Epithelial Tissue

Every cell in our body is specialised to perform a certain function. Similar kinds of cells constitute a tissue, which is classified based on the structure and functions performed by them. The human body has four main types of tissue; epithelial, connective, neural and muscular.

What Is Epithelial Tissue?

Epithelial tissue or epithelium forms the outer covering of the skin and also lines the body cavity. It forms the lining of respiratory, digestive, reproductive and excretory tracts. They perform various functions such as absorption, protection, sensation and secretion.

Structure of Epithelial Tissue

Epithelial tissue is formed from a tightly fitted continuous layer of cells. One surface of the epithelial tissue is exposed to either the external environment or the body fluid. The other surface is attached to tissue by a membrane, which consists of fibres and polysaccharides secreted by epithelial cells.

There is little intercellular material present between cells. There are specialised junctions present between the cells of the epithelium that link individual cells.

Tight junctions- prevent leakage across tissues

Adhering junctions- keep the neighbouring tissues well cemented together

Gap junctions- facilitate the movement of ions and molecules across the tissue

Epithelial cells form membranes. The epithelial membrane consists of a layer of epithelial tissue and has underlying connective tissue. There are two types of epithelial membranes, mucous membrane and serous membrane.

Mucous membrane: It is also known as mucosa. There are goblet cells present, which secrete mucus. The mucus helps in lubrication, protection and easy movement of materials. It prevents tissues from drying. It lines the body cavities such as respiratory and digestive tracts, which open outside the body.

Serous membrane: The serous membrane lines the body cavities, which do not open outside the body, such as the lining of the pleural cavity, pericardial membranes. These membranes secrete the fluid inside the cavity and are made up of simple squamous epithelium.

Glands are made up of epithelial cells. There are two types of glands, exocrine and endocrine. Exocrine glands secrete their product into a duct, e.g. goblet cells, sweat glands. Endocrine glands are called ductless glands and they release their product directly into the blood or intestinal fluid, e.g. hormones.

Epithelial Tissue Functions

Protection: As it covers the entire body surface, it is the first line of defence against any kind of mechanical injury, chemical exposure, excessive fluid loss and infections. Ciliary projections present in the nose or upper respiratory tract, trap the dust particles and prevent it from entering the body

Absorption: The epithelial lining of the digestive tract absorbs water and nutrients

Exchange of substances: Epithelial tissue regulates the exchange of substances between body and external environment as well as the internal exchange between different parts of the body. Everything that enters the body or enters the bloodstream by absorption has to cross the epithelial barrier

Sensation: Sensory receptors are present in the epithelial tissue of the nose, eyes and ears, taste bud, etc. that help in transmitting signals from the external stimuli to the brain

Secretion: Various glands made up of epithelial cells secrete hormones, enzymes, saliva, mucus, sweat, etc.

Types of Epithelial Tissue

There are three types of epithelial cells, which differ in their shape and function.

Squamous Epithelium – They are thin and flat cells

Cuboidal Epithelium – They are short cylindrical cells, which appear hexagonal in cross-section

Columnar Epithelium – They are long or column-like cylindrical cells, which have nucleus present at the base.

On the basis of the number of layers present, epithelial tissue is divided into the simple epithelium and stratified or compound epithelium

Simple Epithelium- it is composed of one layer of a cell and mostly has a secretory or an absorptive function

Compound (Stratified) Epithelium- it is made up of two or more than two layers of cells and mostly has a protective function.

The glandular epithelium is made up of cuboidal or columnar cells. They are specialised for secretion.

Unicellular- isolated glandular cells, e.g. goblet cells

Multicellular- a cluster of cells, e.g. salivary glands

The columnar or cuboidal epithelium may bear cilia and are referred to as ciliated epithelium. They help in the movement of mucus in a specified direction, e.g. fallopian tubes and bronchioles.

Types of Epithelial Tissue	Location	Structure	Function
Simple squamous	Blood vessel lining, air sac lining of lungs	A single layer of flat cells having irregular boundaries	Transport by diffusion and where minimal protection is required
Simple Cuboidal Epithelium	The tubular lining of kidneys, glandular ducts	A single layer of short cylindrical cells. It may have microvilli as in proximal convoluted tubules	Absorption and secretion
Simple Columnar Epithelium	Digestive tract and upper respiratory tract lining	A single layer of columnar cells (tall and slender) and often ciliated	Protection, absorption, mucus secretion and movement in a specific direction
Stratified Squamous Epithelium	The lining of the mouth and vagina	Made up of several layers of cells, continuously sloughed off and regenerated. The older layer of cells is pushed upwards and becomes flat. The lower layer is columnar and metabolically active	Protection
Stratified Cuboidal	Mammary glands, sweat gland and salivary glands	The upper layer is cuboid and other layers may be cuboidal or other types	Protection of ducts of various glands
Stratified Columnar	Male urethra and lobar ducts of salivary glands	There is a layer of columnar cells present on squamous, columnar or cuboidal epithelial cells	Protection and secretion
Pseudostratified Columnar	Respiratory passage and ducts of many glands	Similar to columnar epithelium but all the cells are not of similar height	Protection, secretion and movement of mucous
Transitional epithelia or urothelium	Urinary bladder, urethra, ureter	Stratified epithelium, which can contract or expand as per the requirement. Cells are cuboidal when not stretched, but when the organ stretches, then tissue gets compressed and cells appear irregular and squamous-shaped	Stretch readily to accommodate the different volume of liquids Act as a barrier and have tight junctions to prevent reabsorption of toxic substances
Keratinised	The outer or apical layer of the cell	Mostly dead and devoid of nucleus and cytoplasm. The cytoplasm gets replaced by keratin, which makes the layer waterproof	Protection against abrasion