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N Holder

Publications and source records attributed to N Holder.

At least 55 records · Page 3Linked to original sources
PubMed1992

Down-regulation of scatter factor in MRC 5 fibroblasts by epithelial-derived cells. A model for scatter factor modulation.

Scatter factor/hepatocyte growth factor (SF/HGF) is a multifunctional cytokine produced by embryonic fibroblasts and other mesenchymal cells that affects the growth and/or the movement of certain epithelia. Here we report that expression of scatter factor activity by MRC 5 cells, a strain of normal human embryonic lung fibroblasts, is greatly reduced as a result of co-culture of these cells with SVK14, an SV40-transformed human keratinocyte cell line. Using a cDNA probe to the beta chain of human HGF, we have found that the fall in SF activity in MRC 5/SVK14 co-cultures is accompanied by the loss of SF/HGF transcripts. As the inhibition of SF activity coincides with the disappearance of SF/HGF transcript, we conclude that inhibition of the SF activity expressed by MRC 5 cells by co-culture with SVK14 involves transcriptional regulation.

Animals↗
PubMed1991

Retinoic acid stimulates neurite outgrowth in the amphibian spinal cord.

There is increasing evidence that retinoic acid (RA), a vitamin A metabolite, plays a role in the development of the nervous system. Here we specifically test this notion by examining the effect of RA on neurite outgrowth from explanted segments of the axolotl spinal cord. We show that there is a threshold concentration in the region of 0.1-1 nM above which neurite outgrowth is stimulated 4-5 fold. Retinol, by contrast, only stimulated the migration of glial cells from the explants. Using HPLC we demonstrate that RA and retinol are present endogenously in the axolotl spinal cord. In addition, we have identified by immunocytochemistry with antipeptide antibodies the cells of the spinal cord that contain the binding proteins for RA (cellular RA-binding protein; CRABP) and retinol (cellular retinol-binding protein; CRBP). CRABP is found in the axons and CRBP is found in the ependyma and glial cells. These results provide strong evidence for a role for RA in the developing nervous system, and we propose a specific hypothesis involving CRBP, CRABP, retinol, and RA in the control of axon outgrowth in the spinal cord.

Ambystoma↗
PubMed1991

Continuous growth of the motor system in the axolotl.

During growth of the axolotl, motor neurons, and muscle fibres are added to the motor system. By double labelling neurons with tritiated thymidine and retrogradely transported HRP, we show that some motor neurons are born at postembryonic stages. Further analysis of motor neurons with the aid of HRP reveals this population of newly born cells relatively frequently in small (5-7 cm long) axolotls, but only rarely in large (7-13 cm long) axolotls. Evidence is presented that suggests that these immature cells are in the process of migrating from close to the ependyma out to the ventral horn. HRP transport also reveals growth cones of advancing axons within spinal nerves in animals up to 6 cm in length. Cell counts by light and electron microscopic methods show that muscle fibres are generated throughout larval life in the iliotibialis, a typical limb muscle. This analysis provides data consistent with the notion that new muscle fibres are added from a localised growth zone situated at the superficial edge of the muscle. These results are discussed in terms of the correlation between continuous growth of the motor system and the ability of the axolotl to functionally repair lesions to the peripheral nervous system.

Ambystoma↗
PubMed1991

Organization of motor units in the axolotl: a continuously growing animal.

The characteristics of motor units in the iliotibialis posterior muscle of the axolotl hindlimb are described. Tension recording and intracellular electrophysiological methods demonstrate that the physiological properties of the population of motor units are continuously distributed rather than grouped into a series of discrete types. Overlap between motor units occurs and this is positively correlated with motor unit size but negatively correlated with differences in time to peak tension. Immunocytochemical staining with antimyosin antibodies combined with histochemical demonstration of actomyosin ATPase activity revealed at least four types of muscle fibre which were distributed asymmetrically within iliotibialis posterior. The results are discussed in terms of the continuous growth of the muscle and the interactions between muscle and nerve in the formation of the axolotl motor system.

Ambystoma↗
PubMed1991

Neuroanatomical and functional analysis of neural tube formation in notochordless Xenopus embryos; laterality of the ventral spinal cord is lost.

Notochordless Xenopus embryos were produced by u.v. irradiation of the uncleaved fertilized egg. The spinal cords were examined using intermediate filament staining for glial cells, retrograde HRP staining for neuronal morphology and an anti-glycinergic antibody to reveal commissural cells and axons. The floorplate cells of the normal cord appear to be absent and their position along the ventral midline of the cord is occupied by motor neurones, Kolmer-Agduhr cells, radial glial cells and a ventrally placed marginal zone containing the longitudinal axons. Motor neurone number is reduced to 15% of control values, and the sensory extramedullary cell number is increased twentyfold. Commissural axons are still able to cross the ventral cord but do so at abnormal angles and some commissural axons continue to grow circumferentially up the contralateral side of the cord rather than turning to grow longitudinally. Extracellular electrophysiological recordings from motor axons reveal that the normal alternation of locomotor activity on the left and right side of the embryo is lost in notochordless animals. These results suggest that the notochord and/or the normal floor plate structure are important for the development of the laterality of spinal cord connections and may influence motor neurone proliferation or differentiation.

Animals↗
PubMed1991

Retinoic acid causes abnormal development and segmental patterning of the anterior hindbrain in Xenopus embryos.

Retinoic acid is a very potent teratogen and has also been implicated as an endogenous developmental signalling molecule in vertebrate embryos. One of the regions of the embryo reliably affected by exogenously applied RA is the hindbrain. In this paper, we describe in detail the hindbrain of Xenopus laevis embryos briefly treated with various levels of RA at gastrula stages. Such treatments lead to development of embryos with loss of anterior structures. In addition, RA has a general effect on rhombomere morphology and specific effects on the development of the anterior rhombomeres. This effect is demonstrated using neurofilament antibodies, HRP staining and in situ hybridisation using a probe for expression of the Xenopus Krox-20 gene. Anatomically it is evident that the development of the hindbrain normally anterior to the otocyst (rhombomeres 1-4) is abnormal following RA treatment. Sensory and motor axons of cranial nerves V and VII form a single root and the peripheral paths of V and VII and IX and X are also abnormal, as is the more anterior location of the otocyst. These anatomical changes are accompanied by changes in the pattern of expression for the gene XKrox-20, which normally expresses in rhombomeres 3 and 5, but is found in a single band in the anterior hindbrain of treated embryos which standardly fail to generate the normal external segmental appearance. The results are discussed in terms of both the teratogenic and possible endogenous roles of RA during normal development of the central nervous system. We conclude that low doses of RA applied during gastrulation have specific effects on the anterior Xenopus hindbrain which appear to be evolutionarily conserved in the light of similar recent findings in zebrafish.

Animals↗
PubMed1991

Retinoic acid modifies development of the midbrain-hindbrain border and affects cranial ganglion formation in zebrafish embryos.

Considerable evidence now suggests that retinoic acid (RA) is an important modulator of patterning events in early neuronal development in vertebrates. In this paper, we describe the effects of exogenously applied RA on early neural development in the zebrafish embryo. Neural anatomy is assessed by immunocytochemical and histochemical analysis of the developing embryo in whole mounts at 24 h post-fertilization. RA was applied for one hour at concentrations ranging from 10(-9) to 10(-6) M to embryos at 50% epiboly, the midgastrula stage, and at 10(-7) M to embryos at early and late gastrula stages. The neuroanatomical analysis shows that 10(-7) M RA causes a defined lesion to the developing central nervous system which corresponds to a loss of a region of the brain in the caudal midbrain-rostral hindbrain area, the precursor of the cerebellum and associated neural structures. The region that fails to develop corresponds to the cranial expression domain of the engrailed protein as assessed by the monoclonal antibody 4D9 (Patel et al. 1989: Expression of engrailed proteins in arthropods, annelids and chordates. Cell 58, 955-968). Structures caudal to rhombomere 4 are unaffected by 10(-7) M RA, as are the cranial midbrain and forebrain: 10(-7) M RA also affects the development of cranial ganglia, principally the Vth, anterior lateral line and VIIIth ganglia, suggesting that RA affects normal development of the cranial neural crest. Effects of RA at stages immediately prior to and after gastrulation show some similar and some distinct features. Results are discussed in terms of the possible role of RA as an endogenous moderator of normal head development.

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PubMed1991

The involvement of retinoic acid in the development of the vertebrate central nervous system.

We discuss here both previously published data and our current experiments which suggest that the vitamin A derivative, retinoic acid (RA), may play a role in the development of the vertebrate central nervous system (CNS). This evidence comes from the following: both an excess and a deficiency of vitamin A causes embryonic defects of the CNS; RA has been detected endogenously in the CNS; RA stimulates neurite outgrowth; the retinoic acid receptors have been detected with interesting distributions in the CNS; the binding protein for retinol, namely cellular retinol binding protein (CRBP) is found in the radial glia of the ventral floor plate; the binding protein for RA, namely, cellular retinoic acid binding protein (CRABP) is found in particular sets of axons in the developing spinal cord, in particular rhombomeres in the developing hindbrain and in the neural crest. Some hypotheses for the possible role of RA in various aspects of CNS development are discussed.

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PubMed1990

Heterogeneity in spinal radial glia demonstrated by intermediate filament expression and HRP labelling.

Considerable evidence indicates that radial glial cells play an active role in guiding growing neurites during development of the vertebrate CNS. In this paper we describe subpopulations of radial glial in the spinal cord of the axolotl. Amphibians maintain radial glia throughout life, and subpopulations are described using anatomical criteria following filling of individual cells with horseradish peroxidase and immunocytochemical staining with a range of intermediate filament antibodies. Radial glial cells in specific regions of the spinal cord stain with a range of antibodies specific to human keratins 8 and 18, and to glial fibrillary acid protein (GFAP). Some of these antibodies show selective localized to specific regions of individual glial cell processes. Immunoblotting analysis indicates that two keratins are present in the axolotl CNS corresponding to the two earliest embryonic keratins of vertebrates, keratin 8 and 18. Comparisons of molecular weight indicate that these may correspond to keratins identified in Xenopus laevis, the genes of which have been cloned. Axolotl GFAP is also identified in Western blots and may be present in two forms of differing molecular weight. These results are discussed in terms of the likely role of radial glial cells, and comparisons are drawn between the keratin and GFAP types seen in the axolotl spinal cord and of those in other vertebrate groups.

Ambystoma↗
PubMed1989

Organization of connective tissue patterns by dermal fibroblasts in the regenerating axolotl limb.

A set of tendons, aponeurotic sheets and retinaculae, which transduce muscle action from proximal limb levels to flexion and extension of the digits, is found in limbs of many vertebrates. This set of structures, here termed the digit tendon complex, is described for the axolotl forelimb. We show that the complex forms autonomously in muscleless axolotl limb regenerates produced from a cuff of unirradiated dermis surrounding an irradiated limb stump, and persists for up to a year after amputation. The pattern of other connective tissue structures, including the skeleton, is also normal. Fibroblast condensations that may represent sets of these cells normally associated with muscles in the extensor and flexor compartments of the carpal region also form in muscleless limbs. The results are discussed in terms of the importance of the dermis in pattern regulation, selforganization of connective tissues in general and autonomous development of the digit tendon complex in particular.

Ambystoma↗
PubMed1989

Homing behaviour of regenerating axons in the amphibian limb.

Following peripheral nerve deviation in the limbs of urodele amphibians axons regrow distally toward their previous target muscles (Holder et al. 1984; Proc. Roy. Soc. Lond. B 222, 477-489). This study describes analysis of this axon regeneration over time following deviation of the forearm flexor nerve in Triturus cristatus and the extensor cranialis nerve in the axolotl. Using horseradish peroxidase (HRP) axonal tracing, electrophysiology and electron microscopy, we describe the sequence of events leading to reestablishment of functional innervation. HRP fills reveal axons leaving the deviated nerve via a number of possible routes and they invariably grow distally. Many axons take a path close to that of the original nerve but others fasciculate forming parallel paths. Electrophysiology and electron microscopy show that axons in the deviated region of the nerve degenerate extensively compared with cut, but undeviated, controls. The results are discussed in terms of the possible axon-growth-promoting mechanisms that result in directed growth.

Ambystoma mexicanum↗
PubMed1988

Regeneration of descending axons in the spinal cord of the axolotl.

Horseradish peroxidase was used to describe the positions and approximate numbers of neurones with axons that descend to the lumbar spinal cord in normal axolotls and axolotls whose spinal cord had been transected 3-23 months previously. Three to 4 months after the transection approximately 10% of the axons had grown across the cut and returned to the lumbar spinal cord whereas 23 months after the transection the number and distribution of these cells were approaching those of the controls.

Ambystoma↗
PubMed1988

Reformation of specific neuromuscular connections during axolotl limb regeneration: evidence that the first contacts are correct.

Retrograde neuronal tracing with horseradish peroxidase was used to determine the position in the spinal cord of the motor neurone pools of a proximal (biceps) and a distal (extensor digitorum) limb muscle at various times during axolotl limb regeneration. It was found that from the earliest stages of muscle redifferentiation (as judged by light and electron microscopic analysis) the vast majority of axons innervating the regenerating muscles came from cells within the bounds of the normal motor neurone pool for each muscle. A few incorrect projections were noted in that the regenerating proximal muscle was sometimes innervated by some cells caudal to its normal motor neurone pool. The results are discussed in terms of mechanisms that may be operating in the regenerating limb to ensure that specific neuromuscular connections are made.

Ambystoma↗
PubMed1987

The pattern of innervation in serially duplicated axolotl limbs: further evidence for the existence of local pathway cues?

The innervation of the biceps muscle was examined in regenerated and vitamin A-induced serially duplicated axolotl forelimbs using retrograde transport of horseradish peroxidase. The regenerated biceps muscle becomes innervated by motor neurones in the same position in the spinal cord as the normal biceps motor pool. In previous experiments in which the innervation of a second copy of a proximal limb muscle was examined in serially duplicated limbs (Stephens, Holder & Maden, 1985), the duplicate muscle was found to become innervated by motor neurones that would normally have innervated distal muscles. In the present study, the innervation of the second copy of biceps was examined under conditions designed to encourage nerve sprouting from 'correct' biceps axons. Following either partial limb denervation or denervation coupled with removal of the proximal biceps, the second copy of the muscle was still innervated by inappropriate motor neurones, which again would normally innervate distal limb muscles. These results are interpreted as evidence for the necessity for an appropriate local environment for axonal growth to allow reformation of a correct pattern of motor innervation in the regenerated limb.

Ambystoma↗
PubMed1987

Reformation of the pattern of neuromuscular connections in the regenerated axolotl hindlimb.

Retrograde neuronal tracing with horseradish peroxidase (HRP) was used to determine the position in the spinal cord of motor neurone pools innervating muscles in the regenerated axolotl hindlimb. This method allows a detailed analysis of the accuracy of reformation of neuromuscular connections. The results show that regenerated distal limb muscles are reinnervated by motor neurones in the same region of the cord as those that innervate normal control distal limb muscles but that proximal muscles are innervated by a mixture of motor neurones in a normal position and motor neurones in a region of the spinal cord that normally supplies innervation to distal limb muscles. This difference between the reinnervation of proximal and distal limb muscles suggests that axons destined for proximal muscles may not enter distal limb territory during reinnervation of the regenerated limb.

Ambystoma↗
PubMed1987

Pathfinding by dorsal column axons in the spinal cord of the frog tadpole.

Sensory fibres from dorsal root ganglia (DRG) enter the spinal cord and run within a clearly defined ipsilateral pathway, the dorsal column, which lies in the dorsal funiculus. We have examined the characteristics of this pathway as a defined substrate for dorsal column axons in Rana temporaria tadpoles by rotating the thoracic spinal cord through 180 degrees from dorsal to ventral. Using HRP as a neuronal tracer we establish that many dorsal column axons from the hindlimb locate the ipsilateral or contralateral dorsal column pathway in the rotated cord. Other axons locate and grow caudally down the contralateral dorsal column returning to the lumbar region. Axons of the dorsal column never take an inappropriate pathway except at the transection sites where they negotiate abnormal routes to reach the contralateral or ipsilateral dorsal columns in normally positioned or rotated cord. The results demonstrate that the dorsal columns act as highly specific pathways for axons from DRG neurones but the axons' interactions with the pathway do not control the craniocaudal or left-right options for growth.

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