Response of astrocytes in the visual system to Wallerian degeneration: an immunohistochemical analysis of laminin and glial fibrillary acidic protein (GFAP)

Eph/ephrin expression in the adult rat visual system following localized retinal lesions: localized and transneuronal up-regulation in the retina and superior colliculus

Following unilateral optic nerve section in adult PVG hooded rat, the axon guidance cue ephrin-A2 is up-regulated in caudal but not rostral superior colliculus (SC) and the EphA5 receptor is down-regulated in axotomised retinal ganglion cells (RGCs). Changes occur bilaterally despite the retino-collicular projection being mostly crossed. Here we investigate the dynamics of Eph/ephrin expression using in situ hybridization and semi-quantitative immunohistochemistry after localized retinal lesions. Unilateral krypton laser lesions to dorso-nasal retina ablated contralaterally projecting RGCs (DN group); ventro-temporal lesions ablated contralaterally and ipsilaterally projecting RGCs (VT group). Lesions of the entire retina served as controls (Total group). Results are compared to normal animals in which tectal ephrin-A2 and retinal EphA5 are expressed, respectively, as shallow ascending rostro-caudal and naso-temporal gradients. In both SCs of DN and Total groups, tectal ephrin-A2 was up-regulated caudally; in the VT group, expression remained normal bilaterally. Unilateral collicular ablation indicated that bilateral changes in ephrin-A2 expression are mediated via intercollicular pathways. EphA5 expression in the VT group was elevated in the intact nasal region of experimental retinae. For each experimental group, EphA5 expression was also elevated in nasal retina of the opposite eye, resulting in uniform expression across the naso-temporal axis. Up-regulation of ephrin-A2 in caudal, but not rostral, SC suggests the enhancement of developmental positional information as a result of injury. Bilateral increases in retinal EphA5 expression demonstrate that signals for up-regulation operate interocularly. The study demonstrates that signals regulating guidance cue expression are both localized and relayed transneuronally.

Changing therapeutic paradigms in glaucoma management

Glaucoma is a family of diseases commonly characterised by progressive optic neuropathy with associated visual field deficits for which elevated intraocular pressure (IOP) is one of the primary risk factors. For more than a century the main goal of glaucoma management has been to eliminate the risk associated with elevated IOP. In recent years, accumulating evidence of pressure-independent causes of glaucomatous optic neuropathy has led to the recognition that lowering IOP alone may often be insufficient for the long-term preservation of visual function. An innovative therapeutic approach is now emerging to prevent progression of glaucomatous optic neuropathy and preserve vision, irrespective of disease aetiology: direct protection of the optic nerve. In addition to reducing the risk associated with elevated IOP, this neuroprotective approach will augment the overall goal of preserving the optic nerve through direct promotion of retinal ganglion cell (RGC) survival and/or prevention of RGC death. Although no currently available compounds have been clinically demonstrated to provide neuroprotective benefit in glaucoma, recent preclinical studies have shown that alpha-adrenergic agonists, such as brimonidine, provide neuroprotective benefits, as well as excellent IOP lowering efficacy. In addition, new agents with promising neuroprotective utility that are emerging from other studies are now being investigated for efficacy in glaucoma. The review discusses recently introduced compounds and new drugs in development with regard to their potential value in conventional and/or neuroprotective strategies for vision sparing in glaucoma.

Sequential loss of myelin proteins during Wallerian degeneration in the rat spinal cord

Axotomy of nerve fibers leads to the subsequent degeneration of their distal part, a process termed Wallerian degeneration (WD). While WD in the peripheral nervous system is usually followed by regeneration of the lesioned axons, central nervous system (CNS) neurons are generally unable to regrow. In this study, we investigated the process of WD in the dorsal columns of the rat spinal cord rostral to a mid-thoracic lesion. We confirm earlier studies describing a very delayed microglial and an early and sustained astroglial reaction finally leading to scar formation. Interestingly, we found a differential time course in the loss of myelin proteins depending on their location. Proteins situated on the periaxonal myelin membrane such as myelin associated glycoprotein disappeared early, within a few days after lesion, concomitantly with cytoskeletal axonal proteins, whereas compact myelin and outer myelin membrane proteins such as MBP and Nogo-A remained for long intervals in the degenerating tracts. Two distinct mechanisms are probably responsible for this difference: processes of protein destruction emanating from and initially probably located in the axon act on a time scale of 1-3 days. In contrast, the bulk of myelin destruction is due to phagocytosis known to be slow, prolonged, and inefficient in the CNS. These results may also have implications for future intervention strategies aiming at enhancing CNS regeneration.

Injury to retinal ganglion cells induces expression of the small heat shock protein Hsp27 in the rat visual system

Optic nerve transection results in apoptotic cell death of most adult rat retinal ganglion cells that begins at 4 days and leaves few surviving neurons at 14 days post-injury [Berkelaar et al. (1994) J. Neurosci. 14, 4368-4374]. The small heat shock protein Hsp27 has recently been shown to play a role in sensory neuron survival following peripheral nerve axotomy [Lewis et al. (1999) J. Neurosci. 19, 8945-8953]. To investigate the role of Hsp27 in injured CNS sensory neurons, we have studied the induction and cell-specific expression of Hsp27 in rat retinal ganglion cells 1-28 days after optic nerve transection. Immunohistochemical results indicate that Hsp27 is not present at detectable levels in the ganglion cell layer of control (uninjured) or sham-operated control rats. In contrast, Hsp27 is detected in retinal ganglion cells from 4 to 28 days following axotomy. Furthermore, the percentage of surviving retinal ganglion cells that are Hsp27-positive increased over the same time period. Hsp27 is also detected in glial fibrillary acidic protein-positive astrocytes in the optic layer of the superior colliculus from 4 to 28 days after optic nerve transection. These experiments demonstrate that transection of the optic nerve results in the expression of Hsp27 in three distinct regions of the rat visual system: sensory retinal ganglion cells in the eye, glial cells of the optic tract, and astrocytes in the optic layer of the superior colliculus. Hsp27 may be associated with enhanced survival of a subset of retinal ganglion cells, providing evidence of a protective role for Hsp27 in CNS neuronal injury.

Plastic reaction of the rat visual corticocollicular connection after contralateral retinal deafferentiation at the neonatal or adult stage: axonal growth versus reactive synaptogenesis

The effects of neonatal or adult enucleation on the final adult pattern of the rat visual corticocollicular (C-Co) connection were studied using the anterograde tracer biotinylated dextranamine 10,000 (BDA) iontophoretically injected in the primary visual cortex. In control animals, column-shaped terminal fields limited to a small portion of the collicular surface were observed. Synaptic boutons were present in all superficial strata of the superior colliculus (SC), with the highest density in the ventral part of the stratum griseum superficiale (SGS). Neonatal enucleation caused a considerable expansion of the contralateral visual C-Co terminal fields, which occupied almost the entire collicular surface, suggesting that axonal sprouting had occurred. In addition, terminal boutons tended to localize more dorsally in these cases compared with controls. Following enucleation in adult animals, no changes were observed with respect to the extension of the terminal fields, although a plastic reaction leading to an increase in the bouton density in the stratum zonale (SZ) and upper SGS was found, reflecting a process of reactive synaptogenesis at these levels. These results show that both neonatal and adult visual C-Co fibers react in response to retinal ablation, although this reaction shows distinct characteristics. Molecular factors, such as growth-associated cytoskeletal proteins operating in the cortical origin, and extracellular matrix components and myelin-associated axonal growth inhibitors acting on the collicular target very likely account for these differences.

Increased expression of GABA transporters, GAT-1 and GAT-3, in the deafferented superior colliculus of the rat

GABA transporters (GATs) play a critical role in the translemmal transport of GABA in neurons and glial cells. Two major brain GATs, GAT-1 and GAT-3, are found in astrocytes in the adult brain. Astroglia demonstrate morphological and molecular changes in response to brain injury and deafferentation. The present study was designed to determine whether the expression of GATs changes after nerve deafferentation using the rat superior colliculus (SC) as a model. The immunoreactivity for GAT-1 and GAT-3, as well as GABA and glutamic acid decarboxylase (GAD)-65 and GAD-67, was studied in the SC of control rats and rats with unilateral optic nerve transections. Immunolabeling for both GAT-1 and GAT-3 was increased in the neuropil of the denervated SC as compared to that for the SC of control rats or for the unaffected SC of experimental rats. In contrast, immunoreactivity for GABA, GAD-65 and GAD-67 was not altered. The change in the immunolabeling of GAT-1 and GAT-3 was detectable at 1 day postlesion and became more evident between 3-30 days postlesion. At the electron microscopic level, immunoreactivity for both GAT-1 and GAT-3 in the unaffected SC was localized to astrocytic processes, whereas GAT-1 immunolabeling was also present in synaptic terminals. In the deafferented SC, immunolabeling for both GATs was elevated in the somata and processes of hypertrophied astrocytes as compared to that in the unaffected SC, whereas GAT-1 labeling in neuronal profiles was largely unchanged. A substantial increase of GAT-1 and GAT-3 in astrocytes following optic nerve transection suggests that these cells play a role in modulating GABA's action in the deafferented SC.

The astroglial response to Wallerian degeneration after spinal cord injury in humans

We describe the changes exhibited by astrocytes in areas of Wallerian degeneration after spinal cord injury in humans using glial fibrillary acidic protein immunohistochemistry correlated to standard histology at time points ranging from 8 days to 23 years after injury. Astrocytes were slow to react; a slight increase in immunoreactivity was observed at 4 months. Over time they began to lose immunoreactivity in both the somata and the processes as the debris from the degenerative process was cleared. By 1 year after injury the staining intensity had decreased to levels which were lower than in normal areas of the cord. This hypointense staining persisted for at least 23 years after injury. These findings are significantly different from those observed in animal studies and emphasize the need for additional pathological studies of human spinal cord injury.

Relationship between injury-induced astrogliosis, laminin expression and axonal sprouting in the adult rat brain

Lesion-induced regenerative sprouting of CNS axons is accompanied by structural and metabolic changes of astrocytes. In order to evaluate the effects of these astrocytic changes on axonal regeneration, we investigated the spatio-temporal relationship of gliosis, laminin expression and axonal sprouting in the postcommissural fornix of the adult rat. Using immunocytochemical methods we observed (1) a perilesional area with a transient lack of astrocytes and axons, (2) the reappearance of reactive astrocytes followed by the ingrowth of sprouting fibres and finally an increase in laminin-immunoreactivity, (3) the absence of lesion-induced laminin-expression in reactive astrocytes and (4) the formation and long-lasting (at least 28 months) persistence of a dense plexus of laminin-immunopositive blood vessels at the site of transection and in the proximal and distal stumps. These data indicate that astrogliosis is permeable for regrowing axons and that injury-induced axonal sprouting in the transected postcommissural fornix may be mediated by laminin-independent mechanisms.

Immunohistochemical staining for glial fibrillary acidic protein (GFAP) after deafferentation or ischemic infarction in rat visual system: features of reactive and damaged astrocytes

Immunohistochemical staining for glial fibrillary acidic protein (GFAP) is standard for visualization of reactive astrocytes in tissue sections, whereas various forms of astrocytic damage remain to be described in detail. In this study we tested differences in GFAP labeling in reactive astrocytes and in glial cells damaged by ischemia and edema. Studies were performed in the anatomically well defined visual system of rat. Basic staining patterns for GFAP were established in subcortical visual nuclei and visual cortex. In the first model, deafferentation of visual centers was performed by unilateral optic nerve lesion, and characteristic changes of GFAP labeling in reactive astrocytes were studied at 0.5, 1, 1.5, 2, 4, 8 and 21 days after lesion. Initial changes were seen in the deafferented superior colliculus at 1 day after deafferentation with a diffuse increase and stellate types of reactive cells formed at 2-8 days. In the second model, small ischemic infarcts were produced in the visual cortex of rats using the method of photochemically-induced thrombosis. GFAP labeling with a polyclonal antiserum was massively enhanced in the infarct at 4 hr. Characteristic morphological changes in damaged astrocytes were seen which were also identified in experiments with simulated global ischemia. In the surround of the infarct, swelling of astrocytes also caused increased labeling. At 3-4 days infarction typical reactive astrocytes surrounded the lesioned area. In conclusion, these immunohistochemical studies on GFAP in rat visual system allow for the following classifications. (a) Normal astrocytes vary in labeling at different anatomical localizations. (b) Reactive astrocytes show enhanced labeling and larger cell-size within an interval of 1-2 days after lesion. (c) Astrocytes damaged by ischemia reveal increased labeling of disintegrating cellular elements within hours after a lesion. (d) Swollen astrocytes undergo enhanced labeling in areas with vasogenic edema.

Immunohistochemical changes of neuronal calcium-binding proteins parvalbumin and calbindin-D-28k following unilateral deafferentation in the rat visual system

The neuron-specific calcium-binding proteins, parvalbumin and calbindin-D-28k, were studied in the subcortical visual system of normal and unilaterally deafferented albino rats. Immunohistochemistry with monoclonal antibodies was used on vibratome sections through optic tract (OT), dorsal lateral geniculate nucleus (dLGN), olivary pretectal nucleus (OPN), and superior colliculus (SC). In controls, OT stained strongly for parvalbumin and weakly for calbindin-D-28k. The dLGN contained a plexus of parvalbumin-positive fibers. In dLGN, calbindin-D-28k-antibodies showed strong labeling of some neurons with long dendrites and weak staining of the cytoplasm in other neurons. In OPN, parvalbumin stained a ring of neurons and terminals in the shell region, whereas calbindin-D-28k was contained in medial cell populations. In SC, parvalbumin was contained in fibers, terminals, and neurons throughout the visual layer. Calbindin-D-28k showed a laminar distribution of neurons with a predominance in deep portions of superficial grey matter and in ventral portions of stratum opticum. Following unilateral deafferentation induced by optic nerve section, retinal axons showed immunohistochemical changes related to Wallerian degeneration and target neurons reacted by changes of calcium-binding proteins. Parvalbumin and calbindin-D-28k immunostaining decreased during Wallerian degeneration of OT. In the deafferented dLGN, immunohistochemical labeling for calbindin-D-28k declined in strongly stained neurons from 4 to 21 days after lesion. Measurement of dendritic length per number of cells or per area of dLGN showed a significant decline for the contralateral side at 4, 8, and 21 days (ANOVA, P less than 0.05). In deafferented OPN, terminal-like staining for parvalbumin decreased and neuronal labeling was enhanced. In deafferented SC, the neuronal and dendritic staining for parvalbumin increased beginning from Day 1 on and persisting at Day 21, whereas fibers and terminal-like elements decreased in staining. Measurement of parvalbumin-positive neurons per area of SC showed a significant increase of labeling in the contralateral side from Day 1 to Day 21 (ANOVA, P less than 0.05). These studies show that cellular responses to deafferentation of visual neurons involve a regulation of calcium-binding proteins. The decline in staining for calbindin-D-28k in dLGN may relate to reduced retinal afferent activity. The progressive cellular changes in parvalbumin staining may be related to unmasking of intrinsic neurons after removal of parvalbumin-containing, afferent fibers and terminals. Additionally, the changes of parvalbumin labeling in SC neurons may reflect a plastic reorganization of local circuits known to occur in rat SC in response to deafferentation.

Laminin-like antigen in rat CNS neurons: distribution and changes upon brain injury and nerve growth factor treatment

Using several antibodies against rat or human laminin and an avidin-biotin immunocytochemical protocol, laminin-like immunoreactivity was detectable in the rat nervous system in expected locations, i.e., associated with blood vessels and reactive astrocytes. However, laminin staining was also abundantly present within neuronal cell bodies in most parts of the developing and adult rat CNS. Medial septum neuronal immunoreactivity was lost after septo-hippocampal disconnection, but could be preserved or even restored by intraventricular administration of nerve growth factor. Thus, at least for medial septum neurons, this laminin-like molecule can be accumulated or produced independent of direct hippocampal (target) contact. It remains to be determined whether CNS neuronal "laminin" processes activities similar to those found for laminin in vitro.

Astroglial cell alteration caused by neurotoxins: immunohistochemical observations with antibodies to glial fibrillary acidic protein, laminin, and tyrosine hydroxylase

Kainic acid or 6-hydroxydopamine (6-OHDA) was injected into rat striatum, and their effects on astrocytes, laminin, and catecholamine fibers were examined temporally by immunohistochemical methods in an attempt to understand the roles of reactive astrocytes and laminin on the restoration of central nervous tissue. Kainic acid injection caused a severe neuronal degeneration in the striatum but catecholamine fibers were spared with only transient loss of tyrosine hydroxylase immunoreactivity. Reactive astrocytes appeared around the lesioned area soon after the kainic acid injection, then migrated into that area, and finally covered the lesioned striatum. Laminin immunoreactivity was found only in the lesioned area before the migration of reactive astrocytes and disappeared when the area was covered by astrocytes. 6-OHDA injection, on the other hand, resulted in a severe degeneration of catecholamine fibers, but striatal neurons were mostly spared. From 7 to 28 days after injection, regenerating fibers were found to enter the affected region. In this period reactive astrocytes were seen in the affected region but were only slightly more numerous than those found in control (saline injected) striatum. Laminin-immunoreactive blood vessels seemed to show a distribution similar to that in control striatum. These observations indicate that reactive astrocytes may play an important role in areas of neuronal cell loss and that laminin may aid their migration into such areas. Laminin and reactive astrocytes may not, however, be essential for the regeneration of dopamine fibers.

Critical period-dependent alterations of the transient body image in the rodent cerebral cortex

The present study demonstrates that the boundary patterns of cell surface-associated molecules detected with lectins in the barrel cortex of neonatal rodents are altered, as are the boundary patterns of cortical glia, following perturbation of large vibrissae in the contralateral mystacial face pad. The alterations in the transiently expressed molecular patterns of lectin-receptors provide data that are consistent with the idea that the periphery plays a prominent role in the establishment of functional cytoarchitecture in the developing cortex. The data are also consistent, however, with the notion that factors intrinsic to the cerebrum, such as the immature cortical glial cells, are of considerable importance in this respect and a direct or indirect interaction of thalamocortical afferents with glial cells in the somatosensory cortex of the neonate are indicated. It is suggested therefore that a critical period in early barrel development, a time in which the cortical neuronal architecture is malleable in response to altered afferent input, is directly related to the presence of these cellular and molecular boundaries. The transient barrel boundaries, it is argued, are the morphological and molecular substrates that form the physical basis of the critical period.

Conditioned media of regenerating fish optic nerves modulate laminin levels in glial cells

Adult injured optic nerves of mammals show an increased distribution of laminin in the extracellular matrix after application of soluble substances in the form of conditioned media originating from growing nerves (Zak, et al., 1987). This increase could result from direct or an indirect activation of glial cells or from activation of other cellular elements. In the present study, we show that the conditioned media derived from regenerating fish optic nerves contain factors that directly modulate the level of laminin in C-6 glioma cells. The identity of the laminin was confirmed by metabolic labeling of the treated cells with [35S]methionine and by subsequent immunoprecipitation and gel electrophoresis. The level of other extracellular matrix proteins, such as fibronectin, is also increased. The significance of the presence of glial-activating substances in the conditioned media of regenerating nerves for the process of regeneration is discussed.

Patterns of gliosis in Alzheimer's disease and aging cerebrum

The distribution of astrocytic gliosis in Alzheimer's disease (AD) and aging cerebrum, as marked by immunoperoxidase staining for glial fibrillary acidic protein (GFAP), was examined in whole-hemisphere coronal sections. Cortical gliosis in AD had an obvious laminar pattern. There were two heavy bands of staining, one in layers II-III and another in layer V. Normal aging cases sometimes displayed considerable cortical gliosis, but no specific patterns were apparent. Most AD cases, and some normal aging cases, displayed hypertrophy of immunoreactive astrocytes at grey matter-white matter interfaces, especially the cortico-medullary junction. Subcortical grey matter gliosis was common in both normal aging and AD, but there was no consistent pattern in either group. The deep cerebral white matter, which is stained evenly and heavily in young, healthy individuals, showed uneven staining in both normal elderly and AD brains. In both AD and aging, perivascular gliosis was prominent throughout the cerebrum and especially in the putamen. In conclusion, both AD and aging cerebri show extensive gliosis: AD cortical gliosis has a specific laminar pattern, but there does not appear to be an AD-specific pattern of subcortical gliosis.

Astrocytes are important for sprouting in the septohippocampal circuit

Damage to the fimbria-fornix, and separately to the perforant path, leads to distinct and dramatic time-dependent increases in glial fibrillary acidic protein immunoreactivity (GFAP-IR) in specific areas of the hippocampal formation. Specifically, fimbria-fornix lesions resulted in an increase in the GFAP-IR in the pyramidal and oriens area of the CA3 as well as the inner molecular layer of the dentate gyrus. In addition, in the septum ipsilateral to the lesion, there was a rapid and robust increase in GFAP-IR in the dorsal lateral quadrant of the septum, but not in the medial region. Only after 30 days did the GFAP-IR reach the medial septum. Following perforant path lesions, there was a selective increase in GFAP-IR in the outer molecular layer of the dentate gyrus. Most of these changes were transient and had disappeared by 30 days postlesion. We speculate that the increase in GFAP-IR in these target areas is a necessary requirement for the sprouting responses that are observed. This hypothesis is supported by the fact that astrocytes secrete NGF in vitro and that NGF activity increases in these target areas following these same lesions. A mechanism for the selective activation of the astrocytes through the initial activation of microglia and secretion of interleukin-1 is postulated.

Direct evidence that complex experience increases capillary branching and surface area in visual cortex of young rats

Rats housed in complex environments with toys and other rats generate new synapses, and the expanding neuropil tends to spread apart existing blood vessels. Previous work demonstrated that weanling rats kept in complex environments had more closely packed capillaries, suggesting that new capillaries had sprouted into the newly added neuropil. The present study directly investigates the issue of new branching by using india ink perfusions of weanling rats kept for 30 days in a complex environment (EC), paired in standard caging (SC), or individual cages (IC) to examine the density of capillary branch points and the capillary surface area per unit tissue volume. EC rats had a greater density of branch points than the SC and IC littermates, a finding consistent with increased capillary sprouting. Capillary surface area per unit tissue volume and the number of branch points per unit of capillary surface area were also higher for EC rats. This suggests that blood vessels of EC rats branch off more often than those of animals kept in more standard conditions, and provides further evidence that complex experience can increase angiogenesis in cerebral cortex of postweanling rats.

Laminin: molecular organization and biological function

Laminin, the most abundant glycoprotein molecule found in basement membrane, has multiple functions in eukaryotic tissues. It serves to attach epithelial cells to basement membrane, aids development and migration of specific cell types in growth and maturation, and has been implicated in tumor metastasis and some types of infection. Current concepts of the molecular organization and myriad functions of the laminin molecule are reviewed.

Laminin and heparan sulphate proteoglycan in the lesioned adult mammalian central nervous system and their possible relationship to axonal sprouting

Several in vitro studies indicate that the extracellular matrix (ECM) glycoprotein laminin can promote neurite outgrowth from CNS (central nervous system) neurons. Laminin has been detected immunohistochemically in astrocytes in the embryonic but not the uninjured adult mammalian CNS. In the injured adult CNS, it is found in some reactive astrocytes located near the site of CNS lesions. In the present study, we have attempted to examine the relationship between these laminin+ astrocytes and the axonal sprouting that occurs after CNS injuries. This was studied in the intracranially transected adult rat optic nerve which consists of a cranial segment devoid of all retinal ganglion cell axons, and a retinal segment attached to the retina which contains some viable axons that undergo sprouting. Laminin+ reactive astrocytes were found in the cranial segment, but not in the retinal segment. In addition, the cut ends of the retinal and cranial segments were capped by an intensely laminin+, glial fibrillary acidic protein negative (GFAP) region. Axonal sprouts from the transected retinal ganglion cell axons, identified by anterogradely transported rhodamine isothiocynate (RITC), were confined to laminin-, GFAP+ regions of the retinal segment. These results suggest that injury-induced axonal sprouting in the adult mammalian CNS in vivo may be promoted by molecules other than laminin, that may be associated with astrocytes. The presence of heparan sulphate proteoglycan HSP G was also examined in the transected optic nerve because the neurite outgrowth promoting factors found in conditioned media derived from several cell types in vitro have been shown to consist of a complex of laminin and heparan sulphate proteoglycan. No significant changes in heparan sulphate proteoglycan-like immunoreactivity was observed after transection. The presence of laminin and HSPG were also examined in the lesioned adult rat cerebral cortex.

N-cadherin and integrins: two receptor systems that mediate neuronal process outgrowth on astrocyte surfaces

Receptor-mediated interactions between neurons and astroglia are likely to play a crucial role in the growth and guidance of CNS axons. Using antibodies to neuronal cell surface proteins, we identified two receptor systems mediating neurite outgrowth on cultured astrocytes. N-cadherin, a Ca2(+)-dependent cell adhesion molecule, functions prominently in the outgrowth of neurites on astrocytes by E8 and E14 chick ciliary ganglion (CG) neurons. beta 1-class integrin ECM receptor heterodimers function less prominently in E8 and not at all in E14 neurite outgrowth on astrocytes. The lack of effect of integrin beta 1 antibodies on E14 neurite outgrowth reflects an apparent loss of integrin function, as assayed by E14 neuronal attachment and process outgrowth on laminin. N-CAM appeared not to be required for neurite outgrowth by either E8 or E14 neurons. Since N-cadherin and integrin beta 1 antibodies together virtually eliminated E8 CG neurite outgrowth on cultured astrocytes, these two neuronal receptors are probably important in regulating axon growth on astroglia in vivo.