An Idea On Epithelium

All living things, including bacteria, animals, and plants, have cells. Microorganisms are unicellular, whereas plants and animals are often multicellular. Multicellular organisms organise their cells into tissues. Tissues are made up of the same cell types that execute the same functions. They can be segregated into four major groups: epithelial, neural, connective, and muscular tissues. The epithelium or epithelial tissue comprises epithelial cells and covers all of your body’s external and internal surfaces. It also lines organs and body cavities and is the primary tissue found in glands. This article will discuss one of the four major animal tissues, the epithelial tissue, in more detail.

Characteristics of Epithelium

The characteristics of epithelium or epithelial tissue are mentioned below.

• The epithelial cells comprising the epithelium can divide and regenerate throughout their lives.
• A cementing substance consisting of glycoprotein holds the epithelial cells together. They are tightly packed together to form continuous layers.
• The epithelium is devoid of blood vessels. The lymph vessels are unable to puncture this tissue due to the lack of intercellular gaps. Therefore, blood circulation is nonexistent in the epithelium.
• The epithelial cells’ free surfaces can either be smooth or have delicate hair-like structures called cilia or microvilli.
• Epithelial tissues can perform specialised activities like secretion, absorption, sensory reception, filtration, diffusion, and excretion, along with protection.

Epithelium: Types

Epithelial tissue is classified into several categories based on the number of sheets or layers it forms and the morphology of each cell that comprises those layers. Based on the shapes, the epithelium can be of the following types:

• Squamous

Squamous epithelium cells are broader as compared to height. So they have a flat, scale-like appearance. They are located in the lining of the oesophagus, arteries, alveoli of the lungs, and inner mouth lining. 

• Columnar

The cells of the columnar epithelium are taller than their width. They have a column-like appearance. There are two types of columnar epithelial cells: glandular columnar epithelial cells and ciliated columnar epithelial cells.

• Cuboidal

Cuboidal epithelium comprises cells that are roughly the same height and width. They have a cube-like appearance.

Depending on the number of layers formed, the epithelium can be further divided into;

• Simple epithelium

A single layer of epithelial cells lies on top of a basement membrane in the simple epithelium. They can be either squamous, columnar, or cuboidal based on the shape of the cells.

• Simple squamous epithelium

This form of single-layered, flattened epithelial cells lines blood vessels and bodily cavities and controls the entry of substances into the tissue beneath. It can be located in the linings of the glomeruli (kidneys), alveoli (lungs), serous membranes (mesothelium), the heart (endocardium), etc.

• Simple cuboidal epithelium

It is a single cube-shaped cell layer. This form of epithelium provides more protection than the simple squamous epithelium because of its considerable thickness. Due to its organelle-rich cytoplasm, it also has absorptive, secretory, and excretory properties.

• Simple columnar epithelium

It is a single column-shaped cell layer. The simple columnar epithelial tissue also features apical specialisations like cilia or microvilli for better absorptive function. This kind of epithelium is mainly located in the inner parts of the stomach, especially the stomach wall. It is also present in the intestinal and gall bladder walls.

• Stratified epithelium

Two or more layers of cells make up stratified epithelium. They are further characterised as squamous, cuboidal, or columnar based on the morphology of their apical layer of cells. Furthermore, there are two kinds of specialised stratified epithelium. They are transitional and keratinised stratified epithelium.

• Stratified squamous epithelium

They are multiple layers of flat squamous epithelial cells that protect against dehydration and abrasion. It can be further categorised into non-keratinised and keratinised. The oral cavity, oesophagus, vagina, larynx, anal canals, and the cornea’s outer layer are lined with this type of epithelium.

• Stratified cuboidal epithelium

It also acts as a protective layer of epithelial tissues. It enveloped the ovarian follicles. The stratified cuboidal epithelium is also found in the lining of the sweat gland’s excretory duct and the anorectal junction.

• Stratified columnar epithelium

This form of epithelium is less prevalent and can be found in the mucous membrane that lines the eyelids. It serves as both a protective and mucus-secreting layer. Additionally, the stratified columnar epithelium can be located in the exocrine gland’s large ducts. The sensory epithelium is a particular type of stratified columnar epithelium. The apical specialisations of these columnar cells have been adjusted to detect the many sorts of stimuli obtained by human sensory organs. Therefore, the sensory epithelium is found in the tongue’s taste bud, organ of the Corti, the retina of the eye, etc.

Significant functions of epithelium

Epithelial tissue’s principal role is to provide protection. Semi-permeable, tight junctions connect the cells that make up an epithelial layer, establishing a barrier between the outside environment and the organ it protects. In addition to its protective function, epithelial tissue has specialised tasks such as absorption, secretion, and excretion. Epithelial tissue protects organs from injury, dehydration, and pathogenic microorganisms.

Conclusion

The animal tissues are of four major types: epithelial tissues or epithelium, neural tissues, muscular, and connective tissues. The epithelial tissues are made up of epithelial cells and primarily offer protection to the body. It also performs specialised functions like absorption, secretion, excretion, filtration, sensory reception, and diffusion. The above article concerns the different types of epithelium based on cell morphology and layers formed by the epithelial cells.