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Epithelial Tissues

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Epithelial Tissues - Lesson Summary

Many animals are present on our planet, ranging from unicellular to multicellular. In unicellular organisms single cell carries out all the body functions. However, in multicellular organisms, different cells perform different functions.
A group of similar cells that performs a specific function is called a tissue. All complex animals including human beings have only four basic types of tissues. These tissues are arranged in a specific proportion to form organs.  Two or more organs together form an organ system.
Based on function, animal tissues are broadly classified into four types, namely epithelial, connective, muscular and neural.
Epithelial cells have a free surface, which either faces the body fluid or the outer environment. Thus, it serves as a protective covering.  The epithelium and other tissues have specialised junctions. These junctions are of three types, namely tight, adhering and gap. Tight junctions prevent leakage, adhering junctions help cement adjoining cells together and gap junctions contain pores, which permit the exchange of ions.
Epithelial tissues are of two types – simple and compound. Simple epithelium consists of a single layer of cells and serves as a lining for body cavities. Compound epithelium consists of two or more layers of cells. Its chief function is protection against mechanical and chemical stress and it plays a limited role in secretion and absorption.  It is present on the outermost layer of the skin, the moist surface of the buccal cavity and pharynx, and the inner lining of the ducts of the salivary glands and pancreas.
Based on shape, simple epithelium is further categorised into squamous, cuboidal and columnar. Squamous epithelium is composed of a single, thin layer of flat and irregularly shaped cells. Cuboidal epithelium consists of a single layer of cube-like cells.
Columnar epithelium consists of a  single layer of tall cells with their nuclei at the base. Some cuboidal and columnar cells have cilia on their free surface and are therefore called ciliated epithelium. Some cuboidal and columnar cells are specialised for secretion and are therefore called glandular epithelium. Glandular epithelium is of two types – unicellular and multicellular. Unicellular glandular epithelium consists of isolated cells,multi cellular glandular epithelium consists of a cluster of cells. The epithelial tissues also form many glands. These glands are of two types – exocrine and endocrine. The exocrine glands secrete their products through ducts or tubes while the endocrine glands secrete their hormones directly into the bloodstream.

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