Substrate pathways demonstrated by transplanted Mauthner axons

A Subtle Network Mediating Axon Guidance: Intrinsic Dynamic Structure of Growth Cone, Attractive and Repulsive Molecular Cues, and the Intermediate Role of Signaling Pathways

A fundamental feature of both early nervous system development and axon regeneration is the guidance of axonal projections to their targets in order to assemble neural circuits that control behavior. In the navigation process where the nerves grow toward their targets, the growth cones, which locate at the tips of axons, sense the environment surrounding them, including varies of attractive or repulsive molecular cues, then make directional decisions to adjust their navigation journey. The turning ability of a growth cone largely depends on its highly dynamic skeleton, where actin filaments and microtubules play a very important role in its motility. In this review, we summarize some possible mechanisms underlying growth cone motility, relevant molecular cues, and signaling pathways in axon guidance of previous studies and discuss some questions regarding directions for further studies.

Drosophila motor axons recognize and follow a Sidestep-labeled substrate pathway to reach their target fields

During development of the Drosophila nervous system, migrating motor axons contact and interact with different cell types before reaching their peripheral muscle fields. The axonal attractant Sidestep (Side) is expressed in most of these intermediate targets. Here, we show that motor axons recognize and follow Side-expressing cell surfaces from the ventral nerve cord to their target region. Contact of motor axons with Side-expressing cells induces the down-regulation of Side. In the absence of Side, the interaction with intermediate targets is lost. Misexpression of Side in side mutants strongly attracts motor axons to ectopic sites. We provide evidence that, on motor axons, Beaten path Ia (Beat) functions as a receptor or part of a receptor complex for Side. In beat mutants, motor axons no longer recognize Side-expressing cell surfaces. Furthermore, Beat interacts with Side both genetically and biochemically. These results suggest that the tracing of Side-labeled cell surfaces by Beat-expressing growth cones is a major principle of motor axon guidance in Drosophila.

Are pioneer axons guided by regulatory gene expression domains in the zebrafish forebrain? High-resolution analysis of the patterning of the zebrafish brain during axon tract formation

Although the principles of axon growth are well understood in vitro the mechanisms guiding axons in vivo are less clear. It has been postulated that growing axons in the vertebrate brain follow borders of neuroepithelial cells expressing specific regulatory genes. In the present study we reexamined this hypothesis by analysing the earliest growing axons in the forebrain of embryonic zebrafish. Confocal laser scanning microscopy was used to determine the spatiotemporal relationship between growing axons and the expression pattern of eight regulatory genes in zebrafish brain. Pioneer axons project either longitudinally or dorsoventrally to establish a scaffold of axon tracts during this developmental period. Each of the regulatory genes was expressed in stereotypical domains and the borders of some were oriented along dorsoventral and longitudinal planes. However, none of these borders clearly defined the trajectories of pioneer axons. In two cases axons coursed in proximity to the borders of shh and pax6, but only for a relatively short portion of their pathway. Only later growing axons were closely apposed to the borders of some gene expression domains. These results suggest that pioneer axons in the embryonic forebrain do not follow continuous pathways defined by the borders of regulatory gene expression domains.

Axons of Xenopus neural tube respond to reversals of neural tube orientation

Axonal trajectories of the Kolmer-Agduhr (KA) neurons of Xenopus embryos, were observed after anterior-posterior (A-P) inversions of neural tube grafts to determine whether KA axons follow cell-inherent directional cues, cues from their immediate environment, or rostrocaudal signals from the embryo. KA axons form one of the earliest ascending spinal pathways in Xenopus and are visible in the lateral marginal zone of whole mounts processed for GABA immunoreactivity. Grafts were made at trunk levels at stages 22-24, 3-5 h before the first KA neurons were detectable and prior to axonal out-growth. Embryos were fixed and immunostained 6-36 h later. KA trajectories within and adjacent to reversed grafts were compared to those of nonrotated control grafts and to neural tube lengths comparable in position and in length in unoperated embryos. Most KA axons within rotated grafts followed the graft's orientation. However, others changed direction, taking novel routes, including turning to conform to the orientation of the host embryo. Reorientations were most common near the posterior host/graft interface. Some host KA cells also reoriented, always within a few hundred microns of the graft interface. Taken together, these growth patterns show that most KA axons within the grafts grow normally with respect to the original polarity of the graft neural tube and maintain that direction even into tissue of opposite polarity, suggesting that their routes are mainly determined by cell-intrinsic and/or local tissue factors. However, the reorientation of many other axons, particularly near graft seams, implies that KA axons can respond to local fluctuations in directional or segment identity signals generated in both host and graft after this perturbation.

Ultrastructure of an identified array of growth cones and possible substrates for guidance in the embryonic medicinal leech, Hirudo medicinalis

The oblique muscle organizer (Comb- or C-cell) in the embryonic medicinal leech, Hirudo medicinalis, provides an amenable situation to examine growth cone navigation in vivo. Each of the segmentally iterated C-cells extends an array of growth cones through the body wall along oblique trajectories. C-cell growth cones undergo an early, relatively slow period of extension followed by later, protracted and rapid directed outgrowth. During such transitions in extension, guidance might be mediated by a number of factors, including intrinsic constraints on polarity, spatially and temporally regulated cell and matrix interactions, physical constraints imposed by the environment, or guidance along particular cells in advance of the growth cones. Growth cones and their environment were examined by transmission electron microscopy to define those factors that might play a significant role in migration and guidance in this system. The ultrastructural examination has made the possibility very unlikely that simple, physical constraints play a prominent role in guiding C-cell growth cones. No anatomically defined paths or obliquely aligned channels were found in advance of these growth cones, and there were no identifiable physical boundaries, which might constrain young growth cones to a particular location in the body wall before rapid extension. There were diverse associations with many matrices and basement membranes located above, below, and within the layer in which growth cones appear to extend at the light level. Additionally, a preliminary examination of myocyte assembly upon processes proximal to the growth cones further implicates a role for matrix-associated interactions in muscle histogenesis as well as process outgrowth during embryonic development.

The disposition of early-generated neurons in the rat embryo predicts the pattern of major axonal tracts

During embryogenesis, the fiber tracts grow in a highly stereotyped pattern. A very small number of predetermined paths, preceding the growth of fasciculi, are present in the young neural tube (10-12, 15). What is the origin of these substrate pathways defined by Katz et al. (16) as "... a set of similar guidance cues which are aligned in a continuous discrete pathway..."? Could the first neurons play a role in the guidance of early nerve fibers? Observations in the brain stem revealed the presence of two longitudinal columns of early-generated neurons. These longitudinal columns were associated with well-differentiated marginal zones, characterized by cell-free spaces and representing the prospective site of the medial longitudinal (mlf) and lateral longitudinal (llt) tracts. Nerve fibers were also traced in the brain stem of young embryos. Axons were seen to travel in the early mlf and llt, in close proximity to the regions of early-generated neuronal columns. The data suggest that the precocious neurons that are organized in a definite pattern could somehow be involved in the guidance of some longitudinal axonal tracts, either by directly promoting the formation of an adequate terrain in the marginal layer, or by inducing other cells to do so.

Fate of B1-B2 and B3 rhombencephalic cells transplanted into the transected spinal cord of adult rats: light and electron microscopic studies

Embryonic cell suspensions (14-day embryos) containing either B3 or B1-B2 serotonergic cell groups were obtained by microdissection of specific rhombencephalic regions and transplanted into the transected spinal cord of adult male Sprague-Dawley rats. After 3 months of survival, the animals were sacrificed and the spinal cords processed for the immunocytochemical detection of serotonin (5-HT). 5-HT-immunoreactive fibers from B1-B2-grafted cells were selectively distributed in the ventral horn and the intermediolateral cell column (IML) where they established conventional synaptic contacts. However, B3 5-HT cells grew and extended their processes into the dorsal horn where in addition we observed scarce synaptic contacts as in the normal spinal cord. These results suggest that the specificity of the 5-HT innervation of the spinal cord by grafted neurons is due, at least partly, to the presence of local mechanisms mediating guidance and cell recognition, possibly operating in conjunction with preexisting substrate pathways.

Development of serotoninergic system in the brain and spinal cord of the chick

(1) Development of serotonin positive cells and fibers was immunohistochemically studied by the use of an antibody against serotonin. (2) Serotoninergic neurons were first observed in the immature rohmbencephalon raphe nuclei on embryonic day (E)4, where two clusters of serotonin positive neurons were located: one observed at the rostral part of the rohmbencephalon corresponding to the dorsal raphe nuclei had many serotonin positive cells: the other located at the caudal part of the rohmbencephalon corresponding to the medullary raphe nuclei of the adult animals had only a small number of serotoninergic cells. (3) By E8 the number of serotonin positive cells in the brain stem increased, and virtually all the raphe nuclei found in an adult animal were located. (4) Serotonin positive fibers in the marginal layer reached up to the diencephalon and telencephalon on E6 and E8, respectively. (5) Serotonin positive cells were found beside the midline regions in the ventral part of the spinal cord of the embryonic as well as posthatching chick. (6) Because almost all the serotoninergic fibers in the spinal cord originated from the brain stem raphe nuclei, propriospinal serotonin positive cells were considered as phylogenetic vestiges. (7) Serotoninergic fibers were first found in the marginal layer of the cervical and lumbar spinal cord on E6 and E8, respectively. (8) There was a waiting period of a few days before they penetrated into the mantle layer. (9) Terminal arbolization of the serotoninergic fibers started from late embryonic periods (E16 less than), and was maximized within one week of hatching. (10) Thereafter the density of serotonin positive fibers decreased in all the regions of the spinal cord. (11) Developmental changes of the density of serotonin determined with a high performance liquid chromatography were the same as those determined through immunohistochemistry. Namely the density of serotonin increased linearly from E6 to hatching period, and reached the maximum value one week posthatching. (12( The density of the serotonin in the adult spinal cord was about half of the maximum value. (13) It is to say that the densities of serotonin and serotoninergic fibers transiently increased around one week posthatching. (14) Following the transient increase serotoninergic fibers were eliminated from the neuropil, the fibers were localized in the specific regions of the motor nucleus: motor neuron pools of extensor muscles of the hip joint in the lumbosacral spinal cord.(ABSTRACT TRUNCATED AT 400 WORDS)

Organization of ascending projections from the optic tectum and mesencephalic pretectal gray in Rana pipiens

The ascending projections from the dorsal mesencephalon to the thalamus and pretectum in Rana pipiens were investigated by using the anterograde and retrograde transport of HRP with regard to two major issues: (1) the degree of tectotopic organization in the projections, and (2) their cells of origin. The results indicate that the spatial organization of the tecto-thalamic tract is specifically related to the laminar organization of the contributing tectal efferent neurons. Axons of neurons in the superficial portion of tectal layer 8 exit the tectum through layer 9 and travel in the superficial portion of the dorsal and ventral tecto-thalamic tracts and innervate the nucleus lentiformis mesencephali, the posterior lateral dorsal nucleus, and corpus geniculatum. The distribution of terminals within these structures varied with the tectal HRP-injection site. HRP injections in the ventral tecto-thalamic tract retrogradely labeled neurons in the superficial portion of tectal layer 8 across the lateral and caudal portion of the tectal lobe. HRP injections into the dorsal tecto-thalamic tract, at the level of the pretectum, retrogradely labeled pyriform neurons in the superficial portion of tectal layer 8 in the rostral and medial portions of the tectal lobe. With regard to the deep tectal layers, axons from pyramidal neurons in layer 6 and ganglionic neurons in layer 8 leave the tectum through layer 7, travel in both the dorsal and ventral tecto-thalamic tracts, and are located internal to the axons of the pyriform neurons of superficial tectal layer 8. The majority of the ganglionic neurons project to the posterior lateral ventral nucleus and the anterior lateral nucleus. The distribution of terminals within these nuclei did not display a tectotopic organization. A second major projection to the thalamus originates from the mesencephalic pretectal gray and innervates the nucleus lentiformis mesencephali, the posterior lateral dorsal nucleus, the anterior lateral nucleus, dorsal and ventral divisions of the ventral lateral thalamus, and the nucleus of Bellonci. Other axons from the mesencephalic pretectal gray terminate in the contralateral, medial portions of the posterior lateral dorsal thalamus, the ventral lateral thalamus, and the anterior lateral nucleus. The isthmo-tectal projection was also retrogradely labeled following tectal injections of HRP. This pathway travels in the most ventral portion of the ventral tecto-thalamic tract; its axons passed over the lateral margin of the endopeduncular nucleus bilaterally, and crossed the midline in the caudal portion of the optic chiasm. Extensive, bead-like varicosities were observed on these axons both in the endopeduncular nucleus and in the posterior optic chiasm.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of precocious and delayed afferent arrival on synapse localization on the amphibian Mauthner cell

Afferents often form synapses on restricted regions of their target cells. The connections between vestibular axons and the Mauthner cell are an example of this sort of specificity. The Mauthner cells are a pair of identifiable central neurons in certain fish and amphibians. In the axolotl (Ambystoma mexicanum), the large vestibular axons form club endings on only one portion of one dendrite of the Mauthner cell. During development, this specific distribution might result from nothing more than when and where the growing axons and dendrite meet (spatiotemporal model). Alternatively, the distribution might reflect constraints on where the developing axons may grow (axon segregation model) or form synapses (active recognition model). As a test of the spatiotemporal model, ears and vestibular ganglia from older embryos (Harrison stage 33/34-) were unilaterally grafted in place of prospective ear/ganglion placodes of younger embryos (stage 23). Light and electron microscopic examination revealed that the axons precociously entered the brain and formed synapses on the Mauthner cell. Similarly, grafts from younger to older embryos resulted in a delay. In both situations, mapping the club endings on the mature Mauthner cells of 21-mm larvae revealed a distribution similar to that of the contralateral control cells. Thus precocious and delayed afferent arrival did not alter the eventual localization of the synapses. These results conflict with the spatiotemporal model and imply that the vestibular axons are either restricted to a certain region of the neuropil or capable of recognizing a specific region of the Mauthner cell surface.

Development of the rat corticospinal tract through an altered glial environment

The major corticospinal tract (CST) in the rat is located at the base of the dorsal funiculus. It is a late-developing tract, and the growth of its axons into the lumbosacral region of the spinal cord does not occur until postnatal days 5 and 6. This delay is taken advantage of in this study in order to evaluate the effects of a markedly reduced glial population on ingrowth of the CST axons into the lumbosacral spinal cord. A reduction of the glial population is achieved by exposure of this region of spinal cord to X-radiation at 3 days of age. Growth of CST axons into and through the lumbosacral spinal cord in rats in which this region has undergone a radiation-induced depletion of glial cells is compared with that in their non-irradiated littermate controls by axonal tracing techniques using horseradish peroxidase (HRP). The HRP was applied directly to the motor cortices of normal and irradiated rats, and at all ages studied, there was anterograde filling of CST axons and their growth cones. At 3 days postnatally, the age when the lumbosacral spinal cord was irradiated in the experimental animals, CST axons were present in the more rostral thoracic levels. CST axons were observed in the lumbar region of non-irradiated rats on day 5, and by day 7 they were present at sacral levels.(ABSTRACT TRUNCATED AT 250 WORDS)

Regeneration of an identifiable motoneuron in the crayfish. II. Patterns of reconnection and synaptic strength established in the presence of an extra nerve

The regeneration of neuromuscular connections to the superficial flexor muscle system in the crayfish has been studied under a variety of experimental manipulations. These have provided insight into the factors that can influence the regeneration program of neurons. In this work the regeneration of the largest excitor motoneuron was studied under two different conditions: (1) when the original neuron and a transplanted neuron were growing simultaneously into a denervated target, and (2) when a transplanted neuron was growing into a target that had its original nerve supply intact. In condition 1 both the transplanted and the original neuron formed normal patterns of connectivity and synaptic strength in comparable periods of time. In condition 2 the rate of growth of the transplanted neuron is significantly reduced and does not extend into the lateral fibers of the muscle. It is concluded that the regeneration program of this neuron is not affected by the presence of other neurons growing at the same time into a denervated muscle. Since regeneration is seriously affected if growth occurs into a fully innervated target area, it is suggested that lack of growth stimuli from the target or competitive interactions between established and growing synaptic terminals could influence the regeneration program of this neuron.

Spatial and temporal correlation between early nerve fiber growth and neuroepithelial cell death in the chick embryo retina

The distribution of cell death in the ventral pycnotic zone of the chick embryo retina was studied in Hamburger-Hamilton's stages 16 to 25 (2 1/2 to 4 1/2 days of incubation). The number of fragments appearing in the retina increases notably from stage 20 at which stage they are limited almost exclusively to the optic disc region. At the same time optic fibers are seen in this area for the first time. In stage 24 cell death phenomena are numerous in the ventral retina, and become even more extensive in the following stage. Stage 25 meanwhile sees a drop in cell death in the dorsal retina. The overall picture presented by cell remains and young ganglion cells indicates that in stages 19-23 cell death occurs mainly in the zone between the ganglion cells of the posterior pole and the optic stalk. In the stage 25 retina most of the cell fragments of the ventral retina are found on either side of the fissure, while ganglion cells in the process of sending out axons toward the fissure appear laterally (nasally and temporally) to these zones of degeneration. Hence a spatial and temporal correlation is established between fiber growth and neuroepithelial cell degeneration, allowing us to construct a hypothesis with regard to the role that cell death might play in setting up an initial pattern of optic fiber growth.

Regeneration studies on a crayfish neuromuscular system. I. Connectivity changes after intersegmental nerve transplants

The superficial flexor muscles of the crayfish are a neuromuscular system of a few muscle cells innervated by six neurons in a precise position-dependent pattern. The neurons are capable of regenerating their normal connectivity patterns within a short span of time when conditions are favorable. The superficial flexor muscles of the second and third segments, despite their similarities in neuronal and muscle cell size and number, have distinctive connectivity patterns; some homologous neurons form similar patterns but other homologous neurons form patterns that are reversed between segments. We transplanted each segment's nerve into each other's muscle in order to observe regeneration of the nerves into a target area that differed in connectivity patterns from their original muscle. During the first weeks of regeneration all neurons formed a connectivity pattern with more connections medially and declining connections laterally, a pattern determined by the medial location of the nerve transplant. This pattern is maintained for most of the neurons, but for some there is an eventual reduction in medial connections as maximum synapse formation shifts to the lateral muscle fibers. Three of the eight neurons studied were able to regenerate connectivity patterns that corresponded to their segment of origin and not to their host muscle. This suggests that intersegmental muscle differences are not influencing the formation of these connectivity patterns, so the neurons will follow their inherent synaptogenesis program.

Expression of the proto-oncogene int-1 is restricted to specific neural cells in the developing mouse embryo

We have used in situ hybridization and computer-aided reconstruction to study the spatial distribution of expression of the mammary tumor proto-oncogene int-1 during mouse embryogenesis. int-1 RNA accumulation is restricted to specific regions of the neural plate and its derivatives between 9 and 14.5 days of development. int-1 RNA accumulates throughout the neural plate at the anterior head folds of the 9 day embryo but only at its lateral tips in more posterior regions. Following neural tube closure, int-1 expression is restricted to specific regions of the dorsal wall of the brain ventricles and spinal cord, the ventral wall of the midbrain and the diencephalon, and the lateral walls of the neuroepithelium at the midbrain-hindbrain junction. These data suggest that int-1 has a role in the early stages of central nervous system development in the mouse embryo.

Pathway formation and the terminal distribution pattern of the spinocerebellar projection in the chick embryo

Pathway formation and the terminal distribution pattern of spinocerebellar fibers in the chick embryo were examined by means of an anterograde labelling technique with wheat germ agglutinin conjugated horseradish peroxidase (WGA-HRP). Spinocerebellar fibers, which originate in the lumbar spinal cord and are located in the marginal layer of the spinal cord, reach the dorsal part of the cerebellar plate on embryonic day (E)8. On the way to the cerebellum the fibers form one distinct bundle, that suggests that gross projection errors probably do not occur during the formation of the spinocerebellar pathway. On E10, labelled fibers are located mostly in the medullary zone of the anterior lobe. By E12, the number of labelled fibers increases greatly in the inner granular and molecular layers. In transverse sections labelling was distributed throughout the mediolateral extent of the medullary zone. By E14, sagittal strips of labelling were clearly recognized in lobules II-IV; however, labelled terminals were present throughout lobule I. Although the adult pattern of terminal distribution is attained by E14, the mossy fiber terminals are still quite immature. The density of labelling decreased greatly by E16, and small terminal varicosities were first recognized. Structural differentiation of mossy fiber terminals continues to the end of the embryonic or the newly posthatched period.

Control of neuron shape during development and regeneration

We have examined the ability of Mueller reticulospinal neurons in the CNS of the larval sea lamprey to sprout following axonal and dendritic injury. Axotomy induces regenerative sprouting exclusively from the axon stump if it occurs at a site distant from the soma in the spinal cord. However, axotomy within the hindbrain at a site close to the soma results in profuse neuritic sprouting from the dendrites. The gross morphology and trajectories of these "dendritic" sprouts resemble those of regenerating axons. Amputation of Mueller cell dendrites (dendrotomy) without axotomy does not result in neuritic sprouting from either the axon or dendrites, indicating that axotomy is specifically required for sprouting to occur. However, dendrotomy is capable of altering the distribution of sprouting in a previously axotomized Mueller cell by inducing sprouting at the site of the dendrotomy lesion. Sprouts of both dendritic and axonal origin tend to follow linear, rostrocaudally oriented paths along or near the ventral surface of the hindbrain. Some sprouts form very large, palmate growth cones on the marginal surface, which in turn give rise to many branches that continue to grow either rostrally or caudally along the surface of the brain. We discuss the possibility that both dendritic and axonal sprouts evoked by axotomy of Mueller neurons are recapitulating initial axonal development during embryogenesis, and that their trajectories are determined by developmental guidance cues persisting in the ventral hindbrain.

On the development of the pyramidal tract in the rat. II. An anterograde tracer study of the outgrowth of the corticospinal fibers

An anterograde tracer study has been made of the developing corticospinal tract (CST) in the rat using wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP). Analysis of normal Rager stained material revealed that corticospinal axons reach upper cervical spinal cord levels at the day of birth (PO). Postnatal rats ranging in age from one (P1) to fourteen (P14) days received multiple WGA-HRP injections into the cortex of their left hemisphere and were allowed to survive for 24 h. The first labeled CST fibers caudally extend into the third thoracic spinal cord segment at P1; into the eighth thoracic segment at P3; into the first or second lumbar segment at P7 and into the second to third sacral segment at Pg. Thus the outgrowth of the leading 'pioneer' fibers of the CST is completed at P9 but later developing axons are continuously added even beyond P9. Quantitative analysis of the amount of label along the length of the outgrowing CST revealed a characteristic pattern of labeling varying with age. The most striking features of that pattern are: the formation of two standing peaks at the level of the cervical and lumbar enlargements respectively and the transient presence of a smaller running peak which moves caudally with the front of the outgrowing bundle. The standing peaks are ascribed to the branching of the axon terminals at both intumescences, whereas the running peak probably arises by the accumulation of tracer within the growth cones at the tips of the outgrowing CST axons. Factors such as the number of axons, the varying axon diameters, the branching collaterals, the presence of varicosities, the transport rate of the tracer, the uptake of the tracer at the injection site, which possibly may affect the amount of label present in both the entire bundle and in the individual axons are discussed. Current research is focused upon an analysis of the relation between the site of injection within the cortex and the pattern of labeling of the CST. A delay of two days was found between the arrival of the CST axons at a particular spinal cord level and their outgrowth into the adjacent spinal gray. However, combined HRP and electronmicroscopic experiments are necessary to determine the factors behind the maturation of the CST as well as the maturation of the spinal gray.

Axonal branch shapes

To distinguish the relative roles of the intrinsic and the extrinsic determinants of axonal branching shapes, a number of key branch parameters were measured under a variety of conditions. Branch shapes of frog and of chick axons were analyzed in tissue culture, and these in vitro patterns were compared with in vivo branch patterns of axons in tadpole tail fins. The shape of a branch junction can be characterized by the sizes of its branch angles. In all cases, branch junctions had only two branches (3 branch angles), and the bifurcation angle between them was usually the smallest. The shape of the branch junction was constant in a wide variety of environments, but the exact branch angles, as well as the numbers of branches per axon and the numbers of axons per neuron, could be modulated by changes in the substrate adhesivity.

Development of the major pathways for neurite outgrowth in the chick hindlimb

To elucidate mechanisms that may control development of the gross anatomical nerve pattern, motoneuron outgrowth into the chick hindlimb was examined using orthograde labeling, scanning and transmission electron microscopy, and Alcian blue staining. Results show that growth cones are not guided by contact with oriented extracellular fibrils, aligned mesenchyme cells, the myotome, or the vasculature. Pathways are not delineated by cell-free space or channels of lower cell density; however, densely packed mesenchyme may form barriers that channel outgrowth. In addition, abundant mesenchymal cell death was seen at the nerve front. This cell death may provide space that encourages growth cone advancement. Pathways often lie along interfaces between areas that stain darkly and lightly with Alcian blue, which specifically stains glycosaminoglycans, and growth cones never penetrate areas that stain intensely, such as the pelvic girdle, which is known to be a barrier to outgrowth. Leading growth cones form specialized contacts with mesenchyme cells, but the predominant contacts are interneuronal. It is proposed that the anatomical pattern of outgrowth is determined by the distribution of preferred substrata, the most preferred substratum being other neurites. Further, neurites tend to prefer loose mesenchyme to dense mesenchyme or areas rich in glycosaminoglycans.

Early axon patterns of the spinal cord: experiments with a computer

In the vertebrate central nervous system, most axons appear in one of two elemental patterns--sheets or bundles. Many developmental mechanisms are involved in the formation of the elemental axon patterns, and these mechanisms often act simultaneously. The major axon-patterning mechanisms include differential adhesivity, internal growth constraints on axons, and initial orientation of axonal outgrowth. To evaluate the effects of these mechanisms on the formation of axon patterns, a computer was used to model axonal growth. Experiments with the computer model suggest that axon sheets are produced by the cooperative action of more than one mechanism. Furthermore, in the appropriate combination, these mechanisms produce orderly axon sheets even on patternless substrates. On the other hand, to transform the sheets into axon bundles, the substrate must be patterned.

Intensifier for Bodian staining of tissue sections and cell cultures

Intensification of the standard Bodian silver stain with a poststaining photographic enhancer produces high resolution of fine cell processes such as axonal growth cones. This technique can be used on tissue sections and is especially useful for visualizing individual cells fixed in tissue cultures.

Guidance of neurite outgrowth by pathways of substratum-adsorbed laminin

Neurite outgrowth is guided by narrow pathways of bioactive laminin. These pathways are created by ultraviolet light irradiation of laminin-coated coverslips masked with electron microscope grids. Patterned outgrowth of neurites is independent of gross mechanical guidance and guidance caused by substrate limitation. Cells on unirradiated laminin are less readily displaced by shear forces than cells on irradiated laminin. This study suggests that ultraviolet light alters the adhesive properties of laminin and that differential cell-substratum adhesion may guide extending neurites on the purified naturally occurring substance, laminin.

Steady growth cone currents revealed by a novel circularly vibrating probe: a possible mechanism underlying neurite growth

The rate and direction of neurite growth have been shown in a number of studies to be determined by the distribution of adhesive sites on the growth cone. Recent evidence showing that the application of extrinsic electric fields can redistribute membrane molecules and alter both the rate and direction of neurite growth have raised the question whether endogenous electric fields might be produced by steady currents in growth cones. To investigate this question, we have devised a novel circularly vibrating microprobe capable of measuring current densities in the range of 5 nA/cm2 (near the theorectical limit of sensitivity), with a spatial resolution of 2 micron. The design of this device and the development of a novel algorithm for computing current vectors on-line is described. Using this probe we have found that cultured goldfish retinal ganglion cell growth cones generate steady inward currents at their tips. The measured currents, in the range of 10-100 nA/cm2, appear to flow into the filopodia at their tips and back outward near the junctures of the filopodia and the growth cone. The currents appear to be produced only during active growth. Ion substitution experiments support the conclusion that the majority of this current is carried by Ca2+ ions, which we postulate flow through a population of activated voltage-sensitive Ca2+ channels located on the filopodial tips. Calculation of the transmembrane current density (4 X 10(-6) nA/cm2) leads to an estimate of channel density (10 channels/micron2) in close agreement with the measured density of Ca2+ channels in other systems. The assumption that calcium channel proteins are conveyed to nerve terminals by active transport, whereas sodium channel proteins are conveyed passively by a slower somatofugal diffusion process [Strichartz et al, 1984], would explain why developing neurons tend to display Ca2+-sensitive electrogenesis at their growing tips, and Na+-sensitive action potentials later in development. In order to gain some insight into the possible role of these steady growth currents, we estimated the membrane depolarization and axial voltage gradient they produce. It is likely that the currents produce sufficient membrane depolarization (approximately equal to 4 mV) to cause autogenous activation of ion channel permeabilities. Similarly, the axial voltage gradient (approximately equal to 4 mV/cm) would be expected to move intracytoplasmic vesicles by electrophoresis at a rate (20-40 microns/hr) very close to that at which the filopodia are observed to grow.(ABSTRACT TRUNCATED AT 400 WORDS)

Selection of pathways by regenerating spinal cord fiber tracts

We have traced fiber tracts in the regenerating teleost spinal cord and have found that the regenerative capabilities of fiber tracts coming from different parts of the CNS vary considerably. Some descending tracts can regenerate up to 5.6 mm beyond a transection, while other fiber tracts cannot even penetrate the scar tissue. In spite of a great disturbance of their normally ordered pathways as they grow through the lesion site, those tracts which can regenerate can find their appropriate funiculi in the distal spinal cord.

CNS effects of mechanically produced spina bifida

Neural-tube defects were produced by one of two mechanical perturbations in an amphibian model system: newly-formed neural tubes were slit dorsally, or an intrinsic mass (an eye primordium transplanted to the neural plate) was introduced to prevent dorsal closure. Electron microscopic analyses showed that the resulting CNS distortions invariably included paired spinal-cord tubes that developed ciliated central canals surrounded by cellular layers and rimmed laterally by axon-filled marginal zones. Specific axon tracts grew in appropriate locations through the regions of simple diastematomyelia. Even in the most histologically distorted regions, ventral roots developed, axons were myelinated, and neuromuscular synapses formed in adjacent muscles. On the other hand, an excess of cells populated the defects and in histologically distorted regions the axon tracts were clearly less well organized than in normal animals. Neural-crest cells and the notochord did not appear to be affected. In addition, distant neurons and axon tracts appeared normal. These mechanical lesions could produce spina bifida without hydrocephalus. It is concluded that mechanical lesions to the forming neural tube will produce mainly local effects with a strong tendency toward diastematomyelia.

Stereotyped and variable growth of redirected Mauthner axons

To assay the axon tract organizing capabilities of different regions of the vertebrate CNS, Mauthner axons were redirected by grafting supernumerary hindbrains in Xenopus embryos. The 63 redirected Mauthner axons thus produced included donor axons projecting into the host CNS and host axons that grew through the graft or that were redirected in the host CNS. Two major phenomena were observed. Caudal to the optic chiasm, the Mauthner axons followed a single ipsilateral stereotyped route--the basal substrate pathway--extending in the ventral and ventrolateral marginal zone from the diencephalon to the caudal spinal cord. In contrast, rostral to the optic chiasm, these same Mauthner axons followed variable ipsilateral and contralateral routes. Even pairs of Mauthner axons entering the optic chiasm side-by-side eventually followed different routes in normal forebrains. The contrasting behaviors of the Mauthner axons growing in the rostral diencephalon and telencephalon and of the same Mauthner axons growing elsewhere suggest that there are differences in the effective guidance cues between these two regions of the developing brain. This is consistent with other types of neuroanatomical and neuroembryological evidence indicating a fundamental division between the rostral and the caudal diencephalon.

Axonal projections of mechanosensory neurons in the connectives and peripheral nerves of the leech, Haemopis marmorata

The axonal projections of the seven mechanosensory neurons in the connectives and peripheral nerves were examined by horseradish peroxidase (HRP) injections. In the connective the closely functionally related mechanosensory neurons travel in two bilaterally symmetrical regions, which comprise less than 10% of the connective's cross-sectional area. This grouping may reflect the similar functional properties and synaptic connections of the cells. Serial sections through the neuropil-connective junction zone showed that the organization of the axons is independent of glial fasciculation. Fascicles are not rigid structures; they were found to part and fuse in a seemingly random manner. The glial cell seems therefore to have only a supportive structural function and not to play any role in axonal guidance or to define any specific nerve fascicles in the leech. The projections of the mechanosensory cells in the peripheral roots were not as stereotyped as in the connective. However, examination of the branching of the medial and lateral P cells in the posterior root showed that the roots are highly structured and that some axons may be confined to tracts which have specific positions related to the branching of the root. The possible role of the P cells in pioneering these tracts is discussed.

Plasticity of the corticospinal tract following midthoracic spinal injury in the postnatal rat

Rats received a midthoracic spinal cord "overhemisection" including right hemicord and left dorsal funiculus at birth (neonatal operates, N = 15) or 21 days of age (weanling operates, N = 14). In a second experiment neonatal (N = 6), 6-day (N = 3), and 12-day (N = 7) rats sustained a right sensorimotor cortex (SmI) ablation to destroy the left corticospinal tract (CST) at the same time as the spinal injury (double lesion operates). Later (3-12 months) injections of 3H-proline and autoradiography were used to label the left or right CST. The results of the first experiment showed that most right CST axons failed to grow around the spinal lesion in neonatal operates (N = 9). There was an increase in the density of label, mainly to CST projection areas, in a 1-mm zone rostral to the lesion. However, left CST axons bypassed the lesion by growing through the intact tissue in neonatal operates (N = 6). These displaced axons were consistently located within the dorsal portion of the lateral funiculus (dLF) and remained within that location caudal to the lesion, an area normally containing only a few CST axons. In spite of this abnormal position, these axons terminated bilaterally throughout the remainder of the cord in normal CST sites. In weanling operates, CST axons severed by the lesion did not regenerate around the lesion site. An increased density of label over the few spared axons within the left dLF and in CST projection zones immediately caudal to the lesion site suggested axonal sprouting by these axons. The results of the second experiment showed that the lack of growth of right CST axons around this injury in neonatal operates was, at least partially, due to an interaction with left CST axons. In neonatal double lesion operates, right CST axons grew around the spinal injury for a varying distance within the left dLF and distributed bilaterally to normal CST sites. The number of right CST axons bypassing the lesion was related to the configuration of the lesion site. A smaller number of right CST axons bypassed the lesion in 6-day double lesion operates and most terminated within 2-3 mm of the lesion site. Right CST axons failed to grow around this injury in 12-day double lesion operates.(ABSTRACT TRUNCATED AT 400 WORDS)

Ontophyletics of the nervous system: development of the corpus callosum and evolution of axon tracts

The evolution of nervous systems has included significant changes in the axon tracts of the central nervous system. These evolutionary changes required changes in axonal growth in embryos. During development, many axons reach their targets by following guidance cues that are organized as pathways in the embryonic substrate, and the overall pattern of the major axon tracts in the adult can be traced back to the fundamental pattern of such substrate pathways. Embryological and comparative anatomical studies suggest that most axon tracts, such as the anterior commissure, have evolved by the modified use of preexisting substrate pathways. On the other hand, recent developmental studies suggest that a few entirely new substrate pathways have arisen during evolution; these apparently provided opportunities for the formation of completely new axon tracts. The corpus callosum, which is found only in placental mammals, may be such a truly new axon tract. We propose that the evolution of the corpus callosum is founded on the emergence of a new preaxonal substrate pathway, the "glial sling," which bridges the two halves of the embryonic forebrain only in placental mammals.

Trajectories of axons in ectopic VIIIth nerves

Transplantation of the acousticolateral placode to the evacuated eye position in embryos of the frog Rana pipiens has been used to force axons of the VIIIth cranial nerve to penetrate the diencephalon. These ectopic axons establish a growth trajectory that is strikingly similar to their normal growth trajectory within the medulla oblongata despite the fact that no other axons within the diencephalon normally follow this route. The result is discussed in terms of the "blueprint" and substrates pathway hypotheses which have been advanced to explain the initial development of axon tracts within the central nervous system.

Regeneration of rat hippocampal fimbria fibers after fimbria transection and peripheral nerve or fetal hippocampal implantation

After a unilateral hippocampal fornix-fimbria transection in adult rats, either autologous peripheral nerve or fetal hippocampus was implanted into the transection site. After 2 to 4 weeks, 2 to 3 months, and 6 to 8 months fimbria fiber regeneration was analyzed by acetylcholinesterase (AChE) histochemistry and retrograde transport of horseradish peroxidase after injection into the denervated host hippocampus. Prominent innervation of both types of implant by central AChE-staining axons occurred by 2 to 3 weeks postimplantation and was sustained to at least 8 months. Reinnervation of the adjacent host hippocampal terminal zone was also apparent, but was sparse compared with innervation of implants.

Development of the marginal zone in the rhombenecephalon of Xenopus laevis

The marginal zone is the most superficial component of the developing central nervous system. In this study we describe the pattern of formation of the marginal zone in Xenopus laevis as seen with the light microscope and correlate its appearance with developing axon bundles seen with the electron microscope. The marginal zone was first identified at stage 26 on the ventrolateral aspects of the neural tube in restrictive foci throughout the medulla. By stage 28 this presumptive fiber area had expanded to occupy a longitudinal zone along the ventrolateral aspect of the rhombencephalon. The marginal zone continued expanding rostrocaudally, mediolaterally, and dorsoventrally coming to occupy a substantial area along the periphery of the central nervous system by the time of hatching (stage 36). Electron microscopic (EM) observations allowed us to identify axons and growth cones on the lateral borders of the neural tube at stage 22. Foci of marginal zones identified in the caudal medulla at stage 24 consisted of several fascicles grouped together on the ventrolateral surface. These fascicles, separated and surrounded by ependymal processes contained axons and growth cones. The pattern of marginal zone development reflected the addition of axons within fascicles and also the addition of new fascicles of axons.

Observations on the development of descending pathways from the brain stem to the spinal cord in the clawed toad Xenopus laevis

Anurans such as the clawed toad Xenopus laevis offer a unique opportunity to study the ontogeny of descending pathways to the spinal cord. Their transition from aquatic limbless tadpole to juvenile toad occurs over a protracted period time during which the animal is accessible for experimental studies. In Xenopus laevis tadpoles the development of descending pathways has been studied from early limb-bud stage on (stage 50) with the aid of HRP slow-release gels. In stage 50, cells of origin of descending supraspinal pathways were already present throughout the reticular formation (including the interstitial nucleus of the fasciculus longitudinalis medialis) and in the vestibular nuclear complex. Also the giant Mauthner cells project to the cord at this stage. A spinal projection from the anuran homologue of the nucleus ruber of higher vertebrates does not appear before stage 58, i.e., when the hindlimbs are used for locomotion. Hypothalamospinal projections appear for the first time at stage 57. These observations in Xenopus laevis tadpoles suggest that reticulospinal and vestibulospinal projections innervate spinal segments very early in development, whereas the anuran red nucleus projects spinal ward definitely later in development.

Axonal guidance during development of the optic nerve: the role of pigmented epithelia and other extrinsic factors

It is well established that a congenital lack of ocular melanin (albinism) can lead to developmental abnormalities of the central visual pathways. However, it is yet unknown how the pigmentation per se acts to influence formation of the optic projection. In order to study the possible interaction between eye pigment and optic axons during development, we have examined, with the use of serial section techniques, a series of timed embryos at stages when the ocular pigment and outgrowing axons first become apparent. Our results have demonstrated that, in mice and rats, the upper wall of the distal half of the primitive eye stalk (a region which lies along the potential route to be taken by the earliest developing nerve fibers) is transiently pigmented prior to and during the migration of the pioneer optic axons. All outgrowing neurites avoid this stretch of melanotic tissue and instead grow preferentially through a system of extracellular tunnels in the ventral, pigment-free zones of the distal eye stalk. The stalk remains unpigmented from about its midpoint and continuing toward the brain. At the pigment/pigment-free interface many of the axons shift upward from their ventral positions, forming a marginal annulus. In the chick, on the contrary, pigmentation of the stalk does not occur and as the optic axons exit the globe they grow immediately in an annulus configuration. In Xenopus, the entire stalk becomes pigmented and the optic fibers congregate in one discrete bundle of fascicles along the length of the stalk's most ventral margin. These observations suggest that melanin-producing stalk cells may play a role in controlling the topographic patterning of optic fibers within the developing nerve by inhibiting the lateral spread of axonal growth cones into or within their territory. To test this hypothesis we have charted the distribution of optic fibers in the developing optic stalks of timed albino rat embryos. Indeed, as fibers leave the mutant eye, it was found that a small but consistent number of pioneering axons (day E15) become ectopic and immediately invade nonpigmented regions (those normally pigmented and axon-free) in the distal optic stalk. Thus, the usual topographic arrangement of the collection of pioneer optic fibers is altered in the albino.