The extracellular matrix (ECM) is the network of proteins and associated molecules in tissues that serve a variety of roles in support of cells. ECM is most commonly found in fibrous form, with different protein components serving different functions. As a taste, the ECM provides mechanical support, cell adhesion sites for motility, physical connections between tissues, an anchor for various chemokines, and can act as a physical barrier, as in the basement membrane.
The following are examples of common ECM proteins: Collagen-large fiber that provides mechanical stability. It is also the most common protein in the human body.
Elastin-provides elasticity, which is essential in tissues that expand or contract. Hence, this protein is common in tissues such as blood vessels, the lungs, and any epithelial tissues.
Fibronectin-serves as an intermediary between cells and the large collagen supports. It serves as the cell's main point of contact to the ECM.
Laminin-forms a fibrous web or network instead of arrays of fibers. This is an important structural protein in that the web can act as a binder for other heterogeneous ECM constructs.
Cells adhere to the ECM via a collection of proteins called focal adhesions, the most important class of proteins being integrins. Integrins are heterodimers that bind to different peptide sequences on ECM (the most common is the RGD sequence). Based on the binding to these peptide sequences, the integrins can activate downstream signaling cascades that control a host of functions.
Applications/Connections to Soft Matter
ECM proteins are receiving much attention within tissue engineering, as this field strives to recreate physiological environments such that cells organize into tissues and organs. In striving to achieve more accurate environments, engineers have tweaked various properties of the ECM, from the number and types of binding sites, to the degree of crosslinking in the polymer scaffolds. There has even been significant work in taking other hydrogels, such as alginate, and combining them with other ECM proteins to make hybrid scaffolds.
Recent work has also focused on how cells exert traction forces on and remodel surrounding ECM to communicate. Instead of paracrine or synaptic signaling, cells could potentially sense the altered strain field in the matrix caused by the matrix remodeling.
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