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Epithelial Tissues

Structure Organisation in Animals

All complex animals consist of only four basics types of tissue: epithelial, connective, muscular, and neural tissue.

Epithelial tissues

  • Epithelial tissues protect the surface of the organ, face, body fluid, or the outside environment and form glands. They usually rest on thin noncellular basement membranes. They usually lack blood vessels and receive their nutrition from the underlying connective tissue by diffusion through basement membranes.

Simple epithelium

  • Simple epithelium is unicellular or unilaminar and occurs at surfaces where transfers of materials by secretion, absorption or diffusion, etc., is more important a function than protection. It can be of the following types :
  1. Squamous epithelium: also called payment epithelium. It is made of a single thin layer of flattened cells with irregular boundaries. They are found in the walls of blood vessels and air sacs of lungs. They are involved in diffusion.
  2. Cuboidal epithelium: composed of a single layer of cube-like cells, commonly found in ducts of glands and tubular parts of nephrons in the kidney and its main function are secretion and absorption. Cells often have microvilli, which increases the absorptive surface area.
  3. Columnar epithelium: composed of a single layer of tall and slender cells with their nuclei at the base. In the intestine, this layer has microvilli to increase the absorptive surface area and is called brush border columnar epithelium. It lines the stomach, intestine, gall balder, and bile duct.

When the columnar or cuboidal cells bear cilia on their free surface, they are called the ciliated epithelium. They help in the movement of mucus, urine, egg, and cerebrospinal fluid.

Pseudostratified epithelium

  • The cells are columnar but unequal in size. The long cells extend up to the free surface. Although epithelium is one cell thick, yet it appears to be multi-layered, which is due to the fact that the nuclei lie at different levels in different cells. It is of 2 types: pseudostratified columnar epithelium (in large ducts of paranoid salivary glands and urethra) and pseudostratified columnar ciliated epithelium (in trachea and bronchi).

Compound epithelium

  • The compound epithelium is a multicellular thicker and stronger than simple epithelia. They cover the surface where constant replacement of cells is needed due to rapid wear and tear by friction. They are two types
  1. Transitional epithelium: consist of fewer layers of cells, less flattened surface cells, and has remarkable flexibility. It is found in the ureters, urinary bladder, and urethra, a remarkably adapted for distension of these organs.
  2. Stratified epithelium: consists of many layers of cells. The cells of the deepest layer are of low columnar or cuboidal type, but those of the outer or surface layer vary in nature. It may be keratinized or non-keratinized.

Figure: Epithelium tissue

 

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