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EDITORIAL
In the April issue, in a mini-review entitled "Cell type-specific response to growth on soft materials," Drs. P. Georges and P. Janmey present an examination of various techniques used to study the influence of extracellular mechanics on individual cells, as well as cell type specificity of this response. The test systems these authors discuss include protein-based extracellular matrix gels such as fibrin and collagen, as well as synthetic substrates, particularly polyacrylamide, used to create two- or three-dimensional cell culture substrates of controlled stiffness. Living cells are able to evaluate the matrix mechanical properties in their environment and respond to these signals. However, not all cell types react in a similar fashion to changes in the stiffness of the extracellular matrix. In fact, responses between cell types may differ even within the same tissue system. These authors examine the specificity of cellular responses to matrix compliance in endothelial, liver, and central nervous system-derived cell types.
In a mini-review entitled "Force sensing and generation in cell phases: analyses of complex functions," Dr. H.-G. Döbereiner and colleagues discuss the interactions of motility, force sensing, and force generation that drive cellular morphology and propose a new phase-based model of cell motility. Evident in several different contexts, spreading fibroblasts exhibit distinct motile phases that are punctuated by sharp transitions. Such phases are inherent to cellular force sensing machinery, which is thought to be controlled by local activation of distinct protein modules.
In the May issue, in a mini-review entitled "Mechanical signal transduction in skeletal muscle growth adaptation," Dr. J. Tidball explores how the insights gained from learning how muscle mass and phenotype respond to exogenous, chemical signals have provided a framework for learning how the mechanical environment induces similar adaptive responses. Particularly, over the past decade, growing knowledge of the mechanisms through which IGF-I modulates muscle adaptation has provided insight as to how mechanical perturbations influence muscle adaptation through similar and overlapping pathways. Most recently, studies have shown that mechanical activation of the signaling pathways that influence muscle adaptation occurs independently of exogenous chemical signals, suggesting that mechanical signals act as a first messenger in these pathways.
Discussions of the mechanical properties of the lung parenchyma have been featured in the Journal since the 1950s. Advances in cellular and molecular biology shifted attention somewhat away from classical air and tissue mechanics; however, such advances have also been influential in revealing that mechanical properties of the extracellular matrix are critical to all cellular functions and, in turn, induce a variety of effects on matrix composition and properties. In a mini-review entitled "Biomechanics of the lung parenchyma: critical roles of collagen and mechanical forces," Dr. B. Suki and colleagues discuss the classical mechanical properties of lung tissue within this new framework. These authors state that to fully appreciate normal function and understand the spatial development of diseases, it is essential to uncover the relationships among cell signaling, matrix composition, and biomechanical properties of tissue components, particularly those of collagen.
In the June issue, in a mini-review entitled "Mechanical, biochemical and extracellular matrix effects on vascular smooth muscle cell phenotype," Dr. J. Stegemann and colleagues focus on how mechanical and biochemical stimulation, as well as cell-matrix interactions, affect function and phenotype of vascular smooth muscle cells. Recent studies applying mechanical, biochemical, and extracellular matrix stimulation have illustrated that the combination of these factors induce phenotypic changes. These authors emphasize in vitro experimental studies that combine multiple stimuli, especially in three-dimensional culture systems, and discuss the implications of such culture systems on vascular tissue engineering aimed at designing and creating living vascular tissue substitutes.
Although our understanding of the molecular basis of mechanotransduction has been much advanced in recent years, the fundamental molecular processes by which cells detect and transduce mechanical signals are still not established. Studies of bone and skeletal muscle have provided a framework for understanding how mechanical loads influence the functional properties of tissues, and insights into the cell biology of the mechanotransduction process and the molecular responses of cells to mechanical stimuli have allowed us to better appreciate the physiological effects of mechanical loads on tissues, whether under normal or abnormal conditions. Realizing the effects of external mechanical stimuli on cellular phenotype, cell matrix expression, and tissue structure has enabled the advent of synthetic tissue engineering, which, in turn, provides invaluable insights into the mechanisms of normal tissue development, as well as healing and tissue remodeling under pathophysiological conditions.
The Associate Editors and I hope that this Highlighted Topics series will serve to welcome the future publication of work in this ever-expanding area of research in the Journal of Applied Physiology.
Journal of Applied Physiology
April 2005, Volume 98
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