A comparison of postlesion growth of retinotectal and corticotectal axons after superior colliculus transections in neonatal rats

We examined, in neonatal rats, the postinjury response of two different axonal systems that project to a common target area in the visual system. Transections across the rostral part of the left superior colliculus (SC) were made in 2- or 6-day-old rats (P2, P6). Lesioned animals were randomly selected into short- or long-term groups. The short-term group was used to determine the efficacy of the lesion technique; 2-6 days after transections, right (contralateral) eyes were injected with horseradish peroxidase (HRP). Complete deafferentation of the SC was achieved in 73% of P2 (n = 22) and 53% of P6 (n = 10) short-term animals. In the long-term group (examined 2-7 months after transection), retinotectal and corticotectal projections were assessed in each animal by using [3H]proline and wheat germ agglutin-HRP, respectively. Examination of a series of sagittal sections revealed that the cut had extended across the entire SC in 63% of P2 (n = 19) and 55% of P6 (n = 12) long-term rats. Despite this, retinal and cortical axons were seen in appropriate layers in postlesion SC in all P2 lesioned animals. Cortical projections caudal to the cut were seen in all P6 rats; however, in these animals, the retinal projection was sparse and not always present. Differences in lesion geometry led to consistent differences in the pattern and extent of ingrowth of retinal and cortical axons into postlesion SC neuropil. The two axonal populations also followed different paths as they grew between prelesion and postlesion SC. It is likely that a number of factors influenced the patterns of postlesion growth, including the relative maturity of the axons and the neuropil into which they were growing. There was also, however, clear evidence of competitive interactions between retinal and cortical axons in postlesion SC that consistently led to greater than normal segregation of the two populations and hence restricted their terminal distributions.     

Differential withdrawal of retinal axons induced by a secreted factor

To understand the development of the topographic map in the chick retinotectal projection, we studied the long-term interactions between retinal axons and tectal cell processes using a novel coculture system, the ryomen chamber. Both nasal and temporal retinal axons initially grew equally well on a substrate consisting of posterior tectal cell processes; however, subsequently most temporal axons withdrew from this surface, whereas most nasal axons did not. Experiments using conditioned media indicate that posterior tectal cells induced withdrawal of the temporal axons by secreting a soluble factor. This withdrawal seems to be distinct from the immediate repulsive effect of ephrin-A2 (ELF-1) and ephrin-A5 (RAGS) seen in the stripe assay because (1) the withdrawal-inducing factor was diffusible, whereas ephrin-A2 and -A5 are membrane-bound, and (2) the withdrawal-inducing factor appeared later in development than ephrin-A2 and -A5. Furthermore, sensitivity to the withdrawal-inducing factor decreased continuously from the temporal to nasal retina. These results suggest that target cell-induced axonal withdrawal may be involved during a late stage of the development of the retinotectal map.     

Eph family transmembrane ligands can mediate repulsive guidance of trunk neural crest migration and motor axon outgrowth

The establishment of cell and fibre layers and the specification of different cell types are crucial processes during development of the central nervous system. Here we investigated the developmental architecture of radial glia cells in these processes using so-called spheroids that arise from dissociated chicken embryonic neural cells in rotation culture. We were able to produce retinal, tectal, and telencephalic spheroids from E6 embryos and cerebellar spheroids from E10 embryos. Cell and fibre differentiation can be observed in all types of spheroids, however, it is most abundant in retinal spheroids. Moreover, only in retinal spheroids a histotypic organization can be detected. Using immunohistochemistry and electron microscopy, we assign this -at least partially- to the capacity of Müller cells to form radial scaffolds, since we observe a congruency between these radial scaffolds and the presence of rosettes formed by photoreceptor precursors and Müller cells. Tectal, telencephalic and cerebellar spheroids do not show organized radial glia scaffolds, instead, the radial glia cells are randomly arranged and the spheroids do not show histotypical organization. The application of the specific gliotoxin 6-aminonicotinamide to growing retinal spheroids leads to a significant decrease in the number and size of the rosettes. Concomitantly, the degree of histotypical organization is also drastically reduced. This organizing capacity of Müller cells in vitro now strongly suggests the presence of a comparable function also in vivo. Moreover, since non-retinal radial glia cells are not able to re-organize an histotypic organization in vitro, Müller cells seem to be qualitatively different from other radial glia cells. In future studies we want to untangle these differences.     

Morphology, axonal projection pattern, and response types of tectal neurons in plethodontid salamanders. I: tracer study of projection neurons and their pathways

In three salamander species (Hydromantes italicus, H. genei, Plethodon jordani), the tectobulbospinal and tectothalamic pathways and their cells of origin were studied by means of anterograde and retrograde biocytin and tetramethylrhodamine tracing. In plethodontid salamanders, five types of tectal projection neurons were identified. TO1 neurons have widefield dendritic trees that arborize in the layers of retinal afferents and form a neuropil in the superficial layer; axons constitute the crossed tectospinal tract. Dendrites of TO2 cells have the largest dendritic trees that arborize in the intermediate and deep layers of retinal afferents; axons constitute a lateral uncrossed tectospinal tract. TO3 cells have widefield dendritic trees that arborize in the deep layer of retinal afferents and in the layer of tectal efferents; axons constitute a superficial uncrossed tectospinal tract. TO4 cells have slender primary dendrites and small-field dendritic trees that arborize in the intermediate layers of retinal afferents; axons constitute another lateral uncrossed tectospinal tract. TO2, TO3, and TO4 cells also have ascending axons that run to the ventral and dorsal thalamus. TO5 cells have slender primary dendrites and small-field dendritic trees that extend into the superficial layers of retinal afferents; their fine axons constitute the bulk of the pathways ascending to the ipsilateral and contralateral thalamus. These morphological types of projection neurons and their ascending and descending axonal pathways closely resemble those found in frogs, reptiles, and birds. Their role in visual and visuomotor functions is discussed.     

Axonal versus dendritic outgrowth is differentially affected by radial glia in discrete layers of the retina

Formation of neural cell polarity defined by oriented extension of axons and dendrites is a crucial event during the development of the nervous system. Ganglion cells of the chicken retina extend axons exclusively into the inner retina, whereas their dendrites grow into the outer retina. To analyze guidance cues for specific neurite extension, novel in vitro systems were established. Ganglion cells were purified by enzymatically facilitated detachment of the ganglion cell layer. A newly developed retrograde labeling technique and the expression analysis of the cell type-specific 2A1 antigen were used to monitor ganglion cell purification. In highly purified ganglion cells explanted onto retinal cryosections (cryoculture), axon formation was induced when the cells were positioned on the inner retina. In contrast, on outer layers of the developing retina dendritic outgrowth was prevalent. Because radial glia have been demonstrated to be instructive in neuritogenesis, distinct glial cell compartments located in inner and outer retina, respectively, were isolated for functional assays. Glial end feet were purified by a physical detachment technique. Glial somata were purified by complement mediated cytolysis of all nonglial cells. When ganglion cells were cultured on different glial compartments, axon formation occurred on end feet but not on glial somata. In striking contrast, on glial somata dendrites were formed. The data support the notion that ganglion cell polarity is affected by the retinal microenvironment, which in turn is possibly influenced by radial glia, being themselves polarized.     

Development of the retinotectal system in the pigeon: a cytoarchitectonic and tracing study with cholera toxin

The optic tectum of the pigeon (Columba livia) is marked by morphological dorso-ventral and left-right differences. Both features seem to be related to functional specializations, but the responsible developmental mechanisms are unclear. Since the visual system becomes functional only after hatching, the developmental processes might be extended into the post-hatching period. The development of the asymmetries in the tectofugal system, however, depends on an asymmetric light stimulation acting already before hatching. As a first attempt to resolve this discrepancy, we examined the ontogeny of the retinotectal system by labeling the developing retinal projection with cholera toxin subunit B, in conjunction with an analysis of the cytoarchitectonic differentiation of the optic tectum. The data demonstrate that the first fibers to penetrate all retinoreceptive tectal layers could be observed from embryonic day 15 onwards, indicating that visual information could in principle be already processed before hatching. The afferent projection already exhibited the adult lamination pattern directly at the beginning of the invasion of the tectal layers; a surprising finding, since at that time the lamination pattern of the tectal layers did not have an adult appearance. The differentiation of the outer retinoreceptive laminae started only when the whole optic tectum was occupied by retinal fibers, 4 days after hatching, and was finished a week later. The dorso-ventral differences in the thickness of layers 4 and 5 were not apparent before the first week after hatching. The late appearance of these differences indicates that their maturation may be influenced by retinal input.     

Involvement of heat shock elements and basal transcription elements in the differential induction of the 70-kDa heat shock protein and its cognate by cadmium chloride in 9L rat brain tumor cells

The EphA3 receptor tyrosine kinase has been implicated in guiding the axons of retinal ganglion cells as they extend in the optic tectum. A repulsive mechanism involving opposing gradients of the EphA3 receptor on retinal axons and its ligands, ephrin-A2 and ephrin-A5, in the tectum influences topographic mapping of the retinotectal projection. To investigate the overall role of the Eph family in patterning of the visual system, we have used in situ hybridization to localize nine Eph receptors in the chicken retina and optic tectum at Embryonic Day 8. Three of the receptors examined correspond to the novel chicken homologs of EphA2, EphA6, and EphA7. Unexpectedly, we found that many Eph receptors are expressed not only in retinal ganglion cells, but also in tectal cells, In particular, EphA3 mRNA is prominently expressed in the anterior tectum, with a pattern reciprocal to that of ephrin-A2 and ephrin-A5. Similarly, ephrin-A5 is expressed not only in tectal cells but also in the nasal retina, with a pattern reciprocal to that of its receptor EphA3 and partially overlapping with that of its other receptor EphA4. Consistent with the even distribution of EphA4 and the polarized distribution of EphA4 ligands in the retina, probing EphA4 immunoprecipitates from different sectors of the retina with anti-phosphotyrosine antibodies revealed spatial differences in receptor phosphorylation. These complex patterns of expression and tyrosine phosphorylation suggest that Eph receptors and ephrins contribute to establishing topography of retinal axons through multiple mechanisms, in addition to playing a role in intraretinal and intratectal organization.     

Specificity and plasticity of retinotectal connections: a computational model

A computational model is presented which simulates the development and regeneration of orderly connections between retinal fibers and tectal cells in frogs and goldfish. The model distinguishes two aspects of retinotectal connectivity: (1) the contact adhesion between retinal fibers and tectal cells as mediated by fixed chemospecific markers and (2) the formation of modifiable synapses between them. Chemospecificity is assumed to be an intrinsic property of both the retina and tectum and is modeled as a graded distribution of a binding determinant or marker. Synapse formation depends upon the timing of neural activity as well as on the intinsic chemospecificity of retinotectal contacts. In addition to the normal development and regeneration of the retinotectal map, the model simulates the compressed, expanded, translocated, and rotated maps that have been found in surgically manipulated contexts. There examples of plasticity in the retinotectal map can be simulated without assuming any changes in the marker distributions. Moreover, the model demonstrates that a very shallow gradient of a single marker suffices to organize retinotectal connections in a variety of contexts.     

Serotonin modulates induced synaptic activity in the optic tectum of the frog

The extent to which retinal signals are modulated at central sites is unknown. We sought to determine the effects of serotonin, a neurotransmitter present in the retinorecipient layers of the frog tectum, on retinotectal transmission. Acute electrical stimulation delivered to the retinorecipient layer of optic tectum brain slices was used to model the activation of tectal neurons by visual inputs. This stimulation evoked either a monosynaptic or a polysynaptic current response in patch-clamped tectal neurons. External application of serotonin blocked both of these induced currents as did 5-carbotryptamine (5-CT), a nonselective agonist of 5-HT1 receptors. Alpha-methylserotonin, a nonselective agonist of 5-HT2 receptors, also blocked polysynaptic responses but was less effective than either serotonin or 5-CT in blocking monosynaptic ones. Lateral synaptic interactions between tectal cells, modeled by acute electrical stimulation in the main cellular layer of the tectum, were also blocked by serotonin, 5-CT or alpha-methylserotonin. The presented data suggest that endogenous serotonin may strongly affect visual signal processing by modulating synaptic transmission between both the retina and the tectum as well as between tectal neurons. This modulation is likely to be due, at least in part, to a demonstrated outward current induced by serotonin in a subpopulation of tectal cells.     

Distribution of glial fibrillary acidic protein and vimentin-immunopositive elements in the developing chicken brain from hatch to adulthood

The present study describes the distribution of glial fibrillary acidic protein (GFAP) and vimentin-immunopositive structures in the brain of the domestic chicken (Gallus domesticus) from hatching to maturity. The telencephalon is penetrated by a vimentin-immunopositive radial fibre system, representing a modified form of radial glia, in day-old chicks. Numerous fibres of this system persist until adulthood, mainly in the lobus parolfactorius, lamina medullaris dorsalis and lamina frontalis superior. GFAP immunoreactivity also appears in the course of development in these fibres. The distribution of GFAP-immunopositive astrocytes in the post-hatch telencephalon is like that found in adult chicken, except for the ectostriatum, in which an adult-like GFAP-immunostaining only develops during week three. This delay may be associated with a relatively slow maturation of this visual centre. In the diencephalon and in the mesencephalic tegmentum of day-old chicks GFAP-immunopositive astrocytes are confined to the border zone of several nuclei. In these areas as well as in the pons most GFAP positive astrocytes only appear gradually during the first two post-hatch weeks, although radial fibres occur only sparsely at hatch. Summarizing these results, a gradual replacement of radial fibres by astrocytes, typical of mammals, cannot be found in chicken. In the nucleus laminaris we observed a characteristic palisade of non-ependymal glia, reactive to GFAP but not to vimentin, which almost completely disappears by adulthood. We suggest that this glial system is instrumental in the development of the dendritic organisation of this nucleus. The optic tectum displays a dense array of GFAP-immunopositive radial glia at hatching, similar in this to the situation found in reptiles. However, in the tectum of reptiles this radial glia persists for the lifetime, whereas in the chick it disappears from the superficial tectal layers. This phenomenon may reflect the fact that there is no replacement of tectal cells or regeneration of retinotectal pathways in the chicken. In the early stage, the large cerebral tracts were found to contain dense accumulations of GFAP-positive cells, with peculiarly long outgrowths accompanying nerve fibres. No vimentin-immunopositivity was found in these glial elements; however vimentin was present in the glia situated at the optic chiasm, the anterior commissure and at other decussations. These structures, as well as the raphe, displayed the most intense vimentin-immunopositivity in the post-hatch chicken. This characteristic glial population may represent glial elements that have been reported to regulate fibre-crossing at the midline.     

Do oculomotor neuroblasts migrate across the midline in the retal rat brain?

Toluidine blue-stained semithin sections and Cajal-Castro preparations are used to study in rat fetuses whether oculomotor neuroblasts migrate across the midline at a certain period of development. In confirmation of previous studies, a group of oculomotor neuroblasts was detected which first grow cytoplasmic processes into the mesencephalic midline, and afterwards translocate their somata towards the midline, between the 12th and the 15th days of gestation. At this moment a midline mass of neuroblasts characterizes the meeting at this landmark of both left and right migrating neuroblastic groups. No crossing oculomotor axons yet are demonstrable with reduced silver techniques. In further stages of development the neuroblasts continue their migration until they arrive at the contralateral nucleus at the 16th and 17th day of gestation. At the midline the mass of neuroblasts disappears gradually and crossed oculomotor axons become visible. The electron microscope was then used to study ultrastructurally the migrating motoneurons. It was discovered that no preexisting structure guides their movement by contact. Their leading processes show no filopodial activity, and contain abundant microtubules and thick bundles of neurofilaments in eccentric position. The neuroblasts carry their axon across the midline as a trailing process.     

The effect of superior colliculus lesions upon the visual fields of cats with cortical ablations

The visual fields of 18 cats were measured before and after various lesions. Preoperatively, all cats had identical fields. With both eyes open, they saw from 90 degrees left to 90 degrees right; with one eye, from 90 degrees ipsilateral to 45 degrees contralateral. Thus the field for nasal retina extends from 90 degrees ipsilateral through to the midline; for temporal retina, from the midline through to 45 degrees contralateral. In summary, postoperative testing led to two major conclusions. (1) Large occipito-temporal cortical lesions produce a stable field blindness, but the blindness is alleviated by a transection of the commissure of the superior colliculus (or a unilateral collicular ablation). This transection yields the same result whether it occurs in an operation before, during, or after the cortical lesion. These data confirm and extend the Sprague effect. (2) Cats made dependent upon retinotectal pathways due to cortical ablations responded much better to stimulation of nasal retina than to stimulation of temporal retina. This presumably is related to the preponderance of nasal retina as a source of the retinotectal pathway. Since even smaller cortical lesions limited to areas 17, 18, and 19 produce this nasal/temporal retinal difference, it is concluded that integrity of the geniculocortical pathways is necessary for good temporal retinal vision as determined by these methods.     

Different cell surface areas of polarized radial glia having opposite effects on axonal outgrowth

During neuronal development neurites are likely to be specifically guided to their targets. Within the chicken retina, ganglion cell axons are extended exclusively into the optic fibre layer, but not into the outer retina. We investigated, whether radial glial cells having endfeet at the optic fibre layer and somata in the outer retina, might be involved in neurite guidance. In order to analyse distinct cell surface areas, endfeet and somata of these glial cells were purified. Glial endfeet were isolated from flat mounted retina by a specific detachment procedure. Glial somata were purified by negative selection using a monoclonal antibody/complement mediated cytolysis of all non-glial cells. Retinal tissue strips were explanted either onto pure glial endfeet or onto glial somata. As revealed by scanning and fluorescence microscopy, essentially no ganglion cell axons were evident on glial somata, whereas axonal outgrowth was abundant on glial endfeet. However, when glial somata were heat treated and employed thereafter as the substratum, axon extension was significantly increased. Time-lapse video recording studies indicated that purified cell membranes of glial somata but not of endfeet induced collapse of growth cones. Collapsing activity was destroyed by heat treatment of glial membranes. The collapsing activity of retinal glia was found to be specific for retinal ganglion cell neurites, because growth cones from dorsal root ganglia remained unaffected. Employing four different kinase inhibitors revealed that the investigated protein kinase types were unlikely to be involved in the collapse reaction. The data show for the first time that radial glial cells are functionally polarized having permissive endfeet and inhibitory somata with regard to outgrowing axons. This finding underscores the pivotal role of radial glia in structuring developing nervous systems.     

Experimental down-regulation of the NMDA channel associated with synapse pruning

The N-methyl-D-aspartate (NMDA) receptor has been implicated in activity-dependent synapse stabilization, but its role as a detector of correlated activity during development is debated. In the amphibian retinotectal system, synaptic sorting and stabilization occur throughout larval life, and map refinement is dependent on continuous NMDA receptor function. Moreover, tadpole tecta chronically treated with NMDA selectively fail to maintain retinal synapses wherever their activity correlations are lowest. To determine whether this synapse elimination is associated with a specific down-regulation of NMDA receptor function, whole cell voltage-clamp recordings were made from single neurons in tectal slices. After chronic NMDA treatment, decreases in the magnitude of NMDA currents were detected in glutamatergic synaptic currents, in agonist-evoked currents, and in single-channel currents activated by NMDA. The results suggest that the efficacy of NMDA receptors on tectal neurons determines the amount of correlation required to stabilize sets of tectal inputs during formation of the retinotectal projection.     

Effects of polar carotenoids on the shape of the hydrophobic barrier of phospholipid bilayers

Based on recent experimental studies of complementary gradients of receptor density (R) on the retinal surface and ligand density (L) on the tectal surface, and mapping of the high point on the receptor gradient to the low point on the ligand and vice versa, the servomechanism model was constructed involving a mechanism for the retinal axon to reach its target automatically sensing a difference between the signal strength (R.L) and the standard value (S). Computer simulations based on the model demonstrated desired two-dimensional topographic mapping of the retinal axons on the tectum, and explained three strange behaviors of the retinal axons that had been observed in stripe assays for retinal axons using stripes composed of tectal membrane fragments: repulsive behaviors of the retinal axons by the ligand substances, uncertainty of the nasal axons whether or not they show regional selectivity between substances of anterior and posterior tecta, and abrupt transition of growth of the axons originating at continuously varied retinal positions on the stripes having graded ligand density. Finally we suggested what is to be improved in stripe assays with the artificial gradient of the tectal membrane fragments.     

Ascending projections from the optic tectum in the lizard Podarcis hispanica

The ascending projections of the optic tectum, including their cells of origin, have been studied in the lizard Podarcis hispanica by means of a two-step experimental procedure. First, tracers were injected in the tectum to study the anterograde labeling in the forebrain. Second, the cells of origin of these projections have been identified by analyzing the retrograde labeling after tracer injections in the thalamus, hypothalamus, and pretectum. Three main tectal ascending pathways have been described: the dorsal tecto-thalamic tract (dtt), the medial tecto-thalamic tract (mtt), and the ventral tecto-thalamic tract (vtt). The dtt originates in radial cells of layers 5 and 7 and bipolar cells of layers 8 and 10 that project to the lateral neuropile of the dorsal lateral geniculate nucleus (GLD), to the intergeniculate leaflet (IGL), and to the ventral lateral geniculate nucleus (GLV). The mtt arises from radial neurons of layers 3 and 5 and bilaterally reaches the putative reticular thalamus and its boundary with the hypothalamus, the rostral IGL, and the area triangularis (AT). The vtt is composed of fibers from ganglion and multipolar cells of the layer 7 that project bilaterally to the nucleus of the vtt, the ventrolateral thalamic nucleus, the medial posterior thalamic nucleus (MP), the nucleus rotundus (Rot), the IGL, and the cell plate of the GLD. Therefore, the GLD receives not only direct retinal afferents but also two different tectal inputs, thus constituting a convergence point in the two visual pathways to the telencephalon. Moreover, different tectal cells specifically project to the ventrolateral thalamus and to pretectal nuclei. These results are discussed from comparative and functional viewpoints.     

Expression of multiple cadherins and catenins in the chick optic tectum

Cadherins form a large family of homophilic cell adhesion molecules that are involved in numerous aspects of neural development. The best-studied neural cadherin, N-cadherin, is concentrated at synapses made by retinal axons in the chick optic tectum and is required for the arborization of retinal axons in their target (retinorecipient) laminae. By analogy, other cadherins might mediate arborization or synaptogenesis in other tectal laminae. Here we consider which cadherins are expressed in tectum, which cells express them, and how their expression is regulated. First, using N-cadherin as a model, we show that synaptic input regulates both cadherin gene expression and the subcellular distribution of cadherin protein. Second, we demonstrate that N-, R-, and T-cadherin are each expressed in distinct laminar patterns during retinotectal synaptogenesis and that N- and R- are enriched in nonoverlapping synaptic subsets. Third, we show that over 20 cadherin superfamily genes are expressed in the tectum during the time that synapses are forming and that many of them are expressed in restricted groups of cells. Finally, we report that both beta-catenin and gamma-catenin (plakoglobin), cytoplasmic proteins required for cadherin signaling, are enriched at synapses and associated with N-cadherin. However, beta- and gamma-catenins are differentially distributed and regulated, and form mutually exclusive complexes. This result suggests that cadherin-based specificity involves multiple cadherin-dependent signaling pathways as well as multiple cadherins.     

Migration of glial cells into retinal axon target field in Drosophila melanogaster

During the development of the Drosophila visual system, photoreceptor (retinal) axons (R axons) project retino-topically to their targets in the optic lobes. The establishment of this precise pattern of connections does not depend on interactions between adjacent axon bundles, suggesting that R axons rely on environmental signals for proper pathfinding. Glial cells that are located along the R-axon trajectory are likely candidates to provide guidance cues for R-axon navigation. This study defines the origin of lamina glia (L glia), and demonstrates that L glia migrate into the lamina over a considerable distance. Glia are located in positions at which the R axons make critical growth choices. In the absence of cues from the eye, several classes of glia migrate to their final positions within the optic lobe anlage and begin to differentiate. Our results are consistent with a role for the glia in providing guidance cues to the R axons.     

Developmentally regulated and spatially restricted antigens of radial glial cells

Radial glial cells, present in many parts of the embryonic vertebrate central nervous system (CNS), have been implicated in the guidance of neuroblasts from the ventricular zone to their laminar destinations. Moreover, radial glial cells may be progenitors of some CNS neurons and glia. To gain new insight into the structure and development of these cells, we have generated and characterized a panel of monoclonal antibodies that recognize radial glial cells of the chick optic tectum. Mice were immunized with homogenates of embryonic day (E) 10 tectum, and antibodies were analyzed by immunofluorescence and immunoblotting. We describe here three pairs of antibodies. 1) H5 and a previously generated antibody, R5 (Dr?ger et al., J. Neurosci. 4:2025, 1984), stain the whole extent of the radial glial cell from E7 to E20. In cultures prepared from E10 tecta, both stain a filamentous meshwork in glial cells but not in neurons. On immunoblots, both recognize a protein of approximately 52 kD that is closely related (or identical) to vimentin. 2) H28 and H29 stain radial glia between E7 and E14, but not later. Moreover, H28 and H29 staining is markedly more intense in the ventricular and intermediate zones than in the laminae of the tectal plate. Both of these antibodies recognize an intracellular epitope in cultured glial cells and a protein of approximately 35 kD on immunoblots. 3) H2 and H27 recognize antigens concentrated in the most superficial processes and endfeet of radial glia in late (E16-E20) embryos. They stain distinct structures in cultured glia, suggesting that they recognize distinct antigens. H27 recognizes a protein of approximately 29 kD on immunoblots. Thus antibodies H5 and R5 are good markers of radial glial cells at all stages, whereas the others define antigens that are developmentally regulated and localized to discrete domains. Together, these antibodies can be used to study temporal and spatial specializations of radial glia.     

Target- as well as source-derived factors direct the morphogenesis of anomalous retino-thalamic projections

Neonatal tectal lesions in hamsters result in the elimination of a major central target of retinal axons, massively denervate the lateral posterior nucleus of the thalamus (LP), and lead to a marked increase of the retino-LP projection. In such animals, retino-LP axons show all of the normally-occurring terminal types. In addition, large clusters of varicosities, whose tubular configuration resembles the major type of tecto-LP terminals observed in normal animals, are also noted if the tectal lesion is made on the day after birth (P1). If, however, the neonatal lesion occurs on P5 rather than on P1, terminals resembling normal tecto-LP endings are rarely observed; rather, the distribution and morphology of retino-LP terminals bear a greater resemblance to those seen in normal hamsters, but the size and complexity of the terminals, particularly those that form string-like arrangements, is significantly increased. Our findings suggest that the altered morphology of some abnormally induced retino-LP terminals may be orchestrated by target-associated signals. However, there are age-related limitations on the degree to which afferent systems can vary their terminal morphology; these restrictions may derive from the target, or may be a function of intrinsic changes within the cells of origin of the afferent fibers.