Extracellular Matrix (ECM)
Structure
The ECM is a network of extracellular molecules which are secreted locally
to ensure cell and tissue cohesion. The ECM also serves as a reservoir
for extracellular signaling molecules that control cell growth,
migration, and differentiation. The major classes of ECM molecules are
proteoglycans, collagens and multi-adhesive matrix proteins (e.g.
laminin, fibronectin). In mammals, the ECM is commonly known as
“connective tissue”. ECM components are linked to each other through diverse protein and carbohydrate-binding domains. For stability in tissues, cells are linked to the ECM through cell adhesion receptors (e.g. integrins).
Function
Cell binding to ECM components also mediates the assembly of cell-matrix adhesions during cell migration. For example; filopodia attach to ECM components through integrin receptors, allowing these structures to probe the stiffness of the environment around them and promote migration [1]. Integrin molecules accumulate within filopodia to mediate the initial cell-matrix adhesions [2]. In addition, basal adhesions to laminin anchor the filopodial base, which usually remains immobile despite considerable flexibility in the shaft [3].Tension that is generated between the cytoskeletal network (via the action of contractile stress fibers), linked ECM and focal adhesions controls the cells ability to migrate and protrude filopodia. Cell-matrix adhesion therefore functions as a molecular ‘clutch’ to convert intracellular cytoskeletal assembly into protrusion and movement (see Figures under “Adherence” in filopodia) [4]. Cells also interact with and modify ECM components mechanically, as well as chemically, to alter their alignment and composition in ways that influence cell fate, movement, polarization, and shape. E.g. through the secretion of matricellular proteins that alter ECM composition, which in turns affects cell morphology [5].
