Adult Rat Optic Nerve Research Articles

Transplanted Glial Scar Impedes Olfactory Bulb Reinnervation

The olfactory system is the only region of the mammalian central nervous system in which degeneration of the primary sensory neurons results in the development of new neurons and reinnervation of the secondary sensory neurons. Axotomy of the olfactory nerve at the cribriform plate does not cause the formation of a glial scar which blocks nerve regeneration. The purpose of this study was to determine whether a glial scar that formed in the optic nerve would suppress axonal regeneration when transplanted to the site of olfactory nerve axotomy. Primary olfactory neurons were axotomized along the cribriform plate in adult rats. A compact glial scar formed by transection of an adult rat optic nerve 50 to 60 days prior to removal was transplanted into the olfactory nerve axotomy site. The rats were allowed to survive for 1, 2, 3, or 4 weeks. Transport of horseradish peroxidase (HRP) from the nasal cavity by the olfactory neurons was used to examine the temporal and spatial pattern of regeneration of the olfactory nerve after axotomy and axotomy followed by glial scar transplantation. Twenty-one days after olfactory nerve axotomy, HRP was found in the glomerular layer where the primary olfactory axons synapse on the apical dendrites of the secondary olfactory neurons. In the presence of the transplanted glial scar, HRP labeling was not found in certain glomeruli even at 4 weeks postaxotomy. Glial scars formed within the optic nerve impede reinnervation of the olfactory bulb by neurons which have an exceptional regenerative capacity due in part to the ensheathing glia.

Retinal Ganglion Cell Axons Recognize Specific Guidance Cues Present in the Deafferented Adult Rat Superior Colliculus

During development, retinal ganglion cell axons establish a topographically ordered projection from the retina to the superior colliculus (SC). The putative guidance activities for retinal axons that operate during embryonic development are not detectable in the normal adult SC. However, these cues reappear upon transection of the optic nerve of adult rats. In the present study, we used a modified version of the "stripe assay," in which membranes from either anterior or posterior SC alternated with laminin stripes. Temporal embryonic retinal axons consistently avoid membranes from embryonic posterior SC, but only rarely from adult deafferented SC. However, they are attracted to membranes from both embryonic and adult deafferented anterior SC. Nasal retinal axons only show a significant preference for membranes from posterior SC after deafferentation. When retinal axons were offered a choice to grow on membranes either from their embryonic or their deafferented target regions, they showed a preference for the deafferented SC. On carpets consisting of laminin and membranes from normal SC (not deafferented) or nontarget regions (inferior colliculus), temporal and nasal axons grow either in a random fashion or show preferences for the laminin stripes. Our modified version of the classic stripe assay shows specific growth preferences of embryonic retinal axons for membrane lanes from their appropriate embryonic or deafferented adult target regions. These findings suggest that the deafferentation of the SC in adult rats triggers the reexpression of specific guidance activities for retinal axons. Those "attractive" guidance cues appear to be differentially expressed in the developing and deafferented SC.

Loss and Subsequent Recovery of Local Cerebral Glucose Use in Visual Targets after Controlled Optic Nerve Crush in Adult Rats

A mild crush of the adult rat optic nerve serves as a model to study the restoration of function after traumatic brain injury. It causes a progressive degeneration of retinal ganglion cells, but visually guided behavior is partially restored within 2–3 weeks. The purpose of this study was to determine to what extent local cerebral glucose use (LCGU) decreases and if it recovers in retinofugal targets following unilateral optic nerve crush. At intervals of 2, 9, and 22 days after crush, LCGU was monitored in rats in which the visual system was stimulated by a strobe-light and pattern. In the ipsilateral retinofugal targets there was only a minimal loss of LCGU use, but in the contralateral retinofugal targets, LCGU was reduced at Postlesion Day 2: to 50% in the superior colliculus (SC), to 60% in the lateral geniculate nucleus of the thalamus (LGN), and to 87% in the visual cortex. On Postoperative Days 9 and 22 we observed a partial restoration of LCGU in the contralateral SC and LGN to 68 and 79%, respectively. As recovery of visual performance is known to follow a similar time course, we conclude that restoration of metabolic activity in target structures may contribute to the restoration of vision after optic nerve crush.

HU-211, a nonpsychotropic cannabinoid, produces short- and long-term neuroprotection after optic nerve axotomy.

HU-211 is a novel synthetic analog of tetrahydrocannabinol that was recently shown in animal models to be nonpsychotropic. In this study we show that HU-211 can potentially be used as a neuroprotective compound in the CNS. Using a calibrated crush injury of adult rat optic nerve, we show that HU-211 can reduce injury-induced metabolic and electrophysiological deficits. Energy metabolism was monitored by measuring the intramitochondrial nicotine-amine adenine dinucleotide redox state hourly for 6 h after injury and treatment. Electrophysiological activity was assessed by compound action potential and visual evoked potential response. Beneficial effects were dose-dependent, being optimal at 7 mg/kg, administered intraperitoneally. The time window during which treatment was effective was found to be from the time of injury for at least 5 h, with treatment most effective at the time of injury. These results strongly suggest that HU-211, given immediately after CNS injury at the optimal dosage, may possess neuroprotective activities.

NADPH Diaphorase Expression in the Rat Retina after Axotomy—a Supportive Role for Nitric Oxide?

The large majority of mammalian retinal ganglion cells degenerate following section of their axons in the optic nerve. It has been suggested that some axotomized retina ganglion cells die because of toxic agents produced within their immediate environment. Our hypothesis was that nitric oxide might be one of the toxic factors implicated in the death of adult retinal ganglion cells post-axotomy. In the first instance, we determined whether there were any changes in the retinal expression of NADPH diaphorase both 3 and 14 days following intraorbital section of the optic nerve in adult rats. Secondly, if nitric oxide was indeed implicated in the death of ganglion cells, then trophic factors which rescue these neurons might do so by decreasing the expression of nitric oxide synthase. Recently, we found that a collicular proteoglycan purified from the major target of retinal ganglion cells, the superior colliculus, rescued a greater proportion of adult ganglion cells from axotomy-induced death than most other known trophic factors. We thus injected this proteoglycan intraocularly after section of the optic nerve and examined its effect on the expression of NADPH diaphorase in the retina. Thirdly, an inhibitor of nitric oxide synthetase was repeatedly injected into the eye following the section of the optic nerve in order to determine if such a treatment might improve the survival of retinal ganglion cells. The present results indicate that section of the optic nerve does not alter the overall levels of NADPH diaphorase within the adult rat retina. Intraocular injections of the collicular proteoglycan actually increased the number of neurons expressing NADPH diaphorase, particularly in the ganglion cell layer. Finally, inhibition of nitric oxide synthetase following axotomy resulted in increased loss of retinal ganglion cells over a 2 week period when compared with controls. Our findings indicate that, rather than being toxic, small amounts of nitric oxide may be important for the survival of a proportion of injured retina ganglion cells.

Regeneration in the optic nerve of adult rats: influences of cultured astrocytes and optic nerve grafts of different ontogenetic stages

We have studied the effects of transplanted optic nerves of different ontogenetic stages (E19 to adult), and cultured astrocytes from P2 cerebral cortex on the regeneration of axons in the optic nerve of adult rats. Regeneration was visualized by anterograde tracing with rhodamine-iso-thiocyanate. Grafts were identified with Nuclear Yellow. Astroglia within both the cut optic nerve and the transplants were detected by anti-glial fibrillary acidic protein staining. In control animals (cut optic nerve, 2-3 mm behind the optic disc), only a few neurites were found 15 days after the operation which grew randomly for short distances into the surrounding meningeal sheaths. Perinatal (E19 to P2) optic nerves induced a massive outgrowth of RITC-filled axons from the host optic nerve. The regenerating fibres grew for up to 3 mm towards the graft, ahead of glial fibrillary acidic protein-positive astroglia emanating from the host optic nerve that seemed to follow them. Although the regenerating fibres reached the grafts, they did not penetrate them. Optic nerve grafts of increasing age elicited smaller growth responses; e.g. grafts from P8 promoted only a very limited (several 100 microns) growth response, grafts from P12 and later induced outgrowth comparable with that of control animals. Grafted astrocytes from P2 donors that had previously been grown in culture, were also capable of promoting outgrowth of rhodamine-iso-thiocyanate-filled axons from the host optic nerve. These findings suggest that only astrocytes at an immature stage of differentiation are capable of inducing axon growth from the adult optic nerve. Furthermore, the absence of an obvious cellular bridge between host and graft suggests that the graft effect is probably mediated by the release of astroglia-derived diffusible neurite growth promoting factors.

SCHWANN CELLS AND THE REGROWTH OF AXONS IN THE MAMMALIAN CNS: A REVIEW OF TRANSPLANTATION STUDIES IN THE RAT VISUAL SYSTEM

1. We have used peripheral nerve transplants or cultured Schwann cells grafted in association with different types of polymer to study axonal regrowth in the rat visual system. In some instances the glia were co-grafted with fetal tectal tissue. 2. The studies have two main aims: (i) to determine whether retinal axons can be induced to regrow at a site distant from their cell soma, that is, after damage to the brachial region of the optic tract; (ii) to determine whether retinal axons exposed to Schwann cells retain the ability to recognize their appropriate target neurons in CNS tissue. 3. In brachial lesion studies, Schwann cells were placed in the lesion site in association with nitrocellulose papers, within polycarbonate tubes in the presence or absence of a supporting extracellular matrix (ECM), or within polymer hydrogel scaffolds. Autologous sciatic nerve grafts were also used. Immunohistochemical studies revealed the presence of regenerating axons within all polymer bridges. Regrowth of retinal axons was also seen, however, growth was not extensive and was limited to the proximal 1-1.5 mm of the implants. 4. In target innervation experiments, two surgical paradigms were developed. In one experiment, a segment of sciatic nerve was autografted onto the transected optic nerve in adult rats and the distal end of each graft was placed adjacent to fetal tectal (target) tissue implanted into the frontal cortex. To date, we have not been able to demonstrate selective recognition of target regions within tectal transplants by retinal axons exiting the sciatic nerve implants. 5. In the second experiment, Schwann cells were mixed with fetal tectal cells and co-grafted to the midbrain of newborn host rats. Schwann cells altered the characteristic pattern of host retinal growth into tectal grafts; in some cases axons were induced to grow away from appropriate target areas by nearby co-grafted Schwann cells. 6. In summary, Schwann cell/polymer scaffolds may provide a useful way of promoting the regrowth of damaged axons in the CNS, however: (i) in adults, at least, their effectiveness is reduced if they are located at a distance from the cell bodies giving rise to regenerating axons; (ii) in some circumstances exposure to a peripheral glial environment may affect the capacity of regenerating axons to recognize appropriate target cells in the CNS neuropil.

Adult rat optic nerve oligodendrocyte progenitor cells express a distinct repertoire of voltage- and ligand-gated ion channels.

Cultured oligodendrocyte progenitor cells derived from the developing central nervous system (CNS) express a pattern of ion channels that is distinct from mature oligodendrocytes and other cell types of the CNS. In the present study, we used the whole-cell patch-clamp technique and the fura-2-based Ca++ imaging system to study the ion channel expression of oligodendrocyte progenitor cells derived from the optic nerves of adult rats. We found that the adult oligodendrocyte progenitor cell membrane is dominated by K+ currents, both delayed outward and inward rectifying. The inwardly rectifying K+ currents were often as large as the outward delayed rectifying K+ currents. The delayed rectifying outward currents were partially blocked by 50 mM tetraethylammonium or 1 mM 4-aminopyridine, but not by 2 or 5 mM BaCl2. This suggests that the delayed rectifier channels expressed by adult progenitor cells are different from those expressed by perinatal cells. Most adult oligodendrocyte progenitor cells showed no or only small A-type K+ currents. Both Ca++ and Na+ channels were also detected in these cells. Furthermore, adult progenitor cells responded to the neurotransmitters GABA and kainate and the pharmacology of these responses indicated that these cells express GABAA receptors and kainate receptors that are Ca(++)-permeable. Our study suggests that adult oligodendrocyte progenitor cells are electrophysiologically distinct and that these cells share electrophysiological characteristics with both perinatal progenitor cells and immature oligodendrocytes.

Norepinephrine modulates excitability of neonatal rat optic nerves through calcium-mediated mechanisms.

We report that norepinephrine markedly increases excitability of neonatal rat optic nerves. To investigate the mechanisms of the norepinephrine-induced excitability increase, we studied isolated optic nerves from 42 neonatal (< three days old) and five adult (> three months old) Long-Evan's hooded rats. Norepinephrine (10(-6), 10(-5) and 10(-4) M) rapidly and reversibly increased the amplitude (mean +/- S.D.: 3.5 +/- 1.7%, 12.1 +/- 2.8% and 35.6 +/- 8.4%) of compound action potentials elicited by submaximal stimulation of neonatal optic nerves. The beta-1 adrenoceptor antagonist atenolol (10(-5) M) blocked the norepinephrine-induced increase in excitability but the alpha antagonist phentolamine (10(-5) M) did not. The beta agonist isoproterenol (10(-5) and 10(-4) M) increased response amplitudes (8.7 +/- 4.1% and 25.8 +/- 4.6%) but the alpha-1 agonist methoxamine and alpha-2 agonist clonidine did not. The beta antagonist propranolol blocked the isoproterenol effect. Replacing Ca2+ with Mg2+ or adding 0.8 mM of Cd2+ reversibly blocked the norepinephrine effects. Extracellular K+ concentrations did not change in optic nerves during norepinephrine application. Blockade of K+ channels with apamin (10(-6) M) or tetraethylammonium (10(-3) M) did not prevent the excitatory effects of norepinephrine. Adult rat optic nerves were insensitive to both norepinephrine (10(-4) M) and isoproterenol (10(-4) M). Our results indicate that norepinephrine increases neonatal optic axonal excitability through Ca(2+)-dependent mechanisms. The data suggest that the adrenoceptors are situated on the axons, that the excitability changes are not due to changes in extracellular K+ concentration or K+ channels sensitive to apamin or tetraethylammonium. The sensitivity of rat optic nerves to norepinephrine declined with age. Axonal adrenoceptors may play a role in optic axonal development and injury.

Astrocytes from adult rat optic nerves are nonpermissive for regenerating retinal ganglion cell axons

We have directly compared the abilities of astrocytes from newborn and adult rats to support or inhibit the growth of regenerating axons in vitro. Astrocytes prepared from newborn rats were able to promote retinal ganglion cell (RGC) axon growth from embryonic and adult rat and from adult fish retinal explants. Retinal axons from E16 rat retinae grew significantly faster on astrocytes from neonatal rats than those from E18 or adult rat retinae with growth rates comparable to RGC axons from adult fish retinae. RGC regeneration from adult rat retinae was almost completely inhibited on adult rat optic nerve astrocytes. Only axons from adult fish retinae were able to extend onto monolayers from these reactive astrocytes, although their growth rates were significantly reduced. We conclude that the failure of mammalian RGC axons to regrow within the lesioned optic nerve environment is, at least in part, due to nonpermissive aspects of adult “reactive” optic nerve astrocytes. However, the cell intrinsic growth potential of RGCs also appears to influence their ability to extend axons on cellular substrates.

Strongly GD3+ cells in the developing and adult rat cerebellum belong to the microglial lineage rather than to the oligodendrocyte lineage

A recent study has shown that ramified microglia in the adult rat optic nerve express the ganglioside GD3 [Wolswijk Glia 10:244-249, 1994], thereby raising the possibility that some GD3+ in the developing rat central nervous system (CNS) belong to the microglial lineage rather than to the oligodendrocyte lineage, as previously thought. To examine this possibility, sections of postnatal and adult cerebellum were double-labelled with markers for rat microglia [the B4 isolectin derived from Griffonia simplicifolia (GSI-B4), the ED1 monoclonal antibody (mAb), and the OX-42 mAb] and anti-GD3 mAbs (the mAbs R24 and LB1). These immunolabellings showed that ramified microglia as well as amoeboid microglia are strongly GD3+ in vivo. Moreover, most, if not all, cells that express high levels of GD3 in sections of developing cerebellum appear to belong to the microglial lineage. These observations contradict previous suggestions that the strongly GD3+ cells in the putative white matter regions of the developing brain are oligodendrocyte-type-2 astrocyte (O-2A) progenitor cells; the cells that give rise to oligodendrocytes in the CNS. The present study did, however, confirm that some O-2A progenitor cells in sections of postnatal cerebellum are weakly GD3+ in vivo. Amoeboid microglia are present in areas of the developing cerebellum where newly generated oligodendrocytes are found, suggesting that these cells play a role in the phagocytosis of the large numbers of oligodendrocytes that die as part of CNS development.

Influence of fetal brain grafts on axotomized retinal ganglion cells

The neurotrophic activity of fetal brain grafts next to the proximal stump of the transected optic nerve of adult rats was investigated. Axotomy ( n= 25) reduced the original retinal ganglion cell population by 90% and the mean neuron size by one-third within 30 days. No increase in number or size of the surviving neurons could be found in any group receiving an embryonic brain graft (E16; spinal cord: n=16, cerebral cortex: n=16, tectum: n=15, thalamus: n= 17) at the site of optic nerve transection, despite good survival of transplants. Although no positive effect on axotomized retinal ganglion cells could be found after transplantation of embryonic central nervous system structures, the same experimental model of the optic nerve seems to be valuable for further investigations of degeneration of traumatized retinal ganglion cells.

Type II sodium channels in spinal cord astrocytes in situ: immunocytochemical observations.

The expression of sodium channel alpha-subunit isoforms in astrocytes in adult rat spinal cord and optic nerve was examined utilizing immunocytochemical methods with antibodies generated against conserved and subtype-specific sequences of the sodium channel. In adult rat spinal cord, astrocytes within the dorsal and ventral funiculi were immunolabelled with antibody SP20, which recognizes a conserved sequence within sodium channel types I, II, and III. In addition, astrocytes within these spinal cord white matter tracts were immunostained with antibody SP11-II, which recognizes sodium channel type II. Antibodies SP11-I and SP32-III, which are directed against subtype-specific sequences in sodium channel types I and III, respectively, did not label astrocytes in the dorsal and ventral funiculi of the spinal cord. In optic nerves, astrocytes were immunostained with antibody SP20. However, no detectable labelling of cells within the optic nerve was observed with antibodies SP11-I, SP11-II, and SP32-III. These observations demonstrate that sodium channel II is expressed by astrocytes in spinal cord white matter. Moreover, these data suggest that regional factors regulate the level of sodium channel isoform expression in astrocytes.

Differential regulation of c-JUN expression in rat retinal ganglion cells after proximal and distal optic nerve transection

In adult rats, the distance between the site of axotomy and the cell body influences the ability of retinal ganglion cells (RGCs) to survive and regenerate an axon. Optic nerves of adult rats were transected 2 mm or 6 mm from the optic bulb and RGCs labeled with the fluorescent marker fluorogold. The time course of c-JUN expression was monitored immunocytochemically between 3 days and 4 weeks after axotomy. c-JUN expression began later and declined faster after distal axotomy than after proximal transection of the optic nerve. However, at the same time significantly more RGCs survived after distal than after proximal axotomy 3 and 4 weeks after axotomy. These findings suggest that survival and expression of c-JUN in rat RGCs are independently regulated by the length of the optic nerve stump after lesion.

Recovery of Visual Response of Injured Adult Rat Optic Nerves Treated with Transglutaminase

Failure of axons of the central nervous system in adult mammals to regenerate spontaneously after injury is attributed in part to inhibitory molecules associated with oligodendrocytes. Regeneration of central nervous system axons in fish is correlated with the presence of a transglutaminase. This enzyme dimerizes interleukin-2, and the product is cytotoxic to oligodendrocytes in vitro. Application of this nerve-derived transglutaminase to rat optic nerves, in which the injury had caused the loss of visual evoked potential response to light, promoted the recovery of that response within 6 weeks after injury. Transmission electron microscopy analysis revealed the concomitant appearance of axons in the distal stump of the optic nerve.

GD3+ cells in the adult rat optic nerve are ramified microglia rather than 0–2Aadult progenitor cells

The adult central nervous system (CNS) contains a population of adult oligodendrocyte-type-2 astrocyte (O-2A) progenitor cells (O-2Aadult progenitor cells). These cells may provide a source of the new oligodendrocytes that are needed to repair demyelinated lesions. In order to examine the role of O-2Aadult progenitor cells in the regeneration of the oligodendrocyte population following demyelinating damage, it is essential to be able to identify such cells unambiguously in sections of adult CNS tissue. The present study examined whether antibodies to the ganglioside GD3 specifically label O-2Aadult progenitor cells in cultures and sections of adult optic nerve, since previous studies on the developing CNS had suggested that O-2Aperinatal progenitor cells were GD3+ in vitro and in vivo. Evidence is presented indicating that, although O-2Aadult progenitor cells in vitro were labelled with the R24 mAb (an anti-GD3 mAb), all GD3+ cells in sections of adult optic nerve bound the OX-42 mAb and the B4 isolectin derived from Griffonia Simplicifolia, and thus were not O-2Aadult progenitor cells, but ramified microglia. The data suggest that O-2Aadult progenitor cells become GD3+ when placed in culture and that ramified microglia lose GD3-expression in vitro.

Tenascin in the injured rat optic nerve and in non‐neuronal cells in vitro: Potential role in neural repair

The distribution of tenascin was examined in the lesioned adult rat optic nerve and central nervous system (CNS) non-neuronal cells in vitro, by means of a double immunofluorescence technique. Tenascin-like immunoreactivity is localized to the leptomeninges and astrocytes that border the site of optic nerve transection. Anti-tenascin labeling was observed as early as 24 hours after transection, when it appeared as a fine interface between leptomeninges and neural tissue. The anti-tenascin labeling increased in the cells at this border zone during the next 2 weeks, and disappeared 18-21 days after transection. In vitro studies further confirmed that both astrocytes and leptomeningeal cells express tenascin as detected by immunofluorescence labeling with anti-tenascin antibodies. However, the pattern of immunolabeling associated with the two cell types differed. Astrocytes showed exclusively punctate labeling of the cell surface, while leptomeningeal cells showed mainly coarse, fibrillary, matrix-like deposits. Astrocytes and leptomeningeal cells remained segregated when cocultured. In these cultures, an increased amount of the fibrillary, matrix-like deposits of tenascin was also observed in the region of the interface between astrocytes and leptomeningeal cells when these two cell types contact each other. Given the antiadhesive and antispreading properties of tenascin, these in vivo and in vitro results suggest that tenascin might play a role in the initial segregation of leptomeningeal cells from neural tissue at the site of CNS trauma during the first 2 weeks after injury, i.e., prior to the formation of a fully differentiated glia limitans. Therefore, tenascin may influence the early stages in the formation of the glia limitans, and thus prevent the indiscriminate migration of leptomeningeal cells into CNS tissue after injury.

Immunolocalization of proteolipid protein peptide 103-116 in myelin.

Determination of the topographic orientation of proteolipid protein (PLP) within myelin is part of an overall understanding of the functions of PLP and the roles of its multiple domains in diseases that primarily affect central nervous system (CNS) myelin. As part of an analysis of PLP orientation, two mouse monoclonal antibodies (mAb) and a rabbit antiserum against a synthetic peptide corresponding to PLP residues 103-116 (YKTTICGKGLSATV) were tested for their reactivity on compact CNS myelin. By ELISA, the antibodies react with intact PLP and PLP residues 103-116, but not with other PLP peptides. Ultrathin cryosections of adult rat optic nerve were immunostained and antibody binding was localized using appropriate second antibodies coupled to 1 nm gold particles that were visualized by silver enhancement. Localization of the particles on the major or intermediate dense lines was determined by three independent observers. Using the PLP peptide mAb and the polyclonal antibody, we demonstrated that > or = 71% of the particles were localized on the major dense line. At least 66% of particles directed against myelin basic protein, which is known to occur on the major dense line, were also found in that location. These semiquantitative morphologic observations suggest that PLP residues 103-116 occur on the cytoplasmic face of the myelin membrane.

Contacts between regenerating axons and the Schwann cells of sciatic nerve segments grafted to the optic nerve of adult rats.

The relation between Schwann cells, basal laminae and axons during retinal ganglion cell regeneration was studied by using cellular, acellular and partially acellular sciatic nerve autografts into the optic nerve. Acellular grafts were achieved by temporary compression which eliminates living Schwann cells and axons. The compressed sciatic nerve together with the intact portion was used as a partially acellular graft. The compressed portion was anastomosed to the optic nerve and the intact portion was situated distally. After 3-21 days post-operation, the grafts were studied by thin sectioning and freeze-fracture. Axons were seen to regenerate into cellular grafts in contact with Schwann cells after one week, but not into acellular grafts for the entire period. In the partially acellular grafts, regenerating axons were first observed after two weeks and were always in contact with Schwann cells migrating from the intact portion. Moreover, membrane specializations, fuzzy materials in the space between apposed membranes, and putative tight junctions, were found between regenerated axons including growth cone and Schwann cells, and between adjoining Schwann cells. An extensive meshwork of putative tight junctions was displayed between reforming perineurial cells surrounding the groups of Schwann cells and associated axons. Gap junctions were seen between adjoining Schwann cells, and between reforming perineurial cells. These results suggest that the axonal contact with Schwann cell surfaces plays an important role in retinal ganglion cell regeneration.

A New Method for Expressing Axonal Size: Rat Optic Nerve Analysis

Analysis of the shape of the cross sections of adult rat optic nerve axons reveals that the majority of axons do not have a true circular shape. Therefore, determination of axonal size has to utilize methods of approximation. The method presented here utilizes three calculated parameters for expression of axonal size: (i) axonal diameter, as calculated from its area, or (ii) axonal diameter, as calculated from its perimeter, both assuming axonal shape to be a perfect circle and (iii) axonal shape factor, which represents the divergence of the axon from a perfect circular shape. The use of the calculated axonal diameter, with a correction for its shape factor, provides a normalized way of expressing axonal size.