connective tissue

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Tissue arising chiefly from the embryonic mesoderm that is characterized by a highly vascular matrix and includes collagenous, elastic, and reticular fibers, adipose tissue, cartilage, and bone. It forms the supporting and connecting structures of the body.

One of the four primary tissues of the body. It differs from the other three tissues in that the extracellular components (fibers and intercellular substances) are abundant. It cannot be sharply delimited from the blood, whose cells may give rise to connective tissue cells, and whose plasma components continually interchange with and augment the ground substance of connective tissue. Bone and cartilage are special kinds of connective tissue.

The functions of connective tissues are varied. They are largely responsible for the cohesion of the body as an organism, of organs as functioning units, and of tissues as structural systems. The connective tissues are essential for the protection of the body both in the elaborate defense mechanisms against infection and in repair from chemical or physical injuries. Nutrition of nearly all cells of the body and the removal of their waste products are both mediated through the connective tissues. Connective tissues are important in the development and growth of many structures. Constituting the major environment of most cells, they are probably the major contributor to the homeostatic mechanisms of the body so far as salts and water are concerned. They act as the great storehouse for the body of salts and minerals, as well as of fat. The connective tissues determine in most cases the pigmentation of the body. Finally, the skeletal system (cartilage and bones) plus other kinds of connective tissue (tendons, ligaments, fasciae, and others) make motion possible.

The connective tissues consist of cells and extracellular or intercellular substance (see illustration). The cells include many varieties, of which the following are the most important: fibroblasts, macrophages (histiocytes), mast cells, plasma cells, melanocytes, and fat cells. Most of the cells of the connective tissue are developmentally related even in the adult; for example, fibroblasts may be developed from histiocytes or from undifferentiated mesenchymal cells.

Components of connective tissue.

The extracellular components of connective tissues may be fibrillar or nonfibrillar. The fibrillar components are reticular fibers, collagenous fibers, and elastic fibers. The nonfibrillar component of connective tissues appears amorphous with the light microscope and is the matrix in which cells and fibers are embedded. It consists of two groups of substances: (1) those probably derived from secretory activity of connective tissue cells including mucoproteins, protein-polysaccharide complexes, tropocollagen, and antibodies; and (2) those probably derived from the blood plasma, including albumin, globulins, inorganic and organic anions and cations, and water. In addition, the ground substance contains metabolites derived from, or destined for, the blood.

All the manifold varieties of connective tissue may contain all the cells and fibers discussed above in addition to ground substance. They differ from each other in the relative occurrence of one or another cell type, in the relative proportions of cells and fibers, in the preponderance and arrangement of one or another fiber, and in the relative amount and chemical composition of ground substance. They are classified as:

1. Irregularly arranged connective tissue—which may be loose (subcutaneous connective tissue) or dense (dermis). The dominant fiber type is collagen.

2. Regularly arranged connective tissue—primarily collagenous—with the fibers arranged in certain patterns depending on whether they occur in tendons or as membranes (dura mater, capsules, fasciae, aponeuroses, or ligaments).

3. Mucous connective tissue—ground substance especially prominent (umbilical cord).

4. Elastic connective tissue—predominance of elastic fibers or bands (ligamentum nuchae) or lamellae (aorta).

5. Reticular connective tissue—fibers mostly reticular, moderately rich in ground substance, frequently numerous undifferentiated mesenchymal cells.

6. Adipose connective tissue—yellow or brown fat cells constituting chief cell type, reticular fibers most numerous.

7. Pigment tissue—melanocytes numerous.

8. Cartilage—cells exclusively of one type, derived from mesenchymal cells.

9. Bone—cells are predominantly osteocytes, but also include fibroblasts, mesenchymal cells, endothelial cells, and osteoclasts.

Connective tissue links, separates, supports, embeds, and protects the body's cells, tissues, and organs, and imparts varying degrees of fluidity, elasticity, or rigidity. There are many types. All contain non-cellular fibres together with the living cells which manufacture them and a matrix which ranges from near-liquid to solid. There are fibres of collagen in all types and also of elastin in many. Loose areolar tissue is the least solid, with gelatinous material among its fibres. Fatty (adipose) tissue has lipid-containing cells in a network of collagenous fibres. Fibrous connective tissue forms a framework among muscle fibres, and among nerve fibres, as well as sheaths for whole muscles and whole nerves; it also forms membranes such as the thin sheets of fascia which separate whole tissue layers from their neighbours, and stronger capsules for individual organs and ligaments which support joints. Elastic fibres in the lungs are crucial for the mechanics of breathing, and in the major arteries for their rhythmic expansion and recoil. Tendons (‘sinews’) are strong, resilient straps of collagenous and elastic fibres which connect a muscle to a bony attachment. Cartilage is a rigid connective tissue; it encircles the windpipe and stiffens the nostrils to keep the airway open, keeps the ears from flapping, and covers the ends of bones in the joints. Finally, bone is obviously the most rigid in the category, with mineral deposits laid down in a collagen fibre framework.

— Sheila Jennett

Consists of the protein collagen which in fish is found between the muscle segments (myotomes); in meat it is spread through the muscle, uniting the muscle fibres into bundles and supporting the blood vessels (a kind of soft skeleton), and consists of both collagen and elastin.

A high content of connective tissue results in tougher meat.

Collagen is insoluble; it is converted to soluble gelatine by moist heat, so making the food more tender. Tough meat is softened to some extent by stewing, but roasting or frying has little effect. Elastin is unaffected by heating, and remains tough, elastic, and insoluble.

A tissue found in all parts of the body. Its functions include support, storage, and protection. All types of connective tissue have a good blood supply. Most types, especially bones, cartilage, tendons, and ligaments, are strengthened by collagen fibres. This enables the tissues to endure high levels of abuse from physical trauma and abrasions. Adipose tissue is a fairly loose connective tissue that is not very strong. Its main function is to store fat.

The binding and supportive tissue of the body; derived from the mesoderm; depending on its location and function, it is composed of fibroblasts, primitive mesenchymal cells, collagen fibers, and elastic fibers, with associated blood and lymphatic vessels and nerve fibers.

For more information on connective tissue, visit Britannica.com.

A vascularized tissue composed mainly of extracellular material. It has a number of functions including support, storage, and protection. Connective tissue is found in all parts the body, and includes blood, lymph, bone, cartilage, and adipose (fat) tissue. Joint structures, such as tendons and ligaments, contain connective tissue that is rich in collagen (see dense regular connective tissue). This enables the tissue to endure high tensile stresses.

Body tissue that serves to connect or support other tissues or parts. Cartilage, tendons, and bone are all kinds of connective tissue.

A fibrous type of body tissue with varied functions. The connective tissue system supports and connects internal organs, forms bones and the walls of blood vessels, attaches muscles to bones, and replaces tissues of other types following injury.
Connective tissue consists mainly of long fibers embedded in noncellular matter, the ground substance. The density of these fibers and the presence or absence of certain chemicals make some connective tissues soft and rubbery and others hard and rigid. Compared with most other kinds of tissue, connective tissue has few cells. The fibers contain a protein called collagen.
Connective tissue can develop in any part of the body, and the body uses this ability to help repair or replace damaged areas. Scar tissue is the most common form of this substitute. See also collagen diseases.

• elastic c. t. — found especially in supportive tissues, e.g. some ligaments, and tendons, e.g. nuchal ligament or tunica flava.
• reticular c. t. — a type of connective tissue found principally in myeloid and lymphatic organs; they account for reticular meshwork.

Connective tissue is one of the four types of tissue in traditional classifications (the others being epithelial, muscle, and nervous tissue.) It 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.
• Derived from mesoderm, usually.
• Characterized largely by the traits of non-living tissue.

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.

Collagen is the main protein of connective tissue in animals and the most abundant protein in mammals, making up about 25% of the total protein content.[1]

Classification

Connective tissue proper

Connective tissue proper

• Areolar (or loose) connective tissue holds organs and epithelia in place, and has a variety of proteinaceous fibres, including collagen and elastin.
• Dense connective tissue (or, less commonly, fibrous connective tissue) forms ligaments and tendons. Its densely packed collagen fibers have great tensile strength.

Specialized connective tissues

Specialized connective tissues

• 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 makes up virtually the entire skeleton in adult vertebrates.
• 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.
• Adipose tissue contains adipocytes, used for cushioning, thermal insulation, lubrication (primarily in the pericardium) and energy storage. [fat]
• Reticular connective tissue is a network of reticular fibres (fine collagen, type III) that form a soft skeleton to support the lymphoid organs (lymph nodes, bone marrow, and spleen.)

Embryonic connective tissues

• Mesenchymal connective tissue
• Mucous connective tissue

Fiber types

Fiber types as follows:

• collagenous fibers
• elastic fibers
• reticular fibers

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 - deficient type III collagen- a genetic disease causing progressive deterioration of collagens, with different EDS types affecting different sites in the body, such as joints, heart valves, organ walls, arterial walls, etc.
• Loeys-Dietz syndrome - a genetic disease related to Marfan syndrome, with an emphasis on vascular deterioration.
• Pseudoxanthoma elasticum - an autosomal recessive hereditary disease, caused by calcification and fragmentation of elastic fibres, affecting the skin, the eyes and the cardiovascular system.
• Systemic lupus erythematosus - a chronic, multisystem, inflammatory disorder of probable autoimmune etiology, occurring predominantly in young women.
• 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.

Staining of connective tissue

For microscopic viewing, the majority of the connective tissue staining techniques color tissue fibers in contrasting shades. Collagen may be differentially stained by any of the following techniques:

• Van Gieson's stain
• Masson's Trichrome stain
• Mallory's Aniline Blue stain
• Azocarmine stain
• Krajian's Aniline Blue stain

See also

• Zootomy

External links

• connective+tissue at eMedicine Dictionary
• t_12/12810256 at Dorland's Medical Dictionary
• Overview at kumc.edu
• UIUC Histology Subject 230
• Connective tissue atlas at uiowa.edu

Biological tissue
Animals	Epithelium - Connective - Muscular - Nervous
Plants	Dermal - Vascular - Ground

Histology: connective tissue
Classification	proper (loose/areolar, dense, adipose brown and white, reticular) embryonic (mucous, mesenchymal) specialized (cartilage, bone, blood)
Extracellular matrix	ground substance (tissue fluid) fibers (collagen, reticular fiber, elastic fibers)
Cells	resident (fibroblast, adipocyte, chondroblast, osteoblast), wandering cell

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