Oriented extracellular channels and axonal guidance in the embryonic chick retina

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

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

Laminin-coated multifilament entubulation, combined with Schwann cells and glial cell line-derived neurotrophic factor, promotes unidirectional axonal regeneration in a rat model of thoracic spinal cord hemisection

Biomaterial bridging provides physical substrates to guide axonal growth across the lesion. To achieve efficient directional guidance, combinatory strategies using permissive matrix, cells and trophic factors are necessary. In the present study, we evaluated permissive effect of poly (acrylonitrile-co-vinyl chloride) guidance channels filled by different densities of laminin-precoated unidirectional polypropylene filaments combined with Schwann cells, and glial cell line-derived neurotrophic factor for axonal regeneration through a T10 hemisected spinal cord gap in adult rats. We found that channels with filaments significantly reduced the lesion cavity, astrocytic gliosis, and inflammatory responses at the graft-host boundaries. The laminin coated low density filament provided the most favorable directional guidance for axonal regeneration which was enhanced by co-grafting of Schwann cells and glial cell line-derived neurotrophic factor. These results demonstrate that the combinatorial strategy of filament-filled guiding scaffold, adhesive molecular laminin, Schwann cells, and glial cell line-derived neurotrophic factor, provides optimal topographical cues in stimulating directional axonal regeneration following spinal cord injury. This study was approved by Indiana University Institutional Animal Care and Use Committees (IACUC #:11011) on October 29, 2015.

Hippocampal Neurons’ Alignment on Quartz Grooves and Parylene Cues on Quartz Substrate

Alignment and patterning of neurons have great importance in some research fields, especially for regenerative medicine and for the formation of artificial neural networks. Alignment of neurons on quartz grooves and parylene cues on quartz substrate was evaluated in this work. The neurons’ alignment on quartz grooves is considered to be topographical alignment, while the neurons’ alignment on parylene cues on quartz substrate is considered to be chemical alignment. Both quartz grooves’ and parylene cues’ widths were fabricated in a range from 2 µm to 8 µm; quartz grooves’ heights were in a range from 0.25 µm to 4 µm, while parylene cues’ heights were only 0.25 µm. Neurons were dissociated hippocampal neurons from rat E18. Neurons were cultivated on test substrates for 7 days before alignment evaluation. As expected, neurons aligned according to the direction of grooves and cues; however, transversal growth direction was also observed with much smaller tendency. Chemical alignment was found to be more effective than topographical alignment. If parylene cues are thin and distanced enough, then neurons have a tendency to follow the direction of individual parylene cues; however, neurons on quartz grooves have a tendency just to follow a preferable direction than individual quartz grooves.

Micropatterned coumarin polyester thin films direct neurite orientation

Guidance and migration of cells in the nervous system is imperative for proper development, maturation, and regeneration. In the peripheral nervous system (PNS), it is challenging for axons to bridge critical-sized injury defects to achieve repair and the central nervous system (CNS) has a very limited ability to regenerate after injury because of its innate injury response. The photoreactivity of the coumarin polyester used in this study enables efficient micropatterning using a custom digital micromirror device (DMD) and has been previously shown to be biodegradable, making these thin films ideal for cell guidance substrates with potential for future in vivo applications. With DMD, we fabricated coumarin polyester thin films into 10×20 μm and 15×50 μm micropatterns with depths ranging from 15 to 20 nm to enhance nervous system cell alignment. Adult primary neurons, oligodendrocytes, and astrocytes were isolated from rat brain tissue and seeded onto the polymer surfaces. After 24 h, cell type and neurite alignment were analyzed using phase contrast and fluorescence imaging. There was a significant difference (p<0.0001) in cell process distribution for both emergence angle (from the body of the cell) and orientation angle (at the tip of the growth cone) confirming alignment on patterned surfaces compared to control substrates (unpatterned polymer and glass surfaces). The expected frequency distribution for parallel alignment (≤15°) is 14% and the two micropatterned groups ranged from 42 to 49% alignment for emergence and orientation angle measurements, where the control groups range from 12 to 22% for parallel alignment. Despite depths being 15 to 20 nm, cell processes could sense these topographical changes and preferred to align to certain features of the micropatterns like the plateau/channel interface. As a result this initial study in utilizing these new DMD micropatterned coumarin polyester thin films has proven beneficial as an axon guidance platform for future nervous system regenerative strategies.

Stimulation of olfactory ensheathing cell motility enhances olfactory axon growth

Axons of primary olfactory neurons are intimately associated with olfactory ensheathing cells (OECs) from the olfactory epithelium until the final targeting of axons within the olfactory bulb. However, little is understood about the nature and role of interactions between OECs and axons during development of the olfactory nerve pathway. We have used high resolution time-lapse microscopy to examine the growth and interactions of olfactory axons and OECs in vitro. Transgenic mice expressing fluorescent reporters in primary olfactory axons (OMP-ZsGreen) and ensheathing cells (S100ß-DsRed) enabled us to selectively analyse these cell types in explants of olfactory epithelium. We reveal here that rather than providing only a permissive substrate for axon growth, OECs play an active role in modulating the growth of pioneer olfactory axons. We show that the interactions between OECs and axons were dependent on lamellipodial waves on the shaft of OEC processes. The motility of OECs was mediated by GDNF, which stimulated cell migration and increased the apparent motility of the axons, whereas loss of OECs via laser ablation of the cells inhibited olfactory axon outgrowth. These results demonstrate that the migration of OECs strongly regulates the motility of axons and that stimulation of OEC motility enhances axon extension and growth cone activity.

The development of retinal ganglion cells in a tetraploid strain of Xenopus laevis: a morphological study utilizing intracellular dye injection

The morphological development of retinal ganglion cells was examined in a tetraploid strain of Xenopus frogs. The enlarged cells of the tetraploid strain facilitate the application of intracellular techniques. Using an in vitro retinal preparation and Nomarski optics, intracellular recording and dye injection were carried out under visual control on ganglion cells in central retina from 2 days of development (stage 24) to metamorphosis (stage 64). We identified three phases in the morphological differentiation of ganglion cells. During the first phase (stages 24-30), all cells were neuroepitheliallike in form and possessed robust resting potentials in the range of -35 to -60 mV, and dye-coupling was occasionally observed between neighboring cells. During the second phase of ganglion cell development (stages 31-45) the neurons had begun to elaborate axons and dendrites. These cells possessing neurites had resting potentials between -15 and -30 mV, and no dye-coupling was observed between neighbors. During the third and final phase of maturation, from stage 46 onward, three distinct morphological types of ganglion cells could be identified. Type I cells had the smallest somata and the smallest-diameter dendritic arborizations. The profusely branched dendrites of these cells ramify extensively throughout the inner plexiform layer. Type II cells had large somata, intermediate-diameter dendritic fields, and a highly elaborate dendritic branching pattern. These cells were seen to arborize within two sublamina in the inner plexiform layer. Type III cells had large somata, the largest-diameter dendritic fields, and a dendritic arbor with long primary branches but little higher-order branching. These large dendritic fields were confined to a single sublamina of the inner plexiform layer, abutting the inner nuclear layer. While most phase 3 cells showed radial axon trajectories from the soma to the optic disc, a minority of cells (1-5%) with erratic and nonradial axon trajectories were also observed. Our data provide a morphological description of ganglion cell maturation in the central retina of Xenopus. We show that very early in development (as early as stage 46) three distinct morphological types of retinal ganglion cells are present, which correspond to the three classes of ganglion cells previously described in adult Xenopus (Chung et al., '75).

Effect of filament diameter and extracellular matrix molecule precoating on neurite outgrowth and Schwann cell behavior on multifilament entubulation bridging devicein vitro

At present there is no clinically effective treatment for injuries or pathological processes that disrupt the continuity of axons in the mature central nervous system. However, a number of studies suggest that a tremendous potential exists for developing biomaterial based therapies. In particular, biomaterials in the form of bridging substrates have been shown to support at least some level of axonal regeneration across the lesion site, but display a limited capacity for directing axons toward their targets. To improve the directionality and outgrowth rate of the axonal regeneration process, filaments and tubes appear promising, but the technology is far from optimized. As a step toward optimization, the influence of filament diameter and various extracellular matrix coatings on nerve regeneration was evaluated in this article using a dorsal root ganglion (DRG) explant model. An increasing pattern of alignment and outgrowth of neurites in the direction parallel the long axis of the packed filament bundles with decreasing filament diameters ranging from supracellular and beyond (500 to 100 mum), cellular (30 mum), down to subcellular size (5 mum) was observed. Such effects became most prominent on filament bundles with individual filament diameters in the range of cellular size and below (5 and 30 mum) where highly directional and robust neuronal outgrowth was achieved. In addition, laminin-coated filaments that approached the size of spinal axons support significantly longer regenerative outgrowth than similarly treated filaments of larger diameter, and exceed outgrowth distance on similarly sized filaments treated with fibronectin. These data suggested the feasibility of using a multifilament entubulation bridging device for supporting directional axonal regeneration.

Integration of topographical and biochemical cues by axons during growth on microfabricated 3-D substrates

During embryonic neural development, axon tips ("growth cones") are guided through a dynamic three-dimensional (3-D) landscape by soluble chemotropic factors and by immobilized, growth-permissive or growth-inhibiting contact cues present in the extracellular matrix and on the surface of surrounding cells. It has been difficult to probe the search algorithms of growth cones in response to multiple contact cues during 3-D navigation using traditional two-dimensional (2-D) substrates. Here, we present an in vitro study in which the axons of murine embryonic cortical neurons are challenged with competing growth options, using 3-D substrates that feature variations in permissiveness and microtopography. As 3-D substrates, we used poly-D-lysine (PDL) coatings on microfabricated steps of polydimethylsiloxane (PDMS) and complementary features of Matrigel. We found that axons display a preference for PDL over Matrigel and for the straightest path within a distance consistent with the exploratory range of the growth cone. When these two preferences are in conflict, axons choose to grow straight into Matrigel; when the straight path is not permissive, the axon turns in the direction that minimizes the turning angle. These results suggest that growth cones make 3-D navigation decisions by integrating permissiveness and topographical cues.

Immunohistological Localization of Tenascin in the Developing and Lesioned Adult Mouse Optic Nerve

To gain insight into the morphogenetic functions of the recognition molecule tenascin in the central nervous system, we have studied its localization in the developing and lesioned adult mouse optic nerve using light and electron microscopic immunocytochemistry. Since tenascin is a secreted molecule, we have analysed the tenascin-synthesizing cells in tissue sections of retinae and optic nerves by in situ hybridization. A weak and homogeneous tenascin immunoreactivity was detectable in the developing retinal nerve fibre layer and optic nerve of 14-day-old mouse embryos, the earliest developmental age investigated. In the optic nerve of neonatal and 1-week-old animals, a high number of tenascin messenger RNA (mRNA)-containing cells were present, and antibodies to tenascin labelled the surfaces of astrocytes and unmyelinated retinal ganglion cell axons. With increasing age, expression of tenascin in the optic nerve was down-regulated at the mRNA and protein levels. At the fourth postnatal week, blood vessels in the optic nerve and collagen fibrils in the vicinity of meningeal fibroblast-like cells still showed significant immunoreactivity, but the optic nerve tissue proper no longer did so. In adult animals, tenascin was no longer detectable in association with blood vessels located in the myelinated part of the optic nerve, and meninges were only weakly immunoreactive. Also, tenascin mRNA-containing cells were no longer detectable in the myelinated part of the adult mouse optic nerve and few labelled cells were found in the meninges. In the retina, ganglion cells contained no detectable levels of tenascin mRNA at any of the developmental ages analysed. No significant up-regulation of tenascin expression was seen in the nerve tissue proper of transected proximal (i.e. retinal) and distal (i.e. cranial) optic nerve stumps of adult mice during the first 4 weeks after lesioning, the time period studied. However, collagen fibrils associated with meningeal fibroblast-like cells and located near the lesion site became strongly tenascin-immunoreactive 2 days after lesioning. Also, some blood vessels at the lesion site became immunoreactive. We conclude that tenascin in the optic nerve is synthesized by glial cells and not by retinal ganglion cells. The detectability of tenascin at embryonic ages suggests that it may mediate neurite growth in vivo. The absence of a strong, lesion-induced up-regulation of tenascin expression in the regeneration-prohibitive mouse optic nerve contrasts with the lesion-induced pronounced up-regulation in the regeneration-permissive peripheral nervous system, and may indicate a functional involvement of tenascin in regenerative processes. The high tenascin positivity of collagen fibrils at early postnatal ages and after lesioning suggests that tenascin expression may be correlated with mitotic activity of the associated meningeal fibroblast-like cells. Finally, tenascin may be involved in the process of vascularization, since the molecule is associated with blood vessels in developing and adult lesioned, but not intact adult, optic nerves.

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.

The polysialic acid moiety of the neural cell adhesion molecule is involved in intraretinal guidance of retinal ganglion cell axons

We have characterized the antigen recognized by mab10, a monoclonal antibody that has been shown to modify outgrowth of thalamic and cortical axons in vitro, and investigated the influence of this antibody on axonal growth in the chicken retina in vivo. Immunopurification, peptide sequencing, and biochemical characterization proved the epitope recognized by mab10 to be polysialic acid (PSA), associated with the neural cell adhesion molecule (NCAM). Intravitreal injections of antibody-secreting hybridoma cells were combined with whole-mount studies using the fluorescent tracer 1,1'-dioctadecyl-3,3,3', 3'-tetramethylindocarbocyanine perchlorate (DiI). Pathfinding at the optic fissure was affected, resulting in a failure of axons to exit into the nerve. Misprojections also occurred in more peripheral areas of the retina; however, axons eventually oriented toward the center. Similar projection errors were observed after enzymatic removal of PSA by injecting endoneuraminidase N (endo N). Quantitative measurements of the optic nerve diameter as well as the width of the optic fiber layer confirmed that many axons failed to leave the retina and grew back in the optic fiber layer of the retina. Our findings suggest that NCAM-linked PSA is involved in guiding ganglion cell axons in the retina and at the optic fissure.

The retinal axon's pathfinding to the optic disk

Retinal ganglion cell (RGC) axons travel in radial routes unerringly toward the optic disk, their first intermediate target in the center of the eye. The path of the RGC growth cone is restricted to a narrow zone subjacent to the endfeet of Müller glial cells and the vitreal basal lamina. The present survey indicates that RGC growth cones are guided by many molecular cues along their pathway which are recognized by receptors on their surface. Growth-promoting molecules on Müller glial endfeet and in the basal lamina assist growth cones in maintaining contact with these elements. The repellant character of deeper retinal laminae discourages them from escaping the RGC axon layer. Cell adhesion/recognition proteins enable growth cones to fasciculate with preformed axons in their vicinity. It is still unclear whether the optic disk emits long range guidance components which enable the growth cones to steer toward it. Recent evidence in fish indicates the existence of an axonal receptor (neurolin) for a guidance component of unknown identity. Receptor blockade causes RGC axons to course in aberrant routes before they reach the disk. At the disk, axons receive signals to exit the retina. Contact with netrin-1 at the optic disk/nerve head encourages growth cones to turn into the nerve. This response requires the axonal netrin receptor DCC, laminin-1, beta-integrin and most likely the UNC5H netrin receptors which convert the growth encouraging signal into a repulsive one which drives growth cones into the nerve.

The tracheal system of the developing primary olfactory pathway of Manduca sexta: tracheae do not play a guidance or targeting role for ingrowing receptor axons

Axons navigate to their targets by detecting signals within the environment through which they are growing. The surfaces of tracheae, which are prominent features of the insect body plan, could be detected as favorable pathways for sensory axons growing toward the brain. The pattern of the tracheal investment of the adult antennal lobe of the moth Manduca sexta suggested two specific possibilities for interaction between tracheae and axons during development: that tracheae might be involved in guiding olfactory receptor axons to their target region of the brain, the antennal lobe; and that tracheae could provide an address system within the lobe that defines the sites of glomeruli, which are olfactory-axon target areas within the lobe. To determine whether tracheae contribute to development of the primary olfactory pathway, the distribution of tracheae in the adult and developing antennal lobes was examined with both confocal and electron microscopes. During the major stages in which axons are growing into the antennal lobe and in which glomeruli are forming, the tracheal investment of the nerve and lobe was found to be minimal. Tracheae thus cannot serve as axon guides or as local address sites for newly forming glomeruli during the initial targeting of receptors onto the antennal lobe.

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.

Embryonic lens repels retinal ganglion cell axons

During development of the vertebrate visual system, retinal ganglion cell (RGC) axons follow a precise path toward their midbrain targets. Although much is known about the cues that direct RGC axons once they have left the optic disc, less is known about the guidance of axons at earlier stages, when RGCs first send out their axons to navigate within the developing retina. Using collagen gel coculture experiments, we find that the embryonic lens produces a powerful diffusible repulsive activity for RGC axons. We also find that this activity is localized to the lens epithelium and not the lens fiber layer, while the pigmented epithelium and vitreous humour are devoid of activity. The further observation that the lens also chemorepels primary sensory axons, but does not repel olfactory bulb axons, shows that this activity is specific for subsets of axons. Our experiments have excluded two candidate repellents for RGC axons (collapsin-1/sema III and chondroitin sulfate proteoglycans). These results implicate the lens in the earliest stages of RGC axon guidance. One function of the lens repellent may be to prevent aberrant targeting toward the lens, and it may also be involved in the directional guidance of RGC axons toward the optic disc.

Neurolin Ig domain 2 participates in retinal axon guidance and Ig domains 1 and 3 in fasciculation

The optic disk-directed growth of retinal ganglion cell axons is markedly disturbed in the presence of polyclonal antineurolin antibodies, which mildly affect fasciculation (Ott, H., M. Bastmeyer, and C.A.O. Stuermer, 1998. J. Neurosci. 18:3363-3372). New monoclonal antibodies (mAbs) against goldfish neurolin, an immunoglobulin (Ig) superfamily cell adhesion/recognition molecule with five Ig domains, were generated to assign function (guidance versus fasciculation) to specific Ig domains. By their ability or failure to recognize Chinese hamster ovary cells expressing recombinant neurolin with deletions of defined Ig domains, mAbs were identified as being directed against Ig domains 1, 2, or 3, respectively. Repeated intraocular injections of a mAb against Ig domain 2 disturb the disk-directed growth: axons grow in aberrant routes and fail to reach the optic disk, but remain fasciculated. mAbs against Ig domains 1 and 3 disturb the formation of tight fascicles. mAb against Ig domain 2 significantly increases the incidence of growth cone departure from the disk-oriented fascicle track, while mAbs against Ig domains 1 and 3 do not. This was demonstrated by time-lapse videorecording of labeled growth cones. Thus, Ig domain 2 of neurolin is apparently essential for growth cone guidance towards the disk, presumably by being part of a receptor (or complex) for an axon guidance component.

Disruption of the retinal basal lamina during early embryonic development leads to a retraction of vitreal end feet, an increased number of ganglion cells, and aberrant axonal outgrowth

Bacterial collagenase was injected into the vitreous of the eye of chick and quail embryos. Immunocytochemical and ultrastructural studies revealed that the collagenase dissolved the retinal basal lamina of the injected eye. The basal lamina disruption was first detectable 1 hour after enzyme injection and was complete within 3 hours. With further development, the retinal basal lamina was not reestablished; newly developing neuroepithelium in the peripheral retina, however, generated an intact basal lamina. Western blot analysis showed that Clostridial collagenase degraded various collagens but spared noncollagenous proteins. Basal lamina disruption of embryonic day 3 to 6 retinae led to the retraction of the end feet of the neuroepithelial cells, caused an increase in the number of Islet-1+ cells (most likely ganglion cells), an increase in the thickness of the optic fiber layer, and aberrant growth of optic axons on their way toward the optic disc. None of these changes were observed when retinal basal laminae were disrupted at later stages of development. The present data demonstrate that the retinal basal lamina, by anchoring the neuroepithelial cells to the pial surface of the retina, has an important function in the development of the normal cytoarchitecture of this structure. It is proposed that the altered extracellular environment in the vitreal part of the retina, resulting in the retraction of the neuroepithelial end feet, is responsible for the increased number of Islet-1+ cells and the aberrant axonal navigation.

Neurolin, the goldfish homolog of DM-GRASP, is involved in retinal axon pathfinding to the optic disk

Young axons of new retinal ganglion cells (RGCs) in the continuously growing goldfish retina fasciculate with one another and their immediate forerunners on their path toward the optic disk and along the optic nerve. They express the immunoglobulin superfamily cell adhesion molecules (CAMs) neurolin (DM-GRASP) and the L1-like E587 antigen. Repeated injections of Fab fragments from polyclonal antisera against neurolin (neurolin Fabs) into the eye of 3. 4-cm-long and rapidly growing goldfish caused highly aberrant pathways of young RGC axon subfascicles in the dorsal retina. Many axons grew in circles and failed to reach the optic disk. In contrast, E587 Fabs, used in parallel experiments, disrupted the fascicles but did not interfere with the disk-directed growth. Neurolin Fabs also disturbed axonal fasciculation in vivo as well as in vitro but less severely than E587 Fabs. Coinjections of both Fabs increased defasciculation of the dorsal axons in both aberrant and disk-directed routes. They also disrupted the order of young RGC axons in the optic nerve more severely than E587 Fabs alone. This demonstrates that the development of tight and orderly fascicles in the dorsal retina and in the optic nerve requires both E587 antigen and neurolin. More importantly, our results suggest an involvement of neurolin in RGC axonal guidance from the retinal periphery to the optic disk. Because disrupted fascicles and errant axon routes were found only in the dorsal retinal half, a cooperation with so-called positional markers may be conceived.

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 early neuronal organization predicts the path followed by some major axonal bundles in the embryonic brainstem

In the embryonic CNS, preformed pathways precede the growth of axonal fasciculi [Katz M. J. and Lasek R. J. (1980) Cell Motil. 1, 141-157; Katz M. J. et al. (1980) Neuroscience 5, 821-833]. What are the developmental events that lead to the elaboration of these preformed pathways? To answer this question, we investigated the organization of the primitive neural tube and more particularly the arrangement of the early-generated cells using [3H]thymidine autoradiography or bromodeoxyuridine. Our data suggest that the position of early-generated cells might be involved in the setting of such pathways. In the brain stem of E12(0) (12 days and 0 h) and E12(15) rat embryos, the first-generated cells were organized into three longitudinal columns associated with glycoconjugate-rich extracellular spaces in the adjacent primitive marginal layer. Also, axons traced with 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) were contiguous to the early-generated cellular columns and represented the primordium of the medial longitudinal fasciculus, the lateral longitudinal tract and the mesencephalic trigeminal tract. Our results show a correlation between the organization of early-generated cells, likely neurons, and the pattern of extracellular spaces in the marginal layer where axons grow. It has been reported in the literature that neurons produce elements of the extracellular matrix such as growth-modulating molecules or space-creating molecules. We therefore suggest that the position of early-generated neurons could be involved in the elaboration of a template for the setting of some major longitudinal tracts during embryonic development of the brainstem.

The human fetal retinal nerve fiber layer and optic nerve head: a DiI and DiA tracing study

The organization of the primate nerve fiber layer and optic nerve head with respect to the positioning of central and peripheral axons remains controversial. Data were obtained from 32 human fetal retinae aged between 15 and 21 weeks of gestation. Crystals of the carbocyanine dyes, DiI or DiA, and fluorescence microscopy were used to identify axonal populations from peripheral retinal ganglion cells. Peripheral ganglion cell axons were scattered throughout the vitreal-scleral depth of the nerve fiber layer. Such a scattered distribution was maintained as the fibers passed through the optic nerve head and along the optic nerve. There was a rough topographic representation within the optic nerve head according to retinal quadrant such that both peripheral and central fibers were mixed within a wedge extending from the periphery to the center of the nerve. There was no indication that the fibers were reorganized in any way as they passed through the optic disc and into the nerve. The present results suggest that any degree of order present within the fiber layer and optic nerve is not an active process but a passive consequence of combining the fascicles of the retinal nerve fiber layer. Optic axons are not instructed to establish a retinotopic order and the effect of guidance cues in reordering fibers, particularly evident prechiasmatically and postchiasmatically, does not appear to be present within the nerve fiber layer or optic nerve head in humans.

Retinotopy of the human retinal nerve fibre layer and optic nerve head

The organisation of the primate nerve fibre layer and optic nerve head with respect to eccentricity or the positioning of central and peripheral axons remains controversial. Crystals of the carbocyanine dyes DiI (1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate), or DiA (4-[4-didecylaminostryryl]-N-methylpridiniumiodide) were used to trace retinal ganglion cell axons within the nerve fibre layer, optic nerve head, and optic nerve. The present study demonstrated that peripheral retinal axons were scattered throughout the vitreal-scleral depth of the nerve fibre layer. This scattered distribution was maintained as the fibres passed through the optic nerve head and into the optic nerve. Axons of the arcuate bundles showed a bias towards the scleral portions of the nerve fibre layer and a variable degree of fibre scatter across the nerve fibre layer which was not as evident in labelling from other retinal regions. There was a rough topographic representation within the optic nerve head according to retinal circumference such that both peripheral and central fibres were mixed within a wedge extending from the periphery to the centre of the nerve. Foveal fibres occupied a large proportion of the temporal aspect of the optic nerve head and nerve, whereas fibres from areas temporal to the fovea appeared to be displaced to more superior and inferior regions. Consistent with the scleral bias seen in the retina, arcuate fibres maintained a peripheral position as they passed through the optic nerve head and occupied a more peripheral position in the nerve. The present results suggest that any degree of order present within the optic nerve is not an active process; optic axons are not instructed to establish a retinotopic order within the initial portions of the visual pathway.

Organization of retinal ganglion cell axons in the optic fiber layer and nerve of fetal ferrets

Previous authors have hypothesized that retinotopic projections may be influenced by 'preordering' of the axons as they grow towards their targets. In some nonmammalian species, axons are reorganized at or near the optic nerve head to establish a retinotopic order. Data are ambiguous concerning the retinotopy of the mammalian retinal nerve fiber layer and whether fibers become reorganized at the optic nerve head. We have examined this question in fetal and newborn ferrets (Mustela putorius furo) by comparing the arrangement of axons in the retinal nerve fiber layer with that in the optic nerve. Dil or DiA crystals were implanted into fixed tissue in the innermost layers of the retinal periphery, or at a location midway between the periphery and the optic nerve head. Fluorescence labelling was examined in 100-200 microns Vibratome sections, or the eyecup and nerve were photooxidized and 1-2 microns longitudinal or transverse sections were examined. Regardless of fetal age, eccentricity or quadrant of the implant site, a segregation of labelled peripheral axons from unlabelled central ones was not detected within the nerve fiber layer. Axons coursed into the nerve head along the margin of their retinal quadrant of origin, often entering the optic nerve as a radial wedge, thus preserving a rough map of retinal circumference. However, peripheral axons were in no way restricted to the peripheral (nor central) portions of the nerve head or nerve, indicating that the optic axons do not establish a map of retinal eccentricity. Our results demonstrate that (1) the nerve fiber layer is retinotopic only with respect to circumferential position and (2) optic axons are not actively reorganized to establish a retinotopic ordering at the nerve head. The present results suggest that any degree of order present within the optic nerve is a passive consequence of combining the fascicles of the retinal nerve fiber layer; optic axons are not instructed to establish, nor constrained to maintain, a retinotopic order within the optic nerve.

The behavior of optic axons on substrate gradients of retinal basal lamina proteins and merosin

To study the behavior of optic axons to continuously changing concentrations of their substrate, explants from embryonic retina were placed across gradients of retinal basal lamina proteins and merosin. The following growth patterns of axons in response to the substrate gradients were found: (1) Axons that grew up gradients, i.e., from low to high substrate concentrations, became longer and less fasciculated with increasing concentration of the substrate. On shallow basal lamina gradients, the axons also showed a directional response that resulted in guidance to higher substrate concentrations. (2) Axons that grew down gradients, i.e., from high to low substrate concentrations, became shorter and more fasciculated with decreasing concentrations of the substrate. On gradients of merosin, a significant alteration in the axonal growth direction toward higher substrate concentrations was detected. Axons heading down gradients never U turned to higher substrate concentrations. (3) Axons confronted with discontinuous substrates were confined to the borders of the substrate exclusively, whereas axons confronted with substrate gradients were able to cross into the territory beyond the substrate. (4) The growth patterns of axons on substrate gradients of basal lamina proteins and merosin were similar but not identical, indicating that axons may respond to substrate gradients dependent on its chemical composition. The present results show that substrate gradients can regulate length and fasciculation of neurites and have a limited capability to direct axons to higher substrate concentrations.

Expression of the axonal cell adhesion molecules axonin-1 and Ng-CAM during the development of the chick retinotectal system

Cell surface glycoproteins expressed on growth cones and axons during brain development have been postulated to be involved in the cell-cell interactions that guide axons into their target area. Nevertheless, an unequivocal description of the mechanism by which such molecules exert control over the pathway of a growing axon has not been done. As a crucial requirement in support of a relevant involvement of an axonal surface molecule in growth cone guidance, this molecule should be expressed in the growth cone. The developing retinotectal system provides an excellent opportunity to test whether a particular neuronal surface molecule fulfills the requirement of the spatiotemporal coincidence between its appearance and the emergence of growth cones because its setup follows the rule of chronotopy, i.e., the position of axons in a certain site is determined by the time of their arrival. We have analyzed axonin-1 and the neuron-glia cell adhesion molecule (Ng-CAM), two axonal surface molecules that promote neurite growth in vitro, for their expression in the retina and in the retinotectal system of the chick throughout its development. At stage 18, both axonin-like (A-LI) and Ng-CAM-like immunoreactivity (Ng-CAM-LI) are clearly present in the area where first retinal ganglion cells (RGCs) are generated. The immunoreactivity spreads synchronously with the formation of RGCs over the developing retina. From stage 32 on, the inner plexiform layer is also stained according to its temporospatial gradient of maturation. In later stages, the outer plexiform layer and the inner segments of photoreceptors also show immunoreactivity. The development of A-LI and Ng-CAM-LI along the optic nerve, chiasm, optic tract, and in the superficial layers of the optic tectum follows the chronotopic pattern of axons, as was found by earlier morphological investigations. Older axons loose their A-LI. This allows to localize the position of newly formed axons. The fact that A-LI and Ng-CAM-LI parallel the formation and maturation of axons suggests that axonin-1 and Ng-CAM may play an important role in the organization of the retinotectal system.

Formation of alternating tiers in the optic chiasm of the chick embryo

BACKGROUND

When the fibers of the two optic nerves of the chick cross to the contralateral side at the prospective chiasmatic region, they segregate into clearly defined bundles. These bundles form horizontally oriented tiers which alternate between the right and the left optic nerve.

METHODS

We have analyzed the development of these tiers qualitatively and quantitatively using light and electron microscopy between embryonic days (E) 4 and E19.

RESULTS

The formation of the chiasm begins on E4. In the course of E4, tiers become visible for the first time. Their number increases rapidly until E7. Then the increase is slowed down and the final value (32 +/- 1) is approximated by E18/19. Growing axons allow one to distinguish three different segments: the growth cone, the distal, and the proximal segment. The latter originates in the perikaryon. Growth cones and distal segments are found predominantly in the ventralmost tiers. Their frequency decreases from ventral to dorsal. Proximal segments which indicate the presence of older axons appear first in the dorsal tiers and later also in more ventrally located tiers.

CONCLUSION

Based on these criteria it is concluded that newly formed axons contribute primarily but not exclusively to the ventral tiers. There is a gradient of maturity of axons from ventral to dorsal whose slope becomes steeper with age until the last growth cones have arrived by E18. Thus, the formation of the chiasm corresponds to the spatiotemporal pattern of ganglion cell formation in the retina. The process of cell death of retinal ganglion cells is also seen in the chiasm but probably does not lead to a transitory diminution in the number of tiers.

Cell death in the developing olfactory epithelium of rat embryos

Cell death process in the developing olfactory epithelium was studied by light and electron microscopy in rat embryos from embryonic days 12-18. A massive wave of cell death was observed at embryonic days 12 and 13 and two types of dying cells were seen coexisting. The first type of dying cells exhibited morphological features of apoptosis while the second showed characteristics of a cell death by non-lysosomal disintegration with cytoplasmic swelling and absence of phagocytosis. From embryonic day 14 onward, only apoptotic figures could be still occasionally observed. The significance of such a massive wave of cell death occurring during the earliest developmental stages of the rat olfactory epithelium is discussed in relation with the morphogenesis of the olfactory system.

Ultrastructure of an identified array of growth cones and possible substrates for guidance in the embryonic medicinal leech, Hirudo medicinalis

The oblique muscle organizer (Comb- or C-cell) in the embryonic medicinal leech, Hirudo medicinalis, provides an amenable situation to examine growth cone navigation in vivo. Each of the segmentally iterated C-cells extends an array of growth cones through the body wall along oblique trajectories. C-cell growth cones undergo an early, relatively slow period of extension followed by later, protracted and rapid directed outgrowth. During such transitions in extension, guidance might be mediated by a number of factors, including intrinsic constraints on polarity, spatially and temporally regulated cell and matrix interactions, physical constraints imposed by the environment, or guidance along particular cells in advance of the growth cones. Growth cones and their environment were examined by transmission electron microscopy to define those factors that might play a significant role in migration and guidance in this system. The ultrastructural examination has made the possibility very unlikely that simple, physical constraints play a prominent role in guiding C-cell growth cones. No anatomically defined paths or obliquely aligned channels were found in advance of these growth cones, and there were no identifiable physical boundaries, which might constrain young growth cones to a particular location in the body wall before rapid extension. There were diverse associations with many matrices and basement membranes located above, below, and within the layer in which growth cones appear to extend at the light level. Additionally, a preliminary examination of myocyte assembly upon processes proximal to the growth cones further implicates a role for matrix-associated interactions in muscle histogenesis as well as process outgrowth during embryonic development.

Position, guidance, and mapping in the developing visual system

Positional identity in the visual system affects the topographic projection of the retina onto its central targets. In this review we discuss gradients and positional information in the retina, when and how they arise, and their functional significance in development. When the axons of retinal ganglion cells leave the eye, they navigate through territory in the central nervous system that is rich in positional information. We review studies that explore the navigational cues that the growth cones of retinal axons use to orient towards their target and organize themselves as they make this journey. Finally, these axons arrive at their central targets and make a precise topographic map of visual space that is crucial for adaptive visual behavior. In the last section of this review, we examine the topographic cues in the tectum, what they are, when, and how they arise, and how retinal axons respond to them. We also touch on the role of neural activity in the refinement of this topography.

Development of radial glia and astrocytes in the spinal cord of the North American opossum (Didelphis virginiana): an immunohistochemical study using anti-vimentin and anti-glial fibrillary acidic protein

We have shown previously that rubrospinal axons grow around a lesion of their pathway in developing opossums and that a critical period exists for that plasticity. As a first step toward addressing the possibility that glial maturation and/or the development of an astrocytic response to lesioning contribute to loss of rubrospinal plasticity, we have studied the normal development of radial glia and astrocytes in the spinal cord of the opossum by immunostaining for vimentin (Vim) and glial fibrillary acidic protein (GFAP). Vim-like immunoreactivity (Vim-LI) was present in radial glia throughout the spinal cord at birth (12 days after conception), whereas GFAP-like immunoreactivity (GFAP-LI) was limited to cells of comparable morphology in the ventral part of the cervical cord. The subsequent appearance of GFAP-LI followed ventral to dorsal and rostral to caudal gradients and by postnatal day (PD) 15, it was found in radial glia throughout the cord. At the same age, processes immunostained by either antibody had lost their radial orientation in the ventral horn of the cervical cord. The subsequent transformation from radial glia to astrocytes also followed ventral to dorsal and rostral to caudal gradients. By PD30, mature appearing astrocytes were immunostained by both antibodies at thoracic levels of the spinal cord, the levels lesioned in the plasticity experiments referred to above, and by PD41, they were found at all levels of the cord.(ABSTRACT TRUNCATED AT 250 WORDS)

Early axon outgrowth of retinal ganglion cells in the fetal rhesus macaque

Employing retinal explants and retrograde transport techniques, we studied the formation of the arcuate fascicles by examining the growth of the central retina, the emergence of the adult fiber layer pattern, and the projections of retinal ganglion cells in the central and peripheral retina. Sixty days prior to foveal pit formation, the distance from the incipient fovea to the optic disk was equal to the adult, even though the retinal area was only 8% of the adult. Arcuate fibers, at this age, were observed to avoid the incipient fovea, with no fascicles and few axons projecting over this region. A small population of 15.2% of the ganglion cells located within 2 mm of the incipient fovea possessed an axon with an aberrant trajectory that wound around and projected 50 to several hundred microns away from the optic disk, compared to only 3% at other retinal locations. The incidence of disorder decreased with increasing fetal age, establishing mature values in late fetal periods. These findings suggest that the area of the central retina does not increase after embryonic day 60 and that guidance factors are present that allow outgrowing ganglion cell axons to distinguish and avoid that portion of the retina that will become the fovea.

The influence of fibronectin and laminin during Schwann cell migration and peripheral nerve regeneration through silicon chambers

The ability of extracellular proteins to influence the regenerative process was examined in Sprague-Dawley rats. Silicon chambers, filled with sterile saline solutions of cytochrome-c, fibronectin, laminin, a combination of fibronectin and laminin, or nerve growth factor were surgically implanted between the severed ends of sciatic nerves to form gaps of 18 mm. Four months later, the various groups were examined to determine the success of regeneration. The incidence of cable formation that bridged the gap was similar in all groups. The group of animals that had implants containing the combination of fibronectin/laminin had increased numbers of myelinated axons in the regenerated segment within the chamber and in the distal sciatic tributary nerves. Horseradish peroxidase labelling demonstrated that increased numbers of sensory and motor neurons in the fibronectin/laminin group had regenerated axons across the gap into the distal tributaries of the sciatic nerve. The effect of the various agents on non-neuronal cells was measured by immunohistochemical staining with S-100 antibodies to determine the effects on Schwann cell migration. Silicon chambers, filled with sterile saline solutions of fibronectin, laminin, fibronectin/laminin, nerve growth factor, or cytochrome-c, were surgically implanted to form 5 mm gaps between severed sciatic nerve ends. Ten days later, Schwann cell migration into the bridging cables was examined in each group. Analysis revealed a greater influx of Schwann cells migrating into the regenerating segments in the fibronectin, the laminin, and the combination fibronectin/laminin groups compared to the control group (cytochrome-c).

Immunocytochemical demonstration of early appearing astroglial structures that form boundaries and pathways along axon tracts in the fetal brain

During normal development of the mammalian forebrain, the paired cerebral hemispheres are initially separated midsagittally by the connective tissue-filled longitudinal fissure. During subsequent stages, the hemispheres fuse as basal lamina is remodeled and fibroblasts are eliminated from the fissure to create new central nervous system (CNS) territory in the midline. Two axon pathways, the corpus callosum and dorsal callosal stria, eventually use this region as part of their pathway. In order to assess the possible role of glial cells in the fusion process and in the guidance of axons in this and several other areas of the forebrain, we have analyzed the developing brain in timed cat and mouse embryos with immunohistochemical and morphological techniques. With the use of astroglial-specific antibodies and electron microscopy, we have visualized two distinct, primitive astroglial structures associated with the cerebral midline, and seven more associated with other specific brain regions. The way in which one of these structures moves as a column along the hemispheric midline in synchrony with seam formation suggests the possibility that during morphogenesis of the telencephalon, astrocytes may aid in the fusion process. In addition, the compact assemblage, early appearance and location of this and the other glial structures in relation to well defined neuroanatomical landmarks or axon pathways suggest that they may transiently compartmentalize relatively large regions of the CNS and organize certain developing fiber systems by acting as guides or barriers at critical stages of ontogeny.

Position of growth cones within the retinal nerve fibre layer of fetal ferrets

Optic axons are added to the retinal nerve fibre layer of fish along its vitreal border in a chronotopic manner. Likewise, the optic tract of all vertebrate species acquires axons preferentially along the superficial surface of the pathway. We have examined the developing retina of fetal ferrets (Mustela putorius furo) aged between embryonic day 27 (E27) and E34 to see whether a similar segregation of growth cones is apparent within the mammalian retinal nerve fibre layer. The distributions of growth cone, "wrist" (thick trailing portion of the growth cone), axonal, and glial profiles were determined from electron micrographs, and expressed as a percentage of neural profiles for several retinal locations. The retinal nerve fibre layer of fetal ferrets contains radially elongated bundles of fibres composed of axonal, wrist, and growth cone profiles. Glial processes of varying density divide the adjacent bundles, occasionally subdividing them in the plane of the retina, and give rise to endfeet lining the basal lamina and separating the optic axons from the latter. Growth cones within the developing fibre layer represented about 2.4% of profiles at E28, while at later developmental stages (E34), this value fell to about 0.6%. During this period of axonal outgrowth, growth cones were not preferentially segregated toward the vitreal basal lamina or the glial endfeet within the nerve fibre layer. Rather, they were found scattered throughout the axon bundles of the fibre layer. While there were differences in the proportion of immature profiles found within the vitreal half compared to the scleral half of the fibre layer, such that more growth cones and wrists were found vitreally, there was no clear accumulation of them in association with features of the vitreal margin. The present results show that young and old optic axons course together throughout the depth of the nerve fibre layer. A chronotopic mode of pathway genesis such as seen in the optic fibre layer of fish or in the optic tract of mammals is not present in the nerve fibre layer of ferrets. Differences in growth cone behaviour in the optic fibre layer and tract indicate that the mechanisms governing pathway formation differ along its course.

The structural and neurochemical development of the fetal guinea pig retina and optic nerve in experimental growth retardation

In this study we have examined structural and neurochemical aspects of retinal and optic nerve development in experimentally growth-retarded fetal guinea pigs following maternal unilateral artery ligation. Eye weight (n = 4) and total retinal area (n = 6) at 62 days gestation (term approximately 66 days) were both relatively spared when expressed as a percentage of body weight but in absolute terms were significantly reduced by 18% (P less than 0.001) and 13% (P less than 0.05) respectively when compared with age-matched controls. The numerical density of neurons in the ganglion cell layer was significantly higher at both 52 days (n = 4) and 62 days (n = 4) in growth-retarded fetuses compared with controls. However, there was no difference between the groups in the total number of neurons in this retinal layer at either age, since retinal areas are reduced in growth retardation. The area of neuronal somata in the ganglion and inner nuclear layers was significantly reduced in growth-retarded fetuses compared with controls. There was a concomitant reduction in the width of the cellular layers in the retina and also in the plexiform (synaptic) and photoreceptor layers. The growth of the outer segments of the photoreceptor layer was particularly affected in peripheral retina. The higher packing density of cells and the reduced growth of the plexiform layers suggests a reduction in the growth of the neuropile in growth-retarded fetuses compared with controls. The radial bundling of ganglion cell axons coursing across the retina to enter the optic nerve head was poorly defined in growth retardation. In addition myelination was delayed in the optic nerve with the numerical density of myelinated axons being significantly reduced (P less than 0.005) in growth-retarded fetuses compared with controls. There was a significant reduction (P less than 0.01) in the number of amacrine cells in the inner plexiform layer expressing Substance P-like immunoreactivity in growth-retarded fetuses compared with controls. Glutamate-like immunoreactivity was most intense in the five laminae of the inner plexiform layer and in the outer plexiform layer and less pronounced in photoreceptors, ganglion cells and their axons. There was no qualitative difference in glutamate immunoreactivity between control and growth-retarded fetuses in any of these structures. Thus we have shown that intrauterine growth retardation has specific effects on the development of the fetal guinea pig retina, reducing the growth of several types of neurons and their processes and affecting the expression of the neuropeptide substance-P in amacrine cells.

Elevated protein tyrosine phosphorylation in the optic tract of the chick embryo

Antibodies specific for protein phosphotyrosyl residues were used to localize sites of protein tyrosine kinase activity in the optic tract of the developing chick by immunoperoxidase staining. In the stage 34 (day 8) chick embryo, phosphotyrosine-modified proteins were abundant within outgrowing neuronal processes in the optic nerve head and nerve fiber layer of the retina, and in the developing stratum opticum at the surface of the optic tectum. These sites corresponded to regions where migrating growth cones and fasciculating bundles of some, but not all, retinal ganglion cell axons were located. Phosphotyrosine-modified proteins were also abundant in and highly restricted to the process-rich layers of the embryonic optic tectum. Phosphotyrosine immunoreactivity decreased dramatically in the corresponding regions of the optic tract of the adult chicken, indicating that protein tyrosine phosphorylation occurred principally in developing, rather than mature, neuronal processes. These findings are in accord with the idea that protein tyrosine phosphorylation may be important in cell-cell or cell-substratum interactions of ganglion cell axons.

Relationship of glia to GnRH axonal outgrowth from third ventricular grafts in hpg hosts

The homozygous mutant hypogonadal (hpg) mouse lacks a functional gene for the neuropeptide gonadotropin releasing hormone (GnRH). The consequence of this defect is an infantile reproductive tract in adulthood. This condition can be reversed by the implantation of normal fetal preoptic area tissue that contains GnRH neurons. Reversal is always preceded by the outgrowth of GnRH axons into the host target tissue, the median eminence, by a stereotyped pathway. In the current experiments we investigated the cellular nature of the path taken by early emerging GnRH axons focusing on their relationship with astrocytic components and with the specialized ependymal population of this area, the tanycytes. In control tissue glial fibrillary acid protein (GFAP) immunoreactivity was confined to the exterior of cerebral blood vessels and glial limitans. Both GFAP and vimentin, another intermediate filament protein, marked the specialized ependymal cells of this region, the tanycytes. There was a robust reactive astrocytic response to the injury of transplantation in both the donor and host tissue within 5 days of implantation and the reactive astrocytes persisted for 60 days. These cells were GFAP-positive and were present in many areas of the host along the cannula tract and not confined to the area of GnRH axonal outgrowth. Vimentin, another intermediate filament, marked only the specialized ependymal cells of this region, the tanycytes, in both control and grafted tissue. Despite the profound reactive gliosis, GnRH axons were shown to exit the implant as early as 5 days after grafting suggesting that the gliotic process did not constitute a barrier to this phenomenon. At the light microscopic level, double label immunocytochemical studies did not reveal any specific association between GFAP or vimentin-positive cellular processes and these pioneer GnRH fibers. However, since normal GnRH axons had been reported to travel in tanycytic channels through the medial basal hypothalamus we reinvestigated the pattern of early emerging GnRH axons at the ultrastructural level. With this higher resolution, GnRH axons were found adjacent to glial elements along their entire traverse from the graft-host interface, through the host basal hypothalamus to their termination on the hypophysial portal capillaries. At the interface, GnRH-positive axons appeared to exit via glial channels similar to those described in other developing and regenerating systems. In the host, GnRH immunoreactive axonal profiles were surrounded by glial processes though the latter could not be further defined as tanycytic or astroglial. Other, immunonegative, axons were frequently seen in axonal bundles or fascicles and not necessarily in contact with glia.(ABSTRACT TRUNCATED AT 400 WORDS)

R-cadherin: a novel Ca(2+)-dependent cell-cell adhesion molecule expressed in the retina

cDNAs encoding a novel member of the cadherin cell adhesion receptor family were cloned. This cadherin is expressed in the retina of the chicken and is termed R-cadherin. It is similar to other cadherins in its primary structure, but most resembles N-cadherin, showing 74% amino acid identity. Cells expressing R-cadherin can adhere to those expressing N-cadherin when mixed, but they form homotypic clusters within their chimeric aggregates. In the development of the neural retina, R-cadherin begins to be expressed around embryonic day 8 in both neuronal and glial cells, and this expression continues up to the hatching stage. The pattern of the expression of R-cadherin was different from that of N-cadherin, suggesting distinctive roles in retinal morphogenesis.

Topographical features of the substratum for growth of pioneering neurons in the Manduca wing disc

The sensory neurons of the Manduca wing form a planar network nestled between the wing's upper and lower monolayers. The pioneering axons of this network grow in a distal-to-proximal direction over the basal surface of the upper epithelial monolayer. The basal surface of this monolayer has been examined ultrastructurally during the period of axonal outgrowth. The cellular terrain traversed by axons shows a graded distribution of epithelial processes, with the number of processes increasing in a proximal direction. Growth cones of axons, therefore, encounter increasing surface areas for contact with their substratum as they move toward the base of the wing. Because a basal lamina is laid down over these epithelial processes after axons have pioneered the neural pathways of the wing, axonal guidance cues apparently lie on surfaces of these basal processes. At branch points of the neural pathway examined in this study, axons avoid pathways in which the basal surfaces of cells in the upper wing monolayer interdigitate with basal surfaces of underlying tracheal cells. This interaction between wing epithelial cells and tracheal epithelial cells could act as a physical barrier to axonal outgrowth.

Aberrant axon growth of the chick embryo retina during normal development

Axon growth behavior in the optic nerve was examined using a carbocyanine dye, DiI, as a tracer, DiI facilitated clear visualization of the whole growth pattern of the optic nerve, i.e. the initial association of axons, fasciculated growth within the optic fiber layer and flattened growth cones in both living and fixed chick embryo retinae. Retrograde labelling with DiI in fixed retinae revealed that a considerable number of ganglion cells were apparently misdirected, extending their axons toward the periphery of the retina during normal development. The maximum proportion of aberrant ganglion cells reached about 15% of the total upon staining with a single DiI crystal. Misdirection was predominantly observed in retinae prepared from 6- to 8-day-old chick embryos. In embryos more than 9 days old, however, distinction of aberrant ganglion cells from normal ones became difficult, so that any degeneration of misdirected ganglion cells could not be clarified. Almost all of the misdirected ganglion cells were oriented centrifugally to the retinal periphery. These results indicate that misdirection occurs spontaneously during normal development even within the retina.

Evidence that the B2 chain of laminin is responsible for the neurite outgrowth-promoting activity of astrocyte extracellular matrix

Extracellular matrix (ECM) derived from cerebral cortical astrocytes stimulates neurite outgrowth from pheochromocytoma (PC12) cells in the absence of the classical nerve growth factor (NGF). We have shown here that astrocyte ECM can also stimulate neurite outgrowth from primary cultures of central nervous system (CNS) neurons. Using PC12 cells for a quantitative assay, we also demonstrated that the neurite growth-promoting activity increased as the astrocytes matured in vitro: ECM from older astrocytes (3-12 weeks in vitro) exhibited two-fold more neurite growth-promoting activity than ECM for younger astrocytes (5 days to 2 weeks in vitro). We applied various antibodies to identify the neurite growth-promoting factor of astrocyte ECM and found that anti-laminin inhibited neurite outgrowth by 50%, whereas anti-fibronectin and anti-NGF had no effect. Immunoblots, using laminin chain-specific antibodies, and cDNA hybridization of laminin mRNA demonstrated that cultured astrocytes synthesize only the B2 chain of laminin. This suggests that the B2 chain of laminin suffices to stimulate neurite outgrowth.

Development of the rat corticospinal tract through an altered glial environment

The major corticospinal tract (CST) in the rat is located at the base of the dorsal funiculus. It is a late-developing tract, and the growth of its axons into the lumbosacral region of the spinal cord does not occur until postnatal days 5 and 6. This delay is taken advantage of in this study in order to evaluate the effects of a markedly reduced glial population on ingrowth of the CST axons into the lumbosacral spinal cord. A reduction of the glial population is achieved by exposure of this region of spinal cord to X-radiation at 3 days of age. Growth of CST axons into and through the lumbosacral spinal cord in rats in which this region has undergone a radiation-induced depletion of glial cells is compared with that in their non-irradiated littermate controls by axonal tracing techniques using horseradish peroxidase (HRP). The HRP was applied directly to the motor cortices of normal and irradiated rats, and at all ages studied, there was anterograde filling of CST axons and their growth cones. At 3 days postnatally, the age when the lumbosacral spinal cord was irradiated in the experimental animals, CST axons were present in the more rostral thoracic levels. CST axons were observed in the lumbar region of non-irradiated rats on day 5, and by day 7 they were present at sacral levels.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of cell adhesion molecules in neurite outgrowth on Müller cells

The roles of neural cell adhesion molecule (NCAM), L1, N-cadherin, and integrin in neurite outgrowth on various substrates were studied. Antibodies against these cell surface molecules were added to explants of chick retina and the neurites from retinal ganglion cells were examined for effects of the antibodies on neurite length and fasciculation. On laminin, an anti-integrin antibody completely inhibited neurite outgrowth. The same antibody did not inhibit neurite outgrowth on polylysine or Müller cells. Antibodies to NCAM, L1, and N-cadherin did not significantly inhibit neurite outgrowth on laminin but produced significant inhibition on Müller cells. The inhibition of neurite outgrowth on glia by anti-L1 antibodies supports the hypothesis that L1 is capable of acting in a heterophilic binding mechanism. On laminin, both anti-N-cadherin and anti-L1 caused defasciculation of neurites from retinal ganglion cells, while anti-NCAM did not. None of these antibodies produced defasciculation on Müller cells. The results indicate that these three cell adhesion molecules may be very important in interactions with glia as axons grow from the retina to the tectum and may be less important in axon-axon interactions along this pathway. No evidence was found supporting the role of integrins in axon growth on Müller cells.

Neuronal potentialities of cells in the optic nerve of the chicken embryo are revealed in culture

Neuronal potentialities in neuroepithelial cells of the chicken embryonic optic nerve were studied in culture by using neurofilament antibodies as neuronal markers. Embryonic day-4 and -5 (E4 and E5) optic stalks were explanted in vitro. Within the first few days of culture, numerous morphologically identifiable neurons extending long neurites developed. These neurons and their processes were specifically labeled with neurofilament antibodies. Similar results were obtained by explanting only the medial portion of E7 optic stalks away from possibly contaminating cerebral or retinal tissue. To determine whether neuronal potentialities persisted at later embryonic stages, cultures of dissociated optic stalks were established at E11, E15, and E18. Neurons labeled with the various neurofilament antibodies appeared in all cultures of E11 and E15 optic stalks. However, typical neurons could not be recognized in cultures of E18 optic nerves. These results indicate that cells with neuronal potentialities are present in the embryonic optic nerve from early stages of development and persist until at least E15. Since the adult optic nerve is devoid of nerve cell bodies, our observations are consistent with the hypothesis that axons of retinal ganglion cells, which course through the optic stalk, repress neuronal potentialities within a subpopulation of precursor cells during normal development.

Selective growth of sensory nerve fibers on metal oxide pattern in culture

Metal oxides were used to study how the sensory nerve fibers recognize surface properties. Neurites selectively grow on the metal oxides deposited on silica glass, being guided along the axial direction of the patterns. The guiding ability depends on the electronegativity of the metal in metal oxide. Aluminum oxide or indium oxide patterns showed a remarkable ability to guide the growth direction. Neurites recognize the differences in surface properties (which are reflected by electronegativity) between metal oxides when the metal oxide substrata are only 1 micron in width.

Development of layer I and the subplate in the rat neocortex

Development of layer I and the subplate of the rat neocortex was examined with [3H]thymidine autoradiography. The experimental animals used for neurogenesis were the offspring of pregnant females injected with [3H]thymidine on 2 consecutive days: Embryonic Day (E) 13-E14, E14-E15, . . . E21-E22, respectively. On Postnatal Day 5, the proportion of layer I and subplate cells originating during 24-h periods were quantified at three anteroposterior levels. Presumptive Cajal-Retzius cells (large horizontal cells) are generated mainly on E14 and subplate cells on E14 and E15 ("outside-in" gradient). Both populations are generated earlier than cells in the cortical plate, which has an "inside-out" gradient. The subplate also has a ventrolateral/older to dorsomedial/younger neurogenetic gradient. The small- to medium-sized horizontal cells in layer I have an extensive period of neurogenesis with an "outside-in" gradient. To study morphogenesis, pregnant females were given single injections of [3H]-thymidine during gestation and embryos were removed in successive 24-h intervals (sequential-survival). On E15 and E16, cells accumulate outside the neuroepithelium in the primordial plexiform layer with older presumptive Cajal-Retzius cells superficial and younger presumptive subplate cells deep. The Cajal-Retzius cells permanently settle superficially among a first system of extracellular channels that appears on E14. Before reaching their final settling sites, subplate cells form the incipient cortical plate in the ventrolateral neocortex on E16. On E17, a seocnd system of extracellular channels appears below the cortical plate. On E18 and E19, subplate cells leave the cortical plate and permanently settle among the deep extracellular channels in a separate layer.

Monoclonal antibody markers for amphibian oligodendrocytes and neurons

Few immunocytochemical probes have been developed for cold-blooded vertebrates, thus hampering analyses of cellular processes in these species. Those developed from mammalian and avian tissue often fail either to react or to show similar specificities in poikilotherms. Therefore, we have begun raising monoclonal antibodies (mabs) in mice against frog and tadpole brain tissue. The following analyses of two of these mabs suggest that these antibodies represent specific probes for frog axons and oligodendrocytes. Mab Olig recognizes all the myelinated axon tracts of the mature frog brain and spinal cord, as well as the tracts of the developing tadpole CNS once they have become myelinated. Axons cut in cross section show characteristic o-shaped staining around individual axons when processed with this antibody. Particularly easy to visualize in the tadpole are immunoreactive cell bodies and processes, seen in continuity with the myelin sheath. Occasionally, in this developing tissue, cells with highly branched processes characteristic of immature oligodendrocytes are observed. No other cells or processes within the brain or spinal cord react with this antibody. Mab Linc stains numerous filaments in all axonal projections. Occasionally, a thin rim of filamentous staining is observed in cell somata, but many regions rich in neuronal somata or dendrites are unreactive to this antibody. This in vivo staining pattern suggests that the Linc antigen is differentially distributed within neurons and exhibits a high concentration in axons. Linc immunoreactivity is robust in the processes of a subpopulation of dissociated tectal cells in culture. These Linc-positive cells are characterized as neurons on morphological criteria. Also, intense Linc immunoreactivity is present in the bundles of retinal axons that extend from retinal explants. Olig immunoreactivity, however, has not been detected in tectal cultures or retinal explants. Improved staining following Triton X-100 treatment of tissue sections suggests that neither of the mabs recognizes lipid antigens and that both are probably localized within the cell cytoplasm. Only the Linc mab reacts on Western blots of denatured brain protein. Linc consistently recognizes two Triton X-100-insoluble proteins with apparent molecular weights of 56 and 58 kD. The epitopes recognized by the Olig and Linc mabs have been surveyed in terms of their resistance to optic nerve crush and their consequent value in studies requiring such procedures. Possible homologies to known cell-type-specific molecules are discussed.