extracellular matrix

Meaning: The ECM is the meshwork secreted by cells, such as fibroblasts, that fills the space around them. Its parts divide the work:

Collagen, a triple helix rich in glycine and proline, resists stretching (tensile strength). Elastin lets tissue stretch and recoil. Proteoglycans, proteins heavily decorated with glycosaminoglycans, trap water into a hydrated gel that resists compression. Fibronectin and laminin are adhesive glycoproteins that bind cells to the matrix.

Integrins are transmembrane receptors that bind these proteins outside the cell and connect to the cytoskeleton inside. The basal lamina is a thin ECM sheet, rich in type IV collagen and laminin, beneath epithelia.

On the MCAT: Passages ask which component explains a property (strength, elasticity, cushioning) or what happens when one is defective. Another common setup blocks or removes an integrin or fibronectin and asks about cell detachment, poor migration or altered growth. Questions may also contrast cell-cell junctions (cadherins, desmosomes) with cell-matrix attachment (integrins, hemidesmosomes). Matrix metalloproteases that degrade ECM appear in wound healing and tumor invasion.

Common confusion: The ECM is not just passive filler, and it is not the cytoskeleton. It is outside the cell, and integrins pass signals in both directions, so the matrix can influence cell shape, survival, division and movement.