A tissue is made up of a group of cells that usually look similar to one another and come from the same region in a developing embryo. The group of cells that make up a tissue have physiological functions that work together in a coordinated way to support special functions. The special function of a tissue is also influenced by the kind of material that surrounds the tissue and by communication among the cells of the tissue. Different kinds of tissue have different physical properties. Tissues may be hard (bone), soft (muscle), or even liquid (blood).
In the structural organization of the body, tissues are located between the cell and organ levels of organization. Individual cells are a lower level of organization. Tissues are made up of many individual cells. Groups of different kinds of tissues are organized together to form organs, which have special functions with characteristic shapes and functional properties.
There are four kinds of tissues based on differences in their anatomy and function: epithelial tissue, connective tissue, muscle tissue, and nervous tissue. Epithelial tissue is made of layers of cells that are joined together and may cover the surface of the body (epidermis of the skin), line spaces in the body (lining of the abdominal cavity) and hollow structures (lining of blood vessels), or form glands (sweat glands). Connective tissue is usually made of cells and extracellular fibers that hold structures together (tendons), protect them (cartilage), store energy (fat), or produce blood.
Muscular tissue is made of cells that are organized to shorten and produce force when they contract (smooth skeletal and cordine muscle). Nervous tissue is made of neurons and accessory cells. Neurons are the cells that carry information in the form of electric action potentials . Accessory cells protect and support the function of neurons.
Michael G. Scott
Tortora, Gerard J., and Sandra R. Grabowski. Principles of Anatomy and Physiology. New York: John Wiley & Sons, Inc., 2000.