Cells make a number of key decisions by actively adhering to a substrate and applying forces. Naive mesenchymal stem cells (MSCs) from human bone marrow will be shown to specify lineage and commit to phenotypes on collagen-coated hydrogels with tissue-level elasticity. Soft matrices that mimic brain appear neurogenic, stiffer matrices that mimic muscle are myogenic, and comparatively rigid matrices that mimic collagenous bone prove osteogenic. Inhibition of myosin blocks all elasticity directed lineage specification – without strongly perturbing many other aspects of cell function and shape. Physical studies of nuclei suggest an unusual degree of plasticity for stem cell nuclei, and a "Cysteine Shotgun" Mass Spectrometry method for in situ labeling of the 'foldome' reveals distinct structural differences attributable to unfolding and/or dissociation of cellular proteins. The results have significant implications for understanding physical effects of the in vivo microenvironment and also – as will be shown – for therapeutic uses of stem cells such as in muscle repair. A. Engler, S. Sen, H.L. Sweeney, and D.E. Discher. Matrix elasticity directs stem cell lineage specification. Cell 126: 677-689 (2006). J.D. Pajerowski, K.N. Dahl, F.L. Zhong, P.J. Sammak, and D.E. Discher. Physical plasticity of the nucleus in stem cell differentiation. PNAS (Proceedings of the National Academy of Science – USA) 104: 15619-15624 (2007). C.P. Johnson, H-Y. Tang, C. Carag, D.W. Speicher, and D.E. Discher. Forced unfolding of proteins within cells. Science 317: 663-666 (2007). A. Engler, C. Carag, C. Johnson, M. Raab, H-Y. Tang, D. Speicher, J. Sanger, J. Sanger, and D.E. Discher. Embryonic cardiomyocytes beat best on a matrix with heart-like elasticity: scar-like rigidity inhibits beating. Journal of Cell Science 121: 3794-3802 (2008). D.E. Discher, D.M. Mooney, P. Zandstra. Growth factors, matrices, and forces combine and control stem cells. Science 324: 1673-1677 (2009).