This type of tissue is made up of cells of different shape. It covers the body layers as skin and the internal layers of hollow organs like- stomach, uterus, cavities, blood vessels, etc. it forms organs’ boundary, free surfaces and lining membrane. All the epithelial cells are stuck with each other through substance called mucoprotein. A basement membrane is often found in such types of tissues, on which cells are located. These tissues act as absorption, secretion, protection and excretion.
There are mainly two types of epithelial tissue according to the shape and layers (membranes) of cells-
1. Simple epithelium-
- Columnar epithelium.
- Ciliated epithelium.
- Pavement or squamous epithelium.
- Cuboidal epithelium.
- Glandular epithelium.
2. Compound epithelium-
- Stratified squamous non-cornified.
- Stratified columnar.
- Stratified squamous cornified.
- Stratified columnar ciliated.
Simple epithelium- Only one layer of cells is found in simple epithelium. Normally, this layer is found on absorptive or secretive layers. This epithelium is very delicate and found on such places where minimum injury takes place. It is of five types whose name varies according to the shape and functions of cells.
Pavement or squamous epithelium- It is made up of only one layer of flattened cells and all the cells are situated on a basement membrane. Normally, nucleus is found in the center of the cell. This type of epithelium is found in alveoli, serous membranes like- peritoneum, pleura, etc., internal (bored) cardiac layer, internal layer of the blood vessels, cornea, tympanic membrane, etc. it is known as endothelium in the lungs, serous membranes, blood vessels and lymphatic vessels while as endocardium in the heart. The main function of this epithelium is to provide protection; as well as, the exchange of gases and liquids also take place by this epithelium.
Cuboidal epithelium- Its cells are cuboidal and situated on the basement membrane only in one layer. This type of epithelium is found in small terminal respiratory bronchioles, inside the digestive glands, salivary glands, in the layers of thyroid and ovaries. This type of epithelium protects the body organs and also acts somewhere as secretion, storage, etc.
Columnar epithelium- The cells of columnar epithelium are higher (longer) than their width i.e. are rectangular in shape. In this epithelium too, the cells are situated on the basement membrane only in one layer. This type of epithelium is found in stomach, intestinal layer, alveoli and most of glands’ tubes. In this epithelium, the shape of the cells varies in different places. This epithelium is known as very important in the view of function. It does two important works like absorption and secretion.
Ciliated epithelium- The cells of this epithelium are often columnar, but somewhere are also cubical. On the end of every cell, hairy structure of 20 to 30 hairs is found, called cilia or flagellae. The end, on which this type of hairy structure located, has also a line of basal particles and a cilium is attached with every basal particle. These basal particles are the fragments of the centriol of cell. Normally, this type of epithelium is found in fallopian tubes, trachea (respiratory path) and in the central tube of spinal cord. The cilia do their own ciliary movement that goes on taking place all the time in every living cell. Ciliary movement takes place 10-20 times per second. In this movement, the cilia bend once, known as effective phase and then come back in their straight position, known as return phase. By this ciliary movement, the ovum moves from fallopian tube towards uterus and dust, mucus, etc. are go on entering towards throat through trachea (respiratory path).
Glandular epithelium- This type of epithelium forms the layer of alveoli and mammary glands, sweat glands, salivary glands and few parts of the tubes of sebaceous glands and few parts of internal glands and the alveoli of the thyroid gland, etc. normally, the cells of this epithelium are cubical, columnar or polyhedral and often situated only in one layer. The cells of this epithelium produce any new substance and send them in its different types of secretions. It is their very important job.
Goblet cells- Goblet cells secrete mucus. This type of cells are often columnar in shape and found between cilia like trachea, gastro-intestinal tract, etc. in the deep of cell, nucleus and cytoplasm are found, while on the apical part, mucinogen granules are located secreting mucus. It produces glycoprotein mucin that expands gradually and secretes mucus after bursting. This process goes on taking place frequently (again and again). The secretion secreted from goblet cells acts as lubrication, forming protective shield (layer) on mucus membrane, neutralizing acid and alkaline, and as trapping bacteria, foreign substances, etc.
Compound epithelium- There are many layers of cells are found in the compound epithelium. The chief function (job) of it to protect the structures situated below it. These are as follow-
Transitional epithelium- The transitional epithelium has 3-4 layers of cells and located in between one layered simple epithelium and multilayered stratified epithelium. Hence, it is called transitional epithelium. The cells of its first line are big, flat, zigzag and octagonal. Two nucleuses are often found in them. In the second line (layer), pyriform cells are located, whose apexes (ends) are round in shape outwards and remain merged in the deep of the apex of lining cells below it. In the next one or two layers below it, small polyhedral cells are located that remain packed in between the pointed ends of pyriform cells of the second line. This epithelium is found in pelvis of kidneys, ureters, urinary bladder and in the upper part of urethra. It stops excretory substance to be absorbed again in the system.
Stratified squamous cornified epithelium-
This type of epithelium is made up of many layers of cells. The superficial layer is become horny due to the decomposition of keratin. It is found in skin. Hairs, nails, enamel of teeth, etc. are the epithelium tissue of this category (type). The next layer of the skin is made up of flat, scaly, compressed cells. In the layer next of it, the cells are wide and polyhedral. The cells present in the deep of further next line are small and columnar which are inter-connected with uncountable intercellular fibrils and protoplasmic processes (seem like horns). Due to only this horny structure, these cells are also known as prickle cells. Superficial layered cells go on falling down into pieces (damaging) by rubbing and go on repairing by the cell division of layered cells located in deep. This epithelium protects the structure situated under it from environmental effects, mechanical pressure, getting rub and hurt (injury). The skin is known as the typical example of it.
Stratified squamous, non-cornified- According to histology, its structure is like stratified squamous cornified. Only difference between both of them is that the upper layer of it is not made up of keratin. This type of epithelium is found in cornea, mouth, pharynx, oesophagus, anal canal, in the lower parts of urethra, vocal cords, vagina and cervix (the neck of womb). The body organs get protection from this epithelium.
Stratified columnar epithelium- This type of epithelium is rare and found only in few locations (places). It keeps small body parts as covered like- fornix of conjunctiva, pharynx, epiglottis, anal mucosa, cavernous part of urethra (male), etc.
Stratified columnar ciliated epithelium- This epithelium too is found only in few small places like- soft palate of nasal surface, in the few part of larynx, etc.