epithelial tissue

• tissue are masses of similar cells that combine with other tissues to form membranes or organs
• histology is the systematic study of tissues type
• epithelial tissue composed of polygonal cells closely positioned together with little or no extracellular materials in between
• covering or lining epithelium is found in sheets that cover body structures or line body spaces
• covering- free to face outward
• lining- free to face inward
• non-free face epithelial sheet is attached to underlying connective tissue by the basement membrane
• basement membrane- thin, glue-like layer that holds the epithelium in place while remaining highly permeable to water and other substances
• epithelia do not have their own blood supply
• glandular epithelium- forms the functional portions of exocrine glands and endocrine glands
• exocrine glands-secretes substances into ducts that empty onto epithelial surfaces
• endocrine glands- secrete substances that diffuse into the bloodstream
• simple- have only one layer of cells
• stratified- with more than one layer of cells
• pseudostratified- looks as if it were stratified but it is only one layer deep.
• squamous- cells are shaped like fish scales, these cells are much wider than they are tall
• cuboidal- cells are cube-shaped, being as wide as they are tall
• columnar- cells are taller than they are wide
• transitional- cells change shape as the need arises, being stretched or compressed into any of the three shapes. These cells are found in membranes subject to a great degree of stretching and recoil. Transitional epithelia are always stratified
• cilia- short, hair-like projection from each cell's free surface that propels material along the epithelial surface. Pseudostratified epithelium may be ciliated or no-ciliated
• goblet cells- secrete mucus which lubricates the passageway and traps foreign particles
• microvilli- tiny finger-like projections on the apical surfaces of some cells. They increase the surface area available for absorption.
• in some stratified squamous tissue, the upper layers of cells are dead and filled with a tough waterproof material called keratin.
• Non-keratinized samples have nucleated squamous cells along the free edge
• Simple squamous- found in the endothelium of blood vessels, alveoli, and mesothelium covers the body cavities. It is made up of thin, flat cells that are fitted closely together to form a continuous sheet.
• stratified squamous- with multiple layers of cells; columnar cells along the basement membrane topped by cuboidal cells then by squamous cells. Because of its thickness and constant renewal, it is well adapted for protection. It is found in the outer part of the skin and mucous lining of the mouth, vagina and esophagus.
• simple cuboidal- composed of single layer of almost cubic cells, this type is found in secreting organs, such as glands. It also forms the kidney tubules, where it is specialized for water reabsorption and ion movement.
• simple columnar- forms linings specialized for absorption and secretion. Lines portion of the reproductive tract, digestive tract, respiratory tract and excretory ducts.
• Goble cell- often interspersed among other columnar cells. It has a large cuplike vesicle that may open onto the free surface. It produces and secretes mucus in great quantity.
• mucus- lubricates and protects the epithelial lining
• ciliated pseudostratified- a single layer of columnar cells that all attach to the basement membrane. It may be ciliated or non-ciliated. It is found in the upper throat, upper respiratory tract, and parts of the male urinary and reproductive tracts.