The structure of cuboidal or columnar or squamous epithelium

The structure of cuboidal or columnar or squamous epithelium

Squamous Epithelium:

This is made up of flat disk like polygo­nal cells that are closely packed leav­ing no intercellular spaces. They appear like the arrangement of the tiles on the floor hence the epithelium is also known as the pavement epithelium. The cells of the tissue are connected by fine pro­toplasmic bridges and are slightly thicker at the centre. The nucleus is slightly flattened and is located at the centre of the cell.

Simple Squamous Epithelium is present in the inner lining of Bowman’s¹ capsule in the kidneys, alveoli of the lungs, blood vessels etc.

Cuboidal Epithelium:

This tissue consists of cells that are usually squarish with a polygonal outline. In a surface view they appear as sheet of cells under the micro­scope. The cells have single nucleus located in the centre. The cubiodal epithelial cells are lo­cated in thyroid follicles, small salivary and pan­creatic ducts, lining of the inner ear and proxi­mal parts of the urinary tubules etc. Cubodial epithelium is also found in sweat glands, pig­mented layer of retina and in the duct.

In the urinary tubules the cubodial cells have numerous tiny fingers like outgrowths called microvilli; these appear like a brush and the epithelium is often called brush bordered cuboid epithelium. The villi are supposed to in­crease the area for reabsorption of materials. The cuboidal epithlum that lines the ovaries and the seminiferous tubules in known as germinal epithe­lium. Cubodial cells play an active role in secretion, absorption and excretion.

Columunar Epithelium:

As the very name indicates this epithelium consists of columnar cells that are longer (taller) than broad. The tissue is made up of a single layer of cells. The outlines of the cells appear polygonal in a surface view. Each cell has a flattened and an oval end. There is a single nucleus in each cell which is elongated and usually lo­cated either in the centre or a little away from the centre. Usually some mucous secreting cells called Goblet cells are found in between the columnar epithe­lial cells that line the stomach and intestine.

The goblet cells have vacoule filled with mucous. The epithelial tissue containing the mucous secreting cells along with the underlying supporting connective tissue is usually referred to as the mucous membrane or mucosa.

Simple Columnar epithelial cells are found distributed in various organs like the lining of the gall bladder, bile ducts, gastric glands, intestinal galnds, pancreatic lobules etc.

In the intestine, the columnar epithelial cells that line the inner cavity are called microvilli and they are specialised for the absorption of water and digested food. The microvilli increase the surface area of absorption. These cells present in the small intestine constitute the brash bordered columnar epithelium.