Epithelial tissues are the thin tissue that lines the interior of hollow organs and cover exposed body surfaces such as skin the airways, the digestive tract, as well as the urinary and reproductive systems. It is one of the four types of tissues in animals. Epithelial tissue consists of epithelial cells that arrange in the continuous sheet, in either single or multiple layers that are underlain by the basement membrane.
This basement membrane is the extracellular matrix that provides structural support to the epithelial tissue. The basement membrane consists of two layers basal lamina, and the reticular lamina. The basal lamina is the mixture of proteins such as glycoproteins, laminin, and collagen as well as proteoglycans secreted by the epithelial tissue on which they sit and helps the epithelial cells to hold the one side of connective tissue. Reticular lamina is secreted by connective tissue.
No blood vessels cross the basement membrane to enter the tissue because they are completely avascular. Nutrients must come by diffusion or absorption from underlying tissues or the surface inside the tissue.
Epithelial cell membranes have three regions which are following:
- The free surface is a project towards the external surface
- Help to interact with cells and form tightly packed contiguous cells
- connected with Basel membrane
Classification Based In Function
Epithelial tissue can be divided based on function into
Surface epithelium tissues are also known as lining or covering epithelium. Surface epithelium specialized layer of tissues formed by closely aggregated cells that forms the outer covering of the skin and some internal organs. The surface epithelium also forms the inner lining of blood vessels, ducts, body cavities, as well as the inner lining of the respiratory, digestive, urinary, and reproductive systems. Surface epithelial helps in protection against mechanical chemical and harmful effects.
The glandular epithelium is a type of epithelial tissue that covers the glands (both exocrine and endocrine) and also that makes up the secreting portion of glands Like the thyroid gland, adrenal glands, sweat glands, and digestive glands.
The main function of glandular epithelium is secretion such as digestive juice, sweat, sebum, breast milk, mucous (mucins). Endocervical glandular epithelium (Glandular epithelium in the reproductive tract) helps in sexual functions by secreting lubricating fluid during sexual excitation.
The glandular epithelium in the intestine helps in the absorption of nutrients and thus aids in digestion. Thus, the glandular epithelium is a multifunctional tissue that performs vital functions of the body with the connection to other tissues.
Classification Based On Cells Layer And Shape
Epithelial tissues are also classified on the basis of shape and number of layers present. Epithelial tissue can be:
Simple Epithelial Tissue
Simple epithelial tissues consist of only one layer of cells with all cells attached to the basement membrane. They are further divided into three main types
Simple Squamous Epithelium
The simple squamous epithelium consists of a single layer of flat cells and has the appearance of thin scales. Epithelial tissue that lines vessels of the lymphatic and cardiovascular system, and is made up of a single layer of squamous cells called the endothelium.
The function of simple squamous epithelium is to secrete a lubricating substance, allows diffusion and filtration. Because of the thinness of the cell, Simple squamous epithelium present where the rapid passage of chemical compounds is observed.
They present in blood and lymphatic vessels (lining of capillaries), air sacs of lungs (Alveoli), lining of the heart, glomerular (Bowman’s) capsule of kidneys and the inner surface of the tympanic membrane (eardrum).
Simple Cuboidal Epithelium
Simple cuboidal epithelium is a single layer of cube-shaped cells and has a nucleus of the box-like cells that appears round and is generally located near the center of the cell. Simple cuboidal epithelium allows excretion, secretion, or absorption of molecules. Simple cuboidal epithelia are found in the lining of the kidney tubules and in the ducts of small glands and also covers the surface of the ovary, lines the anterior surface of the capsule of the lens of the eye, forms pigmented epithelium at the posterior surface of retina of the eyes and surface of the ovary.
Simple Columnar Epithelium
The simple columnar epithelium consists of a single layer of closely packed elongated cells and located in the basal end of the cells. These tissues can be further classified based on the presence of motile cilia; ciliated or non-ciliated columnar epithelium. Ciliated tissues including the respiratory system (bronchi); helps to remove particulate matter, uterine tubes, and uterus.
Non-ciliated is found in the digestive system (lining the stomach and lining the small intestine is covered with microvilli) and parts of the female reproductive tract. This epithelium is active in the absorption and secretion (secrete mucous and enzymes) of molecules and also provides a barrier against the non-specific movement of luminal substances.
Pseudostratified Columnar Epithelium
The pseudostratified columnar epithelium contains a tuft of cilia tops each columnar cell. Pseudostratified columnar epithelia are tissues formed by a single layer of cells that give the appearance of being made from multiple layers (stratified). Nuclei of neighboring cells appear at different levels rather than clustered in the basal end leading to the illusion of being stratified and each cell is in contact with the basement membrane. The pseudostratified columnar epithelium is found in the respiratory tract (nasal cavity, trachea, and bronchi).
Pseudostratified columnar epithelium with stereocilia found in the epididymis, a highly coiled genital duct of the male larger ducts of many glands, and parts of the male urethra.
A stratified epithelium consists of several stacked layers of cells of various shapes, and basement membranes are usually absent and the basal layer is the only one that is in contact with the basal lamina. This epithelium has a mechanical or protective role against physical and chemical wear and tear.
- Stratified squamous epithelium
- Stratified cuboidal epithelium
- Stratified columnar epithelium
Stratified Squamous Epithelium
The stratified squamous epithelium has two or more layers of cells that are squamous at the superficial layer, whereas the basal layer contains either columnar or cuboidal cells. They can be keratinized or non-keratinized stratified squamous epithelium on the basis of the cytoskeletal structures found within the cell.
The apical segment or several layers deep to it may be covered with dead cells filled with keratin helps to form the waterproof tough layer prevents drying of the live cells present underneath. This epithelium forms the superficial layer of the skin. Non keratinized epithelium does not contain large amounts of keratin in the apical layer.
The lining of the mouth cavity (lining of mouth, esophagus, part of the epiglottis, part of the pharynx, and vagina and tongue) is an example of a nonkeratinized, stratified squamous epithelium.
Stratified Cuboidal Epithelium
The stratified cuboidal epithelium multiple layers of cells that are cuboidal at the superficial layer, whereas the basal layer contains either columnar or cuboidal cells. They are not common in the human body but found in the excretory ducts of salivary and sweat glands.
Stratified Columnar Epithelium
The stratified columnar epithelium multiple layers of cells that are columnar at the superficial layer, whereas the basal layer contains either columnar or cuboidal cells. They are seen in the conjunctiva of the eyes, parts of the urethra, and the small area in the anal mucosa.
Transitional epithelium is a stratified, multi-layered, collection of transitional epithelial cells so-called because they can undergo a change in their shape, structure, and composition. They are found urinary bladder, urethra, and ureters.
As the bladder fills with urine this epithelium is less stratified and transit to cuboidal to squamous shape but when the bladder is filled with urine his epithelium is convoluted and has cuboidal apical cells with convex, umbrella shaped, apical surfaces.