explain epithelial tissue in detail class 9
Answers
Epithelial tissues are thin tissues that cover all the exposed surfaces of the body. They form the external skin, the inner lining of the mouth, digestive tract, secretory glands, the lining of hollow parts of every organ such as the heart, lungs, eyes, ears, the urogenital tract, as well as the ventricular system of the brain and central canals of the spinal cord.
The cells making up epithelia are often closely bound to one another through specialized structures called tight junctions. They are also free from blood vessels and nerves and are supported by a connective tissue called the basement membrane. They have polarity with a distinct basal domain facing the basement membrane and the other apical surface facing the lumen of an organ or the external environment.
Epithelial tissue has a number of functions, which include protection against abrasion, radiation damage, chemical stress and invasion by pathogens. A single organ can have different types of epithelial tissue based on the substances to which different surfaces are exposed. Protective tissue tends to be thicker, made of multiple layers of cells and often has inclusions such as keratin to provide mechanical strength and resistance. The skin of most mammals contains layers of thick keratinized dead epithelial cells protecting them against water loss and other stresses. Similarly, the esophagus is also exposed to a wide range of different textures, pH levels and chemical compositions from food and drink. Therefore, it also contains protective epithelium. Due to its involvement in the digestive process, however, it remains non-keratinized, and secretes mucus to smoothen the passage of food.
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Epithelial tissues are widespread throughout the body. They form the covering of all body surfaces, line body cavities and hollow organs, and are the major tissue in glands. Epithelial cells may be squamous, cuboidal, or columnar in shape and may be arranged in single or multiple layers.
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