Changes in astroglial scar formation in rat optic nerve as a function of development

Developing Extracellular Matrix Technology to Treat Retinal or Optic Nerve Injury(1,2,3)

Adult mammalian CNS neurons often degenerate after injury, leading to lost neurologic functions. In the visual system, retinal or optic nerve injury often leads to retinal ganglion cell axon degeneration and irreversible vision loss. CNS axon degeneration is increasingly linked to the innate immune response to injury, which leads to tissue-destructive inflammation and scarring. Extracellular matrix (ECM) technology can reduce inflammation, while increasing functional tissue remodeling, over scarring, in various tissues and organs, including the peripheral nervous system. However, applying ECM technology to CNS injuries has been limited and virtually unstudied in the visual system. Here we discuss advances in deriving fetal CNS-specific ECMs, like fetal porcine brain, retina, and optic nerve, and fetal non-CNS-specific ECMs, like fetal urinary bladder, and the potential for using tissue-specific ECMs to treat retinal or optic nerve injuries in two platforms. The first platform is an ECM hydrogel that can be administered as a retrobulbar, periocular, or even intraocular injection. The second platform is an ECM hydrogel and polymer "biohybrid" sheet that can be readily shaped and wrapped around a nerve. Both platforms can be tuned mechanically and biochemically to deliver factors like neurotrophins, immunotherapeutics, or stem cells. Since clinical CNS therapies often use general anti-inflammatory agents, which can reduce tissue-destructive inflammation but also suppress tissue-reparative immune system functions, tissue-specific, ECM-based devices may fill an important need by providing naturally derived, biocompatible, and highly translatable platforms that can modulate the innate immune response to promote a positive functional outcome.

Spinal cord regeneration

Three theories of regeneration dominate neuroscience today, all purporting to explain why the adult central nervous system (CNS) cannot regenerate. One theory proposes that Nogo, a molecule expressed by myelin, prevents axonal growth. The second theory emphasizes the role of glial scars. The third theory proposes that chondroitin sulfate proteoglycans (CSPGs) prevent axon growth. Blockade of Nogo, CSPG, and their receptors indeed can stop axon growth in vitro and improve functional recovery in animal spinal cord injury (SCI) models. These therapies also increase sprouting of surviving axons and plasticity. However, many investigators have reported regenerating spinal tracts without eliminating Nogo, glial scar, or CSPG. For example, many motor and sensory axons grow spontaneously in contused spinal cords, crossing gliotic tissue and white matter surrounding the injury site. Sensory axons grow long distances in injured dorsal columns after peripheral nerve lesions. Cell transplants and treatments that increase cAMP and neurotrophins stimulate motor and sensory axons to cross glial scars and to grow long distances in white matter. Genetic studies deleting all members of the Nogo family and even the Nogo receptor do not always improve regeneration in mice. A recent study reported that suppressing the phosphatase and tensin homolog (PTEN) gene promotes prolific corticospinal tract regeneration. These findings cannot be explained by the current theories proposing that Nogo and glial scars prevent regeneration. Spinal axons clearly can and will grow through glial scars and Nogo-expressing tissue under some circumstances. The observation that deleting PTEN allows corticospinal tract regeneration indicates that the PTEN/AKT/mTOR pathway regulates axonal growth. Finally, many other factors stimulate spinal axonal growth, including conditioning lesions, cAMP, glycogen synthetase kinase inhibition, and neurotrophins. To explain these disparate regenerative phenomena, I propose that the spinal cord has evolved regenerative mechanisms that are normally suppressed by multiple extrinsic and intrinsic factors but can be activated by injury, mediated by the PTEN/AKT/mTOR, cAMP, and GSK3b pathways, to stimulate neural growth and proliferation.

Traumatology of the optic nerve and contribution of crystallins to axonal regeneration

Within a few decades, the repair of long neuronal pathways such as spinal cord tracts, the optic nerve or intracerebral tracts has gone from being strongly contested to being recognized as a potential clinical challenge. Cut axonal stumps within the optic nerve were originally thought to retract and become irreversibly necrotic within the injury zone. Optic nerve astrocytes were assumed to form a gliotic scar and remodelling of the extracellular matrix to result in a forbidden environment for re-growth of axons. Retrograde signals to the ganglion cell bodies were considered to prevent anabolism, thus also initiating apoptotic death and gliotic repair within the retina. However, increasing evidence suggests the reversibility of these regressive processes, as shown by the analysis of molecular events at the site of injury and within ganglion cells. We review optic nerve repair from the perspective of the proximal axon stump being a major player in determining the successful formation of a growth cone. The axonal stump and consequently the prospective growth cone, communicates with astrocytes, microglial cells and the extracellular matrix via a panoply of molecular tools. We initially highlight these aspects on the basis of recent data from numerous laboratories. Then, we examine the mechanisms by which an injury-induced growth cone can sense its surroundings within the area distal to the injury. Based on requirements for successful axonal elongation within the optic nerve, we explore the models employed to instigate successful growth cone formation by ganglion cell stimulation and optic nerve remodelling, which in turn accelerate growth. Ultimately, with regard to the proteomics of regenerating retinal tissue, we discuss the discovery of isoforms of crystallins, with crystallin beta-b2 (crybb2) being clearly upregulated in the regenerating retina. Crystallins are produced and used to promote the elongation of growth cones. In vivo and in vitro, crystallins beta and gamma additionally promote the growth of axons by enhancing the production of ciliary neurotrophic factor (CNTF), indicating that they also act on astrocytes to promote axonal regrowth synergistically. These are the first data showing that axonal regeneration is related to crybb2 movement within neurons and to additional stimulation of CNTF. We demonstrate that neuronal crystallins constitute a novel class of neurite-promoting factors that probably operate through an autocrine and paracrine mechanism and that they can be used in neurodegenerative diseases. Thus, the post-injury fate of neurons cannot be seen merely as inevitable but, instead, must be regarded as a challenge to shape conditions for initiating growth cone formation to repair the damaged optic nerve.

Focal cerebral ischemia upregulates SHP-1 in reactive astrocytes in juvenile mice

The role of the tyrosine phosphatase SHP-1 in the hematopoietic system has been well studied; however, its role in the central nervous system (CNS) response to injury is not well understood. Previous studies in our laboratory have demonstrated increased immunoreactivity for SHP-1 in a subset of reactive astrocytes that do not appear to enter the cell cycle following deafferentation of the chicken auditory brainstem. In order to determine whether mammalian astrocytes also upregulate SHP-1 immunoreactivity following CNS injury, a mouse model of focal cerebral ischemia was utilized to study SHP-1 expression. The brains of 3-week-old mice were analyzed at four time points following permanent middle cerebral artery occlusion (MCAO): 1, 3, 7, and 14 days. Our results demonstrate consistent infarct volumes within surgical groups, and infarct volumes decrease as a function of time from 1 day (maximum infarct volume) to 14 days (minimum infarct volume) post-MCAO. In addition, SHP-1 protein levels are upregulated following cerebral ischemia and this increase peaks at 7 days post-MCAO. Analysis of confocal images further reveals that immunoreactivity for SHP-1 occurs predominantly in GFAP+ reactive astrocytes, although a small percentage of F4-80+ microglia are also double labeled for SHP-1 at early times post-MCAO. These SHP-1+ reactive astrocytes do not appear to enter the cell cycle (as defined by PCNA immunoreactivity), confirming our previous studies in the avian auditory brainstem. These results suggest that SHP-1 plays an important role in the regulation of glial activation and proliferation in the ischemic CNS.

Extent of retinal ganglion cell death in the frog Litoria moorei after optic nerve regeneration induced by lesions of different sizes

Some amphibian retinal ganglion cells die during optic nerve regeneration. Here we have investigated whether ganglion cell death in the frog Litoria moorei is associated with the lesion site. For one experimental series, the optic nerve lesion extended for 0.15 mm; in the other, it extended for 1.5 mm. The extent of ganglion cell death was estimated from cresyl violet-stained whole mounts at 24 weeks post lesion. In other animals, individual regenerating axons were visualised in the optic nerve by horseradish peroxidase (HRP) labelling from 1 day to 24 weeks post lesion; counterstaining with cresyl violet allowed examination of cells that repopulated the lesion site. Ganglion cell numbers fell significantly more after an extensive than after a localised lesion, long-term losses being 50% and 34%, respectively (P < 0.05). Regenerating axons were delayed in their passage across the cell-poor extensive lesion compared with the relatively cell-rich localised lesion. The differing rates of regeneration between series were matched by greater delay after extensive lesion in the return of visually guided behaviour as assessed by optokinetic horizontal head nystagmus. We suggest that delays in regeneration after an extensive lesion exacerbate ganglion cell death, indicating that conditions within the lesion are associated with the death of some ganglion cells.

Genetic dysmyelination alters the molecular architecture of the nodal region

We have examined the molecular organization of axons in the spinal cords of myelin-deficient (md) rats, which have profound CNS dysmyelination associated with oligodendrocyte cell death. Although myelin sheaths are rare, most large axons are at least partially surrounded by oligodendrocyte processes. At postnatal day 7 (P7), almost all node-like clusters of voltage-gated Na+ channels and ankyrinG are adjacent to axonal segments ensheathed by oligodendrocytes, but at P21, many node-like clusters are found in axonal segments that lack oligodendrocyte ensheathment. In P21 wild-type (WT) rats, the voltage-gated Na+ channels Na(v)1.2, Na(v)1.6, and Na(v)1.8, are found in different subpopulations of myelinated axons, and md rats have a similar distribution. The known molecular components of paranodes--contactin, Caspr, and neurofascin 155--are not clustered in md spinal cords, and no septate-like junctions between oligodendrocyte processes and axons are found by electron microscopy. Furthermore, Kv1.1 and Kv1.2 K+ channels are not spatially segregated from the node-like clusters of Na+ channels in md rats, in contrast to their WT littermates. These results suggest the following: node-like clusters of voltage-gated Na+ channels and ankyrinG form adjacent to ensheathed axonal segments even in the absence of a myelin sheath; these clusters persist after oligodendrocyte cell death; dysmyelination does not alter the expression of different nodal of voltage-gated Na+ channels; the absence of paranodes results in the mislocalization of neurofascin155, contactin, and Caspr, and the aberrant localization of Kv1.1 and Kv1.2.

Reactive glia support and guide axon growth in the rat thalamus during the first postnatal week. A sharply timed transition from permissive to non-permissive stage

The present study demonstrates a supportive and guiding effect of the reactive glia on the postlesional axon growth in vivo, and offers a model system to compare permissive and non-permissive forms of the glial reaction. After stab wounds in early postnatal (P2-P9) rats, the reactive glia and the nerve fibers were detected by the immunohistochemical staining of glial fibrillary acidic protein (GFAP) and neurofilament protein, respectively. In the thalamus of the animals lesioned at P5 or earlier, an extraordinary bundle of fibers immunoreactive to neurofilament protein was found, corresponding to the lesion track marked by reactive glia. This bundle persisted up to 2 months, as shown by electron microscopy. When the animals were lesioned at P7 or later, the lesion track was immunonegative to neurofilament protein. Following P6 lesions, an intermediate situation was found, the strip of immunoreactive neurofilament protein was missing, or short and weak. GFAP immunostaining demonstrated a typical reactive glia in every case. As a result of the same operation, reactive glia plus a deficiency of neurofilament protein immunostaining was found in every animal in the cortex and the corpus callosum, independently from the age at lesion. The results demonstrate that the permissive nature of the glial reaction depends on the lesioned area as well, and changes to a non-permissive effect in a short time interval.

Expression of the gene encoding the chemorepellent semaphorin III is induced in the fibroblast component of neural scar tissue formed following injuries of adult but not neonatal CNS

This study evaluates the expression of the chemorepellent semaphorin III (D)/collapsin-1 (sema III) following lesions to the rat CNS. Scar tissue, formed after penetrating injuries to the lateral olfactory tract (LOT), cortex, perforant pathway, and spinal cord, contained numerous spindle-shaped cells expressing high levels of sema III mRNA. The properties of these cells were investigated in detail in the lesioned LOT. Most sema III mRNA-positive cells were located in the core of the scar and expressed proteins characteristic for fibroblast-like cells. Neuropilin-1, a sema III receptor, was expressed in injured neurons with projections to the lesion site, in a subpopulation of scar-associated cells and in blood vessels around the scar. In contrast to lesions made in the mature CNS, LOT transection in neonates did not induce sema III mRNA expression within cells in the lesion and was followed by vigorous axonal regeneration. The concomitant expression of sema III and its receptor neuropilin-1 in the scar suggests that sema III/neuropilin-1-mediated mechanisms are involved in CNS scar formation. The expression of the secreted chemorepellent sema III following CNS injury provides the first evidence that chemorepulsive semaphorins may contribute to the inhibitory effects exerted by scars on the outgrowth of injured CNS neurites. The vigorous regrowth of injured axons in the absence of sema III following early neonatal lesions is consistent with this notion. The inactivation of sema III in scar tissue by either antibody perturbation or by genetic or pharmacological intervention could be a powerful means to promote long-distance regeneration in the adult CNS.

Capability for reactive gliosis develops prenatally in the diencephalon but not in the cortex of rats

In this study, the glial reactions to stab wounds were investigated on a large population of newborn (P0) and fetal rats, by the immunohistochemical staining of the glial fibrillary acidic protein. The lesions penetrated both the cortex and the diencephalon. The fetuses were lesioned in utero from the 17th embryonic day (E17) and were born on E22 or E23 in the natural way. In the cortex usually no reactive gliosis developed although definitive tissue destructions remained after the lesion. Weak and incomplete glial reactions were observed in a few cases of E20 or P0 lesions only. In the diencephalon, however, the same stabbings provoked massive glial reactions. The timing and the morphology of this reaction were similar to those found in adult animals. At E17 the lesion did not result in reactive gliosis even in the diencephalon. Our study highlights two phenomena: (i) depending on the brain area servere glial reactions can already follow fetal lesions, and (ii) the appearance of the capability for glial reactions may be a stage of the local tissue maturation in every brain area and cannot be considered as a function of brain development in general. Probably, the capability for glial reactions can take place only when certain histogenetic processes (e.g., cell migration, axon growth, apoptosis) have been at least mostly accomplished, but which of the local development events are the determining ones remains to be investigated.

Astrocyte reactivity in neonatal mice: apparent dependence on the presence of reactive microglia/macrophages

In neonatal mice, an acute injury produced by a stab wound to the cortex results in minimal astrocyte reactivity, as has been observed by others. However, if the source of the stab wound, a piece of nitrocellulose (NC) membrane, were now implanted in the cortex for a period of time (chronic NC implant injury), then extensive astroglial reactivity in the neonatal brain ensues. The astrogliosis is manifested by increased mRNA, protein content, and immunoreactivity for GFAP, and by ultrastructural changes. Given the previous reports that inflammatory cytokines are possible mediators of astrocyte reactivity (e.g., Balasingam et al: J Neurosci 14:846, 1994), we examined the brain parenchyma of neonatal mice following an NC stab or implant injury, with minimal or extensive astrogliosis, respectively, for a possible differential representation of inflammatory cells. A significant correlation (r = 0.87, P < 0.05) was observed between the occurrence of astrogliosis and the presence of reactive microglia/macrophages; no other inflammatory cell type was detected in the brain parenchyma of neonatal mice following NC implant injury. We suggest that reactive microglia/macrophages are required for the evolution of cells into reactive astrocytes following insults to the neonatal brain.

Astrocytes and extracellular matrix following intracerebral transplantation of embryonic ventral mesencephalon or lateral ganglionic eminence

Transplantation of embryonic neurons to the adult mammalian central nervous system (CNS) offers the possibility of re-establishing neural functions lost after traumatic injuries or neurodegenerative disease. In the adult CNS, however, transplanted neurons and their growing neurites can become confined to the graft region, and there may also be a relative paucity of afferents innervating grafted neurons. Because glia may influence the development and regeneration of CNS neurons, the present study has characterized the distribution of astrocytes and developmentally regulated glycoconjugates (chondroitin-6-sulfate proteoglycan and tenascin) within regions of the embryonic mouse CNS used as donor tissues, and in and around these grafts to the adult striatum and substantia nigra. Both chondroitin-6-sulfate proteoglycan and tenascin are present in the embryonic ventral mesencephalon (in association with radial glia and their endfeet, and glial boundaries that cordon off the ventral mesencephalon dopamine neuron migratory zone) and lateral ganglionic eminence before transplantation, and they are conserved within grafts of these tissues to the adult mouse. Neostriatal grafts exhibit a heterogeneous pattern of astrocyte and extracellular matrix molecule distribution, unlike ventral mesencephalon grafts, which are rather homogeneous. There is evidence to suggest that, in addition to variation in astroglial/extracellular matrix immunostaining within different compartments in striatal grafts to either adult striatum or substantia nigra, there are also boundaries between these compartments that are rich in glial fibrillary acidic protein/extracellular matrix components. Substantia nigra grafts, with cells immunoreactive for tyrosine hydroxylase, are also rich in immature astroglia (RC-2-immunopositive), and as the astroglia mature (to glial fibrillary acidic protein-positive) over time the expression of chondroitin-6-sulfate proteoglycan and tenascin is also reduced. These same extracellular matrix constituents, however, are only slightly up-regulated in an area of the adult host which surrounds the grafted tissue. Glial scar components exhibit no obvious differences between grafts from different sources to homotopic (e.g., striatum to striatum) or heterotopic (e.g., substantia nigra to striatum) sites, and likewise grafts of non-synaptically associated structures (e.g., cerebellum to striatum), needle lesions or vehicle injections all yield astroglial/extracellular matrix scars in the host that are indistinguishable. Studies utilizing the ROSA-26 transgenic (beta-galactosidase-positive) mouse as a host for non-5-bromo-4-chloro-3-indolyl-beta-d-galactopyranoside-labeled grafts indicate that the early astroglial/extracellular matrix response to the graft is derived from the surrounding host structures. Furthermore, biochemical analysis of one of the "boundary molecules", tenascin, from the developing ventral mesencephalon versus adult striatal lesions, suggests that different forms of the molecule predominate in the embryonic versus lesioned adult brain. Such differences in the nature and distribution of astroglia and developmentally regulated extracellular matrix molecules between donor and host regions may affect the growth and differentiation of transplanted neurons. The present study suggests that transplanted neurons and their processes may flourish within graft versus host regions, in part due to a confining glial scar, but also because the extracellular milieu within the graft site remains more representative of the developmental environment from which the donor neurons were obtained [Gates M. A., et al. (1994) Soc. Neurosci. Abstr. 20, 471].

Expression of myelin proteins in the opossum optic nerve: late appearance of inhibitors implicates an earlier non-myelin factor in preventing ganglion cell regeneration

The pattern of appearance of myelin-associated proteins in the visual system of the Brazilian opossum Monodelphis domestica is described. Whole mounts of optic nerve, chiasm, and optic tract were sectioned horizontally and incubated with antibodies to myelin basic protein (MBP), proteolipid protein (PLP), myelin-associated glycoprotein (MAG), "Rip," and the neurite inhibitory protein (IN-1), followed by visualization with diaminobenzidine and a peroxidase-conjugated secondary antibody. PLP is first detectable 24 days after birth (P24) at the centre of the optic chiasm. MBP, MAG, Rip, and IN-1 appear first in the same area at P26. By P28 the distribution of all proteins is similar, occupying the entire chiasm, optic tracts, and prechiasmatic portion of the optic nerves. Protein expression progresses along the optic nerve to reach the lamina cribrosa by P34, coincident with the time of eye opening. A critical period in which the retinofugal pathway has a regenerative capacity has recently been observed in Monodelphis. This period ends at P12, 2 weeks before the appearance of the myelin-associated inhibitory proteins MAG and IN-1. These results therefore suggest that regeneration in the developing retinofugal projection of the opossum is restricted by an earlier non-myelin factor, which is in contrast to current literature on the spinal cord.

Glial cells in transected optic nerves of immature rats. II. An immunohistochemical study

The glia response to Wallerian degeneration was studied in optic nerves 21 days after unilateral enucleation (PED21) of immature rats, 21 days old (P21), using immunohistochemical labelling. Nerves from normal P21 and P42 nerves were also studied for comparison. At PED21, there was a virtual loss of axons apart from a few solitary fibres of unknown origin. The nerve comprised a homogeneous glial scar tissue formed by dense astrocyte processes, oriented parallel to the long axis of the nerve along the tracks of degenerated axons. Astrocytes were almost perfectly co-labelled by antibodies to glial fibrillary acid protein and vimentin in both normal and transected nerves. However, there was a small population of VIM+GFAP- cells in normal P21 and P42 nerves, and we discuss the possibility that they correspond to O-2A progenitor cells described in vitro. Significantly, double immunofluorescence labelling in transected nerves revealed a distinct population of hypertrophic astrocytes which were GFAP+VIM-. These cells represented a novel morphological and antigenic subtype of reactive astrocyte. It was also noted that the number of oligodendrocytes in transected nerves did not appear to be less than in normal nerves, on the basis of double immunofluorescence staining for carbonic anhydrase II, myelin oligodendrocyte glycoprotein, myelin basic protein, glial fibrillary acid protein and ED-1 (for macrophages), although it was not excluded that a small proportion may have been microglia. A further prominent feature of transected nerves was that they contained a substantial amount of myelin debris, notwithstanding that OX-42 and ED1 immunostaining showed that there were abundant microglia and macrophages, sufficient for the rapid and almost complete removal of axonal debris. In conclusion, glial cells in the immature P21 rat optic nerve reacted to Wallerian degeneration in a way equivalent to the adult CNS, i.e. astrocytes underwent pronounced reactive changes and formed a dense glial scar, oligodendrocytes persisted and were not dependent on axons for their continued survival, and there was ineffective phagocytosis of myelin possibly due to incomplete activation of microglia/macrophages.

Glial cells in transected optic nerves of immature rats. I. An analysis of individual cells by intracellular dye-injection

The glial response to Wallerian degeneration was studied in optic nerves following unilateral enucleation in immature rats, aged 21 days old (P21). The three-dimensional morphology of dye-filled glia was determined in intact nerves, at post-enucleation day 21 in normal nerves from untreated P21 rats, by correlating laser scanning confocal microscopy and camera lucida drawings of single cells. In normal and transected nerves, the majority of dye-filled cells comprized astrocytes (54% and 65%, respectively). In normal P21 nerves, the predominant astrocyte form had a complex stellate morphology and had a centrally-located cell body from which branching processes extended randomly. Two other distinct forms were transverse and longitudinal astrocytes, which had a polarized process extension in a plane perpendicular or parallel to the long axis of the nerve, respectively. These forms were recognized in transected nerves also, but astrocytes in transected nerves had a simple morphology on the whole, and extended few, dense processes which branched infrequently. Quantitative analysis of astrocyte morphology confirmed that individual astrocytes underwent considerable remodelling in response to Wallerian degeneration. A prominent reaction was that astrocytes had withdrawn radial processes and extended a greater proportion of processes longitudinally, parallel to the long axis of the nerve and along the course of degenerated axons. A further, notable feature of transected nerves was the development of novel longitudinal forms and of hypertrophic astroglia. These results indicated that all astrocytes became reactive following enucleation and that glial scar formation was not the function of a single astrocyte subtype. Oligodendrocytes in transected nerves had lost their myelin sheaths and appeared as small cells with numerous bifurcating processes which extended radially, but a small number of oligodendrocytes were recognized which apparently supported myelin sheaths (9%, compared to 40% in normal nerves). In addition, there was a significant population of indeterminate cells in transected nerves (26%, compared to 6% in normal nerves) and, although some of these were identified as microglia/macrophages, it was concluded that many were likely to be dedifferentiated oligodendrocytes.

Development and role of retinal glia in regeneration of ganglion cells following retinal injury

AIMS/BACKGROUND

Recent observations have shown that the glial scar resulting from a surgical lesion of the immature retina differs from elsewhere in the central nervous system, in that it permits the through growth and reconnection of regenerating axons. This study in the opossum examines in detail the development and reaction to injury of retinal glia at different developmental stages, and specifically examines the distribution of the gliosis related inhibitory molecule, chondroitin sulphate proteoglycan (CSPG), making comparisons with a control site of gliosis in the cerebral cortex.

METHODS

A linear slit was cut into the retina or cortex with a fine tungsten probe. After a variable time delay, immunocytochemistry of the resulting gliosis was employed to detect astrocytes with glial fibrillary acidic protein (GFAP), Müller cells with vimentin, and CSPG with CS-56 antibodies. GFAP was also used at different ages to examine the normal development of astrocytes in the retina of this species.

RESULTS

Astrocytes entered the retina 12 days after birth (P12), closely associated with blood vessels in the nerve fibre layer. In experiments at all ages studied, cellular continuity was re-established across the lesioned retina, which did not result in a significant astrocyte proliferation or CSPG expression. In contrast, cortical injury led to the development of a cystic cavity surrounded by astrocytes and CSPG. Müller cells expressed GFAP but not CSPG in the lesioned retina.

CONCLUSION

Successful regrowth of ganglion cells through a retinal lesion may be partly the result of the scarcity of astrocytes in the retina, which results in minimal gliosis, or of their apparent inability to express inhibitory molecules.

Ultrastructural study of the optic nerve in blind mole-rats (Spalacidae, Spalax)

The optic nerve in two species of subterranean mole-rats (Spalacidae) has been examined at the ultrastructural level. The axial length of the eye and the diameter of the optic nerve are 1.9 mm and 52.5 microns in Spalax leucodon, and 0.7 mm and 80.8 microns in Spalax ehrenbergi, respectively. An anti-glial fibrillary acidic protein postembedding procedure was used to distinguish glial cell processes from axons. In both species, the optic nerve is composed exclusively of unmyelinated axons and a spatial distribution gradient according to the size or the density of fibers is lacking. The optic nerve of S. leucodon contains 1790 fibers ranging in diameter from 0.07-2.30 microns (mean = 0.57 microns), whereas in S. ehrenbergi, only 928 fibers, with diameters of 0.04-1.77 microns (mean = 0.53 microns) are observed. In S. ehrenbergi, a higher proportion of glial tissue is present and the fascicular organization of optic fibers is less obvious. Distribution gradients according to size frequency or density of fibers in the optic nerve are absent in both species. Comparison with other mammals suggests that although ocular regression in microphthalmic species is correlated with a significant decrease in the total number of optic fibers and the relative proportion of myelinated fibers, no difference in the absolute size range of unmyelinated axons is observed. The total absence of myelinated fibers in Spalax may be related to the subcutaneous location of the eyes.(ABSTRACT TRUNCATED AT 250 WORDS)

Dolphin peripheral visual pathway in chronic unilateral ocular atrophy: complete decussation apparent

Components of the peripheral visual pathway were examined in two bottlenose dolphins, Tursiops truncatus, each with unilateral ocular degeneration and scarring of 3 or more years' duration. In both animals, the optic nerve associated with the blind eye (right eye in Tg419 and left eye in Tt038) had a translucent, gel-like appearance upon gross examination. This translucency was also evident in the optic tract contralateral to the affected eye. In Tg419, myelinated axons of varying diameters were apparent in the left optic nerve, whereas the right optic nerve, serving the blind eye, appeared to be devoid of axons. In Tt038, myelinated axons were associated with the right optic nerve (serving the functional eye) and left optic tract but were essentially absent in the left optic nerve and right optic tract. Examined by light microscopy in serial horizontal sections, the optic chiasm of Tt038 was arranged along its central plane in segregated, alternating pathways for the decussation of right and left optic nerve fibers. Ventral to this plane, the chiasm was comprised of fibers from the left optic nerve, whereas dorsal to the central plane, fibers derived from the right optic nerve. Because of this architectural arrangement, the right and left optic nerves grossly appeared to overlap as they crossed the optic chiasm with the right optic nerve coursing dorsally to the left optic nerve. At the light and electron microscopic levels, the optic nerves and tracts lacking axons were well vascularized and dominated by glial cell bodies and glial processes, an expression of the marked glial scarring associated with postinjury axonal degeneration. The apparent absence of axons in one of the optic tract pairs (right in Tt038 and left in Tg419) supports the concept of complete decussation of right and left optic nerve fibers at the optic chiasm in the bottlenose dolphin.

Structure of reticulospinal axon growth cones and their cellular environment during regeneration in the lamprey spinal cord

The large larval sea lamprey is a primitive vertebrate that recovers coordinated swimming following complete spinal transection. An ultrastructural study was performed in order to determine whether morphologic features of regenerating axons and their cellular environment would provide clues to their successful regeneration compared to their mammalian counterparts. Three larval sea lampreys were studied at 3, 4 and 11 weeks following complete spinal transection and compared with an untransected control. Müller and Mauthner cells or their giant reticulospinal axons (GRAs) were impaled and injected with horseradish peroxidase (HRP). Alternating thick and thin sections were collected for light and electron microscopy. A total of 9 neurites were examined. At all times, growth cones of GRAs differed from those of cultured mammalian neurons in being packed with neurofilaments and in lacking long filopodia, suggesting possible differences in the mechanisms of axon outgrowth. Morphometric analysis suggested that GRA growth cones contact glial fibers disproportionately compared to the representation of glial surface membranes in the immediate environment of these growth cones. No differences were found between glial cells in regenerating spinal cords and those of untransected control animals with regard to the size of the cell body and nucleus and the packing density of their intermediate filaments. Glial fibers in control animals and glial fibers located far from a transection were oriented transversely. Glial cells adjacent to the transection site sent thickened, longitudinally oriented processes into the blood clot at the transection site. These longitudinal glial processes preceded the regenerating axons. Desmosomes were observed on glia adjacent to the lesion but were scarce in the lesion during the first four weeks post-transection. These findings suggest that longitudinally oriented glial fibers may serve as a bridge along which axons can regenerate across the lesion. The presence of desmosomes might prevent migration of astrocytes near the transection, thus stabilizing the glial bridge.

Development of an astrocytic response to lesions of the spinal cord in the North American opossum: an immunohistochemical study using anti-glial fibrillary acidic protein

We have shown previously that rubral axons grow around a lesion of their pathway in developing opossums and that a critical period exists for that plasticity. The critical period begins when rubral axons first reach the level of the lesion and ends sometime between postnatal days (PD) 26 and 30. The aim of the present study was to examine the development of an astrocytic response to lesioning the spinal cord to determine if there is a temporal correlation between the development of such a response and the end of the critical period. The astrocytic response was examined immunohistochemically, 2 and 4 weeks after hemisecting the thoracic spinal cord, using an antibody to glial fibrillary acidic protein (GFAP). A response was first seen at PD21 in the 2-week series. The response was relatively mild, however, and limited to the white matter. When the lesion was made at PD26, the response was still restricted to the white matter, but hypertrophied astrocytes were found at the gray/white matter junction and cystic cavities were present. When the lesion was made at PD41, the response had spread to the gray matter and it occupied a larger area rostral and caudal to the lesion than at earlier ages. The animals allowed to survive 4 weeks after lesioning were subjected to a second operation 4-5 days before sacrifice so that Fast Blue could be injected bilaterally two to three segments caudal to the lesion. When the hemisection was made at PD15, a response was present in the ventral and ventrolateral funiculi, but not in that part of the lateral funiculus that contains rubrospinal axons.(ABSTRACT TRUNCATED AT 250 WORDS)

Axon dependent glial changes during optic fiber regeneration in the goldfish

During optic fiber regeneration in the goldfish, astrocytes in the visual system undergo a number of changes. These include hypertrophy of cell processes, increased reactivity with anti-intermediate filament antisera, and expression of cytoskeletal antigens not usually seen in these cells. In the present study, I have asked how much of this response might be due to interactions of glial cells with regenerating optic axons. Animals with and without a retina (regenerating and nonregenerating animals, respectively) had their optic nerve crushed and were then examined at various postoperative times with immunohistochemical methods. Three major differences between these two groups of animals were observed. First, in nonregenerating animals the crush lesion is not repopulated by immunoreactive glial cells while in regenerating animals it is. Second, the nature of the glial hypertrophy in the optic nerve is different in regenerating and nonregenerating animals. Finally, there is marked submeningeal swelling in regenerating nerves that is absent from nonregenerating nerves. Thus, these three aspects of the cellular response to optic nerve crush in the goldfish--wound healing, optic nerve gliosis, and non-neural cellular responses--appear to depend on interactions between regenerating optic axons and astrocytes or other non-neuronal cells of the visual paths for their expression.

Reactive astrocytes in explant cultures of glial scars derived from lesioned rat optic nerve: an ultrastructural study

Explant cultures of glial scars generated by surgical removal of the retina in 3-60-day-old rats were used to determine if reactive astrocytes survive in vitro and how closely reactive astrocytes in culture resemble their in vivo counterparts. Characterization of the composition of age matched glial scars in vivo and in vitro showed that reactive astrocytes survived in glial scar explants even after several weeks in culture. Reactive astrocytes in both neonatal and adult glial scars retained ultrastructural features characteristic of reactive astrocytes in vivo. However, fewer reactive astrocytes survived in culture when explants were prepared from adult rat glial scars. The results of this study demonstrate that tissue culture is a viable model for the study of reactive astrocytes. A critical factor in the survival of reactive astrocytes in culture was the complete removal of myelin debris prior to the establishment of the culture. This outcome suggests that it will be important to clarify why myelin debris persists in culture and how it affects the survival of reactive astrocytes.

The glial framework of central white matter tracts: segmented rows of contiguous interfascicular oligodendrocytes and solitary astrocytes give rise to a continuous meshwork of transverse and longitudinal processes in the adult rat fimbria

The cellular skeleton of the adult rat fimbria consists of regularly spaced interfascicular glial rows of considerable length, running in the longitudinal (axonal) axis of the tract. Each row consists of a series of repeated segments made up of a stretch of interfascicular oligodendrocytes lying in direct contact with each other, and separated from the adjacent segments by usually solitary interfascicular astrocytes. A typical segment would be around 60 microns long, and have an axial core of about eight contiguous oligodendrocytes surrounded by a shell of about 1,200 axons, 70% of which are myelinated. In the transverse plane of the tract, adjacent segments are stacked together with a core-to-core distance of around 15 microns. The interfascicular oligodendrocytes have radial stem processes (in a plane transverse to the axonal axis) which give rise to the longitudinal myelinating (internodal) processes. Both transverse and longitudinal oligodendrocytic processes are longer than the dimensions of the segment (in which their cell bodies lie) and its axonal shell. They thus cooperate in myelinating axons of adjacent segments in both planes. The interfascicular astrocytes have three distinct types of processes: radial, longitudinal, and vascular (bearing end feet). The radial astrocytic processes are thick and tapering, and the processes of individual astrocytes extend transversely (in the plane of the original embryonic radial glial processes) for a total of at least 100 microns. The considerably more numerous longitudinal astrocytic processes arise from all parts of the cell bodies and radial processes. They are up to at least 30 microns long, thin, untapering, and largely unbranched, and are interdigitated among the fimbrial axons. In the radial plane, the astrocytic radial processes spread out through a wide swathe of adjacent segments, so that the integrated meshwork of interpenetrating longitudinal processes arising from overlapping radial processes of astrocytes from many different interfascicular rows provides a continuous longitudinal substrate for the fimbrial axons.

Differential proliferative response of human and mouse astrocytes to gamma-interferon

We have previously shown that gamma-interferon promoted the proliferation of adult human astrocytes isolated from brain biopsy specimens. In contrast, in the present study, astrocytes derived from neonatal mouse brains and treated with recombinant murine gamma-interferon responded by a decrease (average of 50% at 100 U/ml) in proliferation. The basal rate of proliferation as assessed by bromodeoxyuridine incorporation was markedly increased in neonatal mouse astrocytes when compared to the adult human cells, suggesting that age, and the corresponding metabolic activity of cells, could be important determinants in the mitogenic response of astrocytes to cytokines. However, subsequent examinations of fetal human and adult mouse astrocytes, with comparable basal rate of proliferation to neonatal mouse and adult human cells respectively, showed gamma-interferon to promote DNA synthesis in fetal human astrocytes while inhibiting that of adult mouse astrocytes. The results suggest species differences in the proliferative response of human and mouse astrocytes to the cytokine gamma-interferon.

Morphometric analysis of blood vessels in chronic experimental spinal cord injury: hypervascularity and recovery of function

A model of spinal cord trauma in guinea pigs, based on compression to a set thickness, was described previously. Compression injuries of the lower thoracic cord were produced in 11 anesthetized, adult guinea pigs, and the outcome monitored, using successive behavioral tests and morphometry of the lesion at 2-3 months. This report describes changes in the vascularity of the spinal cord, based on light microscopic analysis of 1 micron plastic transverse sections through the center of the lesion. Mean blood vessel density in these lesions was approximately twice that found in equivalent regions of normal, uninjured spinal cords, and hypervascularity of the white matter extended at least four spinal cord segments cranially and caudally from the lesion center. Capillary diameter distribution was significantly shifted to larger values and large perivascular spaces surrounded most capillaries and pre- and post-capillary vessels. Extent of hypervascularity was not correlated with the overall severity of the injury, but there was a significant positive correlation between the density of blood vessels in the outer 400 microns of the white matter and secondary loss of neurological function below the lesion, seen between one day and eight weeks after injury. This suggests that hypervascularization of the lesion is related to secondary pathological mechanisms in spinal cord injury, possibly inflammatory responses, that are relatively independent of the primary mechanical injury but more closely connected with loss and recovery of function.

Astroglia in CNS injury

The astroglial response to CNS injury is considered in the context of neuron-glial relationships. Although previous models suggested that astroglial cells present in "scars" impede axon regrowth owing to irreversible changes in the glial cell following injury, recent in vivo and in vitro studies indicate that astroglial cells exhibit considerable plasticity, elevating expression of the glial filament protein and altering expression of properties which support axons, including extracellular matrix components and cell surface adhesion systems. Both in vivo and in vitro studies on neuron-glia interactions in different brain regions suggest that glia express region-specific properties, including ion channels, neurotransmitter uptake and receptor systems, and cell surface adhesion systems. Together these findings suggest that a more detailed analysis of glial response to injury in different brain regions will lead to an appreciation of the diversity of the astroglial response to injury, and its regulation by neuron-glia relationships.