cell behavior is regulated by numerous distinct cues that impinge on them in vivo . alongside chemical gradients geard and willadsen , 2009 ; niehrs , 2010 ; ben - zvi et  al . , 2011 ; gershenson , 2012 ) and physical forces ( beloussov and grabovsky , 2006 ; beloussov , 2008 ; nelson , 2009 ; von dassow and davidson , 2011 ; davidson , 2012 ) , cell activity is orchestrated toward the creation and repair of high - order anatomical structures by a set of bioelectrical cues ( levin , 2012a , b ; levin and stevenson , 2012 ) . here bioelectricity refers to endogenous electrical signaling via ion channels and pumps at the plasma membrane ; specifically excluded due to length constraints is the rich literature on external electromagnetic fields ( funk et  al . , 2009 ; cifra et  al . , 2011 ; hronik - tupaj and kaplan , 2012 ) , ultraweak photon emission ( farhadi et  al . , 2007 ; fels , 2009 ; sun et  al . , 2010 ; beloussov , 2011 ) , and subcellular organelle potentials ( bustamante et  al . , 1995 ; the importance of bioelectricity for cells beyond excitable nerve and muscle was realized long ago , and solid functional data implicate steady ion currents in embryogenesis and wound healing ( burr and northrop , 1935 ; lund , 1947 ; jaffe and nuccitelli , 1977 ; nuccitelli et  al . , 1986 ; borgens et  al . , 1989 ; hotary and robinson , 1992 ) . by tracking developmental currents and applying physiological - strength electric fields , it was shown that transepithelial electric fields regulate cell migration , orientation , and nerve growth ( jaffe and poo , 1979 ; patel and poo , 1982 ; borgens et  al . , 2005 ; nishiyama et  al . , 2008 ; cao et  al . , 2011 , 2013 ; ozkucur et  al . , 2011 ; pullar , 2011 ; reid et  al . , 2011b ; vieira et  al . , 2011 ; pan and borgens , 2012 ; zhao et  al . , 2012 ; yamashita , 2013 ) . however , recent advances and development of molecular - level techniques ( adams , 2008 ; adams and levin , 2013 ; levin , 2013 ; tseng and levin , 2013 ) have identified a new aspect of bioelectricity that regulates individual cell function and helps coordinate the embryogenesis and regenerative repair of complex structures . this review focuses on the instructive cues mediated by spatiotemporal patterns of voltage potentials across the membranes ( vmem ; figure 1a ) of nonneural cells and the roles these play in coordinating cell behavior during regeneration , development , and cancer . bioelectrical signaling at the cell and organism levels , at the level of single cells , bioelectrical signals are produced by ion channel proteins , transduced into second - messenger responses , and alter key aspects of cell behavior . ( a ) the voltage potential ( vmem ) at the cell membrane is produced by the movement of ions through across a cell membrane . ions move via many different ion channels and pumps , under the control of concentration and electric gradients . ( b ) change of vmem is transduced into cellular effector cascades by a range of mechanisms , including voltage - sensitive phosphatases , voltage - gated calcium channels , and voltage - sensitive transporters of signaling molecules such as serotonin and butyrate . ( diagram modified , with permission , from figure 1b of levin , 2007 . ) ( c ) bioelectrical signals feed into epigenetic and transcriptional cascades and thus trigger changes in cell properties such as proliferation , differentiation , migration , shape change , and programmed cell death . ( d ) voltage reporter dye reveals gradients of vmem across the anterior - posterior axis of planarian flatworms . ( taken , with permission , from figure 2b of beane et  al . , 2013 . ) ( e ) in amputated worms , a circuit composed of proton and potassium conductances sets the voltage states at each blastema , which in turn determines the anatomical identity of each end of a regenerating fragment . ( diagram taken , with permission , from figure 7c of beane et  al . , 2011 . ) ( f ) manipulating this circuit in amputated planaria using pharmacological or genetic techniques that target ion flux allows the programming of stem cell  mediated morphogenesis to specific anatomical outcomes , such as the creation of two - head animals shown here . in general , terminally differentiated , quiescent cells tend to be strongly polarized ( bearing a more - negative resting potential ) , whereas embryonic , stem , and tumor cells tend to be depolarized ( closer to zero ; binggeli and weinstein , 1986 ) . the picture is complicated by two still poorly understood factors : the relationship of overall vmem state to the cell cycle  dependent ( sinusoidally varying ) changes in voltage potential ( arcangeli et  al . , 1995 ; higashimori and sontheimer , 2007 ; aprea and calegari , 2012 ) and the fact that many cells in fact do not have a single vmem but bear a set of distinct voltage domains over their surface ( o'connell and tamkun , 2005 ; o'connell et  al . , 2006 ; levin , 2012a ) . crucially , vmem is not simply a readout but is also a functional determinant of cell behavior , such as proliferative state and plasticity ( table 1 ) , due to a number of mechanisms that functionally couple voltage potential changes to downstream cascades ( figure 1 , b and c ) . these data derive from genetic experiments , as well as pharmacological screens designed to identify compounds that regulate stem cell differentiation or cancer progression ( alves et  al . , 2011 ; sun et  al . , 2013 ) differentiation and proliferation are controlled by changes in vmem , as shown in human mesenchymal stem cells ( sundelacruz et  al . , 2008 , 2013 ; cardiomyocytes ( lan et  al . , 2014 ) , inhibitory postsynaptic currents ( jiang et  al . , 2009 ) , vascular muscle ( jia et  al . , 2013 ) , embryonic stem cells ( ng et  al . , 2010 ; du et  al . , 2013 ) , myoblasts ( in which hyperpolarization driven by the kir2.1 channel plays a key role ; hinard et  al . , 2008 ; li et  al . , 2010 ) , the specification of neurotransmitter types ( root et  al . , 2008 ) , and the control of precursor differentiation ( van vliet et  al . , 2010 ; yasuda and adams , 2010 ; lange et  al . , 2011 ; liebau et  al . , 2011 ; ring et  al . , 2012 ; given the known roles of vmem in regulating normal migration , differentiation , and proliferation ( aprea and calegari , 2012 ; ding et  al . , 2012 ; zhang et  al . , 2012 ; cao et  al . , 2013 ; yamashita , 2013 ) , it is not surprising that control of ion flux ( park et  al . , 2008 ; house et  al . , 2010 ) and membrane voltage ( morokuma et  al . , 2008a ; blackiston et  al . , 2011 ; chernet and levin , 2013a , 2013b ; yang and brackenbury , 2013 ) are also increasingly implicated in the cell dysregulation of cancer ( table 2 ) . the differential activation of voltage - responsive transduction mechanisms on opposite sides of a cell allows bioelectric signals to regulate cell polarity . this was long ago shown in the symmetry breaking and control of outgrowth point in the algae fucus ( jaffe , 1966 , 1968 ) and has been recently shown using high - resolution imaging and genetic techniques in yeast ( minc and chang , 2010 ) and pollen tubes ( certal et  al . , 2008 ; michard et  al . , the cytoskeleton is one target of such signaling ( chifflet et  al . , 2003 ; , 2006 ; sekulic et  al . , 2011 ; campetelli et  al . , positional information can likewise be dictated by voltage properties of cells ( baglioni et  al . , 2012 ) and their neighbors ( shi and borgens , 1995 ) . studies of embryonic left right patterning of the xenopus embryo have revealed how bioelectrical processes link individual cell dynamics to axial patterning of the entire body plan ( levin and palmer , 2007 ; aw and levin , 2009 ) : cytoskeletal chirality within the fertilized egg drives asymmetric distribution of ion transporter proteins in the early blastomeres , and the resulting gradient drives unidirectional ( preneural ) serotonin flow through cell fields , eventually triggering differential gene expression on the left versus right sides of the body ( levin , 2006 ; levin et  al . , 2006 ; aw et  al . , 2008 ; lobikin et  al . , 2012b ; vandenberg et  al . , 2012 , 2013 ) . the dissection and synthesis of such systems at the genetic and physiological levels is beginning to reveal the properties of biophysical pathways by which individual cell polarity is integrated into large - scale patterning outcomes ( marshall , 2011 ) . the first step in analyzing a bioelectric signal is the characterization of the spatiotemporal distributions of ionic parameters and a determination of how they correlate with patterning events . vmem in cells can be quantified using several approaches ; unlike mrna and protein levels revealed by sequencing or immunohistochemistry , bioelectric properties are only ascertainable in vivo and can not be analyzed in fixed tissue . voltage gradients can now be visualized continuously in situ using fluorescent reporters of transmembrane potential ( adams and levin , 2012a , b ; figure 1d ) and more exotic nanoscale materials ( tyner et  al . , 2007 ) suitable for use in any optically accessible tissue ( steinberg et  al . , 2007 ; yun et  al these are a significant improvement on physiological impalement of single cells : far less invasive , and able to report multiple vmem values across tissues and even within cell membrane subdomains ( lechleiter et  al . , 1991 ; adams and levin , 2013 ) . reagents include cell - permeant dyes such as cc2-dmpe and disbac2(3 ) ( adams et  al . , 2006 ; adams and levin , 2012b ; oviedo et  al . , 2008 ; ozkucur et  al . , 2010 ) and genetically encoded protein reporters ( tsutsui et  al . , 2008 ; additional tools for the characterization of bioelectrical events include highly sensitive ion - selective extracellular electrode probes ( reid et  al . , 2007 ; smith et  al . , 2007 ) that reveal ion flux , microelectrode arrays ( aryasomayajula et  al . , 2010 ; schonecker et  al . , 2014 ) , and reporters of individual ion species such as protons ( tantama et  al . , 2011 ) and sodium ( tseng et  al . , 2010 ; dubach et  al . significant opportunities exist for the development of specific , bright , ratiometric dyes that localize exclusively to the desired subcellular locale ( e.g. , plasma membrane or nucleus ) . especially exciting will be the use of multiple physiological dyes in fluorescence - activated cell sorting experiments to identify subpopulations of  pure  stem and other cell types that differ in key bioelectric properties ( mello de queiroz et  al . , 2008 ) , as has been observed for human endothelial cells ( yu et  al . , 2002 ) . of importance , such experiments on dissociated cells will clearly highlight properties that are cell autonomous versus those physiological conditions that can only be maintained within a group context . the mechanistic investigation of bioelectric cues and their interactions with canonical biochemical pathways has been enriched by several new functional techniques ( adams and levin , 2006b , 2013 ; reid et  al . the comprehensive workflow for probing developmental bioelectricity can be illustrated by two examples . in the first , a tiered pharmacological screen ( adams and levin , 2006a ) implicated a proton pump and two channels as specifically required for tail regeneration but not for wound healing or development of the primary tail ( adams et  al . , 2007 ) . these loss - of - function data were confirmed using reagents with molecular specificity by misexpression of a dominant - negative form of a v - atpase subunit protein . marker analysis was used to show why tails failed to regenerate in v - atpase  inhibited tails ( loss of regeneration - specific gene up - regulation , lack of the obligate increase of mitosis near the wound , and abrogation of innervation into the regenerate ) . fluorescent dye imaging provided physiomic profiling of the changes of vmem during the stages of regeneration and confirmed that the unique voltage changes characteristic of the regenerating state were blocked by v - atpase inhibition and were absent during stages at which tadpoles normally are not competent to regenerate their tails . on the basis of these findings , to develop a gain - of - function application , a yeast p - type proton pump was misexpressed in regeneration - incompetent animals , leading to restoration of mitosis , gene expression ( msx-1 , notch ) , innervation , and morphological regeneration of a complete tail . additional rescue experiments using net - electroneutral proton exchangers allowed the independent testing of ph versus voltage signaling . one key result was that the anatomical outcome ( regeneration rescue ) can be induced by a completely heterologous hyperpolarizing pump , which has no sequence or structural homology to the native xenopus protein endogenously driving regeneration . this demonstrated that the necessary and sufficient trigger for regeneration is not a specific gene product ( v - atpase ) , but a bioelectrical state , which can be implemented using a variety of different reagents . this finding facilitated development of a purely pharmacological method of modulating ion flows in the wound to induce tail ( tseng et  al . , 2010 ) and leg ( tseng and levin , 2013 ) regeneration without the need for gene therapy . the available tools enable a multistep strategy that combines pharmacological screening , physiological imaging , and molecular - genetic tools to generate loss- and gain - of - function data showing how a bioelectric pathway normally works and how it can be exploited to trigger pattern formation . a similar approach was taken with an initial gain - of - function screen , misexpressing ion channels in frog embryogenesis . one of the outcomes was the finding that a specific vmem range was necessary and sufficient to trigger ectopic eye development ( pai et  al . , 2012 ) . dye imaging data showed that the location of the endogenous eyes is demarcated by a prepattern of vmem states in the anterior neurectoderm and that experimental alteration of this prepattern results in abnormal craniofacial gene expression and eye and facial malformations ( vandenberg , 2011 ; pai et  al . , 2012 ) . to complement the data showing that bioelectric states are an endogenous component of eye development , it was then shown that driving eye - specific vmem states in other body regions ( by misexpression of ion channels ) was sufficient to induce anatomically complete ( well - formed ) ectopic eyes ( figure 2a ) . marker analysis revealed that this occurs via establishment of a positive feedback loop between hyperpolarization and rx1/pax6 expression , whereas a suppression screen of transduction mechanisms implicated voltage - gated calcium signaling as the transduction mechanism . eye genes such as pax6 do not produce eyes outside the head in vertebrates ( chow et  al . moreover , as with the tail , individual cell types appropriate to the eye did not have to be specified . together these data revealed the unique properties of bioelectric triggers to reprogram body regions at the level of organ identity and overcome lineage specification limits observed with biochemical inducers . ( a ) targeted vmem change , via misexpression of ion channels in the frog embryo , induces the formation of ectopic structures such as complete eyes , even in regions normally not competent to form eyes ( such as on the gut ) . ( used , with permission , from figure 3 g of pai et  al . , ( b ) tracking the ion channel expression using a lineage marker reveals that the effect is not cell - autonomous : in a lens created in the tail of a tadpole by ion channel expression , only about half of the ectopic cells express the heterologous ion channel ( revealed by blue lacz staining ) ; the other half of the induced structure consists of host cells recruited to participate in making the appropriate shape but not themselves targeted by the vmem - altering reagent . ( c ) melanocytes seen in a cross section of a xenopus tadpole are normally few in number , round , and confined to their normal locations . ( d ) depolarization induced by ion channel modulation induces these cells to overproliferate , acquire an elongated shape , and invade many organs ( red arrow ) . of importance , this effect is also not cell autonomous , as seen in the melanocyte phenotype , which results when cells ( marked by ion channel expression construct lineage label in blue ) are depolarized at a considerable distance from the melanocytes . ( taken , with permission , from figure 6a of chernet and levin , 2013b . ) ( e ) a normal planarian has a head and tail and regenerates each at the appropriate end of an amputated fragment . when it is cut into thirds and the middle fragment is briefly exposed to octanol , which temporarily blocks long - range bioelectrical signaling between the wound and mature tissues , a two - headed worm results ( f ) . remarkably , upon further rounds of cutting in plain water ( long after the octanol has left the tissues , as confirmed by hplc ) , the two - headed form results ( h , i ; images of two - headed worms provided by fallon durant , tufts university , medford , ma ) . this change in the animal 's target morphology ( the shape to which it regenerates upon damage ) appears to be permanent and persists across the animal 's normal reproductive mode ( fissioning ) , despite the fact that the genomic sequence has not been altered . chromatin modifications alone do not explain this , because the posterior wound cells , which could have been epigenetically reprogrammed to a head fate , are discarded at each cut : the information encoding a bipolar two - head animal is present even in the normal gut fragment  it is distributed throughout the body . we propose that this information is a kind of memory , encoded in electrical networks of somatic cells coupled by gap junctions , and is stored at the level of bioelectrical dynamics . ( e  i taken , with permission , from figure 2 of levin , 2014 ; photographs of planaria taken by taisaku nogi , children 's health research institute , canada , and fallon durant . ) of interest , many forward genetic approaches have identified ion channel genes responsible for patterning phenotypes , as have unbiased transcriptional network analyses in development ( langlois and martyniuk , 2013 ) and cancer ( house et  al . , 2010 ) . these include patterning of the face , limb , brain , and viscera in a range of model systems and a number of channelopathies that form an important class of human birth defects ( table 3 ) . thus upstream of endogenous bioelectrical signaling lie a set of ion channel and pump proteins that establish resting potential and alter it in response to physiological , transcriptional , and mechanical signals . such data often come from studies that , unlike the previously discussed two examples , did not set out to investigate bioelectricity , and the overall structure of developmental bioelectric signaling is starting to emerge from the synthesis of bioelectric projects investigating molecular mechanisms and molecular biology efforts that implicate ion channel activity in instructive roles . downstream of voltage change lie two types of endpoints  at the mrna and chromatin modification levels . transcriptional responses to depolarization include genes such as notch , bmp , sox10 , nurr1 , slug , fos , jun , npy , and wnt ( bartel et  al . , 1989 ; higuchi et  al . , 1990 ; raya et  al . , 2004 ; morokuma et  al . , 2008a ; he et  al . , 2011 ; lange et  al . , 2011 ; tseng et  al . , 2011 ; dahal et  al . , 2012 ; swapna and borodinsky , 2012 ; adams et  al . , epigenetic responses are triggered by movement of butyrate through an ion - dependent transporter , slc5a8 ; butyrate is an hdac1 inhibitor , and this allows voltage change to regulate chromatin acetylation ( davie , 2003 ; tong et  al . , 2004 ; this is believed to mediate control of tumorigenesis by depolarization and is also implicated in bioelectrical signaling during tail regeneration in xenopus ( tseng et  al . , 2011 ; chernet and levin , 2013a , 2014 ) . a set of transduction mechanisms has been identified by which changes of resting potential affect events at the nucleus ( figure 1 , b and c , and table 4 ) . one involves voltage - gated calcium channels , which convert voltage change into signaling via this versatile second - messenger molecule ( nilius et  al . , 1993 ; dolmetsch et  al . , 1998 ; this mode has been implicated in control of growth - cone turning ( nishiyama et  al . , 2008 ) , eye patterning ( pai et  al . , 2012 ) , and flatworm regeneration ( nogi et  al . , 2009 ; beane et  al . another uses the voltage gradients among cells to move small signaling molecules such as serotonin through gap junction  coupled cell fields , as occurs in right patterning ( fukumoto et  al . , 2005b ; adams et  al . , 2006 ) and control of neuronal pathfinding ( blackiston et  al . , 2015 ) . finally , voltage - sensitive phosphatases couple vmem change to the plethora of events regulated by pten phosphatases ( murata et  al . , 2005 ; okamura and dixon , 2011 ) . known transduction mechanisms by which ion flows affects cell behavior . of interest , when they conflict , bioelectrical cues tend to trump chemical signals . one example is the guidance of cell motility : if a chemical gradient and an electric field are set up in opposite directions , the bioelectric vector trumps the chemical cue in directing cell movement ( zhao , 2009 ; cao et  al . , 2011 ) . another example is the differentiation of human mesenchymal stem cells ( hmscs ) , which normally hyperpolarize as they differentiate ; despite the presence of potent chemical inducers , hmscs will not differentiate if kept artificially depolarized ( sundelacruz et  al . , 2008 ) . indeed , the voltage state can even partially reverse the differentiation state , inducing plasticity in differentiated hmscs ( sundelacruz et  al . , 2013 ) . by identifying the specific ion channel genes that set vmem states , the transduction mechanisms that sense vmem change , and the downstream transcriptional or epigenetic targets ( which include ion channels themselves ) , recent work has established the causal chain integrating bioelectrical cues with chemical pathways ( table 5 ) . neither signaling mode is entirely  upstream  of the other  cellular processes are regulated by the continuous cyclical interplay between transcriptional control of ion channel profiles within cells and the regulation of transcription by voltage dynamics . future work will identify new ion channel genes important for specific functions , additional transduction mechanisms by which cells sense their depolarization and hyperpolarization , and genome - wide ( next - generation sequencing [ ngs ] or microarray ) profiles of transcriptional programs triggered by specific vmem change . cells can sense the voltage states of their neighbors through gap junctions ( gjs)versatile ( and themselves voltage - sensitive ) channels allowing the direct sharing of current and other small molecules between cells ( palacios - prado and bukauskas , 2009 ; pereda et  al . the importance of gj - mediated cues for cellular decision making has been shown , for example , in the development of the neocortex ( sutor and hagerty , 2005 ) and more broadly in setting up the patterns of chemical synapses ( anava et  al . cells can also read the bioelectrical state of distant regions via the chemical molecules redistributed ( and transported or diffused ) across long distances by bioelectric state change . this was long ago suggested by burr , who used voltage readings at remote locations of the body to detect transplanted or induced tumors ( burr et  al . recent data in the frog model implicate long - range signaling via bioelectrical control of butyrate ( chernet and levin , 2014 ) and serotonin ( blackiston et  al . , 2011 ; lobikin et  al . , 2012a ) in tumorigenesis and metastatic induction . additional modes for nonlocal bioelectrical signaling include tunneling nanotubes ( chinnery et  al . , 2008 ; wittig et  al . , 2012 ) and exosomes , which contain numerous ion channels ( lotvall and valadi , 2007 ; valadi et  al . , 2007 ; wahlgren et  al . , 2012 ) and could regulate bioelectric states of cells that incorporate them . because bioelectrical gradients mediate signaling beyond the single - cell level , they form a versatile medium for carrying information . spatiotemporal gradients of vmem among cells in vivo are now known to regulate organ identity , positional information , size control , and polarity of anatomical axes . much like hox genes , whose combinatorial patterns of gene expression encode specific body regions during development , it has recently been shown that bioelectric prepatterns in the developing face of the frog and planarian models regulate the gene expression , size , and shape of craniofacial components ( vandenberg et  al . , 2011 ; in the frog , for example , patterns of hyperpolarization in the nascent face reveal the prospective locations of the eyes and other structures ; experimental perturbation of these distributions alters the boundaries of expression of face patterning genes such as frizzled , with the expected effects on craniofacial anatomy . right axis in frog and chick embryos ( levin et  al . , 2002 ; adams et  al . , 2006 ) and set the size of regenerating structures in segmented worms and regenerating zebrafish tails ( kurtz and schrank , 1955 ; beane et  al . , 2013 ; perathoner et  al . , ion transporters , such as the v - atpase , are required for normal left right patterning in several vertebrate models ( adams et  al . , 2006 ) , zebrafish fin regeneration ( monteiro et  al . , 2014 ) , and zebrafish eye development ( nuckels et  al . , 2009 ) . these examples illustrate that bioelectric patterns can be necessary aspects of development because , when they are specifically disrupted , predictable and coherent changes in morphogenesis occur . of importance , many of these data sets used distinct ion species ( potassium , sodium , chloride , or protons ) to show that the necessary parameter is indeed the voltage potential , not any one channel gene ( which could have had scaffold or binding roles ) or even any one ion type ( which could have had chemical , not electrical , roles ) . as with the gain - of - function examples discussed later , the voltage is what matters for the outcome , not which ion or channel was used to set it . in addition to specifying directly the pattern of subsequent anatomy , some bioelectric signals seem to trigger whole developmental modules . in the case of tail regeneration in xenopus , genetic , optogenetic , and pharmacological experiments have been used to recapitulate a regeneration - specific bioelectric state in nonregenerative animals and induce complete regrowth of this complex neuromuscular appendage ( adams et  al . not only could appropriate vmem state overcome physiological , chemical , and age - dependent blockade of regenerative capacity , but it was seen that a very simple ( low information content ) stimulus , such as  pump protons ,  could be sufficient to trigger a complete and self - limiting cascade of events that rebuilt the appendage ( tseng and levin , 2013 ) , in essence providing a  build whatever normally goes here  signal . these examples reveal that bioelectric state can function as a sufficient signal or master regulator ; this bodes well for the use of this approach in regenerative medicine , as we may not need to micromanage the morphogenesis of complex structures but instead rely on patterning subroutines already present in the host . f ) can be directed to make heads or tails by appropriate modulation of resting potential ( beane et  al . , 2011 , 2013 ) . in vertebrates , whole - eye formation can be induced ectopically , far outside the head , even in mesoderm or endoderm ( figure 2a ) by misexpression of specific ion channels in vivo ( pai et  al . , 2012 ) ; this process is mediated by a feedback loop between hyperpolarization and expression of eye - specific genes such as rx1 and pax6 , which in its absence can not initiate eye formation outside of the head . it is also interesting that this signaling is not cell autonomous : cells with unique voltage characteristics serve as organizers , recruiting wild - type host tissues to participate in the ectopic morphogenesis ( figure 2b ) . these examples illustrate the fact that bioelectric state provides instructive information to patterning processes and reveal that cell groups can be programmed at the level of complex organs , not only at the level of specifying individual cell types . understanding in detail the mapping between bioelectric states and the anatomical outcomes  quantitatively cracking the bioelectric code  is a major open direction in this field . possibilities for the parameters that functionally determine distinct organ types include spatial distribution of absolute vmem values within a cell group , relative differences in vmem across cell borders , and/or time - dependent changes of vmem within cells . one technology that is likely to be instrumental in testing hypotheses about the bioelectric code is optogenetics ( knopfel et  al . , 2010 ; liu and tonegawa , 2010 ) , which will facilitate the reading and writing of bioelectric patterning information in vivo . the first steps have been taken , showing regulation of stem cells via optogenetic signaling ( stroh et  al . , 2010 ; wang et  al . , 2014 ) , and a recent report showed the induction of tail regeneration by optical modulation of bioelectric state after amputation ( adams et  al . , the information - bearing signal ( the necessary and sufficient trigger ) for events such as eye induction , head determination , and tail regeneration via vmem change is a physiological state , not a gene product ( levin , 2013 ; tseng and levin , 2013 ) . studies reveal that the exact identity of the channel or pump used to trigger such morphological changes is often irrelevant many sodium , potassium , chloride , or proton conductances can be used , as long as the appropriate vmem state is reached . this means that the actual cause of the given morphological change can be a bioelectrical property not necessarily in 1:1 correspondence with any genetic locus . because channels and pumps can open and close posttranslationally , two cells expressing precisely the same mrna and protein can be in very different bioelectrical states . thus rich patterns of bioelectrical gradients can exist in a transcriptionally homogeneous tissue and be completely invisible to protein and mrna profiling until they trigger distinct downstream transcriptional targets . conversely , cells with very different channel and pump complements may have the same vmem , since resting potential is an ensemble state that is a function of many different ion flows . the implication is that mrna and protein profiling approaches are insufficient to detect and characterize important biophysical determinants of morphogenesis , and knockout screens may completely miss bioelectric pathways , since knockouts of single ion channels will be subject to compensation and redundancy by other channels contributing to vmem . one context in which bioelectric and genetic state information can diverge is cancer ( yang and brackenbury , 2013 ; chernet and levin , 2013b ) . dependent invasion of body tissues , and drastic arborization ) can be induced in genetically normal melanocytes by depolarization of somatic cells ( blackiston et  al . , 2011 ; this effect is not cell autonomous ( figure 2 , c and d ) , showing that the bioelectric state of cells at considerable distance can trigger metastatic behavior . conversely , the formation of tumors by human oncogenes such as p53 and kras mutations can be suppressed , despite the strong presence of oncogene protein within the cells , by artificially preventing the depolarization that occurs during oncogenic transformation ( chernet and levin , 2013a ) . these examples reveal the potential dissociation between genetic state and disease outcome ; an implication of these data is that the neoplastic state can not always be predicted from examination of the genome , transcriptomes , or proteome , although in some cases , ion channel expression is altered ( onkal and djamgoz , 2009 ; becchetti , 2011 ; lang and stournaras , 2014 ) . on the other hand , the functionally determinative voltage states can not be seen in fixed tissue , stressing the importance of gathering real - time in vivo bioelectric information over and above analysis of mutations , mrna profiles , and protein levels . another implication for cancer biology is that although expression of some ion channel might be a useful marker ( wang , 2004 ; fraser et  al . 2006 ) , there will also be many cases in which the transcriptional profile reveals nothing ( because of signaling via posttranslational gating of channel state ) , and drugs targeting one specific channel type ( arcangeli et  al . , 2009 , 2012 ) may have no effect ( due to compensation and redundancy of channel types ) . if indeed cancer is augmented or induced by a depolarized bioelectric state ( binggeli and weinstein , 1986 ; olivotto et  al . , 1996 ; yang and brackenbury , 2013 ) , we will have to think less about individual ion channels as oncogenes ( pillozzi et  al . , 2004 ; lallet - daher et  al . , 2013 ; than et  al . , 2013 ) and focus instead on the way in which many channels contribute to a bioelectrical oncostate , to develop strategies for dominating the resting potential irrespective of native channel identity ( sharmeen et  al . bioelectric patterns are clearly important drivers of cell behavior and pattern formation , but how do these patterns originate ? diverse resting potentials across a tissue can arise from preexisting differences in ion channel transcription , but that is not the only way ( justet et  al . , 2013 ) . such regionalized patterns of vmem can also form de novo in transcriptionally and proteomically identical cells because cells coupled by gap junctions ( electrical synapses ) form a ( slow ) electrically excitable medium ; this is a particularly interesting aspect because such media are known to have powerful computational capabilities ( fenton et  al . , 1999 ; gorgcki and gorgcka , 2007 ; adamatzky et  al . , 2011 ) . positive feedback loops implemented by elements such as voltage - gated ion channels , which both set and respond to vmem changes , can drive spontaneous symmetry breaking and amplification of physiological noise . considerable self - organization dynamics can take place without a need for preexisting chemical prepattern ( toko et  al . , 1987 ; schiffmann , 1991 , 1997 ; palacios - prado and bukauskas , 2009 ) or transcriptional activity ; for example , human red blood cells have a physiological , not genetic , circadian clock rhythm driven by a slow ionic oscillation ( chakravarty and rizvi , 2011 ; o'neill and reddy , 2011 ) . such dynamics has been studied in nerve and muscle ( zykov , 1990 ; chen et  al . , 1997 ; , 2009 ; boettiger and oster , 2009 ) , and turing - type self - organization has long been appreciated in chemical signaling ( takagi and kaneko , 2005 ; muller et  al . , 2012 ; sheth et  al . , 2012 however , capabilities and properties of self - organization of voltage patterns in groups of nonneural cells remain to be formally analyzed . quantitative analysis of in silico models of bioelectric dynamics will need to be integrated with deep new data sets from appropriate physiomic technologies to fully understand and control developmental patterning in vivo . one unexpected recent finding illustrates the storage of patterning information in physiological networks and has significant implications for evolution . planarian flatworms have the remarkable ability to regenerate completely from partial body fragments ( reddien and sanchez alvarado , 2004 ; salo et  al . , 2009 ; lobo et  al . , the cells at one edge make a tail , whereas those at the other edge make a head , revealing that the adult stem cells that implement regeneration are not locally controlled ( since the cells were direct neighbors until the scalpel separated them ) but must communicate with the remaining tissue to decide what anatomical structures must be formed . it was shown that this long - range communication occurs via gap junction  mediated electrical synapses ( scemes et  al . 2013 ) , and works together with a bioelectric circuit that determines head versus tail identity in each end 's blastema ( beane et  al . , 2011 , 2013 ) . brief inhibition of this gap junction  mediated communication results in worms developing heads at both ends ( nogi and levin , 2005 ; oviedo et  al . , 2010 ) . i ) is that weeks later , when these two - headed animals have their heads and tails amputated again ( in just water , with no further perturbation ) , the same two - headed phenotype results , and this is repeated upon subsequent amputations . thus a transient perturbation of physiological cell : cell communication stably changes the pattern to which the animal regenerates upon damage , despite normal genomic sequence . this again illustrates the potential divergence of genetic versus physiological information , especially since the phenotype is stable across fission ( this animal 's most frequent reproductive mode ) , and thus could have significant implications for evolution . although epigenetic processes may be involved , chromatin modification mechanisms alone are not a sufficient explanation , since the ectopic heads ( tissue that might be suggested to have been epigenetically reprogrammed into a head state from its original tail identity ) are thrown away at each generation of cutting . what remains is a gut fragment , which somehow knows that it is to form two heads , not one , upon further cutting ; the information about basic anatomical polarity and body organization must be stored in a distributed form throughout the animal . quantitative , field - like models of this circuit remain to be developed to understand precisely how information guiding specific shape outcomes is encoded in ( represented by ) bioelectric states among cells . major open questions for future progress include the mechanisms by which cells compare bioelectric state across distances , additional molecular details of the interactions of bioelectrical signals with chemical gradients and physical forces , and the development of quantitative models of bioelectric circuits that store stable patterning information during morphogenesis . expansions of the toolkit of synthetic biology will soon allow the rational top - down programming of bioelectric circuits , which will have important implications for regenerative medicine , cancer biology , and bioengineering ( reid et  al . optogenetics , once expanded to facilitate the control of stable vmem in large , nonexcitable cell groups , will play a large part , and there is significant room for advances in better voltage reporters and techniques for in vivo modulation of bioelectric state . one hypothesis for the development of deep , quantitative theory in this field is that pattering information may be stored within nonneural bioelectric cell networks using the same molecular mechanisms and information - processing algorithms that underlie behavioral memory in the nervous system . it is thus possible that the techniques such as those now used to extract mental imagery from electrical measurements of living human brains ( nishimoto et  al . , 2011 ) may shed crucial light on the encoding of anatomical pattern in the electrical circuits of somatic cells ; conversely , the cracking of the bioelectric code in development and regeneration may have important benefits for the understanding of the semantics of electric states in the brain . in practical terms , the molecular biologist needs to consider not only transcriptional and protein profiles when working to understand regulation of single - cell behavior and pattern formation . significant instructive information is generated at the level of bioelectricity ; ion channels and gap junctions are the molecular elements of such circuits , but bioelectrical signaling has its own unique dynamics that will become increasingly tractable with development of new technology specifically targeting stable vmem states . the existence of bioelectric signaling among most cell types , not only neurons , suggests that the field of applicability of electroceuticals ( famm et  al . , 2013 ; sinha , 2013 ; birmingham et  al . , 2014 ) more broadly , to the extent that the data of developmental bioelectricity are erasing artificial distinctions between neural and nonneural cell types , the insights of computational neuroscience and cognitive science will become relevant to cell and developmental biology . it is possible that the most effective ways to understand high - order ( anatomical - level ) outcomes will involve not only bottom - up models of molecular pathways but also top - down models in which information and control theory concepts play central roles . in this way , molecular bioelectricity may be revealing a mechanistic path toward understanding the intelligence exhibited by cell behavior and harnessing it toward transformative advances in biomedicine and the information sciences ( albrecht - buehler , 1985 ; rubenstein et  al .