Connective tissue is a term used to describe the tissue of mesodermal origin that that forms a matrix beneath the epithelial layer and is a connecting or supporting framework for most of the organs of the body.
In contrast to epithelia, connective tissue is sparsely populated by cells and contains an extensive extracellular matrix consisting of protein fibers, glycoproteins, and proteoglycans. The function of this type of tissue is to provide structural and mechanical support for other tissues, and to mediate the exchange of nutrients and waste between the circulation and other tissues. These tissues have two principal components, an extracellular matrix and a variety of support cells. Extracellular matrix is comprised of two non-cellular biological materials, the fiber component and ground substance. In addition, there is a cellular component to connective tissue that is comprised of the cells that produce and maintain the extracellular matrix and that cells that migrate from the associated vasculature to reside in the extracellular matrix.