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R B Borgens

Publications and source records attributed to R B Borgens.

At least 19 recordsLinked to original sources
PubMed1997

The responses of mammalian spinal axons to an applied DC voltage gradient.

We have imposed a steady, rostrally negative, weak (ca 0.4 mV/mm) voltage gradient across transections of ascending white matter tracts in the adult guinea pig using an implanted stimulator and electrodes for about 1 month. We have evaluated the projections of these axons relative to the transection approximately 2 months postinjury by anterograde transport of injected tetramethylrhodamine-conjugated dextran and the use of an indwelling marker device which locates the plane of the original transection. Tract tracing was accomplished with conventional epifluorescence microscopy and confocal laser microscopy. Sham-treated control spinal cords contained well-filled lateral and dorsal column ascending tracts terminating caudal to the lesion which formed at the level of the hemisection. Electric field-treated spinal cords contained similarly labeled columns of axons that penetrated the lesion within the caudal segment of the spinal cord, branched within it, and in some cases such branches projected across the plane of transection. Ascending axons also passed around the lesion through undamaged parenchyma, branched repeatedly at the plane of the hemisection, and passed into the rostral segment of the spinal cord. Spear-shaped endings typical of growth cones were found at the terminals of these processes which often branched again within the rostral segment. Centrally projecting fibers, their processes, and the overall level of branching in these projections was not observed in our previous studies using high molecular weight horseradish peroxidase tracers.

Animals↗
PubMed1996

Reduction of the current of injury leaving the amputation inhibits limb regeneration in the red spotted newt.

Immediately following amputation of the limb in salamanders, a strong, steady, and polarized flow of ionic current is produced by the injury. Current flows in a proximodistal direction within the limb stump and is associated with a fall in electrical potential of about 50 mV/mm near the stump's end. This current is electrogenically driven by the Na(+)-dependent, internally positive transcutaneous voltage of the intact skin of the limb stump. Reduction of this EMF, the skin's battery, by topical application of Na+ blocking agents leads to inhibition or disruption of normal limb regeneration. This suggests electrical factors are a critical control of limb regeneration. Here we test another means to reduce the injury current and its associated electrical field within the forelimb stump of red spotted newts. A fine (40 gauge), insulated, multistrand wire was inserted beneath the skin of the animal's back, with the uninsulated portion terminating either at the shoulder region or at the base of the tail. When this cathodal (negative) electrode is connected to a regulated current source, sufficient current was pulled into the stump end from an external anode (placed in the water the animal was immersed in) to markedly reduce or null the endogenous current for the first 8 days following amputation. The extent of limb regeneration in sham-treated and experimentally treated animals was determined 1 month following amputation at the elbow. Sham-treated animals regenerated normally, with most producing digits within this time. Limb regeneration was completely arrested, or caused to be strikingly hypomorphic, in half of the experimentally treated animals. This effect was independent of where the subcutaneous electrode was placed and suggests that electrical (physiological) factors are indeed a critical control of limb regeneration in urodeles.

Animals↗
PubMed1995

Three-dimensional gradients of voltage during development of the nervous system as invisible coordinates for the establishment of embryonic pattern.

We are interested in the generation of endogenous electric fields associated with ionic currents driven through the vertebrate embryo by the transepithelial potential of its surface ectoderm. Using a non-invasive vibrating electrode for the measurement of ionic current, we have provided measurements of currents traversing amphibian embryos, and a preliminary report of the internal, extracellular voltage gradient under the neural plate which polarizes the embryo in the rostral/caudal axis (Metcalf et al. [1994] J. Exp. Zool. 268:307-322). Here we complete a description of this gradient in electrical potential (ca. 10 mV/mm, caudally negative), describe a simultaneous gradient organized in the medial/lateral axis (ca. 5-18 mV/mm, negative at the margins of the neural folds), and describe their appearance and disappearance during ontogeny of the axolotl embryo. Both voltage gradients are not expressed until neurulation, and disappear at its climax. This appearance and disappearance correlates with the shunting of current out of the lateral margins of the neural folds in rostral regions of the embryo beginning at stage 15, and is not associated with a more substantial current leak from the blastopore which appears at gastrulation. A steady blastopore current is still present after neural tube formation when intra-embryonic electric fields have been extinguished. We discuss the direct experimental tests supporting the hypothesis that these extracellular electric fields both polarize the early vertebrate embryo and serve as cues for morphogenesis and pattern.

Animals↗
PubMed1995

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.

Ambystoma↗
PubMed1995

Plasma and cerebrospinal fluid concentrations of 4-aminopyridine following intravenous injection and metered intrathecal delivery in canines.

Potassium channel blockade by 4-aminopyridine (4-AP) has been shown to initiate modest levels of functional recovery in spinal-injured dogs and people following intravenous administration; however, the relevant central nervous system (CNS) concentration mediating these effects is not known. We have determined the concentrations of 4-aminopyridine in plasma and cerebrospinal fluid following intravenous administration (0.5 mg/kg) in large (> 22 kg) dogs, using liquid column chromatography. Plasma levels are initially high (> 1 microgram/mL) and fall rapidly to levels less than 100 ng/mL by about 2 h postinjection. A characteristic secondary peak in plasma 4-AP is observed at about 1 h postinjection. Corresponding concentrations of 4-AP in CSF were relatively stable for nearly 2 h, never exceeding (as a mean) 50 ng/mL within the first 2 h postinjection. We suggest behavioral recovery in clinical cases of spinal cord injury in both dogs and humans is mediated by such low (< 50 ng/mL) concentrations of 4-AP bathing the lesion. Since the adverse side effects that accompany IV administration of the drug limit its potential clinical usefulness, we have evaluated the feasibility of an alternate route of administration, continuous metered delivery of 4-AP into the spinal cord's subarachnoid space. This is accomplished by using a surgically implantable pump and delivery catheter. The pump itself can be interrogated, and is fully programmable, by noninvasive telemetry. Intrathecal delivery rates of between 1 and 60 micrograms of 4-AP per hour never produced detectable levels of the drug in plasma or cervically sampled CSF in dogs independent of the amount or duration of infusion (hours to days). The levels of 4-AP in lumbar samples of CSF near the lumbar delivery site suggest a very steep gradient of the drug, with local concentrations easily reaching 1 microgram/mL or higher (10- to 20-fold higher than can be safely produced by IV administration). The most frequent adverse reaction to intrathecal 4-AP delivery was a mild hindlimb tremor, fully reversible following reduction in the rate of drug delivery or termination of delivery. This route of drug administration relative to clinical spinal cord injury is discussed.

4-Aminopyridine↗
PubMed1994

Embryonic neuroepithelial sodium transport, the resulting physiological potential, and cranial development.

We have shown that the amiloride/novobiocin-sensitive sodium transport system of adult animal integuments is first observed in embryonic surface ectoderm and show here that this physiology is retained in this ectoderm following the closure of the neural folds. Unidirectional transport of Na+ out of the neural tube lumen results in a potential difference on the order of 40-90 mV, negative with respect to the abluminal surface. This transneural tube potential can be collapsed by iontophoresis of Na+ channel blockers amiloride or benzamil into the lumen, leading to severe cranial defects and incomplete morphogenesis. Modestly increasing the transneural tube potential with injection of novobiocin into the lumen also produces a lesser degree of developmental abnormality. We discuss the ways in which this physiology may help control the organization of the early nervous system.

Ambystoma↗
PubMed1994

Mammalian cortical astrocytes align themselves in a physiological voltage gradient.

Astrocytes obtained from primary cultures of newborn rat cerebral cortex show a marked structural rearrangement to weak (50-500 mV/mm) applied voltage gradients. Astrocytes reorient their processes so that the cells are aligned perpendicular to the voltage gradient. At field strengths of 100 mV/mm or greater, this realignment occurs in over 90% of the cell population. Furthermore, these magnitudes of electric fields completely eliminate any parallel alignments originally observed prior to application of the voltage. Realignment usually occurs by a withdrawal, followed by an extension, of cell processes. These responses occur at voltage gradients within the physiological range that naturally exist across the neural tube during early development. We suggest the possibility that architectural arrangements of developing glia and, subsequently, neurons may be regulated by endogenous transepithelial potentials that exist across embryonic neuroepithelium.

Animals↗
PubMed1993

Grafting in acute spinal cord injury: morphological and immunological aspects of transplanted adult rat enteric ganglia.

We have studied allogeneic transplants of adult rat enteric ganglia in order to evaluate their use as donor tissue for eventual autografts in rodent spinal cord injury models. Female Sprague-Dawley rats of similar weights served either as transplant donors or as recipients. A glass micropipette of 0.8 mm diameter was used to create a local penetrating injury of the lower thoracic spinal cord and the transplant material was pressure injected through the pipette within the neural parenchyma. Ganglia of the myenteric plexus adhering to the stratum longitudinal muscularis were dissected from portions of the jejunum and ileum. Following partial enzymatic digestion and mechanical disruption of the myenteric plexus and muscle tissue (labeled with adherent rhodamine conjugated microbeads), reaggregates of myenteric plexus and muscle were suspended in growth medium and cultured in vitro for one to two days prior to transplantation. Transplants were examined at three, four, six, and eight weeks after surgery. Some of the donor tissue was grown in vitro, in order to determine its cellular composition. These cultured explants were fixed after 10 days, and like myenteric plexus and muscle grafts, were stained histochemically for acetylcholinesterase and observed by fluorescence and light microscopy. At the earlier post-transplantation periods, grafts contained several clusters of enteric ganglion cells that were positive for acetylcholinesterase and exhibited ultrastructural features characteristic of the enteric nervous system. They had well-defined boundaries. Reactive astrocytes and their processes remained located within the host spinal cord adjacent to the boundary region of the grafts. Likewise, macrophages were located in areas abutting the graft. Newly formed vasculature penetrated the graft interior and appeared to be continuous with the host vessels. Grafts grown for at least eight weeks were characterized by interdigitating boundaries. Finger-like protrusions of graft tissue containing fibroblasts and collagen intermixed with adjacent gray and white matter of the host cord. Such transplants also had reactive astrocytes and ED1-positive macrophages. At this later stage, several groups of ganglion cells were identified that were intensely acetylcholinesterase-positive; however, only two of four grafts were recovered, whereas two of the transplants degenerated. We postulate that degeneration of allogeneic grafts may occur as a result of ongoing immune responses of the host which could be prevented by use of autogeneic enteric ganglia. Our studies show that fully differentiated enteric ganglia can survive transplantation to acutely injured spinal cord of adult rats.

Acetylcholinesterase↗
PubMed1991

The regeneration of electroreceptors in Kryptopterus.

The regeneration of ampullary electroreceptors was studied in the living catfish, Kryptopterus, by differential interference contrast optics. Electroreceptors in this transparent catfish are found, among other places, along the proximal portion of each anal fin ray, while the distal portion does not contain electroreceptors. Upon interruption of the sensory innervation, the electroreceptors disappear but regenerate when the skin is reinnervated. In this study, we tested the role of the skin and nerve in receptor regeneration with the following two experiments. First, a plug of fin containing electroreceptors was removed to determine whether electroreceptors could form in regenerated skin after the complete removal of all of the receptors within an interradial zone of the anal fin. Second, a portion of anal fin that contained electroreceptors was excised and a graft of electroreceptor-free (EF) fin was sutured in its place to determine whether epidermis that does not normally contain these receptors can be induced to form them. These grafts were compared to control grafts taken from proximal electroreceptor-containing (EC) fin. By 2 weeks following surgery, receptors were found in regenerated fin tissue and within the EC grafts. Electroreceptors also formed within most of the EF grafts. As electroreceptor regeneration does not require the presence of degenerated organs, and as electroreceptors can form in fin that normally does not contain receptors, we suggest that the formation of electroreceptors does not require old target sites and that epidermal cells can be induced to form receptors upon contact by regenerating axons. We discuss as well the factors that influence the pattern of receptor reappearance.

Animals↗
PubMed1991

Patterning in the regeneration of electroreceptors in the fin of Kryptopterus.

The influence of the target tissue on afferent nerve regeneration was studied in the adult glass catfish, Kryptopterus. In this fish, electroreceptors in the anal fin are distributed in a characteristic pattern in the proximal part of the fin and are absent in the distal portion of the fin. We tested whether axons were more likely to induce electroreceptors in certain regions of fin epidermis than in others. We rotated fin transplants so that the location of the degenerating electroreceptors was altered with respect to the regenerating axons in the host tissue dorsal to the fin. The effects of these rotations were observed in the living animal with differential interference contrast optics over a period of 10 weeks. When transplants were reversed rostrocaudally, new electroreceptors formed in the caudal half of the interradial zone, where degenerating electroreceptors were at the time of transplantation. When transplants were rotated so that the dorsoventral and rostrocaudal axes were reversed, some new receptors formed in the old target site regions that were located in the caudal interradial zones (in the distal half of the graft with respect to the host). Regenerating axons reached these regions of the transplant by taking unusual routes around the electroreceptor-free regions of fin. Very few electroreceptors formed in the distal/caudal or proximal/caudal interradial quadrants of grafts where the original orientation of the tissue was maintained. We suggest that old target sites have a neurotropic influence on the regenerating afferent axons and discuss the possibility that the distal fin epidermis is not as permissive to electroreceptor formation as proximal fin epidermis.

Animals↗
PubMed1990

Cutaneus trunci muscle reflex of the guinea pig.

The cutaneus trunci muscle reflex in guinea pigs was studied with a combination of video analysis, electromyography, lesioning, and light microscopy. The muscle forms a bilateral, subdermal sheet over much of the trunk. Local contractions of the dorsal part of the muscle are produced in response to brief tactile or electrical stimulation of the skin and consist of a twitch centered 1-2 cm rostral of the stimulus site. The reflex receptive field covers most of the thoracic and lumbar dorsal surface. The sensory information is carried via segmental dorsal cutaneous nerves. Receptive fields of adjacent nerves overlap and form rectangular areas perpendicular to the midline, at thoracic levels. Motor innervation projects through the lateral thoracic nerves of the brachial plexus. The motoneurons are located near the cervical thoracic junction (C7-T1). Lesions of the lower thoracic cord indicate that ascending sensory information is carried to the motor nuclei via the ventral half of the lateral funiculus. This pathway conveys information primarily from ipsilateral skin. There is a weaker input from contralateral skin, crossing at segmental levels. Electromyographic responses to brief electrical stimulation of lower thoracic skin occur usually as 10-12 msec bursts at latencies of 10-20 msec, and do not readily habituate or fatigue at stimulus frequencies below 10 Hz. The reflex persists under light pentobarbital anesthesia. This combination of characteristics makes the reflex useful for a variety of physiological and pathophysiological studies.

Animals↗
PubMed1990

Functional recovery after spinal cord hemisection in guinea pigs: the effects of applied electric fields.

Right lateral hemisection of the lower thoracic spinal cord was performed in 216 adult guinea pigs. Animals that proved suitable for the study were divided into one control and two experimental groups. Experimental animals were implanted with intraperitoneal stimulators delivering regulated current of 35 or 50 microA through electrodes placed 1 cm rostral and caudal of the hemisection. The cathode was cranial to the lesion in one group (n = 67) and caudal in the other (n = 33). Control animals (n = 62) were implanted with sham stimulators and electrodes delivering no current. The functional status of the animals was measured by tactile stimulation of the back skin to elicit the cutaneus trunci muscle reflex, and by the vestibulospinal free-fall response. The cutaneous response ipsilateral and caudal to the lesion was lost following hemisection and did not recover in any of the control animals or in animals with cathode caudal to the lesion. Recovery of the response was found in 9 of 67 animals in the cathode rostral group, between 56 and 139 days after injury. Toe spreading recovered spontaneously in 80-90% of animals in all groups. Of the possible mechanisms of skin reflex recovery, most current evidence points to regrowth of ascending nerve fibers in the lateral funiculus of the spinal cord local to the lesion.

Animals↗
PubMed1988

Eye regeneration in the mystery snail.

Mystery snails (Family Ampullariidae) are aquatic prosobranchs which possess structurally complex eyes at the tip of a cephalic eyestalk. No other sensory organs are found in association with this stalk. These snails possess the ability to regenerate the eye completely after amputation through the mid-eyestalk. Amputation induces gross changes in the cellular character of the entire eyestalk; in particular, an invagination of integumentary epithelium at the apex of the eyestalk stump produces a shallow cleft or "eyecup." Differentiation of all components of the eye apparently occurs by transdetermination of these epithelial cells. Retinal differentiation and the appearance of a new lens is observed as soon as 14 days postamputation. Complete eyes (by external observation), although smaller than the originals, have regenerated by 25 days postamputation. We compare this regeneration to the reconstruction of other animal tissues, in particular the regeneration of amphibian limbs.

Animals↗
PubMed1988

Electrical responses to amputation of the eye in the mystery snail.

Immediately following amputation through the eyestalk of the mystery snail (Pomacea), a persistent ionic current enters the apical amputation surface of the eyestalk stump. The circuit is completed by current driven from undamaged integument of the eyestalk stump and other body regions. The current is relatively steady during the first 10 hours following amputation. Currents subsequently begin a slow decline to base line levels by 60 hours postamputation--a time coincident with wound healing processes. The "battery" driving this ionic current is the internally negative transepidermal potential existing across the snail integument--perhaps the result of a net inward pumping of chloride across the skin. This system is compared to other regeneration models such as the amphibian limb, bone fracture repair, and skin wound healing. We suggest that ionic current may be a control of eye regeneration in the snail.

Animals↗
PubMed1988

Voltage gradients and ionic currents in injured and regenerating axons.

When an axon is damaged, a strong, persistent, injury current enters the damaged area, driven by the ionic pumps of the healthy portion of the cell. This current is associated with an electrical field, existing within the damaged axon and in the extracellular space around it. Such currents and fields are a common feature of neuronal damage and probably play a role in Ca2+-mediated degenerative events soon after transection, retrograde die-back, a structuring of the axonal terminus into zones, and perhaps organelle movement within damaged fibers. The long-term persistent currents may be a common feature of growing axons and might play a role in development and regeneration. Altogether, electrical effects of injury provide an alternative to conjecture concerning the physiological basis for immediate cellular responses to axonal injury. They also provide a possible basis for modulating the responses of neurons to injury by applied electric fields.

Action Potentials↗
PubMed1987

Behavioral recovery induced by applied electric fields after spinal cord hemisection in guinea pig.

Applied electric fields were used to promote axonal regeneration in spinal cords of adult guinea pigs. A propriospinal intersegmental reflex (the cutaneous trunci muscle reflex) was used to test lateral tract function after hemisection of the thoracic spinal cord. An electrical field (200 microvolts per millimeter, cathode rostral) applied across the lesion led to functional recovery of the cutaneous trunci muscle reflex in 25 percent of experimental animals, whereas the functional deficit remained in control animals, which were implanted with inactive stimulators.

Animals↗
PubMed1987

Anatomy of axolotl flank integument during limb bud development with special reference to a transcutaneous current predicting limb formation.

We have compared the anatomy of immature axolotl integument from limb-forming regions with adjacent non-limb-forming regions of the flank, concentrating on the earliest stages of limb bud development. We have extended these observations to include prominent buds just prior to their differentiation. At the ultrastructural level, we note striking differences between these two regions of skin, including a complete loss of hemidesmosomes and tonofilaments in the basal cells of the epidermis; a marked deterioration of the basal lamella; and focal areas of desquamating cells in the apical regions of the bud-all characteristics of limb-forming regions. These observations were made in the same larvae which provided measurements of a steady endogenous electric (ionic) current that either was coincident with or predicted the area of limb bud outgrowth (Borgens et al.: J. Exp. Zool. 228:491-503, 1983). We discuss these physiological measurements, the changes in the anatomy of the bud-forming region, and the relevance of these observations to our theory of early limb formation.

Ambystoma↗