Tissue is a biological term that refers to groupings of cells that have the same general structure and function. In the biological levels of organization, tissue is categorized between cells and organisms, as tissue is composed of cells and organisms are composed of tissue. There are many types of tissue in the body that serve many different purposes, from simply holding parts of the body together to moving nutrients and other materials throughout the body. There are four main tissue types: connective tissue, muscle tissue, nervous tissue, and epithelial tissue. These tissue types all work in conjunction to make a complete organism.
Connective tissue supports and holds other kinds of tissue in place. It tends to be fibrous in nature and often works with various nonliving structural components in the body to provide support. Bones and blood are both considered to be connective tissues.
Muscle tissue is significant because of its ability to contract and relax in order to exert force on the outside world. There are several different types of tissue that fall into the category of muscle tissue. Smooth muscle tissue, for example, lines the insides of some organs and skeletal muscular tissue is used for movement and for purposeful force. The heart is composed of a specific kind of muscle tissue known as cardiac muscle.
Nervous tissue is used for sending signals throughout the body. Nervous tissue connects to most of the other tissue types in the body, providing commands and information where needed. Both the central nervous system, which is composed of the brain and the spinal cord, and the peripheral nervous system, which is made up of the other nerves in the body, are made of nervous tissue.
Epithelial tissue serves as a lining for many other tissue types. Skin, for example, is a kind of epithelial tissue that covers the whole surface of the body. Some kinds of epithelial tissue form a single layer over whatever it happens to be coating while others form thick layers.
Another variety of tissue that is very important is scar tissue. When other tissue types, particularly epithelial tissue, are damaged, they may never be restored to their original state. In this case, fibrous tissue known as scar tissue forms. Scarring is a normal part of the healing process; almost all wounds with the exception of very minor cuts and scrapes tend to scar eventually. In spite of this, scar tissue is generally not as effective as the tissue it replaces.