Contrast the roles of collagen and proteoglycans in the extracellular space. How

Question

Contrast the roles of collagen and proteoglycans in the extracellular space. How do fibronectin and laminin contribute to embryonic development

Explanation / Answer

Collagens are, in most animals, the most abundant protein in the ECM. In fact, collagen is the most abundant protein in the human body[10][11] and accounts for 90% of bone matrix protein content.[12] Collagens are present in the ECM as fibrillar proteins and give structural support to resident cells. Collagen is exocytosed in precursor form (procollagen), which is then cleaved by procollagen proteases to allow extracellular assembly. Diseases such as osteogenesis imperfecta and epidermolysis bullosa are linked with genetic defects in collagen-encoding genes.[4] The collagen can be divided into several families according to the types of structure they form: >>>Fibrillar (Type I,II,III,V,XI) >>>Facit (Type IX,XII,XIV) >>>Short chain (Type VIII,X) >>>Basement membrane (Type IV) >>>Other (Type VI,VII, XIII) Proteoglycans GAGs are carbohydrate polymers and are usually attached to extracellular matrix proteins to form proteoglycans (hyaluronic acid is a notable exception, see below). Proteoglycans have a net negative charge that attracts positively charged sodium ions (Na+) which attracts water molecules via osmosis, keeping the ECM and resident cells hydrated. Proteoglycans may also help to trap and store growth factors within the ECM. Described below are the different types of proteoglycan found within the extracellular matrix. a) Heparan sulfate b) Chondroitin sulfate c) Keratan sulfate Cellular Fibronectin is an extracellular glycoprotein that exists in a soluble form in body fluids and in an insoluble form in the extracellular matrix. Its one of the primary cell adhesion molecules and plays a major role in many important physiological processes, such as embryogenesis, wound healing, hemostasis and thrombosis. It is secreted as a dimer with a monomer molecular weight of 220–250 kDa. Altered fibronectin expression, degradation, and organization has been associated with a number of pathologies, including cancer and fibrosis. It is synthesized by, and present around, fibroblasts, endothelial cells, chondrocytes, glial cells, and myocytes. It represents an active element in the process of T cell activation in the immune cascade triggered by organ transplantation. Different forms of fibronectin appear to be generated from tissue specific splicing of fibronectin mRNA, transcribed from a single gene. Multiple domains show binding affinities for collagen, fibrin, heparin, and specific cell membrane receptors. The most notable domain, Arg-Gly-Asp (RGD), is recognized by integrins and mediates cell adhesion. Fibronectin is involved in widespread interactions and functions, such as the attachment and migration of many cell types, cytoskeletal assembly, tyrosine phosphorylation, and metastasis.

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