The tissue that always has a free surface exposed to the internal or external environment is epithelia.
A group of cells that perform similar functions and have a similar structure can be referred to as a tissue. Tissues are of four types- epithelial, connective, muscle, and nervous tissue. Epithelial tissue acts as a protective sheath, connective tissue joins other tissues and keeps them in place, muscle tissues help in voluntary and involuntary movements, and nerve tissues carry signals to and from various parts of the body.
The epithelium is an animal tissue that covers the surface of organs, blood vessels, and cavities of the internal organs. It covers the body surfaces that are exposed to the external environment and lines the body cavities exposed to the internal environment.
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