Progress of topographic regulation of the visual projection in the halved optic tectum of adult goldfish

1. The patterns of re-established visual projections on to the rostral half-tectum are studied following excision of the caudal tectum at various intervals after section of either the contralateral optic nerve or the ipsilateral optic tract in adult goldfish. 2. The pattern of a newly restored retinotectal projection depends on the duration of the post-operative period given to the halved tectum before it is re-innervated by regenrating optic fibres from the retina. 3. When the duration is such that regenerating optic fibres invade the denervated rostral half-tectum at about 40 days or longer after excision of the caudal tectum, the remaining half-tectum is able to accommodate incoming optic fibres not only from the appropriate temporal hemi-retina but also from the foreign nasal hemiretina in an orderly compressed topographic pattern. 4. If the surgical operations are timed so that the halved tectum receive regenerating optic fibres earlier than 33 days after excision of the caudal tectum, the halved tectum initially accommodates only those optic fibres originating from the temporal half of the retina at this early stage. 5. This normal (uncompressed) pattern of the newly regenerated visual projection, however, eventually changes into an orderly compressed pattern at a later period. Post-operative dark-deprivation of the operated fish has no significant effect on the temporal transition. 6. The temporal transition from an initially normal pattern into an orderly compressed pattern may reflect the time course of progressive and systematic changes involved in topographic regulation of the halved tectum into a whole.     

Synergistic neurite-outgrowth promoting activity of two related axonal proteins, Bravo/Nr-CAM and G4/Ng-CAM in chicken retinal explants

In the developing chicken retina, optic fibres migrating to the tectum express on their surfaces several cell adhesion molecules, including Bravo/Nr-CAM and G4/Nr-CAM and G4/Ng-CAM. We have previously described differential distribution along the retinotectal projection and differential modulation by environmental cues for Bravo and G4 and here we further compare the characteristics of these immunoglobulin superfamily molecules. From day 6 of embryonic development (E6) to 20 (E20), Bravo and G4 were found to coexist in the retinal optic fibre layer. However, while G4 staining was confined to that layer, as development proceeded Bravo staining spread to plexiform layers and some radial structures of the retina. G4 displayed a dose-dependent neurite-outgrowth promoting activity for E6 retinal explants, while Bravo did not support neurite growth. Surprisingly, when the retinal explants were grown on mixtures of the two molecules, a much more vigorous growth of neurites was seen, revealing a synergistic effect. We propose that Bravo and G4, as well as other axonal surface molecules, affect axonal growth in different ways when they are present in combination than when they are alone.     

Predictors of medication compliance after hospital discharge in adolescent psychiatric patients

Serotonin amacrine cells have been described in the retina of vertebrates, except mouse and rat. Moreover, serotonin immunoreactive fibers have been reported in the optic nerve of rodents, frog and stingray. The purpose of this work was to study serotonin dorsal raphe nucleus projections to the retina, and to determine whether this pathway occurs in teleosts. The existence of specific connections was investigated in the rat and in the goldfish by the use of the neurotoxic substance 5,7-dihydroxytryptamine followed by monoamines determination in the retina by HPLC. The administration of 5,7-dihydroxytryptamine into the central or the lateral area of the rat dorsal raphe nucleus decreased serotonin levels in the raphe area and in the hippocampus, but only the bilateral injection into the dorsal raphe nucleus decreased it in the retina. In the goldfish, the injection of 5,7-dihydroxytryptamine into the optic tectum decreased serotonin concentration in situ and in the retina. The binding of [3H]paroxetine, a marker of serotonin transporter, was reduced in the retina of both species after the central treatment with the neurotoxic substance. In addition, the administration of the serotonin precursor 5-hydroxytryptophan into the optic tectum increased serotonin levels in the site of the injection and in the retina. The intraocular administration of 5,7-dihydroxytryptamine produced a big decrease in the content of retinal serotonin. This indoleamine and 5-hydroxyindoleacetic acid were detected in the optic nerves of rat and goldfish. The results indicate the existence of serotonergic retinopetal fibers in the rat and in the goldfish, a pathway that was not specifically demonstrated for the rat and was not previously proposed for the fish. The study of these serotonergic projections from the brain to the retina could be of interest in the understanding of the functional role of serotonin in the retina.     

Developmentally regulated spontaneous activity in the embryonic chick retina

Even before birth and the onset of sensory experience, neural activity plays an important role in shaping the vertebrate nervous system. In the embryonic chick visual system, activity in the retina before vision has been implicated in the refinement of retinotopic maps, the elimination of transient projections, and the survival of a full complement of neurons. In this study, we report the detection of a physiological substrate for these phenomena: waves of spontaneous activity in the ganglion cell layer of the embryonic chick retina. The activity is robust and highly patterned, taking the form of large amplitude, rhythmic, and wide-ranging waves of excitation that propagate across the retina. Activity waves are most prominent and organized between embryonic days 13-18, coinciding with the developmental period during which retinal axons refine their connections in their targets. The spatial and temporal features of the patterns observed are consistent with the role of activity patterns in shaping eye-specific projections and retinotopic maps but inconsistent with the hypothesis that they specify lamina-specific projections in the tectum. Antagonists of glutamatergic and glycinergic transmission and of gap junctional communication suppress spontaneous activity, whereas antagonists to GABAergic transmission potentiate it. Based on these results, we propose that spontaneous activity in the ganglion cells is regulated by chemical inputs from both bipolar and amacrine cells and by gap junctional coupling involving ganglion cells.     

Synchronizing retinal activity in both eyes disrupts binocular map development in the optic tectum

Spatiotemporal correlations in the pattern of spontaneous and evoked retinal ganglion cell (RGC) activity are believed to influence the topographic organization of connections throughout the developing visual system. We have tested this hypothesis by examining the effects of interfering with these potential activity cues during development on the functional organization of binocular maps in the Xenopus frog optic tectum. Paired recordings combined with cross-correlation analyses demonstrated that exposing normal frogs to a continuous 1 Hz of stroboscopic illumination synchronized the firing of all three classes of RGC projecting to the tectum and induced similar patterns of temporally correlated activity across both lobes of the nucleus. Embryonic and eye-rotated larval animals were reared until early adulthood under equivalent stroboscopic conditions. The maps formed by each RGC class in the contralateral tectum showed normal topography and stratification after strobe rearing, but with consistently enlarged multiunit receptive fields. Maps of the ipsilateral eye, formed by crossed isthmotectal axons, showed significant disorder and misalignment with direct visual input from the retina, and in the eye-rotated animals complete compensatory reorientation of these maps usually induced by this procedure failed to occur. These findings suggest that refinement of retinal arbors in the tectum and the ability of crossed isthmotectal arbors to establish binocular convergence with these retinal afferents are disrupted when they all fire together. Our data thus provide direct experimental evidence that spatiotemporal activity patterns within and between the two eyes regulate the precision of their developing connections.     

The optic tectum of birds: Mapping our way to understanding visual processing.

Over the past few decades there has been a massive amount of research on the geniculo-striate visual system in primates. However, studies of the avian visual system have provided a rich source of data contributing to our understanding of visual processing. In this paper we review the connectivity and function of the optic tectum (homolog of the superior colliculus) in birds. We highlight the retinotopic projections that the optic tectum has with the isthmal nuclei, and the functional topographic projections that the optic tectum has with the nucleus rotundus and entopallium (homologs of the pulvinar and extrastriate cortex, respectively) where retinotopy has been sacrificed. This work has been critical in our understanding of basic visual processes including attention, parallel processing, and the binding problem. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

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.     

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.     

Semaphorins act as attractive and repulsive guidance signals during the development of cortical projections

Members of the semaphorin family have been implicated in mediating axonal guidance in the nervous system by their ability to collapse growth cones and to function as chemorepellents. The present findings show that recombinant Semaphorin D has similar effects on cortical axons and, in addition, inhibits axonal branching. In contrast, semaphorin E acts as an attractive guidance signal for cortical axons. Attractive effects were only observed when growth cones encountered increasing concentrations or a patterned distribution of Semaphorin E, but not when they are exposed to uniform concentrations of this molecule. Specific binding sites for Semaphorin D and Semaphorin E were present on cortical fibers both in vitro and in vivo at the time when corticofugal projections are established. In situ hybridization analysis revealed that the population of cortical neurons used in our experiments express neuropilin-1 and neuropilin-2, which are essential components of receptors for the class III semaphorins. Moreover, semD mRNA was detected in the ventricular zone of the neocortex whereas semE mRNA was restricted to the subventricular zone. Taken together, these results indicate that semaphorins are bifunctional molecules whose effects depend on their spatial distribution. The coordinated expression of different semaphorins, together with their specific activities on cortical axons, suggests that multiple guidance signals contribute to the formation of precise corticofugal pathways.     

The central projections in the retina in Necturus maculosus

The projections of the retina in Necturus maculosus were studied by injecting radioactive proline into one eye. Labeling was seen in both the contralateral and ipsilateral diencephalon and tectum. The contralateral fibers are divided into three major tracts: the marginal, axial, and basal. The ipsilateral fibers separate into a marginal and an axial optic tract. The contralateral and ipsilateral axial optic tracts have a similar distribution. The contralateral and ipsilateral marginal optic tracts projecting to the diencephalon also have a similar distribution. However, in the tectum the ipsilateral marginal optic tract ends in the anterior third while the contralateral extends almost the entire length of the tectum. The retinotectal ipsilateral projection ends in clumps as has been described in other vetebrates. A direct ipsilateral retinotectal projection has not been described in any other amphibian.     

Similarities and differences in the cytoarchitecture of the tectum of frogs and salamanders

Frogs exhibit a morphologically complex (multiply laminated) optic tectum, while salamanders have one of the morphologically simplest tecta among vertebrates. In a comparative approach, the morphology of tectal projection neurons is investigated in three salamander species, Hydromantes italicus, H. genei and Plethodon jordani, and two frog species, Discoglossus pictus and Eleutherodactylus coqui, by means of retrograde Biocytin labeling complemented by intracellular Biocytin staining of cells. Despite striking differences in the gross anatomy of the tectum, salamanders and frogs have the same types of tectal neurons with respect to their dendritic arborization and the pattern of ipsilaterally and bilaterally ascending (to praetectum and thalamus) and ipsilaterally or contralaterally descending projections (to nucleus isthmi, medulla oblongata and rostral spinal cord). In the light of these findings, the relationship between morphological complexity of the tectum and behavioral complexity (feeding behavior) is discussed.     

Organisation of the tectorotundal and SP/IPS-rotundal projections in the chick

The organisation of the neural projections from the optic tectum and pretectal nuclei complex, n. subpretectalis / n. interstitio-pretecto-subpretectalis (SP/IPS), to the nucleus rotundus (Rt) in chicks was studied by using retrograde tracing techniques. After the injection of fluorescent retrograde tracers, rhodamine-conjugated latex microspheres, fluorescein-conjugated latex microspheres, True Blue, Fluoro-Gold, or rhodamine B isothiocyanate, into different regions of Rt and its middorsal extension, the nucleus triangularis (T), the distribution of retrogradely labelled neuronal cell bodies in the tectum and pretectal nuclei was assessed. Both the ipsilateral and contralateral tectorotundal projections were found to be organised topographically in as much as different sublaminas of the stratum griseum centrale (SGC) project in an orderly manner to Rt and T. The deepest stratum of SGC overlapping into the stratum album centrale projects to T. Deep SGC projects to the dorsal Rt and superficial SGC to the ventral Rt. A band running through the centre of Rt receives input from the central sublamina of SGC, and the caudal central Rt receives input from a deeper sublamina than does the rostral central Rt. The SP/IPS projects to the ipsilateral Rt only and the projection order is dorsal SP to dorsal Rt, ventral SP to ventral Rt and middle SP to the central band of Rt. The neurones in IPS and the nucleus of the tractus tectothalamicus project to T. Thus, Rt and T receive topographically both tecto- (excitatory) and SP/IPS- (inhibitory) projections. The possible functional implications for parallel information processing in these projections are discussed.     

Stereotaxic coordinates for the Rhesus monkey thalamus and mesencephalon referencing visual afferents and cytoarchitecture

When using a stereotaxic instrument for visual field stimulation we found that electrode placements in the thalamus and mesencephalon of prone rhesus monkeys with the aid of avaiable atlases showed considerable errors. As these animals are valuable for primate visual system reseach an atlas was constructed with methods that have not been used before for rhesus. In addition, the specific connections from the visual cortices, superior colliculus and retina to the thalamus and mesencephalon are also shown. Anesthetized monkeys of specific body dimensions had a matrix of pins inserted into the brain before fixation. A matrix was used so that the penetrations seen in the sectioned brain could be cross related as a control for accurate measurements of the stereotaxic planes throughout the brain. The surface of the whole brain frozen blocks were photographed on the microtome just before a cut section was taken. These calibrated pictures formed the "floor plan" of the atlas as they represent more accurately the brain geometry than individual sections which are distorted by cutting, staining and mounting. Cytoarchitectural (Nissl stain) and axonal connectional (Fink-Heimer stain) information was transferred and adjusted onto the block pictures from their corresponding stained sections. Follow up experiments showed that the present coordinates are accurate for these monkeys of restricted body dimensions. In addition, referencing visual axonal projections onto the same cytoarchitectural map in stereotaxic coordinates provides an atlas for localizing areas of the thalamus, on a basis other than cytoarchitecture, which receive combinations of visual inputs for further anatomical and physiological studies of the rhesus monkey visual system. The atlas further demonstrates that projections do not necessarily follow the cytoarchitectural definition of an area, but rather redefine the thalamus on the basis of specific axonal connections.     

Development of long-distance efferent projections from fetal hippocampal grafts depends upon pathway specificity and graft location in kainate-lesioned adult hippocampus

Fetal hippocampal cells grafted into the excitotoxically lesioned hippocampus of adult rats are capable of extending axonal projections into the host brain. We hypothesize that the axonal growth of grafted fetal cells into specific host targets, and the establishment of robust long-distance efferent graft projections, require placement of fetal cells in close proximity to appropriate axon guidance pathways. Intracerebroventricular administration of kainic acid in adult rats leads to a specific loss of hippocampal CA3 pyramidal neurons. We grafted 5'-bromodeoxyuridine-labeled embryonic day 19 hippocampal cells into adult hippocampus at four days post-kainic acid lesion, and quantitatively measured the projection of grafted cells into the contralateral hippocampus and the septum after three to four months survival using Fluoro-Gold and 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine (Dil) tracing. Grafts located in or near the degenerated CA3 cell layer exhibited numerous neurons which established commissural projections with the contralateral hippocampus. However, such projection did not occur in intrahippocampal grafts located away from the CA3 cell layer. In contrast, neurons in all grafts established robust projections into the septum regardless of location within hippocampus although grafts located near the degenerated CA3 cell layer displayed more neurons with such projections. Location of grafted cells clearly influences the development of efferent graft projections into distant targets in the adult host brain, particularly access to axon guidance pathways to facilitate the formation of projections. The establishment of robust long-distance commissural projections of fetal hippocampal grafts is clearly dependent on their placement in or near the degenerated CA3 cell layer, suggesting that appropriate axon guidance pathways for commissural pathways are tightly focussed near this cell layer. However, the establishment of septal projections of these grafts was not dependent on specific location within the CA3 cell layer, suggesting that axonal guidance mechanisms to the septum are more diffuse and not limited to the CA3 dendritic layers. The results underscore that fetal hippocampal grafts are capable of partly restoring lesioned hippocampal circuitry in adult animals when appropriately placed in the host hippocampus.     

Projection status of calbindin- and parvalbumin-immunoreactive neurons in the superficial layers of the rat's superior colliculus

Immunocytochemistry and retrograde labeling were used to define the thalamic projections of calbindin- and parvalbumin-containing cells in superficial layers of the rat's superior colliculus (SC). Quantitative analysis revealed that 90.8 +/- 2.2% (mean +/- standard deviation) of the calbindin-immunoreactive neurons in the stratum griseum superficiale (SGS) projected to the dorsal lateral geniculate nucleus (LGNd) and that 91.3 +/- 4.3% of calbindin-immunoreactive neurons in the stratum opticum (SO) projected to the lateral posterior nucleus (LP). In contrast, only 17.3 +/- 2.5% of parvalbumin-immunoreactive neurons in the SGS were found to project to the LGNd and 16.5 +/- 3.1% of the parvalbumin-immunoreactive SO cells were retrogradely labeled after LP injections. Few of the parvalbumin-immunoreactive neurons in either the SGS (7.2 +/- 2.5%) or the SO (9.2 +/- 2.5%) were GABA positive. The retrograde-labeling results suggest that parvalbumin-immunoreactive neurons in the rat's SO and SGS may either be primarily interneurons or have descending projections, while calbindin-containing cells are primarily thalamic projection neurons. These results are consistent with data from other rodents, but almost exactly the opposite of data that have been reported for the cat for these same populations of SC projection neurons. Such interspecies differences raise questions regarding the functional importance of expressing one calcium-binding protein versus another in a specific neuronal population.     

Larval development of tectal efferents and afferents in Xenopus laevis (Amphibia Anura)

The development of tectal connections in Xenopus laevis had been investigated using the degeneration technique to demonstrate the efferent pathways and the retrograde HRP transport to label the afferent pathways. Bilateral tectal efferents were present as soon as the beginning of metamorphosis. Ascending efferents originated from the anterior tectal part terminate in the secondary visual thalamic centres whereas the descending efferents coming from the posterior tectal part reached the tegmentum and the medulla oblongata. At this same time, the optic tectum already received secondary visual afferents originating in the ipsilateral pretectum and non-visual afferents from the ipsilateral semicircular torus and tegmentum. Some sparse bilateral isthmotectal connections were also present. Later, efferent pathways showed an increasing number of fibres whereas the sites of origin of afferents became more diversified: the dorsal thalamus, the suprachiasmatic area, the tegmental nuclei and in the medulla oblongata, the reticular and octavolateral areas sent bilateral projections to the optic tectum. At the end of metamorphosis, we noted ipsilateral olivotectal fibres and reciprocal connections between the tectum and the area of the Vth nerve. These last findings and the presence of the following direct projections, not previously reported in Anurans: the reciprocal connections between the tectum and the semicircular torus or the octavolateral area, underline the implication of the optic tectum in the multisensory (visual, acoustic, vibratory) integration elicited during the larval behavior. Also, the relations between the optic tectum and the lateral line system are particularly examined in the discussion.     

Positional determination of the naso-temporal retinal axis coincides with asymmetric expression of proteins along the anterior-posterior axis of the eye primordium

We used two different methodologies to examine at what stage development retinal positional specificity is established and which molecules are responsible. The first goal was achieved by removing parts of the presumptive temporal primary optic vesicle at stage 11 (40 to 45 hr of incubation) and fate mapping of tissue with presumptive nasal properties that shifted into the wound during the events of wound-healing. Participation of the shifted tissue in the healing resulted in assembly of a temporal retina with mosaic-like projection properties, as examined by retrograde double staining of the retinal ganglion cells from the optic tectum. In addition to cells with normal temporal-rostral projections, clusters of ganglion cells with nasal-like projection identities appeared labelled within the temporal hemiretina. The number of clusters increased with the amount of resected tissue, and by almost complete ablation of the presumptive temporal anlage, a temporal hemiretina with predominantly nasal retinotectal specificity was created. These neuroanatomical results suggested that neuroepithelial cells had fixed nasal and temporal positional specificities at stage 11. To examine differences in the cells derived of either half of the eye cup, we performed biochemical one- and two-dimensional gel electrophoresis of the hemianlagen at stage 11. In addition, incorporation of 35S-methionin into newly synthesized peptides was investigated. Both techniques revealed the exclusive expression of one major and three less-abundant proteins within the presumptive nasal anlage. The most abundant of these proteins has a molecular weight of about 40 kDa and is clearly distinguishable both in gel electrophoresis and autoradiography. The asymmetric protein patterns had disappeared when the retina was analysed with the same methods at the more advanced embryonic days E4 and E6. The asymmetry in the expression of proteins in the retinal primordium may be the biochemical correlate of an early positional specification of the retinal neuroepithelium. The difference in the protein expression may explain that mixing the positionally specified cells of either origins results in projection mosaics.     

Reelin regulates the development and synaptogenesis of the layer-specific entorhino-hippocampal connections

Here we examine the role of Reelin, an extracellular protein involved in neuronal migration, in the formation of hippocampal connections. Both at prenatal and postnatal stages, the general laminar and topographic distribution of entorhinal projections is preserved in the hippocampus of reeler mutant mice, in the absence of Reelin. However, developing and adult entorhinal afferents show severe alterations, including increased numbers of misrouted fibers and the formation of abnormal patches of termination from the medial and lateral entorhinal cortices. At perinatal stages, single entorhinal axons in reeler mice are grouped into thick bundles, and they have decreased axonal branching and decreased extension of axon collaterals. We also show that the number of entorhino-hippocampal synapses is lower in reeler mice than in control animals during development. Studies performed in mixed entorhino-hippocampal co-cultures combining slices from reeler and wild-type mice indicate that these abnormalities are caused by the lack of Reelin in the target hippocampus. These findings imply that Reelin fulfills a modulatory role during the formation of layer-specific and topographic connections in the hippocampus. They also suggest that Reelin promotes maturation of single fibers and synaptogenesis by entorhinal afferents.     

A role for tectal midline glia in the unilateral containment of retinocollicular axons

Retinal fibers approach close to the tectal midline but do not encroach on the other side. Just before the entry of retinal axons into the superior colliculus (SC), a group of radial glia differentiates at the tectal midline; the spatiotemporal deployment of these cells points to their involvement in the unilateral containment of retinotectal axons. To test for such a barrier function of the tectal midline cells, we used two lesion paradigms for disrupting their radial processes in the neonatal hamster: (1) a heat lesion was used to destroy the superficial layers of the right SC, including the midline region, and (2) a horizontally oriented hooked wire was inserted from the lateral edge of the left SC toward the midline and was used to undercut the midline cells, leaving intact the retinorecipient layers in the right SC. In both cases, the left SC was denervated by removing its contralateral retinal input. Animals were killed 12 hr to 2 weeks later, after intraocular injections of anterograde tracers to label the axons from the remaining eye. Both lesions resulted in degeneration of the distal processes of the tectal raphe glia and in an abnormal crossing of the tectal midline by retinal axons, leading to an innervation of the opposite ("wrong") tectum. The crossover occurred only where glial cell attachments were disrupted. These results document that during normal development, the integrity of the midline septum is critical in compartmentalizing retinal axons and in retaining the laterality of the retinotectal projection.