human body. The cells of this tissue are arranged in a manner where very little intercellular space is present between the cells. The epithelial tissue or the epithelium is present on the outer layer of blood vessels and different organs. It performs the function of protecting tissues and organs beneath it from various radiations, damage, harmful chemicals and also pathogens. It is further divided into eight types based on the shape of the cells, function of the cells and the arrangement of cells in layers.

Epithelial Tissue Structure

Epithelial tissue is made up of cells which are closely packed against one another with very little or no intercellular space in between. Out of the two surfaces of the epithelial tissue, one faces the outer environment while the other is attached to various tissues of the membrane. It is present on the outer surface of tissues and also in the inner surface of some cavities. 

Endothelium is a specialised type of epithelium which covers the inner side of the heart and blood vessels and is known as vascular endothelium. The epithelium present in lymphatic vessels is known as lymphatic endothelium. The cuticle of the arthropods is also formed from epithelial ectoderm. Epithelial tissues also contain cell junctions which allow or restrict movement of substances. Epithelial tissues are also capable of forming membranes which are attached to the connective tissues. 

Epithelial Membranes

Epithelial tissues are divided into 2 types of membrane

• Mucous membrane which is also known as mucosa. There are specialised cells which are responsible for secreting mucus known as the goblet cells. The mucus is useful as it provides lubrication for movement of substances and also provides protection from pathogens. Due to its lubricating nature it also prevents the tissues from drying out. It can be seen in respiratory nasal passage and in digestive tracts also.
• Serous membrane which forms the lining of the cavities present in the body is made up of squamous epithelium and secretes fluid inside the cavities of the body. 

Types

The epithelial cells are classified into 3 types based on the shape

• Squamous cells- these cells are thin and flat. The cells do not contain intercellular spaces which leads to formation of smooth surface area. This allows fluids and substances to move over it smoothly. The nuclei present in the squamous cells are oval in shape. It is generally present in the skin or alveoli of the lung.
• Cuboidal cells- The cells are cuboidal in structure. The nucleus of these cells are present in the centre and are large and spherical in shape. It can be found in the exocrine glands. It is known for its role in protection and movement of material in and out the lumen.
• Columnar cells- these cells are elongated and in shaped like columns. Their height is more compared to their width. The nuclei of these cells are also elongated but are present at the base of the cell. Some of these cells also have microvilli on their surface to increase the surface area. Due to this feature this type of cell is found in gastrointestinal tracts.

Epithelial cells are also classified on the layers of cells present into 2 types

• Simple epithelium – the cells in simple epithelium form a single layer only. It is responsible for secretions and absorptions.
• Compound or Stratified Epithelium- the cells can form 2 or more than 2 layers and perform the function of protection.

Conclusion

Epithelial tissue is also termed as epithelium. It is the one of the simplest tissues with which the lining of the different parts, cavities, organs, etc are made. The cells in epithelial tissue are arranged closely to each other leaving behind little or no intercellular space.  This tissue has a free surface, which faces either a body fluid or the outside environment and thus provides a covering or a lining for some part of the body. The cells are compactly packed with little intercellular matrix. There are two types of epithelial tissues namely simple epithelium and compound epithelium. Simple epithelium is composed of a single layer of cells and functions as a lining for body cavities, ducts, and tubes. The Compound epithelium consists of two or more cell layers and has a protective function as it does in our skin.