calibration or titration can be possible, molecular probes are typically semi-quantitative, whereas electrodes provide absolute bioelectric values. another advantage of fluorescence and other probes is their less-invasive nature and spatial multiplexing, enabling the simultaneous monitoring of large areas of embryonic or other tissues in vivo during normal or pathological pattering processes. role in early development work in model systems such as xenopus laevis and zebrafish has revealed a role for bioelectric signalling in the development of heart, face, eye, brain, and other organs. screens have identified roles for ion channels in size control of structures such as the zebrafish fin, while focused gain-of-function studies have shown for example that body parts can be re-specified at the organ level – for example creating entire eyes in gut endoderm. as in the brain, developmental bioelectrics can integrate information across significant distance in the embryo, for example such as the control of brain size by bioelectric states of ventral tissue. and the control of tumorigenesis at the site of oncogene expression by bioelectric state of remote cells. human disorders, as well as numerous mouse mutants show that bioelectric signalling is important for human development (tables 1 and 2). those effects are pervasively linked to channelopathies, which are human disorders that result from mutations that disrupt ion channels. several channelopathies result in morphological abnormalities or congenital birth defects in addition to symptoms that affect muscle and or neurons. for example, mutations that disrupt an inwardly rectifying potassium channel kir2.1 cause dominantly inherited andersen-tawil syndrome (ats). ats patients experience periodic paralysis, cardiac arrhythmias, and multiple morphological abnormalities that can include cleft or high arched palate, cleft or thin upper lip, flattened philtrum, micrognathia, dental oligodontia, enamel hypoplasia, delayed dentition eruption, malocclusion, broad forehead, wide set eyes, low set ears, syndactyly, clinodactyly, brachydactyly, and dysplastic kidneys. mutations that disrupt another inwardly rectifying k+ channel girk2 encoded by kcnj6 cause keppen-lubinsky syndrome which includes microcephaly, a narrow nasal bridge, a high arched palate, and severe generalized lipodystrophy (failure to generate adipose tissue). kcnj6 is in the down syndrome critical region such that duplications that include this region lead to craniofacial and limb abnormalities and duplications that do not include this region do not lead to morphological symptoms of down syndrome. mutations in kcnh1, a voltage gated potassium channel lead to temple-baraitser (also known as zimmermann- laband) syndrome. common features of temple-baraitser syndrome include absent or hypoplastic of finger and toe nails and phalanges and joint instability. craniofacial defects associated with mutations in kcnh1 include cleft or high arched palate, hypertelorism, dysmorphic ears, dysmorphic nose, gingival hypertrophy, and abnormal number of teeth. mutations in cav1.2, a voltage gated ca2+ channel, lead to timothy syndrome, which causes severe cardiac arrhythmia (long-qt) along with syndactyly and similar craniofacial defects to andersen-tawil syndrome including cleft or high-arched palate, micrognathia, low set ears, syndactyly and brachydactyly. while these channelopathies are rare, they show that functional ion channels are important for development. furthermore, in utero exposure to anti-epileptic medications that target some ion channels also cause increased incidence of birth defects such as oral cleft. the effects of both genetic and exogenous disruption of ion channels lend insight into the importance of bioelectric signalling in development. role in wound healing and cell guidance one of the best-understood roles for bioelectric gradients is at the tissue-level endogenous electric fields utilized during wound healing. it is challenging to study wound-associated electric fields, because these fields are weak, less fluctuating, and do not have immediate biological responses when compared to nerve pulses and muscle contraction. the development of the vibrating and glass microelectrodes, demonstrated that wounds indeed produced and, importantly, sustained measurable electric currents and electric fields. these techniques allow further characterization of the wound electric fields/currents at cornea and skin wounds, which show active spatial and temporal features, suggesting active regulation of these electrical phenomena. for example, the wound electric currents are always the strongest at the wound edge, which gradually increased to reach a peak about 1 hour after injury. at wounds in diabetic animals, the wound electric fields are significantly compromised. understanding the mechanisms of generation and regulation of the wound electric currents/fields is expected to reveal new approaches to manipulate the electrical aspect for better wound healing. how are the electric fields at a wound produced? epithelia actively pump and differentially segregate ions. in the cornea epithelium, for example, na+ and k+ are transported inwards from tear fluid to extracellular fluid, and cl− is transported out of the extracellular fluid into the tear fluid. the epithelial cells are connected by tight junctions, forming the major electrical resistive barrier, and thus establishing an electrical gradient across the epithelium – the transepithelial potential (tep). breaking the epithelial barrier, as occurs in any wounds, creates a hole that breaches the high electrical resistance established by the tight junctions in the epithelial sheet, short-circuiting the epithelium locally. the tep therefore drops to zero at the wound. however, normal ion transport continues in unwounded epithelial cells beyond the wound edge (typically <1 mm away), driving positive charge flow out of the wound and establishing a steady, laterally-oriented electric field (ef) with the cathode at the wound. skin also generates a tep, and when a skin wound is made, similar wound electric currents and fields arise, until the epithelial barrier function recovers to terminate the short-circuit at the wound. when wound electric fields are manipulated with pharmacological agents that either stimulate or inhibit transport of ions, the wound electric fields also increase or decrease, respectively. wound healing can be speed up or slowed down accordingly in cornea wounds. how do electric fields affect wound healing? to heal wounds, cells surrounding the wound must migrate and grow directionally into the wound to cover the defect and restore the barrier. cells important to heal wounds respond remarkably well to applied electric fields of the same strength that are measured at wounds. the whole gamut of cell types and their responses following injury are affected by physiological electric fields. those include migration and division of epithelial cells, sprouting and extension of nerves, and migration of leukocytes and endothelial cells. the most well studied cellular behavior is directional migration of epithelial cells in electric fields – electrotaxis. the epithelial cells migrate directionally to the negative pole (cathode), which at a wound is the field polarity of the endogenous vectorial electric fields in the epithelium, pointing (positive to negative) to the wound center. epithelial cells of the cornea, keratinocytes from the skin, and many other types of cells show directional migration at electric field strengths as low as a few mv mm−1. large sheets of monolayer epithelial cells, and sheets of stratified multilayered epithelial cells also migrate directionally. such collective movement closely resembles what happens during wound healing in vivo, where cell sheets move collectively into the wound bed to cover the wound and restore the barrier function of the skin or cornea. how cells sense such minute extracellular electric fields remains largely elusive. recent research has started to identify some genetic, signaling and structural elements underlying how cells sense and respond to small physiological electric fields. these include ion channels, intracellular signaling pathways, membrane lipid rafts, and electrophoresis of cellular membrane components. role in animal regeneration in the early 20th century, albert mathews seminally correlated regeneration of a cnidarian polyp with the potential difference between polyp and stolon surfaces, and affected regeneration by imposing countercurrents. amedeo herlitzka, following on the wound electric currents footsteps of his mentor, du bois-raymond, theorized about electric currents playing an early role in regeneration, maybe initiating cell proliferation. using electric fields overriding endogenous ones, marsh and beams astoundingly generated double-headed planarians and even reversed the primary body polarity entirely, with tails growing where a head previously existed. after these seed studies, variations of the idea that bioelectricity could sense injury and trigger or at least be a major player in regeneration have spurred over the decades until the present day. a potential explanation lies on resting potentials (primarily vmem and tep), which can be, at least in part, dormant sensors (alarms) ready to detect and effectors (triggers) ready to react to local damage. following up on the relative success of electric stimulation on non-permissive frog leg regeneration using an implanted bimetallic rod in the late 1960s, the bioelectric extracellular aspect of amphibian limb regeneration was extensively dissected in the next decades. definitive descriptive and functional physiological data was made possible owing to the development of the ultra-sensitive vibrating probe and improved application devices. amputation invariably leads to a skin-driven outward current and a consequent lateral electric field setting the cathode at the wound site. although initially pure ion leakage, an active component eventually takes place and blocking ion translocators typically impairs regeneration. using biomimetic exogenous electric currents and fields, partial regeneration was achieved, which typically included tissue growth and increased neuronal tissue. conversely, precluding or reverting endogenous electric current and fields impairs regeneration. these studies in amphibian limb regeneration and related studies in lampreys and mammals combined with those of bone fracture healing and in vitro studies, led to the general rule that migrating (such as keratinocytes, leucocytes and endothelial cells) and outgrowing (such as axons) cells contributing to regeneration undergo electrotaxis towards the cathode (injury original site). congruently, an anode is associated with tissue resorption or degeneration, as occurs in impaired regeneration and osteoclastic resorption in bone. despite these efforts, the promise for a significant epimorphic regeneration in mammals remains a major frontier for future efforts, which includes the use of wearable bioreactors to provide an environment within which pro-regenerative bioelectric states can be driven and continued efforts at electrical stimulation. recent molecular work has identified proton and sodium flux as being important for tail regeneration in xenopus tadpoles, and shown that regeneration of the entire tail (with spinal cord, muscle, etc.) could be triggered in a range of normally non-regenerative conditions by either molecular-genetic, pharmacological, or optogenetic methods. in planaria, work on bioelectric mechanism has revealed control of stem cell behavior, size control during remodelling, anterior-posterior polarity, and head shape. gap junction-mediated alteration of physiological signaling produces two-headed worms in dugesia japonica; remarkably, these animals continue to regenerate as two-headed in future rounds of regeneration months after the gap junction-blocking reagent has left the tissue. this stable, long-term alteration of the anatomical layout to which animals regenerate, without genomic editing, is an example of epigenetic inheritance of body pattern, and is also the only available “strain” of planarian species exhibiting an inherited anatomical change that is different from the wild-type. voltage changes can be transduced to downstream effector mechanisms via a variety of 2nd messenger processes, including vmem-dependent movement of small signaling molecules like serotonin through transporters or gap junctions, voltage-sensitive phosphatases, voltage-gated calcium channels (which trigger calcium-signaling cascades), and dimerization of receptors in the cell surface. bioelectricity and genetic expression work together in an integrated fashion; nothing is downstream. misexpression of specific ion channels in diverse areas of frog embryos can induce the creation of ectopic organs, such as eyes on gut tissue. role in cancer defection of cells from the normally tight coordination of activity towards an anatomical structure results in cancer; it is thus no surprise that bioelectricity – a key mechanism for coordinating cell growth and patterning – is a target often implicated in cancer and metastasis. indeed, it has long been known that gap junctions have a key role in carcinogenesis and progression. channels can behave as oncogenes and are thus suitable as novel drug targets. recent work in amphibian models has shown that depolarization of resting potential can trigger metastatic behavior in normal cells, while hyperpolarization (induced by ion channel misexpression, drugs, or light) can suppress tumorigenesis induced by expression of human oncogenes. depolarization of resting potential appears to be a bioelectric signature by which incipient tumor sites can be detected non-invasively. refinement of the bioelectric signature of cancer in biomedical contexts, as a diagnostic modality, is one of the possible applications of this field. excitingly, the ambivalence of polarity – depolarization as marker and hyperpolarization as treatment – make it conceptually possible to derive theragnostic (portmanteau of therapeutics with diagnostics) approaches, designed to simultaneously detect and treat early tumors, in this case based on the normalization of the membrane polarization. role in pattern regulation recent experiments