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). A specialized form of extracellular matrix that underlies the basal side of polarized epithelial cell sheets to separate them from the underlying connective tissue is the basal lamina .
The key constituents found in the basal lamina are glycoproteins (i.e. laminin, collagen) and proteoglycans (i.e. perlecan), however, the precise composition varies from tissue to tissue and various other molecules (e.g. fibronectin) can also be found .
* not all adhesions may be present within the same cell at the same time
* the relative level of adhesion types may vary (e.g. during cell motility, differentiation)
* the presence (or relative level) of a particular adhesion may be cell type- or tissue-specific integrin receptors, allowing these structures to probe the stiffness of the environment around them and promote migration . Integrin molecules accumulate within filopodia to mediate the initial cell-matrix adhesions . In addition, basal adhesions to laminin anchor the filopodial base, which usually remains immobile despite considerable flexibility in the shaft .
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 . 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 .
The physical state of the extracellular matrix also influences the formation of fibrillar adhesions  in the same way as on focal adhesions. The translocation of pliant matrix components (such as fibronectin) and increased cellular tension from the actin cables to the translocating integrins and associated fibronectin molecules is suggested to initiate fibronectin fibrillogenesis and FB assembly .
Fibrillar adhesions are distinguished from FAs by the high levels of tensin and low levels or absence of phosphotyrosine [30, 22, 35]. They also lack attachment to stress fibres and do not diassemble when the force is relaxed . Integrin linked kinase (ILK) and the complex it forms with cytoskeleton adaptor proteins, PINCH1 and parvin, are thought to be essential for the transition from early focal adhesions to fibrillar adhesions . This IPP complex functions to reinforce α5β1-actin linkage and provide a platform for tensin and zyxin recruitment.
Controversial evidences exist for the role of FAK-Src signaling pathway in regulating fibrillar adhesions and fibrillogenesis. Some studies report that loss of Src family kinases or FAK activity increases tensin recruitment , while in others, similar mutants show reduced efficiency in assembling fibronectin into fibrils [38, 39].