The neural tube of the Xenopus embryo maintains a potential difference across itself

Multifunctional scaffolds for bone repair following age-related biological decline: Promising prospects for smart biomaterial-driven technologies

The repair of large bone defects due to trauma, disease, and infection can be exceptionally challenging in the elderly. Despite best clinical practice, bone regeneration within contemporary, surgically implanted synthetic scaffolds is often problematic, inconsistent, and insufficient where additional osteobiological support is required to restore bone. Emergent smart multifunctional biomaterials may drive important and dynamic cellular crosstalk that directly targets, signals, stimulates, and promotes an innate bone repair response following age-related biological decline and when in the presence of disease or infection. However, their role remains largely undetermined. By highlighting their mechanism/s and mode/s of action, this review spotlights smart technologies that favorably align in their conceivable ability to directly target and enhance bone repair and thus are highly promising for future discovery for use in the elderly. The four degrees of interactive scaffold smartness are presented, with a focus on bioactive, bioresponsive, and the yet-to-be-developed autonomous scaffold activity. Further, cell- and biomolecular-assisted approaches were excluded, allowing for contemporary examination of the capabilities, demands, vision, and future requisites of next-generation biomaterial-induced technologies only. Data strongly supports that smart scaffolds hold significant promise in the promotion of bone repair in patients with a reduced osteobiological response. Importantly, many techniques have yet to be tested in preclinical models of aging. Thus, greater clarity on their proficiency to counteract the many unresolved challenges within the scope of aging bone is highly warranted and is arguably the next frontier in the field. This review demonstrates that the use of multifunctional smart synthetic scaffolds with an engineered strategy to circumvent the biological insufficiencies associated with aging bone is a viable route for achieving next-generation therapeutic success in the elderly population.

Wireless control of nerve growth using bipolar electrodes: a new paradigm in electrostimulation

Electrical activity underpins all life, but is most familiar in the nervous system, where long range electrical signalling is essential for function. When this is lost (e.g., traumatic injury) or it becomes inefficient (e.g., demyelination), the use of external fields can compensate for at least some functional deficits. However, its potential to also promote biological repair at the cell level is underplayed despite abundant in vitro evidence for control of neuron growth. This perspective article considers specifically the emerging possibility of achieving cell growth through the interaction of external electric fields using conducting materials as unwired bipolar electrodes, and without intending stimulation of neuron electrical activity to be the primary consequence. The use of a wireless method to create electrical interactions represents a paradigm shift and may allow new applications in vivo where physical wiring is not possible. Within that scheme of thought an evaluation of specific materials and their dynamic responses as bipolar unwired electrodes is summarized and correlated with changes in dynamic nerve growth during stimulation, suggesting possible future schemes to achieve neural growth using bipolar unwired electrodes with specific characteristics. This strategy emphasizes how nerve growth can be encouraged at injury sites wirelessly to induce repair, as opposed to implanting devices that may substitute the neural signals.

Possible Synergies of Nanomaterial-Assisted Tissue Regeneration in Plasma Medicine: Mechanisms and Safety Concerns

Cold atmospheric plasma and nanomedicine originally emerged as individual domains, but are increasingly applied in combination with each other. Most research is performed in the context of cancer treatment, with only little focus yet on the possible synergies. Many questions remain on the potential of this promising hybrid technology, particularly regarding regenerative medicine and tissue engineering. In this perspective article, we therefore start from the fundamental mechanisms in the individual technologies, in order to envision possible synergies for wound healing and tissue recovery, as well as research strategies to discover and optimize them. Among these strategies, we demonstrate how cold plasmas and nanomaterials can enhance each other's strengths and overcome each other's limitations. The parallels with cancer research, biotechnology and plasma surface modification further serve as inspiration for the envisioned synergies in tissue regeneration. The discovery and optimization of synergies may also be realized based on a profound understanding of the underlying redox- and field-related biological processes. Finally, we emphasize the toxicity concerns in plasma and nanomedicine, which may be partly remediated by their combination, but also partly amplified. A widespread use of standardized protocols and materials is therefore strongly recommended, to ensure both a fast and safe clinical implementation.

Physiological Electric Field: A Potential Construction Regulator of Human Brain Organoids

Brain organoids can reproduce the regional three-dimensional (3D) tissue structure of human brains, following the in vivo developmental trajectory at the cellular level; therefore, they are considered to present one of the best brain simulation model systems. By briefly summarizing the latest research concerning brain organoid construction methods, the basic principles, and challenges, this review intends to identify the potential role of the physiological electric field (EF) in the construction of brain organoids because of its important regulatory function in neurogenesis. EFs could initiate neural tissue formation, inducing the neuronal differentiation of NSCs, both of which capabilities make it an important element of the in vitro construction of brain organoids. More importantly, by adjusting the stimulation protocol and special/temporal distributions of EFs, neural organoids might be created following a predesigned 3D framework, particularly a specific neural network, because this promotes the orderly growth of neural processes, coordinate neuronal migration and maturation, and stimulate synapse and myelin sheath formation. Thus, the application of EF for constructing brain organoids in a3D matrix could be a promising future direction in neural tissue engineering.

Morphogenic fields: A coming of age

Morphogenesis, the coming-into-being of living organisms, was first described in the 4th century BC by Aristotle, progenitor of biology and embryology. Over the centuries it has been the subject of innumerable commentaries by philosophers, theologians and scientists but no consensus has ever been reached as to its causes. In the late 19th century, along with the emergence of cellular and molecular biology, embryology underwent a renaissance and became a topic of great interest and research. Early on the discipline divided into two opposing factions, those who attempted to explain fetal development on the basis of cellular and molecular mechanisms, and those who invoked the presence of organizing fields. The morphogenic field was first articulated in the early decades of the 20th century by multiple researchers independently of each other. The field became an extremely useful conceptual tool by which to explain a wide range of developmental phenomena. While embryology and genetics originally formed a unified discipline, during the 1930s  40 s geneticists became progressively skeptical of the field notion. The discovery of the DNA structure by Watson and Crick in the early 1950s decisively settled matters and thereafter the two disciplines pursued different lines of inquiry. After World War II embryology and the field concept went into a decades-long decline. By the 1980s an increasing number of scientists began to critically reexamine the morphogenic field concept and it underwent a second renaissance. In this paper I examine the development and evolution of the field concept, both experimentally and conceptually, and highlight the failure of genetic mechanisms to explain morphogenesis. I provide three instances from the medical literature of developmental phenomena which are only explainable on the basis of morphogenic field dynamics and argue that the field concept must be readmitted into mainstream scientific discourse.

Bioelectrical control of positional information in development and regeneration: A review of conceptual and computational advances

Positional information describes pre-patterns of morphogenetic substances that alter spatio-temporal gene expression to instruct development of growth and form. A wealth of recent data indicate bioelectrical properties, such as the transmembrane potential (Vmem), are involved as instructive signals in the spatiotemporal regulation of morphogenesis. However, the mechanistic relationships between Vmem and molecular positional information are only beginning to be understood. Recent advances in computational modeling are assisting in the development of comprehensive frameworks for mechanistically understanding how endogenous bioelectricity can guide anatomy in a broad range of systems. Vmem represents an extraordinarily strong electric field (∼1.0 × 106 V/m) active over the thin expanse of the plasma membrane, with the capacity to influence a variety of downstream molecular signaling cascades. Moreover, in multicellular networks, intercellular coupling facilitated by gap junction channels may induce directed, electrodiffusive transport of charged molecules between cells of the network to generate new positional information patterning possibilities and characteristics. Given the demonstrated role of Vmem in morphogenesis, here we review current understanding of how Vmem can integrate with molecular regulatory networks to control single cell state, and the unique properties bioelectricity adds to transport phenomena in gap junction-coupled cell networks to facilitate self-assembly of morphogen gradients and other patterns. Understanding how Vmem integrates with biochemical regulatory networks at the level of a single cell, and mechanisms through which Vmem shapes molecular positional information in multicellular networks, are essential for a deep understanding of body plan control in development, regeneration and disease.

Controlling Nerve Growth with an Electric Field Induced Indirectly in Transparent Conductive Substrate Materials

Innovative neurostimulation therapies require improved electrode materials, such as poly(3,4-ethylenedioxythiophene) (PEDOT) polymers or IrO_x mixed ionic-electronic conductors and better understanding of how their electrochemistry influences nerve growth. Amphibian neurons growing on transparent films of electronic (metal) conductors and electronic-ionic conductors (polymers and semiconducting oxides) are monitored. Materials are not connected directly to the power supply, but a dipole is created wirelessly within them by electrodes connected to the culture medium in which they are immersed. Without electrical stimulation neurons grow on gold, platinum, PEDOT-polystyrene sulfonate (PEDOT-PSS), IrO_x , and mixed oxide (Ir-Ti)O_x , but growth is not related to surface texture or hydrophilicity. Stimulation induces a dipole in all conductive materials, but neurons grow differently on electronic conductors and mixed-valence mixed-ionic conductors. Stimulation slows, but steers neurite extension on gold but not on platinum. The rate and direction of neurite growth on PEDOT-PSS resemble that on glass, but on IrO_x and (Ir-Ti)O_x neurites grow faster and in random directions. This suggests electrochemical changes induced in these materials control growth speed and direction selectively. Evidence that the electric dipole induced in conductive material controls nerve growth will impact electrotherapies exploiting wireless stimulation of implanted material arrays, even where transparency is required.

The bioelectric code: An ancient computational medium for dynamic control of growth and form

What determines large-scale anatomy? DNA does not directly specify geometrical arrangements of tissues and organs, and a process of encoding and decoding for morphogenesis is required. Moreover, many species can regenerate and remodel their structure despite drastic injury. The ability to obtain the correct target morphology from a diversity of initial conditions reveals that the morphogenetic code implements a rich system of pattern-homeostatic processes. Here, we describe an important mechanism by which cellular networks implement pattern regulation and plasticity: bioelectricity. All cells, not only nerves and muscles, produce and sense electrical signals; in vivo, these processes form bioelectric circuits that harness individual cell behaviors toward specific anatomical endpoints. We review emerging progress in reading and re-writing anatomical information encoded in bioelectrical states, and discuss the approaches to this problem from the perspectives of information theory, dynamical systems, and computational neuroscience. Cracking the bioelectric code will enable much-improved control over biological patterning, advancing basic evolutionary developmental biology as well as enabling numerous applications in regenerative medicine and synthetic bioengineering.

The Role of Direct Current Electric Field-Guided Stem Cell Migration in Neural Regeneration

Effective directional axonal growth and neural cell migration are crucial in the neural regeneration of the central nervous system (CNS). Endogenous currents have been detected in many developing nervous systems. Experiments have demonstrated that applied direct current (DC) electric fields (EFs) can guide axonal growth in vitro, and attempts have been made to enhance the regrowth of damaged spinal cord axons using DC EFs in in vivo experiments. Recent work has revealed that the migration of stem cells and stem cell-derived neural cells can be guided by DC EFs. These studies have raised the possibility that endogenous and applied DC EFs can be used to direct neural tissue regeneration. Although the mechanism of EF-directed axonal growth and cell migration has not been fully understood, studies have shown that the polarization of cell membrane proteins and the activation of intracellular signaling molecules are involved in the process. The application of EFs is a promising biotechnology for regeneration of the CNS.

TiO2 surfaces support neuron growth during electric field stimulation

TiO₂ is proposed here for the first time as a substrate for neural prostheses that involve electrical stimulation. Several characteristics make TiO₂ an attractive material: Its electrochemical behaviour as an insulator prevents surface changes during stimulation. Hydration creates -OH groups at the surface, which aid cell adhesion by interaction with inorganic ions and macromolecules in cell membranes. Its ability to neutralize reactive oxygen and nitrogen species that trigger inflammatory processes confers biocompatibility properties in dark conditions. Here, physicochemical characterization of TiO₂ samples and their surfaces was carried out by X-ray diffraction, X-ray photoelectronic emission spectroscopy, scanning electron microscopy, atomic force microscopy and by contact angle measurements. Its properties were related to the growth parameters and morphology of amphibian spinal neurons cultured on TiO₂ samples. Neurons adhered to and extended neurites directly on TiO₂ surfaces without pre-coating with adhesive molecules, indicating that the material permits intimate neuron-surface interactions. On TiO₂ surfaces the distal tips of each extending neurite and the neurite shafts themselves showed more complex filopodial morphology compared with control cultures on glass. Importantly, the ability of TiO₂ to support neuron growth during electric field exposure was also tested. The extent of growth and the degree of neurite orientation relative to the electric field on TiO₂ approximated that on glass control substrates. Collectively, the data suggest that TiO₂ materials support neuron growth and that they have potential utility for neural prosthetic applications incorporating electric field stimulation, especially where intimate contact of neurons with the material is beneficial.

Electric Signals Regulate the Directional Migration of Oligodendrocyte Progenitor Cells (OPCs) via β1 Integrin

The guided migration of neural cells is essential for repair in the central nervous system (CNS). Oligodendrocyte progenitor cells (OPCs) will normally migrate towards an injury site to re-sheath demyelinated axons; however the mechanisms underlying this process are not well understood. Endogenous electric fields (EFs) are known to influence cell migration in vivo, and have been utilised in this study to direct the migration of OPCs isolated from neonatal Sprague-Dawley rats. The OPCs were exposed to physiological levels of electrical stimulation, and displayed a marked electrotactic response that was dependent on β1 integrin, one of the key subunits of integrin receptors. We also observed that F-actin, an important component of the cytoskeleton, was re-distributed towards the leading edge of the migrating cells, and that this asymmetric rearrangement was associated with β1 integrin function.

The effect of pulsed electric fields on the electrotactic migration of human neural progenitor cells through the involvement of intracellular calcium signaling

Endogenous electric fields (EFs) are required for the physiological control of the central nervous system development. Application of the direct current EFs to neural stem cells has been studied for the possibility of stem cell transplantation as one of the therapies for brain injury. EFs generated within the nervous system are often associated with action potentials and synaptic activity, apparently resulting in a pulsed current in nature. The aim of this study is to investigate the effect of pulsed EF, which can reduce the cytotoxicity, on the migration of human neural progenitor cells (hNPCs). We applied the mono-directional pulsed EF with a strength of 250mV/mm to hNPCs for 6h. The migration distance of the hNPCs exposed to pulsed EF was significantly greater compared with the control not exposed to the EF. Pulsed EFs, however, had less of an effect on the migration of the differentiated hNPCs. There was no significant change in the survival of hNPCs after exposure to the pulsed EF. To investigate the role of Ca²⁺ signaling in electrotactic migration of hNPCs, pharmacological inhibition of Ca²⁺ channels in the EF-exposed cells revealed that the electrotactic migration of hNPCs exposed to Ca²⁺ channel blockers was significantly lower compared to the control group. The findings suggest that the pulsed EF induced migration of hNPCs is partly influenced by intracellular Ca²⁺ signaling.

Physiological electrical signals promote chain migration of neuroblasts by up-regulating P2Y1 purinergic receptors and enhancing cell adhesion

Neuroblasts migrate as directed chains of cells during development and following brain damage. A fuller understanding of the mechanisms driving this will help define its developmental significance and in the refinement of strategies for brain repair using transplanted stem cells. Recently, we reported that in adult mouse there are ionic gradients within the extracellular spaces that create an electrical field (EF) within the rostral migratory stream (RMS), and that this acts as a guidance cue for neuroblast migration. Here, we demonstrate an endogenous EF in brain slices and show that mimicking this by applying an EF of physiological strength, switches on chain migration in mouse neurospheres and in the SH-SY5Y neuroblastoma cell line. Firstly, we detected a substantial endogenous EF of 31.8 ± 4.5 mV/mm using microelectrode recordings from explants of the subventricular zone (SVZ). Pharmacological inhibition of this EF, effectively blocked chain migration in 3D cultures of SVZ explants. To mimic this EF, we applied a physiological EF and found that this increased the expression of N-cadherin and β-catenin, both of which promote cell-cell adhesion. Intriguingly, we found that the EF up-regulated P2Y purinoceptor 1 (P2Y1) to contribute to chain migration of neuroblasts through regulating the expression of N-cadherin, β-catenin and the activation of PKC. Our results indicate that the naturally occurring EF in brain serves as a novel stimulant and directional guidance cue for neuronal chain migration, via up-regulation of P2Y1.

Directed migration of embryonic stem cell-derived neural cells in an applied electric field

Spinal cord injury or diseases, such as amyotrophic lateral sclerosis, can cause the loss of motor neurons and therefore results in the paralysis of muscles. Stem cells may improve functional recovery by promoting endogenous regeneration, or by directly replacing neurons. Effective directional migration of grafted neural cells to reconstruct functional connections is crucial in the process. Steady direct current electric fields (EFs) play an important role in the development of the central nervous system. A strong biological effect of EFs is the induction of directional cell migration. In this study, we investigated the guided migration of embryonic stem cell (ESC) derived presumptive motor neurons in an applied EF. The dissociated mouse ESC derived presumptive motor neurons or embryoid bodies were subjected to EFs stimulation and the cell migration was studied. We found that the migration of neural precursors from embryoid bodies was toward cathode pole of applied EFs. Single motor neurons migrated to the cathode of the EFs and reversal of EFs poles reversed their migration direction. The directedness and displacement of cathodal migration became more significant when the field strength was increased from 50 mV/mm to 100 mV/mm. EFs stimulation did not influence the cell migration velocity. Our work suggests that EFs may serve as a guidance cue to direct grafted cell migration in vivo.

Exploration of molecular pathways mediating electric field-directed Schwann cell migration by RNA-seq

In peripheral nervous systems, Schwann cells wrap around axons of motor and sensory neurons to form the myelin sheath. Following spinal cord injury, Schwann cells regenerate and migrate to the lesion and are involved in the spinal cord regeneration process. Transplantation of Schwann cells into injured neural tissue results in enhanced spinal axonal regeneration. Effective directional migration of Schwann cells is critical in the neural regeneration process. In this study, we report that Schwann cells migrate anodally in an applied electric field (EF). The directedness and displacement of anodal migration increased significantly when the strength of the EF increased from 50 mV/mm to 200 mV/mm. The EF did not significantly affect the cell migration speed. To explore the genes and signaling pathways that regulate cell migration in EFs, we performed a comparative analysis of differential gene expression between cells stimulated with an EF (100 mV/mm) and those without using next-generation RNA sequencing, verified by RT-qPCR. Based on the cut-off criteria (FC > 1.2, q < 0.05), we identified 1,045 up-regulated and 1,636 down-regulated genes in control cells versus EF-stimulated cells. A Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis found that compared to the control group, 21 pathways are down-regulated, while 10 pathways are up-regulated. Differentially expressed genes participate in multiple cellular signaling pathways involved in the regulation of cell migration, including pathways of regulation of actin cytoskeleton, focal adhesion, and PI3K-Akt.

ARP2/3 complex is required for directional migration of neural stem cell-derived oligodendrocyte precursors in electric fields

Introduction

The loss of oligodendrocytes in a lesion of the central nervous system causes demyelination and therefore impairs axon function and survival. Transplantation of neural stem cell-derived oligodendrocyte precursor cells (NSC-OPCs) results in increased oligodendrocyte formation and enhanced remyelination. The directional migration of grafted cells to the target can promote the establishment of functional reconnection and myelination in the process of neural regeneration. Endogenous electric fields (EFs) that were detected in the development of the central nervous system can regulate cell migration.

Methods

NSCs were isolated from the brains of ARPC2^+/+ and ARPC2^−/− mouse embryo and differentiated into OPCs. After differentiation, the cultured oligospheres were stimulated with EFs (50, 100, or 200 mV/mm). The migration of OPCs from oligospheres was recorded using time-lapse microscopy. The cell migration directedness and speed were analyzed and quantified.

Results

In this study, we found that NSC-OPCs migrated toward the cathode pole in EFs. The directedness and displacement of cathodal migration increased significantly when the EF strength increased from 50 to 200 mV/mm. However, the EF did not significantly change the cell migration speed. We also showed that the migration speed of ARPC2^−/− OPCs, deficient in the actin-related proteins 2 and 3 (ARP2/3) complex, was significantly lower than that of wild type of OPCs. ARPC2^−/− OPCs migrated randomly in EFs.

Conclusions

The migration direction of NSC-OPCs can be controlled by EFs. The function of the ARP complex is required for the cathodal migration of NSC-OPCs in EFs. EF-guided cell migration is an effective model to understanding the intracellular signaling pathway in the regulation of cell migration directness and motility.

Electronic supplementary material

The online version of this article (doi:10.1186/s13287-015-0042-0) contains supplementary material, which is available to authorized users.

A review of oscillating field stimulation to treat human spinal cord injury

OBJECTIVE

To report the results of use of a human oscillating field stimulator (OFS) in a phase 1 trial of 14 human patients with complete motor and sensory spinal cord injury.

METHODS

Entry criteria were complete spinal cord injury between C5 and T10 in patients 18-65 years old with no transection on magnetic resonance imaging. All patients received the National Acute Spinal Cord Injury Study III methylprednisolone protocol. Cord compression or instability was treated before entry. All patient injuries remained complete (based on American Spinal Cord Injury scoring) with no somatosensory evoked potentials (SSEPs) below the injury after surgery or for 48 hours. All patients were implanted with the OFS within 18 days. Patients were checked every 2 weeks after implantation. The OFS was explanted at 15 weeks. Independent neurologic examinations (American Spinal Cord Injury score, visual analog scale for pain, and SSEPs) were done at 6 weeks, 6 months, and 1 year. Statistical analyses were done by Wilcoxon rank sum test and analysis of variance (ANOVA).

RESULTS

There were no complications at insertion, and one wound infection occurred after explant for a 3.5% infection rate. One patient was lost to follow-up after 6 months. All 14 patients had a mean visual analog scale score of 8 at implant and 2 at 6 months, and 13 remained a mean score of 2 at 1 year. Mean improvement in light touch score at 1 year was 25.9 points (ANOVA, P < 0.001; Wilcoxon, P = 0.02). Mean improvement in pinprick score at 1 year was 15.2 points (ANOVA, P < 0.001; Wilcoxon, P = 0.02). Mean improvement in motor score was 6.9 (ANOVA, P < 0.01; Wilcoxon, P = 0.02). Of eight patients with cervical cord injuries, six had improvement in arm SSEPs, and one recovered a tibial SSEP. Of six patients with thoracic injuries, one recovered an abnormal lower SSEP.

CONCLUSIONS

Treatment of human spinal cord injury with an OFS is safe, reliable, and easy. Compared with National Acute Spinal Cord Injury Study III compliant paralyzed patients, our results suggest efficacy.

Regulation of cell behavior and tissue patterning by bioelectrical signals: challenges and opportunities for biomedical engineering

Achieving control over cell behavior and pattern formation requires molecular-level understanding of regulatory mechanisms. Alongside transcriptional networks and biochemical gradients, there functions an important system of cellular communication and control: transmembrane voltage gradients (V(mem)). Bioelectrical signals encoded in spatiotemporal changes of V(mem) control cell proliferation, migration, and differentiation. Moreover, endogenous bioelectrical gradients serve as instructive cues mediating anatomical polarity and other organ-level aspects of morphogenesis. In the past decade, significant advances in molecular physiology have enabled the development of new genetic and biophysical tools for the investigation and functional manipulation of bioelectric cues. Recent data implicate V(mem) as a crucial epigenetic regulator of patterning events in embryogenesis, regeneration, and cancer. We review new conceptual and methodological developments in this fascinating field. Bioelectricity offers a novel way of quantitatively understanding regulation of growth and form in vivo, and it reveals tractable, powerful control points that will enable truly transformative applications in bioengineering, regenerative medicine, and synthetic biology.

Electric field-guided neuron migration: a novel approach in neurogenesis

Effective directional neuron migration is crucial in development of the central nervous system and for neurogenesis. Endogenous electrical signals are present in many developing systems and crucial cellular behaviors such as neuronal cell division, cell migration, and cell differentiation are all under the influence of such endogenous electrical cues. Preclinical in vivo studies have used electric fields (EFs) to attempt to enhance regrowth of damaged spinal cord axons with some success. Recent evidence shows that small EFs not only guide axonal growth, but also direct the earlier events of neuronal migration and neuronal cell division. This raises the possibility that applied or endogenous EFs, perhaps in combination, may direct transplanted neural stem cells, or regenerating neurons, to the desired site after brain injury or neuron degeneration. The high complexity of both structure and function of the nervous system, however, poses significant challenges to techniques for applying EFs to promote neurogenesis. The evolution of functional biomaterials and nanotechnology may provide promising solutions for the application of EFs in guiding neuron migration and neurogenesis within the central nervous system.

The influence of electric fields on hippocampal neural progenitor cells

The differentiation and proliferation of neural stem/progenitor cells (NPCs) depend on various in vivo environmental factors or cues, which may include an endogenous electrical field (EF), as observed during nervous system development and repair. In this study, we investigate the morphologic, phenotypic, and mitotic alterations of adult hippocampal NPCs that occur when exposed to two EFs of estimated endogenous strengths. NPCs treated with a 437 mV/mm direct current (DC) EF aligned perpendicularly to the EF vector and had a greater tendency to differentiate into neurons, but not into oligodendrocytes or astrocytes, compared to controls. Furthermore, NPC process growth was promoted perpendicularly and inhibited anodally in the 437 mV/mm DC EF. Yet fewer cells were observed in the DC EF, which in part was due to a decrease in cell viability. The other EF applied was a 46 mV/mm alternating current (AC) EF. However, the 46 mV/mm AC EF showed no major differences in alignment or differentiation, compared to control conditions. For both EF treatments, the percent of mitotic cells during the last 14 h of the experiment were statistically similar to controls. Reported here, to our knowledge, is the first evidence of adult NPC differentiation affected in an EF in vitro. Further investigation and application of EFs on stem cells is warranted to elucidate the utility of EFs to control phenotypic behavior. With progress, the use of EFs may be engineered to control differentiation and target the growth of transplanted cells in a stem cell-based therapy to treat nervous system disorders.

Hormetic electric field theory of pattern formation

The hormetic morphogen theory of curvature (Fosslien 2009) proposes that hormetic morphogen concentration gradients modulate the synthesis of adenosine triphosphate (ATP) by cells along the gradients (field cells) and thus regulate their proliferation and induce curvature such as vascular wall curvature; however, it is unclear whether such morphogen gradients can also determine the histological pattern of the walls. Here, I propose that the ATP gradients modulate export of H(+) by vacuolar H(+)-ATPase (V-ATPase) located on the surface of field cells and generate extracellular ion concentration gradients, ion currents and electrical fields along the paths of morphogen gradients. In vitro, electrical fields can induce directional migration and elongation of vascular cells and align the cells with their long axis perpendicular to electrical field vectors (Bai et al. 2004). I suggest that likewise, in vivo vascular transmural electrical fields induced by hormetic morphogen concentration gradients can modulate cell shape i.e. cell elongation and cell curvature, and determine cell orientation. Moreover, I suggest that the electrical fields can modulate bidirectional cell migration and cell sorting via dynamic hormetic galvanotaxis analogous to in vitro isoelectric focusing in proton gradients, thus, hormetic morphogen gradients can determine the curvature of vessel walls and their histological patterns.

Electrical dimensions in cell science

Cells undergo a variety of physiological processes, including division, migration and differentiation, under the influence of endogenous electrical cues, which are generated physiologically and pathologically in the extracellular and sometimes intracellular spaces. These signals are transduced to regulate cell behaviours profoundly, both in vitro and in vivo. Bioelectricity influences cellular processes as fundamental as control of the cell cycle, cell proliferation, cancer-cell migration, electrical signalling in the adult brain, embryonic neuronal cell migration, axon outgrowth, spinal-cord repair, epithelial wound repair, tissue regeneration and establishment of left-right body asymmetry. In addition to direct effects on cells, electrical gradients interact with coexisting extracellular chemical gradients. Indeed, cells can integrate and respond to electrical and chemical cues in combination. This Commentary details how electrical signals control multiple cell behaviours and argues that study of the interplay between combined electrical and chemical gradients is underdeveloped yet necessary.

Perpendicular organization of sympathetic neurons within a required physiological voltage

In vitro, postganglionic sympathetic neurons (PSNs) profoundly organize their anatomy according to cues provided by an extracellular voltage. Over 90% of PSNs retract neurites that are parallel/tangential to a gradient of approximately 400 mV/mm. Complete neurite retraction takes approximately 20-40 minutes. Subsequently, neurites grow out from the soma, but now perpendicular to the lines of force while branching profusely. The complete restructuring of the neurons anatomy takes 2-3 hours at 35 degrees C. The maintenance of this asymmetrical anatomy requires the continuous presence of the extracellular electrical field (Ef). We discuss this observation relative to the organization of neurons residing in natural voltage gradients that exist across all epithelia in which neurons are born, mature, or migrate.

Bioelectric mechanisms in regeneration: Unique aspects and future perspectives

Regenerative biology has focused largely on chemical factors and transcriptional networks. However, endogenous ion flows serve as key epigenetic regulators of cell behavior. Bioelectric signaling involves feedback loops, long-range communication, polarity, and information transfer over multiple size scales. Understanding the roles of endogenous voltage gradients, ion flows, and electric fields will contribute to the basic understanding of numerous morphogenetic processes and the means by which they can robustly restore pattern after perturbation. By learning to modulate the bioelectrical signals that control cell proliferation, migration, and differentiation, we gain a powerful set of new techniques with which to manipulate growth and patterning in biomedical contexts. This chapter reviews the unique properties of bioelectric signaling, surveys molecular strategies and reagents for its investigation, and discusses the opportunities made available for regenerative medicine.

Why the embryo still matters: CSF and the neuroepithelium as interdependent regulators of embryonic brain growth, morphogenesis and histiogenesis

The key focus of this review is that both the neuroepithelium and embryonic cerebrospinal fluid (CSF) work in an integrated way to promote embryonic brain growth, morphogenesis and histiogenesis. The CSF generates pressure and also contains many biologically powerful trophic factors; both play key roles in early brain development. Accumulation of fluid via an osmotic gradient creates pressure that promotes rapid expansion of the early brain in a developmental regulated way, since the rates of growth differ between the vesicles and for different species. The neuroepithelium and ventricles both contribute to this growth but by different and coordinated mechanisms. The neuroepithelium grows primarily by cell proliferation and at the same time the ventricle expands via hydrostatic pressure generated by active transport of Na(+) and transport or secretion of proteins and proteoglycans that create an osmotic gradient which contribute to the accumulation of fluid inside the sealed brain cavity. Recent evidence shows that the CSF regulates relevant aspects of neuroepithelial behavior such as cell survival, replication and neurogenesis by means of growth factors and morphogens. Here we try to highlight that early brain development requires the coordinated interplay of the CSF contained in the brain cavity with the surrounding neuroepithelium. The information presented is essential in order to understand the earliest phases of brain development and also how neuronal precursor behavior is regulated.

Electromagnetic effects - From cell biology to medicine

In this review we compile and discuss the published plethora of cell biological effects which are ascribed to electric fields (EF), magnetic fields (MF) and electromagnetic fields (EMF). In recent years, a change in paradigm took place concerning the endogenously produced static EF of cells and tissues. Here, modern molecular biology could link the action of ion transporters and ion channels to the "electric" action of cells and tissues. Also, sensing of these mainly EF could be demonstrated in studies of cell migration and wound healing. The triggers exerted by ion concentrations and concomitant electric field gradients have been traced along signaling cascades till gene expression changes in the nucleus. Far more enigmatic is the way of action of static MF which come in most cases from outside (e.g. earth magnetic field). All systems in an organism from the molecular to the organ level are more or less in motion. Thus, in living tissue we mostly find alternating fields as well as combination of EF and MF normally in the range of extremely low-frequency EMF. Because a bewildering array of model systems and clinical devices exits in the EMF field we concentrate on cell biological findings and look for basic principles in the EF, MF and EMF action. As an outlook for future research topics, this review tries to link areas of EF, MF and EMF research to thermodynamics and quantum physics, approaches that will produce novel insights into cell biology.

Small applied electric fields guide migration of hippocampal neurons

Effectively directed neuron migration is critical for development and repair in the central nervous system (CNS). Endogenous electric fields (EFs) are widespread in developing and regenerating tissues and regulate a variety of cell behaviors including directed cell migration. Electrically-directed neuronal migration has not been tested previously and we show that an applied EF directs migration of hippocampal neurons toward the cathode at a field strength of 120 mV/mm, close to the physiological range. Reversal of the field polarity reversed the direction of neuron migration. Neuron migration from an explant also was directed by an applied EF. Mechanistically, EF-guided migration was transduced by activation of the second messenger molecules ROCK (Rho-associated protein kinase) and PI3 kinase (phosphoinositide-3 kinase) since their pharmacological inhibition decreased the directedness and speed of neuron migration. This work demonstrates that rat hippocampal neurons respond to applied EFs with directional migration and raises the possibility that EFs may be used as a cue to direct neuronal migration in novel strategies to repair the CNS.

Electric field effects on human spinal injury: Is there a basis in the in vitro studies?

An important basis for the clinical application of small DC electric current to mammalian spinal injury is the responses of neurons in culture to applied electric fields. Our recent finding that zebrafish neurons were unresponsive to applied fields prompted us to critically examine previous results. We conclude that compelling evidence for neuronal guidance and directional stimulation of growth toward either the cathode or anode in an electric field exists only for cultured Xenopus neurons, and not for any mammalian neurons. No basis for the reported success in treating spinal injury exists in the in vitro studies, and considerable research will be required if the conditions of field application in mammalian spinal injury are to be optimized.

Temporally and spatially coordinated roles for Rho, Rac, Cdc42 and their effectors in growth cone guidance by a physiological electric field

Although it is known that neuronal growth cones migrate towards the cathode of an applied direct current (DC) electric field (EF), resembling the EF present in the developing nervous system, the underlying mechanism remains unclear. Here, we demonstrate temporally and spatially coordinated roles for the GTPases Rac, Cdc42 and Rho and their effectors. Growth cones of cultured Xenopus embryonic spinal neurons turned towards the cathode but collective inhibition of Rho, Rac and Cdc42 attenuated turning. Selective inhibition of Rho, Cdc42 or Rac signalling revealed temporally distinct roles in steering by an electrical gradient. Rho, Rac and Cdc42 are each essential for turning within the initial 2 hours (early phase). Later, Rho and Cdc42 signals remain important but Rac signalling dominates. The EF increased Rho immunofluorescence anodally. This correlated spatially with collapsed growth cone morphology and reduced anodal migration rates, which were restored by Rho inhibition. These data suggest that anodally increased Rho activity induces local cytoskeletal collapse, biasing growth cone advance cathodally. Collapse might be mediated by the Rho effectors p160 Rho kinase and myosin light chain kinase since their inhibition attenuated early turning. Inhibitors of phosphoinositide 3-kinase, MEK1/2 or p38 mitogen-activated protein kinase (MAPK) did not affect turning behaviour, eliminating them mechanistically. We propose a mechanism whereby Rac and Cdc42 activities dominate cathodally and Rho activity dominates anodally to steer growth cones towards the cathode. The interaction between Rho GTPases, the cytoskeleton and growth cone dynamics is explored in the companion paper published in this issue. Our results complement studies of growth cone guidance by diffusible chemical gradients and suggest that growth cones might interpret these co-existing guidance cues selectively.

Growth cone steering by a physiological electric field requires dynamic microtubules, microfilaments and Rac-mediated filopodial asymmetry

Electric fields (EFs) resembling those in the developing and regenerating nervous systems steer growth cones towards the cathode. Requirements for actin microfilaments, microtubules and their interactions during EF growth cone steering have been presumed, but remain unproven. Here, we demonstrate essential roles for dynamic microfilaments and microtubules in cathode-directed migration. Cathodal turning of growth cones on cultured Xenopus embryonic spinal neurons was attenuated significantly by nanomolar concentrations of the microfilament inhibitor latrunculin, the microtubule-stabilising drug taxol, or the microtubule-destabilising drugs vinblastine or nocodazole. Dynamically, the cathodal bias of filopodia preceded cathodal turning of the growth cone, suggesting an instructive role in EF-induced steering. Lamellipodial asymmetry accompanied turning. Filopodia and lamellipodia are regulated by the GTPases Cdc42 and Rac, respectively, and, as shown in the companion paper in this issue, peptides that selectively prevented effector binding to the CRIB domains of Cdc42 or Rac abolished cathodal growth cone turning during 3 hours of EF exposure. Here, the Rac peptide suppressed lamellipodium formation, increased the number of filopodia, abolished cathodal filopodial orientation, and prevented cathodal steering. The Cdc42 peptide suppressed filopodium formation, increased lamellipodial area and prevented cathodal steering. The cathodal bias of lamellipodia was independent of Cdc42 CRIB activity and was not sufficient for cathodal steering in the absence of filopodia, but the cathodal bias of filopodia through Rac CRIB activity was necessary for cathodal turning. Understanding the mechanism for cathodal growth cone guidance will enhance the emerging clinical effort to stimulate human spinal cord regeneration through EF application.

Nerve regeneration and wound healing are stimulated and directed by an endogenous electrical field in vivo

Biological roles for naturally occurring, extracellular physiological electric fields have been proposed over the past century. However, in the molecular era, many biologists presume that electric fields have little physiological relevance because there has been no unequivocal demonstration of their importance at the single-cell level in vivo. We have used an in vivo rat corneal model, which generates its own endogenous electric field and show that nerve sprouting, the direction of nerve growth and the rate of epithelial wound healing are controlled coordinately by the wound-induced electric field.

Controlling cell behavior electrically: current views and future potential

Direct-current (DC) electric fields are present in all developing and regenerating animal tissues, yet their existence and potential impact on tissue repair and development are largely ignored. This is primarily due to ignorance of the phenomenon by most researchers, some technically poor early studies of the effects of applied fields on cells, and widespread misunderstanding of the fundamental concepts that underlie bioelectricity. This review aims to resolve these issues by describing: 1) the historical context of bioelectricity, 2) the fundamental principles of physics and physiology responsible for DC electric fields within cells and tissues, 3) the cellular mechanisms for the effects of small electric fields on cell behavior, and 4) the clinical potential for electric field treatment of damaged tissues such as epithelia and the nervous system.

Oscillating field stimulation for complete spinal cord injury in humans: a phase 1 trial

OBJECT

An electrical field cathode (negative pole) has trophic and tropic effects on injured spinal cord axons in in vitro and in vivo models of sea lamprey, rodent, and canine spinal cord injury (SCI) and it improves functional outcome. A human oscillating field stimulator (OFS) was built, a Food and Drug Administration (FDA) exemption number was obtained, and institutional review board approval was given for a Phase 1 trial to study 10 humans with complete motor and sensory SCI.

METHODS

Entry criteria were complete SCI between C-5 and T-10 in patients 18 to 65 years of age and no transection demonstrated on magnetic resonance imaging. All participants received the National Acute Spinal Cord Injury Study (NASCIS) III methylprednisilone protocol. Cord compression and/or vertebral instability was treated before study entry. After treatment complete SCI (according to the American Spinal Injury Association [ASIA] score) remained in all patients with no somatosensory evoked potentials (SSEPs) below the injury level after surgery or for 48 hours. All patients underwent implantation of the OFS within 18 days. Patients underwent evaluation every 2 weeks postimplantation; the OFS was explanted at 15 weeks. Independent neurological status was assessed based on the ASIA score, visual analog scale (VAS) pain score, and SSEPs at 6 weeks, 6 months, and 1 year. Statistical analyses were performed using the two-tailed Wilcoxon test and analysis of variance (ANOVA). There were no complications at insertion of the OFS; there was one case of wound infection after explantation (5% infection rate). One patient was lost to follow up after 6 months. In all 10 patients the mean VAS pain score was 8 at implantation, 2 at 6 months, and in the nine attending follow up for 1 year it remained 2. At 1 year, the mean improvement in light touch was 25.5 points (ANOVA p < 0.001, Wilcoxon test p = 0.02), the mean improvement in pinprick sensation was 20.4 points (ANOVA p < 0.001, Wilcoxon test p = 0.02), and the mean improvement in motor status was 6.3 points (ANOVA p < 0.01, Wilcoxon test p = 0.02). Of five cases involving cervical cord injuries, bilateral upper-extremity SSEPs were normal in one, unilateral upper-extremity SSEPs were recovered in four, bilateral upper-extremity SSEPs were recovered in one, and abnormal lower-extremity SSEPs resolved in one case. In one of the five cases involving thoracic injuries an abnormal lower-extremity SSEP resolved.

CONCLUSIONS

The use of OFS treatment in patients with SCI is safe, reliable, and easy. Compared with the outcomes obtained in compliant NASCIS III plegic patients, the results of the present study indicate efficacy, and the FDA has given permission for enrollment of 10 additional patients.

Bioelectromagnetics in morphogenesis

Understanding the factors that allow biological systems to reliably self-assemble consistent, highly complex, four dimensional patterns on many scales is crucial for the biomedicine of cancer, regeneration, and birth defects. The role of chemical signaling factors in controlling embryonic morphogenesis has been a central focus in modern developmental biology. While the role of tensile forces is also beginning to be appreciated, another major aspect of physics remains largely neglected by molecular embryology: electromagnetic fields and radiations. The continued progress of molecular approaches to understanding biological form and function in the post genome era now requires the merging of genetics with functional understanding of biophysics and physiology in vivo. The literature contains much data hinting at an important role for bioelectromagnetic phenomena as a mediator of morphogenetic information in many contexts relevant to embryonic development. This review attempts to highlight briefly some of the most promising (and often underappreciated) findings that are of high relevance for understanding the biophysical factors mediating morphogenetic signals in biological systems. These data originate from contexts including embryonic development, neoplasm, and regeneration.

Has electrical growth cone guidance found its potential?

Many neurobiologists spurn the existence and use of direct-current (DC) electric fields (EFs) in nervous system development and regeneration. This is despite direct measurement of EFs in embryos and adults, and evidence that EFs are required for normal development, dramatically influence the rate and direction of nerve growth in vitro, and promote nerve regeneration in vivo. The notion that growth cones use EFs as guidance cues was dismissed partly because there was no convincing evidence that naturally occurring EFs influence nerve growth at the single-cell level in vivo. Recent work indicates that growth cones can be guided by EFs in vivo and, intriguingly, that in vitro guidance by chemotropic gradients and EFs might invoke similar mechanisms. Ongoing clinical trials to assess the effectiveness of DC EFs in promoting the regeneration of human spinal cord could allow EFs to achieve their potential.

The cytological implications of primary respiration

Observing the macroscopic complexities of evolved species, the exceptional continuity that occurs among different cells, tissues and organs to respond coherently to the proper set of stimuli as a function of self/species survival is appreciable. Accordingly, it alludes to a central rhythm that resonates throughout the cell; nominated here as primary respiration (PR), which is capable of binding and synchronizing a diversity of physiological processes into a functional biological unity. Phylogenetically, it was conserved as an indispensable element in the makeup of the subkingdom Metazoa, since these species require a high degree of coordination among the different cells that form their body. However, it does not preclude the possibility of a basal rhythm to orchestrate the intricacies of cellular dynamics of both prokaryotic and eukaryotic cells. In all probability, PR emerges within the crucial organelles, with special emphasis on the DNA (5), and propagated and transduced within the infrastructure of the cytoskeleton as wave harmonics (49). Collectively, this equivalent vibration for the subphylum Vertebrata emanates as craniosacral respiration (CSR), though its expression is more elaborate depending on the development of the CNS. Furthermore, the author suggests that the phenomenon of PR or CSR be intimately associated to the basic rest/activity cycle (BRAC), generated by concentrically localized neurons that possess auto-oscillatory properties and assembled into a vital network (39). Historically, during Protochordate-Vertebrate transition, this area circumscribes an archaic region of the brain in which many vital biological rhythms have their source, called hindbrain rhombomeres. Bass and Baker (2) propose that pattern-generating circuits of more recent innovations, such as vocal, electromotor, extensor muscle tonicity, locomotion and the extraocular system, have their origin from the same Hox gene-specified compartments of the embryonic hindbrain (rhombomeres 7 and 8) that produce rhythmically active cardiac and thoracic respiratory circuits. Here, it implies that PR could have been the first essential biological cadence that arose with the earliest form of life, and has undergone a phylogenetic ascent to produce an integrated multirhythmic organism of today. Finally, in its full manifestation, the breathing DNA (1) of the zygote could project itself throughout the cytoskeleton and modify the electromechanical properties of the plasma lamella (26), establishing the primordial axial-voltage gradients for the physiological control of development (53).

Neurotrophins enhance electric field-directed growth cone guidance and directed nerve branching

Neurotrophins play major roles in the developing nervous system in controlling neuronal differentiation, neurite outgrowth, guidance and branching, synapse formation and maturation, and neuronal survival or death. There is increasing evidence that nervous system construction takes place in the presence of dc electric fields, which fluctuate dynamically in space and time during embryonic development. These have their origins in the neural tube itself, as well as in surrounding skin and gut. Early disruption of these endogenous electric fields causes failure of the nervous system to form, or else it forms aberrantly. Nerve growth, guidance, and branching are controlled tightly during pathway construction and in vitro dc electric fields have profound effects on each of these behaviours. We have used cultured neurones to ask whether neurotrophins and dc electric fields might interact in shaping neuronal growth, given their coexistence in vivo. Electric field-directed nerve growth generally was enhanced by the simultaneous presentation of several neurotrophins to the growth cone. Under certain circumstances, more nerves turned cathodally, they turned faster, further, and in lower field strengths. Intriguingly, other combinations of dc electric field and neurotrophins (low field strength and neurotrophin 3 (NT-3) switched the direction of growth cone turning. Additionally, cathodally directed nerve growth was faster and directed branching was much more common when electric fields and neurotrophins interacted with neuronal growth cones. Given such profound changes in growth cone behaviour in vitro, neurotrophins and endogenous electric fields are likely to interact in vivo.

Disruption of proteoglycans in neural tube fluid by beta-D-xyloside alters brain enlargement in chick embryos

Following neurulation, the anterior end of the neural tube undergoes a dramatic increase in size due mainly to the enlarging of the brain cavity. This cavity is filled with so-called neural tube fluid (NTF), whose positive pressure has been shown to play a key role in brain morphogenesis. This fluid contains a water-soluble matrix, rich in chondroitin sulfate (CS), which has been proposed as an osmotic regulator of NTF pressure genesis. The purpose of the present study is to observe the influence of CS on NTF osmolality and its relation to NTF hydrostatic pressure and brain expansion. NTF was obtained by means of microaspiration from the mesencephalic cavity of chick embryos. The osmolality of NTF between H.H. stages 20 and 29 was measured on the basis of its cryoscopic point. CS synthesis was disrupted by using beta-D-xyloside and the induced variations in brain volume were measured by means of morphometry. We also measured the variations in NTF osmolality, hydrostatic pressure, and the concentration of CS and sodium induced by means of beta-D-xyloside. Our data reveal that, at the earliest stages of development analyzed, variations in NTF osmolality show a characteristic pattern that coincides with the developmental changes in the previously described fluid pressure. Chick embryos treated with beta-D-xyloside, a chemical that disrupts CS synthesis, displayed a notable increase in brain volume but no other apparent developmental alterations. Morphometric analysis revealed that this increase was due to hyperenlargement of the brain cavity. Beta-D-xyloside brings about specific changes in the biochemical composition of NTF, which entails a large increase in CS concentration, mainly in the form of free chains, and in that of sodium. As a result, the fluid's osmolality and brain intraluminal pressure increased, which could account for the increase in size of the brain anlage. These data support the hypothesis that the intraluminal pressure involved in embryonic brain enlargement is directly dependent on NTF osmolality, and that the concentrations of CS and its associated microions could play a key role in the regulation of this process.

The direction of neurite growth in a weak DC electric field depends on the substratum: contributions of adhesivity and net surface charge

We investigated the influence of the growth surface on the direction of Xenopus spinal neurite growth in the presence of a dc electric field of physiological magnitude. The direction of galvanotropism was determined by the substratum; neurites grew toward the negative electrode (cathode) on untreated Falcon tissue culture plastic or on laminin substrata, which are negatively charged, but neurites growing on polylysine, which is positively charged, turned toward the positive electrode (anode). Growth was oriented randomly on all substrata without an electric field. We tested the hypothesis that the charge of the growth surface was responsible for reversed galvanotropism on polylysine by growing neurons on tissue culture dishes with different net surface charges. Although neurites grew cathodally on both Plastek substrata, the frequency of anodal turning was greater on dishes with a net positive charge (Plastek C) than on those with a net negative charge (Plastek M). The charge of the growth surface therefore influenced the frequency of anodal galvanotropism but a reversal in surface charge was insufficient to reverse galvanotropism completely, possibly because of differences in the relative magnitude of the substratum charge densities. The influence of substratum adhesion on galvanotropism was considered by growing neurites on a range of polylysine concentrations. Growth cone to substratum adhesivity was measured using a blasting assay. Adhesivity and the frequency of anodal turning were graded over the range of polylysine concentrations (0 = 0.1 < 1 < 10 = 100 microg/ml). The direction of neurite growth in an electric field is therefore influenced by both substratum charge and growth cone-to-substratum adhesivity. These data are consistent with the idea that spatial or temporal variation in the expression of adhesion molecules in embryos may interact with naturally occurring electric fields to enhance growth cone pathfinding.

Physiological electrical fields modify cell behaviour

Steady direct current (dc) electric fields exist in many biological systems over many hours. At these times cells are dividing, differentiating, moving to final locations and extending motile processes. Each of these events may be influenced by physiological electric fields in tissue culture and when electric fields are disrupted in vivo, major developmental abnormalities arise. The likelihood of physiological electric fields playing a role in cell behaviours and some potential mechanisms are outlined.

Uncoupling histogenesis from morphogenesis in the vertebrate embryo by collapse of the transneural tube potential

We have shown that unidirectional pumping of Na+ out of the neural tube's luminal fluids in amphibian embryos produces a large potential difference (40-90 mV, lumen negative to the abluminal surface). This transneural tube potential (TNTP) is analogous to the Na+ dependent transepithelial potential (TEP) that exists across surface ectoderm. This TEP is retained in ectoderm after it is internalized when the neural folds fuse to form the neural tube. The TNTP can be markedly reduced for several hours by injection of the Na+ channel blockers amiloride or benzamil into the lumen by iontophoresis through microelectrodes. Here we describe the effect of TNTP modification on developmental anatomy. Axolotl embryos possessing a fused and closed neural tube (stage 21-23) were injected with either amiloride or benzamil and allowed to continue development for 36-52 hr. These were compared to control embryos injected with vehicle alone, or to embryos in which amiloride or benzamil was iontophoresed just beneath surface ectoderm. All embryos in which the TNTP was reduced were grossly defective. These were characterized by a disaggregation of the cells comprising the structures that had already begun to form (otic primordia, brain, spinal cord, notochord) as well as a failure in the development of new structures. Remarkably, some of these embryos displayed continuing development of external form in the complete absence of concomitant internal histogenesis. We discuss the ways in which a large endogenous voltage gradient associated with an epithelial potential difference (the TNTP) may be required both for the structural integrity of the early neuroepithelium, and a prerequisite for normal morphogenesis.

A computerized 2-dimensional vibrating probe for mapping extracellular current patterns

We describe a computer-assisted 2-dimensional vibrating probe system for mapping endogenous electric current patterns in biological preparations. This system overcomes some of the main limitations of the original 1-dimensional vibrating probe design and adds several new capabilities. Two piezo-electric bender elements mounted perpendicularly are used to vibrate the probe in a circle by applying 2 sine waves (1 to each element) that are 90 degrees out-of-phase with each other. The circular rotation of the probe allows it to detect simultaneously the 2 orthogonal components of a current in the horizontal plane. The voltages measured by the probe are digitized and analyzed by a computer and are used to calculate a current vector. A graphical representation of the current vector is then superimposed on a video image of the experimental preparation. This probe system responds to known currents in the expected manner and exhibits a low inherent noise level. Also included in this paper are some preliminary measurements made with this instrument on neurulating Xenopus embryos and on transected larval sea lamprey (Petromyzon marinus) spinal cords.