EPITHELIAL TISSUES

Epithelial tissue primarily appears as large sheets of cells covering all surfaces of the body exposed to the external environment and lining internal body cavities. In addition, epithelial tissue is responsible for forming a majority of glandular tissue found in the human body.
Epithelial tissues provide the body’s first line of protection from physical, chemical, and biological damage. The cells of an epithelium act as gatekeepers of the body, controlling permeability by allowing selective transfer of materials across its surface. All substances that enter the body must cross an epithelium.
Many epithelial cells are capable of secreting mucous and other specific chemical compounds onto their apical surfaces. For example, the epithelium of the small intestine releases digestive enzymes and cells lining the respiratory tract secrete mucous that traps incoming microorganisms and particles