Connective tissue is one of the four types of tissue in traditional classifications (the others being epithelial, muscle, and nervous tissue.) It is characterized by abundant extracellular matter (intercellular substances and fibers) encasing relatively few cells. The various combinations of the elements in this intercellular matrix give the tissue its character, resulting in different kinds of connective tissue.
As the name implies, connective tissues provide structure and support. These tissues provide integrity to the organs and the major environment of most cells, protect the body, act as a storehouse of minerals and fats, and allow motion through such tissues as bones, cartilage, tendons, ligaments, and so forth.
Connective tissue involves the complex cooperation of a variety of cells and materials (fibers, ground substance) in the intercellular matrix. There may be a macrophage, fibroblast, fat cell, mast cell, and plasma cell in a section of connective tissue, along with collagenous, elastic, and reticular fibers passing through the ground substance. When a greater ability to stretch is needed, such as in ligaments, then the connective tissue will contain more elastic fibers than in that combination of cells and intercellular matrix that forms cartilage or bone. The harmony among the cells and intercellular components in the tissue is astounding, and all the more remarkable considering that all of this originally developed from one fertilized egg cell.
Blood, cartilage, and bone are usually considered connective tissue, but because they differ so substantially from the other tissues in this class, the phrase "connective tissue proper" is commonly used to exclude those three. There is also variation in the classification of embryonic connective tissues; on this page they will be treated as a third and separate category.
Connective tissue is largely a category of exclusion rather than one with a precise definition, but all or most tissues in this category are similarly involved in structure and support and usually derived from embryonic mesoderm.
Connective tissue is composed of different types of cells (fibroblasts, plasma cells, fat cells, melanocytes, mast cells) combined with large amounts of intercellular matter. The intercellular matrix includes fibers (collagenous, elastic, and reticular fibers) and nonfibrillar components (the ground substance in which the cells and fibers are embedded).
There are a variety of types of connective tissues (such as loose connective tissue and dense connective tissue). The types of connective tissue result from variation in the relative proportion of the diverse cell types and substances in the intercellular matrix. Among the variables are how many fibers there are to cells, the numbers and proportions of the different cell types in a given volume, the proportions and arrangements of the different fibers, the composition of the ground substance, and so forth.
The three basic fiber types are:
- Collagenous fibers. Collagen is the main protein of connective tissue in animals and the most abundant protein in mammals. Tough bundles of collagen called collagen fibers or collagenous fibers are a major component of the extracellular matrix that supports most tissues and gives cells structure from the outside. Collagen involves the harmonization of three polypeptide chains into the form of a triple helix, and is characterized by the regular arrangement of amino acids in each of the three chains. Under tension, the triple helix coils tight, resisting stretching, and making collagen valuable for structure and support, while giving bones some elasticity. Collagen is common in loose connective tissue, dense connective tissue, reticular connective tissue, bone, and cartilage.
- Elastic fibers. Elastic fibers, or yellow fibers, are bundles of proteins (elastin) found in connective tissue and produced by fibroblasts and smooth muscle cells in arteries. These fibers can stretch up to 1.5 times their length, and snap back to their original length when relaxed. Elastin is a protein in elastic fibers and it elastic, allowing many tissues in the body to resume their shape after stretching or contracting.
- Reticular fibers. Reticular fibers or reticulin is a type of structural fiber in some connective tissues that involves fine meshwork (reticulim) of glycosylated collagen III and other components. Networks of these fibers make up stroma of lymphatic and hemopoietic tissues, such as the thymus, lymph nodes, spleen, bone marrow, and adipose tissue, and also parenchyma of liver, testis, and lung.
Connective tissue proper
- Areolar (or loose) connective tissue. Areolar connective tissue holds organs and epithelia in place, and has a variety of proteinaceous fibers, including collagen and elastin. Areolar connective tissue is the most widely distributed connective tissue type in vertebrates. It is a pliable, mesh-like tissue with a fluid matrix and functions to cushion and protect body organs. Cells called fibroblasts are widely dispersed in this tissue; they are irregular branching cells that secrete strong fibrous proteins and proteoglycans as an extracellular matrix. The cells of this type of tissue are generally separated by quite some distance by a gel-like gelatinous substance primarily made up of collagenous and elastic fibers. Areolar connective tissue can be found in the skin as well as in places that connect epithelium to other tissues. The areolar tissue is found beneath the dermis layer and is also underneath the epithelial tissue of all the organ systems that have external openings. It is a component of mucus membranes found in the digestive, respiratory, reproductive, and urinary systems, and surrounds the blood vessels and nerves.
- Adipose tissue. Adipose tissue contains adipocytes (fat cells). Its main role is to store energy in the form of fat, although it also is used for cushioning, thermal insulation, and lubrication (primarily in the pericardium.
- Dense connective tissue. Dense connective tissue (or, less commonly, fibrous connective tissue) has collagen fibers as its main matrix element. Crowded between the collagen fibers are rows of fibroblasts, fiber-forming cells, that manufacture the fibers. Dense connective tissue forms strong, rope-like structures such as tendons and ligaments. Tendons attach skeletal muscles to bones; ligaments connect bones to bones at joints. Ligaments are more stretchy and contain more elastic fibers than tendons. Dense connective tissue also make up the lower layers of the skin (dermis), where it is arranged in sheets.
- Reticular connective tissue. Reticular connective tissue is a network of reticular fibers (fine collagen, type III) that form a soft skeleton to support the lymphoid organs (lymph nodes, bone marrow, thymus, and spleen.) Reticular fibers are synthesized by special fibroblasts called reticular cells. The fibers are thin branching structures. Adipose tissue is held together by reticular fibers. Reticular connective tissue resembles areolar connective tissue, but the only fibers in its matrix are reticular fibers. Although reticular fibers are widely distributed in the body, reticular tissue is limited to certain sites.
Specialized connective tissues
- Blood. Blood functions in transport. Its extracellular matrix is blood plasma, which transports dissolved nutrients, hormones, and carbon dioxide in the form of bicarbonate. The main cellular component is red blood cells.
- Bone. Bone makes up virtually the entire skeleton in adult vertebrates.
- Cartilage. Cartilage makes up virtually the entire skeleton in chondrichthyes. In most other vertebrates, it is found primarily in joints, where it provides cushioning. The extracellular matrix of cartilage is composed primarily of collagen.
Embryonic connective tissues
- Mesenchymal connective tissue
- Mucous connective tissue
Disorders of connective tissue
Various connective tissue conditions have been identified; these can be both inherited and environmental.
- Marfan syndrome - a genetic disease causing abnormal fibrillin.
- Scurvy - caused by a dietary deficiency in vitamin C, leading to abnormal collagen.
- Ehlers-Danlos syndrome - a genetic disease, involving deficient type III collagen, causing progressive deterioration of collagens, with different types affecting different sites in the body, such as joints, heart valves, organ walls, arterial walls, and so forth.
- Loeys-Dietz syndrome - a genetic disease related to Marfan syndrome, with an emphasis on vascular deterioration.
- Osteogenesis imperfecta (brittle bone disease) - caused by insufficient production of good quality collagen to produce healthy, strong bones.
- Fibrodysplasia ossificans progressiva - disease of the connective tissue, caused by a defective gene, which turns connective tissue into bone.
- Spontaneous pneumothorax - collapsed lung, believed to be related to subtle abnormalities in connective tissue.
- Sarcoma - a neoplastic process originating within connective tissue.
- Larkin, M. 1992. Coping with connective tissue diseases. FDA Consumer 26: 28.
- Ross, M. H., G. I. Kaye, and W. Pawlina. 2003. Histology: A Text and Atlas. Philadelphia, Pa: Lippincott Williams & Wilkins. ISBN 0683302426.
- Towle, A. 1989. Modern Biology. Austin, TX: Holt, Rinehart and Winston. ISBN 0030139198.
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