It is well known that

It is well known that specific ion fluxes are necessary for tissue regeneration, and that EMF with frequencies below 100Hz induce physiological effects as a result of ionic interactions (Funk and Monsees, 2006; Gartzke and Lange, 2002). Adams et al. reported that active up-regulation of a pump mechanism is specifically required during regeneration (Adams et al., 2007), in contrast to passive injury currents that result from trauma to polarized epithelia during limb regeneration in frogs and salamanders (Borgens, 1984). In general, regeneration is accompanied by a stimulation of endogenous currents; and the inhibition of endogenous currents specifically prevents regeneration (Becker, 2002; Levin, 2007). Due to this phenomenon, an exogenous application of fields, such as EMF, can induce a significant degree of regeneration in normally non-regenerating tissues (Becker, 2002; Nuccitelli, 2003). This holds important relevance to the regeneration of tissues in adult organisms. Yamada et al. showed that mild stimulation using EF strongly influences embryonic stem EZ Cap Reagent GG (ESCs) to assume a neuronal state (Yamada et al., 2007). They reported that induction of Ca influx is required for the formation of embryoid bodies from ES cells. Because Ca is one of the most important signaling ions, many downstream pathways may be involved, such as Ca involvement in the Wnt signaling pathway. Yamada further suggests that physical alteration of cell surface membranes may initiate signaling, even though innate signaling mechanisms take over later. The ion flux signals differentiation in early development through receptor-ligand signaling systems that have evolved to stabilize and refine environmental cues imposed on cells. Sun et al. reported that a DC 0.1V/cm stimulus (30min/day for 10days) applied with osteogenic induction factors, stimulated hBMSC differentiation into the osteogenic cell lineage by reducing the Ca wave frequency, which is typically found in the differentiation processes (Sun et al., 2007). These naturally occurring fluctuations in Ca or other metabolic or signaling waves, can be accessible to appropriate EMF impulses because cells recognize the Ca oscillations through sophisticated mechanisms that decode the information embedded in the Ca dynamics. For example, where rapid and localized changes of Ca (known as Ca spikes) occur, inter- and intra-cellular propagations known as Ca waves control slower responses (Sun et al., 2007). Here the frequency of the Ca oscillations reflects the extracellular stimulus of the EMF. Examples of this phenomenon are Ca-binding proteins such as troponin C in skeletal muscle cells and CaM in eukaryotic cells that serve as transducers of Ca signals by changing their activity as a function of the Ca oscillation frequency (Chawla, 2002). These frequency-modulated responses determine the qualitative and quantitative nature of genomic responses which can be translated into frequency-dependent cell responses such as differentiation (Dolmetsch et al., 1998). The parameters modulating hBMSC differentiation processes depend on the osteogenic markers of interest. As shown in Table 2, there is a trend for the 15Hz field to increase osteogenic differentiation in vitro with field strengths at 1mT. This was measured via increases of early osteogenic markers such as intracellular Ca, ALP, RUNX2, GAGs, COL2A1, BMP2, MMP1 and MMP3; however, application times vary greatly, anywhere from 30min/day for 21days to continuously for 14days. An in vitro comparison of 5, 25, 50, 75, 100 and 150Hz at 1.1mT for 30min/day for 21days, reported that 50Hz was most effective in differentiation of hBMSCs to osteoblasts via significant increase in ALP, OSTEOCALCIN, COLLAGEN, and Ca (Luo et al., 2012). While fewer investigations have been conducted to study the effect of EMF on chondrogenesis, 15Hz, 5mT significantly increased the chondrogenic markers GAGs and COLLAGEN II in a human cell model, compared with controls (Mayer-Wagner et al., 2011).