Groups of tissues that work together are called organs. A single organ may consist of several different types of tissue that are organized to carry out a specific set of functions. Organs, in turn, are organized into systems.
Tissues are made up of cells of the same type and origin and are classified by their primary functions. In animals, there are four basic types. Epithelial tissues consist of closely packed cells that line organs, vessels and cavities and also form the outer surface of the body. Connective tissues support and bind other tissues and typically consist of cells occupying positions in a non-living matrix. Muscle tissues allow voluntary and involuntary movements, while nerve tissue allows the transmission of neural impulses.