Connective tissues, collagen fibers, Ehlers-Danlos syndrome, elastic fibers, Marfan syndrome, amyloidosis, skin

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Colorado State University
Biomedical Science
BMS 460
D.Rao Veeramachaneni

6 September Connective Tissues Cells and Fibers of Loose Connective Tissue In addition to its function of literally connecting the cells and tissues of the body to each other, CT provides a compartment, or “highway”, in which cells are many macromolecules can travel (in principle) throughout the body without having to cross a basal lamina. Ground substance Plasma cells Adipose cells Macrophages, called histiocytes in tissues Collagen fibers Mast cells Elastic fibers Fibroblasts, produce fibers Endothelial cells/pericytes of capillaries Connective Tissue Proper Relatively few cells in large volume of extracellular matrix, ECM Fibroblasts Extracellular fibers Collagen Elastin ECM Amyloid Loose and dense Loose CT, in particular, houses blood vessels and nerves The outer covering of blood vessels and nerves consists of a CT sheath Collagen Fibers Collagen fibers are inelastic, flexible fibers. Collagen is a three-chain fibrous protein in which the chains coil around each other (called a coiled-coil structure) like the strands of a rope. This triple-helix molecular organization generates a protein with considerable tensile strength. A variety of cells produce more than 20 biochemically different types of collagen – fibroblasts in the connective tissue and their equivalents the osteoblast (bone), chrondroblast (cartilage), and odontoblast (teeth) synthesize collagen. A mutation in COL1A1 and COL1A2 genes, encoding the α and α cha1ns of 2ype 1 collagen, respectively, involves cleavage sites for the N-terminal region of the molecule and interferes with the conversion of precollagen to collagen. This leads to defective cross-linking and a consequent reduction in the tensile strength of tendons (rich in type I collagen). This mutation is observed in some clinical forms of Ehlers-Danlos syndrome. Strickler syndrome is characterized by myopia, hypoplasia of the lower jaw, and arthritis associated with dysplasia of the epiphyses. Type II collagen is abundant in cartilage and vitreous humor (eye). The COL2A1 gene is mutated. Osteogenesis imperfect type 1 is associated with bone fragility. COL1A1 point mutations determine a reduction in the production of type 1 collagen required for normal ossification. Ehlers-Danlos syndrome Intracellular Procollagen → tropocollagen Extracellular Tropocollagen → crosslinks → collagen fibrils → fibers → bundle An inherited defect in the procollagen peptidase – triggered removal of the nonhelical endings of procollagen results in the formation of defective collagen fibrils. Another form of the syndrome involves a mutation of the gene encoding the enzyme lysyl hydroxylase, required for the post-translational modification of lysine into hydroxylysine. Lysyl oxidase stabilizes the staggered array of tropocollagen molecules by catalyzing the formation of aldol cross-links between hydroxylysine side chains. Defective hydroxylation of lysine decreases the strength of the collagen molecule in Ehlers-Danlos syndrome. This syndrome can be divided into several clinically distinct subtypes, most of them characterized by joint dislocation (hip and other large joints) and hyperelasticity of the skin. Quarter Horse stallion: Hyperelastosis cutis (HS), also known as hereditary equine regional dermal asthenia (HERDA), is a recessive disorder. There is lack of adhesion within the dermis (the deep layer of the skin) due to a collagen defect. In dramatic cases, the skin splits along the back and even rolls down the sides. The average lifespan for an HC horse is two to four years. Elastic fibers (do not contain collagen) Fibroblasts (in skin and tendons), chondroblasts, chondrocytes (in elastic cartilage of the auricle of the ear, epiglottis, larynx, and auditory tubes), and smooth muscle cells (in large blood vessels like the aorta and in the respiratory tree) produce elastic fibers Elastin contains two characteristic but uncommon amino acids: desmosine and isodesmosine. These amino acids are responsible for cross-linking mature elastic fibers and enable their stretching and recoil, like rubber bands. Proelastin, the precursor of elastin, is secreted as tropoelastin. In the extracellular space, tropoelastin interacts with fibrillin, a 35 kd glycoprotein, to organize immature elastic fibers, which aggregate to form ma
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