Cells with similar structure and function are grouped into tissues. A group of tissues with a specialized function is called an organ. There are four main types of tissue in vertebrates: epithelial, connective, muscle, and nervous.
Epithelial tissue consists of thin sheets of cells and includes the skin and the linings of internal organs and body cavities. Epithelial cells are tightly packed, providing a barrier against injury, infection, and water loss. Epithelial tissue can be a single layer called simple epithelium, or multiple layers called stratified epithelium. In stratified epithelium, such as the skin, the outer cells—which are subject to damage—are replaced through the division of cells underneath. Epithelial cells have a variety of shapes, including squamous (flattened), cuboid, and columnar. Some epithelial tissues absorb or secrete substances, such as the lining of the intestines.
Connective tissue is composed of cells within an extracellular matrix and includes loose connective tissue, fibrous connective tissue, adipose (fat) tissue, cartilage, bone, and blood. Although the characteristics of connective tissue vary greatly, their general function is to support and attach multiple tissues. For example, tendons are made of fibrous connective tissue and attach muscle to bone. Blood transports oxygen, nutrients and waste products, and exerts immune functions, to support the needs of other tissues.
Muscle tissue consists of groups of long, thin muscle cells—called muscle fibers. Muscle cells can contract and expand, allowing the body and internal organs to move. The main types of muscle tissue are the cardiac muscle of the heart; the skeletal muscles that attach to bones, such as the limb muscles; and smooth muscle found in visceral organs such as the intestines.
Nervous tissue is composed of neurons—specialized cells that send, transport, and receive information through electrochemical signaling—and supporting cells called glial cells. A bundle of neurons is a nerve. A brain is the concentration of nervous tissue. Besides controlling movement through the muscles, nervous tissue detects sensory stimuli and is responsible for directing many of the activities of the body.