Epithelial Tissue

A structural organisation in animals begins with the smallest fundamental unit - the cell. Tissues are made up of a collection of cells that all serve the same purpose. When tissues join together to perform specific functions, organs form. All organisms in the animal kingdom are multicellular, but their cell organisation does not follow the same pattern.

Every cell in our bodies is trained to perform a specific function. A tissue is made up of similar types of cells that are classified based on their structure and functions. The human body is made up of four different types of tissue: epithelial, connective, neural, and muscular.

Epithelial Tissue

Large sheets of cell-based tissue known as epithelial tissue typically cover all exposed body surfaces and line internal body cavities. Furthermore, epithelial tissue is responsible for the formation of the vast majority of glandular tissue in the human body. All three major embryonic layers produce epithelial tissue. 

• The ectoderm gives rise to the epithelial tissue that makes up the cutaneous membranes. 
• The endoderm is where the majority of the mucous membrane epithelial tissue originates. 
• Mesoderm is the source of epithelial tissue, which lines vessels and open spaces in the body. 

The endothelium is the epithelial tissue that lines vessels in the lymphatic and cardiovascular systems, whereas the mesothelium is the epithelial tissue that forms the serous membranes lining true cavities.

General Characteristics of Epithelial Tissue

• The epithelium is made up of closely packed, flattened cells that line the inside or outside of body cavities. There isn't a lot of intercellular material.
• The tissue is avascular, which means it lacks blood vessels. Diffusion transports nutrients and waste between neighbouring connective tissues.
• The epithelium's upper surface is free or exposed to the outside of the body or an internal body cavity. The basal surface is supported by connective tissue. Between the epithelial and connective tissue layers, a thin extracellular layer known as the basement membrane forms.

Functions of Epithelial Tissue

• Substance exchange: Epithelial tissue regulates the exchange of substances between the body and the external environment, as well as the exchange of substances within the body.
• Protection: It is the first line of defence against mechanical injury, chemical exposure, excessive fluid loss, and infections because it covers the entire body surface.
• The Secretion Procedure: The majority of epithelial cells can produce secretive macromolecules. The glandular epithelium is one of the best examples. Because the endocrine gland is secretive in nature, it secretes hormones directly into the bloodstream, some of which are insulin, thyroxin, and others. Exocrine glands, which are also secretive in nature, release hormones into ducts, such as skin sebum and digestive enzymes in the small intestine.
• Process of Absorption: Surface epithelial primarily perform absorption with the assistance of microvilli, which increase surface area for absorption. The columnar epithelium of the small intestine is another example. Their primary function in the digestive tract is nutrient absorption.
• Sensation: Sensory receptors are found in the epithelial tissue of the nose, eyes, and ears, as well as taste buds, and aid in the transmission of signals from external stimuli to the brain.

Classification of Epithelial Tissue

Classification of epithelial tissue is mainly based on the Shape, Layers and Specialized functions as follows:

Classification Based on Shape

• Epithelium Squamous: This type of tissue has cells that are longer than they are tall. They are mostly found in the mouth, oesophagus, alveoli of the lungs, and blood vessels.
• Epithelium Cuboidal: This type of tissue has a similar width and height, making it primarily cube-shaped.
• Epithelium Columnar: This type of tissue has a column-like shape and is taller than it is wide. There are two types of columnar epithelium: ciliated columnar epithelium and glandular columnar epithelium.

Classification Based on Layers

Epithelial tissue is classified as simple or stratified or compound epithelium based on the number of layers present.

Simple Epithelium

Simple epithelium is composed of a single layer of identical cells that are typically found on secretory and absorptive surfaces, where the single layer improves these processes. Simple epithelium is divided into three major types, which are named after the shape of the cells, which differ in function.

• Simple Squamous Epithelium: Their cells resemble scales and have a flattened or round shape. Capillary walls, pericardial, pleural, and peritoneal cavity linings, and lung alveolar linings are examples.
• Simple Cuboidal  Epithelium: These cells can perform secretory, absorptive, or excretory functions. The collecting ducts of the kidney, pancreas and salivary gland are examples.
• Simple Columnar  Epithelium: These cells can be secretory, absorbent, or excretory in nature. Some of these tissues, known as the simple glandular columnar epithelium, contain goblet cells. These are found in the stomach, colon, and rectum and secrete mucus.
• Pseudostratified: Some columnar epithelia become so crowded that some nuclei become displaced and several rows of nuclei appear in sections of the cell sheet, despite the fact that each cell remains attached to the basement membrane. It is difficult to trace the continuity of different cells from base to apex in these cases, and the epithelium may appear stratified despite containing only one layer of cells.

This type of epithelia is known as pseudostratified columnar epithelia. The most common location for pseudostratified columnar epithelia is in the respiratory airways. Cilia are found on the apical surface of these cells. Cilia are motile, meaning they beat in unison to move fluid in a straight line. Cilia in the trachea propel mucous and particles into the pharynx and out of the airways. Cilia are significantly longer than microvilli.

Stratified Epithelium

They may be ciliated or nonciliated. The respiratory epithelium is an example of the ciliated tissue. A stratified epithelium is made up of several layers of different-shaped cells, and basement membranes are usually absent. Daughter cells resulting from cell divisions push older cells upward toward the apical layer as basal cells divide. They become dehydrated and less metabolically active as they move toward the surface and away from blood supply in underlying connective tissue. Tough proteins predominate as cytoplasm decreases, and cells transform into tough, hard structures that eventually die.

The stratified epithelium is classified into two types: stratified squamous, stratified cuboidal, and stratified columnar.

• Stratified Squamous Epithelium: They primarily protect against abrasion and water loss. They are mostly found in the oral cavity, oesophagus, larynx, and other areas. Cuboidal epithelium stratified:
• Columnar Epithelium Stratified: Their primary functions are secretion and defence. They are found in the conjunctival covering of the eye.
• Cuboidal Stratified Epithelium: In nature, their primary function is to protect. They primarily line sweat gland excretory ducts and large excretory gland ducts.

Classification Based on Specialized Functions

Epithelial cells can also be classified based on their special functions, which include:

• Transitional Epithelium: (also known as urothelium) is made up of several layers of cells that flatten out when stretched. It lines the majority of the urinary tract and allows the bladder to expand.
• Glandular Epithelium: It specialised in the production and secretion (release) of substances. It is found in your glands, which are specialised organs capable of producing, storing, and/or releasing substances such as hormones, proteins, and water.

Structure of Epithelial Tissue

• A tightly packed continuous layer of cells forms epithelial tissue. One epithelial tissue surface is exposed to either the external environment or body fluid.
• The other surface is connected to the tissue by a membrane made up of fibres and polysaccharides secreted by epithelial cells.
• There is very little intercellular material between cells. Individual cells are linked by specialised junctions found between epithelial cells.
• Tight junctions prevent tissue leakage.
• Adhering junctions keep adjacent tissues well cemented together.
• Gap junctions allow ions and molecules to move across tissue more easily.
• Membranes are formed by epithelial cells. The epithelial membrane is made up of a layer of epithelial tissue with connective tissue underneath. Mucous membranes and serous membranes are the two types of epithelial membranes.
  • Mucous Membrane
    • It is also referred to as mucosa. Goblet cells are present and secrete mucus. Mucus aids in lubrication, protection, and material movement. It keeps tissues from drying out. It lines the body cavities that open outside the body, such as the respiratory and digestive tracts.
  • Serous Membrane
    • The serous membrane lines body cavities that do not open to the outside world, such as the pleural cavity and pericardial membranes. These membranes are made of simple squamous epithelium and secrete fluid inside the cavity.

Location of the Epithelial Tissue

Outside and inside cavities exist in the epithelium. Skin is one of the external cavities. Body Lumina is also possessed. The outside layer is human skin, as previously stated, and it is made up of:

1. Keratinized skin cells
2. Cells of the epithelium
3. Squamous cells that are stratified

Non-keratinized squamous is found in the oesophagus, the inside of the mouth, the vagina, and the rectum. Other epithelial cells are covered in the lungs and gastrointestinal tract. A type of epithelium known as vascular endothelium is a specialised type of epithelium. It is formed within the heart and blood vessels. The fast-growing and regenerating epithelium cells can be seen in the cornea's outer layers. The mesothelium is responsible for the formation of the peritoneum and pericardium walls.