Epithelial Tissue Definition

One of the four kinds of tissue found in animals—the others being epithelial, muscular, connective, and nervous—epithelial tissue is made up of tightly clustered polyhedral cells that adhere strongly to one another to create cellular sheets that fill empty organs and coat the surface of the body. Epithelial tissue comprises cells arranged in either one or several layers of continuous sheets (plural epithelial tissue).

Characteristics of Epithelial Tissue

Even while the shape and functionality of the epithelial tissue found throughout the body might vary, they all have basic traits.

The following list includes some of these qualities:

1. Size and Shape

• Epithelial cells come in a variety of sizes and forms, including tall columnar, cuboidal, and low squamous.
• Depending upon the function, the cells have a certain shape and size.

2. Polarity

• Typically, organelles and membrane proteins are distributed unevenly inside epithelial cells, which exhibit polarity.
• An epithelial cell’s apical (free) surface is present in the direction of the body’s surface. Its chamber, organ lumen, or gland duct receives cell secretions. On the apical surfaces, cilia and microvilli can be seen.
• Intercellular adhesion and other connections may be present on the lateral surfaces of an epithelial cell that are exposed to the neighboring cells on either side.
• The basal surface of an epithelial cell adheres to extracellular substances such as the basement membrane, an inert connective tissue formed by the epithelial cells themselves.

3. Basement Membrane

• Two layers that normally constitute the basement membrane form a thin extracellular layer. The two layers are the basal lamina and the reticular lamina.
• Laminin, collagen and a few glycoproteins and proteoglycans are all found in the basal lamina, which is closer to and secreted by epithelial cells.
• The collagen protein is created by fibroblasts, which are connective tissue cells, and is found beneath and closer to the connective tissue in the reticular lamina.

4. Intercellular Adhesion and Other Junctions

• Cell adhesion and communication are provided by a number of membrane-associated structures.
• The most apical connections, also known as zonulae occludens, create a band that fully encircles each cell.
• The adherens junction, also known as the zonula adherens, or second kind of junction, encircles the epithelial cell, typically just under the tight junction.
• The desmosome or macula adherens, which are disc-shaped structures on the surface of a cell that correspond to similar structures on the surface of a neighboring cell, adhere to one another. This is another type of anchoring connection.
• Rather than facilitating cell adhesion or occlusion, gap junctions mediate intercellular communication.

5. Avascular

• Because it lacks blood vessels, epithelial tissue depends on the blood vessels of the nearby connective tissue to transport nutrients and eliminate waste.
• Through diffusion, most chemicals move from epithelial cells to connective tissue.

6. Innervated

• In other words, epithelial tissue is innervated and has its own supply of nerves.

7. Renew and Repair

• Because epithelial cells divide quickly, the epithelial tissue may continuously renew and repair itself by shedding off old, damaged cells and replacing them with fresh ones.

You may also like to read: Stratified Cuboidal Epithelium

Functions of Epithelial Tissue

• The functions of epithelial tissue vary depending on the location. Following are some of the functions:

1. Protection

• Protection is one of the most important roles played by epithelial tissue. It shields the cells below against damage caused by radiation, desiccation, pathogen invasion, poisons, and physical stress.
• Bleeding in the tissue after abrasion is prevented by the absence of blood vessels in epithelial tissue.

2. Transportation

• With the help of several pumps located inside the epithelial tissue, different chemicals may be transported into and out of the cells.
• Additionally, it enables the interchange of chemicals between the underlying cells and the body cavity, capillaries, and ducts in the respiratory, urinary, and digestive systems.

3. Secretion

• The glandular epithelium secretes a wide range of macromolecules, including hormones, which are involved in several biological processes.
• Numerous endocrine and exocrine glands assist various organ activities and the upkeep of the skin on the body’s exteriors (digestive system).

4. Absorption

• Epithelial tissue also facilitates the absorption of many molecules by expanding the surface area through the action of several specialized structures like cilia and microvilli on the surface of cells.
• The small intestine’s columnar cells aid in the digestive system’s absorption of water and various other nutrients.

5. Receptor Function

• Certain epithelial tissue cells are trained to carry out sensory tasks, allowing them to recognize sensory data and translate it into brain impulses.
• The olfactory sense is made possible by the apical cilia found in epithelial tissue, such as the pseudostratified columnar epithelium of the olfactory mucosa.

Types / Classification with Examples and Location

• Epithelial tissue is divided into two types:
• The coating and lining epithelium sometimes referred to as the surface epithelium, is responsible for the inner linings of blood arteries, ducts, body cavities, and the respiratory, digestive, urinary, and reproductive systems. It also creates the outer covering of the skin and some internal organs.
• The glandular epithelium comprises the secreting portion of glands such as the thyroid, adrenal, sweat, and digestive systems.
• Various kinds of covering and lining epithelial tissue are also classified based on the arrangement and shape of the cells that compose them.

Simple epithelium

• Simple epithelium is composed of a single layer of identical cells and is typically found on secretory or absorbent surfaces since the single layer facilitates these functions.
• There are three main categories of simple epithelium identified by the morphology of the cells, which vary according to their roles.

a. Simple Squamous Epithelium

• The simple squamous epithelium consists of a single layer of flat cells that imitate floor tiles with a central nucleus that is flattened, oval, or spherical when viewed from an apical surface.
• This epithelium, also known as the endothelium, is most frequently seen lining the circulatory and lymphatic systems (heart, blood vessels, lymphatic vessels), as well as the epithelial layer of serous membranes (peritoneum, pleura, pericardium).
• Additionally, the inner surface of the tympanic membrane, the glomerular (Bowman’s) capsule of the kidneys, and the air sacs of the lungs contain it (eardrum).

b. Simple Cuboidal Epithelium

• Simple cuboidal epithelium is a single layer of spherical cube-shaped cells with a nucleus in the middle.
• It 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 the retina of the eye, lines the ducts of certain glands, like the thyroid gland, and lines some ducts of other glands, like the pancreas. It also lines the kidney tubules and smaller ducts of various glands.

c. Simple Columnar Epithelium

• A single layer of rectangular-shaped cells forms the columnar epithelium on top of a foundation membrane.
• This epithelium, which covers numerous organs, is frequently developed to better fit a particular purpose.
• The stomach is lined with columnar epithelium without any surface features.
• The small intestine’s columnar epithelium, on the other hand, has microvilli covering its free surface. It provides a substantial surface area for nutrient absorption from the small intestine.
• The columnar epithelium of the trachea is ciliated. Additionally, it has goblet cells that exude mucus, and eggs are driven into the uterus by cilia in the uterine tubes.

d. Stratified Epithelium

• A stratified epithelium is made up of many layers of cells with different morphologies, and basement membranes are typically not present.
• When basal cells divide, daughter cells produced during basal cell divisions push older cells upward and toward the apical layer 
• As they move closer to the surface and far from the connective tissue’s blood supply, they get dehydrated and much less metabolically active.
• As the volume of cytoplasm shrinks, cells become stiff, rigid entities that eventually perish. Strong proteins are in charge.
• At the apical layer, dead cells that have lost their cell connections are shed off, and new basal cells are constantly replacing them.
• Stratified squamous, Stratified cuboidal, and Stratified columnar epithelium are the two basic forms of stratified epithelium.

Stratified Squamous Epithelium

• The stratified squamous epithelium has two or more layers of cells.
• The apical layer and numerous layers below it are made of squamous cells, although cells in lower levels might be cuboidal or columnar.

Keratinized Stratified Squamous Epithelium

• The apical portion of the cells in this epithelium grows a thick coating of keratin that extends many layers below it.
• As cells distance themselves from the nutritive blood supply and the organelles finally perish, the relative amount of keratin in the cells rises.
• The keratin creates a robust, mostly waterproof protective covering that stops the living cells below from drying out.
• The keratinized stratified squamous epithelium makes up the top layer of skin.
• This type of epithelium constitutes the epidermis of the skin.
• This epithelium is many layers thick and regularly hydrated by mucus from salivary and mucous glands. It does not include significant levels of keratin in the apical layer.
• The tongue is covered by nonkeratinized stratified squamous epithelium, which also borders the mouth, esophagus, a portion of the epiglottis, the throat, and the vagina.

Stratified Cuboidal Epithelium

• The apical layer of the stratified cuboidal epithelium is made up of cuboidal cells, but the lower layer can either be cuboidal or columnar.
• Salivary and sweat gland excretory ducts have stratified cuboidal epithelium.

Stratified Columnar Epithelium

• The stratified columnar epithelium comprises many layers of cells, with columnar cells making up the apical layer and cuboidal or columnar cells making up the lower layer.
• This kind of epithelium may be found in the anal mucosa, the urethra, and the conjunctiva of the eyes.

Pseudostratified Columnar Epithelium

• The nuclei of the cells are present at different levels, giving the pseudostratified epithelium the appearance of having many layers.
• Despite the fact that all the cells are attached to the basement membrane in a single layer, certain cells do not attain the apical surface.
• These characteristics give it the appearance of a multilayered tissue, even though it is only a single epithelium.
• This epithelium lines the airways of the majority of the upper respiratory tract, the male urethra, the epididymis, and the bigger ducts of several glands.

Transitional Epithelium Tissue

• Transitional epithelium’s (urothelium’s) appearance differs (transitional).
• Similar to stratified cuboidal epithelium when relaxed or unstretched, with the difference that the cells in the apical layer tend to be big and rounded.
• As tissue is stretched, cells flatten out, giving the look of stratified squamous epithelium. It is ideal for lining hollow structures such as the bladder that are prone to internal expansion because of its numerous layers and pliability.

Glandular Epithelium

• In epithelia with other major roles or glands and specialized organs, epithelial cells primarily operate to manufacture and secrete different macromolecules.
• Simple cuboidal, columnar, and pseudostratified epithelia frequently include scattered secretory cells, sometimes known as unicellular glands.
• By cell proliferation, development into the underlying connective tissue, and subsequent differentiation, glands form from the covering epithelia in the fetus.

Endocrine Glands

• Hormonal secretions from endocrine glands diffuse into the circulation after entering the interstitial fluid without passing via a duct.
• Because endocrine secretions are transported by circulation throughout the body, they have wide-ranging effects.
• The pituitary gland at the base of the brain, the pineal gland in the brain, the thyroid and parathyroid glands close to the larynx (voice box), the adrenal glands above the kidneys, the pancreas close to the stomach, the ovaries in the pelvic cavity, the testicles in the scrotum, and the thymus in the thoracic cavity are examples of endocrine glands.

Exocrine Glands

• Exocrine glands produce secretions that are released into ducts and onto the surface of organs like the skin or the lumen of a hollow organ.
• Exocrine gland secretions have just a few impacts, and several of them would be dangerous if they got into circulation.
• The skin’s sweat, oil, and earwax glands, as well as digestive glands like the pancreas and salivary glands (which discharge into the small intestine), are examples of exocrine glands.

References

• Epithelial Tissue
• What is the epithelium?