Mechanical Changes of MSCs and Fibroblasts studies, soluble factors [32], cell shape [32], and extracellular matrix rigidity [31] regulated the lineage commitment of MSCs through RhoA signaling pathways. This study sought to understand the underlying mechanical differences in MSCs and fibroblasts as they migrate toward tumors. In order to best simulate the signals migrating cells would receive in vivo, we utilized 4T1 breast tumor cell conditioned media (TCM) to stimulate the cells in vitro. We found that after treatment with TCM, MSCs underwent an exaggerated response as compared to fibroblasts, and that this response could potentially be explained by altered gene expression of Rho GTPases Cdc42 and RhoA. We further demonstrate that even one hour incubation with TCM increases cell motility, indicating a novel ‘mechanical priming’ achieved by short-term TCM preconditioning. Results Characterization of Bone Marrow Derived MSCs To verify the phenotype of MSCs, cells were assayed for both cell surface marker expression and differentiation capacity. After purification by adherence to plastic, MSCs were negative for myeloid and hematopoietic stem cell markers CD11b and CD45 and positive for Sca1, CD29, CD61, and VCAM1 (Fig. 1A, Left). Fibroblast expression is provided for comparison (Fig. 1A, Right). Figure 1. Characterization of bone marrow isolated MSCs and fibroblasts. Phenotypic analysis was performed by flow cytometry was performed on adherent bone marrow cells and Swiss 3T3 fibroblasts with positive populations in red given with S.E.M.(A). Purified MSCs differentiated into adipocytes (B) and osteoblasts (C) within 3 weeks in lineage-specific differentiation media as shown both my staining and RT-PCR (scale bar = 100 mm). doi:10.1371/journal.pone.0033248.g001 PLoS ONE | www.plosone.org 2 March 2012 | Volume 7 | Issue 3 | e33248