Epithelial tissue forms the covering on surface. It forms the covering of all body surfaces, line body cavities and hollow organs, and are the major tissue in glands.Definition of Epithelial Tissue
The word ‘epithelium’ means a covering. The epithelial tissues cover the surfaces of the body (both outer and inner) and hence the cells are arranged in the form of a layer or lining.
The cells of the epithelial tissue are closely packed and stick together by their outer borders.
There is no intercellular space and very little or no intercellular substance (ground substance or matrix) is present. The cells are placed on a very thin and supporting connective tissue, called the basement membrane.
The epithelial tissues are devoid of any blood vessel or lymph vessel and are of two main types such as simple epithelium and compound epithelium.
The epithelium is said to be simple or unilaminar when the cells forming the epithelium are arranged in a single layer. It is said to be compound or stratified or multilaminar when the cells are arranged in several layers.
General Functions of Epithelial Tissue
- The main function of epithelial tissue is to protect the underlying tissues, as the former remains nearest to the external or internal environment. The examples are epithelium of skin and esophagus.
- The epithelium is also concerned with the prevention of dehydration, absorption, excretion of waste products, secretion, respiration, and the transportation of spermatozoa or ova.
There are mainly two kinds of epithelium. One is the Simple Epithelium and the other one is the compound epithelium. Let us know about them in detail.
Types of Epithelial Tissue
Epithelial tissue is broadly divided into two categories, namely, simple epithelium and compound epithelium.
Simple epithelium maybe squamous, cuboidal, columnar, ciliated, glandular, sensory and germinal, whereas compound epithelium maybe stratified and transitional.
Let us know about all these epithelium one by one.
(A) Simple epithelium
Simple Epithelium consists of a single layer of epithelial cells, situated on the basement membrane. It is found on the surface which generally remains moist. According to the shape and function of the cells, simple epithelia (singular: epithelium) may be of the following types:
1. Squamous Epithelium
Squamous Epithelium is composed of thin, broad, and flat cells with distinct nuclei. They are closely placed edge to edge like the marble tiles of a floor and are therefore called pavement epithelium.
This epithelium forms the peritoneum, lining of the coelom, alveoli of lungs, Bowman’s capsule of the kidney, and the endothelium forming the inner surface of a blood vessel, etc. It is mainly protective in function forming barriers in regions where diffusion or filtration takes place.
2. Cuboidal or Cubical Epithelium
The cells of this tissue are cuboidal in shape with the central round nucleus. The cells appear polygonal on the surface view.
This type of epithelium is found in the thyroid glands, uriniferous tubules, and other ducts.
It plays important role in secretion, protection of inner tissues, and storage of glycogen and metal ions.
3. Columnar Epithelium
The cells of this tissue are tall, pillar-like, closely set, and placed with their long axes at a right angle to the free surfaces. Their nuclei are generally elongated and located almost at the same level.
This epithelium forms the inner lining o the alimentary canal from the stomach to the rectum. These cells along with their basement membrane form the mucous membrane. Their main function is the absorption of digested food.
The intestinal and gastric glands are lined with the columnar cells which secrete digestive enzymes. It is also found in the inner lining of the gall bladder, bile ducts, vas deferens, and some parts of the respiratory tracts.
4. Ciliated Epithelium
These cells are generally columnar or cuboidal in shape. The free end of each cell bears numerous fine protoplasmic hair-like processes, called cilia.
These cilia can move back and forth producing a peculiar ciliary motion which can be compared with the wave produced in a paddy field by a gentle breeze.
This type of epithelium forms the inner lining of nasal cavities, trachea, bronchi, mouth cavity of frog, oviduct, and ureter of vertebrates. Their main function is to produce wavy movement that sweeps mucous and other substances in a particular direction.
5. Glandular Epithelium
Glandular epithelium resembles columnar epithelium but the cells are larger and produce various types of secretory or excretory materials.
These cells also form the lining of glands which are of two types: unicellular and multicellular.
a. Unicellular Glands: These are composed of single cells. The various types of unicellular glands and salivary glands include mucous cells in the skin of fish and the goblet cells of the small and large intestine.
b. Multicellular Glands: These glands may be simple or compound. They are formed of many glandular cells and are situated deep in the body. The mucous glands in the skin of frogs, the sweat glands, and the salivary glands of mammals are multicellular.
6. Sensory Epithelium
Sensory Epithelium is generally composed of columnar cells. Each usually bears one or more sensory hairs at its free end.
It is connected by a sensory nerve fiber with the central nervous system. Therefore, they are known as neurosensory cells and the epithelium is called the neurosensory epithelium.
This type of epithelium is found in the nasal passage, internal ear, and retina. Its main function is the reception of stimuli of different kinds.
7. Germinal Epithelium
Germinal Epithelium is the modified columnar epithelium. This epithelium is found in the testis and ovary. The cells of the germinal epithelium are found in the testis and ovary.
The cells of germinal epithelium undergo repeated cell division and give rise to gametes, i.e., female sex cells (ova) or male sex cells (sperms).
(B) Compound Epithelium
The compound epithelium consists of more than one layer of cells, of which the deepest layer is situated on the basement membrane.
This type of epithelium is found on the surface of the body which generally remains dry and where cells are destroyed due to friction. It is mainly of two types :
- Stratified Epithelium
- Transitional Epithelium
1. Stratified Epithelium
This is composed of several layers of cells. The innermost layer is placed on a basement membrane and consists of large cells capable of undergoing mitosis.
These cells continuously divide to form new cells and the innermost layer is therefore called the germinative layer. The new layers formed, keep on moving to the surface, whereas the old outermost layer dies and separates off due to friction.
This type of epithelium is found in the epidermis of the skin, nasal cavity, buccal cavity, esophagus, and vagina of mammals. Sometimes cilia are present at the outer border of the stratified ciliated epithelium.
2. Transitional epithelium:
Transitional epithelium is thin, elastic, and is found in the walls of the urethra of mammals. It forms the inner lining of the urinary bladder, ureters, and consists of 3 to 4 layers of cells. It allows stretching up to one or two cells thickness.
In some cases, a simple epithelium may appear to be stratified. Such epithelium is known as the pseudostratified epithelium. This happens when there is a layer of tall cells with nuclei at different levels or with cell bodies expanded at different levels.
Sometimes there are two types of cells on a basement membrane but one type reaches the surface. The urinary bladder is a common example.