All connective tissues have a large proportion of extracellular matrix as well as cells. This matrix consists of extracellular macromolecules containing collagens, elastins, non-collagenous glycoproteins and proteoglycans.
These consist of three polypeptide chains (a chains) wound around one another in a triple helical confirmation. These a chains contain repeating sequences of Gly-X-Y triplets where X and Yare often prolyl and hydroxyprolyl residues. There is much genetic heterogeneity in collagen fibres and the gene for different chains is located on several chromosomes.
Functionally the tissue depends on the type of collagen. Thirteen distinct collagen types have been described so far, although the exact function of many remains undefined. For example, type I collagen fibres are seen in structures with a high tensile strength, e.g. tendons, type II collagen molecules form the cartilaginous structures and type III collagen is seen in more distensible tissues such as blood vessels.
Elastic fibres in the extracellular matrix consist of elastin and microfibrils. Elastin is an insoluble protein polymer and its gene has been characterized. Its precursor, tropoelastin,is synthesized by vascular smooth muscle cells and skin fibroblasts. Elastin fibres are cross-linked with desmosine and isodesmosine which are specific to elastin.
Fibronectin is the major non-collagenous glycoprotein in the extracellular matrix. Its molecule contains a series of functional domains, or cell recognition sites, that bind ligands and are involved in cell adhesion. A synthetic peptide sequence (Arg-Gly-Asp), which mimics some of the functions of fibronectin, is also found in other adhesion proteins, e.g. vitronectin, laminin and collagen type VI.
Fibronectin plays a major role in tissue remodelling. It is stimulated by interferon-y and transforming growth factor-/ 3 and inhibited by tumor necrosis factor and interleukin-L
These are of different shapes and sizes. Their function isthat of a multipurpose glue in that they bind extra cellular matrix together, mediate cell binding and inhibit soluble molecules. Osteogenesis imperfecta (fragilitas ossium, brittle bone syndrome) This rare group of inherited disorders is due to an abnormality of connective tissue. The major feature is very fragile and brittle bones; other collagen-containing tissues are also involved, such as tendons, the skin and the eyes. Osteogenesis imperfecta tarda is a mild, dominantly inherited condition with milder bony deformities, blue sclerae, defective dentine, early-onset deafness, hypermobility of joints, and heart valve disorders. More severe forms present with multiple fractures and gross deformities. Prognosis is variable, depending on the severity of the disease.
This is one of a group of heterogeneous disorders that affect the normal development of bone and cartilage. Achondroplasia (dwarfism) is diagnosed in the first years of life. The disease is inherited in an autosomal dominant manner. The trunk is of normal length but the limbs are very short and broad. The vault of the skull is enlarged, the face is small and the nose bridge is flat. Intelligence is normal.