Axonal guidance during development of the optic nerve: the role of pigmented epithelia and other extrinsic factors

Expanding the mutational spectrum of FHONDA syndrome

AIM

The aim of the study is to present a rare case of Foveal Hypoplasia, Optic Nerve Decussation defects, and Anterior segment dysgenesis (FHONDA) confirmed by genetic testing with two separate pathogenic mutations in the SLC38A8 gene.

MATERIALS AND METHODS

This was a case report.

RESULTS

A 3-month-old female presented to a neuro-ophthalmology clinic with nystagmus. Her past medical and family history was unremarkable. Her examination demonstrated horizontal pendular nystagmus and small optic nerves with foveal hypoplasia bilaterally. Neuroimaging was unremarkable. She underwent an examination under anesthesia and electroretinogram (ERG). Her anterior segment examination was normal, and dilated fundus examination demonstrated foveal hypoplasia with diffuse pigment granularity. The ERG was normal. Genetic testing revealed two mutations in the SLC38A8 gene, p.Glu233Lys:c.697 G>A (pathogenic) and p.Asp283Ala:c.848A>C (likely pathogenic) with positive parental segregation analysis. Therefore, she was diagnosed with FHONDA.

CONCLUSIONS

To our knowledge, this is the first report of a patient with FHONDA who is compound heterozygous for these two SLC38A8 mutations, which represents an expansion of the known mutational spectrum associated with this syndrome. Moreover, it may provide guidance into genetic counseling for patients and parents with these mutations.

Regulation of axon pathfinding by astroglia across genetic model organisms

Glia and neurons are intimately associated throughout bilaterian nervous systems, and were early proposed to interact for patterning circuit assembly. The investigations of circuit formation progressed from early hypotheses of intermediate guideposts and a "glia blueprint", to recent genetic and cell manipulations, and visualizations in vivo. An array of molecular factors are implicated in axon pathfinding but their number appears small relatively to circuit complexity. Comprehending this circuit complexity requires to identify unknown factors and dissect molecular topographies. Glia contribute to both aspects and certain studies provide molecular and functional insights into these contributions. Here, I survey glial roles in guiding axon navigation in vivo, emphasizing analogies, differences and open questions across major genetic models. I highlight studies pioneering the topic, and dissect recent findings that further advance our current molecular understanding. Circuits of the vertebrate forebrain, visual system and neural tube in zebrafish, mouse and chick, the Drosophila ventral cord and the C. elegans brain-like neuropil emerge as major contexts to study glial cell functions in axon navigation. I present astroglial cell types in these models, and their molecular and cellular interactions that drive axon guidance. I underline shared principles across models, conceptual or technical complications, and open questions that await investigation. Glia of the radial-astrocyte lineage, emerge as regulators of axon pathfinding, often employing common molecular factors across models. Yet this survey also highlights different involvements of glia in embryonic navigation or pioneer axon pathfinding, and unknowns in the molecular underpinnings of glial cell functions. Future cellular and molecular investigations should complete the comprehensive view of glial roles in circuit assembly.

Signaling - transcription interactions in mouse retinal ganglion cells early axon pathfinding -a literature review

Sending an axon out of the eye and into the target brain nuclei is the defining feature of retinal ganglion cells (RGCs). The literature on RGC axon pathfinding is vast, but it focuses mostly on decision making events such as midline crossing at the optic chiasm or retinotopic mapping at the target nuclei. In comparison, the exit of RGC axons out of the eye is much less explored. The first checkpoint on the RGC axons' path is the optic cup - optic stalk junction (OC-OS). OC-OS development and the exit of the RGC pioneer axons out of the eye are coordinated spatially and temporally. By the time the optic nerve head domain is specified, the optic fissure margins are in contact and the fusion process is ongoing, the first RGCs are born in its proximity and send pioneer axons in the optic stalk. RGC differentiation continues in centrifugal waves. Later born RGC axons fasciculate with the more mature axons. Growth cones at the end of the axons respond to guidance cues to adopt a centripetal direction, maintain nerve fiber layer restriction and to leave the optic cup. Although there is extensive information on OC-OS development, we still have important unanswered questions regarding its contribution to the exit of the RGC axons out of the eye. We are still to distinguish the morphogens of the OC-OS from the axon guidance molecules which are expressed in the same place at the same time. The early RGC transcription programs responsible for axon emergence and pathfinding are also unknown. This review summarizes the molecular mechanisms for early RGC axon guidance by contextualizing mouse knock-out studies on OC-OS development with the recent transcriptomic studies on developing RGCs in an attempt to contribute to the understanding of human optic nerve developmental anomalies. The published data summarized here suggests that the developing optic nerve head provides a physical channel (the closing optic fissure) as well as molecular guidance cues for the pioneer RGC axons to exit the eye.

Prospective Study of the Phenotypic and Mutational Spectrum of Ocular Albinism and Oculocutaneous Albinism

Albinism encompasses a group of hereditary disorders characterized by reduced or absent ocular pigment and variable skin and/or hair involvement, with syndromic forms such as Hermansky-Pudlak syndrome and Chédiak-Higashi syndrome. Autosomal recessive oculocutaneous albinism (OCA) is phenotypically and genetically heterogenous (associated with seven genes). X-linked ocular albinism (OA) is associated with only one gene, GPR143. We report the clinical and genetic outcomes of 44 patients, from 40 unrelated families of diverse ethnicities, with query albinism presenting to the ocular genetics service at Moorfields Eye Hospital NHS Foundation Trust between November 2017 and October 2019. Thirty-six were children (≤ 16 years) with a median age of 31 months (range 2-186), and eight adults with a median age of 33 years (range 17-39); 52.3% (n = 23) were male. Genetic testing using whole genome sequencing (WGS, n = 9) or a targeted gene panel (n = 31) gave an overall diagnostic rate of 42.5% (44.4% (4/9) with WGS and 41.9% (13/31) with panel testing). Seventeen families had confirmed mutations in TYR (n = 9), OCA2, (n = 4), HPS1 (n = 1), HPS3 (n = 1), HPS6 (n = 1), and GPR143 (n = 1). Molecular diagnosis of albinism remains challenging due to factors such as missing heritability. Differential diagnoses must include SLC38A8-associated foveal hypoplasia and syndromic forms of albinism.

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.

The cellular bases of choroid fissure formation and closure

Defects in choroid fissure (CF) formation and closure lead to coloboma, a major cause of childhood blindness. Despite genetic advances, the cellular defects underlying coloboma remain poorly elucidated due to our limited understanding of normal CF morphogenesis. We address this deficit by conducting high-resolution spatio-temporal analyses of CF formation and closure in the chick, mouse and fish. We show that a small ventral midline invagination initiates CF formation in the medial-proximal optic cup, subsequently extending it dorsally toward the lens, and proximally into the optic stalk. Unlike previously supposed, the optic disc does not form solely as a result of this invagination. Morphogenetic events that alter the shape of the proximal optic cup also direct clusters of outer layer and optic stalk cells to form dorsal optic disc. A cross-species comparison suggests that CF closure can be accomplished by breaking down basement membranes (BM) along the CF margins, and by establishing BM continuity along the dorsal and ventral surfaces of the CF. CF closure is subsequently accomplished via two distinct mechanisms: tissue fusion or the intercalation of various tissues into the inter-CF space. We identify several novel cell behaviors that underlie CF fusion, many of which involve remodeling of the retinal epithelium. In addition to BM disruption, these include NCAD downregulation along the SOX2⁺ retinal CF margin, and the protrusion or movement of partially polarized retinal cells into the inter-CF space to mediate fusion. Proximally, the inter-CF space does not fuse or narrow and is instead loosely packed with migrating SOX2⁺/PAX2⁺/Vimentin⁺ astrocytes until it is closed by the outgoing optic nerve. Taken together, our results highlight distinct proximal-distal differences in CF morphogenesis and closure and establish detailed cellular models that can be utilized for understanding the genetic bases of coloboma.

Delayed neurogenesis leads to altered specification of ventrotemporal retinal ganglion cells in albino mice

Background

Proper binocular vision depends on the routing at the optic chiasm of the correct proportion of retinal ganglion cell (RGC) axons that project to the same (ipsilateral) and opposite (contralateral) side of the brain. The ipsilateral RGC projection is reduced in mammals with albinism, a congenital disorder characterized by deficient pigmentation in the skin, hair, and eyes. Compared to the pigmented embryonic mouse retina, the albino embryonic mouse retina has fewer RGCs that express the zinc-finger transcription factor, Zic2, which is transiently expressed by RGCs fated to project ipsilaterally. Here, using Zic2 as a marker of ipsilateral RGCs, Islet2 as a marker of contralateral RGCs, and birthdating, we investigate spatiotemporal dynamics of RGC production as they relate to the phenotype of diminished ipsilateral RGC number in the albino retina.

Results

At embryonic day (E)15.5, fewer Zic2-positive (Zic2⁺) RGCs are found in the albino ventrotemporal (VT) retina compared with the pigmented VT retina, as we previously reported. However, the reduction in Zic2⁺ RGCs in the albino is not accompanied by a compensatory increase in Zic2-negative (Zic2⁻) RGCs, resulting in fewer RGCs in the VT retina at this time point. At E17.5, however, the number of RGCs in the VT region is similar in pigmented and albino retinae, implicating a shift in the timing of RGC production in the albino. Short-term birthdating assays reveal a delay in RGC production in the albino VT retina between E13 and E15. Specifically, fewer Zic2⁺ RGCs are born at E13 and more Zic2⁻ RGCs are born at E15. Consistent with an increase in the production of Zic2⁻ RGCs born at later ages, more RGCs at E17.5 express the contralateral marker, Islet2, in the albino VT retina compared with the pigmented retina.

Conclusions

A delay in neurogenesis in the albino retina is linked to the alteration of RGC subtype specification and consequently leads to the reduced ipsilateral projection that characterizes albinism.

Macrophage and microglia ontogeny in the mouse visual system can be traced by the expression of Cathepsins B and D

Here, we show a detailed chronotopographical analysis of cathepsin B and D expression during development of the mouse visual system. Both proteases were detected in large rounded/ameboid cells usually located in close relationship with prominent sites of extensive physiological cell death. In concordance with their morphological features and topographical distribution, we demonstrate that expressing cells corresponded with macrophages and microglial precursors. We found that as microglial precursors differentiated the expression of both cathepsins was down-regulated. Of interest, cathepsin B and D transcripts were never observed in degenerating cells. Our findings point to a role for cathepsin D and B in cell debris degradation after apoptotic processes rather than promoting cell death, as proposed for other developmental models. Additionally their pattern of expression suggests a role in the maturation of the microglial precursors.

Control of division and differentiation in oligodendrocyte-type-2 astrocyte progenitor cells

Oligodendrocyte-type-2 astrocyte (O-2A) progenitor cells give rise to oligodendrocytes and type-2 astrocytes in cultures of rat optic nerve. These progenitors are one of the few cell types in which most aspects of proliferation and differentiation can be manipulated in a defined in vitro environment. When exposed to platelet-derived growth factor (PDGF), O-2A progenitors divide a limited number of times before clonally related cells differentiate into oligodendrocytes with a timing similar to that seen in vivo. In contrast, O-2A progenitors grown in the absence of mitogen do not divide but differentiate prematurely into oligodendrocytes, and progenitors exposed to appropriate inducing factors differentiate into type-2 astrocytes. O-2A progenitors can become immortalized through at least two different mechanisms. First, when O-2A progenitors are exposed to a combination of PDGF and basic fibroblast growth factor (bFGF) these cells undergo continuous self-renewal in the absence of differentiation. In contrast, the application of bFGF alone is associated with premature oligodendrocytic differentiation of dividing O-2A lineage cells. Thus, cooperation between growth factors can modulate O-2A progenitor self-renewal in a defined chemical environment by eliciting a novel programme of division and differentiation which cannot be predicted from the effects of either factor examined in isolation. A further mechanism which allows prolonged self-renewal in the O-2A lineage is the generation of a stem cell. O-2A progenitors isolated from optic nerves of perinatal rats also have the capacity to give rise to a population of cells called O-2Aadult progenitors, which differ from their perinatal counterparts in many characteristics. Most importantly, O-2Aadult progenitors have a slow cell cycle, divide and differentiate asymmetrically and appear to have the capacity for prolonged self-renewal. Thus, immortalization in this lineage can also be achieved by the generation of a cell with stem cell-like characteristics from a rapidly dividing progenitor population.

Early development of the optic nerve in the turtle Mauremys leprosa

We show the distribution of the neural and non-neural elements in the early development of the optic nerve in the freshwater turtle, Mauremys leprosa, using light and electron microscopy. The first optic axons invaded the ventral periphery of the optic stalk in close relationship to the radial neuroepithelial processes. Growth cones were thus exclusively located in the ventral margin. As development progressed, growth cones were present in ventral and dorsal regions, including the dorsal periphery, where they intermingled with mature axons. However, growth cones predominated in the ventral part and axonal profiles dorsally, reflecting a dorsal to ventral gradient of maturation. The size and morphology of growth cones depended on the developmental stage and the region of the optic nerve. At early stages, most growth cones were of irregular shape, showing abundant lamellipodia. At the following stages, they tended to be larger and more complex in the ventral third than in intermediate and dorsal portions, suggesting a differential behavior of the growth cones along the ventro-dorsal axis. The arrival of optic axons at the optic stalk involved the progressive transformation of neuroepithelial cells into glial cells. Simultaneously with the fiber invasion, an important number of cells died by apoptosis in the dorsal wall of the optic nerve. These findings are discussed in relation to the results described in the developing optic nerve of other vertebrates.

The corpus callosum as an evolutionary innovation

The corpus callosum (CC) is the major interhemispheric fibre bundle in the eutherian brain and has been described as a true evolutionary innovation. This paper reviews the current literature with regard to functional, developmental and genetic concepts that may help elucidate the evolutionary origin of this structure. It has been suggested that the CC arose in the eutherian brain as a more direct and, therefore, more effective system for the interhemispheric integration of topographically organized sensory cortices than the anterior commissure (AC) and hippocampal commissure (HC) already present in nonplacental mammals. It can also be argued, however, that the ability of the CC to integrate the newly evolving motor cortices of placental mammals may have played a role in the evolutionary fixation of this structure. Investigations into the developmental mechanism involved in the formation of the CC and their underlying patterns of gene expression make it possible to formulate a tentative hypothesis about the evolutionary origin of this commissure. This paper suggests that changes in the developmental patterns of the expression of certain regulatory genes may have allowed a first group of callosal pioneering axons to cross the cortical midline. These pioneering fibres may have used the axons of the HC to find their way across the midline. Additional callosal fibres may then have fasciculated with these pioneers. Once the CC had formed in this way, more complex systems of axonal guidance may have evolved over time, thus enabling a gradual increase in the size and complexity of the CC.

Amyloid β up-regulates brain-derived neurotrophic factor production from astrocytes: Rescue from amyloid β-related neuritic degeneration

Astrocytes, the most abundant type of glia in the brain, are considered to play a key role in Alzheimer's disease (AD) pathologies. In a cell culture study, we have previously shown that astroglial responses against amyloid beta (Abeta) occur before obvious neuronal damage could be detected, suggesting the possibility that astrocytes might be an attractive therapeutic target for treating AD. In the present study, we investigated astroglial gene expression changes in response to Abeta to elucidate further the role of astrocytes in Abeta toxicity. By using real-time PCR and ELISA analyses, we found that Abeta rapidly induced astrocytes to produce brain-derived neurotrophic factor (BDNF). Abeta42 was more effective than Abeta40 in increasing astroglial BDNF production. Moreover, BDNF treatment rescued the neuronally differentiated human neuroblastoma cells from neuritic degeneration caused by Abeta toxicity. This is the first study to demonstrate that astrocytes are capable of increasing the production of a particular neurotrophic factor in response to Abeta. Our findings also identify BDNF as a potential therapeutic agent for preventing Abeta-related neuritic degeneration.

Comparative study of eye defective worm ‘menashi’ and regenerating wild-type in planarian,Dugesia ryukyuensis

Inbreeding of the sexualized planarian, Dugesia ryukyuensis, produces eye-defective worms, menashi, in the F1 population. To study the effects of this mutation on the eye, we observed the eye-region of menashi using electron microscopy and compared it with the regenerating eye in wild-type worms. The intact eye of wild-type planarians consisted of a few pigment cells and a number of visual cells. Pigment cells containing spherically-shaped electron-dense melanosomes contacted each other and enclosed rhabdomes of visual cells. Rhabdomes had numerous tubular microvilli extending radially and touching the pigment cells. However, in menashi, various lengths of tubular microvilli were irregularly distributed near the pigment cells, which contained numerous electron-lucent premelanosomes, and no adhesive structures were found between the pigment cells. The premelanosomes of menashi were equal in size to those seen after 2 days of regeneration in wild-type planarians and were similar in maturation to those found after 3 days of regeneration in wild-type planarian. These results suggest that menashi is defective in the mechanism(s) of developing pigment granules and constructing visual cells. These findings also suggest that pigment cells in menashi are defective in the mechanism(s) involved with cell adhesion.

Mechanisms of axon guidance in the developing nervous system

The human brain assembles an incredible network of over a billion neurons. Understanding how these connections form during development in order for the brain to function properly is a fundamental question in biology. Much of this wiring takes place during embryonic development. Neurons are generated in the ventricular zone, migrate out, and begin to differentiate. However, neurons are often born in locations some distance from the target cells with which they will ultimately form connections. To form connections, neurons project long axons tipped with a specialized sensing device called a growth cone. The growing axons interact directly with molecules within the environment through which they grow. In order to find their targets, axonal growth cones use guidance molecules that can either attract or repel them. Understanding what these guidance cues are, where they are expressed, and how the growth cone is able to transduce their signal in a directionally specific manner is essential to understanding how the functional brain is constructed. In this chapter, we review what is known about the mechanisms involved in axonal guidance. We discuss how the growth cone is able to sense and respond to its environment and how it is guided by pioneering cells and axons. As examples, we discuss current models for the development of the spinal cord, the cerebral cortex, and the visual and olfactory systems.

The astroglial cell that guides nerve fibers from growth cone to synapse in organotypic cultures of the fetal mouse spinal cord

We present electron microscopic and autoradiographic studies done using organotypic cultures of spinal cord explants excised from 15 days of gestation mouse embryos. Nerve fibers growing from the spinal cord explant carry at their tips immature mitotic astrocytic cells that lead their growth cones. These glial cells divide only during the active phase of neuronal growth, and correspond ultrastructurally to radial glia. They provide a specific cellular substrate for neuronal growth. Some growth cones form axoglial synapses with smooth membranes of immature glial cells. In contrast, maturing glial cells sprout cytoplasmic processes that tightly wrap individual growth cones and effectively arrest their growth. Next, the processes gather nerve endings into islets and nerve fibers into bundles. After internalizing nerve endings, the glial processes withdraw, bringing the endings into contact with each other. The direct neuronal appositions lead to the transformation of growth cones into presynaptic endings, signaled by their collection of presynaptic vesicles. Clustering of the vesicles at presynaptic axoglial or axodendritic membranes indicates the onset of synaptogenesis-completed by differentiation of spinous and compound synapses. Concomitant with the progress of synaptogenesis, astrocytic investment within the neuropil progressively diminishes. The differentiating astrocytic processes show secretory and tethering activity toward nerve fibers and their endings. Our observations demonstrate that astroglial cells-depending on their developmental stage-first promote and then arrest neuronal growth, and induce synaptogenesis. Thus, at any time, the growing nerve fibers are not only supported but also controlled by the astroglial cells.

Modifications of the falciform process in the eye of beloniformes (Teleostei: Atherinomorpha): evolution of a curtain-like septum in the eye

In order to comparatively analyze curtain-like septa in the eyes of visually orientated "close-to-surface-predators" among atherinomorph teleosts, we examined the eyes of 24 atherinomorph species under a binocular microscope with regard to the falciform process and related structures in the vitreous cavity. Additionally, falciform process samples were analyzed by transmission electron microscopy. All the studied representatives of the Cyprinodontiformes and Atheriniformes, and of one of the beloniform suborder, Adrianichthyioidei, possess a "typical" processus falciformis. In the eyes of the representatives of the other beloniform suborder, Belonoidei, however, pigmented structures that originate in the region of the optic disc and protrude into the vitreous cavity were noted. In the Hemiramphidae (halfbeaks) and Exocoetidae (flying fishes) these pigmented structures have a more cone-like shape, whereas in the Belonidae (needlefishes) and Scomberesocidae (sauries) horizontally oriented heavily pigmented curtain-like septa occur that divide the vitreous cavity dorsoventrally. It is suggested that the "typical" processus falciformis represents a plesiomorphic feature within the Atherinomorpha, whereas the pigmented modifications of the falciform process must be seen as a synapomorphic character state of the Belonoidei. The curtain-like septum of the Belonidae and Scomberesocidae might have evolved from the cone-like structures that are found in the Exocoetoidea. The functional significance of the pigmented structures in the eye is as yet not clear, except for the curtain-like septum found in Belonidae. It might play a role in visual orientation near the water surface at Snell's window.

Astrocyte apoptosis: implications for neuroprotection

Astrocytes, the most abundant glial cell types in the brain, provide metabolic and trophic support to neurons and modulate synaptic activity. Accordingly, impairment in these astrocyte functions can critically influence neuronal survival. Recent studies show that astrocyte apoptosis may contribute to pathogenesis of many acute and chronic neurodegenerative disorders, such as cerebral ischemia, Alzheimer's disease and Parkinson's disease. We found that incubation of cultured rat astrocytes in a Ca(2+)-containing medium after exposure to a Ca(2+)-free medium causes an increase in intracellular Ca(2+) concentration followed by apoptosis, and that NF-kappa B, reactive oxygen species, and enzymes such as calpain, xanthine oxidase, calcineurin and caspase-3 are involved in reperfusion-induced apoptosis. Furthermore, we demonstrated that heat shock protein, mitogen-activated protein/extracellular signal-regulated kinase, phosphatidylinositol-3 kinase and cyclic GMP phosphodiesterase are target molecules for anti-apoptotic drugs. This review summarizes (1) astrocytic functions in neuroprotection, (2) current evidence of astrocyte apoptosis in both in vitro and in vivo studies including its molecular pathways such as Ca(2+) overload, oxidative stress, NF-kappa B activation, mitochondrial dysfunction, endoplasmic reticulum stress, and protease activation, and (3) several drugs preventing astrocyte apoptosis. As a whole, this article provides new insights into the potential role of astrocytes as targets for neuroprotection. In addition, the advance in the knowledge of molecular mechanisms of astrocyte apoptosis may lead to the development of novel therapeutic strategies for neurodegenerative disorders.

Retinal ganglion cell-derived sonic hedgehog signaling is required for optic disc and stalk neuroepithelial cell development

The development of optic stalk neuroepithelial cells depends on Hedgehog (Hh) signaling, yet the source(s) of Hh protein in the optic stalk is unknown. We provide genetic evidence that sonic hedgehog (Shh) from retinal ganglion cells (RGCs) promotes the development of optic disc and stalk neuroepithelial cells. We demonstrate that RGCs express Shh soon after differentiation, and cells at the optic disc in close proximity to the Shh-expressing RGCs upregulate Hh target genes, which suggests they are responding to RGC-derived Shh signaling. Conditional ablation of Shh in RGCs caused a complete loss of optic disc astrocyte precursor cells, resulting in defective axon guidance in the retina, as well as conversion of the neuroepithelial cells in the optic stalk to pigmented cells. We further show that Shh signaling modulates the size of the Pax2(+) astrocyte precursor cell population at the optic disc in vitro. Together, these data provide a novel insight into the source of Hh that promotes neuroepithelial cell development in the mammalian optic disc and stalk.

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.

Characterization of a new brain-derived proteoglycan inhibiting retinal ganglion cell axon outgrowth

A proteoglycan was identified and isolated from physiological saline extracts of chick embryo brains by using a new monoclonal antibody (hybridoma clone mab Te38). The purified proteoglycan displayed an apparent molecular mass of 2500-3500 kDa, which became reduced to 370 and 600 kDa after digestion with chondroitinase ABC or chondroitinase AC. After additional treatment with keratanase the 600-kDa band was no longer detectable in Western blots. The specific epitope recognized by mab Te38 is an O-linked carbohydrate associated with the core protein. Tenascin-C, an extracellular matrix protein known to associate with several proteoglycans, copurified with the mab Te38 proteoglycan on the immunoaffinity column. Mab Te38 binds to the surface of nonneuronal cells; in sections from the primary visual system, expression is restricted to cells in the optic fissure, the dorsal optic nerve, and the chiasm. No retinal cells were found to express the mab Te38 epitope. The isolated molecule inhibited axon outgrowth from retinal explants when offered bound to a substrate consisting of either matrigel or collagen, chondroitinase treatment did not alter the inhibitory properties. The distribution and in vitro function of the Te38 proteoglycan indicate that it may serve a role in guidance of retinal ganglion cell axons.

Architecture of the optic chiasm and the mechanisms that sculpt its development

At the optic chiasm the two optic nerves fuse, and fibers from each eye cross the midline or turn back and remain uncrossed. Having adopted their pathways the fibers separate to form the two optic tracts. Research into the architecture and development of the chiasm has become an area of increasing interest. Many of its mature features are complex and vary between different animal types. It is probable that numerous factors sculpt its development. The separate ganglion cell classes cross the midline at different locations along the length of the chiasm, reflecting their distinct periods of production as the chiasm develops in a caudo-rostral direction. In some mammals, uncrossed axons are mixed with crossed axons in each hemi-chiasm, whereas in others they remain segregated. These configurations are the product of different developmental mechanisms. The morphology of the chiasm changes significantly during development. Neurons, glia, and the signals they produce play a role in pathway selection. In some animals fiber-fiber interactions are also critical, but only where crossed and uncrossed pathways are mixed in each hemi-chiasm. The importance of the temporal dimension in chiasm development is emphasized by the fact that in some animals uncrossed ganglion cells are generated abnormally early in relation to their retinal location. Furthermore, in albinos, where many cells do not exit the cell cycle at normal times, there are systematic chiasmatic abnormalities in ganglion cell projections.

TGF-β modulates programmed cell death in the retina of the developing chick embryo

Programmed cell death (PCD) is a key phenomenon in the regulation of cell number in multicellular organisms. We have shown that reduction of endogenous transforming growth factor β (TGF-β) prevents apoptotic PCD of neurons in the developing peripheral and central nervous system, suggesting that TGF-β is an important mediator of ontogenetic neuron death. Previous studies suggested that there are other pro-apoptotic molecules, nerve growth factor (NGF) and brain-derived neurotrophic factor, that induce cell death in the nervous system. In the developing chick retina, NGF induces PCD by activation of the p75 receptor. We have studied the role of TGF-β and its putative interdependence with NGF-mediated PCD in the chick retina. We found that TGF-β is present in the developing chick retina during the period of PCD and is essentially required to regulate PCD of retinal cells. TGF-β2, TGF-β3 and the ligand-binding TGF-β receptor can be detected immunocytochemically in the central retina, a region where apoptosis is most prominent during the early period of PCD. Application of a TGF-β-neutralizing antibody to chick embryos in ovo resulted in a decrease in the number of TUNEL-positive cells and a reduction of free nucleosome levels. In terms of magnitude, reduction of PCD caused by the neutralization of endogenous TGF-β was equivalent to that seen after anti-NGF application. Neutralization of both factors did not result in a further decrease in apoptosis, indicating that NGF and TGF-β may act on the same cell population. Furthermore, neutralization of TGF-β did not affect the expression of NGF or the p75-receptor. Our results suggest that TGF-β and NGF are both required to regulate cell death in the chick retina in vivo.

Comparison of techniques for detecting visually evoked potential asymmetry in albinism

PURPOSE

We compared techniques for analyzing visually evoked potential (VEP) asymmetry in children with albinism to find one that could be used effectively and efficiently.

METHOD

Subjects included 21 child volunteers, ages 10 months to 6 years (control group) and 21 children with albinism, ages 2 months to 6 years (albinism group). Five-channel flash VEP was performed on all subjects. Electrodes were positioned at Oz, O1, O2, O3, and O4 (10/20 system). Data were analyzed by use of techniques previously described. These included inspection of the VEP waveforms, measurement of hemispheric waveform parameters, calculation of an asymmetry index, and use of a bipolar derivation between left and right hemispheric responses (interhemispheric difference potential). In addition, we quantified the interhemispheric difference potential by use of Pearson's correlation coefficient. Measurements of sensitivity and specificity determined the success of the 5 analysis paradigms. The accuracy of each paradigm represented the ability to classify the data according to volunteer or albinism group and is derived from both sensitivity and specificity measures.

RESULTS

Measurement of hemispheric differences in VEP waveform parameters was the least sensitive measure method for detecting multichannel VEP asymmetry in albinism. Comparison of left and right eye interhemispheric difference potential increased accuracy to 67%. Nonquantitative inspection of waveform demonstrated an accuracy of 76%. The asymmetry index and Pearson's correlate measure yielded accuracy rates of 79% and 83%, respectively.

CONCLUSION

The efficiency and capability of Pearson's correlate measure in quantifying interhemispheric difference potentials to detect albinotic misrouting makes this a useful and practical technique in a pediatric clinic.

Slit2 is a repellent for retinal ganglion cell axons

We set out to isolate inhibitory guidance cues that affect retinal ganglion cell (RGC) axons in vitro and that could potentially be involved in RGC pathfinding decisions. Here we describe the biochemical purification of an RGC growth cone collapsing factor from bovine brain membranes and its identification as Slit2. Recombinant human Slit2 collapses and repels RGC growth cones from all quadrants of the chick retina. In the developing mouse visual system, slit2 is expressed in the eye, in the optic stalk, and in the ventral diencephalon. Slit2 expression is strong in anterior ventral diencephalic structures but is absent from the ventral midline where the optic chiasm forms. The putative receptors for Slits, robo1 and robo2, are expressed in the inner retinal layer in which RGCs are located. A comparison of the expression patterns of Slit2 and retinal axon trajectories suggests that slit2 acts as a short range repellent for retinal ganglion cell axons.

The course of later generated axons in the developing optic nerve of the chick embryo. A morphometric electron microscopic study

The topographic position of growth cones (GCs) shows the course of ingrowing axons within the optic nerve and allows to draw conclusions with respect to the fiber order in this pathway. Therefore, the topographic distribution and frequency of GCs as well as the proximal and distal axon shaft segments were studied within cross-sections of the distal, middle, and prechiasmatic part of the nerve of 3-8-day-old embryos using electron microscopy. The ingrowth of GCs was not confined to a particular region. Initially, GCs were found near the ventral periphery. With increasing age, simultaneous ingrowth occurred within an area that expanded dorsally. In parallel, GCs also occurred in dorsal regions and eventually in the dorsal periphery. GCs intermingled everywhere with more mature axon profiles. However, youngest profiles predominated ventrally, oldest dorsally. Hence, maturity increased from ventral to dorsal. This indicated that the time of arrival of axons and the topographic position in the cross-section correlated significantly. It is concluded that axons are chronotopically organized, but in a probabilistic sense. The predominant ingrowth of axons in the ventral part may be associated largely with the first wave of neurogenesis of retinal ganglion cells. The ingrowth in dorsal regions of the cross section may be related to later generated axons that enter the nerve following older axons of the same retinal sector as well as axons of neighboring ganglion cells which continue to leave the mitotic cycle while the front of neurogenesis has spread into the periphery.

The morphogenesis of the zebrafish eye, including a fate map of the optic vesicle

We have examined the morphogenesis of the zebrafish eye, from the flat optic vesicle at 16 hours post fertilization (hpf) to the functional hemispheric eye at 72 hpf. We have produced three-dimensional reconstructions from semithin sections, measured volumes and areas, and produced a fate map by labeling clusters of cells at 14-15 hpf and finding them in the 24 hpf eye cup. Both volume and area increased sevenfold, with different schedules. Initially (16-33 hpf), area increased but volume remained constant; later (33-72 hpf) both increased. When the volume remained constant, the presumptive pigmented epithelium (PE) shrank and the presumptive neural retina (NR) enlarged. The fate map revealed that during 14-24 hpf cells changed layers, moving from the PE into the NR, probably through involution around the margin of the eye. The transformation of the flat epithelial layers of the vesicle into their cup-shaped counterparts in the eye was also accompanied by cellular rearrangements; most cells in a cluster labeled in the vesicle remained neighbors in the eye cup, but occasionally they were separated widely. This description of normal zebrafish eye development provides explanations for some mutant phenotypes and for the effects of altered retinoic acid.

Extrinsic modulation of retinal ganglion cell projections: analysis of the albino mutation in pigmentation mosaic mice

Tyrosinase is a key enzyme involved in the synthesis of melanin in the retinal pigment epithelium (RPE). Mice that are homozygous for the albino allele at the tyrosinase locus have fewer retinal ganglion cells with uncrossed projections at the optic chiasm. To determine the site of the albino gene action we studied the projections of retinal ganglion cells in two types of pigmentation mosaic mice. First, we generated mosaic mice that contain a translocated allele of the wild-type tyrosinase on one X chromosome but that also have the lacZ reporter transgene on the opposite X chromosome. In these lacZ/tyrosinase mice, which are homozygous for the albino allele on chromosome 7, X-inactivation ensures that tyrosinase cannot be functional within 50% of the retinal ganglion cells and that these individual cells can be identified by their expression of the lacZ reporter gene product, beta-galactosidase. The proportion of uncrossed retinal ganglion cells expressing beta-galactosidase was found to be identical to the proportion that did not express it, indicating that the albino mutation associated with axonal behavior at the optic chiasm must affect ganglion cells in a cell-extrinsic manner. Second, to determine whether the RPE is the source of the extrinsic signal, we generated aggregation chimeras between pigmented and albino mice. In these mosaic mice, the extent of the uncrossed projection corresponded with the amount of pigmented cells within the RPE, but did not correspond with the genotypes of neural retinal cells. These studies demonstrate that the albino mutation acts indirectly upon retinal ganglion cells, which in turn respond by making axonal guidance errors at the optic chiasm.

Dorsal-ventral patterning defects in the eye of BF-1-deficient mice associated with a restricted loss of shh expression

Brain factor 1 (BF-1) is a winged-helix transcription factor with restricted expression in the anterior optic vesicle and in the telencephalic neuroepithelium of the neural tube. We have previously found that targeted disruption of the BF-1 gene results in hypoplasia of the cerebral hemispheres, which is more severe in structures derived from the ventral telencephalon. Here we show that the loss of BF-1 leads to multiple developmental anomalies of the eyes. The most ventral structure arising from the optic vesicle, the optic stalk, is missing and is replaced by an expanded retina. Ventral closure of the optic cup and choroid fissure does not occur. These dorsal-ventral patterning defects are not limited to the BF-1-expressing (anterior) cells, but also involve the cells of the posterior optic vesicle. Sonic hedgehog (shh) expression within the ventral telencephalic neuroepithelium is specifically lost in the BF-1(-/-) mutant. Taken together, these findings suggest that shh produced at this site plays a role in patterning the developing eye. This localized deficit in shh expression may also contribute to the prominence of the ventral defects in the telencephalon of the BF-1(-/-) mutant.

The retinal pigment epithelium as a developmental regulator of the neural retina

Melanin-related agents regulate the development of the mammalian neural retina, because in albinos there are a range of retinal deficits including abnormal connections between the eye and brain, an underdeveloped central retina and a rod deficit. These deficits may arise because gradients of retinal development in the albino are delayed and the retina is abnormally proliferative, but also goes through a subsequent period of excessive cell death. This may be caused by a reduction in ocular DOPA in albinos as this is in the synthetic pathway of melanin and is a known cell cycle regulator.

Axon order in the visual pathway of the quokka wallaby

Axon order throughout the visual pathway of the quokka wallaby (Setonix brachyurus) was determined after localised retinal applications of the tracers DiI and/or DiASP. Postnatal days (P) 22-90 were studied to encompass the development and refinement of retinal projections. Order was essentially similar at all stages. Axons entered the optic nerve head true to their sector of retinal origin. In the optic nerve, nasal and temporal axons continued to reflect their retinal origin, dominating, respectively, the medial and lateral halves. By contrast, dorsal and ventral axons exchanged locations between the retrobulbar level and one-third the distance along the nerve; thus, the inversion of the dorsoventral retinal axis, imposed by the lens, was corrected. Decussating axons maintained their relative locations through the chiasm. At the base of the optic tract, nasal and temporal axons underwent an axial rotation to lie on the medial and lateral sides, respectively; thus nasal overlapped with ventral axons and temporal with dorsal axons. Axons maintained their alignments throughout the tract, and as a result, nasal and ventral axons invaded the superior colliculus medially, whereas temporal and dorsal axons invaded laterally. Each retinal quadrant terminated preferentially in its retinotopically appropriate sector of the colliculus. The arrangement of axons in the quokka visual pathway displays several novel features. Axon order is distinct throughout, involving a well-demarcated exchange of dorsal and ventral axons in the nerve and an axial rotation of nasal and temporal axons at the base of the tract; these relocations suggest decision regions for growing axons. The organisation presumably underlies the less extensive searching within the developing superior colliculus to generate retinotopic maps in the quokka and also in tammar wallaby [Marotte, J. Comp Neurol. 293:524-539, 1990] than in the rat [Simon and O'Leary, J. Neurosci. 12:1212-1232, 1992].

Turning of retinal growth cones in a netrin-1 gradient mediated by the netrin receptor DCC

Netrin-1 promotes outgrowth of axons in vitro through the receptor Deleted in Colorectal Cancer (DCC) and elicits turning of axons within embryonic explants when presented as a point source. It is not known whether netrin-1 alone can elicit turning nor whether DCC mediates the turning response. We show that Xenopus retinal ganglion cell growth cones orient rapidly toward a pipette ejecting netrin-1, an effect blocked by antibodies to DCC. In vitro, netrin-1 induces a complex growth cone morphology reminiscent of that at the optic nerve head, a site of netrin-1 expression in vivo. These results demonstrate that netrin-1 can function alone to induce turning, implicate DCC in this response, and support the idea that netrin-1 contributes to steering axons out of the retina.

Netrin-1 and DCC mediate axon guidance locally at the optic disc: loss of function leads to optic nerve hypoplasia

Embryonic retinal ganglion cell (RGC) axons must extend toward and grow through the optic disc to exit the eye into the optic nerve. In the embryonic mouse eye, we found that immunoreactivity for the axon guidance molecule netrin-1 was specifically on neuroepithelial cells at the disk surrounding exiting RGC axons, and RGC axons express the netrin receptor, DCC (deleted in colorectal cancer). In vitro, anti-DCC antibodies reduced RGC neurite outgrowth responses to netrin-1. In netrin-1- and DCC-deficient embryos, RGC axon pathfinding to the disc was unaffected; however, axons failed to exit into the optic nerve, resulting in optic nerve hypoplasia. Thus, netrin-1 through DCC appears to guide RGC axons locally at the optic disc rather than at long range, apparently reflecting the localization of netrin-1 protein to the vicinity of netrin-1-producing cells at the optic disc.

The mouse Pax2(1Neu) mutation is identical to a human PAX2 mutation in a family with renal-coloboma syndrome and results in developmental defects of the brain, ear, eye, and kidney

We describe a new mouse frameshift mutation (Pax2(1Neu)) with a 1-bp insertion in the Pax2 gene. This mutation is identical to a previously described mutation in a human family with renal-coloboma syndrome [Sanyanusin, P., McNoe, L. A., Sullivan, M. J., Weaver, R. G. & Eccles, M. R. (1995) Hum. Mol. Genet. 4, 2183-2184]. Heterozygous mutant mice exhibit defects in the kidney, the optic nerve, and retinal layer of the eye, and in homozygous mutant embryos, development of the optic nerve, metanephric kidney, and ventral regions of the inner ear is severely affected. In addition, we observe a deletion of the cerebellum and the posterior mesencephalon in homozygous mutant embryos demonstrating that, in contrast to mutations in Pax5, which is also expressed early in the mid-hindbrain region, loss of Pax2 gene function alone results in the early loss of the mid-hindbrain region. The mid-hindbrain phenotype is similar to Wnt1 and En1 mutant phenotypes, suggesting the conservation of gene regulatory networks between vertebrates and Drosophila.

Pax2 contributes to inner ear patterning and optic nerve trajectory

During gestation, the paired box-containing gene Pax2 is expressed in the mid-hindbrain area, developing eye and inner ear. We generated Pax2 null mutant mice, which show the requirement of Pax2 for the establishment of axonal pathways along the optic stalks and ventral diencephalon. In mutant brains, the optic tracts remain totally ipsilateral due to agenesis of the optic chiasma. Furthermore, Pax2 mutants show extension of the pigmented retina into the optic stalks and failure of the optic fissure to close resulting in coloboma. In the inner ear, Pax2 mutants show agenesis of the cochlea and the spiral ganglion, i.e., the parts of the organ responsible for auditory function and in whose primordium Pax2 is expressed. Our results identify Pax2 as a major regulator of patterning during organogenesis of the eye and inner ear and indicate its function in morphogenetic events required for closure of the optic fissure and neural tube.

Transillumination of iris and subnormal visual acuity--ocular albinism?

BACKGROUND

A common clinical sign in children with subnormal visual acuity or slow visual development was iris transillumination. This was used as the inclusion criterion in a study of children shown to have a subnormal visual acuity in a general health examination at age 4 years.

METHODS

Refraction values, stereopsis, fundus photography, macular and nerve head appearance, and visual evoked response (VER) recordings were studied in 18 children. The clinical results were compared with 64 controls referred to the eye clinic because of subnormal vision from the general health examination or from school health care.

RESULTS

Eight children had VERs showing asymmetry typical for albinism. Another four had only small asymmetries on the VER, indicating a lower degree of decussation abnormality. No simple correlation of visual acuity, degree of iris transillumination, stereopsis, or macular pathology and VER asymmetries were found. However, marked iris transillumination in all four quadrants, absence of a foveal reflex, and low visual acuity were weakly correlated.

CONCLUSIONS

In a rather homogeneous group of children with iris transillumination and subnormal visual acuity eight of 18 had typical albino VERs. The findings of small atypical VER asymmetries in four children and no asymmetry in six children suggest that albinism may be considered as a description of a heterogeneous group of conditions including maximal decussation rate (100%) in the chiasma to a condition with almost normal (> or = 50%) decussation rate.

The optic tract in embryonic hamsters: fasciculation, defasciculation, and other rearrangements of retinal axons

The early development of the optic tract in hamsters was studied by labeling retinal axons with Dil applied to the eye, and then examining the labeled axons in flatmount preparations of the rostral brain stem. This technique permits a panoramic view of the entire retinal projection, from the chiasm to the caudal end of the superior colliculus. In the E11 embryo, retinal axons have reached the chiasm. They defasciculate as they emerge from the nerve, prior to reaching the ventral midline of the diencephalon, then converge again as they pass over to the opposite side. At the midline, many axonal trajectories crisscross, implying some shuffling of relative positions. Retinal axons are tightly bundled within the optic tract. Upon reaching the ventral border of the lateral geniculate body (LGB), they splay out over the nucleus, revealing a wavefront of pioneer axons individually distributed across the rostro-caudal extent of the LGB. Later-emerging retinal axons course over the surface of the thalamus in waves; subsequent waves of axons interdigitate between the lead fibers without fasciculating along them. Past the LGB, the axons undergo a second change in relative positions as the ribbon of fibers swerves caudally, prior to entering the superior colliculus. Retinal axons are tipped with growth cones of varying morphologies. No strong correlation is evident between the structural complexity of the growth cone and its position within the tract. In the majority of cases, ipsilaterally and contralaterally directed axons follow a similar developmental course along the optic tract, without any indication of a temporal lag in the ipsilateral projection as claimed in earlier reports. Understanding the changes in spatial distribution of embryonic retinal axons as they navigate along the optic tract provides a further step towards elucidating how point-to-point projections form in developing sensory systems.

Retinal axon divergence in the optic chiasm: midline cells are unaffected by the albino mutation

The visual pathway in albino animals is abnormal in that there is a smaller number of ipsilaterally projecting retinal ganglion cells. There are two possible sites of gene action that could result in such a defect. The first site is the retina where the amount of pigmentation in the retinal pigment epithelium is correlated with the degree of ipsilateral innervation (La Vail et al. (1978) J. Comp. Neurol. 182, 399–422). The second site is the optic chiasm, the site of retinal axon divergence. We investigated these two possibilities through a combination of in vivo and in vitro techniques. Our results demonstrate that the growth patterns of retinal axons and the cellular composition of the optic chiasm in albino mice are similar to those of normally pigmented mice, consistent with the albino mutation exerting its effects in the retina, and not on the cells from the chiasmatic midline. We directly tested whether the albino mutation affects the chiasm by studying ‘chimeric’ cultures of retinal explants and chiasm cells isolated from pigmented and albino mice. Crossed and uncrossed axons from pigmented or albino retinal explants display the same amount of differential growth when grown on either pigmented or albino chiasm cells, demonstrating that the albino mutation does not disrupt the signals for retinal axon divergence associated with the albino optic chiasm. Furthermore, in vitro, a greater proportion of albino retinal ganglion cells from ventrotemporal retina, origin of uncrossed axons, behave like crossed cells, suggesting that the albino mutation acts by respecifying the numbers of retinal ganglion cells that cross the chiasmatic midline.

Unilateral containment of retinal axons by tectal glia: a possible role for sulfated proteoglycans

(1) A distinct group of radial glia resides along the roofplate of the mesencephalon. Results of experiments, in which the neonatal tectum is manipulated surgically, point to the involvement of these glia in compartmentalizing retinotectal axons to one side of the midbrain. (2) Immunohistochemical studies document that the GAGs CS and KS are expressed along these midline glia during development: their expression occurs after the intertectal axons grow across the midline, but is coincident with the time of ingrowth of retinotectal axons, which fail to cross the midline. Together with results of in vitro experiments from other laboratories, these observations suggest that CS and KS are involved in the barrier function of the midline cells. (3) Preliminary data on biochemical characterization of PGs in developing tectum indicate that similar PG core proteins are found in the midline region as well as in the lateral tectum, whereas metabolic labeling shows a significantly higher uptake of radioactive sulfates along the midline. Thus differential glycosylation of proteins along the midline is likely, along with the possibility that it is the sugar chains which contribute to the barrier function of the raphe glia. Taken in the context of what we currently know about the biochemical heterogeneity of PGs, their developmental expression, and their functions in relation to the growth of axons from a variety of different neuronal cell types, it is clear that the analyses of interactions between PGs and growing axons must occur at several different levels, not the least of which involves a detailed understanding of the milieu in vivo within which these interactions take place.

Spatiotemporal pattern of retinal ganglion cell differentiation revealed by the expression of neurolin in embryonic zebrafish

The expression of neurolin, the fish homologue of the cell adhesion molecule DM-GRASP/BEN/SC-1, is dynamically regulated. Here we demonstrate that the expression of neurolin correlates with early events of retinal ganglion cell (RGC) differentiation in zebrafish embryos. Neurolin mRNA first appears [28 h postfertilization, (PF)] in nasoventral cells, representing the first RGCs, then in dorsal, central (34 to 40 h PF) and temporal RGCs. After differentiation of RGCs in the central portion of the retina, RGCs exhibiting neurolin mRNA form rings. These rings move toward the retinal periphery and encompass older (central) RGCs. Thereafter, such as at 3.5 days PF, neurolin mRNA expressing RGCs are confined to the annular growth zone at the retinal peripheral margin. Two hours after onset of mRNA expression, RGCs acquire antineurolin immunoreactivity on the surface of their somata and on their axons as they extend to the tectum. The mRNA signal in RGCs decreases significantly within 20 h after its appearance, which correlates with the arrival of axons in the tectum. This is followed by weakening of neurolin immunoreactivity on RGCs and axons. This pattern of RGC differentiation in zebrafish revealed by the expression of neurolin is unique among vertebrates. The spatiotemporal expression pattern of neurolin suggests a functional significance of this cell adhesion molecule in RGC recognition and RGC axon growth.

Glial environment in the developing superior colliculus of hamsters in relation to the timing of retinal axon ingrowth

We have examined the developmental changes of glial cell organization in the superior colliculus of embryonic and neonatal hamsters in reference to the known sequence of retinal axon ingrowth and arborization in the midbrain. Immunolocalization of vimentin, a marker for neuronal and glial cell precursors, reveals a uniform distribution of radially oriented cells, with perikarya located at the ventricular surface and thin, elongated processes fanning out toward the pia. These vimentin-positive cells, referred to as the lateral radial cells, are present in the tectum from embryonic day (E) 10 (earliest day examined) until approximately postnatal day (P) 5. Vimentin expression in the lateral radial cells decreases markedly during the second week of postnatal life: application of DiI to the ventricular surface reveals that the pial attachment of the lateral radial cells is withdrawn and that the radial processes are gradually pulled back toward the ventricular zone. By P14, virtually no vimentin-positive radial cells are detectable in the superior colliculus. At no time during development are the lateral radial cells immunopositive for the glial fibrillary acidic protein (GFAP); however, shorter, vimentin-positive astrocytic profiles can be seen in the tectum around the time the radial fibers have been withdrawn, suggesting that at least some radial cells are transformed into astrocytes that will colonize the mature colliculus. At approximately E12, a second group of cells, referred to as the midline radial glia, is detected at the tectal midline. These cells are tightly bundled, forming a raphe in the tectum. They are intensely vimentin positive from E13 until at least P14. From the time of birth, the midline radial cells also exhibit intense immunoreactivity for GFAP. The lateral radial cells are present in the superior colliculus prior to and during the period of neurogenesis but remain well past the time when collicular neuronal migration is completed. Pial processes of the lateral radial cells are present within the superficial tectal layers during the time retinal axons are entering this target; they may be involved in directing the growth and initial collateralization of retinotectal axons. Their withdrawal from retinorecipient collicular zones begins at about the time arbors are being elaborated on retinal axons. In contrast, the midline glia become distinct just prior to the time retinal axons enter the superior colliculus and persist during the time retinotectal projections are being fully established. These raphe glia may be involved in maintaining the laterality of the retinotectal projection.

Pathfinding by retinal ganglion cell axons: transplantation studies in genetically and surgically blind mice

Optic axons show a highly stereotypical intracranial course to attain the visual centers of the brainstem. Here we examine the course followed by axons arising from embryonic retinae implanted in neonatal ocular retardation mutant mice in which there had been no prior innervation of the visual centers. Retinae placed on the ventrolateral brainstem adjacent to the normal site of the optic tract send axons dorsolaterally toward the ipsilateral superior colliculus, which they innervate along with a number of other subcortical visual centers. Somewhat unexpectedly, axons also course ventrally to cross at the level of the suprachiasmatic nucleus or, less frequently, caudal to the mammillary body to follow the route of the optic tract and innervate contralateral visual centers. Retinae implanted along the course of the internal capsule emit axons that follow projection fibers through the striatum to innervate the lateral geniculate nucleus and other optic nuclei. These grafts also appear to project to the lateral part of the ventrobasal nucleus of the thalamus. The results show that prior existence of an optic projection is not necessary for axons derived from ectopic retinae to attain visual nuclei, not only on the side of implantation but also on the contralateral side of the brain. The cues that these growing axons follow appear to be stable temporally. The fact that axons can also follow highly anomalous routes, such as through the internal capsule, to attain target nuclei in the brainstem suggests that the normal optic pathway is not an obligatory route for optic outgrowth.

Abnormal ipsilateral visual field representation in areas 17 and 18 of hypopigmented cats

We compared the central projections of retinal ganglion cells in temporal retina and the cortical representation of visual fields in areas 17 and 18 in cats with various hypopigmentation phenotypes (albino, heterozygous albino, Siamese, and heterozygous Siamese). In all cats studied, we found that the extent of abnormal ipsilateral visual field representation varied widely, and more of the ipsilateral visual field was represented in area 18 than in area 17. The greatest degree of ipsilateral visual field representation was found in albino cats, followed by Siamese, heterozygous albino and heterozygote Siamese cats, respectively. Additionally, in the different groups there was wide variation in the numbers of contralaterally projecting alpha and beta ganglion cells in temporal retina. In all cases, however, contralaterally projecting alpha cells were found to extend further into temporal retina than beta cells. We found that in each cat studied, the maximum extent of the abnormal ipsilateral visual field representation in areas 18 and 17 corresponded to the location of the 50% decussation line (i.e., the point where 50% of the ganglion cells in temporal retina project to the contralateral hemisphere) for alpha and beta cells, respectively, for that cat. Our results suggest that the extent of the abnormal visual field representations in visual cortex of hypopigmented cats reflects the extent of contralaterally projecting retinal ganglion cells in temporal retina.

Precocious invasion of the optic stalk by transient retinopetal axons

This study demonstrates that the fetal optic nerve contains a conspicuous population of transient retinopetal axons. Implants of the carbocyanine dye, DiI, were made into the retina or diencephalon of fetal ferrets to label the retinopetal axons retrogradely or anterogradely, respectively, and sections were immunostained for beta-tubulin to label the early differentiating axons in the optic nerve. Dye implants into the optic nerve head, but not the retinal periphery, retrogradely labeled somata in the ventrolateral diencephalon, provided the implants were made before embryonic day (E) 30. When dye implants were made into the ventrolateral diencephalon, these same retinopetal axons were anterogradely labeled, coursing through the optic nerve but never invading the retina. The axons course as 2-5 fascicles from their cells of origin and turn laterally to enter the optic nerve where it joins the future hypothalamus. The retinopetal cells can be retrogradely labeled as early as E20, before optic axons have left the retina. The optic nerve and fiber layer are immunoreactive for beta-tubulin on E24 and thereafter, whereas on E20 and E22, they are immunonegative. Yet at these early embryonic ages, immunopositive fascicles of axons course from the diencephalon into the optic stalk, confirming the precocious nature of the retinopetal projection. Implants of dye made into the future optic nerve head at these very early stages also retrogradely label retinopetal cells in the future chiasmatic region. These cells are distributed primarily on the side ipsilateral to the midline, but a few can be found contralateral to it. Both these, as well as the retinopetal axons arising from the ventrolateral diencephalon, may serve a transient guidance function for later developing optic axons.

Glial cell types, lineages, and response to injury in rat and fish: implications for regeneration

Axons of the mammalian central nervous system do not regenerate spontaneously after axonal injury, unlike the central nervous system axons of fish and amphibians and the peripheral nervous system of mammals, which possess a good regenerative ability (Grafstein: The Retina: A Model for Cell Biology Studies, Part II, 1986; Kiernan: Biol Rev 54:155-197, 1979; Murray: J Comp Neurol 168:175-196, 1976; Ramón y Cajal: Degeneration and Regeneration of the Nervous System, 1928; Reier and Webster: J Neurocytol 3:591-618, 1974; Sperry: Physiol Zool 23:351-361, 1948). It was previously believed that intrinsic differences between the central nervous system neurons of mammals and fish account for their differences in regenerative ability. The past decade, however, has seen an accumulation of evidence, indicating that mammalian central nervous system neurons are able to regenerate injured axons, at least to some extent. This was first demonstrated by Aguayo and colleagues (David and Aguayo: Science 214:931-933, 1981; Kierstead et al: Science 246:255-257, 1989), who showed that injured mammalian central nervous system axons can grow for a considerable distance into an autograft of a peripheral nerve. It was also demonstrated that injured rabbit optic axons can regenerate into their own environment (i.e., into the distal part of the injured optic nerve), if the injured nerve is treated so as to make it conducive for growth (Lavie et al: J Comp Neurol 298:293-314, 1990; Eitan et al: Science 264:1764-1768, 1994).(ABSTRACT TRUNCATED AT 250 WORDS)