Significant efforts have been directed to understanding the factors that influence the lineage commitment of stem cells. This paper demonstrates that cell shape, independent of soluble factors, has a strong influence on the differentiation of human mesenchymal stem cells (MSCs) from bone marrow. When exposed to competing soluble differentiation signals, cells cultured in rectangles with increasing aspect ratio and in shapes with pentagonal symmetry but with different subcellular curvature-and with each occupying the same area-display different adipogenesis and osteogenesis profiles. The results reveal that geometric features that increase actomyosin contractility promote osteogenesis and are consistent with in vivo characteristics of the microenvironment of the differentiated cells. Cytoskeletal-disrupting pharmacological agents modulate shape-based trends in lineage commitment verifying the critical role of focal adhesion and myosin-generated contractility during differentiation. Microarray analysis and pathway inhibition studies suggest that contractile cells promote osteogenesis by enhancing c-Jun N-terminal kinase (JNK) and extracellular related kinase (ERK1/2) activation in conjunction with elevated winglesstype (Wnt) signaling. Taken together, this work points to the role that geometric shape cues can play in orchestrating the mechanochemical signals and paracrine/autocrine factors that can direct MSCs to appropriate fates. were initially isolated from bone marrow and noted for their ability to differentiate into bone cells (osteoblasts), cartilage cells (chondrocytes), and fat cells (adipocytes) (1, 2). As an autologous source of stem cells, MSCs are under considerable scrutiny for regenerative therapies (3). A combination of physical, chemical, and biological cues present in the stem cell microenvironment have been implicated as directors of stem cell fate in vivo (4). This concept of a stem cell "niche" has motivated empirical studies to identify optimal combinations of extracellular matrix, culture conditions, and temporally administered growth factors to direct stem cell fate in the laboratory (5-7). However, the physical forces and geometry of the MSC microenvironment remain poorly defined and have begun to emerge as critical parameters for regulating cell fate (8, 9).Physical cues, which were postulated as important factors in tissue development over a century ago (10), have also been recognized as important factors in controlling cell function. Research along these lines has been driven in the past decades with the maturation of microengineering techniques (11-13). Ingber, Whitesides, and colleagues demonstrated the use of substrates that were geometrically patterned to control the microenvironment of individual cells and in turn the decision of cells to initiate apoptosis (14). Patterned substrates have also been used to study cytoskeletal dynamics (15-18) and motility (19-22) with singlecell resolution. Chen and coworkers recently demonstrated the important role that cell shape and size can play ...

In this article, we report the functionalization of alkyne-terminated alkyl monolayers on Si(100) using "click" chemistry, specifically, the Cu(I)-catalyzed Huisgen 1,3-dipolar cycloaddition reaction of azides with surface-bound alkynes. Covalently immobilized, structurally well-defined acetylene-terminated organic monolayers were prepared from a commercially available terminal diyne species using a one-step hydrosilylation procedure. Subsequent derivatization of the alkyne-terminated monolayers in aqueous environments with representative azide species via a selective, reliable, robust cycloaddition process afforded disubstituted surface-bound [1,2,3]-triazole species. Neither activation procedures nor protection/deprotection steps were required, as is the case with more established grafting approaches for silicon surfaces. Detailed characterization using X-ray photoelectron spectroscopy and X-ray reflectometry demonstrated that the surface acetylenes had reacted in moderate to high yield to give surfaces exposing alkyl chains, oligoether anti-fouling moieties, and functionalized aromatic structures. These results demonstrate that click immobilization offers a versatile, experimentally simple, chemically unambiguous modular approach to producing modified silicon surfaces with organic functionality for applications as diverse as biosensors and molecular electronics.

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