It refers to a group of cells that are arranged in layers to form an outer or inner covering of the body or organs.
- The epithelial tissue covers the free surface of the organs.
- It is derived from the ectoderm, mesoderm, and endoderm.
- It consists of compactly arranged cells that are in one or more layers.
- There is little amount of intercellular terrestrial substances between cells. This ground substance contains hyaluronic acid and Ca ++ secreted by cells.
- The epithelium is attached to a thin, non-cellular, gelatinous basement membrane. The basal region of epithelial tissue consists of 3 components. They are basal tools, reticular fibers, proteins and polysaccharides.
- Epithelial cells lack blood vessels.
- The plasma membranes of adjacent epithelial cells are held together by interdigitations, tight junctions, desmosomes, and intercellular bridges.
The main function of the epithelium is to protect the underlying tissue and it also helps in absorption, secretion, excretion and filtration in different organs.
Based on structure and function, epithelial tissue is of 2 types:
- Covering epithelium.
- Glandular epithelium.
The covering epithelium
Covers both exterior and interior surfaces.
- Simple epithelium.
- Composite epithelium.
- Transitional epithelium.
The simple epithelium is made up of a single layer of cells.
Squamous epithelium : it is composed of a layer of flat cells similar to a scale. Therefore, the name is scaly. Also called pavement epithelium. Squamous epithelium with wavy cell outlines is called testa epithelium.
Main distribution : forms the endothelium in blood vessels, the peritoneum in the coelom, the lung alveoili. It is also found in Bowman’s capsule and Henle’s Nephron handle.
Function : protects the underlying tissue. It has a filtering function. It allows the easy passage of liquids or gases through it.
Cubic epithelium : it is made up of a single layer of cuboidal cells that have the same dimension on each side and are placed on the basement membrane.
Distribution : it forms the germinal epithelium of the ovary and the internal part of the digestive and salivary glands.
Function : protection, secretion, storage of glycogen and metal ions are the functions of this tissue.
Columnar epithelium : it is formed by elongated cells of the tall pillar type. The nuclei are placed in the basal part. In the gastrointestinal tract and proximal convoluted tubules of the nephron, its free hosts are longitudinally striated.
Hence it is called brush border epithelium. Under the electron microscope, the edges of the brush are finger-shaped protections called microvilli. Microvilli increase the surface area for absorption of food materials.
Distribution : Found in the lining of the stomach, intestine, gallbladder, and proximal convoluted tubules of the nephron.
Function : its main function is the absorption of food and digested secretions.
Ciliated epithelium : here each cell has 20-30 cilia on its free surface. Each cilium arises from a basal granule found in the cytoplasm. The membrane of a typical cilium is continuous with the cell membrane.
Distribution : found in the lining of: the nose, larynx , trachea , small bronchi, bronchioles, and fallopian tubes.
Function : the beating of the cilia produces mucus, liquid, suspended particles, ovules, etc.
Pseudostratified epithelium : it is composed of cells with a single layer. Many layers due to uneven cell height. They are usually ciliates.
Distribution : found in the urethra and trachea.
Function : carries out all categories like ciliated epithelium.
Modified epithelium: consists of:
- Germinal epithelium: produces gametes in the ovary and testes.
- Sensory epithelium: transmits stimuli.
- Myoepithelium: In the salivary glands and their ducts, cells contain actin and myosin filaments and contract to release secretory products from the glands.
- Pigmented epithelium: In the retina, cells contain pigmented granules of colored melanin.
- Ependymal epithelium – forms a lining in the cavities of the brain and spinal cord .
- Cuticular epithelium: secretes the cuticle, the tough non-cellular protective layer of the body in invertebrates.
This epithelium is made up of many layers of cells. The innermost cells and layers are alive and divide mitotically to replace the outermost cells.
Function : protects the underlying tissue from mechanics, friction and injury.
Stratified squamous cornus : the outermost cells become dead and hard due to the deposition of scleroprotein keratin. The innermost dividing layer, the germinal layer, replaces the outermost lost cells, for example, skin, hair, horn, hoof, and nails.
Non-keratinized stratified squamous : here the surface cells are not keratinized, for example, the oral cavity, pharynx, vagina, cervix, anal canal, etc.
Stratified cuboidal : the cells of the outermost layer are cubic in shape, for example, the Graafian follicle.
Stratified column : the outermost cells are columnar cells of a columnar shape, for example, part of the urethra, epiglottis, etc.
Stratified ciliates – Cells beyond have cilia, for example the larynx.
This is found between the simple single-layer epithelium and many stratified stratified epithelia. Therefore, the name is transitory. It consists of 3 or 4 layers of cells. The cells of the superficial layer are irregularly quadriform.
The next layer consists of pyriform cells that are found in one or two layers of polyhedral cells.
Distribution : Found in the pelvis of the kidney, ureter, urinary bladder, and upper urethra.
Function : prevents the reabsorption of excreted materials into the system. It prevents the drainage of water from the blood and tissues by the higher osmotic pressure of the urine.
It is classified into two unicellular glands and multicellular glands. Goblet cells are single-celled glands and secrete mucus.
Types of glands based on the mode of secretion
- Apocrine gland: secretions are released from the apical part of the cell, for example, the mammary glands.
- Holocrine gland: the whole cell is discharged during secretion, eg sweat gland, salivary gland.
- Merocrine gland: Secretions are released through the plasma membrane by exocytosis, eg sweat gland and salivary gland.
Based on the nature of the secretion
Serous gland : watery, fluid secretion, rich in proteins of an enzymatic nature, for example, sweat gland, gland gland and parotid gland.
Mucous gland : secretes viscous secretions, for example, goblet cells, gastric glands and colon.
Mixed gland : secretes serous and mucous secretions, for example the pancreas and most of the gastric glands .