Embryonic and regenerating Xenopus retinal fibers are intrinsically different

Regulation of intrinsic neuronal properties for axon growth and regeneration

Regulation of neuritic growth is crucial for neural development, adaptation and repair. The intrinsic growth potential of nerve cells is determined by the activity of specific molecular sets, which sense environmental signals and sustain structural extension of neurites. The expression and function of these molecules are dynamically regulated by multiple mechanisms, which adjust the actual growth properties of each neuron population at different ontogenetic stages or in specific conditions. The neuronal potential for axon elongation and regeneration are restricted at the end of development by the concurrent action of several factors associated with the final maturation of neurons and of the surrounding tissue. In the adult, neuronal growth properties can be significantly modulated by injury, but they are also continuously tuned in everyday life to sustain physiological plasticity. Strict regulation of structural remodelling and neuritic elongation is thought to be required to maintain specific patterns of connectivity in the highly complex mammalian CNS. Accordingly, procedures that neutralize such mechanisms effectively boost axon growth in both intact and injured nervous system. Even in these conditions, however, aberrant connections are only formed in the presence of unusual external stimuli or experience. Therefore, growth regulatory mechanisms play an essentially permissive role by setting the responsiveness of neural circuits to environmental stimuli. The latter exert an instructive action and determine the actual shape of newly formed connections. In the light of this notion, efficient therapeutic interventions in the injured CNS should combine targeted manipulations of growth control mechanisms with task-specific training and rehabilitation paradigms.

cAMP regulates axon outgrowth and guidance during optic nerve regeneration in goldfish

Increased cAMP improves neuronal survival and axon regeneration in mammals. Here, we assess cAMP levels and identify activated pathways in a spontaneously regenerating central nervous system. Following optic nerve crush in goldfish, almost all retinal ganglion cells (RGC) survive and regenerate retinotectal topography. Goldfish received injections of a cAMP analogue (CPT-cAMP), a protein kinase A (PKA) inhibitor (KT5720), both compounds combined, or PBS (control). RGC survival in experimental groups was unaffected at any stage. The rate of axon regeneration was accelerated by the activator and decelerated both by the inhibitor and by combined injections, suggesting a PKA-dependent pathway. In addition, errors in regenerate retinotectal topography were observed when agents were applied in vivo and RGC response to the guidance cue ephrin-A5 in vitro was altered by the inhibitor. Our results highlight that therapeutic manipulation of cAMP levels to enhance axonal regeneration in mammals must ensure that topography, and consequently function, is not disrupted.

Insights into activity-dependent map formation from the retinotectal system: A middle-of-the-brain perspective

The development of orderly topographic maps in the central nervous system (CNS) results from a collaboration of chemoaffinity cues that establish the coarse organization of the projection and activity-dependent mechanisms that fine-tune the map. Using the retinotectal projection as a model system, we describe evidence that biochemical tags and patterned neural activity work in parallel to produce topographically ordered axonal projections. Finally, we review recent experiments in other CNS projections that support the proposition that cooperation between molecular guidance cues and activity-dependent processes constitutes a general paradigm for CNS map formation.

Age-related changes underlie switch in netrin-1 responsiveness as growth cones advance along visual pathway

Retinal axons are led out of the eye by netrin-1, an attractive guidance cue which is secreted at the optic nerve head. In the optic pathway, however, netrin-1 is expressed in areas that exclude retinal axon growth. This suggests that axons may change in their responsiveness to netrin-1 as they advance along the pathway. Indeed, in our 'whole-pathway' preparation in Xenopus, a gradual change from attraction to repulsion occurred as retinal axons emerged from progressively distal points along the pathway. We also found that axons that were aged in culture without pathway experience underwent a similar change, which correlated with a decline in cyclic AMP (cAMP) and netrin-1 receptor expression. Cyclic AMP elevators and adenosine A2b receptor agonists rejuvenated the behavior of old growth cones, causing them to regain attraction to netrin-1, whereas antagonists caused young growth cones to be repelled. These findings show that netrin-1 responsiveness is developmentally regulated and suggest that intrinsic changes that lower cAMP levels underlie this regulation.

Is the capacity for optic nerve regeneration related to continued retinal ganglion cell production in the frog?

In the central nervous system of fish and frogs, some, but not all, axons can regenerate. Retinal ganglion cells are among those that can. The retinae of fish and frogs produce new retinal neurons, including ganglion cells, for months or years after hatching. We have evaluated the hypothesis that retinal axonal regeneration is obligatorily linked to continued production of new ganglion cells. We used bromodeoxyuridine immunocytochemistry to assess retinal neurogenesis in juvenile, yearling, and 10 year old Xenopus laevis. Retinal ganglion cell genesis was vigorous in the marginal retina of the juveniles, but in the yearlings and the 10 year olds, no new ganglion cells were produced there. Cellular proliferation in the central retina was evident at all three ages, but none of the cells produced centrally were in the ganglion cell layer. Regeneration was examined in vivo by cutting one optic nerve and then, weeks later, injecting the eye with tritiated proline. Autoradiographs of brain sections showed that the optic nerves of all three ages regenerated. Regeneration in vitro was assessed using retinal explants from frogs of all three ages. In all cases, the cultures produced neurites, with some age-specific differences in the patterns of outgrowth. We conclude that retinal axonal regeneration is not linked obligatorily to maintained neurogenesis.

A protein kinase A-dependent molecular switch in synapsins regulates neurite outgrowth

Cyclic AMP (cAMP) promotes neurite outgrowth in a variety of neuronal cell lines through the activation of protein kinase A (PKA). We show here, using both Xenopus laevis embryonic neuronal culture and intact X. laevis embryos, that the nerve growth-promoting action of cAMP/PKA is mediated in part by the phosphorylation of synapsins at a single amino acid residue. Expression of a mutated form of synapsin that prevents phosphorylation at this site, or introduction of phospho-specific antibodies directed against this site, decreased basal and dibutyryl cAMP-stimulated neurite outgrowth. Expression of a mutation mimicking constitutive phosphorylation at this site increased neurite outgrowth, both under basal conditions and in the presence of a PKA inhibitor. These results provide a potential molecular approach for stimulating neuron regeneration, after injury and in neurodegenerative diseases.

Raphe-spinal neurons display an age-dependent differential capacity for neurite outgrowth compared to other brainstem-spinal populations

Functional regeneration of brainstem-spinal pathways occurs in the developing chick when the spinal cord is severed prior to embryonic day (E) 13. Functional spinal cord regeneration is not observed in animals injured after E13. This developmental transition from a permissive to a restrictive repair period may be due to the formation of an extrinsic inhibitory environment preventing axonal growth, and/or an intrinsic inability of mature neurons to regenerate. Here, we investigated the capacity of specific populations of brainstem-spinal projection neurons to regrow neurites in vitro from young (E8) versus mature (E17) brainstem explants. A crystal of carbocyanine dye (DiI) was implanted in ovo into the E5 cervical spinal cord to retrogradely label brainstem-spinal projection neurons. Three or 12 days later, discrete regions of the brainstem containing DiI-labeled neurons were dissected to produce explant cultures grown in serum-free media on laminin substrates. The subsequent redistribution of DiI into regenerating processes permitted the study of in vitro neurite outgrowth from identified brainstem-spinal neurons. When explanted on E8, i.e., an age when brainstem-spinal neurons are normally elongating through the spinal cord and are capable of in vivo functional regeneration, robust neurite outgrowth was observed from all brainstem populations, including rubro-, reticulo-, vestibulo-, and raphe-spinal neurons. In contrast, when explanted on E17, robust neurite outgrowth was seen only from raphe-spinal neurons. Neurite outgrowth from raphe-spinal neurons was 5-hydroxy-tryptamine immunoreactive. This study demonstrates that in growth factor-free environments with permissive growth substrates, neurite outgrowth from brainstem-spinal neurons is dependent on both neuronal age and phenotype.

Tenascin-R inhibits the growth of optic fibers in vitro but is rapidly eliminated during nerve regeneration in the salamander Pleurodeles waltl

Tenascin-R is a multidomain molecule of the extracellular matrix in the CNS with neurite outgrowth inhibitory functions. Despite the fact that in amphibians spontaneous axonal regeneration of the optic nerve occurs, we show here that the molecule appears concomitantly with myelination during metamorphosis and is present in the adult optic nerve of the salamander Pleurodeles waltl by immunoblots and immunohistochemistry. In vitro, adult retinal ganglion cell axons were not able to grow from retinal explants on a tenascin-R substrate or to cross a sharp substrate border of tenascin-R in the presence of laminin, indicating that tenascin-R inhibits regrowth of retinal ganglion cell axons. After an optic nerve crush, immunoreactivity for tenascin-R was reduced to undetectable levels within 8 d. Immunoreactivity for the myelin-associated glycoprotein (MAG) was also diminished by that time. Myelin was removed by phagocytosing cells at 8-14 d after the lesion, as demonstrated by electron microscopy. Tenascin-R immunoreactivity was again detectable at 6 months after the lesion, correlated with remyelination as indicated by MAG immunohistochemistry. Regenerating axons began to repopulate the distal lesioned nerve at 9 d after a crush and grew in close contact with putative astrocytic processes in the periphery of the nerve, close to the pia, as demonstrated by anterograde tracing. Thus, the onset of axonal regrowth over the lesion site was correlated with the removal of inhibitory molecules in the optic nerve, which may be necessary for successful axonal regeneration in the CNS of amphibians.

Beta 1 integrins regulate axon outgrowth and glial cell spreading on a glial-derived extracellular matrix during development and regeneration

In the present study we have investigated functional roles for beta 1 integrin receptors in regulating axon outgrowth, and glial cell adhesion and spreading in the Xenopus retina. The XR1 glial cell line, isolated from Xenopus retinal neuroepithelium, deposits a proteinaceous extracellular matrix (ECM) with potent neurite outgrowth promoting activity. To investigate a potential role of the integrins as cellular receptors for these glial cell-derived ECM components, embryonic and regenerating retinal explants were cultured in the presence of polyclonal antibodies directed against the beta 1 integrin receptor complex. The IgGs and Fabs of the anti-beta 1 integrin antibody strongly inhibited ganglion cell axon outgrowth on the glial cell-derived ECM, although axons grew freely across the surfaces of glial cells surrounding the explants. The antibodies also inhibited outgrowth on purified laminin containing substrates in a dose-dependent fashion. In addition, the anti-beta 1 antibodies were effective at inhibiting the spreading of glial cells that migrated out from the embryonic explants, and also inhibited attachment and spreading of Xenopus XR1 glial cells on ECM substrates. These results show that the beta 1 integrins play important functional roles in axon outgrowth during development and regeneration, and also serve in regulating retinal glial cell attachment and spreading in vitro, and thus are likely to play similar roles in vivo.

Amphibian-specific regulation of polysialic acid and the neural cell adhesion molecule in development and regeneration of the retinotectal system of the salamander Pleurodeles waltl

Antibodies specific to the neural cell adhesion molecule (NCAM-total), the 180 x 10(3) M(r) component of NCAM (NCAM-180) and polysialic acid (PSA) were used in immunohistochemistry and Western blots to detect the spatiotemporal dynamics of these molecules in development and regeneration of the retinotectal system of Pleurodeles waltl. NCAM-total and NCAM-180 are continuously expressed in the retina, optic nerve, and tectum of the developing and adult salamander. This is also found for the 140 x 10(3) M(r) component of NCAM in Western blots of the retina. In the larval retina, PSA is present in the inner plexiform layer (IPL) and a few cells in all nuclear layers. At metamorphosis, PSA expression in the retina strongly increases in the layer of cone photoreceptor somata. Several cells in the inner nuclear layer and Müller cell processes also begin to express PSA. This pattern persists into adulthood. The optic nerve and the tectum are strongly PSA-immunoreactive throughout development. In the adult optic nerve and optic fiber pathway in the brain, PSA expression is selectively downregulated. In the crush-lesioned adult optic nerve, regenerating fibers are NCAM-180-positive but PSA-negative. This demonstrates a molecular difference between growing nerve fibers of Pleurodeles in development and in regeneration. PSA regulation is closely correlated with metamorphosis, thus suggesting that PSA expression may be under hormonal control. Some aspects of PSA and NCAM isoform expression patterns in the retinotectal system of salamanders differ considerably from that of other vertebrates. The sustained expression of NCAM isoforms in adult salamanders might be due to secondary simplification (paedomorphosis).

Cellular and subcellular distribution of HNK-1 immunoreactivity in the neural tube of Xenopus

The HNK-1 antigen, a carbohydrate moiety bound to many cell adhesion and recognition molecules, is implicated in cell-cell and cell-substrate interactions during neural development. HNK-1 immunoreactivity (HNK1-IR) appears on neurons of the Xenopus neural tube very early in their development (Nordlander, Devel. Brain Res., 50:147-153, 1989). The distribution and onset of expression of the HNK-1 epitope on and within individual neurons is examined in this study. HNK-1 labels developing neurons and their processes, and focal areas of other structures which are directly contacted by neurons, such as neuroepithelial cell surfaces, basal lamina, and culture surfaces. HNK1-IR first appears in the Golgi apparatus and subsequently on the cell surface and in streams of punctate material directed toward the site of axon initiation and into the developing axon and its growth cone. The entire neuron is coated with a thin (20-30 nm) surface layer of HNK1-IR. In addition, the surface is dotted with small (100-250 nm) boluses of HNK1-IR material. Such boluses also occur within cytoplasmic vesicles, and extracellularly on basal lamina and culture substrata in proximity to neurons or their processes. The subcellar distribution of HNK1-IR in this tissue is compatible with a role for the HNK-1 epitope in axonal outgrowth and guidance.

Factors influencing the regeneration of axons in the central nervous system

Damage to the central nervous system (CNS) causes damage to neurons. This damage can result in the complete death of neurons, or in them becoming disconnected from their inputs or target structures due to disruption of axons. The main reason why damage to the human CNS is so disastrous and disabling is that axons will not in general regenerate in the mammalian brain, and neurons once lost are not replaced. In order, therefore, to repair the CNS, techniques will have to be developed to replace dead neurons, and induce axon regrowth. Central to the technologies necessary for brain repair is the ability to induce and control the growth of axons, since in a damaged brain both surviving and newly implanted neurons must grow axons to make or remake appropriate synaptic connections. Worthwhile treatments, however, do not necessarily require the repair of all the damaged circuits in the CNS, it may be possible to substantially improve the function of patients with relatively few reconnected axons, if those axons are ones which mediate particularly important behaviours, such as respiration, bladder control, or hand and arm movements.

Axotomy enhances the outgrowth of neurites from embryonic rat septal-basal forebrain neurons on a laminin substratum

Previous studies have suggested that embryonic (nonaxotomized) and regenerating central nervous system neurons differentially respond to the same substrata. In the present study, we have used an in vitro model to test the ability of laminin and type I collagen to promote the outgrowth of neurites from nonaxotomized and axotomized, embryonic septal-basal forebrain (SBF) neurons. Neurons within explants derived from Embryonic Day (E) 15 rats extended neurites that demonstrated similar growth characteristics on a collagen or laminin substratum. E15 neurons could be induced to extend longer neurites on laminin if they were axotomized in vitro and subsequently replated onto a laminin substratum. The carbocyanine dye DiI indicated that neurons which were axotomized could survive and regenerate processes. These regenerating neurites grew 27% longer on laminin than they did on collagen. Similarly, neurons that were axotomized in situ, i.e., E18 SBF neurons, extended neurites that were 29% longer on a laminin substratum. In contrast, E15 explants that were maintained in suspension culture prior to being plated onto a substratum exhibited similar growth on laminin or collagen. The increase in regeneration by E15 neurons on laminin was augmented, by 22%, if nerve growth factor was supplemented to the culture medium. These results demonstrate that laminin is a better substratum, as compared to collagen, for the elongation of neurites from axotomized SBF neurons. Nonaxotomized neurites, on the other hand, do not appear to prefer one substratum over the other. Furthermore, regeneration from embryonic, SBF neurons on laminin is augmented if NGF is used simultaneously.

Glial influences on axonal growth in the primary olfactory system

Neurogenesis in the olfactory epithelium continues throughout the entire life of mammals, and it is the axons of these newly formed olfactory receptor neurons that grow into the target tissue after the first cranial nerve is injured, not the regenerating axons of mature cells. These axons are able to enter and grow within the CNS of adult animals, unlike regenerating axons in injured dorsal roots, the majority of which are prevented from penetrating very far into the spinal cord. One reason why the olfactory axons are so successful in entering the CNS may be due, at least partially, to the fact that they are ensheathed by a type of glial cell (the ensheathing cell) that expresses phenotypic features of both astrocyte and Schwann cells. The presence of both L1/Ng-CAM and N-CAM in the plasma membranes of both ensheathing cells and immature olfactory receptor neurons would enable the olfactory axons to use the glial cell surfaces as a substratum on which to grow. It is probably also true that ensheathing cells synthesize and secrete laminin, thus providing an additional adhesive substrate for the olfactory axons, as well as glia-derived nexin and nerve growth factor, both of which are neurite-promoting agents.

Monoclonal antibody markers for amphibian oligodendrocytes and neurons

Few immunocytochemical probes have been developed for cold-blooded vertebrates, thus hampering analyses of cellular processes in these species. Those developed from mammalian and avian tissue often fail either to react or to show similar specificities in poikilotherms. Therefore, we have begun raising monoclonal antibodies (mabs) in mice against frog and tadpole brain tissue. The following analyses of two of these mabs suggest that these antibodies represent specific probes for frog axons and oligodendrocytes. Mab Olig recognizes all the myelinated axon tracts of the mature frog brain and spinal cord, as well as the tracts of the developing tadpole CNS once they have become myelinated. Axons cut in cross section show characteristic o-shaped staining around individual axons when processed with this antibody. Particularly easy to visualize in the tadpole are immunoreactive cell bodies and processes, seen in continuity with the myelin sheath. Occasionally, in this developing tissue, cells with highly branched processes characteristic of immature oligodendrocytes are observed. No other cells or processes within the brain or spinal cord react with this antibody. Mab Linc stains numerous filaments in all axonal projections. Occasionally, a thin rim of filamentous staining is observed in cell somata, but many regions rich in neuronal somata or dendrites are unreactive to this antibody. This in vivo staining pattern suggests that the Linc antigen is differentially distributed within neurons and exhibits a high concentration in axons. Linc immunoreactivity is robust in the processes of a subpopulation of dissociated tectal cells in culture. These Linc-positive cells are characterized as neurons on morphological criteria. Also, intense Linc immunoreactivity is present in the bundles of retinal axons that extend from retinal explants. Olig immunoreactivity, however, has not been detected in tectal cultures or retinal explants. Improved staining following Triton X-100 treatment of tissue sections suggests that neither of the mabs recognizes lipid antigens and that both are probably localized within the cell cytoplasm. Only the Linc mab reacts on Western blots of denatured brain protein. Linc consistently recognizes two Triton X-100-insoluble proteins with apparent molecular weights of 56 and 58 kD. The epitopes recognized by the Olig and Linc mabs have been surveyed in terms of their resistance to optic nerve crush and their consequent value in studies requiring such procedures. Possible homologies to known cell-type-specific molecules are discussed.

Growth cone interactions with a glial cell line from embryonic Xenopus retina

We have isolated a nonneuronal cell line from Xenopus retinal neuroepithelium (XR1 cell line). On the basis of immunocytochemical characterization using monoclonal antibodies generated in our laboratory as well as several other glial-specific antibodies, we have established that the XR1 cells are derived from embryonic astroglia. A monolayer of XR1 cells serves as an excellent substrate upon which embryonic retinal explants attach and elaborate neurites. This neurite outgrowth promoting activity appears not to be secreted into the medium, as medium conditioned by XR1 cells is ineffective in promoting outgrowth. Cell-free substrates were prepared to examine whether outgrowth promoting activity is also associated with the XR1 extracellular matrix (ECM). Substrates derived from XR1 cells grown on collagen are still capable of promoting outgrowth following osmotic shock and chemical extraction. This activity does not appear to be associated with laminin or fibronectin. Scanning electron microscopy was used to examine growth cones of retinal axons on XR1 cells and other substrates that supported neurite outgrowth. Growth cones and neurites growing on a monolayer of XR1 cells, or on collagen conditioned by XR1 cells, closely resemble the growth cones of retinal ganglion cells in vivo. A polyclonal antiserum (NOB1) generated against XR1 cells effectively and specifically inhibits neurite outgrowth on XR1-conditioned collagen. We therefore propose that neurite outgrowth promoting factors produced by these cells are associated with the extracellular matrix and may be glial specific.

In vitro growth properties of Xenopus retinal neurons undergo developmental modulation

To determine whether Xenopus retinal neurons undergo intrinsic developmental changes in growth properties, retinal explants from embryos and tadpoles of different stages were grown on laminin, fibronectin, and collagen I in serum-free media. Growth was assayed in terms of a neurite growth index (NGI) and the appearance of clockwise bundles, or a clockwise growth index (CGI). The first neurites from stage 25 optic vesicles are pioneers and display a unique growth phenotype; they emerge rapidly, survive for a short time, show little substrate preferences for growth (they grow almost as well on BSA as they do on laminin and fibronectin), and form no clockwise bundles under any conditions. Neurites from progressively older retinas (stages 32-37) share with stage 25 neurites the rapid outgrowth pattern, but begin to show substrate preferences and clockwise growth. From stage 40 to 50, the mature growth pattern is expressed; a lag in initial outgrowth, long-term survival, distinct substrate preferences (they grow 10 times better on laminin and fibronectin than on BSA) and display robust clockwise growth patterns on laminin and fibronectin. The acquisition of clockwise growth is independent of optic fiber contact with the tectum or exposure to diffusible factors from mature brain tissues. The results suggest that retinal neurons undergo developmental modulation of surface adhesive properties and/or cytoskeletal organization.

Characterization of a goldfish antigen during development and regeneration of the visual system

Normal, regenerating, and developing optic nerves of the goldfish were studied utilizing a monoclonal antibody (mAb) 1E1T which has specificity for Müller cells in the retina, radial glial cells in the tectum, and non-neuronal cells in the optic nerve. Sections of the normal optic nerve revealed longitudinally oriented chains of non-neuronal cells, that were 4-8 cells long. The number of chains in the normal nerve was very few. In addition, short acellular septa, probably the connective tissue septa, were also labeled with mAb 1E1T. Sections of crushed optic nerves showed an increase in the antigen recognized by mAb 1E1T within the septa and new septa were now visualized. Furthermore, the existing septa were longer and extended the length of the optic nerve. The formation and elongation of the septa occurred as early as 3 day postcrush. Between 3 and 11 d postcrush, there was heavy labeling of the septa and a large accumulation of non-neuronal cells at the crush site. At 3 months postcrush, the accumulation of non-neuronal cells labeled by mAb 1E1T were no longer visible but heavy labeling of the septa was still apparent.

Age-dependent patterns and rates of neurite outgrowth from CNS neurons on Schwann cell-derived basal lamina and laminin substrata

In the present study, we have examined the growth characteristics of CNS neurons on type I collagen, detergent-treated collagen (dColl), Schwann cell-derived basal lamina (SC-BL), and purified laminin substrata. Neurons from the cerebral cortex, septal basal forebrain, and lumbosacral spinal cord were obtained from embryonic age (E) 15 and E18 rats and grown in vitro as explants on the test substrata. Neurons from either embryonic age displayed radial neurite outgrowth on collagen and dColl substrata. However, pretreatment of collagen with detergents slightly diminished its ability to support neurite outgrowth, as evidence by the 20-40% decrease in the rate of neurite growth on dColl versus the rate calculated for neurons on collagen. In contrast to the similar growth characteristics of E15 and E18 neurons on collagen and dColl, the pattern of neurite outgrowth for CNS neurons on SC-BL and laminin substrata was age dependent. Most E15 neurons grown on SC-BL extended neurites that grew identically to those observed on dColl; these 'non-orienting' neurites maintained a radial orientation to their outgrowth despite encountering interposing channels of SC-BL and grew at rates equal to that calculated for neurons on dColl. E15 neurons placed on laminin substrata showed similar growth patterns and rates equal to that calculated for neurons on dColl. E15 neurons placed on laminin substrata showed similar growth patterns and rates to neurons on collagen. In contrast, neurons from E18 rats exhibited neurites that preferentially grew in intimate association with SC-BL channels once contact with the channels was established. These 'orienting' neurites faithfully elongated within the SC-BL and demonstrated a 1.4- to 2.0-fold increase in growth rate compared with the sister cultures of neurons grown on dColl. Furthermore, E18 neurons exhibited a 1.4-fold increase in growth on laminin compared with E18 neurons grown on collagen. A minor population of neurites exhibiting similar characteristics to orienting neurites was also observed in E15 cultures. It is hypothesized that orienting and non-orienting neurites reflect the outgrowth of 'regenerating' and 'developing' neurons, respectively, and may indicate an inherent difference in the ability of regenerating and developing neurons to recognize and respond to the same guidance signals.

Development of the electromotor system in Torpedo marmorata: cationic staining of the electric organ

The electric organs of embryonic Torpedo marmorata have been reacted with three cationic stains to evaluate the appearance and distribution of anionic sites. Ruthenium red, alcian blue and lysozyme were used at different pHs and found to react in a time-related manner to anionic components within the interelectrocyte space. The basal lamina covering the ventral electrocyte surface possesses the greatest number of anionic sites whereas growth cone, presynaptic terminal and glial membranes displayed almost no staining. Since this lamina serves as the exclusive substrate for ingrowing neurites during synaptogenesis, the results are consistent with the idea that charge distribution on the membrane surface may provide a necessary cue for neurite motility, extension and eventual synaptogenesis.