Prerequisites for tumor cell migration on a 2D substrate and tumor cell invasion in a 3D extracellular matrix. (a) Scanning electron microscopic image of an MDA-MB-231 breast carcinoma cell on a planar glass and (b) transmission electron microscopic image of an MDA-MB-231 cell invaded in 3D collagen gel matrix. (a) For proper 2D motility, a cell needs to adhere/deadhere fast, remodel its cytoskeleton fast, and possess a high CSK-fluidity. (d) For proper 3D motility, a cell needs, in addition to the 2D motility properties, to generate high contractile forces, and the cells needs to overcome the viscous drag of the surrounding extracellular matrix to move forward.