Dynamic aspects of retinotectal map formation revealed by a vital-dye fiber-tracing technique

In the visual system of Xenopus laevis, the axons from the retinal ganglion cells of the eye form a topographic projection onto the optic tectum. Many studies have focused on revealing the mechanisms responsible for this precise and regular projection pattern. In contrast to the static view of the system that one might expect from examining the regularity of the projection, recent work on its regeneration and its changes during larval development indicate that part of the patterning process involves the dynamic behavior of optic fibers. Typically, anatomical and electrophysiological techniques have been used to obtain static views of the developing retinotectal projection which then must be complied to provide a glimpse of any dynamic behavior. Here we report on experiments using a newly developed fiber tracing technique to directly follow the emergence of topography in the developing retinotectal projection. Defined halves of the developing eyebud were labeled with a vital fluorescent dye which fills the growing axons, and the projection of the labeled cells was followed for up to 2 weeks in individual animals. The experiments confirm that dorsal and ventral optic nerve fibers sort out into an ordered projection early in development. In contrast, nasal and temporal fibers initially overlap, and the same sets of prelabeled fibers then sort out into the adult topography over a period of days.     

Retinotectal plasticity in Xenopus: Anomalous ipsilateral projection following late larval eye removal

The left eye was removed from Stage 56 Xenopus tadpoles. Two to 9 months after metamorphosis, electrophysiologic analysis showed that the surviving (right) eye mediated a normal visual field projection to the left (contralateral) optic tectum. In addition, a peripheral region of the same retina innervated the entire right (ipsilateral) tectum. Primary evidence that indicates this anomalous ipsilateral projection was due to direct retina-to-tectum innervation comes from singleunit analysis, latency measurements, and tectal lesion studies. Thus, the peripheral retina simultaneously connected in much different patterns to the two optic tecta, solely on the basis of the presence (in the left tectum) or absence (in the right tectum) of central retinal fibers. This documents a role for fiber-fiber interaction (such as repulsion or competition) acting in combination with fiber-tectum interactions in the formation of the retinotectal map.     

Retinal projections in the chain pickerel (Esox niger Lesueur)

Summary The retinal projections inEsox niger, as determined with the aid of a modified cobalt-lysine method, are considerably more extensive in the diencephalon and pretectum than in other teleost fishes so far examined. Although most retinal axons terminate contralaterally, rare fibers can be traced to the same aggregates ipsilaterally. The retinohypothalamic projection appears larger than hitherto reported in teleosts, and the dorsomedial optic tract issues fibers to a series of cell clusters extending from the rostral thalamus to mid-torus levels. A retinal projection to a presumed ventrolateral optic nucleus (VLO) is described for the first time in a teleost. Other targets of retinal fibers include the nucleus geniculatus lateralis ipse of Meader (GLI), the pretectal nucleus (P), the cortical nucleus and a well-developed ventromedial optic nucleus (VMO). The projection to the optic tectum is principally to the stratum fibrosum et griseum superficiale (SFGS) and stratum marginale (SM), but a considerable number of axons also course through the stratum album centrale (SAC) before terminating there or piercing the stratum griseum centrale (SGC) and terminating in SFGS. Rare terminal arborizations of retinal fibers were also observed in stratum griseum centrale (SGS) and in the stratum griseum periventriculare (SGC) in restricted portions of the tectum. Because of the relatively large size of the visual structures inE. niger it is a potentially useful model for future experimental studies on the visual system.     

nev (cyfip2) is required for retinal lamination and axon guidance in the zebrafish retinotectal system

In the zebrafish retinotectal system, retinal ganglion cells (RGCs) project topographically along anterior-posterior (A-P) and dorsal-ventral (D-V) axes to innervate their primary target, the optic tectum. In the nevermind (nev) mutant, D-V positional information is not maintained by dorsonasal retinal axons as they project through the optic tract to the tectum. Here we present a detailed phenotypic analysis of the retinotectal projection in nev and show that dorsonasal axons do eventually find their correct location on the tectum, albeit after taking a circuitous path. Interestingly, nev seems to be specifically required for retinal axons but not for several non-retinal axon tracts. In addition, we find that nev is required both cell autonomously and cell nonautonomously for proper lamination of the retina. We show that nev encodes Cyfip2 (Cytoplasmic FMRP interacting protein 2) and is thus the first known mutation in a vertebrate Cyfip family member. Finally, we show that CYFIP2 acts cell autonomously in the D-V sorting of dorsonasal RGC axons in the optic tract. CYFIP2 is a highly conserved protein that lacks known domains or structural motifs but has been shown to interact with Rac and the fragile-X mental retardation protein, suggesting intriguing links to cytoskeletal dynamics and RNA regulation.     

Xefiltin,a Xenopus laevis neuronal intermediate filament protein,is expressed in actively growing optic axons during development and regeneration

Neurofilaments are an important structural component of the axonal cytoskeleton and are made of neuronal intermediate filament (nIF) proteins. During axonal development, neurofilaments undergo progressive changes in molecular composition. In mammals, for example, highly phosphorylated forms of the middle- and high-molecular-weight neurofilament proteins (NF-M and NF-H, respectively) are characteristic of mature axons, whereas nIF proteins such as α-internexin are typical of young axons. Such changes have been proposed to help growing axons accommodate varying demands for plasticity and stability by modulating the structure of the axonal cytoskeleton. Xefiltin is a recently discovered nIF protein of the frog Xenopus laevis, whose nervous system has a large capacity for regeneration and plasticity. By amino acid identity, xefiltin is closely related to two other nIF proteins, α-internexin and gefiltin. α-Internexin is found principally in embryonic axons of the mammalian brain, and gefiltin is expressed primarily in goldfish retinal ganglion cells and has been associated with the ability of the goldfish optic nerve to regenerate. Like gefiltin in goldfish, xefiltin in Xenopus is the most abundantly expressed nIF protein of mature retinal ganglion cells. In the present study, we used immunocytochemistry to study the distribution of xefiltin during optic nerve development and regeneration. During development, xefiltin was found in optic axons at stage 35/36, before they reach the tectum at stage 37/38. Similarly, after an orbital crush injury, xefiltin first reemerged in optic axons after the front of regeneration reached the optic chiasm, but before it reached the tectum. Thus, during both development and regeneration, xefiltin was present within actively growing optic axons. In addition, aberrantly projecting retinoretinal axons expressed less xefiltin than those entering the optic tract, suggesting that xefiltin expression is influenced by interactions between regenerating axons and cells encountered along the visual pathway. These results support the idea that changes in xefiltin expression, along with those of other nIF proteins, modulate the structure and stability of actively growing optic axons and that this stability is under the control of the pathway which growing axons follow. © 1997 John Wiley & Sons, Inc. J Neurobiol 33: 811–824, 1997     

Analysis of a repulsive axon guidance molecule, draxin, on ventrally directed axon projection in chick early embryonic midbrain

The mesencephalic V neurons and tectobulbar axons in chick embryo project over long distances that appear during the early development of the chick optic tectum. The mesencephalic V neuron and tectobulbar axonal growth begin at Hamburger and Hamilton stage 14 and stage 18, respectively. Both fibers proceed downward from the dorsal to the ventral side of the lateral wall of the optic tectum and then turn caudally and join the medial longitudinal fasciculus. Their axons appear in the most superficial layer of the tectum at early stages and do not cross the dorsal midline of the tectum. Here, we report the role of draxin, a recently identified axon guidance protein, in the formation of the ventrally directed tectum axonal tracts in chicken embryo. draxin is expressed in a high dorsal to low ventral gradient in chick optic tectum. In vitro experiments show that draxin repels neurite outgrowth from dorsal tectum explants. In vivo overexpression resulted in inhibition or misrouting of axon growth in the tectum. Therefore, draxin may be an important member of the collection of repulsive guidance molecules that regulate the formation of the ventrally directed tectum axon tracts.     

Ephrin-A6, a new ligand for EphA receptors in the developing visual system

In the embryonic visual system, EphA receptors are expressed on both temporal and nasal retinal ganglion cell axons. Only the temporal axons, however, are sensitive to the low concentrations of ephrin-A ligands found in the anterior optic tectum. The poor responsiveness of nasal axons to ephrin-A ligands, which allows them to traverse the anterior tectum and reach their targets in the posterior tectum, has been attributed to constitutive activation of the EphA4 receptor expressed in these axons. EphA4 is highly expressed throughout the retina, but is preferentially phosphorylated on tyrosine (activated) in nasal retina. In a screen for EphA4 ligands expressed in chicken embryonic retina, we have identified a novel ephrin, ephrin-A6. Like ephrin-A5, ephrin-A6 has high affinity for EphA4 and activates this receptor in cultured retinal cells. In the embryonic day 8 (E8) chicken visual system, ephrin-A6 is predominantly expressed in the nasal retina and ephrin-A5 in the posterior tectum. Thus, ephrin-A6 has the properties of a ligand that activates the EphA4 receptor in nasal retinal cells. Ephrin-A6 binds with high affinity to several other EphA receptors as well and causes growth cone collapse in retinal explants, demonstrating that it can elicit biological responses in retinal neurons. Ephrin-A6 expression is high at E6 and E8, when retinal axons grow to their tectal targets, and gradually declines at later developmental stages. The asymmetric distribution of ephrin-A6 in retinal cells, and the time course of its expression, suggest that this new ephrin plays a role in the establishment of visual system topography.     

金鱼再生的视神经在顶盖投射精确化的DiI顺行标记研究

本文用微量显微注射法,在金鱼视网膜的背侧用亲脂类荧光染料DiI标记少量神经节细胞,通过顺行标记研究了视神经再生过程中视网膜顶盖投射的精确化过程。在损伤视神经后的不同时期观察了再生视神经纤维在顶盖整装片上的分布。在再生早期它们以超出正常的途径由背腹两侧进入顶盖,广泛分布。但其中大部分仍分布于顶盖腹侧的靶区。在再生晚期通过精确化,重建如正常鱼一样精确的视网膜顶盖投射。这个精确化过程表现在以下三方面:(1)再生于顶盖错误区域的再生视神经纤维的消失;(2)再生早期视神经纤维主干上生长的侧部分支的消失;(3)到达靶区的再生视神经纤维形成重迭的终末分支。由以上结果推测,顶盖中可能存在两类不同的因子:一类是普通诱向因子,存在于整个顶盖中,它在再生早期引导再生的视神经纤维长入顶盖。另一类是神经营养因子,它具区域特异性,在再生晚期引导视神经纤维到达顶盖靶区,形成精确的视网膜顶盖投射。     

Changes in retinal arbors in compressed projections to half tecta in goldfish

In adult goldfish, electrophysiological studies have shown that the retinotectal projection reorganizes, following removal of half of the tectum, to form a complete but compressed projection over the remaining half tectum. As a result, each fiber terminates more rostrally than normal. Electron microscopic studies suggest a competition between retinal fibers for a fixed number of synaptic sites. The current study examines whether retinal arbors in the compressed projection are smaller than normal in extent or branching and whether the fiber paths in the tectum show the rostral movements and the search strategy that the retinal fibers use. The caudal half tectum was removed without cutting retinal fibers except those at the cut edge. At 3 to 19 months afterward, retinal fibers were labeled with horseradish peroxidase. In whole-mounted tecta, fibers and terminals were drawn under camera lucida and compared with normal arbors. The axonal paths were also traced across the tectum to their termination sites. At 3 to 6 months (early stages of compression), the arbors were rather normal in appearance, although they were actually significantly larger (23%) than normal in linear extent, arborized somewhat deeper and had fewer branches (18%). The fibers normally terminating in the rostral tectum followed normal stereotyped paths, whereas those cut at the edge had grown back and forth loops (apparent searching behavior) with little branching. By 10 months when compression is complete, arbors were significantly smaller than normal (19%), were arborizing significantly deeper, and had significantly fewer branches (19%). The differences were more pronounced in arbors of coarse and medium caliber than in fine caliber axons. The axons still ran in stereotyped fascicles, but included an extrafascicular portion that, unlike any axons in normals, turned back in a rostral direction before branching. This striking effect, present even in far rostral tectum, indicated that arbors had been forced to move rostrally to accomodate those from the ablated half. The small effect on arbor extent suggests that this is influenced by factors other than the magnification factor of the map, perhaps postsynaptic dendritic extent. The increased depth of termination is consistent with the increased thickness of the retinal terminal layer. The decreased number of branches is consistent with the conclusion that the remaining fixed number of synaptic sites shared among the full complement of retinal fibers should result in fewer synapses per retinal fiber. © 1995 John Wiley & Sons, Inc.     

Enkephalin-immunoreactive cells in the mesencephalic tegmentum project to the optic tectum of the teleosts Salmo gairdneri and Salmo salar

Summary Immunocytochemistry using antibodies against Met-enkephalin and Leu-enkephalin has demonstrated a group of large enkephalin-immunoreactive neurons in the nucleus of the rostral mesencephalic tegmentum (mRMT) of two teleost fish, Salmo gairdneri and Salmo salar. Injections of cobalt-lysine in the medial optic tectum retrogradely labeled the above group of tegmental neurons. Tegmental neurons were labeled only ipsilaterally to the injection site. This indicates that enkephalinergic neurons in the nRMT project to the optic tectum, and that at least some of the enkephalinergic axons observed in the optic tectum belong to a tegmento-tectal pathway. Comparable enkephalinergic pathways have been described in reptiles and birds, where pretectal-mesencephalic nuclei contribute to the enkephalin-containing fibers that project to the optic tectum.     

Alterations in the Xenopus retinotectal projection by antibodies to Xenopus N-CAM

The patterned neural projection from the eye to the optic tectum of lower vertebrates (the retinotectal projection) has been proposed to be ordered by interactions between the optic nerve fibers and their surrounding tissues. To investigate the role of one such defined cell interaction, agarose implants containing antibodies to the neural cell adhesion molecule, N-CAM, were inserted into the tectum of the African clawed frog, Xenopus laevis. Both monoclonal and polyclonal antibodies against N-CAM reversibly and specifically distorted the pattern of the retinotectal projection, decreasing the precision of the projection as determined by electrophysiological techniques as well as decreasing the density of retinal innervation of the tectum and the branching of single axons as determined by horseradish peroxidase tracing. The anatomical effects became maximal at 4 to 6 days after implantation and returned to undetectable levels by 2 weeks, whereas the physiological effects became maximal by 8 to 10 days and a normal physiological map was reestablished within 4 weeks. The results are consistent with the hypothesis that anti-N-CAM antibodies perturb the ongoing growth and retraction of the terminal arbors of the optic nerve fibers, such that a region of the tectum becomes largely denuded of fibers. The physiological defects may then be a consequence both of the initial retraction of optic nerve terminals and of the rapid ingrowth of the perturbed and neighboring optic nerve fibers into the denuded region after the antibodies were cleared from the tectum. These results support the concept of a major role for N-CAM-mediated adhesion during map regeneration and maintenance.     

Ipsilateral retinal projections into the tectum during regeneration of the optic nerve in the cichlid fish Haplochromis burtoni: a Dil study in fixed tissue.

Retinal projections were experimentally manipulated in a bony fish to reveal conditions under which considerably enlarged ipsilateral projections developed and persisted. Three experimental groups were studied: animals after unilateral enucleation, after unilateral nerve crush, and after enucleation and crush of the remaining optic nerve. At 29 days after unilateral enucleation alone, no enhanced ipsilateral projection had developed. After nerve crush, however, large numbers of retinal fibers regenerated into the ipsilateral tectum. Retrogradely filled, ipsilaterally projecting ganglion cells were distributed throughout the entire retina. After 15 days regenerating retinal fibers covered the entire ipsilateral tectum. At later stages the ipsilateral projection showed progressive reduction in coverage of the tectum. Combining enucleation with nerve crush led to an ipsilateral projection that covered the tectum at 28 days and later. In this experimental situation the development of an ipsilateral projection appears to be a two-step process: (1) Fibers are rerouted to the ipsilateral side at the diencephalon, and (2) ipsilateral fibers persist in the tectum only in the absence of a contralateral projection while they appear to be eliminated in the other cases.     

Ipsilateral retinal projections into the tectum during regeneration of the optic nerve in the cichlid fish Haplochromis burtoni: A dil study in fixed tissue

Retinal projections were experimentally manipulated in a bony fish to reveal conditions under which considerably enlarged ipsilateral projections developed and persisted. Three experimental groups were studied: animals after unilateral enucleation, after unilateral nerve crush, and after enucleation and crush of the remaining optic nerve. At 29 days after unilateral enucleation alone, no enhanced ipsilateral projection had developed. After nerve crush, however, large numbers of retinal fibers regenerated into the ipsilateral tectum. Retrogradely filled, ipsilaterally projecting ganglion cells were distributed throughout the entire retina. After 15 days regenerating retinal fibers covered the entire ipsilateral tectum. At later stages the ipsilateral projection showed progressive reduction in coverage of the tectum. Combining enucleation with nerve crush led to an ipsilateral projection that covered the tectum at 28 days and later. In this experimental situation the development of an ipsilateral projection appears to be a two-step process: (1) Fibers are rerouted to the ipsilateral side at the diencephalon, and (2) ipsilateral fibers persist in the tectum only in the absence of a contralateral projection while they appear to be eliminated in the other cases. © 1992 John Wiley & Sons, Inc.     

Light-optic, electron microscopic and electrophysiological study of centrifugal fibers of the retina in the lamprey Lampetra fluviatilis

Light and electronmicroscopic studies have been made on retinal structures in the lamprey labeled by horseradish peroxidase injected into the peripheral end of the cut optic nerve or to the midbrain tectum. On total retinal preparations, labeled axons were revealed together with dendrites and ganglionic cell bodies, as well as branching (presumably retinopetal) fibers, fine endings of which come closely to the labeled dendrites of the ganglionic cells. Electron microscopic data indicate that the labeled terminations of afferent fibers from synapses with both labeled and unlabeled dendrites, as well as with unlabeled neuronal bodies. It is concluded that centrifugal fibers in lamprey retina form contacts with the bodies and dendrites of the amacrine cells and dendrites of the ganglionic cells. Results of intracellular registration of responses of various retinal elements to the electrical stimulation of the optic nerve support this conclusion.     

Gradual appearance of a regulated retinotectal projection pattern in Xenopus laevis

The topographic projection pattern formed by the retinal ganglion cell axons in the tectum of the lower vertebrate appears to require positional cues that guide the optic nerve fibers to their appropriate targets. One approach to understanding these positional cues or "positional information" has been to investigate changes in the pattern of the retinotectal projection after surgical manipulation of the embryonic eyebud. Analysis of these apparent changes in the patterns of positional information in the eye, termed "pattern regulation," may provide clues to both the nature of positional information and the mechanisms by which it is assigned to cells in the eyebud. Here we examine pattern regulation in the Xenopus visual system following the replacement of the temporal half of a right eyebud with the temporal half of a left eyebud. This manipulation requires that the left half-eyebud be inverted along its dorsoventral axis. Electrophysiological maps of these compound eyes in postmetamorphic frogs reveal regulated maps; the cells in the temporal half of the NrTl eye project to the tectum with a dorsoventral polarity appropriate for their position in the host eye and not appropriate for the original positions of the grafted cells in the donor eyebud. Paradoxically, the regulated patterns are not apparent in the projections of the original grafted eyebud cells during early larval development. Using fiber-tracing and electrophysiological mapping techniques, we now show that the regulated patterns appear gradually in the projections made by peripheral retinal cells added during mid-larval development. Because the regulation occurs relatively late in development and probably only in the peripheral retinal cells, simple models of epimorphic or morphallactic regulation do not appear to fit this system. Thus, new or more complex models must be invoked to explain the phenomenon of pattern regulation in the developing visual system of Xenopus.     

Anterograde tracing of retinal axons in the avian embryo with low molecular weight derivatives of biotin

Several reactive biotin esters were injected into the eyes of chick and quail embryos at various stages of development. Four of the biotin esters reacted with molecules of the eye tissue and were detected with light and electron microscopy in fluorescein isothiocyanate and peroxidase-avidin incubated sections and whole mounts. Intra and extracellular components of the lens, the vitreous body, and the retina were labeled to different degrees. Three of the biotin esters (biotin-N-hydroxysuccinimidester, biotin-epsilon-aminocaproic acid-N-hydroxysuccinimidester, and desthiobiotin-N-hydroxysuccinimidester) prominently marked the optic fiber layer in the retina and the biotin labels were transported along the optic pathway. The tracers were detected up to the growth cone of axons 24 to 36 hr after injection. Explants from biotin marked retinas were cultured on collagen or basal laminae. During culturing axons grew out from these explants into the substratum showing that labeled tissue and nerve fibers were viable. The development of the optic pathway at the chiasma of quail embryos was studied using the biotin/avidin tracing. The bulk of fibers emerging from the retina crossed as shown by double labeling of both optic nerves in a complex pattern of segregated and interdigitizing axon bundles at the chiasma toward the contralateral side of the brain. From stage 25 onward a minor ipsilateral projection was found. At the same developmental stage a few fibers traveled into the contralateral optic nerve and grew retrogradely toward the contralateral eye. The percentage of specimens having this retino-retinal projection increased during development from 53% (stage 24 to 27; E3.5-E5.5) to 89% (stage 29 to 35; E6-E8) and declined to 40% at late embryogenesis (stage 37 to 41; E9-E12). The fact that all retinal axons were found within predictable pathways with some of them running in the wrong direction suggests that nerve fiber pathways provide accurate positional information, but at best weak directional information for growing nerve fibers.     

Pathways of Retinotectal Projection in Developing Xenopus Tadpoles Revealed by Selectively Labeling of Retinal Axons with Horseradish Peroxidase (HRP)

Anatomical mapping was made of the retinal central pathways from the chiasm to the targets within the tectum in the developing Xenopus tadpoles, after labeling a specific regional population of retinal axons with horseradish peroxidase (HRP). In the tadpoles at stage 50, pathway sorting of retinal axons within the optic tract was clear for the dorsoventral axis of the retina, but not for the nasotemporal axis. Most nasal retinal axons and some dorsal and ventral retinal axons invaded the tectum directly at the diencephalotectal junction, and arrived at their correct sites of innervation after running through ectopic parts of the tectum. These findings indicate that the pathway orientation before targets is not a prerequisite factor for establishment of the orderly map of the retinotectal projection. Rather, a direct interaction between ingrowing retinal axons and tectal cells seems to be a predominant factor for specification of retinal central connections.     

Calcium-binding proteins and cytochrome oxidase activity in the turtle optic tectum with special reference to the tectofugal visual pathway

Using immunohistochemistry and a tracer technique we investigated the distribution in the optic tectum of turtles (Emys orbicularis and Testudo horsfieldi) of the calcium-binding proteins (CaBPr) parvalbumin (PV), calbindin (CB) and calretinin (CR) before and after labeling of the nucleus rotundus (Rot) with horseradish peroxidase. The optic tectum activity of the cytochrome oxidase (CO) was studied in parallel. In the principal link of the tectofugal visual pathway (central gray layer, SGC) in both chelonian species, the sparse PV-ir as well as CB- and CR-ir neurons were found significantly varying both in number and the intensity of immunoreactivity of their bodies and dendrites. In contrast, the superficial (SGFS) and deeper periventricular (SGP) tectal layers comprised numerous cells immunoreactive to all three CaBPr in different proportions. Only few retrogradely labeled tectorotundal SGC neurons expressed PV, CB or CR. The very large PV-ir neurons in SGC and SAC were not retrogradely labeled; morphologically they matched the efferent neurons with descending projections. SGC neurons of two chelonian species differed in the level of CO activity. Intense immunoreactivity to all three CaBPr and high CO activity were detected in both species in SGFS neuropil with some differences in sublaminar distribution patterns. The peculiarities of the CaBPr and CO activity distribution patterns in different segments of SGC neurons are discussed as related to the laminar organization of the turtle tectum and its retinal innervation. It is suggested that in the projection tectorotundal SGC neurons the CaBPr are concentrated mainly in their distal dendrites that contact retinal afferents in the superficial retinorecipient tectal layer.     

Axoplasmic transport of RNA

The transport of RNA from the ganglion cell bodies within the retina to the contralateral optic tectum has been studied in the chick following intraocular injection of radioactive uridine. By tracing the appearance of labeled RNA at the proximal end of the optic nerve as it leaves the eyeball and comparing this to the time of arrival of RNA within the optic tectum, the migratory velocity of axonal RNA has been calculated to be around 12 mm per day. The continuation of RNA migration to the optic tectum in the presence of intracerebrally injected actinomycin-D but not in the presence of the intraocularly injected drug, suggests a retinal site of synthesis of the excess RNA found in the tectum innervated by the injected eye. A study of the rate of disppearance of radioactivity of the transported RNA in the optic lobes, suggested that this RNA turns over more rapidly than the bulk of tectal RNA. The destination of migrating RNA within the optic tectum has been autoradiographically examined. Most radioactive RNA is found in the outer tectal layers in which are found the afferent fibers of the optic tract and most of their synaptic terminations. Label is not confined to these areas however but is also present in the deeper layers of the optic tectum which are not known to contain any primary synapses of the axons from retinal ganglion cells.     

Two homeobox genes define the domain of EphA3 expression in the developing chick retina

Graded expression of the Eph receptor EphA3 in the retina and its two ligands, ephrin A2 and ephrin A5 in the optic tectum, the primary target of retinal axons, have been implicated in the formation of the retinotectal projection map. Two homeobox containing genes, SOHo1 and GH6, are expressed in a nasal-high, temporal-low pattern during early retinal development, and thus in opposing gradients to EphA3. Retroviral misexpression of SOHo1 or GH6 completely and specifically repressed EphA3 expression in the neural retina, but not in other parts of the central nervous system, such as the optic tectum. Under these conditions, some temporal ganglion cell axons overshot their expected termination zones in the rostral optic tectum, terminating aberrantly at more posterior locations. However, the majority of ganglion cell axons mapped to the appropriate rostrocaudal locations, although they formed somewhat more diffuse termination zones. These findings indicate that other mechanisms, in addition to differential EphA3 expression in the neural retina, are required for retinal ganglion axons to map to the appropriate rostrocaudal locations in the optic tectum. They further suggest that the control of topographic specificity along the retinal nasal-temporal axis is split into several independent pathways already at a very early time in development.