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Journal Club Theme of July 1 2008: Mechanics in Neuronal Development
The biological world is part of the physical world around us and obeys its laws. In particular, physical interactions can be as important in determining tissue or cell fate as biochemical stimuli, and they may also contribute to pathological conditions. The application of cell biomechanics contributes to an understanding of many processes that take place in our body such as cell movement, cell division, phagocytosis, and cellular contractility. Therefore, biomechanical investigations contribute to our understanding of the normal functioning of living organisms, help to predict changes which arise due to alterations of their environment, and maybe also to propose methods of artificial intervention.
Most biomechanical studies have so far focused on systems where mechanics obviously plays an important role, such as the locomotor system, the cardiovascular system, and the lung. However, even if not that obvious, biomechanical aspects may also play an important role in the central nervous system (CNS).
Our nervous system consists of two basic cell types, neurons and glial cells. Neurons, which transmit and process information, extend cell processes, typically several dendrites and one axon. The cues determining which neuronal process becomes the axon have long been unclear. In a groundbreaking study from almost 25 years ago Dennis Bray showed that axonal growth can be initiated de novo by the application of mechanical tension, a process he termed “towed growth”:
How tension may influence tissue formation in the CNS is described in the second paper:
In the first part of this very interesting article the author briefly reviews experimental data on passive and active mechanical properties of neurons, mainly found in the lab of Steve Heidemann, and subsequently develops a theory that explains the morphogenesis of the CNS, e.g., the specific folding of the brain or the development of the retinal fovea, by tension and hydrostatic pressure.
The third paper in this Journal Club presents another key study in the field of neuromechanics. The group of Paul Janmey could show that not only active tension but also passive material properties of the cells’ environment may influence their growth and function:
Georges, P. C., W. J. Miller, D. F. Meaney, E. S. Sawyer, and P. A. Janmey. (2006). Matrices with compliance comparable to that of brain tissue select neuronal over glial growth in mixed cortical cultures. Biophys J 90:3012-3018.
The authors show that neurons are capable of sensing the compliance of their environment in vitro and prefer relatively compliant substrates. Interestingly, glial cells in vitro grow better on stiffer materials. Thus, the cells in the CNS seem to be able to feel and to respond to the mechanical properties of their environment.
These data point towards an important contribution of mechanics to the development of the CNS and also to certain pathological processes.
Future research promises to reveal increasingly interesting facts about the influence of mechanics on CNS physiology and pathology, and it seems very likely that this growing knowledge may be exploited for example in the design of new neural implants and in the treatment of injuries to the fragile nervous tissue.