Fig. 2.

Mammary gland tissue becomes increasingly stiffer during tumor progression. Each tissue has a particular ‘stiffness phenotype’ (stiffness measured in Pascals – Pa) and each cell type is finely tuned to the specific tissue in which it resides. For example, fat tissue is much softer than cartilage. Thus, a highly compliant matrix favors adipogenesis, whereas osteoblast differentiation is optimal on stiffer ECM. Similarly, normal mammary gland development is optimally supported by interaction of epithelial cells with a soft matrix. During tumor progression, breast tissue becomes increasingly stiffer and tumor cells become significantly more contractile and hyper-responsive to highly compliance signals. Although breast tumors are much stiffer than the normal mammary gland, the material properties of a breast tumor or any other physiological environment remain significantly softer than those of glass or plastic culture dishes.