IntroductionEpithelial tissue is basically a fundamental and protective tissue in animals and human body but carries various other important functions within the body. Epithelial tissue is found in the form of single or multi-layered sheets which covers the internal and external surfaces of the organs of an organism. Actually, epithelial tissue is derived from all the embryonic germ layers i.e. ectoderm, mesoderm, and endoderm. The epithelium present on the free surface of all body parts is mostly absorptive, secretory or excretory in function or bear sensory cells and nerve endings to specialize themselves for the reception of stimuli.
General characteristic features of a Epithelial tissue
Types of epithelial tissueBasing of the type of cells by which the epithelium is made, epithelium is of the following types:-
Types of Epithelial Tissue (Courtesy:-https://en.wikipedia.org/wiki/Epithelium)
Squamous epithelial tissueThe Simple squamous epithelial tissue is one of the thinnest epithelial tissue than of all the other type of epithelial tissues. This single-layered epithelial tissue is made up of a layer of large irregular shaped or large plate-like cells arranged in the form of overlapping tiles of a roof of a house. The cells of the epithelium are characterized by possessing dense cytoplasm and with a large nucleus. The cells are very thin in nature and due to this when we observe this tissue through a microscope the nucleus of the cells appear to protrude out of the cells.
Simple squamous epithelium can be found in a variety of locations of the human body. They can be seen in capillaries to the alveoli of lungs, nephrons of kidneys, the pleural cavity that lodges lungs, peritoneal cavity (abdominal cavity) which encloses stomach and intestines, mediastinum (thoracic cavity) that include the heart, lungs, trachea, and esophagus. Most of these cells arise from the ectoderm or outermost layer of cells in the embryo. However, some simple squamous epithelial tissues are also derived from the mesoderm or middle layer of embryonic cells.
As squamous epithelial tissue is thin in nature mostly this tissue is involved in absorption, transportation, diffusion, osmosis or filtration. It makes this tissue prominent in nephrons of kidney's, blood capillaries, alveoli of lungs. The blood capillaries which surround the alveoli of lungs are made up of thin single-layered epithelial cells. In the same way, the alveoli are also made up single-layered thin squamous epithelial cells. Because of this fact, respiratory gases get exchanged between alveoli of lungs and blood capillaries. Daily 1200 liters of oxygen get transported from the lungs to the body. Similarly, nephrons of the kidney are composed of squamous epithelial tissue. The Bowman's capsule of the nephron is made up of single-layered epithelial cells. In the same way, a knot of blood capillaries (glomerulus) present in the Bowman's capsule is also composed of single-layered squamous epithelial cells. Nearly 180 liters of blood gets filtered through the squamous epithelial tissue of glomerulus of each nephron in a day which helps to separate the impurities from our blood.
Columnar epithelial tissueColumnar epithelial tissue is made of cells which are very tall with a large elongated nucleus enclosed at the basal end of the cells. These cells are broader at the upper end and narrower at the lower end. The presence of microvilli on the apical surface of this epithelium increases the surface area. Some cells, goblet cells are also interspersed between these tall cells which help to secrete mucus. These goblet cells will have a short lifespan of 2-3 days and will be replaced by new cells from adjacent epithelial cells.
Simple columnar epithelium can be found in the stomach, small intestine, large intestine, rectum.
The columnar epithelial cells present in the lining layer of stomach and intestines help in secreting enzymes that help in digestion, secreting hormones that help in regulating digestion, peristaltic movements, and water balance, some cells help in absorption of digested nutrients, vitamins, minerals and some cells secrete mucus for immunity to the walls of stomach and intestines.
Ciliated columnar epitheliumColumnar epithelium or Cuboidal epithelium bearing cilia at the top end is known as ciliated epithelium. Ciliated epithelium is found in the lining of the respiratory tract or the tubes which leads to lungs and also in the oviduct or fallopian tubes of female reproductive system.
In the lining layer of windpipe or trachea, this epithelium includes goblet cells which secrete mucus. The mucus secreted by this epithelium traps dust particles and germs which may try to enter into the lungs through respiratory passages. The cilia present in this epithelium by their lashing movements sweeps this mucus containing germs and dust particles away from windpipe back into throat which we may spit out. Ciliated epithelium which forms the inner lining layer of the oviduct in female reproductive system. Cilia of the epithelium by its lashing movements push the egg received from ovary into uterus through oviduct.
Glandular epitheliumSome Cuboidal or Columnar epithelium is modified into Glandular epithelium. Goblet cells or aggregates of glandular cells found in the above epithelia serve as Glandular epithelium. This type of epithelium can be found in Salivary, gastric, intestine and sweat glands.
Sensory epitheliumThe cells of this epithelium is usually Columnar type. The outer end of the cells bear neurosensory hairs or sensory papillae and inner ends are attached to nerve fibers. These cells may be found singly or in groups or may be found interspersed with epithelial cells.
This type of epithelium can be found in retina of eye, lining of internal ear, lining of buccal cavity, epithelial covering of tongue etc.
Germinal epithelium The cells of this epithelium are cuboidal in nature. The cells of this epithelium possess an immmense power of cell division. The structural units of the testis are seminiferous tubules and are made of cells of germinal epithelium which produce sperms. Similarly, the cells of germinal epithelium of ovaries produce egg cells.
Stratified epitheliumIf epithelium is present in the form of many layers is known as Stratified epithelium. It is of four kinds:-
Stratified Squamous epitheliumThis type of epithelium consists of two to many layers. Cells found in different layers are not similar and cells present in the surface layer are more flattened. It is the main protective epithelium of the body and is found in the regions of more friction.
Keratinized stratified squamous epithelium (cells of the epithelium filled with insoluble fibrous protein keratin) is found in the epidermis of the skin.
Non-keratinized stratified squamous epithelium where cells lack keratin or include much-reduced keratin. This type of epithelium is found in cornea of eye, lining of buccal cavity, vagina etc.
Stratified cuboidal epitheliumThe cells of outermost layer are cuboidal. It is two to many-layered. It occurs in conjunctiva of eye, lining the ducts of sweat glands, some parts of anal canal etc.
Stratified columnar epitheliumThe cells of the superficial layer of this epithelium are columnar and it rests upon the layer of cuboidal or columnar of type. This epithelium can be found in the ducts of sweat glands, sebaceous glands, and mammary glands. This epithelium is more commonly found in embryo.
Transitional epitheliumCells of this epithelium is found in in 2-6 layers. This epithelium includes partly non-keratinized flattened cells. Cells of superficial layers of this tissue appear in the form of large cuboidal plates but lower layers include cells of small cuboids. This type of epithelium lacks basement membrane. This type of epithelium is mostly found in the the urinary system.