Rules for retinotectal terminal arborizations in the goldfish optic tectum: a whole-mount study

In vivo evidence of microstructural hypo-connectivity of brain white matter in 22q11.2 deletion syndrome

22q11.2 deletion syndrome, or 22q11.2DS, is a genetic syndrome associated with high rates of schizophrenia and autism spectrum disorders, in addition to widespread structural and functional abnormalities throughout the brain. Experimental animal models have identified neuronal connectivity deficits, e.g., decreased axonal length and complexity of axonal branching, as a primary mechanism underlying atypical brain development in 22q11.2DS. However, it is still unclear whether deficits in axonal morphology can also be observed in people with 22q11.2DS. Here, we provide an unparalleled in vivo characterization of white matter microstructure in participants with 22q11.2DS (12-15 years) and those undergoing typical development (8-18 years) using a customized magnetic resonance imaging scanner which is sensitive to axonal morphology. A rich array of diffusion MRI metrics are extracted to present microstructural profiles of typical and atypical white matter development, and provide new evidence of connectivity differences in individuals with 22q11.2DS. A recent, large-scale consortium study of 22q11.2DS identified higher diffusion anisotropy and reduced overall diffusion mobility of water as hallmark microstructural alterations of white matter in individuals across a wide age range (6-52 years). We observed similar findings across the white matter tracts included in this study, in addition to identifying deficits in axonal morphology. This, in combination with reduced tract volume measurements, supports the hypothesis that abnormal microstructural connectivity in 22q11.2DS may be mediated by densely packed axons with disproportionately small diameters. Our findings provide insight into the in vivo white matter phenotype of 22q11.2DS, and promote the continued investigation of shared features in neurodevelopmental and psychiatric disorders.

Decreased Axon Caliber Underlies Loss of Fiber Tract Integrity, Disproportional Reductions in White Matter Volume, and Microcephaly in Angelman Syndrome Model Mice

Angelman syndrome (AS) is a debilitating neurodevelopmental disorder caused by loss of function of the maternally inherited UBE3A allele. It is currently unclear how the consequences of this genetic insult unfold to impair neurodevelopment. We reasoned that by elucidating the basis of microcephaly in AS, a highly penetrant syndromic feature with early postnatal onset, we would gain new insights into the mechanisms by which maternal UBE3A loss derails neurotypical brain growth and function. Detailed anatomical analysis of both male and female maternal Ube3a-null mice reveals that microcephaly in the AS mouse model is primarily driven by deficits in the growth of white matter tracts, which by adulthood are characterized by densely packed axons of disproportionately small caliber. Our results implicate impaired axon growth in the pathogenesis of AS and identify noninvasive structural neuroimaging as a potentially valuable tool for gauging therapeutic efficacy in the disorder.SIGNIFICANCE STATEMENT People who maternally inherit a deletion or nonfunctional copy of the UBE3A gene develop Angelman syndrome (AS), a severe neurodevelopmental disorder. To better understand how loss of maternal UBE3A function derails brain development, we analyzed brain structure in a maternal Ube3a knock-out mouse model of AS. We report that the volume of white matter (WM) is disproportionately reduced in AS mice, indicating that deficits in WM development are a major factor underlying impaired brain growth and microcephaly in the disorder. Notably, we find that axons within the WM pathways of AS model mice are abnormally small in caliber. This defect is associated with slowed nerve conduction, which could contribute to behavioral deficits in AS, including motor dysfunction.

The balance of NMDA- and AMPA/kainate receptor-mediated activity in normal adult goldfish and during optic nerve regeneration

Retinotectal topography is established during development and relies on the sequential recruitment of glutamate receptors within postsynaptic tectal cells. NMDA receptors underpin plastic changes at early stages when retinal ganglion cell (RGC) terminal arbors are widespread and topography is coarse; AMPA/kainate receptors mediate fast secure neurotransmission characteristic of mature circuits once topography is refined. Here, we have examined the relative contributions of these receptors to visually evoked activity in normal adult goldfish, in which retinotectal topography is constantly adjusted to compensate for the continual neurogenesis and the addition of new RGC arbors. Furthermore, we examined animals at two stages of optic nerve regeneration. In the first, RGC arbors are widespread and receptive fields large resulting in coarse topography; in the second, RGC arbors are pruned to reduce receptive fields leading to refined topography. Antagonists were applied to the tectum during multiunit recording of postsynaptic responses. Normal goldfish have low levels of NMDA receptor-mediated activity and high levels of AMPA/kainate. When coarse topography has been restored, NMDA receptor-mediated activity is increased and that of AMPA/kainate decreased. Once topography has been refined, the balance of NMDA and AMPA/kainate receptor-mediated activity returns to normal. The data suggest that glutamatergic neurotransmission in normal adult goldfish is dual with NMDA receptors fine-tuning topography and AMPA receptors allowing stable synaptic function. Furthermore, the normal operation of both receptors allows a response to injury in which the balance can be transiently reversed to restore topography and vision.

Development of the visual system of the chick. II. Mechanisms of axonal guidance

The quest to understand axonal guidance mechanisms requires exact and multidisciplinary analyses of axon navigation. This review is the second part of an attempt to synthesise experimental data with theoretical models of the development of the topographic connection of the chick retina with the tectum. The first part included classic ideas from developmental biology and recent achievements on the molecular level in understanding cytodifferentiation and histogenesis [J. Mey, S. Thanos, Development of the visual system of the chick. (I) Cell differentiation and histogenesis, Brain Res. Rev. 32 (2000) 343-379]. The present part deals with the question of how millions of fibres exit from the eye, traverse over several millimetres and spread over the optic tectum to assemble a topographic map, whose precision accounts for the sensory performance of the visual system. The following topics gained special attention in this review. (i) A remarkable conceptual continuity between classic embryology and recent molecular biology has revealed that positional cellular specification precedes and determines the formation of the retinotectal map. (ii) Graded expression of asymmetric genes, transcriptional factors and receptors for signal transduction during early development seem to play a crucial role in determining the spatial identity of neurons within surface areas of retina and optic tectum. (iii) The chemoaffinity hypothesis constitutes the conceptual framework for development of the retinotopic organisation of the primary visual pathway. Studies of repulsive factors in vitro developed the original hypothesis from a theoretical postulate of chemoattraction to an empirically supported concept based on chemorepulsion. (iv) The independent but synchronous development of retina and optic tectum in topo-chronologically corresponding patterns ensures that ingrowing retinal axons encounter receptive target tissue at appropriate locations, and at the time when connections are due to be formed. (v) The growth cones of the retino-fugal axons seem to be guided both by local cues on glial endfeet and within the extracellular matrix. On the molecular level, the ephrins and their receptors have emerged as the most likely candidates for the material substrate of a topographic projection along the anterior-posterior axis of the optic tectum. Yet, since a number of alternative molecules have been proposed for the same function, it remains the challenge for the near future to define the proportional contribution of each one of the individual mechanisms proposed by matching theoretical predictions with the experimental evidence.

Topographic restriction of TAG-1 expression in the developing retinotectal pathway and target dependent reexpression during axon regeneration

TAG-1, a glycosylphosphatidyl inositol (GPI)-anchored protein of the immunoglobulin (Ig) superfamily, exhibits an unusual spatiotemporal expression pattern in the fish visual pathway. Using in situ hybridization and new antibodies (Abs) against fish TAG-1 we show that TAG-1 mRNA and anti-TAG-1 staining is restricted to nasal retinal ganglion cells (RGCs) in 24- to 72-h-old zebrafish embryos and in the adult, continuously growing goldfish retina. Anti-TAG-1 Abs selectively label nasal RGC axons in the nerve, optic tract, and tectum. Axotomized RGCs reexpress TAG-1, which occurs as late as 12 days after optic nerve lesion, when regenerating RGC axons arrive in the tectum, suggesting TAG-1 reexpression is target contact-dependent. Accordingly, TAG-1 reexpression ceases upon interruption of the regenerating projection by a second lesion. The topographic restriction of TAG-1 expression and its target dependency during regeneration suggests that TAG-1 might play a role in the retinotopic organization and restoration of the retinotectal pathway.

Axonal conduction velocities of functionally characterized retinal ganglion cells in goldfish

1. Visual response properties and conduction velocities of retinal ganglion cells were studied by extracellular recordings in the intact goldfish eye. Visually responsive single units were confirmed as ganglion cells by collision testing, and their receptive fields were mapped. 2. From compound action potentials, we identified groups I-V in the optic nerve, with overall conduction velocities of 11.5 +/- 1.17, 7.1 +/- 0.79, 4.4 +/- 0.56, 3.1 +/- 0.31 and 2.3 +/- 0.18 m s-1 (mean +/- S.D.) at 23 degrees C. 3. Ganglion cells were classified by their receptive fields as off-, on-off- or on-centre. Nearly all confirmed ganglion cells had axonal conduction velocities in groups II, III and IV; none fell in the fastest group, I. 4. Off-centre ganglion cells had conduction velocities only in the fast group, II. On-off-centre cells fell mainly in group III, with some in group, II. On-centre cells fell in groups II-V, but mainly in groups III and IV. 5. Receptive field centre diameters were 5-30 deg measured with a photopic background. The mean diameters for off-, on-off- and on-centres were 24, 15 and 18 deg, respectively. The relatively larger diameter and higher rate of spontaneous firing of the off-centre cells were maintained under different adaptation conditions. 6. The off-centre cells can be identified with an anatomical class of large, alpha-like ganglion cells in the goldfish retina.

Possible involvement of gradients in guidance of receptor cell axons towards their target position on the olfactory bulb

There is increasing evidence for directional guidance of growing axons by molecular gradients in target tissues. Aside from biochemical studies on gradients and their role, the capability of axons to approach their target position from different aspects of a two-dimensional field is itself an indication for guidance by gradients. According to this criterion, such guidance is expected to be involved not only in map-formation in the visual system but also in targeting of receptor cell axons in the olfactory bulb. In this paper, physico-chemical concepts of visual mapping are adapted to olfactory targeting. In both cases there must be sophisticated processing of graded cues in the growing tip of the axon for growth cone navigation. In visual map formation, a target position is determined by influences of cues depending on the position of axonal origin; in olfactory targeting, however, these influences are expected to be based on properties of the receptor-cell-specific molecules (possibly including the receptor molecule itself), as well as by gene regulation affecting the levels of expression. According to this concept, the main role of molecules expressed in a receptor-cell-type specific manner is not matching specific counterparts on the target tissue, but instead quantitative modulation of growth cone steering for sensing the direction towards the target position.

Axonal regrowth after spinal cord transection in adult zebrafish

Using axonal tracers, we characterized the neurons projecting from the brain to the spinal cord as well as the terminal fields of ascending spinal projections in the brain of adult zebrafish with unlesioned or transected spinal cords. Twenty distinct brain nuclei were found to project to the spinal cord. These nuclei were similar to those found in the closely related goldfish, except that additionally the parvocellular preoptic nucleus, the medial octavolateralis nucleus, and the nucleus tangentialis, but not the facial lobe, projected to the spinal cord in zebrafish. Terminal fields of axons, visualized by anterograde tracing, were seen in the telencephalon, the diencephalon, the torus semicircularis, the optic tectum, the eminentia granularis, and throughout the ventral brainstem in unlesioned animals. Following spinal cord transection at a level approximately 3.5 mm caudal to the brainstem/spinal cord transition zone, neurons in most brain nuclei grew axons beyond the transection site into the distal spinal cord to the level of retrograde tracer application within 6 weeks. However, the individually identifiable Mauthner cells were never seen to do so up to 15 weeks after spinal cord transection. Nearly all neurons survived axotomy, and the vast majority of axons that had grown beyond the transection site belonged to previously axotomized neurons as shown by double tracing. Terminal fields were not re-established in the torus semicircularis and the eminentia granularis following spinal cord transection.

Imaging of individual normal and regenerating optic fibers in the brain of living adult goldfish

Retinal arbors in the tectum of living adult goldfish were imaged to determine whether the structural remodelling and refinement that occurs during development continues in adulthood. Individual optic fibers were labelled by making small injections of the lipophilic fluorescent dye DiI into ventral retina and viewing the exposed tectum through a fluorescence microscope equipped with a cooled CCD camera. Arbors were imaged in the living fish every 30-60 minutes for up to 7 hours. Normal adult goldfish showed no evidence of arbor remodelling during this period, though dynamic movements of varicosities present along axon segments were observed. For comparison, regenerating optic fibers were similarly imaged in fish that had undergone optic nerve crush 2-6 weeks previously. In these fish, dynamic structural changes were seen, including branch remodelling, extension and retraction of growth cones, and movement of varicosities.

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.

Activity-driven sharpening of the retinotectal projection in goldfish: development under stroboscopic illumination prevents sharpening

Blocking or synchronizing activity during regeneration of the retinotectal projection prevents both the sharpening of the retinotopic map recorded on tectum and the refinement of the structure of individual arbors within the plane of the map, and this refinement is triggered by N-methyl-D-aspartate (NMDA) receptors. We tested whether activity-driven refinement also occurs during development of the projection in larval and young adult goldfish. Shortly after hatching, larval goldfish were placed into tanks within light-tight chambers illuminated by a xenon strobe at 1 Hz for 14 h of each daily cycle. Fish were reared for 1.5-2 years, until large enough to record in our retinotectal mapping apparatus (6 cm length). Age- and size-matched controls had normal maps with multiunit receptive fields (MURFs) recorded at each tectal point of 10.8 degrees (0.16 S.E.M., n = 5), whereas the strobe-reared fish had only roughly retinotopic maps with much enlarged MURFs averaging 26.7 degrees (1.41 S.E.M., n = 5). This enlargement represents an abnormal convergence onto each tectal point, as the maps failed to sharpen during development. The arbors of individual retinal axons were stained with horseradish peroxidase (HRP) in larval fish and in adult strobe-reared and control fish. They were drawn with camera lucida from tectal whole mounts, and analyzed for spatial extent in the plane of the retinotopic map, order of branching, number of branch endings, depth of termination, and caliber of the parent axon. Arbors from larval fish (1-2 weeks) were small (approximately 50 x 40 microns) with less than 10 branches, occupied a single strata, and could not be separated into different classes by caliber of axon. The 87 arbors stained in control adult fish (6 cm long) were much like previously examined adult arbors, with those from fine, medium, and coarse axons averaging 115, 166, and 194 microns in extent, respectively, and having 17-24 branch endings. The 110 arbors from 12 strobe-reared fish were often abnormal. Although the fasciculation was normal, the extrafascicular routes were abnormal with reversing turns. The axons often had branches along their course, and these branches were scattered across a wider extent, rather than forming a distinct cluster. In contrast, neither the number of branches nor the depths of termination was significantly changed in any group. The coarse caliber arbors were most abnormal, being 64% longer and 30% wider than controls. The fine caliber arbors were also significantly larger by about 20%, but the medium caliber arbors were not enlarged.(ABSTRACT TRUNCATED AT 400 WORDS)

Influence of exogenous gangliosides on the three-dimensional sprouting of goldfish retinal explants in vitro

To investigate the 3-dimensional outgrowth of ganglion cells of normal and regenerating goldfish retina, retinal explants were cultured in a serum free 3-D fibrin matrix. Daily applications of exogenous gangliosides (GM1), injected either intraocularly (i.o.) or intraperitoneally (i.p.) had no significant effect on the sprouting activity of retinal explants prepared from lesion-activated goldfish whose corresponding optic nerve had been transected. However, in normal, unlesioned animals, a local i.o. injection of GM1 or mixed gangliosides led to a significant enhancement of the basal retinal sprouting activity as compared to controls, which were injected with a 0.9% NaCl solution. This ganglioside related stimulation was maximal after i.o. injection of low concentrations (3 micrograms/eye), didn't occur at high concentrations (30 micrograms/eye) and was similar to the response obtained after i.o. injection of NGF or insulin. I.o. injected phospholipids had no or a slightly inhibitory effect on the sprouting activity as compared to NaCl controls. Daily in vivo i.o. injections of the monoclonal antibody Q211, specifically recognizing c-pathway polysialogangliosides, led to a dose dependent inhibition of the in vitro sprouting of goldfish retina explants. In summary, these data suggest an involvement of gangliosides in the complex process of induction of neuronal sprouting.

Neurolin, a cell surface glycoprotein on growing retinal axons in the goldfish visual system, is reexpressed during retinal axonal regeneration

The mAb E 21 recognizes a cell surface glycoprotein selectively associated with fish retinal ganglion cell axons that are in a state of growth. All retinal axons and ganglion cells in goldfish embryos stained for E 21. In adult fish, however, E 21 immunoreactivity exhibited a patterned distribution in ganglion cells in the marginal growth zone of the continuously enlarging fish retina and the new axons emerging from these cells in the retina, optic nerve, and optic tract. The E 21 antigen was absent from older axons, except the terminal arbor layer in the tectum, the Stratum fibrosum et griseum superficiale where it was uniformly distributed. Upon optic nerve transection, the previously unlabeled axons reacquired E 21 positivity as they regenerated throughout their path to the tectum. Several months after ONS, however, E 21 staining disappeared from the regenerated axons over most of their lengths but reappeared as in normal fish in the terminal arbor layer. The immunoaffinity-purified E 21 antigen, called Neurolin, has an apparent molecular mass of 86 kD and contains the HNK1/L2 carbohydrate moiety, like several members of the class of cell adhesion molecules of the Ig superfamily. The NH2-terminal amino acid sequence has homologies to the cell adhesion molecule DM-Grasp recently described in the chicken. Thus, retinal ganglion cell axons express Neurolin during their development and are able to reexpress this candidate cell adhesion molecule during axonal regeneration, suggesting that Neurolin is functionally important for fish retinal axon growth.

Why run parallel fibers parallel? Teleostean Purkinje cells as possible coincidence detectors, in a timing device subserving spatial coding of temporal differences

The present paper explores the possible functional significance of the parallel orientation of parallel fibers in teleostean cerebellar and cerebelloid molecular layers, taking advantage of the restricted width of these molecular layers compared with mammalian ones and several specific configurations of granule cells. These configurations include: (i) a unilateral location, i.e. at only one (lateral) side of the molecular layer, giving rise to parallel fibers without bifurcation in a unidirectional molecular layer, where all parallel fibers conduct signals in the same direction; (ii) a bilateral location at both sides of the molecular layer giving rise to a bidirectional molecular layer where parallel fibers conduct signals in two opposite directions originating from two discrete sources; and (iii) a basal (or sometimes apical) location underneath (or opposite to) the layer of Purkinje cells, giving rise to a bidirectional molecular layer where parallel fibers conduct signals in two opposite directions originating from a continuous range of sources. It is argued that molecular layers with a bilateral location of granule cells, exemplified by the mormyrid lobus transitorius, represent an optimal configuration for the analysis of small temporal differences (up to 4 ms) between inputs to the right and left granule cell mass, by means of detection of the site of coincidence of parallel fiber activity running from left to right and vice versa. Morphological aspects that probably optimize such a function include not only the parallel course and bilateral origin of parallel fibers, but also their small diameter, large number and co-extensive location, as well as the sagittal orientation and the presence of many spines of Purkinje cell dendrites and the presence of stellate and other inhibitory interneurons. The only assumption underlying the present coincidence detection hypothesis is that Purkinje cells are supposed to be maximally stimulated by parallel fiber input when all spines are activated in such a way that their excitatory postsynaptic potentials reach the axon hillock simultaneously. For molecular layers with a unilateral location of granule cells, exemplified by the teleostean torus longitudinalis-tectal marginal parallel fiber system, a similar coincidence detecting mechanism is proposed on the basis of the presence of two populations of parallel fibers with slightly different conduction velocities. Such a system might be suitable to adapt the location of coincidence peaks to topographic maps present in deeper layers of nervous tissue. Molecular layers with basally (or apically) located granule cells as encountered in the teleostean corpus cerebelli, are probably involved in the analysis of specific spatio-temporal input waves directed centripetally towards different Purkinje cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Evidence for a driving role of ingrowing axons for the shifting of older retinal terminals in the tectum of fish

In amphibians and teleosts, retina and tectum grow incongruently. In order to maintain the retinotopy of the retinotectal projection, Gaze, Keating, and Chung (1974) postulated a shifting of terminals throughout growth. In order to test the possibility that ingrowing retinal fibers are the driving force for this shifting, we induced a permanent retinal projection into the ipsilateral tectum in juveniles of the cichlid fish Haplochromis burtoni. The surface of the tectum had increased (11-18 months later) 2.5-5.8 times, and the surface of the retina 8.6-14 times. Filling of ganglion cells with horseradish peroxidase (HRP) retrogradely from the tectum showed ipsilaterally regenerating ganglion cells only in the center of the retina. The position of ganglion cells indicated that the ipsilateral projection derived only from axotomized and regenerating retinal ganglion cells but not from those newly born. Ipsilaterally projecting retinal fibers showed terminals only in the rostral half of the tectum. Comparison of area of terminations of ipsilaterally projecting ganglion cells at various times after the crush provided no evidence for expansion or a shift into caudal tectal areas throughout the period of growth. These findings are compatible with the idea that newly ingrowing fibers induce older terminals to move caudally.

Activity-driven sharpening of the regenerating retinotectal projection: effects of blocking or synchronizing activity on the morphology of individual regenerating arbors

Both blocking activity with intraocular tetrodotoxin (TTX) and synchronizing activity with a xenon strobe light (1 Hz) prevent retinotopic sharpening of regenerating optic projection in goldfish (Meyer, 1983; Schmidt, 1985; Cook and Rankin, 1986). In this study, we tested, in both normal and regenerating projections, the effects of these two treatments on individual optic arbors. Arbors were stained via anterograde transport of HRP, drawn in camera lucida from tectal whole mounts, and analyzed for spatial extent in the plane of the retinotopic map, order of branching, number of branch endings, depth of termination, and the caliber of the parent axon. In normal tectum, fine, medium, and coarse caliber axons gave rise to small, medium, and large arbors, which averaged 127 microns, 211 microns and 275 microns in horizontal extent, and terminated at characteristic depths. All three classes averaged roughly 21 branch endings. Optic arbors that regenerated with normal patterns of activity returned to a roughly normal appearance by 6-11 weeks postcrush: the same three calibers of axons gave rise to the same three sizes of arbors at the same depths, but they were much less stratified and well on average about 16% larger in horizontal extent. At this time point, arbors regenerated under TTX or strobe were on the average 71 and 119% larger, respectively, than the control-regenerated arbors (larger in all classes), although they had approximately the same number of branch endings and were equally poorly stratified. Synapses formed under strobe were also normal in appearance. Thus the only significant effect of both strobe and TTX treatment was to enlarge the spatial extent of arbor branches. Arbors that were not regenerating were very slightly (but significantly) enlarged by TTX block of activity or strobe illumination. As previous staining showed that regenerating axons initially make widespread branches and later retract many of those branches (Schmidt, Turcotte, Buzzard, and Tieman, 1988; Stuermer, 1988), the present findings support the idea that blocking activity or synchronizing activity prevents retinotopic sharpening by interfering with the elimination of some of the errant branches.

Analysis of binary trees when occasional multifurcations can be considered as aggregates of bifurcations

The geometrical properties of neurons are important for the way they function within neural circuits. The arborescent processes of neurons that are necessary for the transmission of the information are formed by branching and elongation of segments. In studies that model the outgrowth the tree structures have generally been considered as binary. However, multifurcations do occur. It will be shown that if the multifurcations can be considered as aggregates of bifurcations they may be included in the topological analysis of neuronal branching patterns.

Target recognition and dynamics of axonal growth in the retinotectal system of fish

Embryonic and regenerating retinal axons in fish are able to seek out their retinotopic target sites in the tectum. Neither a specific preordering of axons in the retinotectal pathway nor activity-dependent axon-target interactions are required for appropriate axonal targeting. Axon-target recognition appears to be predominantly mediated by positional cell surface markers. The discrimination of position-dependent differences by retinal axons in a special in vitro assay is consistent with this concept. To understand retinal axonal regeneration we have analyzed the glial cells of the fish optic nerve and the expression of growth-associated cell surface molecules on the regenerating axons. The surfaces of the glial cells identified as oligodendrocytes are excellent substrates for the elongation of regenerating axons. Raising monoclonal antibodies we have found 3 cell surface proteins specific for growing axons. In the normal adult goldfish optic nerve, these proteins are only expressed by the few new axons from the newborn ganglion cells at the retinal margin. They are re-expressed on all axons during regeneration. A known cell surface molecule, NCAM, is expressed in a similar, specific spatiotemporal pattern on the fish retinal axons and may--in normal nerves--contribute to the establishment of the age-related fiber association. Whether the re-expression of NCAM and the antigens detected by the novel monoclonal antibodies are functionally involved in axonal growth and regeneration remains to be investigated.

Pathfinding and target selection of goldfish retinal axons regenerating under TTX-induced impulse blockade

To define the extent to which impulse blockade interferes with the morphological changes of regenerating retinal axons during their growth through the tectum, axons were deprived of activity by repeated intraocular injections of TTX. At intervals between 24 and 189 days after optic nerve section (ONS), a defined group of TTX-silenced axons and of axons with normal activity (controls) were labeled by applications of HRP to the ventro- or dorsotemporal retina. The trajectories of these labeled axons were traced in DAB processed tectal wholemounts. As in controls, TTX-blocked axons went through a phase of exploratory growth at early regeneration stages (24 to 80 days after ONS). Coursing in abnormal routes, the axons initially distributed their growing endings widely over the tectum. Axons with and without activity extended side branches with growth cones and filopodia over all regions of the tectum. These ramifications were of similar dimensions for the TTX-blocked and control axons. Despite abnormal routes and branching over inappropriate territories, axons showed a preference for the rostral tectum. At late regeneration stages (120-189 days after ONS), axons had lost their side branches and their growth cones. Their preterminal segments exhibited striking bends, suggesting that they had undergone course corrections to achieve access to the retinotopic target. Axonal processes had disappeared from the caudal tectum, and the preferential accumulation of axons over the rostral tectum had increased. The majority of the TTX-blocked and control axons ended in terminal arbors at retinotopic regions. The labeled arbors of the TTX-group were no larger than those of the control group. The arbors of each group lay close together in a continuous cluster in the TTX-group as well as in two-thirds of the control group. In the other one-third of the control group, however, terminal arbors were aggregated into separate patches. The clusters of the TTX-blocked axons covered between 2.2 and 3.9% (mean 2.95%) of the tectal surface and the clusters and/or patches of active axons between 1.9 and 3.4% (mean 2.7%). Thus the terminal arbor clusters of the TTX-silenced axons were not significantly larger than those of the active axons. These data show that retinal ganglion cell impulse activity is required for neither the extension of side branches in the early exploratory phase of regeneration nor for the withdrawal of these branches nor for the establishment of target-directed routes and the deployment of normal-size terminal arbors at retinotopic loci.(ABSTRACT TRUNCATED AT 400 WORDS)

Developing retinotectal projection in larval goldfish

The retinotectal projection in larval goldfish was studied with the aid of anterograde filling of optic fibers with HRP applied to the retina. The results show that optic fibers have already reached the tectum and begun to form terminal arbors in newly hatched fish. The projection is topographic in that fibers from local regions of the retina project to discrete patches of tectum, with the smallest patch covering 3.5% of the total surface area of tectal neuropil. Many fibers in young larvae have numerous short side branches along their length and only some of them show evidence of terminal sprouting. The arbors are approximately elliptical in shape and average about 1,500 microns 2. Growth cones are seen frequently. In older larvae, terminal arbors are larger and more highly branched, and they have begun to resemble those in adult fish. Fibers terminate in two strata; those in the upper layer are smaller (1,800 microns 2 on average) than those in the deeper stratum (4,000 microns 2 on average). The fraction of tectal surface area covered by individual arbors (the "tectal coverage") ranges from 1.5% to 3% of the total surface area of the tectal neuropil. In contrast, the tectal coverage of individual arbors in young adult goldfish is much smaller, ranging from 0.02% to 0.42% of tectal surface area (Stuermer, '84, and unpublished). This apparent increase in precision of the map in older animals is not due to retraction of arbors, which are slightly larger in adults, but is accounted for by overall tectal growth: the tectal neuropil in goldfish increases in area by about 250-fold during this period (Raymond, '86).

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)

Trajectories of regenerating retinal axons in the goldfish tectum: I. A comparison of normal and regenerated axons at late regeneration stages

To visualize and compare the intratectal path of normal and regenerated retinal axons, HRP was applied to localized sites in the dorsotemporal and dorsonasal retina in normal goldfish and in goldfish at 3-12 months after optic nerve section. The anterogradely labeled axons were traced in tectal whole mounts. In normal animals the axons were confined to the appropriate ventral hemitectum. Therein they ran in very orderly routes (Stuermer and Easter: J. Neurosci. 4:1045-1051, '84) and terminated in regions retinotopic to the labeled ganglion cells in the retina. The terminal arbors of dorsotemporal axons resided in the ventrorostral tectum and those of dorsonasal axons in the ventrocaudal tectum. In regenerating animals the terminal arbors also resided at retinotopic regions, where they sometimes formed two separate clusters. In contrast to normal axons, the regenerating ones traveled in abnormal routes through the appropriate and inappropriate hemitectum. From various ectopic positions, they underwent course corrections to redirect their routes toward the retinotopic target region. In their approach toward their target sites, dorsotemporal and dorsonasal axons behaved differently in that the vast majority of dorsotemporal axons coursed over the more rostral tectum whereas dorsonasal axons progressed into the caudal tectal half. This differential behavior of regenerating dorsonasal and dorsotemporal axons was substantiated by a quantitative evaluation of axon numbers and orientations.

Trajectories of regenerating retinal axons in the goldfish tectum: II. Exploratory branches and growth cones on axons at early regeneration stages

HRP was applied to small sites in the dorsotemporal or dorsonasal retina in fish at 10-36 days after optic nerve section. The anterogradely labeled axons were visualized in tectal whole mounts. Axons traveled through all regions of the tectum in various abnormal routes. Misrouted axons were also seen to alter their orientation and to direct their course toward their target. At all regeneration stages the majority of dorsotemporal axons coursed and achieved target-related orientations preferentially within the rostral tectal half whereas dorsonasal axons proceeded into the caudal tectum. The growing axons exhibited various morphologies. All axons in the superficial fascicle layer stratum opticum (SO) and some in the synaptic layer stratum fibrosum et griseum superficiale (SFGS) were unbranched and tipped with a leading growth cone. Other axons in the synaptic layer carried one to several growth cones at their ends and often filopodia proximal to the growth cone, or they had sprouted numerous side branches with growth cones and filopodia on the shaft and on branches. Some axons at retinotopic or ectopic sites gave rise to several long branches of several hundred microns in length, with growth cones and filopodia. From 32 days onward axons ending in terminal arbors at retinotopic sites became apparent. Thus, numerous axons at early regeneration stages go through a phase of exploratory growth on their way toward their target sites.

Evidence for centripetally shifting terminals on the tectum of postmetamorphic Rana pipiens

In larval frogs the retina and tectum grow in topologically dissimilar patterns: new cells are added as peripheral annuli in the retina and as caudal crescents in the tectum. Retinotopy is maintained by the continual caudalward shifting of the terminals of the optic axons. After metamorphosis the pattern of growth changes. The retina continues to add new ganglion cells peripherally, but there is no neurogenesis in the tectum. To maintain retinotopy in postmetamorphic frogs, the terminals of the optic axons must continually shift toward the central tectum. We tested the proposal of centripetally shifting axons by making punctate injections of horseradish peroxidase (HRP) in the tectum of adult Rana pipiens and observing the patterns of filled cells in the contralateral retina, as was done in the goldfish (Easter and Stuermer, '84). Punctate applications of HRP in the tectum should be taken up: 1) by fascicles, and label a partial anulus of cells, 2) by terminals, and label a cluster of cells in the corresponding retinotopic site, and 3) by the extrafascicular axonal segments, and label a band of cells connecting the partial annulus to the cluster. If the terminals have shifted centripetally, the band of cells labeled through their extrafascicular segments should have a spoke-like orientation, with the center of the retina as the hub. As the tectal site moves from rostral to caudal, this band of cells should move, pendulum-like, from temporal to nasal retina. In general, the patterns of HRP-filled retinal cells we observed were consistent with our predictions. In addition, HRP taken up by the oldest (rostral) tectal axons produced more complex patterns of filled cells that indicated that these axons had shifted both caudally before metamorphosis and centripetally after.

Comparative neurology of the optic tectum in ray-finned fishes: patterns of lamination formed by retinotectal projections

Retinotectal projections were studied in 33 different species of Actinopterygii, the ray-finned fishes, with horseradish peroxidase and cobalt tracing techniques. The distribution of retinorecipient layers in the contralateral optic tectum was analyzed. In addition, the degree of differentiation of the stratum periventriculare, and the presence of ipsilateral retinotectal projections was examined. Retinofugal fibers are labeled in the stratum opticum (SO), stratum fibrosum et griseum superficiale (SFGS), stratum griseum centrale (SGC), stratum album centrale (SAC) and stratum periventriculare (SPV). Some species lack the projection to the SO, others lack the projection to the SGC, and a third group of fishes lack both projections. Five different patterns of retinorecipient tectal strata are distinguished. These patterns correlate with the species' taxonomic position. Evolutionary trends of tectal lamination and retinotectal innervation are described. The retinotectal projection patterns provide a useful indicator of phylogenetic relationships. Some of our data suggest different relationships between actinopterygian species than hitherto believed.

Axonal arborization in the developing chick retinotectal system

The growth and arborization of chicken retinal ganglion cell axons have been investigated by means of an intraaxonally transported fluorescent marker in the developing retinotectal system. The fluorescent dye D282 or diI from the carbocyanine group of dyes is taken up by ganglion cells and labels the axon as well as the axonal growth cones and the terminal arborizations on the tectum. Branching and arborization start in the chick retinotectal system on embryonic day 9 (E9). At this stage retinal axons leave the stratum opticum (SO) and invade the stratum griseum et fibrosum superficiale (SGFS), where arborization takes place. On day E12 several axons were found to arborize in the SGFS. At this stage arbors appear to have small branches with less than 4 branching points. The extension of terminal arbors in the anterior/posterior (A/P) and in the dorsal/ventral (D/V) direction was determined for 50 axonal trees at days E13-14 and for 24 arbors at days E15-16. Few axonal terminals were investigated at day E18. The mean A/P extent of axonal terminal trees increases from 0.23 +/- 0.12 to 0.36 +/- 0.22 mm from E13-14 to E15-16 and seems to stay at this order of magnitude on E18. The mean D/V extent increases from 0.23 +/- 0.17 to 0.30 +/- 0.18 mm in the same embryonic period of development. The number of branching points calculated from the same number of axonal trees increases from 7.50 +/- 2.98 at E13-14 to 11.70 +/- 4.10 at E15-16. This number seems to increase further after day E16 achieving values of about 20 to 25 at E18. This was, however, not quantifiable by the technique used here and represents an approximate value estimated from 6 completely labeled terminal fields at E18. The data presented here suggest that the modeling of the final branching pattern in the chick retinotectal system takes place within a relatively short period of embryonic development. Prior to the beginning of terminal arborization two important events contribute to the formation of a retinotopic projection. One event is the change of the D/V position by a minority of axons lying ectopic in terms of retinotopy. Some axons turn at right angles and change their D/V position. The other event is the appearance of side branches along the A/P axis.

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.

Regeneration of the frog optic nerve. Comparisons with development

Developing and regenerating frog optic axons grow within optic pathways and form connections only with optic targets. However, unlike normal development, many regenerating optic axons in the adult frog are misrouted within optic pathways, including axons that grow into the opposite retina. Many of the axons misrouted during regeneration appear to be collaterals of axons that grow in normal directions. Ganglion cell loss of up to 60% occurs after optic nerve damage, beginning prior to reinnervation of optic targets. Massive axonal collateralization also takes place near the point of nerve damage, causing the normal order found within the nerve to be lost. Collaterals are eliminated as selective reinnervation is completed, and the smaller complement of optic cell axons remaining after regeneration form an expanded projection within optic targets. Evidence is reviewed that suggests that factors involved in axonal guidance and target recognition during development remain intact in the adult frog brain. Additional conditions resulting from nerve injury causes axonal guidance to be less successful during regeneration.

Variability of axonal arborizations hides simple rules of construction: a topological study from HRP intracellular injections

Intracellular staining with horseradish peroxidase (HRP) allows the analysis of the extent and diversity of axonal field, as the Golgi techniques did for dendritic fields. In this study, we have used such HRP injections to investigate possible rules of construction that underlie the variability in the observed morphological patterns of axons. The Mauthner cell of the teleost provides a suitable material for such a work since it receives inhibitory inputs from two distinct classes of cells that can be identified physiologically prior to their intracellular staining: those that contribute to a recurrent collateral network and those that are part of the commissural vestibulovestibular pathway. The distribution of their terminal boutons over the M-cell surface was quantified and their axonal arborizations were analyzed topologically with the help of a centripetal method of terminal ordering. The results indicated that both types of interneurons have qualitatively a similar distribution of boutons over this target, encompassing the soma, initial portions of the main dendrites, and the cap-dendrites, and that furthermore, the terminals have a tendency to form clusters, the proportion of which is relatively constant regardless of the total number of boutons established by a given afferent cell. In respect to their projection on the M-cell, the two populations differ mainly in the number of established contacts, which averaged 44.1 +/- 28.2 (n = 10) and 14.4 +/- 11.3 (n = 43) for collateral and commissural interneurons, respectively. These differences reflect the variations in axonal arborizations. A topological analysis has revealed that branching occurs mainly as bifurcations, whereas in contrast three or four segments issuing from a branch point are only occasionally observed at the level of the last segment; for each cell, the distribution of boutons within orders is not random and rather follows an unimodal distribution, centered on the mean order (theta); the terminals of each subset, or class, of neurons have their own pattern of distribution within orders; and finally the mean order (theta) is linked to the number of terminal segments (nT) by the relation theta = 1.28 log2 nT. This parameter is of importance: it characterizes the axonal arborization and it allows one to predict the number of terminal segments of individual neurons. Similar analysis of camera lucida reconstruction of axonal arborizations of various cell types studied by other investigators suggests that this relation can be generalized.(ABSTRACT TRUNCATED AT 400 WORDS)

Retinofugal projections in a weakly electric gymnotid fish (Apteronotus leptorhynchus)

The eyes of weakly electric gymnotid fish are poorly developed in comparison to those of most diurnal teleosts. The tectum and pretectum, despite their usual association with the visual system, are large and well differentiated in gymnotids. We have studied retinal projections in gymnotids in order to define the visual components of the mesencephalon and diencephalon and thus allow comparison with other teleosts in which retinofugal fibers have been extensively mapped. Retinofugal projections reported in this work are based on the anterograde transport of conjugated wheat germ agglutinin horseradish peroxidase, following injection into the posterior chamber of the eye of Apteronotus leptorhynchus (brown ghost knife fish). The results show a remarkable similarity to those of non-electroreceptive teleosts. Although the optic nerves appear to cross completely at the optic chiasm, close scrutiny shows a slender recrossing fascicle which continues from the contralateral tractus opticus medialis through the rostroventral hypothalamus to reach the ipsilateral side, providing a scanty projection to the n. opticus hypothalamicus, n. anterior periventricularis, n. dorsolateralis thalami, and n. commissurae posterioris. A few fibers ascend via the tractus opticus dorsomedialis to the rostral dorsomedial part of the stratum fibrosum et griseum superficiale of the ipsilateral tectum. The main body of the retinal projections in Apteronotus are to the following contralateral target areas: preoptic area, n. opticus hypothalamicus, n. anterior periventricularis, n. dorsolateralis thalami, n. pretectalis, area pretectalis, n. corticalis, n. commissurae posterioris, n. geniculatus lateralis, area and n. ventrolateralis thalami, caudal dorsal tegmentum and the tectum opticum. The retinotectal projection is modest in comparison to that of more vision dependent fish and terminates mainly in the upper half of the stratum fibrosum et griseum superficiale; hardly any retinal fibers reach the caudalmost tectum.

Development of the optic tract in the cichlid fish Haplochromis burtoni

In the teleost fish, Haplochromis burtoni, the optic tract is composed of 3 distinct components: the marginal tract, which projects to the optic tectum and is by far the largest, and the axial and medial tracts which project to diencephalic targets. In this paper we report on the normal development of these pathways in larval H. burtoni, an African cichlid fish. The earliest optic tract fibers are found in what will become the marginal optic tract. These fibers hug the wall of the diencephalon in a cohesive bundle. The first fibers in the axial tract location appear on day 5, increasing in number between days 6 and 18. Like marginal tract fibers, axial tract fibers form a cohesive bundle. It is not clear from these experiments whether the first axial tract fibers actually arrive at this location at day 5, or whether they are fibers arriving earlier that were physically displaced from the marginal tract at day 5. Medial tract fibers are not evident until day 6 of development and the number of medial tract fibers also increases as the animal gets older. Unlike fibers in the other two pathways, medial tract fibers do not travel together in a bundle. Rather, each one follows an independent trajectory to its target site. Comparison of this larval development with the adult optic tract organization which we have studied earlier suggests constraints on the mechanisms of axon guidance.