Growth cone-target interactions in the frog retinotectal pathway

Morphology of developing olfactory axons in the olfactory bulb of the rabbit (Oryctolagus cuniculus): a Golgi study

Transient expression of axon collaterals plays an important role in enabling neurons to find appropriate targets during development. In the olfactory bulb, the numbers of both sensory neurons and their targets, the glomeruli, increase markedly during the postnatal period. In the present study, the morphology of developing olfactory axons in the olfactory bulb of 1-21-day-old rabbits was analyzed using stereological methods and the rapid Golgi technique. The findings demonstrated a change in axon morphology from the olfactory nerve layer to the glomeruli suggestive of a sequence in axon development. In the olfactory nerve layer, axons typically had knob-like growth cones and a few collateral branches. Close to glomeruli, axons increased in thickness, formed rather complex and irregular growth cones, and typically gave off many collaterals. Within glomeruli, the axons formed terminal branches and boutons. Extraglomerular branches were apparently removed once axons had entered a glomerulus, insofar as these branches often displayed morphological signs of degeneration. In contrast, collateral branches ending in the same glomerulus remained, indicating that formation of collaterals may assist olfactory axons in locating glomerular targets.

In vivo observations of timecourse and distribution of morphological dynamics in Xenopus retinotectal axon arbors

Changes in neuronal structure can contribute to the plasticity of neuronal connections in the developing and mature nervous system. However, the expectation that they would occur slowly precluded many from considering structural changes as a mechanism underlying synaptic plasticity that occurs over a period of minutes to hours. We took time-lapse confocal images of retinotectal axon arbors to determine the timecourse, magnitude, and distribution of changes in axon arbor structure within living Xenopus tadpoles. Images of axons were collected at intervals of 3 min, 30 min, and 2 h over total observation periods up to 8 h. Branch additions and retractions in arbors imaged at 3 or 30 min intervals were confined to shorter branches. Sites of additions and retractions were distributed throughout the arbor. The average lifetime of branches was about 10 min. Branches of up to 10 microns could be added to the arbor within a single 3 min observation interval. Observations of arbors at 3 min intervals showed rapid changes in the structure of branchtips, including transitions from lamellar growth cones to more streamlined tips, growth cone collaps, and re-extension. Simple branchtips were motile and appeared capable of exploratory behavior when viewed in time-lapse movies. In arbors imaged at 2-h intervals over a total of 8 h, morphological changes included longer branches, tens of microns in length. An average of 50% of the total branch length in the arbor was remodeled within 8 h. The data indicate that the elaboration of the arbor occurs by the random addition of branches throughout the arbor, followed by the selective stabilization of a small fraction of the new branches and the retraction of the majority of branches. Stabilized branches can then elongate and support the addition of more branches. These data show that structural changes in presynaptic axons can occur very rapidly even in complex arbors and can therefore play a role in forms of neuronal plasticity that operate on a timescale of minutes.

Exogenous nitric oxide causes collapse of retinal ganglion cell axonal growth cones in vitro

We show that nitric oxide (NO) from applied NO-donating chemicals induces collapse of ganglion cell axonal growth cones extending from explants of tadpole retina in culture. Peroxynitrite, a neurotoxic product of NO and superoxide reaction, did not induce collapse, and oxyhemoglobin, which binds NO, blocked the highly effective collapsing activity of the NO donor S-nitrosocysteine. Membrane-permeable analogs of cyclic guanosine monophosphate had no collapsing activity. Inhibitors of NO synthase did not induce collapse. NO is a potential retrograde messenger through which postsynaptic neurons signal to their inputs to modify synaptic efficacy following NMDA receptor activation. Our results suggest a role for NO as such a messenger during development of the retinotectal projection.

Growth behavior of retinotectal axons in live zebrafish embryos under TTX-induced neural impulse blockade

The growth dynamics of individual DiO-labeled retinal axons deprived of normal neural impulse activity by TTX was monitored in the tectum of living zebrafish embryos with time-lapse video microscopy and compared with normal active axons. Growth cones of TTX-blocked axons advance intermittently with an average velocity similar to normal axons. While exploring their local environment, they are broadened and bear ruffling lamellipodia and filopodia, but become streamlined when advancing. The activity-deprived axons grow directly towards their retinotopic target sites in the tectum as do their normal counterparts and very rarely extend branches en route. Much like normal axons, TTX-blocked axons begin to branch and develop their terminal arbors only at their retinotopic target area. They emit and retract numerous short side branches over a period of several hours. The area they contact (the "exploration field") is of similar dimension as that of active axons, covering from 1% to 7.4% of the tectal neuropil surface, but the final arbors cover an area only one-half to one-sixth as large. TTX arbors are as small as arbors of normal active axons and retinotopically correct. Thus, the typical exploratory growth behavior of developing retinal axons in the tectum, the dynamics of terminal arbor formation at retinotopically correct sites, the dimension of the exploration field, and the shaping of the arbors in zebrafish embryos are unaffected by TTX-induced neural impulse blockade.

Corticospinal axons and mechanism of target innervation in rat lumbar spinal cord

The aim of the present study is to investigate the mechanism by which outgrowing rat corticospinal (CS) axons innervate their spinal gray target areas. This study was carried out with the use of anterogradely transported horseradish peroxidase or 1,1-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) after application in the sensorimotor cortex of rat pups varying in age between 5 days postnatal (P5) and 10 days postnatal (P10). The CS axons of neurons situated in the sensorimotor cortex have reached the ventral most parts of the dorsal funiculus at mid-lumbar spinal cord levels at the fifth postnatal day (P5). After a waiting period of 2 days some CS fibers change their direction and directly enter the adjacent spinal gray target areas. One day later, i.e., P8, CS target innervation by the formation of collateral branches can be observed. Finally, the development of collaterals by interstitial budding from their parent axons appears to be the major, but not the exclusive, mechanism by which CS axons innervate the lumbar spinal gray matter target area.

How do retinal axons find their targets in the developing brain?

Tissue culture studies show that cell survival and process outgrowth from retinal ganglion cells depend on the molecular composition of the substrates over which the neurites grow, and on diffusible factors present in the medium. Recent work has begun to show that at least some of these components might be interactive. Since the conditions in a culture dish, as well as the patterns of antigen expression on cells in vitro, can differ considerably from those encountered in vivo, it is important to design experiments in vivo that examine how growing neurites relate to their natural microenvironment. By the use of transplantation techniques, it has been possible to provide evidence for a comparable duality of substrate-dependent and target-derived controls of optic axon growth, which might provide insight into the normal developmental process.

Differences in lipid composition between isolated growth cones from the forebrain and those from the brainstem in the fetal rat

The lipid composition of nerve growth cone membranes isolated from rat fetal forebrain or brainstem by the sucrose density gradient method was analyzed biochemically and immunochemically. In the forebrain, growth cone membrane (GCM) contained lower levels of gangliosides than those from other heavier fractions, but it was not the case in the fetal brainstem at the same developmental stage. The distinctive features in the ganglioside composition of GCM are the predominance of GD3 and the presence of c-series gangliosides that are due to fetal expression in mammals. A unique acidic glycolipid, sulfoglucuronylparagloboside (SGPG), which is not present in adult brains, was first detected in both forebrain and brainstem GCM. Including such minor species, the ganglioside composition in forebrain or brainstem GCM was almost identical to other membrane fractions from the forebrain or brainstem. The compositional ratios of the major lipid classes in membranes, cholesterol and phospholipids, seemed to be common to forebrain GCM and brainstem GCM, as indicated by the identical values of phospholipid-to-protein (PL/Pr), cholesterol-to-protein (Ch/Pr), and cholesterol-to-phospholipid (Ch/PL) ratios for both. This study has revealed that GCM isolated from forebrain which is supposed to be at an earlier stage of neuronal differentiation than brainstem has less amounts of total gangliosides, high proportion of GD3 to GD1a and enriched c-series gangliosides as compared to brainstem GCM.

An aberrant retinal pathway and visual centers in Xenopus tadpoles share a common cell surface molecule, A5 antigen

A monoclonal antibody A5 (MAb-A5), which was raised against Xenopus tadpole tectal cells, recognizes a cell surface-related protein molecule (A5 antigen) expressed on the visual centers of Xenopus tadpoles (S. Takagi, T. Tsuji, T. Amagai, T. Takamatsu, and H. Fujisawa, 1987, Dev. Biol. 122, 90-100). The present immunohistochemistry using MAb-A5 indicated that, in addition to the visual centers, A5 antigen was expressed on the general somatic sensory tract in the medulla and spinal cord of Xenopus tadpoles. As the general somatic sensory tract has been shown to be a pathway for ectopically transplanted retinal axons (M. Constantine-Paton and R. R. Capranica, 1976, J. Comp. Neurol. 170, 17-32; M. J. Katz and R. J. Lasek, 1979, J. Comp. Neurol. 183, 817-832), we examined whether retinal axons transplanted close to the spinal cord or medulla preferentially grow into the A5 antigen-positive general somatic sensory tract. We performed eye transplantation at embryonic stages and detected precise locations and trajectories of transplanted retinal axons within the medulla and spinal cord in tadpoles after filling retinal axons with horseradish peroxidase (HRP). HRP histochemistry in combination with MAb-A5 immunohistochemistry indicated that almost all HRP-filled transplanted retinal axons joined the A5 antigen-positive general somatic sensory tract. These findings suggest the involvement of A5 antigen in specific cell-cell recognition between retinal axons and their targets.

Staining of regenerated optic arbors in goldfish tectum: progressive changes in immature arbors and a comparison of mature regenerated arbors with normal arbors

Individual optic arbors, normal and regenerated, were stained via anterograde transport of HRP and viewed in tectal whole mounts. Camera lucida drawings were made of 119 normal optic arbors and of 242 regenerated arbors from fish 2 weeks to 14 months postcrush. These arbors were analyzed for axonal trajectory, spatial extent in the horizontal plane, degree of branching, number of branch endings, average depth, and degree of stratification. Normal optic arbors ranged in size from roughly 100 to 400 microns across in a continuous distribution, had an average of 20 branch endings with average of fifth-order branching, and were highly stratified into one of three planes within the major optic lamina (SO-SFGS). Small arbors arising from fine-caliber axons terminated in the most superficial plane of SO-SFGS; large arbors from coarse axons terminated in the superficial and middle planes; and medium arbors from medium-caliber axons terminated in the middle and deep planes of SO-SFGS, as well as deeper in the central gray and deep white layers. Arbors from central tectum tended to be much more tightly stratified than those in the periphery. No other differences between central and peripheral arbors were noted. Mature regenerated arbors (five months or more postcrush) were normal in their number of branch endings, order of branching, and depth of termination. Their branches covered a wider area of tectum, partially because of their early branching and abnormal trajectories of branches. Axonal trajectories were often abnormal with U-turns and tortuos paths. Fine-, medium-, and coarse-caliber axons were again present and gave rise to small, medium, and large arbors at roughly the same depths as in the normals. There was frequently a lack of stratification in the medium and large arbors, which spanned much greater depths than normal. Overall, however, regenerates reestablished nearly normal morphology except for axonal trajectory and stratification. Early in regeneration, the arbors went through a series of changes. At 2 weeks postcrush, regenerated axons had grown branches over a wider-than-normal extent of tectum, though they were sparsely branched and often tipped with growth cones. At 3 weeks, the branches were more numerous and covered a still wider extent (average of five times normal), many covering more than half the tectal length or width. At 4-5 weeks smaller arbors predominated, although a few enlarged arbors were present for up to 8 weeks. Additional small changes occurred beyond 8 weeks as the arbors became progressively more normal in appearance.(ABSTRACT TRUNCATED AT 400 WORDS)

Axonal growth cones in the developing amphibian spinal cord

Axonal growth cones in the spinal cord of embryonic and larval Xenopus (stages 24-48) were filled with the anatomical tracer horseradish peroxidase (HRP). Growth cones of lateral and ventral marginal zones, including those of descending spinal and supraspinal pathways, were labeled by application of tracer to the caudal medulla or to one of several levels of the spinal cord. Central axons of sensory neurons were filled via their peripheral processes. Growth cone configuration varied widely but fell into five general categories: complex with both filopodia and veils, filopodia only, lamellipodia only, clavate, and fusiform. Several general patterns emerged from the distribution of these various configurations. Growth cones of younger animals generally were more complex than those of older ones. Growth cones closer to the leading edge of descending fiber bundles were more elaborate than those that followed. Growth cones of the dorsolateral fascicle, which carries ascending central processes of Rohon-Beard and sensory ganglion neurons, were very simple and followed a straight course rostrally, whereas those of descending axons of the lateral fiber areas were more complex and sometimes spread over almost the entire lateral marginal zone. Growth cones of Rohon-Beard central ascending axons were fusiform or clavate, while those of sensory ganglion axons showed several fine filopodia at their tips. Growth cones of both types of sensory axons change configuration as they approached the hindbrain, becoming more complex. This study demonstrates that the configurations of growth cones belonging to the same axonal pathway vary with age and with position along their routes, and that growth cones of different neuron classes exhibit characteristic ranges of morphological variation.

Mode of growth of retinal axons within the tectum of Xenopus tadpoles, and implications in the ordered neuronal connection between the retina and the tectum

Retinal axons of Xenopus tadpoles at various stages of larval development were filled with horseradish peroxidase (HRP), and their trajectories and the patterns of branching within the tectum were analyzed in wholemount preparations. To clarify temporal and spatial modes of growth of retinal axons during larval development, special attention was directed to labeling a restricted regional population of retinal axons with HRP, following reported procedures (H. Fujisawa, K. Watanabe, N. Tani, and Y. Ibata, Brain Res. 206:9-20, 1981; 206:21-26, 1981; H. Fujisawa, Dev. Growth Differ 26:545-553, 1984). In developing tadpoles, individual retinal axons arrived at the tectum, without clear sprouting. Axonal sprouting first began when growing tips of each retinal axon had arrived at the vicinity of its site of normal innervation within the tectum. Thus, the terminals of the newly added retinal axons were retinotopically aligned within the tectum. The retinotopic alignment of the terminals may be due to an active choice of topographically appropriate tectal regions by growth cones of individual retinal axons. The stereotyped alignment of the newly added retinal axons was followed by widespread axonal branching and preferential selection of those branches. Each retinal axon was sequentially bifurcated within the tectum, and old branches that had inevitably been left at ectopic parts of the tectum (owing to tectal growth) were retracted or degenerated in the following larval development. The above mode of axonal growth provides an adequate explanation of cellular mechanisms of terminal shifting of retinal axons within the tectum during development of retinotectal projection. Selection of appropriate branches may also lead to a reduction in the size of terminal arborization of retinal axons, resulting in a refinement in targeting.