One of the primary challenges when designing compression chambers for chronic compression studies is the necessity to provide adequate gas exchange for the cells during pressurization. As the study of mechanotransduction in vivo is very complex, various in-vitro models have been used to study tissues that experience physiological mechanical loading (Blackman et al., 2000 Halka et al., 2008 Matsuzaki et al., 2005 Reilly et al., 2003), as well as for studies of mechanically-induced injury (Bottlang et al., 2007 Gidday et al., 1999 LaPlaca et al., 2005 Pfister et al., 2004 Yang et al., 2004), and to facilitate the engineering of load-bearing tissues (Hasel et al., 2002 Park et al., 2008 Takai et al., 2004). H ydrostatic compression is an important mechanical stimulus that directs cellular activity in several biological tissues including bone (Zhang et al., 1998), intervertebral disc (Setton and Chen, 2006), articular cartilage (Lammi et al., 2004), and the vascular system (Muller et al., 2004). In addition, we find that myelinated Schwann cells proliferate in response to applied hydrostatic compression. Cell membrane integrity assay results show that co-cultures respond differently to hydrostatic pressure, depending on the magnitude and duration of stimulation. Finally, we use the system to model chronic nerve compression injuries, such as carpal tunnel syndrome and spinal nerve root stenosis, with myelinated neuron-Schwann cell co-cultures. Monitoring dissolved oxygen continuously during pressurization, we find that the ensuing response exhibits characteristics of a second- or higher-order system which can be mathematically modeled using a second-order differential equation.
To better isolate the cellular response to long-term compression, we created a model that features continuous gas flow through the chamber during pressurization, and a negative feedback control system to rigorously control dissolved oxygen levels. However, these systems are limited in the magnitude of pressure that can be applied and may require frequent media changes, thereby eliminating critical autocrine and paracrine signaling factors. To address this, several sophisticated compression chamber designs have been developed. Applying compression in vitro may alter the equilibrium of the system and thereby disrupt the gas exchange kinetics. While a variety of in-vitro models have been employed to investigate the response of load-bearing tissues to hydrostatic pressure, long-term studies are limited by the need to provide for adequate gas exchange during pressurization.
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