Cells of the body are organized into four fundamental tissues; epithelial, connective, muscle and nerve. Epithelial tissues line all surfaces of the body: the skin, cavities, ducts, vessels. They are named according to the cell shape on the free surface. Cells of a tissue are held together by the basement membrane and intercellular fibbers. These tissues receive their nutrition by diffusion, for they are without blood vessels. Simple squamous epithelium lines all blood and lymphatic vessel, including the heart, air cells of the lung and certain tubules of the kidney. Filtration or diffusion occurs rapidly across this thin epithelial sheet. Cuboiodal and columnar cells line glands and the digestive tract and are involved in secretion, absorption. Pseudo stratified columnar tissue lines there respiratory tract. Its glands secret mucus, and the cilia stroke the pollutant-laden mucus to the pharynx. Stratified squamous epithelium lines the skin, oral cavity, much of the pharynx, oesophagus, vagina and anal canal. It protects against wear and tear. Stratified columnar is seen in the reproductive tract. Transitional epithelium is seen in the urinary bladder, ureters, and kidney. It is capable of distention and contraction in response to changing volumes of urine. Exocrine glands arise as outpocketings of epithelial tissues. They generally secrete enzymes, mucus, or serous fluid. They are characterized by ducts that open onto the free surface of a cavity of skin. Examples: sebacecus, seat, mammary, pancreatic. The secretory portions may have one of several shapes (tubular, coiled, alveolar; with one duct or many ducts). Endocrine glands arise like exocrine glands but lose their ducts during development. They are intimately related to capillaries, which conduct their secretory products away. Examples: thyroid, pituitary, adrenal.
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