Epithelium

To define epithelium, let us say it is a complex of specialized cellular organizations arrayed into sheets without important intercellular substance that covers the body's internal and external surface and secreting organs. In simple terms, epithelial meaning is a sheet of cells covering the body surface or the lines of a body cavity. Epithelium occurs in both plants and animals. However, epithelial cells can be cuboidal,  flat, or cylindrical and are arranged in either one or several layers depending on their function. Epithelial tissue occurs in two different forms- covering and lining epithelium covering both the outer and inner body surfaces of most organs and glandular epithelium, forming most of the body glands.

What are the Layers of Epithelium?

There are two significant layers of epithelium- simple epithelia and stratified epithelia.

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Simple epithelia comprise a single layer of cells, with each cell in direct contact with the basement membrane. Simple epithelium tissue is composed of elongated columnar cells. 

These cells are often found in places where absorption and filtration are necessary, like lining most of the body's cavities, such as the lining of the female ovary and blood vessels.

Stratified epithelia comprise several layers of cells present on the basal surface and remain attached to the basement membrane. However, they do not come in contact with the basement membrane. These cells offer better protection from external threats, such as harmful toxins. Stratified tissues are found where protection is essential, like the lining of the oesophagus and the lining of the urethra and bladder.

What are the Various Parts of Epithelium?

Based on the cell shape, there are three parts of epithelium- 

Squamous Epithelial Cells: Squamous epithelium is composed of flat, scale-like irregular cells with centrally located disc-shaped nuclei. There are two types of squamous epithelia: simple squamous and stratified squamous epithelium. 

The simple squamous epithelium allows the passage of materials through diffusion or filtration and also secretes mucus. The primary function of stratified squamous epithelium is the protection of the body's internal against abrasion.

Cuboidal Cells: Cuboidal Epithelium is cube-shaped cells with spherical, centrally located nuclei. There are two types of cuboidal epithelium: simple cuboidal epithelium and stratified cuboidal epithelium. The primary function of simple cuboidal cells is secretion and absorption, while stratified cuboidal epithelium serves as a protective tissue.

Columnar Cells: Columnar cells are composed of taller and thinner column-like cells. The nuclei of columnar cells are commonly oval, elongated from top to bottom, and located near the basal surface. There are three primary types of columnar epithelia: simple columnar, pseudostratified columnar, and stratified columnar epithelium. The simple columnar cells secrete mucus and enzymes and absorb nutrients, while pseudostratified columnar secrete and move mucus. The primary function of stratified columnar epithelium protects the body internal against abrasion and secretes mucus.

Functions of Epithelial Tissue

Epithelial tissue definition states that it is a layer of cells closely bound to one another to form continuous sheets that cover surfaces that come into contact with foreign substances. Therefore, the functions of epithelial tissue are as follows-

• Protection of the underlying tissues

• Secretion through the release of molecules from cells

• Absorption through bringing small molecules into cells

• Diffusion through the movement of molecules down their concentration gradient

• Filtration occurs through the passage of small molecules through a sievelike membrane

• Sensory reception

Interesting Characteristics about Epithelial Tissue

Cellularity: Epithelium is composed entirely of cells separated by a minimal amount of extracellular materials, mainly projections of their integral membrane proteins into the narrow spaces between the cells.

Support by Connective Tissue: An underlying connective tissue layer supports all epithelial sheets in the body. 

Specialized Contacts: This characteristic states that the adjacent epithelial cells directly join at many points by particular cell junctions.

Regeneration: Another characteristic feature is that epithelial tissues hold a high regenerative capacity. While some epithelia are exposed to friction, others are destroyed by hostile substances present in the external environment, such as bacteria, acids, and smoke. Their surface cells rub off. However, if the epithelium receives sufficient nutrition, it can quickly replace the lost cells through cell division through mitosis.

Polarity: All epithelia have a free apical surface that is attached to the basal surface. However, the function and the structure of the apical and basal surfaces differ by a characteristic known as polarity. The epithelia's apical surface abuts the open space of a gland, tube, cavity, or hollow organ. In contrast, the basal surface rests on a thin supporting sheet of the basal lamina, which is part of the basement membrane.