Connective tissue holds together and gives shape to body organs and tissues. It also holds organs in place and supports the body. Compared with other types of tissue, connective tissue has relatively few cells and much intercellular substance (material outside the cells). Scientists divide connective tissue into three groups, according to the hardness of the intercellular substance. These groups are (1) soft connective tissue proper, (2) firm cartilage, and (3) hard bone. This article discusses connective tissue proper. For information on the other kinds of connective tissue, see Bone; Cartilage.
There are two chief types of connective tissue proper—loose and dense. Loose connective tissue forms the networks of thin fibers that surround such organs as the heart and lungs. It is also found under the skin and over the muscles. Dense connective tissue has more fibers and greater strength than does loose connective tissue. It forms tough capsulelike coverings that keep such organs as the kidneys in place. Dense connective tissue also forms the ligaments, which hold the bones and joints in place, and the tendons, which connect the muscles to the bones (see Ligament; Tendon).
In addition, connective tissue proper stores fat cells. Specialized kinds of this tissue, called reticular tissue, manufacture red and white blood cells.
The intercellular substance of connective tissue proper consists of long fibers mixed with a jellylike material. The fibers are mostly made up of a protein called collagen (see Collagen). Most collagen fibers resemble strong cords, but some form strands of delicate reticular tissue. Other fibers consist of a protein called elastin, which stretches easily.
See also Marfan syndrome.
