Epithelial Tissue

• The epithelial tissue is also referred as epithelium (pl.: epithelia). This tissue has a free surface, facing either the body fluid or the outside environment and provides a covering/lining for some part of the body. The cells are compactly packed with slight intercellular matrix.

The epithelial tissues are of two types:

· Simple epithelium

· Compound epithelium

Simple epithelium:

• Simple epithelium is a single layer of cells and acts as a lining for body cavities, ducts, and tubes. The compound epithelium consists of two or more cell layers with a protective function as in case of human skin. 

On the basis of structural modification of the cells, simple epithelium is further divided into three types. These are

· Squamous

· Cuboidal

· Columnar

Simple epithelium: (a) Squamous (b) Cuboidal (c) Columnar (d) Columnar cells bearing cilia

Squamous epithelium: The squamous epithelium is a single thin layer of flattened cells with uneven boundaries. They are found in the walls of blood vessels, air sacs of lungs and are involved in functions like forming a diffusion boundary.

Cuboidal epithelium: The cuboidal epithelium is composed of a single layer of cube-like cells and is usually found in ducts of glands and tubular parts of nephrons in kidneys, with secretion and absorption as its main functions. The epithelium of proximal convoluted tubule (PCT) of nephron in the kidney has microvilli.

Columnar epithelium: The columnar epithelium is composed of a single layer of tall and slender cells. Their nuclei are located at the base and free surface may have microvilli. They exist in the lining of stomach and intestine and assist in secretion and absorption. 

If the columnar or cuboidal cells bear cilia on their free surface they are called ciliated epithelium. Their function is to move particles or mucus in a specific direction over the epithelium and are mainly present in the inner surface of hollow organs like bronchioles and fallopian tubes. 

Some of the columnar or cuboidal cells get specialised for secretion and are called glandular epithelium. They are mainly of two types:

· Unicellular - consisting of isolated glandular cells (goblet cells of the alimentary canal).

Glandular epithelium : Unicellular

· Multicellular - consisting of cluster of cells (salivary gland).

Glandular epithelium : Multicellular

On the basis of the mode of pouring of their secretions, glands are divided into two categories namely,

· Exocrine glands

· Endocrine glands

Exocrine glands: Exocrine glands secrete mucus, saliva, earwax, oil, milk, digestive enzymes and other cell products. These products are released through ducts or tubes. 

Endocrine glands: The endocrine glands products called hormones are secreted directly into the fluid bathing the gland and do not have ducts.

Compound epithelium:

• Compound epithelium is of multi-layered cells and has a restricted role in secretion and absorption and their mainly involved to provide protection against chemical and mechanical stresses. They cover the dry surface of the skin, the moist surface of buccal cavity, pharynx, inner lining of ducts of salivary glands and of pancreatic ducts.

Compound epithelium

• All cells in epithelium are held together with little intercellular material. In all animal tissues, specialised junctions provide both structural and functional links between its individual cells.

Three types of cell junctions are found in the epithelium and other tissues and they are called as tight, adhering and gap junctions.

Tight junctions: Tight junctions assists to stop substances from leaking across a tissue.

Adhering junctions: Adhering junctions achieve cementing to keep neighbouring cells together.

Gap junctions: Gap junctions enable the cells to connect with each other by linking the cytoplasm of adjoining cells, for rapid transfer of ions, small molecules and sometimes big molecules.