bioelectricity bioelectricity in developmental biology, bioelectricity refers to the regulation of cell, tissue, and organ-level patterning and behavior as the result of endogenous electrically mediated signalling. cells and tissues of all types use ion fluxes to communicate electrically. the charge carrier in bioelectricity is the ion (charged atom), and an electric current and field is generated whenever a net ion flux occurs. endogenous electric currents and fields, ion fluxes, and differences in resting potential across tissues comprise a signaling system. it functions alongside (in series and in parallel to) biochemical factors, transcriptional networks, and other physical forces to regulate the cell behavior and large-scale patterning during embryogenesis, regeneration, cancer suppression, and many other processes. the morphogenetic field of pattern formation and maintenance during the lifespan. for neuroelectric signalling, see action potential. overview developmental bioelectricity is a sub-discipline of biology, related to, but distinct from, neurophysiology and bioelectromagnetics. developmental bioelectricity refers to the endogenous ion fluxes, transmembrane and transepithelial voltage gradients, and electric currents and fields produced and sustained in living cells and tissues. this electrical activity is often used during embryogenesis, regeneration, and cancer suppression—it is one layer of the complex field of signals that impinge upon all cells in vivo and regulate their interactions during pattern formation and maintenance. this is distinct from neural bioelectricity (classically termed electrophysiology), which refers to the rapid and transient spiking in well-recognized excitable cells like neurons and myocytes (muscle cells); and from bioelectromagnetics, which refers to the effects of applied electromagnetic radiation, and endogenous electromagnetics such as biophoton emission and magnetite. membrane potential and transepithelial potential. electric potential difference across corneal epithelium, and the generation of wound electric fields. distribution of bioelectric potential in the flank of a frog embryo stained with voltage-sensitive fluorescent dye. the inside/outside discontinuity at the cell surface enabled by a lipid bilayer membrane (capacitor) is at the core of bioelectricity. the plasma membrane was an indispensable structure for the origin and evolution of life itself. it provided compartmentalization permitting the setting of a differential voltage/potential gradient (battery or voltage source) across the membrane, probably allowing early and rudimentary bioenergetics that fuelled cell mechanisms. during evolution, the initially purely passive diffusion of ions (charge carriers), become gradually controlled by the acquisition of ion channels, pumps, exchangers, and transporters. these energetically free (resistors or conductors, passive transport) or expensive (current sources, active transport) translocators set and fine tune voltage gradients – resting potentials – that are ubiquitous and essential to life's physiology, ranging from bioenergetics, motion, sensing, nutrient transport, toxins clearance, and signalling in homeostatic and disease/injury conditions. upon stimuli or barrier breaking (short-circuit) of the membrane, ions powered by the voltage gradient (electromotive force) diffuse or leak, respectively, through the cytoplasm and interstitial fluids (conductors), generating measurable electric currents – net ion fluxes – and fields. some ions (such as calcium) and molecules (such as hydrogen peroxide) modulate targeted translocators to produce a current or to enhance, mitigate or even reverse an initial current, being switchers. endogenous bioelectric signals are produced in cells by the cumulative action of ion channels, pumps, and transporters. in non-excitable cells, the resting potential across the plasma membrane (vmem) of individual cells propagate across distances via electrical synapses known as gap junctions (conductors), which allow cells to share their resting potential with neighbors. aligned and stacked cells (such as in epithelia) generate transepithelial potentials (such as batteries in series) and electric fields, which likewise propagate across tissues. tight junctions (resistors) efficiently mitigate the paracellular ion diffusion and leakage, precluding the voltage short circuit. together, these voltages and electric fields form rich and dynamic and patterns inside living bodies that demarcate anatomical features, thus acting like blueprints for gene expression and morphogenesis in some instances. more than correlations, these bioelectrical distributions are dynamic, evolving with time and with the microenvironment and even long-distant conditions to serve as instructive influences over cell behavior and large-scale patterning during embryogenesis, regeneration, and cancer suppression. bioelectric control mechanisms are an important emerging target for advances in regenerative medicine, birth defects, cancer, and synthetic bioengineering. history the modern roots of developmental bioelectricity can be traced back to the entire 18th century. several seminal works stimulating muscle contractions using leyden jars culminated with the publication of classical studies by luigi galvani in 1791 (de viribus electricitatis in motu musculari) and 1794. in these, galvani thought to have uncovered intrinsic electric-producing ability in living tissues or “animal electricity”. alessandro volta showed that the frog's leg muscle twitching was due to a static electricity generator and from dissimilar metals undergoing or catalyzing electrochemical reactions. galvani showed, in a 1794 study, twitching without metal electricity by touching the leg muscle with a deviating cut sciatic nerve, definitively demonstrating “animal electricity”. unknowingly, galvani with this and related experiments discovered the injury current (ion leakage driven by the intact membrane/epithelial potential) and injury potential (potential difference between injured and intact membrane/epithelium). the injury potential was, in fact, the electrical source behind the leg contraction, as realized in the next century. subsequent work ultimately extended this field broadly beyond nerve and muscle to all cells, from bacteria to non-excitable mammalian cells. building on earlier studies, further glimpses of developmental bioelectricity occurred with the discovery of wound-related electric currents and fields in the 1840s, when one of the founding fathers of modern electrophysiology – emil du bois-reymond – reported macroscopic level electrical activities in frog, fish and human bodies. he recorded minute electric currents in live tissues and organisms with a then state-of-the-art galvanometer made of insulated copper wire coils. he unveiled the fast-changing electricity associated with muscle contraction and nerve excitation – the action potentials. at the same time, du bois-reymond also reported in detail less fluctuating electricity at wounds – injury current and potential – he made to himself. some sample cell types and their resting potentials, revealing that actively proliferating and plastic cells cluster in the depolarized end of the continuum, while terminally-differentiated mature cell types tend to be strongly polarized. bioelectricity work began in earnest at the beginning of the 20th century. since then, several waves of research produced important functional data showing the role that bioelectricity plays in the control of growth and form. in the 1920s and 1930s, e. j. lund and h. s. burr were some of the most prolific authors in this field. lund measured currents in a large number of living model systems, correlating them to changes in patterning. in contrast, burr used a voltmeter to measure voltage gradients, examining developing embryonic tissues and tumours, in a range of animals and plants. applied electric fields were demonstrated to alter the regeneration of planaria by marsh and beams in the 1940s and 1950s, inducing the formation of heads or tails at cut sites, reversing the primary body polarity. the introduction and development of the vibrating probe, the first device for quantitative non-invasive characterization of the extracellular minute ion currents, by lionel jaffe and richard nuccittelli, revitalized the field in the 1970s. they were followed by researchers such as joseph vanable, richard borgens, ken robinson, and colin mccaig, among many others, who showed roles of endogenous bioelectric signaling in limb development and regeneration, embryogenesis, organ polarity, and wound healing. c.d. cone studied the role of resting potential in regulating cell differentiation and proliferation and subsequent work has identified specific regions of the resting potential spectrum that correspond to distinct cell states such as quiescent, stem, cancer, and terminally differentiated. although this body of work generated a significant amount of high-quality physiological data, this large-scale biophysics approach has historically been in the shadow of the limelight of biochemical gradients and genetic networks in biology education, funding, and overall popularity among biologists. a key factor that contributed to this field lagging behind molecular genetics and biochemistry is that bioelectricity is inherently a living phenomenon – it cannot be studied in fixed specimens. working with bioelectricity is more complex than traditional approaches to developmental biology, both methodologically and conceptually, as it typically requires a highly interdisciplinary approach. study using electrodes the gold standard techniques to quantitatively extract electric dimensions from living specimens, ranging from cell to organism levels, are the glass microelectrode (or micropipette), the vibrating (or self-referencing) voltage probe, and the vibrating ion-selective microelectrode. the former is inherently invasive, and the two latter are non-invasive, but all are ultra-sensitive and fast-responsive sensors extensively used in a plethora of physiological conditions in widespread biological models. the glass microelectrode was developed in the 1940s to study the action potential of excitable cells, deriving from the seminal work by hodgkin and huxley in the giant axon squid. it is simply a liquid salt bridge connecting the biological specimen with the electrode, protecting tissues from leachable toxins and redox reactions of the bare electrode. owing to its low impedance, low junction potential and weak polarization, silver electrodes are standard transducers of the ionic into electric current that occurs through a reversible redox reaction at the electrode surface. the vibrating probe was introduced in biological studies in the 1970s. the voltage-sensitive probe is electroplated with platinum to form a capacitive black tip ball with large surface area. when vibrating in an artificial or natural dc voltage gradient, the capacitive ball oscillates in a sinusoidal ac output. the amplitude of the wave is proportional to the measuring potential difference at the frequency of the vibration, efficiently filtered by a lock-in amplifier that boosts probe's sensitivity. the vibrating ion-selective microelectrode was first used in 1990 to measure calcium fluxes in various cells and tissues. the ion-selective microelectrode is an adaptation of the glass microelectrode, where an ion-specific liquid ion exchanger (ionophore) is tip-filled into a previously silanized (to prevent leakage) microelectrode. also, the microelectrode vibrates at low frequencies to operate in the accurate self-referencing mode. only the specific ion permeates the ionophore, therefore the voltage readout is proportional to the ion concentration in the measuring condition. then, flux is calculated using the fick's first law. emerging optic-based techniques, for example, the ph optrode (or optode), which can be integrated into a self-referencing system may become an alternative or additional technique in bioelectricity laboratories. the optrode does not require referencing and is insensitive to electromagnetism simplifying system setting up and making it a suitable option for recordings where electric stimulation is simultaneously applied. much work to functionally study bioelectric signaling has made use of applied (exogenous) electric currents and fields via dc and ac voltage-delivering apparatus integrated with agarose salt bridges. these devices can generate countless combinations of voltage magnitude and direction, pulses, and frequencies. currently, lab-on-a-chip mediated application of electric fields is gaining ground in the field with the possibility to allow high-throughput screening assays of the large combinatory outputs. tools for manipulating non-neural bioelectricity include pharmacological and genetic reagents to alter cell connectivity (control gap junctions), cell vmem (control ion channels/pumps), and bioelectrically-guided 2nd messengers (control neurotransmitters and other small molecules). study using fluorescence progress in molecular biology over the last six decades has produced powerful tools that facilitate the dissection of biochemical and genetic signals; yet, they tend to not be well-suited for bioelectric studies in vivo. prior work relied extensively on current applied directly by electrodes, reinvigorated by significant recent advances in materials science and extracellular current measurements, facilitated by sophisticated self-referencing electrode systems. while electrode applications for manipulating neurally-controlled body processes have recently attracted much attention, there are other opportunities for controlling somatic processes, as most cell types are electrically active and respond to ionic signals from themselves and their neighbors. in the early part of the 21st century, a number of new molecular techniques were developed that allowed bioelectric pathways to be investigated with a high degree of mechanistic resolution, and to be linked to canonical molecular cascades. these include: pharmacological screens to identify endogenous channels and pumps responsible for specific patterning events; voltage-sensitive fluorescent reporter dyes and genetically encoded fluorescent voltage indicators for the characterization of the bioelectric state in vivo. panels of well-characterized dominant ion channels that can be mis expressed in cells of interest to alter the bioelectric state in desired ways; and computational platforms that are coming on-line to assist in building predictive models of bioelectric dynamics in tissues. compared with the electrode-based techniques, the molecular probes provide a wider spatial resolution and facilitated dynamic analysis over time. although