Connective tissues are the most abundant, widely distributed, and histologically variable of the primary tissues. They include fibrous tissue, adipose tissue, cartilage, bone, and blood. Such diverse tissues may seem to have little in common, but as a rule, their cells occupy less space than the extracellular matrix. Usually their cells are not in direct contact with each other, but are separated by expanses of matrix. Most connective tissues serve to bind organs to each other, for example, the way a tendon connects muscle to bone, form a structural framework for an organ, or support and protect organs. Connective tissues vary greatly in vascularity, from rich networks of blood vessels in the loose connective tissues to few or no blood vessels in cartilage.
