Retinal axon pathfinding in the optic chiasm: divergence of crossed and uncrossed fibers

A Stay in Friedrich Bonhoeffer's Lab in Tubingen in the Mid-eighties

The main focus of research for which Friedrich Bonhoeffer's work is known in the Neuroscience community was pioneer experiments on how axonal projections could organize into "maps", what mechanisms are involved in axon guidance and involve gradients of guiding molecules, and isolation of the first such molecules, e.g. RAGS (ephrin A5) and RGM (repulsive guidance molecule). Other papers have described in detail these contributions as well as Friedrich Bonhoeffer's personality. In the mid-eighties, I made a 2-year stay in his lab and initiated a line of research on development of binocular connections in Mammals, particularly the guidance of retinal fibers to one or the other side of the brain. In this paper I recall these circumstances as they pertain to Neuroscience as it stood at the time, and explain as best as I can how his lab was a conducive setting for the discoveries made there and how Friedrich Bonhoeffer acted for me as a scientist and a tutor.

Analysis of axon divergence at the optic chiasm in nogo-a knockout mice

Our earlier studies have shown that the axon growth inhibitory molecule Nogo affects axon routing at the optic chiasm likely through a differential regulation of Nogo receptor on the optic axons. Using isoform specific antibodies, we further showed that Nogo-A was predominantly expressed by retinal ganglion cells and their axons, while Nogo-B was highly localized on the radial glia at the midline of the chiasm, suggesting a role of Nogo-B in regulating turning of uncrossed axons. To further investigate the roles of Nogo-A in axon divergence, we analyzed the routing of axons in the chiasm of Nogo-A knockout mice during the growth of axons across the midline. At E13 to E16, there was no significant difference in the contralateral projection (P = 0.6943 for E13; P = 0.9867 for E14; P = 0.4121 for E15 and P = 0.3402 for E16). The results also showed the absence of Nogo-A did not cause any obvious change to the ipsilateral projection at the optic chiasm, both for the early generated uncrossed axons at E13 and E14 and the late cohorts at E15-E16, when compared with the wild-type mice (P = 0.4788 for E13; P = 0.188 for E14; P = 0.3152 for E15 and P = 0.432 for E16). These findings support that Nogo-A is not the major isoform to guide the axon divergence in the mouse optic chiasm.

Retinal Ganglion Cell Axon Wiring Establishing the Binocular Circuit

Binocular vision depends on retinal ganglion cell (RGC) axon projection either to the same side or to the opposite side of the brain. In this article, we review the molecular mechanisms for decussation of RGC axons, with a focus on axon guidance signaling at the optic chiasm and ipsi- and contralateral axon organization in the optic tract prior to and during targeting. The spatial and temporal features of RGC neurogenesis that give rise to ipsilateral and contralateral identity are described. The albino visual system is highlighted as an apt comparative model for understanding RGC decussation, as albinos have a reduced ipsilateral projection and altered RGC neurogenesis associated with perturbed melanogenesis in the retinal pigment epithelium. Understanding the steps for RGC specification into ipsi- and contralateral subtypes will facilitate differentiation of stem cells into RGCs with proper navigational abilities for effective axon regeneration and correct targeting of higher-order visual centers.

Commissural axon guidance in the developing spinal cord: from Cajal to the present day

During neuronal development, the formation of neural circuits requires developing axons to traverse a diverse cellular and molecular environment to establish synaptic contacts with the appropriate postsynaptic partners. Essential to this process is the ability of developing axons to navigate guidance molecules presented by specialized populations of cells. These cells partition the distance traveled by growing axons into shorter intervals by serving as intermediate targets, orchestrating the arrival and departure of axons by providing attractive and repulsive guidance cues. The floor plate in the central nervous system (CNS) is a critical intermediate target during neuronal development, required for the extension of commissural axons across the ventral midline. In this review, we begin by giving a historical overview of the ventral commissure and the evolutionary purpose of decussation. We then review the axon guidance studies that have revealed a diverse assortment of midline guidance cues, as well as genetic and molecular regulatory mechanisms required for coordinating the commissural axon response to these cues. Finally, we examine the contribution of dysfunctional axon guidance to neurological diseases.

Conversations with Ray Guillery on albinism: linking Siamese cat visual pathway connectivity to mouse retinal development

In albinism of all species, perturbed melanin biosynthesis in the eye leads to foveal hypoplasia, retinal ganglion cell misrouting, and, consequently, altered binocular vision. Here, written before he died, Ray Guillery chronicles his discovery of the aberrant circuitry from eye to brain in the Siamese cat. Ray's characterization of visual pathway anomalies in this temperature sensitive mutation of tyrosinase and thus melanin synthesis in domestic cats opened the exploration of albinism and simultaneously, a genetic approach to the organization of neural circuitry. I follow this account with a remembrance of Ray's influence on my work. Beginning with my postdoc research with Ray on the cat visual pathway, through my own work on the mechanisms of retinal axon guidance in the developing mouse, Ray and I had a continuous and rich dialogue about the albino visual pathway. I will present the questions Ray posed and clues we have to date on the still-elusive link between eye pigment and the proper balance of ipsilateral and contralateral retinal ganglion cell projections to the brain.

A Subtle Network Mediating Axon Guidance: Intrinsic Dynamic Structure of Growth Cone, Attractive and Repulsive Molecular Cues, and the Intermediate Role of Signaling Pathways

A fundamental feature of both early nervous system development and axon regeneration is the guidance of axonal projections to their targets in order to assemble neural circuits that control behavior. In the navigation process where the nerves grow toward their targets, the growth cones, which locate at the tips of axons, sense the environment surrounding them, including varies of attractive or repulsive molecular cues, then make directional decisions to adjust their navigation journey. The turning ability of a growth cone largely depends on its highly dynamic skeleton, where actin filaments and microtubules play a very important role in its motility. In this review, we summarize some possible mechanisms underlying growth cone motility, relevant molecular cues, and signaling pathways in axon guidance of previous studies and discuss some questions regarding directions for further studies.

Semi-invasive and non-invasive recording of visual evoked potentials in mice

PURPOSE

Visual evoked potentials (VEPs) are used to assess visual function in preclinical models of neurodegenerative diseases. VEP recording with epidural screw electrodes is a common method to study visual function in rodents, despite being an invasive procedure that can damage the tissue under the skull. The present study was performed to test a semi-invasive (epicranial) and a non-invasive (epidermal) VEP recording technique, comparing them with the classic epidural acquisition method.

METHODS

Flash VEPs were recorded from C57BL/6 mice on three separate days within 2 weeks. Waveforms, latencies and amplitudes of the components were compared between the three different methods, utilizing coefficient of repeatability, coefficient of variation and intersession standard deviation to evaluate reproducibility.

RESULTS

While epidural electrodes succeeded in recording two negative peaks (N1 and N2), epicranial and epidermal electrodes recorded a single peak (N1). Statistical indexes showed a comparable reproducibility between the three techniques, with a greater stability of N1 latency recorded through epicranial electrodes. Moreover, N1 amplitudes recorded with the new less-invasive methods were more reproducible compared to the invasive gold-standard technique.

CONCLUSIONS

These results demonstrate the reliability of semi- and non-invasive VEP recordings, which can be useful to evaluate murine models of neurological diseases.

Spatiotemporal distribution of glia in and around the developing mouse optic tract

In the developing mouse optic tract, retinal ganglion cell (RGC) axon position is organized by topography and laterality (i.e., eye-specific or ipsi- and contralateral segregation). Our lab previously showed that ipsilaterally projecting RGCs are segregated to the lateral aspect of the developing optic tract and found that ipsilateral axons self-fasciculate to a greater extent than contralaterally projecting RGC axons in vitro. However, the full complement of axon-intrinsic and -extrinsic factors mediating eye-specific segregation in the tract remain poorly understood. Glia, which are known to express several guidance cues in the visual system and regulate the navigation of ipsilateral and contralateral RGC axons at the optic chiasm, are natural candidates for contributing to eye-specific pre-target axon organization. Here, we investigate the spatiotemporal expression patterns of both putative astrocytes (Aldh1l1+ cells) and microglia (Iba1+ cells) in the embryonic and neonatal optic tract. We quantified the localization of ipsilateral RGC axons to the lateral two-thirds of the optic tract and analyzed glia position and distribution relative to eye-specific axon organization. While our results indicate that glial segregation patterns do not strictly align with eye-specific RGC axon segregation in the tract, we identify distinct spatiotemporal organization of both Aldh1l1+ cells and microglia in and around the developing optic tract. These findings inform future research into molecular mechanisms of glial involvement in RGC axon growth and organization in the developing retinogeniculate pathway.

Dynamic expression of p75NTR and Lingo-1 during development of mouse retinofugal pathway

Our previous studies showed interaction of Nogo at the midline with its receptor (NgR) on optic axons plays a role in axon divergence at the mouse optic chiasm. Since NgR lacks a cytoplasmic domain, it needs transmembrane receptor partners for signal transduction. In this study, we examined whether the co-receptors of NgR, low-affinity neurotrophic receptor (p75^NTR) and Lingo-1, are localized on axons in the mouse optic pathway. In the retina, p75^NTR and Lingo-1 were observed on neuroepithelial cells at E13 and later on the retinal ganglion cells at E14 and E15. At the optic disc, p75^NTR was observed on the retinal axons, whereas Lingo-1 was found on glial processes surrounding the axon fascicles. Both p75^NTR and Lingo-1 were found on axons in the optic stalk, optic chiasm and optic tract. Furthermore, a transient expression of Lingo-1 was observed on the SSEA-1 positive chiasmatic neurons at E13, but not at later developmental stages. The presence of p75^NTR and Lingo-1 on optic axons provides further supports to the contribution of Nogo/NgR signaling in axon divergence at the mouse optic chiasm.

Receptors of intermediates of carbohydrate metabolism, GPR91 and GPR99, mediate axon growth

During the development of the visual system, high levels of energy are expended propelling axons from the retina to the brain. However, the role of intermediates of carbohydrate metabolism in the development of the visual system has been overlooked. Here, we report that the carbohydrate metabolites succinate and α-ketoglutarate (α-KG) and their respective receptor-GPR91 and GPR99-are involved in modulating retinal ganglion cell (RGC) projections toward the thalamus during visual system development. Using ex vivo and in vivo approaches, combined with pharmacological and genetic analyses, we revealed that GPR91 and GPR99 are expressed on axons of developing RGCs and have complementary roles during RGC axon growth in an extracellular signal-regulated kinases 1 and 2 (ERK1/2)-dependent manner. However, they have no effects on axon guidance. These findings suggest an important role for these receptors during the establishment of the visual system and provide a foundational link between carbohydrate metabolism and axon growth.

Eye-specific segregation and differential fasciculation of developing retinal ganglion cell axons in the mouse visual pathway

Prior to forming and refining synaptic connections, axons of projection neurons navigate long distances to their targets. While much is known about guidance cues for axon navigation through intermediate choice points, whether and how axons are organized within tracts is less clear. Here we analyze the organization of retinal ganglion cell (RGC) axons in the developing mouse retinogeniculate pathway. RGC axons are organized by both eye-specificity and topography in the optic nerve and tract: ipsilateral RGC axons are segregated from contralateral axons and are offset laterally in the tract relative to contralateral axon topographic position. To identify potential cell-autonomous factors contributing to the segregation of ipsilateral and contralateral RGC axons in the visual pathway, we assessed their fasciculation behavior in a retinal explant assay. Ipsilateral RGC neurites self-fasciculate more than contralateral neurites in vitro and maintain this difference in the presence of extrinsic chiasm cues. To further probe the role of axon self-association in circuit formation in vivo, we examined RGC axon organization and fasciculation in an EphB1^-/- mutant, in which a subset of ipsilateral RGC axons aberrantly crosses the midline but targets the ipsilateral zone in the dorsal lateral geniculate nucleus on the opposite side. Aberrantly crossing axons retain their association with ipsilateral axons in the contralateral tract, indicating that cohort-specific axon affinity is maintained independently of guidance signals present at the midline. Our results provide a comprehensive assessment of RGC axon organization in the retinogeniculate pathway and suggest that axon self-association contributes to pre-target axon organization.

Retinal axon guidance at the midline: Chiasmatic misrouting and consequences

The visual representation of the outside world relies on the appropriate connectivity between the eyes and the brain. Retinal ganglion cells are the sole neurons that send an axon from the retina to the brain, and thus the guidance decisions of retinal axons en route to their targets in the brain shape the neural circuitry that forms the basis of vision. Here, we focus on the choice made by retinal axons to cross or avoid the midline at the optic chiasm. This decision allows each brain hemisphere to receive inputs from both eyes corresponding to the same visual hemifield, and is thus crucial for binocular vision. In achiasmatic conditions, all retinal axons from one eye project to the ipsilateral brain hemisphere. In albinism, abnormal guidance of retinal axons at the optic chiasm leads to a change in the ratio of contralateral and ipsilateral projections with the consequence that each brain hemisphere receives inputs primarily from the contralateral eye instead of an almost equal distribution from both eyes in humans. In both cases, this misrouting of retinal axons leads to reduced visual acuity and poor depth perception. While this defect has been known for decades, mouse genetics have led to a better understanding of the molecular mechanisms at play in retinal axon guidance and at the origin of the guidance defect in albinism. In addition, fMRI studies on humans have now confirmed the anatomical and functional consequences of axonal misrouting at the chiasm that were previously only assumed from animal models. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 844-860, 2017.

Dopamine signaling regulates the projection patterns in the mouse chiasm

Ocular albinism (OA) is characterized by inadequate L-3, 4-dihydroxyphenylalanine (L-DOPA) and dopamine (DA) in the eyes. This study investigated DA-related signaling pathways in mouse chiasm projection patterns and the potential role of ocular albinism type 1 (OA1) and dopamine 1A (D1A) receptors in the optic pathway. In embryonic day (E) E13-E15 retina, most L-DOPA and OA1-positive cells were distributed among Müller glial cells on E13 and retinal ganglion cells (RGC) on E14. In the ventral diencephalon, OA1 and L-DOPA were strongly expressed on the optic chiasm (OC) and optic tract (OT), respectively, but weak on the optic stalk (OS). At E13-E15, DA and D1A staining was predominately expressed in radially arranged cells with a neuronal expression pattern. In the ventral diencephalon, DA and D1A were strongly expressed on the OC, OT and OS. Furthermore, L-DOPA significantly inhibited retinal axon outgrowth in both the dorsal nasal (DN) and ventral temporal (VT) groups. DA inhibited retinal axon outgrowth, which was abolished by the D1A antagonist SCH23390. Brain slice cultures indicated that L-DOPA inhibited axons that crossed at the OC of E13 embryos, which was not abolished by DA. L-DOPA also inhibited axons that crossed at the OC of albino mice. Albino mice exhibited reduced ipsilateral retinal projections compared with C57 pigmented mice. No significant difference was identified in the uncrossed projections of albino mice following L-DOPA and DA expression. Furthermore, transcription factor Zic family member 2 (Zic2) upregulated OA1 mRNA expression. Our findings provide critical insights into DA-related signaling in retinal development.

Localization of protein kinase C isoforms in the optic pathway of mouse embryos and their role in axon routing at the optic chiasm

Protein kinase C (PKC) plays a key role in many receptor-mediated signaling pathways that regulate cell growth and development. However, its roles in guiding axon growth and guidance in developing neural pathways are largely unknown. To investigate possible functions of PKC in the growth and guidance of axons in the optic chiasm, we first determined the localization of major PKC isoforms in the retinofugal pathway of mouse embryos, at the stage when axons navigate through the midline. Results showed that PKC was expressed in isoform specific patterns in the pathway. PKC-α immunoreactivity was detected in the chiasm and the optic tract. PKC-βΙΙ was strong in the optic stalk but was attenuated on axons in the diencephalon. Immunostaining for PKC-ε showed a colocalization in the chiasmatic neurons that express a surface antigen stage specific embryonic antigen-1 (SSEA-1). These chiasmatic neurons straddled the midline of the optic chiasm, and have been shown in earlier studies a role in regulation of axon growth and guidance. Expression levels of PKC-βΙ, -δ and -γ were barely detectable in the pathway. Blocking of PKC signaling with Ro-32-0432, an inhibitor specific for PKC-α and -β at nanomolar concentration, produced a dramatic reduction of ipsilateral axons from both nasal retina and temporal crescent. We conclude from these studies that PKC-α and -βΙΙ are the predominant forms in the developing optic pathway, whereas PKC-ε is the major form in the chiasmatic neurons. Furthermore, PKC-α and -βΙΙ are likely involved in signaling pathways triggered by inhibitory molecules at the midline that guide optic axons to the uncrossed pathway.

Forward signaling by EphB1/EphB2 interacting with ephrin-B ligands at the optic chiasm is required to form the ipsilateral projection

EphB receptor tyrosine kinases direct axonal pathfinding through interactions with ephrin-B proteins following axon-cell contact. As EphB:ephrin-B binding leads to bidirectional signals, the contributions of signaling into the Eph-expressing cell (forward signaling) or the ephrin-expressing cell (reverse signaling) cannot be assigned using traditional protein null alleles. To determine if EphB1 is functioning solely as a receptor during axon pathfinding, a new knock-in mutant mouse was created, EphB1(T-lacZ), which expresses an intracellular-truncated EphB1-β-gal fusion protein from the endogenous locus. As in the EphB1(-/-) protein null animals, the EphB1(T-lacZ/T-lacZ) homozygotes fail to form the ipsilateral projecting subpopulation of retinal ganglion cell axons. This indicates that reverse signaling through the extracellular domain of EphB1 is not required for proper axon pathfinding of retinal axons at the optic chiasm. Further analysis of other EphB and ephrin-B mutant mice shows that EphB1 is the preferred receptor of ephrin-B2 and, to a lesser degree, ephrin-B1 in mediating axon guidance at the optic chiasm despite the coexpression of EphB2 in the same ipsilaterally projecting retinal axons.

Appropriate Bmp7 levels are required for the differentiation of midline guidepost cells involved in corpus callosum formation

Guidepost cells are essential structures for the establishment of major axonal tracts. How these structures are specified and acquire their axon guidance properties is still poorly understood. Here, we show that in mouse embryos appropriate levels of Bone Morphogenetic Protein 7 (Bmp7), a member of the TGF-β superfamily of secreted proteins, are required for the correct development of the glial wedge, the indusium griseum, and the subcallosal sling, three groups of cells that act as guidepost cells for growing callosal axons. Bmp7 is expressed in the region occupied by these structures and its genetic inactivation in mouse embryos caused a marked reduction and disorganization of these cell populations. On the contrary, infusion of recombinant Bmp7 in the developing forebrain induced their premature differentiation. In both cases, changes were associated with the disruption of callosal axon growth and, in most animals fibers did not cross the midline forming typical Probst bundles. Addition of Bmp7 to cortical explants did not modify the extent of their outgrowth nor their directionality, when explants were exposed to a focalized source of the protein. Together, these results indicate that Bmp7 is indirectly required for corpus callosum formation by controlling the timely differentiation of its guidepost cells.

Ephrin-B2 elicits differential growth cone collapse and axon retraction in retinal ganglion cells from distinct retinal regions

The circuit for binocular vision and stereopsis is established at the optic chiasm, where retinal ganglion cell (RGC) axons diverge into the ipsilateral and contralateral optic tracts. In the mouse retina, ventrotemporal (VT) RGCs express the guidance receptor EphB1, which interacts with the repulsive guidance cue ephrin-B2 on radial glia at the optic chiasm to direct VT RGC axons ipsilaterally. RGCs in the ventral retina also express EphB2, which interacts with ephrin-B2, whereas dorsal RGCs express low levels of EphB receptors. To investigate how growth cones of RGCs from different retinal regions respond upon initial contact with ephrin-B2, we utilized time-lapse imaging to characterize the effects of ephrin-B2 on growth cone collapse and axon retraction in real time. We demonstrate that bath application of ephrin-B2 induces rapid and sustained growth cone collapse and axon retraction in VT RGC axons, whereas contralaterally-projecting dorsotemporal RGCs display moderate growth cone collapse and little axon retraction. Dose response curves reveal that contralaterally-projecting ventronasal axons are less sensitive to ephrin-B2 treatment compared to VT axons. Additionally, we uncovered a specific role for Rho kinase signaling in the retraction of VT RGC axons but not in growth cone collapse. The detailed characterization of growth cone behavior in this study comprises an assay for the study of Eph signaling in RGCs, and provides insight into the phenomena of growth cone collapse and axon retraction in general.

Development of the retina and optic pathway

Our understanding of the development of the retina and visual pathways has seen enormous advances during the past 25years. New imaging technologies, coupled with advances in molecular biology, have permitted a fuller appreciation of the histotypical events associated with proliferation, fate determination, migration, differentiation, pathway navigation, target innervation, synaptogenesis and cell death, and in many instances, in understanding the genetic, molecular, cellular and activity-dependent mechanisms underlying those developmental changes. The present review considers those advances associated with the lineal relationships between retinal nerve cells, the production of retinal nerve cell diversity, the migration, patterning and differentiation of different types of retinal nerve cells, the determinants of the decussation pattern at the optic chiasm, the formation of the retinotopic map, and the establishment of ocular domains within the thalamus.

Cellular strategies of axonal pathfinding

Axons follow highly stereotyped and reproducible trajectories to their targets. In this review we address the properties of the first pioneer neurons to grow in the developing nervous system and what has been learned over the past several decades about the extracellular and cell surface substrata on which axons grow. We then discuss the types of guidance cues and their receptors that influence axon extension, what determines where cues are expressed, and how axons respond to the cues they encounter in their environment.

The growth-inhibitory protein Nogo is involved in midline routing of axons in the mouse optic chiasm

We have investigated the role of Nogo, a protein that inhibits regenerating axons in the adult central nervous system, on axon guidance in the developing optic chiasm of mouse embryos. Nogo protein is expressed by radial glia in the midline within the optic chiasm where uncrossed axons turn, and the Nogo receptor (NgR) is expressed on retinal neurites and growth cones. In vitro neurite outgrowth from both dorsonasal and ventrotemporal retina was inhibited by Nogo protein, and this inhibition was abolished by blocking NgR activity. In slice cultures of the optic pathway, blocking NgR with a peptide antagonist produced significant reduction in the uncrossed projection but had no effect on the crossing axons. This result was confirmed by treating cultures with an anti-Nogo functional blocking antibody. In vitro coculture assays of retina and optic chiasm showed that NgR was selectively reduced on neurites and growth cones from dorsonasal retina when they contacted chiasm cells, but not on those from ventrotemporal retina. These findings provide evidence that Nogo signaling is involved in directing the growth of axons in the mouse optic chiasm and that this process relies on a differential regulation of NgR on axons from the dorsonasal and ventrotemporal retina.

Retinal axon growth at the optic chiasm: to cross or not to cross

At the optic chiasm, retinal ganglion cell axons from each eye converge and segregate into crossed and uncrossed projections, a pattern critical for binocular vision. Here, we review recent findings on optic chiasm development, highlighting the specific transcription factors and guidance cues that implement retinal axon divergence into crossed and uncrossed pathways. Although mechanisms underlying the formation of the uncrossed projection have been identified, the means by which retinal axons are guided across the midline are still unclear. In addition to directives provided by transcription factors and receptors in the retina, gene expression in the ventral diencephalon influences chiasm formation. Throughout this review, we compare guidance mechanisms at the optic chiasm with those in other midline models and highlight unanswered questions both for retinal axon growth and axon guidance in general.

Diversity in mammalian chiasmatic architecture: ipsilateral axons are deflected at glial arches in the prechiasmatic optic nerve of the eutherian Tupaia belangeri

Permanent ipsilaterally projecting axons approach the chiasmatic midline in rodents but are confined to lateral parts of the optic chiasm in marsupials. Hence, principally different mechanisms were thought to underlie axon pathway choice in eutherian (placental) and marsupial mammals. First evidence of diversity in eutherian chiasmatic architecture came from studies in the newborn and adult tree shrew Tupaia belangeri (Jeffery et al. [1998] J. Comp. Neurol. 390:183-193). Here, as in marsupials, ipsilaterally projecting axons do not approach the midline. The present study aims to clarify how the developing tree shrew chiasm is organized, how glial cells are arranged therein, and the extent to which the tree shrew chiasm is similar to that of marsupials or other eutherians. By using routinely stained serial sections as well as immunohistochemistry with antibodies against glial fibrillary acidic protein, vimentin, and medium-molecular-weight neurofilament protein, we investigated chiasm formation from embryonic day 18 (E18) to birth (E43). From E22 onward, ipsilaterally projecting axons diverged from contralaterally projecting axons in prechiasmatic parts of the optic nerve. They made sharp turns when arriving at glial arches found at the transition from the optic nerve to the chiasm. Thus, during the ingrowth period of axons, Tupaia belangeri and marsupials have specialized glial arrays in common, which probably help to deflect ipsilaterally projecting axons to lateral parts of the chiasm. Our observations provide new evidence of diversity in eutherian chiasmatic architecture and identify Tupaia belangeri as an appropriate animal model for studies on the mechanisms underlying axon guidance in the developing chiasm of higher primates.

Zic2 promotes axonal divergence at the optic chiasm midline by EphB1-dependent and -independent mechanisms

Axons of retinal ganglion cells (RGCs) make a divergent choice at the optic chiasm to cross or avoid the midline in order to project to ipsilateral and contralateral targets, thereby establishing the binocular visual pathway. The zinc-finger transcription factor Zic2 and a member of the Eph family of receptor tyrosine kinases, EphB1, are both essential for proper development of the ipsilateral projection at the mammalian optic chiasm midline. Here, we demonstrate in mouse by functional experiments in vivo that Zic2 is not only required but is also sufficient to change the trajectory of RGC axons from crossed to uncrossed. In addition, our results reveal that this transcription factor regulates the expression of EphB1 in RGCs and also suggest the existence of an additional EphB1-independent pathway controlled by Zic2 that contributes to retinal axon divergence at the midline.

Enzymatic removal of hyaluronan affects routing of axons in the mouse optic chiasm

Perturbations of interaction of hyaluronan (HA) with its receptor CD44 cause multiple errors in axon routing at the mouse optic chiasm. To investigate this interaction further on the chiasm routing, we studied the axon routing after enzymatic removal of HA from slice preparations of the optic pathway. Hyaluronidase treatment produced an obvious reduction in midline crossing of the first generated axons in E13 chiasms, but had no influence on routing ofthe uncrossed axons in E15 and E16 slices. These findings support a direct role of HA, acting probably through CD44, on axon decussation during early phase of chiasm development, but argue against a direct function of HA on the turning of uncrossed axons in the mouse optic chiasm.

Tyrosinase and ocular diseases: Some novel thoughts on the molecular basis of oculocutaneous albinism type 1

Tyrosinase (TYR) is a multifunctional copper-containing glycoenzyme (approximately 80 kDa), which plays a key role in the rate-limiting steps of the melanin biosynthetic pathway. This membrane-bound protein, possibly evolved by the fusion of two different copper-binding proteins, is mainly expressed in epidermal, ocular and follicular melanocytes. In the melanocytes, TYR functions as an integrated unit with other TYR-related proteins (TYRP1, TYRP2), lysosome-associated membrane protein 1 (LAMP1) and melanocyte-stimulating hormone receptors; thus forming a melanogenic complex. Mutations in the TYR gene (TYR, 11q14-21, MIM 606933) cause oculocutaneous albinism type 1 (OCA1, MIM 203100), a developmental disorder having an autosomal recessive mode of inheritance. In addition, TYR can act as a modifier locus for primary congenital glaucoma (PCG) and it also contributes significantly in the eye developmental process. Expression of TYR during neuroblast division helps in later pathfinding by retinal ganglion cells from retina to the dorsal lateral geniculate nucleus. However, mutation screening of TYR is complicated by the presence of a pseudogene-TYR like segment (TYRL, 11p11.2, MIM 191270), sharing approximately 98% sequence identity with the 3' region of TYR. Thus, in absence of a full-proof strategy, any nucleotide variants identified in the 3' region of TYR could actually be present in TYRL. Interestingly, despite extensive search, the second TYR mutation in 15% of the OCA1 cases remains unidentified. Several possible locations of these "uncharacterized mutations" (UCMs) have been speculated so far. Based on the structure of TYR gene, its sequence context and some experimental evidences, we propose two additional possibilities, which on further investigations might shed light on the molecular basis of UCMs in TYR of OCA1 patients; (i) partial deletion of the exons 4 and 5 region of TYR that is homologous with TYRL and (ii) variations in the polymorphic GA complex repeat located between distal and proximal elements of the human TYR promoter that can modulate the expression of the gene leading to disease pathogenesis.

Effects of exogenous hyaluronan on midline crossing and axon divergence in the optic chiasm of mouse embryos

Perturbation of the transmembrane glycoprotein, CD44, has been shown to cause multiple errors in axon routing in the mouse optic chiasm. In a recent report we have shown that the major CD44 ligand, hyaluronan (HA), is colocalized with CD44 at the midline of the chiasm, suggesting a possible contribution to the control of axon routing in the chiasm. We examined this issue by investigating the effects of exogenous HA on routing of axons in the chiasm in slice preparations of the optic pathway. In preparations of the E13 optic pathway, administration of exogenous HA produced a dose-dependent failure in midline crossing of the first generated optic axons. In E15 slices, when the adult pattern of axon divergence develops in the chiasm, anterograde filling of the optic axons showed an obvious reduction in the uncrossed pathway after HA treatment. This reduction was confirmed by retrograde filling of the ganglion cells in E15 slices, and later in E16 pathways where the uncrossed projection is better developed. Furthermore, we have demonstrated in explant cultures of the retina that HA, when presented in soluble or substrate-bound form, does not affect outgrowth and extension of retinal neurites. These findings together indicate the crucial functions of this matrix molecule in regulating midline crossing and axon divergence, probably through interactions with guidance molecules including CD44, at the midline of the chiasm.

Localization of hyaluronan in the optic pathway of mouse embryos

CD44 has been shown to be involved in midline crossing and the generation of ipsilateral projections in the mouse optic chiasm. To determine whether these functions involve hyaluronan, the major ligand of CD44, we examined localization of hyaluronan in the mouse optic pathway. Hyaluronan was deposited mainly in vitreal regions of the retina and the optic disk. In ventral diencephalon, it was localized largely on the chiasmatic neurons that project processes to the chiasmatic midline and the optic tract. Colocalization of hyaluronan and CD44 was observed only in the midline but not lateral domains of the chiasmatic neurons, suggesting a hyaluronan/CD44-mediated mechanism that controls axon routing at the midline but not at the optic tract and the retina.

Distribution of hyaluronan in the central nervous system of the frog

The qualitative and quantitative distribution pattern of hyaluronan (HA), a component of the extracellular matrix (ECM), was studied in the frog central nervous system by using a highly specific HA probe and digital image analysis. HA reaction was observed in both the white and the gray matter, showing a very intense staining around the perikarya and dendrites in the perineuronal net (PN). In the telencephalon, strong reaction was found in different parts of the olfactory system, in the pallium, and in the amygdala. In the diencephalon, intensive staining was found in the nucleus of Bellonci, the dorsal habenula, the lateral and central thalamic nuclei, and the subependymal zone of the third ventricle. In the mesencephalon, layers of optic tectum displayed different intensities, with the strongest reaction in layers B, D, F, 3, and 5. Other structures of the mesencephalon showed regional differences. The PN was especially intensively stained around the perikarya of the toral nuclei, the oculomotor and trochlear nuclei, and the basal optic nucleus. In the rhombencephalon, the granular layer of cerebellum, the vestibulocochlear nuclei, the superior olive, the spinal tract of the trigeminal nerve, and parts of the reticular formation showed the most intense reaction in the PN. In the spinal cord, considerable HA staining was found in the white matter and around the perikarya of motoneurons. The present study is the first description of the HA-positive areas of frog brain and spinal cord demonstrating the heterogeneity of HA distribution in the frog central nervous system.

Variations in the architecture and development of the vertebrate optic chiasm

At the vertebrate optic chiasm there is major change in fibre order and, in many animals, a separation of fibres destined for different hemispheres of the brain. However, the structure of this region is not uniform among all species but rather shows marked variations both in terms of its gross architecture and the pathways taken by different fibres. There also are striking differences in the developmental mechanisms sculpting this region even between closely related animals. In spite of this, recent studies have provided strong evidence for a remarkable degree of conservation in the molecular nature of the guidance signals and regulatory genes driving chiasmatic development. Here differences and similarities in chiasmatic organisation and development between separate groups of animals will be reviewed. While it may not be possible to ascribe a single set of factors that are universal components of the vertebrate chiasm, there are both strikingly similar elements as well as diverse features to the development, organisation and architecture of this region. This review aims to highlight key issues in the organisation and development of the vertebrate optic chiasm with a focus on comparing and contrasting the data that has been gleaned to date from different vertebrate groups.

Chiasmatic neurons in the ventral diencephalon of mouse embryos--changes in arrangement and heterogeneity in surface antigen expression

We have investigated the changes in arrangement of the SSEA-1 immunoreactive chiasmatic neurons in the mouse ventral diencephalon from embryonic day (E) 9 to the end of gestation. A regionally specific staining of SSEA-1 was first detected in the ventricular layer of the caudal diencephalon at E10 and later at E11 on the cells in the subventricular layer. At E12, these cells formed the characteristic V-shaped configuration caudal to the optic axons in the chiasm. At E13-E15, this neuronal array changes gradually to a configuration that facilitates contact with the optic axons only at the midline and the initial segment of the optic tract. Colocalization studies showed that CD44 was localized strongly on the neurons in the central but not lateral domains of the array, suggesting existence of heterogeneity in these neurons in terms of surface antigen presentation. This difference between the central and lateral domains raises the possibility that the chiasmatic neurons may regulate the patterning of axon orders at the midline and the optic tract through presentation of distinct combination of guidance cues at these strategic positions in the optic pathway. Furthermore, exogenous Lewis-x/SSEA-1 inhibited neurite outgrowth from the E14 retinal explants; this inhibition was observed in neurites from both ventral temporal and dorsal nasal retina. These findings suggest an action of this surface carbohydrate on the control of axon growth and guidance in the mouse optic pathway.

Selective inhibition of ventral temporal but not dorsal nasal neurites from mouse retinal explants during contact with chondroitin sulphate

We have determined whether chondroitin sulphate (CS) glycosaminoglycans are sufficient to direct a selective inhibition of neurite growth from ventral temporal (VT) but not from dorsal nasal (DN) retina in mouse embryos; this may underlie the formation of axon divergence in the optic chiasm. Explants from the retinal region of embryonic day-14 mouse were grown on a laminin-polylysine substrate near to a circular spot coated with CS. In control cultures, in which no CS was added to the spot, both VT and DN retinal neurites grew extensively into the coated territory. When presented with spots coated with 10 mg/ml CS, neurite growth from the VT retina into the CS territory was dramatically reduced but that from the DN retina was not significantly affected. The selective inhibition to VT neurites was completely abolished by treatment with chondroitinase ABC, indicating a specific contribution of CS glycosaminoglycan in this regionally specific behaviour. This differential behaviour was not observed in explants presented with a lower or higher concentration of CS or in explants grown on substrate coated with a different laminin concentration. Thus, a critical ratio of CS to laminin seems to be essential to induce this differential behaviour in retinal neurites towards contact with CS. Furthermore, this behavior was not observed in explants cultured directly on a CS-rich substrate, suggesting that contact with growth-promoting molecules is necessary for the selective responses of retinal neurites during subsequent contact with CS. We concluded that CS glycosaminoglycan is sufficient to drive selective inhibition of VT but not DN neurites and that, together with a critical combination of growth-promoting factors, it may control the axon divergence process at the mouse optic chiasm.

Mena and vasodilator-stimulated phosphoprotein are required for multiple actin-dependent processes that shape the vertebrate nervous system

Ena/vasodilator-stimulated phosphoprotein (VASP) proteins regulate the geometry of the actin cytoskeleton, thereby influencing cell morphology and motility. Analysis of invertebrate mutants implicates Ena/VASP function in several actin-dependent processes such as axon and dendritic guidance, cell migration, and dorsal closure. In vertebrates, genetic analysis of Ena/VASP function is hindered by the broad and overlapping expression of the three highly related family members Mena (Mammalian enabled), VASP, and EVL (Ena-VASP like). Mice deficient in either Mena or VASP exhibit subtle defects in forebrain commissure formation and platelet aggregation, respectively. In this study, we investigated the consequence of deleting both Mena and VASP. Mena-/-VASP-/- double mutants die perinatally and display defects in neurulation, craniofacial structures, and the formation of several fiber tracts in the CNS and peripheral nervous system.

Magnitude of Binocular Vision Controlled by Islet-2 Repression of a Genetic Program that Specifies Laterality of Retinal Axon Pathfinding

Pathfinding of retinal ganglion cell (RGC) axons at the midline optic chiasm determines whether RGCs project to ipsilateral or contralateral brain visual centers, critical for binocular vision. Using Isl2tau-lacZ knockin mice, we show that the LIM-homeodomain transcription factor Isl2 marks only contralaterally projecting RGCs. The transcription factor Zic2 and guidance receptor EphB1, required by RGCs to project ipsilaterally, colocalize in RGCs distinct from Isl2 RGCs in the ventral-temporal crescent (VTC), the source of ipsilateral projections. Isl2 knockout mice have an increased ipsilateral projection originating from significantly more RGCs limited to the VTC. Isl2 knockouts also have increased Zic2 and EphB1 expression and significantly more Zic2 RGCs in the VTC. We conclude that Isl2 specifies RGC laterality by repressing an ipsilateral pathfinding program unique to VTC RGCs and involving Zic2 and EphB1. This genetic hierarchy controls binocular vision by regulating the magnitude and source of ipsilateral projections and reveals unique retinal domains.

Expression of phosphacan and neurocan during early development of mouse retinofugal pathway

We have investigated whether the two major brain chondroitin sulfate (CS) proteoglycans (PGs), phosphacan and neurocan, are expressed in patterns that correlate to the axon order changes in the mouse retinofugal pathway. Expression of these proteoglycans was examined by polyclonal antibodies against phosphacan and N- and C-terminal fragments of neurocan. In E13-E15 mouse embryos, when most optic axons grow in the chiasm and the optic tract, phosphacan and neurocan were observed in the inner regions of the retina. In the chiasm and the tract, phosphacan but not neurocan was expressed prominently at the midline and in the deep parts of the tract. Both proteoglycans were observed on the chiasmatic neurons, which have been shown to regulate axon divergence at the chiasmatic midline and the chronotopic fiber ordering in the tract, but phosphacan appeared to be the predominant form that persists to later developmental stages. Intense staining of both proteoglycans was also observed in a strip of glial-like elements in lateral regions of the chiasm, partitioning axons in the stalk from those in the tract. We conclude that phosphacan but not neurocan is likely the major carrier of the CS glycosaminoglycans that play crucial functions in axon divergence and age-related axon ordering in the mouse optic pathway. Furthermore, localization of these carrier proteins in the optic pathway raises a possibility that these two proteoglycans regulate axon growth and patterning not only through the sulfated sugars but also by interactions of the protein parts with guidance molecules on the optic axons.

Potential role of Pax-2 in retinal axon navigation through the chick optic nerve stalk and optic chiasm

The degree of fiber decussation at the optic chiasm differs between species, ranging from complete crossing in lower vertebrates to highly complex patterns of intermingling of the fibers from the two eyes seen in mammals and birds. Understanding the genetic control of fiber guidance through the chiasm is therefore important to unravel the developmental mechanisms within the visual system. Here we first report on early stages of chiasm formation, with pioneering axons from the left eye consistently arriving earlier than their counterparts from the right eye. This initial left-right asymmetry is transient and no functional significance is assigned to it yet. Secondly, we examined formation of the chiasm in relation with the expression of the transcription factor Pax-2 along the ventral eye cup and optic nerve stalk. Finally, in order to examine causal involvement of Pax-2 in chiasm formation, the gene was overexpressed along the neuraxis and in the eye cup at embryonic stages preceding the exit of axons from the eye, and hence arrival of axons at the chiasm. When studied with neuroanatomical tracing, Pax-2 overexpression resulted in visibly anomalous decussation of axons at the chiasm. A likely consequence of this perturbation was erroneous arrival of axons at the tectum, as observed by anterograde staining from the retina. These data suggest that balanced expression of Pax-2 results in the correct formation of the chick chiasm at early stages by imposing accurate pathfinding within the optic stalk and the midline.

Regionally specific expression of L1 and sialylated NCAM in the retinofugal pathway of mouse embryos

We have examined expression of L1 and the polysialic acid-associated form of the neural cell adhesion molecule (PSA-NCAM) in mouse embryos during the major period of axon growth in the retinofugal pathway to determine whether they are expressed in patterns that relate to the changes in axon organization in the pathway. Immunostaining for L1 and PSA-NCAM was found on all axons in the retina and the optic stalk. In the chiasm, while L1 immunoreactivity remained high on the axons, PSA-NCAM staining was obviously reduced. At the threshold of the optic tract, L1 immunoreactivity was maintained only in a subpopulation of axons, whereas PSA-NCAM staining was dramatically elevated in axons at the caudal part of the tract. Further investigations of the tract showed that both L1 and PSA-NCAM were preferentially expressed on the dorsal but not ventral optic axons, indicating a regionally specific change of both adhesion molecules on the axons at the chiasm-tract junction. Moreover, intense PSA-NCAM expression was also observed in the tract of postoptic commissure (TPOC), which lies immediately caudal to the optic tract. Immunohistochemical and retrograde tracing studies showed that these PSA-NCAM-positive axons arose from a population of cells rostral to the CD44-positive chiasmatic neurons. These findings indicate that, in addition to the chiasmatic neurons, these PSA-NCAM-positive diencephalic cells also contribute axons to the TPOC. These early generated commissural axons together with the regionally specific pattern of cell adhesion molecule expression on the optic axons may control formation of the partial retinotopic axon order in the optic tract through homophilic or heterophilic interactions that involve PSA-NCAM.

Multipolar migration: the third mode of radial neuronal migration in the developing cerebral cortex

Two distinct modes of radial neuronal migration, locomotion and somal translocation, have been reported in the developing cerebral cortex. Although these two modes of migration have been well documented, the cortical intermediate zone contains abundant multipolar cells, and they do not resemble the cells migrating by locomotion or somal translocation. Here, we report that these multipolar cells express neuronal markers and extend multiple thin processes in various directions independently of the radial glial fibers. Time-lapse analysis of living slices revealed that the multipolar cells do not have any fixed cell polarity, and that they very dynamically extend and retract multiple processes as their cell bodies slowly move. They do not usually move straight toward the pial surface during their radial migration, but instead frequently change migration direction and rate; sometimes they even remain in almost the same position, especially when they are in the subventricular zone. Occasionally, the multipolar cells jump tangentially during their radial migration. Because the migration modality of these cells clearly differs from locomotion or somal translocation, we refer to their novel type of migration as "multipolar migration." In view of the high proportion of cells exhibiting multipolar migration, this third mode of radial migration must be an important type of migration in the developing cortex.

Perturbation of CD44 function affects chiasmatic routing of retinal axons in brain slice preparations of the mouse retinofugal pathway

Neurons generated early in development of the ventral diencephalon have been shown to play a key role in defining the midline and the caudal boundary of the optic chiasm in the mouse retinofugal pathway. These functions have been attributed to a surface bound adhesion molecule, CD44 that is expressed in these chiasmatic neurons. In this study, we investigated the effects of perturbing normal CD44 functions on axon routing in brain slice preparations of the mouse retinofugal pathway. Two CD44 antibodies (Hermes-1 and IM7) were used that bind to distinct epitopes on the extracellular domain of the molecule. We found that both antibodies produced dramatic defects in routing of the retinal axons that arrive early in the chiasm. In preparations of embryonic day 13 (E13) and E14 pathways, the crossed component in the chiasm was significantly reduced after antibody treatment. However, such reduction in axon crossing was not observed in E15 chiasm, indicating that the lately generated crossed axons lost their responses to CD44. Furthermore, the anti-CD44 treatment produced a reduction in the uncrossed component in the E15 but not in younger pathways, suggesting a selective response of the lately generated axons, mostly from ventral temporal retina, but not those generated earlier, to the CD44 at the chiasmatic midline in order to make their turn for the uncrossed pathway. These findings provide evidence that a normal function of CD44 molecules in the chiasmatic neurons is essential for axon crossing and axon divergence at the mouse optic chiasm.

Enzymatic removal of chondroitin sulphates abolishes the age-related axon order in the optic tract of mouse embryos

Retinal axons undergo an age-related reorganization at the junction of the chiasm and the optic tract. We have investigated the effects of removal of chondroitin sulphate on this order change in mouse embryos aged embryonic day 14, when most axons are growing in the optic tract. Enzymatic removal of chondroitin sulphate but not keratan sulphate in brain slice preparations of the retinofugal pathway abolished the accumulation of phalloidin-positive growth cones in the subpial region of the optic tract. The loss of chronotopicity was further demonstrated by anterograde filling of single retinal axons, which showed a dispersion of growth cones from subpial to the whole depth of the tract. The enzyme treatment neither produced detectable changes in growth cone morphology and growth dynamic of retinal neurites nor affected the radial glial processes in the tract, indicating a specific effect of removal of chondroitin sulphate from the pathway to the axon order in the tract. Although chondroitin sulphate was also found at the midline of the chiasm, growth cone distribution across the depth of fibre layer at the midline was not affected by the enzyme treatment. These results suggest a mechanism in which retinal axons undergo changes in response to chondroitin sulphate at the chiasm-tract junction, but not at the midline, that produce a chronotopic fibre rearrangement in the mouse retinofugal pathway.

Normal chiasmatic routing of uncrossed projections from the ventrotemporal retina in albino Xenopus frogs

Albino mammals lacking melanin in the embryonic retinal pigment epithelium (RPE) have abnormal retinal decussation patterns at the optic chiasm: their uncrossed projections are smaller and arise from fewer, more peripheral temporal retinal ganglion cells than in con-specific wild-types. To determine whether these abnormalities generalize to nonmammalian mutants, we used anterograde and retrograde labeling methods to compare the distribution of retinal projections to the thalamus in adult normal and albino Xenopus frogs. In both pigmentation phenotypes, crossed retinal terminations covered approximately 80% of the neuropil of Bellonci (nB) and corpus geniculatum thalamicum (cgt) and uncrossed inputs occupied, respectively, approximately 75% and 25% of these two main visual centers. In the wild-type frogs and in the albinos, ganglion cells giving rise to the crossed projections were distributed throughout the retina, whereas ipsilaterally projecting cells were confined to a specific ventrotemporal retinal division. This region comprised approximately 40% of the total retinal area, was bordered by a well-defined line of decussation, and contained an average of approximately 3,000 ipsilaterally projecting ganglion cells of equivalent soma sizes in the two pigmentation phenotypes. In summary, we found no evidence of chiasmatic misrouting in the uncrossed retinothalamic projections of albino Xenopus, even though these pathways are substantial in normal frogs and share features in common with mammalian retinogeniculate projections. Our findings suggest that congenital RPE melanin deficiency results in major defects in the development of the retina and its central projections only in mammals.

Development of visual projections follows an avian/mammalian-like sequence in the lizard Ctenophorus ornatus

Development of primary visual projections was examined in a lizard Ctenophorus ornatus by anterograde and retrograde tracing with DiI and by GAP-43 immunohistochemistry. Visual pathway development was essentially similar to that in birds and mammals and thus differed from patterns in fish or amphibians. A number of features characterised the development as mammalian-like. Three phases occurred in rapid succession after laying: outgrowth (2-3 weeks, early), exuberance (4-5 weeks, intermediate), and retraction to the adult pattern (6-8 weeks, late) at about the time of hatching and eye opening. Furthermore, ipsilateral projections developed with only a slight lag relative to the contralateral ones. The dorsally located fovea could be identified from early stages. Optic axons formed transient exuberant projections to the ipsilateral optic tectum, to the opposite optic nerve, and to nonvisual regions. The pattern resembled that formed in the long term by regenerating optic axons in C. ornatus (Dunlop et al. [2000b] J. Comp. Neurol. 416:188-200), suggesting that axons recognise molecular signals associated with the initial exuberant innervation but not those associated with subsequent refinement.

Expression of Vema in the developing mouse spinal cord and optic chiasm

A critical phase of nervous system development is the formation of connections between axons and their synaptic targets. Intermediate targets play important roles in axon pathfinding by supplying growing axons with long- and short- range guidance cues at decision points along their trajectory. We recently identified Vema as a novel membrane-associated protein that is expressed at the ventral midline of the developing vertebrate central nervous system (CNS). We report that Vema is expressed in the floor plate, an intermediate target for pathfinding commissural axons located at the ventral midline of the developing mouse spinal cord. Interestingly, Vema expression overlaps with the position of an unique population of neurons situated at the midline of the ventral diencephalon and that function as intermediate targets for pathfinding retinal ganglion cell axons. The distribution of Vema in the developing spinal cord and optic chiasm resembles the expression patterns of a variety of molecules known to play important roles in axon guidance, including Robo2, Neuropilin2, and SSEA. The expression of Vema at two key choice points for pathfinding axons suggests an important role for this protein in regulating axon guidance at the midline of the developing mouse central nervous system.

Changes in expression of fibroblast growth factor receptors during development of the mouse retinofugal pathway

Retinal axons undergo several changes in organization as they pass through the region of the optic chiasm and optic tract. We used immunocytochemistry to examine the possible involvement of fibroblast growth factor receptors (FGFR) in these changes in retinal axon growth. In the retina, at all ages examined, prominent staining for FGFR was seen in the optic fiber layer and at the optic disk. At embryonic day 15 (E15), FGFR immunoreactivity was also detected in the ganglion cell layer, as defined by immunoreactivity for islet-1. At later developmental stages (E16 to postnatal day 0), FGFR were found in the optic fiber layer and the inner plexiform layer. In the ventral diencephalon, immunostaining for FGFR was first detected at E13 in a group of cells posterior to the chiasm. These cells appeared to match the neurons that are immunopositive for the stage-specific embryonic antigen-1 (SSEA-1). FGFR staining was also found on the retinal axons at E13. At E14-E16, when most axons are growing across the chiasm and the tract, a dynamic pattern of FGFR immunoreactivity was observed on the retinal axons. The staining was reduced when axons reached the midline but was increased when axons reached the threshold of the optic tract. These results suggest that axon growth and fiber patterning in distinct regions of the retinofugal pathway are in part controlled by a regulated expression of FGFR. Furthermore, the axons with elevated FGFR expression in the optic tract have a posterior border of rich FGFR expression in the lateral part of the diencephalon. This region overlaps with a lateral extension of the SSEA-1-positive cells, suggesting a possible relation of these cells to the elevated expression of FGFR.

Slit1 and Slit2 cooperate to prevent premature midline crossing of retinal axons in the mouse visual system

During development, retinal ganglion cell (RGC) axons either cross or avoid the midline at the optic chiasm. In Drosophila, the Slit protein regulates midline axon crossing through repulsion. To determine the role of Slit proteins in RGC axon guidance, we disrupted Slit1 and Slit2, two of three known mouse Slit genes. Mice defective in either gene alone exhibited few RGC axon guidance defects, but in double mutant mice a large additional chiasm developed anterior to the true chiasm, many retinal axons projected into the contralateral optic nerve, and some extended ectopically-dorsal and lateral to the chiasm. Our results indicate that Slit proteins repel retinal axons in vivo and cooperate to establish a corridor through which the axons are channeled, thereby helping define the site in the ventral diencephalon where the optic chiasm forms.

Control of retinal ganglion cell axon growth: a new role for Sonic hedgehog

Retinal ganglion cell (RGC) axons grow towards the diencephalic ventral midline during embryogenesis guided by cues whose nature is largely unknown. We provide in vitro and in vivo evidence for a novel role of Sonic hedgehog (SHH) as a negative regulator of growth cone movement. SHH suppresses both the number and the length of neurites emerging from the chick retina but not from neural tube or dorsal root ganglia explants, without interfering with their rate of proliferation and differentiation. Similarly, retroviral-mediated ectopic expression of Shh along the chick visual pathway greatly interferes the growth of RGC axons. Upon SHH addition to grown neurites, the intracellular level of cAMP decreases, suggesting that the dampening of growth cone extension mediated by SHH may involve interaction with its receptor Patched which is expressed by RGC. Based on these findings, we propose that Shh expression at the chiasm border defines a constrained pathway within the ventral midline which serves to guide the progression of RGC axons.

Heparan sulfate proteoglycan expression in the optic chiasm of mouse embryos

Previous studies have demonstrated that heparan sulfate (HS) proteoglycans (PGs) regulate neurite outgrowth through binding to a variety of cell surface molecules, extracellular matrix proteins, and growth factors. The present study investigated the possible involvement of HS-PGs in retinal axon growth by examining its expression in the retinofugal pathway of mouse embryos by using a monoclonal antibody against the HS epitope. Immunoreactive HS was first detected in all regions of the retina at embryonic day (E) 11. The staining was gradually lost in the central regions and restricted to the retinal periphery at later developmental stages (E12--E16). Prominent staining for HS was consistently found in the retinal fiber layer and at the optic disk, indicating a possible supportive role of HS-PGs in axon growth in the retina. At the ventral diencephalon, immunostaining for HS was first detected at E12, before arrival of any retinal axons. The staining matched closely the neurons that are immunopositive for the stage-specific embryonic antigen 1 (SSEA-1). At E13 to E16, when axons are actively exploring their paths across the chiasm, immunoreactivity for HS was particularly intense at the midline. This characteristic expression pattern suggests a role for HS-PGs in defining the path of early axons in the chiasm and in regulating development of axon divergence at the midline. Furthermore, HS immunoreactivity is substantially reduced at regions flanking both sides of the midline, which coincides spatially to the position of actin-rich growth cones from subpial surface to the deep regions of the optic axon layer at the chiasm. Moreover, at the threshold of the optic tract, immunoreactive HS was localized to deep parts of the fiber layer. These findings indicate that changes in age-related fiber order in the optic chiasm and optic tract of mouse embryos are possibly regulated by a spatially restricted expression of HS-PGs.

[Development of the central nervous system in mammals]

In humans, the nervous system is induced during the third gestational week by molecular signals coming from the mesoderm, which modulate the temporal and spatial expression of specific genes in the cells of the dorsal ectoderm. The induced neural plate closes to form the neural tube where the cells actively proliferate in the germinal zone. The neuroblasts which have completed their last division migrate along the fibers of the radial glia to which they adhere, and this movement is essential to establish the normal cerebral organization. The regional identity of the developing brain is governed by the expression of homeobox genes, and the main central structures are clearly delineated by the end of the fifth week. The cerebral cortex begins to form on the seventh week, and the early specification of the cortical areas, which is under genetic control, would be modulated later on by environmental influences. When the neurons have reached their final position, they extend an axon, using surface molecules or diffusible molecules present along its pathway as cues to reach the appropriate target and form a synapse, and this process is a critical step for the establishment of neuronal relationships. The maturation and stabilization of neural networks is characterized by the apoptotic death of roughly 50% of the neurons, due to insufficient neurotrophic support, and by the remodeling of the initial synaptic connections in the surviving neurons. These regressive events occur late in development and depend on both the interactions with the environment and the resulting neuronal activity.