Epithelial Tissue Epithelial tissue is characterized by closely packed cells with one end of the tissue exposed to the "outside environment." This includes the outer layers of the skin, the linings or the mouth and digestive organs, the linings of the kidneys and urinary organs, the inside linings of the heart and blood vessels (which technically aren't "outside"), the linings of the reproductive organs, and the linings of the respiratory organs. These cells tend to form a flat sheet with the outer surface exposed to the environment or an internal organ cavity and the deeper surface attached to the basement membrane. This attachment is because adhesive membrane proteins on the surface of epithelial cells binds to collagen in the basement membrane. It is rare to find epithelial cells without an underlying connective tissue and most epithelial cells receive oxygen and necessary nutrients through the underlying connective tissues. Types of Epithelial Tissues
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