Target dependence of chick retinal ganglion cells during embryogenesis: cell survival and dendritic development

Neural activity and branching of embryonic retinal ganglion cell dendrites

The shape of a neuron's dendritic arbor is critical for its function as it determines the number of inputs the neuron can receive and how those inputs are processed. During development, a neuron initiates primary dendrites that branch to form a simple arbor. Subsequently, growth occurs by a process that combines the extension and retraction of existing dendrites, and the addition of new branches. The loss and addition of the fine terminal branches of retinal ganglion cells (RGCs) is dependent on afferent inputs from its synaptic partners, the amacrine and bipolar cells. It is unknown, however, whether neural activity regulates the initiation of primary dendrites and their initial branching. To investigate this, Xenopus laevis RGCs developing in vivo were made to express either a delayed rectifier type voltage-gated potassium (KV) channel, Xenopus Kv1.1, or a human inward rectifying channel, Kir2.1, shown previously to modulate the electrical activity of Xenopus spinal cord neurons. Misexpression of either potassium channel increased the number of branch points and the total length of all the branches. As a result, the total dendritic arbor was bigger than for control green fluorescent protein-expressing RGCs and those ectopically expressing a highly related mutant non-functional Kv1.1 channel. Our data indicate that membrane excitability regulates the earliest differentiation of RGC dendritic arbors.

Neurotrophic factors and the regeneration of adult retinal ganglion cell axons

The adult central nervous system (CNS) has only a limited capacity to regenerate axons after injury. This is due to a number of factors including the presence of extrinsic inhibitory factors that limit plasticity, lack of effective trophic support, and intrinsic changes in neuronal responsiveness. In this review, we describe the expression and role of neurotrophins in retinal ganglion cells (RGCs) during development and adulthood, and the receptors and miscellaneous signaling systems that influence axonal regeneration after injury. The impact of exogenous neurotrophic factors on adult RGCs injured at different sites in the visual pathway is described for several modes of delivery, including recombinant factors, viral vectors, cell transplantation, as well as combinatorial treatments involving other pharmacotherapeutic agents. Indirect, off-target effects of neurotrophic factors on RGC axonal regeneration are also considered. There remain unresolved issues relating to optimal delivery of neurotrophic factors, and we emphasize the need to develop safe, reliable methods for the regulation of exogenous supply of these factors to the injured CNS.

Control of programmed cell death by neurotransmitters and neuropeptides in the developing mammalian retina

It has long been known that a barrage of signals from neighboring and connecting cells, as well as components of the extracellular matrix, control cell survival. Given the extensive repertoire of retinal neurotransmitters, neuromodulators and neurotrophic factors, and the exhuberant interconnectivity of retinal interneurons, it is likely that various classes of released neuroactive substances may be involved in the control of sensitivity to retinal cell death. The aim of this article is to review evidence that neurotransmitters and neuropeptides control the sensitivity to programmed cell death in the developing retina. Whereas the best understood mechanism of execution of cell death is that of caspase-mediated apoptosis, current evidence shows that not only there are many parallel pathways to apoptotic cell death, but non-apoptotic programs of execution of cell death are also available, and may be triggered either in isolation or combined with apoptosis. The experimental data show that many upstream signaling pathways can modulate cell death, including those dependent on the second messengers cAMP-PKA, calcium and nitric oxide. Evidence for anterograde neurotrophic control is provided by a variety of models of the central nervous system, and the data reviewed here indicate that an early function of certain neurotransmitters, such as glutamate and dopamine, as well as neuropeptides such as pituitary adenylyl cyclase-activating polypeptide and vasoactive intestinal peptide is the trophic support of cell populations in the developing retina. This may have implications both regarding the mechanisms of retinal organogenesis, as well as pathological conditions leading to retinal dystrophies and to dysfunctional cellular behavior.

Glial and neuronal regulation of the lipid carrier R-FABP

Neuroembryogenesis critically depends on signaling molecules that modulate cell proliferation, differentiation, and the formation of neural networks. In an attempt to identify potential morphogenetic active components that are distributed in a graded fashion in the developing nervous system, we generated substraction libraries of the embryonic nasal and temporal chick retina. Selected clones were analyzed by sequencing, Northern and Western blotting, in situ hybridization, and immunocytochemistry. Retinal fatty acid-binding protein (R-FABP) mRNA displayed the most pronounced topographic gradient. R-FABP was most strongly expressed in nasal retina, though topographic differences were not evident on the protein level. R-FABP expression was subject to a pronounced spatio-temporal regulation. Peak expression was at the period of cell generation/migration and differentiation. To identify the cell types involved in R-FAPB synthesis, ganglion cells as the only retinal projection neurons were enriched by enzymatic delayering. Cell somata, axons, and growth cones were R-FABP immunoreactive. Most interestingly, R-FABP immunoreactivity was critically dependent on the growth substratum. It was abrogated when axons grew on isolated glial endfeet. Radial glia purified by complement-mediated cytolysis also expressed R-FABP at moderate levels. The expression level was significantly increased during mitosis and dropped down again in postmitotic cells. Further on, transient loss of cell-cell and substratum contact induced a subcellular redistribution of R-FABP. In conjunction with the morphogen-binding activity of other FABP family members and their impact on cell migration and tissue differentiation, R-FABP characteristics suggest a regulatory function during retinal histogenesis but not during topographic map formation.

Dendrite development and target innervation of displaced retinal ganglion cells of the chick (Gallus gallus)

The avian accessory optic system (AOS) processes visual signals of translational and rotational flowfields resulting from self-motion. It has been investigated extensively with physiological methods and, because of its anatomical distinction from other retinofugal projections, is well suited for the investigation of dendritic differentiation and axonal pathfinding. Displaced retinal ganglion cells (dRGC) constitute the retinal origin of the AOS. Since little is known about the time course of the development of this projection, we studied the dendritic differentiation of dRGC, their innervation of the nucleus of the basal optic root (nBOR) and the histological development of this target area. dRGC, visualized by retrograde 1,1'-dioctadecyl-3,3,3',3'-tetramethyl-indocarbocyanine perchlorate labeling, migrated into the inner nuclear layer of the retina and subsequently developed their characteristic dendritic morphology between E9 and E14. At this stage, dendrites were unistratified in the inner plexiform layer and displayed characteristic branches with 45-90 degrees angles. The frequency of dendritic branches increased from an average of 44 branches per cell at E9 to an average of 155 at E15. This phase was followed by a period of dendritic pruning, E15-E17, where a large number of small branches were eliminated. At the time of hatching, dRGC were morphologically mature with mean dendritic field sizes of 0.28 mm2 and an average of 108 dendritic branches per cell. Retinal innervation of the nBOR occurred between E8 and E11, and tracer injections at later stages revealed no further changes. In addition to the predominant contralateral projection, we have also described a connection to the ipsilateral nBOR. This ipsilateral pathway persisted at least to juvenile stages (P14). The histological development of the nBOR proceeded such that calretinin-immunoreactive neurons were observed from E10 onwards and morphologically described cell types evolved after E12.

Development of the visual system of the chick. II. Mechanisms of axonal guidance

The quest to understand axonal guidance mechanisms requires exact and multidisciplinary analyses of axon navigation. This review is the second part of an attempt to synthesise experimental data with theoretical models of the development of the topographic connection of the chick retina with the tectum. The first part included classic ideas from developmental biology and recent achievements on the molecular level in understanding cytodifferentiation and histogenesis [J. Mey, S. Thanos, Development of the visual system of the chick. (I) Cell differentiation and histogenesis, Brain Res. Rev. 32 (2000) 343-379]. The present part deals with the question of how millions of fibres exit from the eye, traverse over several millimetres and spread over the optic tectum to assemble a topographic map, whose precision accounts for the sensory performance of the visual system. The following topics gained special attention in this review. (i) A remarkable conceptual continuity between classic embryology and recent molecular biology has revealed that positional cellular specification precedes and determines the formation of the retinotectal map. (ii) Graded expression of asymmetric genes, transcriptional factors and receptors for signal transduction during early development seem to play a crucial role in determining the spatial identity of neurons within surface areas of retina and optic tectum. (iii) The chemoaffinity hypothesis constitutes the conceptual framework for development of the retinotopic organisation of the primary visual pathway. Studies of repulsive factors in vitro developed the original hypothesis from a theoretical postulate of chemoattraction to an empirically supported concept based on chemorepulsion. (iv) The independent but synchronous development of retina and optic tectum in topo-chronologically corresponding patterns ensures that ingrowing retinal axons encounter receptive target tissue at appropriate locations, and at the time when connections are due to be formed. (v) The growth cones of the retino-fugal axons seem to be guided both by local cues on glial endfeet and within the extracellular matrix. On the molecular level, the ephrins and their receptors have emerged as the most likely candidates for the material substrate of a topographic projection along the anterior-posterior axis of the optic tectum. Yet, since a number of alternative molecules have been proposed for the same function, it remains the challenge for the near future to define the proportional contribution of each one of the individual mechanisms proposed by matching theoretical predictions with the experimental evidence.

Group I metabotropic glutamate receptors are expressed in the chicken retina and by cultured retinal amacrine cells

Glutamate is well established as an excitatory neurotransmitter in the vertebrate retina. Its role as a modulator of retinal function, however, is poorly understood. We used immunocytochemistry and calcium imaging techniques to investigate whether metabotropic glutamate receptors are expressed in the chicken retina and by identified GABAergic amacrine cells in culture. Antibody labeling for both metabotropic glutamate receptors 1 and 5 in the retina was consistent with their expression by amacrine cells as well as by other retinal cell types. In double-labeling experiments, most metabotropic glutamate receptor 1-positive cell bodies in the inner nuclear layer also label with anti-GABA antibodies. GABAergic amacrine cells in culture were also labeled by metabotropic glutamate receptor 1 and 5 antibodies. Metabotropic glutamate receptor agonists elicited Ca(2+) elevations in cultured amacrine cells, indicating that these receptors were functionally expressed. Cytosolic Ca(2+) elevations were enhanced by metabotropic glutamate receptor 1-selective antagonists, suggesting that metabotropic glutamate receptor 1 activity might normally inhibit the Ca(2+) signaling activity of metabotropic glutamate receptor 5. These results demonstrate expression of group I metabotropic glutamate receptors in the avian retina and suggest that glutamate released from bipolar cells onto amacrine cells might act to modulate the function of these cells.

Development of the visual system of the chick. I. Cell differentiation and histogenesis

This review summarizes present knowledge on the embryonic development of the avian visual projections, based on the domestic chick as a model system. The reductionist goal to understand formation and function of complex neuroanatomical systems on a causal level requires a synthesis of classic developmental biology with recent advances on the molecular mechanisms of cell differentiation and histogenesis. It is the purpose of this article. We are discussing the processes underlying patterning of the anterior neural tube, when the retina and optic tectum are specified and their axial polarity is determined. Then the development of these structures is described from the molecular to the anatomical level. Following sections deal with the establishment of secondary visual connections, and the developmental interactions between compartments of the retinotectal system. Using this latter pathway, from the retina to the optic tectum, many investigations aimed at mechanisms of axonal pathfinding and connectivity have accumulated a vast body of research, which will be covered by a following review.

Regulatory cell interactions between retinal ganglion cells and radial glia during axonal and dendritic outgrowth

Neuronal differentiation and the formation of cell polarity are crucial events during the development of the nervous system. Cell polarity is a prerequisite for directed information flux within neuronal networks. In this article, we focus on neuro-glial cell interactions that influence the establishment of neural cell polarity and the directed outgrowth of axons versus dendrites. The cellular model discussed in detail is the retinal ganglion cell (RGC) of the chick retina, which is investigated by a comprehensive set of in vitro assays. The experiments demonstrate that retinal microenvironment determines axon vs. dendrite formation of RGCs. The instructive differences in different retinal microenvironments are substantially influenced by radial glia. Different glial domains support or inhibit axon vs. dendrite outgrowth. The data support the notion that neuro-glial interactions are crucial for directed neurite outgrowth.

Brain-derived neurotrophic factor differentially regulates retinal ganglion cell dendritic and axonal arborization in vivo

Expression of the neurotrophin brain-derived neurotrophic factor (BDNF) and its receptor trkB in the ganglion cell layer of the Xenopus retina during retinal ganglion cell (RGC) dendritic arborization indicates that BDNF is spatially and temporally available to influence RGC morphological differentiation (; ). BDNF promotes RGC axon arborization in vivo by acting as a target-derived trophic factor (). To determine whether BDNF also acts locally to regulate RGC dendritic development in vivo, we altered retinal neurotrophin levels at the onset of dendritic arborization and assessed the resulting arbor morphologies of RGCs retrogradely labeled with fluorescent dextrans. Injecting neurotrophins or BDNF function-blocking antibodies coupled to microspheres provided local alterations of retinal neurotrophin levels. BDNF significantly decreased RGC dendritic arbor complexity, whereas neutralizing endogenous BDNF levels with function-blocking antibodies significantly increased dendritic arbor complexity. RGCs exposed to other neurotrophins, as well as RGCs in retinae treated with BDNF but in areas not directly exposed to the neurotrophin, developed dendritic arbors that were indistinguishable from controls, indicating that exogenous BDNF acts specifically and locally. In the tectum, where RGC axons arborize, BDNF had opposite effects. BDNF significantly increased RGC axon arbor complexity and anti-BDNF reduced RGC arborization. Thus, BDNF reduces RGC dendritic arborization within the retina and increases axon arborization in the tectum. These results indicate that BDNF can differentially modulate axonal and dendritic arborization within a single neuronal population in opposing manners and raise the possibility that differential modulation by a neurotrophic factor finely tunes the morphological differentiation program of a neuron.

Genesis, neurotrophin responsiveness, and apoptosis of a pronounced direct connection between the two eyes of the chick embryo: a natural error or a meaningful developmental event?

Unilateral intraocular injections of either of two fluorescent carbocyanine dyes into the embryonic chick eye resulted in both retrograde staining of ganglion cells (GCs) in the eye contralateral to site of injection and anterograde labeling of axons whose cell bodies were located within the injected eye. This prominent retino-retinal projection formed by thousands of GCs having a nasal origin and temporal termination appeared at embryonic day 6 (E6), attained its maximum intensity at E13-E14, and gradually disappeared until E18. The axonal growth cones ended superficially and never penetrated deeper layers of the retina. Treatment of the projection with BDNF resulted in massive terminal branching of the axons within deeper layers of the target retina. Double injection into the eye and the isthmo-optic nucleus showed a concomitant ingrowth of axons in the contralateral retina. Individual GCs died between E9 and E13, but massive apoptotic cell death was mainly monitored at E14 and later. Disintegrated cells showed typical images of apoptosis. Because degenerating cells were prelabeled with the membranophilic fluorescent carbocyanine dye, their death allowed the concomitant visualization of phagocytosing cells, too. Radial Müller glia were the only class of cells observed to become phagocytotic between E9 and E16. These cells became replaced exclusively with microglial cells from E17 on. The results suggest that the topologically restricted retino-retinal projection may have some developmental significance rather than representing a massive erroneous projection. Most likely, the projection may serve as a "template" to guide centrifugal isthmo-optic axons into the retina.

Connexin alpha1 and cell proliferation in the developing chick retina

During the formation of the eye, high levels of connexin alpha1 (connexin 43) are expressed within the tissues of the cornea, lens, and neural retina. In order to determine whether connexin alpha1 plays a role in the regulation of cell proliferation we have used a novel antisense technique to reduce its expression early in development (embryonic days 2-4). Application of Pluronic gel, containing antisense oligodeoxynucleotides (ODNs) to connexin alpha1, to one eye of early chick embryos results in a rapid and significant reduction of alpha1 protein which lasts for 24-48 h. Embryos grown for 48 h, after ODN application to one eye, showed a marked reduction in the diameter of the treated, compared to that of the contralateral untreated, eye. Sections cut from the treated eyes showed that the retina was also reduced in size. TUNEL labeling and staining with propidium iodide showed that apoptosis within the retinae of both treated and untreated eyes was rare and thus that the reduction in the area of the retina brought about by antisense ODNs directed at connexin alpha1 was unlikely to be the result of increased cell death. However, the number of mitotic figures in the ventricular zone of the antisense-treated retinae revealed by propidium iodide staining was significantly reduced (P < 0.0001) to 53 +/- 3.5% (n = 5) of that in the contralateral untreated control eyes. Embryos in which one eye was sham operated, treated with pluronic gel, or treated with sense ODN showed no significant changes in eye size or in the number of mitotic figures within the neural retina. These results point to a role for connexin alpha1-mediated gap-junctional communication in controlling the early wave of neurogenesis in the chick retina.

A biophysical model for the developmental time course of retinal orientation selectivity

A quantitative study of the time course of development of the percentage of orientationally selective and isotropic ganglion cells in turtle retina has recently been performed. This study revealed that as soon as ganglion cells start responding to light, a large percentage of them are selective to the orientations of moving visual stimuli. This percentage decreases with age to reach a minimum around hatching, increases dramatically after birth and finally, decreases again following the first month of life to reach adult level. Concomitantly, the percentage of cells responding isotropically to the orientation of elongated stimuli increases monotonically until about 30 days after birth, stabilizing afterwards. To account for both time courses, we propose a biophysical model implementing features ubiquitous to developing vertebrate retinas. These features include early dendritic and synaptic spatial polarization, dendritic growth, and waves of activity generated spontaneously or by visual stimulation sweeping across the inner plexiform layer (IPL). The model also assumes a physiologically plausible Hebbian rule, which includes long-term potentiation and depression. Computer simulations of this model yield good fits of the data. The quality of these fits confirms and extends results from an earlier model using computationally-simple mechanisms, which suggested that early dendritic polarization might be the seed for mature orientation selectivity.

Regeneration and transplantation of the optic nerve: developing a clinical strategy

Three separate experimental models of optic nerve regeneration have been presented--along the existing pathway in the presence of antibodies to neutralise inhibitory molecules, along peripheral nerve grafts and from retinal transplants. Each offers a theoretical clinical strategy for restoration of vision, if the mechanism of re-establishment of maps and reconnection to appropriate targets during regeneration can be determined. This is the process of axon guidance, and underlines the importance of our research into the molecular determinants that guide normal development of the visual system.

Contributions of the optic tectum and the retina as sources of brain-derived neurotrophic factor for retinal ganglion cells in the chick embryo

Retinal ganglion cells (RGC) are supported by brain-derived neurotrophic factor (BDNF), but it is not known if BDNF acts as a target-derived factor or as an afferent or autocrine trophic factor. Here we demonstrate that BDNF mRNA is expressed in the retinorecipient layer of the chick optic tectum as well as in the inner nuclear layer and ganglion cell layer of the retina. Amacrine cells rather than RGC were the main source of BDNF mRNA in the ganglion cell layer, as determined by in situ hybridization that was combined with retrograde labeling of RGC and destruction of RGC by optic stalk transection, followed by quantitative RT-PCR. Cells in the ganglion cell layer as well as the retinorecipient layers of the optic tectum were BDNF-immunolabeled. After injections into the tectum, radio-iodinated BDNF was transported to the retina where autoradiographic label accumulated in the inner plexiform and ganglion cell layers. After intraocular injection, iodinated BDNF accumulated in these same retinal layers and correlated with the distribution of p75 neurotrophin receptor protein. The majority of cross-linked receptor-bound BDNF in the retina immunoprecipitated with p75 antibodies. No difference in the intensity of BDNF immunolabel was observed in the experimental retina or tectum after optic stalk transection, indicating that most of the BDNF in the RGC was not derived from the optic tectum. These data indicate that a substantial fraction of the BDNF in the ganglion cell layer is derived from local sources, afferents within the retina, rather than from the optic tectum via retrograde transport.

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

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

Axonal transport blockade in the neonatal rat optic nerve induces limited retinal ganglion cell death

Optic nerve section in the newborn rat results in a rapid apoptotic degeneration of most axotomized retinal ganglion cells (RGCs). This massive process of neuronal death has been ascribed mainly to the interruption of a trophic factor supply from target structures rather than to the axonal damage per se. To distinguish between these two possibilities, we induced a reversible axonal transport blockade in the developing optic nerve by topical application of a local anesthetic (lidocaine). Light and electron microscopy showed no alterations in the fine structure of treated optic nerves. Retinae of treated and control rats were stained with cresyl violet and examined at different times after surgery. We found that axonal transport blockade induced only a limited number of pyknotic RGCs. Degeneration of these cells was completely prevented by inhibiting protein synthesis during lidocaine application. We conclude that the rapid degeneration of RGCs after axotomy can be ascribed only partly to the loss of retrogradely transported trophic factors.

Regeneration in the developing optic nerve: correlating observations in the opossum to other mammalian systems

Regeneration of severed axons within the central nervous system of adult mammals does not normally occur with any degree of success. During development, however, newly forming projections must send axons to distant sites and form appropriate connections with their targets: successful regeneration has been observed during this critical period. The opossum central nervous system develops during early postnatal life and has provided a useful experimental model to investigate this specialized mode of axonal regeneration in mammals. The presence of a clear decision point at the optic chiasm has also provided a useful site at which to investigate the navigational capacity of retinal ganglion cells regenerating along the optic nerve during this critical period. Regeneration failure occurs as the central nervous system progresses from this permissive, developing state to a mature, non-permissive adult state. Studies into the behaviour of glial and neuronal elements around this transition period can help elucidate some of the factors that need to be overcome if regeneration is ever to become successful in adult mammals. The regeneration characteristics of a lesioned projection are dependent upon its developmental stage and are also related to the proximity of axotomy along its pathway. A system of staging is proposed to correlate observations in the opossum optic nerve to other mammalian systems.

Possible roles of spontaneous waves and dendritic growth for retinal receptive field development

Several models of cortical development postulate that a Hebbian process fed by spontaneous activity amplifies orientation biases occurring randomly in early wiring, to form orientation selectivity. These models are not applicable to the development of retinal orientation selectivity, since they neglect the polarization of the retina's poorly branched early dendritic trees and the wavelike organization of the retina's early noise. There is now evidence that dendritic polarization and spontaneous waves are key in the development of retinal receptive fields. When models of cortical development are modified to take these factors into account, one obtains a model of retinal development in which early dendritic polarization is the seed of orientation selectivity, while the spatial extent of spontaneous waves controls the spatial profile of receptive fields and their tendency to be isotropic.

Developmentally regulated release of intraretinal neurotrophic factors in vitro

The effects of conditioned media either from aggregates or from explants of embryonic chick retinae and of recombinant neurotrophins were tested upon the survival in vitro of ganglion cells in dissociated cell cultures from the retina of newborn rats. Ganglion cells were identified by the detection of retrogradely transported horseradish peroxidase injected bilaterally into the superior colliculus. Conditioned media increased significantly the survival of ganglion cells after 2 days in culture, at a wide range of plating densities, and had no effect upon adhesion of rat retinal cells. Media conditioned by cell ensembles from chick retinae from embryonic day 8 (E8) to E16 had neurotrophic effects. Release of neurotrophic activity peaked at E10 E12, irrespective of the numbers of cells or total concentration of protein in the conditioned media. The active molecules were non-dialyzable and were released either in the presence or in the absence of fetal calf serum. The neurotrophic activity was abolished by trypsinization, and recovered by salting-out with 25 75% ammonium sulfate. NT-4, BDNF and, to a lesser extent, NT-3, increased the survival of ganglion cells in our assay, while NGF had no effect. The data show that chick retinal cells release soluble trophic proteins according to a developmentally regulated pattern. These neurotrophic factors may be involved in local competitive interactions that help control naturally occurring neuron death among ganglion cells of the vertebrate retina.

Transient retinal ganglion cells in the developing rat are characterized by specific morphological properties

To determine whether dendritic development of mammalian retinal ganglion cells (RGCs) is affected by axonal target specificity, the morphology of three populations of maturing RGCs was examined. These included RGCs that exhibited either a transient, topographically incorrect, projection to the caudal superior colliculus (SC), or a transient projection to the caudal inferior colliculus (IC), in addition to a control group that exhibited a topographically correct projection to the caudal SC. Projection populations were identified by retrograde transport of rhodamine labeled latex microspheres injected into target nuclei. Labeled RGCs were then injected in vitro with Lucifer yellow to reveal the details of their dendritic morphology. Retinal ganglion cells making target errors, most of which ultimately die, were found to undergo a remarkable degree of morphological differentiation and could be categorized according to the adult type I, II, or III criteria. However, the relative proportions of these cell types were different among RGCs making transient connections versus those whose projections were preserved. Approximately half of the RGCs making topographically incorrect projections to the SC belonged to type III, in contrast to 6% that made a topographically correct projection. In addition, the population of cells sending axons to caudal IC did not include type III RGCs, but consisted of small type II neurons. The development of the basic dendritic form of each RGC type was only modestly influenced by its projection pattern; dendritic trees of cells making transient projections were essentially normal with only a slight, but statistically significant, reduction in dimensions. Moreover, dendritic remodeling was evident during maturation of neurons making either transient or normal projections. Together, these findings indicate that target specificity plays a relatively minor role on dendritic development of retinal ganglion cells.

Target-independent diversification and target-specific projection of chemically defined retinal ganglion cell subsets

In diverse vertebrate species, defined subsets of retinal ganglion cells (RGCs, the neurons that project from retina to brain) are distinguishable on the basis of their dendritic morphology, physiological properties, neurotransmitter content and synaptic targets. Little is known about when this diversity arises, whether diversification requires target-derived signals, and how subtype-specific projection patterns are established. Here, we have used markers for two chemically defined RGC subsets in chick retina to address these issues. Antibodies to substance P (SP) and the nicotine acetylcholine receptor (AChR) beta 2 subunit label two small (&lt; 10%), mutually exclusive groups of RGCs in mature retina. SP and AChRs accumulate in distinct RGCs before retinotectal synapses have formed. Moreover, both populations of RGCs form in retinae that develop following tectal ablation or transplantation to the coelomic cavity. Thus, RGC subsets acquire distinct neurotransmitter phenotypes in the absence of extraretinal cues. In the mature optic tectum, SP- and AChR-positive RGC axonal arbors are confined to distinct retinorecipient (synaptic) laminae. In the developing tectum, SP- and AChR-positive axons are initially intermingled in a superficial fiber layer, but then enter and arborize in appropriate laminae soon after those laminae form. Importantly, SP-positive axons, which synapse in a superficial lamina, never extend into the deeper, AChR-positive lamina. Tectal interneurons rich in SP receptors are concentrated in the lamina to which SP-positive RGC axons project, and a set of cholinergic (choline acetyltransferase-positive) tectal projection neurons elaborate dendrites in the lamina to which AChR-positive RGC axons project. These populations of tectal neurons, which are likely targets of the RGC subsets, form in tecta that develop following enucleation. Thus, RGCs and their targets can diversify in each others absence. Accordingly, we propose that the lamina-selective connectivity we observe reflects the presence of complementary cues on RGC subsets and their laminar targets.

Modifications of motoneuron development following transplantation of thoracic spinal cord to the lumbar region in the chick embryo: evidence for target-derived signals that regulate differentiation

In order to examine the role of target cells in the development of spinal motoneurons, the neural tube from thoracic segments was transplanted to the lumbar region on embryonic day (E) 2, and allowed to innervate hindlimb muscles in the chick embryo. When examined at later stages of development, the proportion of white and gray matter in the thoracic transplant was altered to resemble normal lumbar cord. Many thoracic motoneurons were able to survive up to posthatching stages following transplantation. The branching and arborization of dendrites of thoracic motoneurons innervating hindlimb muscles, as well as motoneuron (soma) size, were also increased to an extent approximating that seen in normal lumbar motoneurons. In support of previous studies using a similar transplant model, we have also found that the peripheral (intramuscular) branching pattern of thoracic motoneuron axons innervating hindlimb muscles was similar to that of normal lumbar motoneurons. Axon size and the degree of myelination of transplanted thoracic motoneuron axons were also increased so that these parameters more closely resembled axons of normal lumbar than normal thoracic spinal motoneurons. Virtually all of the changes in motoneuron properties noted above were observed irrespective of whether or not the transplanted spinal cord had developed in anatomical continuity with the host rostral cord. Accordingly, it is unlikely that the changes in the development of transplanted thoracic motoneurons reported here are induced either entirely, or in part, by signals derived from the host central nervous system. Rather, these changes appear to be mediated by interactions between the transplanted motoneurons and the hindlimb. We favor the notion that retrograde trophic signals derived from the hindlimb act to modulate the development of innervating motoneurons. Whether this signal involves a diffusible trophic agent released from target cells, or acts by some other mechanism is presently unknown.

Ganglion cells in the juvenile chick retina and their ability to regenerate axons in vitro

Ganglion cells in the chicken retina fail to regenerate their axons upon mechanical injury. In order to determine whether this failure to regenerate axons is intrinsic to the neurons or is mediated by the environment, we asked whether ganglion cells possess an ability to regrow their injured axons in the absence of their natural environment, namely in vitro. Since the retina contains morphologically different types of ganglion cells, it became desirable to investigate whether all types of ganglion cells contribute to regeneration of axons. Ganglion cells were labelled post-mortem with the fluorescent dye DiI and described morphologically. Morphometric parameters like the sizes of their perikarya, their dendrites, and the patterns of dendritic ramification and stratification were considered for grouping cells. Although a strong classification of the cells could not be achieved because of the high diversity among this population of neurons, the chick retinal ganglion cells could be separated into seven regular groups which have their somata located within the ganglion cell layer and into one group with the somata located within the inner nuclear layer (displaced ganglion cells). The experimental procedure for regeneration combines crush injury of the chick optic nerve in situ with explanation of retinal pieces 1 week later for organ cultures in a serum-free medium. Under these conditions, the ganglion cells extended axons 1 day after explanation on polylysine/laminin. The densities of ganglion cells contributing to the axonal regrowth reached up to 1447 cells mm-2 (mean 1028 cells mm-2; S.D. 237). This density corresponds to 13% of the ganglion cell density in the normal retina, averaged across the total retina area. Although the dendrites of some cells whose axons had regenerated were altered in comparison with the normal ganglion cells, all morphological types of ganglion cells including those with displaced cell bodies contributed proportionally to the regrowth of axons from the explants.

How do retinal axons find their targets in the developing brain?

Tissue culture studies show that cell survival and process outgrowth from retinal ganglion cells depend on the molecular composition of the substrates over which the neurites grow, and on diffusible factors present in the medium. Recent work has begun to show that at least some of these components might be interactive. Since the conditions in a culture dish, as well as the patterns of antigen expression on cells in vitro, can differ considerably from those encountered in vivo, it is important to design experiments in vivo that examine how growing neurites relate to their natural microenvironment. By the use of transplantation techniques, it has been possible to provide evidence for a comparable duality of substrate-dependent and target-derived controls of optic axon growth, which might provide insight into the normal developmental process.

Ontogenetic changes in the regenerative ability of chick retinal ganglion cells as revealed by organ explants

Whereas mature neurons in the central nervous system (CNS) of birds lack the capability of regenerating axons after injury, embryonic nerve cells are able to do so. The time course of this decline of regenerative ability was investigated in ganglion cells from embryonic chick retinae. Retinal strips from 7- to 19-day-old embryos (E7-E19) were explanted and cultured in vitro. The numbers of retinal ganglion cell (RGC) neurites that had extended during the first 22-23 h incubation, their elongation rates, and morphometric parameters of the growth cones were measured to characterize the regenerative behavior. We observed two periods of decline in neuritic growth: the first from E7 to E9, and another from E14 to E19. The first decrease may reflect a gradually disappearing portion of neurons which produced their axons de novo. The second decline coincides with the major period of synaptogenesis by ganglion cell axons in ovo. The time required for initiation of axonal outgrowth increased, accordingly, from less than 3 h in explants from younger retinae (E7-E16) to 10-12 h for E17 and E19 explants. Axonal elongation rates ranged between 36 micron/h and 56 micron/h (mean values) for E7-E13 explants, but were significantly lower for cells from E14-E19 retinae (13-21 microns/h). Morphologically, neurites and growth cones for RGC explanted before E17 were characterized by their high variability. They possessed more filopodia than mature neurons (E17, E19), fasciculated to a higher degree and branched more frequently. In addition, older neurites appeared "stiffer" and were morphologically simpler.(ABSTRACT TRUNCATED AT 250 WORDS)

Retrograde modulation of dendritic geometry in the vertebrate brain during development

The neurons of the chick's isthmo-optic nucleus (ION) are known to innervate the retina. We here show that removing the retinal primordia causes the ION dendritic trees to be much less polarized than normal. Our observations were made at 11 embryonic days, which is before the isthmo-optic neurons become dependent on the retina for survival. Other parameters such as neuronal size were unchanged, so the effect seems to have been specific to dendritic shape. Our interpretation is that early target removal eliminates a retrograde signal that normally enhances dendritic polarization.