Connective Tissue

Two main types of fibrous connective tissue are found in the body: dense and loose. In dense connective tissue, almost all the space between the cells is filled by large numbers of protein fibers. In loose connective tissue, there are fewer fibers between the cells which imparts a more open, loose structure.

Dense connective tissue contains large numbers of collagen fibers, and so it is exceptionally tough. Dense regular connective tissue has parallel bundles of collagen fibers and forms tendons that attach muscles to bone and ligaments that bind bone to bone. Dense irregular connective tissue, with less orderly arranged collagen fibers, forms the tough lower layer of the skin known as the dermis, and encapsulates delicate organs such as the kidneys and the spleen.

Loose connective tissue has fewer collagen fibers than dense connective tissue, and therefore is not as tough. Loose connective tissue (also known as areolar connective tissue) is widely distributed throughout the body and provides the loose packing material between glands, muscles, and nerves.

A scanning electron micrograph (SEM) of hyaline articular cartilage covering the end of a long bone. The smooth, slippery surface of the cartilage enables movement of the joint. Photograph by Prof. P. Motta/Dept. of Anatomy/University "La Sapienza", Rome/Science Photo Library. National Audubon Society Collection/Photo Researchers, Inc. Reproduced by permission.

Two other connective tissues with fibers are adipose tissue and reticular tissue. Adipose tissue is composed of specialized fat cells and has few fibers: this tissue functions as an insulator, a protector of delicate organs and as a site of energy storage. Reticular connective tissue is composed mostly of reticular fibers that form a net-like web, which forms the internal framework of organs like the liver, lymph nodes, and bone marrow.