Tissues are classified into 4 major types, they are epithelium, connective tissues, muscular tissues, and nervous tissues. The epithelium is the most common tissue found in the human body. The epithelial tissues, as the name suggests( Epi-outer) cover the outer surfaces of many organs, blood vessels as well as the skin. It also helps in protecting the visceral coverings of internal organs and the covering layers of mucus membranes. Epithelial layers do not have a direct blood supply from the blood vessels hence they receive nourishment through the diffusion of substances from the underlying connective tissue.
Table Of Contents
2. Classification of epithelium
2.1 Simple epithelium
2.2 Simple columnar epithelium
2.3 Simple cuboidal epithelium
2.4 Stratified epithelium
3. Functions of Epithelial Tissues:
Classification of epithelium
The epithelium is classified based on the shape, its location and functions. There are 2 main types of epithelium, thesimple epitheliaandstratified epithelia. Simple epithelium is made up ofa single layer of cells, and a stratified epithelium has a bundle of layers attached together in a complex pattern. There is one more unusual type of epithelium called pseudostratified type, this is pseudo ( false) because a single layer of cells is presented in a varying pattern that gives the appearance of real stratified epithelium. Based on the shape of the epithelium, they are classified into 5 types; squamous epithelium, cuboidal epithelium, columnar epithelium, ciliated columnar epithelium, and the cuboidal epithelial tissue. Squamous epithelium is flat, ciliated epithelium have ciliary hair-like projections, cuboidal is characterised by cube-like cells with a long and cylindrical shape and the ciliated columnar have cilia at the top their long and cube-like cells. The paragraphs below explain the detailed structure and functions of different types of epithelium.
1. Simple squamous epithelia and Stratified squamous epithelia
2. Simple cuboidal epithelium and Stratified cuboidal epithelia
3. Simple columnar epithelia and Stratified columnar epithelia
4. Pseudo-stratified and transitional epithelia.
Simple and compound Epithelium
The cells in simple squamous epithelium appears like thin scales. The nuclei of simple epithelium tend to be flat, horizontal, and very thin. One of the variants of epithelium is the endothelium that lines the inner walls of blood vessels and the lymphatic vessels. Because of the thin and fine nature of squamous cells, they expand and contract the presenting part hence they are found in the alveoli of lungs to assist in gaseous exchange. They are also seen in certain layers of the kidney where blood vessels need an inner lining. Another class of epithelium is the mesothelium which is a simple squamous epithelium that covers the external layers of serous membrane that lines body cavities and internal organ
Simple columnar epithelium
As the name suggests, simple columnar epithelium has a long, slender and column-like cells that help in the active absorption and the secretory functions. Simple columnar epithelium forms the lining of some sections of the digestive system and parts of the female reproductive tract as well. The ciliated columnar epithelium is a variant of columnar epithelium with their simple columnar epithelial cells containing small and fine hair-like cilia on their apical surfaces.
Simple cuboidal epithelium
Simple cuboidal cells are perfect cube-like cells piled in a systematic fashion. They are found on the surface of ovaries, the lining of nephrons, the walls of the renal tubules, and some surfaces of the eye and thyroid gland.
Stratified Epithelium is formed from several stacked layers of cells. They are the combination of multiple layers of simple epithelium that protect against the physical and chemical barriers that cause wear and tear. Stratified epithelium is again classified as stratified squamous epithelium, stratified columnar tissues, and stratified ciliary columnar epithelium. Stratified epithelium is also present on the apical layers of certain spaces. Another form of stratified epithelium is the transitional epithelium. It is a transitional epithelium found in the bladder, ureters and the urethra.
Functions of Epithelial Tissues
1. Secretory function: Epithelium inside the glands and mucus membrane helps in secretory function
2. Protective function: As a skin ad mucus membrane, epithelium creates a barrier against sunlight, infections and other foreign material.
3. Movement: Helps in respiratory movement, expansion and constriction of the urinary bladder.