There are four types of tissue in the human body.

Epithelial tissue: sheets of closely attached cells which cover the body, make up many glands or organs, or line the inside of body cavities. Epithelial tisue always has a free surface (one side of the cell layer is always exposed to the outside or to an internal, open compartment, such as the digestive tract. Epithelial tissue can be one cell thick or multiple layers thick.

Connective tissue: provides support and points of attachment or connections. Includes bone, cartilage, adipose or fat tissue, blood, and loose connective tissue. All types of connective tissue consist of cells dispersed in a non-cellular matrix. The matrix can be mineralized (as in bone), fibrous (as in cartilage), or liquid (as in blood).

Muscle tissue: cells, which can generate tension or movement (contraction) by means of intracellular contractile proteins. The three basic types of muscle are skeletal (voluntary muscle), cardiac muscle and smooth muscle (involuntary).

Nervous tissue: highly specialized to receive electrical or chemical stimuli (irritability) and to convey this information to all parts of the body (conductivity). Nervous tissue consists of two major cell populations: neuroglia (supporting cells) and neurons (nerve cells). Neurons of the brain and spinal cord have cell extensions that make connections with other nerve cells, muscle cells, or gland cells.