The Extracellular Matrix

The ECM is a complex fibrillar mesh comprised of proteins, proteoglycans, and glycosaminoglycans. The ECM provides the structural support for cells to organize into tissues, and its properties vary widely depending on the particular functional demands placed on it. For example, the fibrillar collagen network and elastic fibers of the lung provide the lung with its unique ability to expand and contract to allow for gas exchange, whereas the mineralized collagen within bone imparts the structural strength required to support the body. While these classic structural roles for the ECM have been appreciated for some time, more recent research has revealed a wide array of activities for the ECM. Indeed, the specific composition of the ECM can directly influence cells through matrix receptors on the cell surface for ECM components that when activated lead to intracellular signaling and altered metabolism and synthesis of the ECM itself. In this way, a sophisticated signaling loop can lead to the development, maintenance, and repair of complex tissues. In addition to these direct ECM-cell interactions, it has also become recognized that the ECM is the medium of communication between cells. Hence, all soluble messenger molecules are regulated to some degree as they encounter the ECM through a range of interactions between the soluble messenger molecules (i.e., growth factors) and the components of the ECM. Thus, the high density of high-affinity binding sites for growth factors within the ECM creates a situation where growth factors can be considered as "transient" components of the ECM. As such, the local phar-macodynamics and metabolism of growth factors can vary quite extensively at the surface of cells on the basis of the local composition of the ECM.

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