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An Introduction to Epithelial Tissue

Epithelial tissue. Generalized functions of epithelial tissue. Epithelial tissue provides the body's first line of protection from physical, chemical, and biological wear and tear. The cells of an epithelium act as gatekeepers of the body, controlling permeability and allowing selective transfer of materials across a physical barrier. All substances that enter the body must cross an epithelium. Some epithelia often include structural features that allow the selective transport of molecules and ions across their membranes. Many epithelial cells are capable of secretion and release mucus and specific chemical compounds onto their apical surfaces. For example, the epithelium of the small intestine releases digestive enzymes. Cells lining the respiratory tract secrete mucus that traps incoming microorganisms and particles. A glandular epithelium contains many secretory cells. The epithelial cell. Epithelial cells are typically characterized by the polarized distribution of organelles and membrane-bound proteins between their basal and apical surfaces. Particular structures, found in some epithelial cells, are an adaptation to specific functions. Certain organelles are segregated to the basal sides, whereas other organelles and extensions, such as cilia, when present, are on the apical side. Cilia are microscopic extensions of the apical cell membrane that are supported by microtubules. They beat in unison and move fluids as well as chopped particles. Ciliated epithelium lines the ventricles of the brain where it helps circulate the cerebrospinal fluid. The ciliated epithelium of your airways forms a muco-ciliary escalator that sweeps particles of dust and pathogens trapped in the secreted mucus, towards the throat. It is called an escalator, because it continuously pushes mucus with trapped particles upward. In contrast, nasal cilia sweep the mucous blanket down towards the throat. In both cases, the transported materials are usually swallowed and end up in the acidic environment of your stomach.