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Epithelium or epithelial tissue is a thin, continuous, protective layer of cells with little extracellular matrix. An example is the epidermis, the outermost layer of the skin. Epithelial (mesothelial) tissues line the outer surfaces of many internal organs, the corresponding inner surfaces of body cavities, and the inner surfaces of blood vessels.
Functions of epithelial tissue: The principle function of epithelial tissues are covering and lining of free surface; The cells of the body's surface form the outer layer of skin. Inside the body, epithelial cells form the lining of the mouth and alimentary canal and protect these organs. Epithelial tissues help in the elimination of waste.
Pseudostratified columnar epithelium is a type of epithelium that, though comprising only a single layer of cells, has its cell nuclei positioned in a manner suggestive of stratified columnar epithelium. A stratified epithelium rarely occurs as squamous or cuboidal.
There is an inner lining of single flattened epithelial cells (simple squamous epithelium) composed of a type of epithelium that is called the endothelium, and the cells are called endothelial cells. This layer functions to mechanically transport fluid and since the basement membrane on which it rests is discontinuous; it leaks easily. [2]
Transitional epithelium is made up of three types of cell layers: basal, intermediate, and superficial. [3] The basal layer fosters the epithelial stem cells in order to provide constant renewal of the epithelium. [4] These cells' cytoplasm is rich in tonofilaments and mitochondria; however, they contain few rough endoplasmic reticulum.
A serous membrane is an epithelial membrane composed of mesodermally derived epithelium called the mesothelium that is supported by connective tissue. [2] These membranes line the coelomic cavities of the body, that is, those cavities that do not open to the outside, and they cover the organs located within those cavities.
Cell junctions are especially abundant in epithelial tissues. Combined with cell adhesion molecules and extracellular matrix , cell junctions help hold animal cells together. Cell junctions are also especially important in enabling communication between neighboring cells via specialized protein complexes called communicating (gap) junctions .
Stratified columnar epithelium is initially present in parts of the gastrointestinal tract in utero, before being replaced with other types of epithelium. For example, by 8 weeks, it covers the lining of the stomach. [7] [8] By 17 weeks, it is replaced by simple columnar epithelium. [7] This is also found in the fetal esophagus. [8]