Cell cortex

The cell cortex is a specialized layer of cytoplasm on the inner face of the plasma membrane that functions as a mechanical support of the plasma membrane. In animal cells it is an actin-rich layer responsible for movements of the cell surface. In plant cells, the cell cortex is reinforced by cortical microtubules underlying the plasma membrane. The direction of these cortical microtubules determines which way the cell elongates when it grows.

A cell cortex is made primarily of spectrin proteins along with lesser amounts of actin microfilaments. Spectrin proteins and actin microfilaments are attached to transmembrane proteins by attachment proteins between them and the transmembrane proteins. The cell cortex is attached to the inner (cytosolic) face of the plasma membrane in cells where the spectrin proteins and actin microfilaments form a mesh-like structure much like a fishnet except that it can be broken and reformed. This breakage and reformation is referred to as "dynamic instability."