CONNECTIVE TISSUE

in physiology, any tissue in the body that maintains the form of the body and its organs and provides cohesion and internal support. The term encompasses bone, ligaments, tendons, cartilage, and adipose tissue. Connective tissue consists of cells and one of three kinds of protein fibres embedded in an extracellular, largely carbohydrate amorphous matrix called ground substance. All connective and supportive tissues originate from the same embryonic layer, the mesoderm. The major components of connective tissue are cells, extracellular fibres, and the ground substance. The amorphous ground substance varies in its physical consistency from a thin gel to a rigid structure, depending on its composition and function. The cells most commonly found in connective tissue are either stationary (fibroblasts, macrophages, and adipose) or motile (mast cells, monocytes, lymphocytes, plasma cells, and eosinophils). The most abundant of these tissue cells is the fibroblast, which is principally responsible for the maintenance and repair of the extracellular fibres. Macrophages are cells that help to destroy bacteria. Adipose cells process and store nutrients and, where present in abundance, form the fatty tissue of the body. The motile cells help regulate the degree to which blood penetrates into tissue, the ability of blood to clot, and the means to fight disease and infection; they also assist in the healing of injuries. There are three types of extracellular fibres: collagenous, elastic, and reticular fibres. Most abundant are the fibres composed of the protein collagen. The arrangement of the collagenous fibres provides them with great tensile strength. Reticular fibres form fine-meshed networks of support and connection around cells and tissues. Elastic fibres differ from collagenous fibres in structural arrangement and chemical composition. They are composed of the protein elastin and are highly extensible and resilient. Loose connective tissue consists of a loosely woven matrix of collagenous, elastic, and reticular fibres. It is found between tissues and organs and is used as connection. In more specialized forms of connective tissue, one component may predominate over the others, depending on the local structural and metabolic requirements. In cartilage, for instance, chondrocytes (cartilage cells) are embedded in an abundance of tough, gelatinous ground substance. Cartilage is a firm, flexible support tissue found mainly in the intervertebral disks, nasal septum, larynx, and parts of the sternum and of the external, middle, and inner ear. Bone matrix contains additional mineral crystals of calcium phosphate, which is responsible for the rigidity and hardness of bone. Adipose tissue is loose connective tissue in which adipose (fat) cells make up the bulk of the tissue. Dense fibrous connective tissue contains closely packed bundles of collagen and fibroblasts but few elastic fibres and little ground substance. Fibrous tissues include tendons (connecting muscles to bones) and ligaments (connecting bone to bone). There are two basic types of connective-tissue diseases, those that are genetically transferred and attack one of the elements of connective tissue (collagen, cells, or ground substance), often impeding normal growth, and those that are acquired (formerly called collagen diseases), having an inflammatory nature or causing irregularities in the body's immune system. The most frequent heritable connective tissue disease is Marfan's syndrome, which causes an excessive skeletal length in the extremities. Homocystinuria is similar to Marfan's syndrome but is usually accompanied by mental degeneration. Alkaptonuria is caused by an overabundance of homogentisic acid due to the inability to synthesize the enzyme that breaks this substance down. It produces a blackening of tissue and a brittleness of cartilage and intervertebral disks. Mucopolysaccharidoses are diseases of the ground substance. Acquired connective-tissue diseases usually involve inflammation of the joints, serous membranes, and small blood vessels. Many also affect the internal organs. Rheumatoid arthritis is a chronic disorder in which the soft tissue in the joint becomes inflamed, damaging joint cartilage and causing deformities. Women are affected about three times as frequently as men. Systemic lupus erythematosus (SLE) is also chronic, involving inflammation of the skin, joints, kidneys, nervous system, and serous membranes. It may attack one or more of the aforementioned locations, and it may affect other organs. SLE most often afflicts people in their 20s, 30s, and 40s. Its severity ranges from mild to fatal. Progressive systemic sclerosis, or scleroderma, is an inflammation of the fibrous tissue, causing a thickening of the skin and deterioration of the joints, muscles, and internal organs. Women contract the disease twice as often as men. Polymyositis involves the inflammation of the musculature, causing debilitation, atrophy, and contracture. The heart is sometimes afflicted, as are the esophagus and larynx muscles. Sjgren's syndrome, or sicca syndrome, is the decrease of tear and saliva secretion, causing dryness of the eyes and mouth. It may involve other mucous membranes. Rheumatic fever follows a group A streptococcal infection, attacking the joints and, in its most dangerous form, the heart. Amyloidosis is an abnormal buildup of amyloid in connective tissue. Osteoarthritis is the degeneration of joint cartilage. Especially vulnerable to this disease are weight-bearing joints. When the cartilage deteriorates, the bones usually enlarge, protruding out of the joint articulation, causing joint damage and pain. Relapsing polychondritis is the inflammation of cartilages, particularly those of the ear and nose, resulting in deformities. The most severe form of the disease afflicts the windpipe, frequently causing recurring pneumonia and blockage of the air passages. in physiology, any tissue in the body that maintains the form of the body and its organs and provides cohesion and internal support. The connective tissues are a heterogeneous group derived from the mesenchyme, a meshwork of stellate cells that develop in the middle layer of the early embryo. They include several types of fibrous tissue that vary only in their density and cellularity, as well as the more specialized variants of bone, ligaments, tendons, cartilage, and adipose tissue. The cells that are responsible for the specific functions of an organ are referred to as its parenchyma, while the delicate fibrous meshwork that binds the cells together into functional units, the fibrous partitions, or septa, that enclose aggregations of functional units, and the dense fibrous capsule that encloses the whole organ collectively make up its connective-tissue framework, or stroma. Blood vessels, both large and small, course through connective tissue, which is therefore closely associated with the nourishment of tissues and organs throughout the body. All nutrient materials and waste products exchanged between the organs and the blood must traverse perivascular spaces occupied by connective tissue. One of the important functions of the connective-tissue cells is to maintain conditions in the extracellular spaces that favour this exchange. Some organs are suspended from the wall of a body cavity by thin sheets of connective tissue called mesenteries; others are embedded in adipose tissue, a form of connective tissue in which the cells are specialized for the synthesis and storage of energy-rich reserves of fat, or lipid. The entire body is supported from within by a skeleton composed of bone, a type of connective tissue endowed with great resistance to stress owing to its highly ordered, laminated structure and to its hardness, which results from deposition of mineral salts in its fibres and amorphous matrix. The individual bones of the skeleton are held firmly together by ligaments, and muscles are attached to bone by tendons, both of which are examples of dense connective tissue in which many fibre bundles are associated in parallel array to provide great tensile strength. At joints, the articular surfaces of the bones are covered with cartilage, a connective tissue with an abundant intercellular substance that gives it a firm consistency well adapted to permit smooth gliding movements between the apposed surfaces. The synovial membrane, which lines the margins of the joint cavity and lubricates and nourishes the joint surfaces, is also a form of connective tissue. Additional reading Additional information on connective tissues may be found in W. Bloom and D.W. Fawcett, Textbook of Histology, 9th ed., ch. 10, pp. 131164, and ch. 12, pp. 347357 (1968); and in A.W. Ham, Histology, 6th ed., pt. 3, ch. 10, pp. 205227 (1969).