Connective tissue is found throughout the body and includes fat, cartilage, bone, and blood. The main functions of the different types of connective tissue include providing support, filling in spaces between organs, protecting organs and aiding in the transport of materials around the body.
Connective tissue is composed of living cells and protein fibers suspended in a gel-like material called matrix. Depending on the type of connective tissue, the fibers are either collagen fibers, reticular fibers, elastin fibers, or a combination of two or more types. The type and arrangement of the fibers gives each type of connective tissue its particular properties.
Of the three types of protein fibers in connective tissue, collagen is by far the most abundant, and accounts for almost one third of the total body weight of humans. Under the microscope, collagen looks like a rope, with three individual protein fibers twined around each other. It is extremely strong but has little flexibility. Reticular fibers are composed of very short, small collagen fibers that form a netlike supporting structure that gives shape to various organs. Elastin fibers have elastic properties and can stretch and be compressed, importing flexibility into the connective tissues where they are found.
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; it 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.
Two other fibrous connective tissues are adipose and reticular tissue. Adipose tissue is composed of specialized fat cells and has few fibers; it 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 netlike web that forms the internal framework of organs like the liver, lymph nodes, and bone marrow.
Connective tissue composed of ground substance and protein fibers
Connective tissue composed of ground substance and protein fibers differs from fibrous connective tissue in that it contains more ground substance. Two main types of this kind of connective tissue are found in the body: cartilage and bone.
Cartilage is composed of cartilage cells and collagen fibers or a combination of collagen and elastin fibers. An interesting characteristic of cartilage is that when it is compressed, it immediately springs back into shape.
Hyaline cartilage is rigid yet flexible, due to evenly spaced collagen fibers. Hyaline cartilage is found at the ends of the ribs, around the trachea (windpipe), and at the ends of long bones that form joints. Hyaline cartilage forms the entire skeleton of the embryo, which is gradually replaced by bone as the newborn grows.
Fibrocartilage contains densely packed, regularly arranged collagen fibers that impart great strength to this connective tissue. Fibrocartilage is found between vertebrae as discs that act as a cushion.
Elastic cartilage contains elastin fibers and is thus more flexible than either hyaline cartilage or fibrocartilage. Elastic cartilage is found in the pinnas of the external ear.
Bone is composed of bone cells (osteocytes), suspended in a matrix of collagen fibers and minerals. The mineral portion imparts great strength and rigidity to bone. Osteocytes are located in depressions called lacunae, which are connected by canals called Haversian canals.
Two types of bone form the mammalian skeleton: cancellous (or trabecular or spongy) bone and compact bone. Cancellous bone is more lattice-like than compact bone and does not contain as many collagen fibers in its matrix. Cancellous bone is lightweight, yet strong, and is found in the skull, the sternum and ribs, the pelvis and the growing ends of the long bones. Compact bone is densely packed with fibers and forms the outer shell of all bones and the shafts of the long bones of the arms and legs. Compact bone is heavier and provides great strength and support.
Blood is a liquid connective tissue composed of a fluid matrix and blood cells, including white blood cells, which function in the immune system, and red blood cells, which transport oxygen and carbon dioxide. The fluid part of blood, the plasma, transports hormones, nutrients, and waste products, and plays a role in temperature regulation.
See also Skeletal system.
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