Growth and myelination of goldfish optic nerve fibers after retina regeneration and nerve crush

Demyelination and shrinkage of axons in the retinal nerve fiber layer in chickens developing deprivation myopia

Placing diffusers in front of the eyes induces deprivation myopia in a variety of animal models. As a result of the low pass filtering of the retinal images, less spatial information is available to the retina which should reduce neural activity. Since it has been found that myelination of axons in the central nervous system is modulated by neuronal activity, we have studied whether ganglion cell axons may shrink in response to the restricted visual input. Young chickens were treated for 5 h or 7 days with frosted diffusers to induce deprivation myopia. Nerve fiber layer thickness was measured in vivo, using B-scan OCT. Refractive states were tracked by IR photoretinoscopy, and UV fundus reflectivity by a custom-built device which flashed an LED centered in the camera aperture and recorded pupil brightness after refractive errors were corrected by trial lenses. Moreover, structure and histology of the retinal nerve fibers layer (RNFL) were analyzed ex vivo using transmission electron microscopy and immunohistochemistry. Since chicks have both non-myelinated and myelinated fibers in their RNFL, the thickness of myelin sheaths (G ratio) was measured, as well as the percentage of myelinated axons and the diameters of unmyelinated axons. Short-term deprivation caused an increase in UV fundus reflectivity already after 5 h (measured as pixel grey levels in the pupil: 28 ± 5 vs. 36 ± 10, p < 0.05) and thinning of the myelin sheaths (higher G ratio), compared to untreated control eyes (0.74 ± 0.01 vs. 0.79 ± 0.03, p < 0.05). Neither axon diameters (0.81 ± 0.05 μm vs. 0.82 ± 0.15 μm) nor thickness of the RNFL had changed after only 5 h (42.9 ± 1.3 μm vs. 42.3 ± 2.5 μm). However, after 7 days of diffuser wear, axons had become thinner (0.56 ± 0.14 μm vs. 0.78 ± 0.09 μm vs, p < 0.05), which could explain the thinning of the RNFL (36.3 ± 2.7 μm vs. 42.1 ± 2.4 μm, p < 0.01). Furthermore, myopic eyes had 38% less myelinated axons than untreated eyes as determined by immunohistochemical labelling against myelin basic protein (immunopositive areas in the central retina 1406 ± 341 μm² vs. 2185 ± 290 μm² in controls, p < 0.001). Myelin sheaths in the remaining axons remained unchanged (G ratio 0.76 ± 0.02 vs. 0.76 ± 0.03). Our study shows that deprivation myopia is associated with a significant loss of myelinated axons and shrinkage of the axon diameters of certain fibers in the RNFL. Early changes were already detected after 5 h and were accompanied by an increased fundus reflectivity in UV light. These parameters could therefore serve as the biomarkers for myopia development, at least in the chicken.

Optic-nerve-transmitted eyeshine, a new type of light emission from fish eyes

BACKGROUND

Most animal eyes feature an opaque pigmented eyecup to assure that light can enter from one direction only. We challenge this dogma by describing a previously unknown form of eyeshine resulting from light that enters the eye through the top of the head and optic nerve, eventually emanating through the pupil as a narrow beam: the Optic-Nerve-Transmitted (ONT) eyeshine. We characterize ONT eyeshine in the triplefin blenny Tripterygion delaisi (Tripterygiidae) in comparison to three other teleost species, using behavioural and anatomical observations, spectrophotometry, histology, and magnetic resonance imaging. The study's aim is to identify the factors that determine ONT eyeshine occurrence and intensity, and whether these are specifically adapted for that purpose.

RESULTS

ONT eyeshine intensity benefits from locally reduced head pigmentation, a thin skull, the gap between eyes and forebrain, the potential light-guiding properties of the optic nerve, and, most importantly, a short distance between the head surface and the optic nerves.

CONCLUSIONS

The generality of these factors and the lack of specifically adapted features implies that ONT eyeshine is widespread among small fish species. Nevertheless, its intensity varies considerably, depending on the specific combination and varying expression of common anatomical features. We discuss whether ONT eyeshine might affect visual performance, and speculate about possible functions such as predator detection, camouflage, and intraspecific communication.

The glial design of a teleost optic nerve head supporting continuous growth

This study demonstrates the peculiarities of the glial organization of the optic nerve head (ONH) of a fish, the tench (Tinca tinca), by using immunohistochemistry and electron microscopy. We employed antibodies specific for the macroglial cells: glutamine synthetase (GS), glial fibrillary acidic protein (GFAP), and S100. We also used the N518 antibody to label the new ganglion cells' axons, which are continuously added to the fish retina, and the anti-proliferating cell nuclear antigen (PCNA) antibody to specifically locate dividing cells. We demonstrate a specific regional adaptation of the GS-S100-positive Müller cells' vitreal processes around the optic disc, strongly labeled with the anti-GFAP antibody. In direct contact with these Müller cells' vitreal processes, there are S100-positive astrocytes and S100-negative cells ultrastructurally identified as microglial cells. Moreover, a population of PCNA-positive cells, characterized as glioblasts, forms the limit between the retina and the optic nerve in a region homologous to the Kuhnt intermediary tissue of mammals. Finally, in the intraocular portion of the optic nerve there are differentiating oligodendrocytes arranged in rows. Both the glioblasts and the rows of developing cells could serve as a pool of glial elements for the continuous growth of the visual system.

Distribution of the phosphorylated form of microtubule associated protein 1B in the fish visual system during optic nerve regeneration

Microtubule associated proteins are a heterogeneous group of proteins that have been implicated in regulating microtubule stability. They play an important role in the organisation of the neuronal cytoskeleton during neurite outgrowth, plasticity and regeneration. The fish visual system presents a considerable degree of plasticity. Thus, the retina grows continually throughout life and the optic nerve regenerates after crush. In the present study, we compared the distribution of the microtubule associated protein 1B in its phosphorylated form (MAP1B-phos) in the normal adult fish visual system with that observed during optic nerve regeneration after adult optic nerve crush using a specific monoclonal antibody mAb-150. Expression of MAP1B-phos was observed in some ganglion cell somata and in developing, growing axons within the control optic nerve. Few immunoreactive terminals were seen in the control optic tectum. After optic nerve crush, we found additional MAP1B-phos expression in regenerating axons throughout the visual system. Our results demonstrate that MAP1B-phos is present in growing and regenerating axons of fish retinal ganglion cells, which suggests that the phosphorylated form of MAP1B may play an important role in developmental and regeneration processes within the fish central nervous system.

Gap junctional coupling between progenitor cells at the retinal margin of adult goldfish

We prepared living slice preparations of the peripheral retina of adult goldfish to examine electrical membrane properties of progenitor cells at the retinal margin. Cells were voltage-clamped near resting potential and then stepped to either hyperpolarizing or depolarizing test potentials using whole-cell voltage-clamp recordings. Electrophysiologically examined cells were morphologically identified by injecting both Lucifer Yellow (LY) and biocytin. All progenitor cells examined (n = 37) showed a large amount of passively flowing currents of either sign under suppression of the nonjunctional currents flowing through K(+) and Ca(2+) channels in the cell membrane. They did not exhibit any voltage-gated Na(+) currents. Cells identified by LY fills were typically slender. As the difference between the test potential and the resting potential increased, 13 out of 37 cells exhibited symmetrically voltage- and time-dependent current decline on either sign at the resting potential. The symmetric current profile suggests that the current may be driven and modulated by the junctional potential difference between the clamping cell and its neighbors. The remaining 24 cells did not exhibit voltage dependency. A gap junction channel blocker, halothane, suppressed the currents. A decrease in extracellular pH reduced coupling currents and its increase enhanced them. Dopamine, cAMP, and retinoic acid did not influence coupling currents. Injection of biocytin into single progenitor cells revealed strong tracer coupling, which was restricted in the marginal region. Immature ganglion cells closely located to the retinal margin exhibited voltage-gated Na(+) currents. They did not reveal apparent tracer coupling. These results demonstrate that the marginal progenitor cells couple with each other via gap junctions, and communicate biochemical molecules, which may subserve or interfere with cellular differentiation.

Functional differentiation of ganglion cells from multipotent progenitor cells in sliced retina of adult goldfish

Multipotent progenitor cells at the retinal margin of adult goldfish give rise to all cell types in the rest of the retina. We took advantage of this spatial arrangement of progenitor and mature cells in slices of peripheral retina, to investigate the appearance and maturation of voltage-activated Na(+) current. We divided the peripheral retina into three broad regions (marginal, intermediate, and mature) on the basis of their morphological development. Whole-cell patch-clamp recordings were performed in ruptured-patch mode, so that cells from which currents were recorded could be identified by Lucifer Yellow fills. No voltage-activated Na(+) current was detected in the slender, peripherally located marginal cells. Voltage-activated Na(+) currents were detected in rounded cells found alongside or near marginal cells, facing the vitreal side of the retina. Some of these "intermediate cells" had a long axon-like process which ran along the vitreal surface. Intermediate cells adjacent to the marginal region tended to have smaller Na(+) currents than intermediate cells closer to the mature region. On average, the maximum Na(+) current amplitude recorded from intermediate cells was roughly 6-fold smaller than that of mature ganglion cells. In addition, the activation threshold of the Na(+) current in intermediate cells was nearly 14 mV more positive than that of mature ganglion cells. The results indicate that voltage-activated Na(+) current, as a possible marker of retinal ganglion cells, begins to develop well before these cells migrate to their adult position within the retina.

Fish optic nerve oligodendrocytes support axonal regeneration of fish and mammalian retinal ganglion cells

Segments from adult fish and rat retinae were explanted on myelin-marker expressing oligodendrocytes derived from the regenerating goldfish optic nerve. Fish axons grew in high density and even rat retinal axons regenerated to considerable length on the surface of the fish oligodendrocytes, suggesting that this type of fish glia has axon-growth promoting surface components that exert their influence across species boundaries. One interesting surface component of the fish oligodendrocytes as demonstrated here is the E 587 antigen, which is related to the L1 family of cell adhesion molecules. In long term cocultures of oligodendrocytes and retinal axons, the fish glial cells were found to enwrap rat axons. This suggests that the oligodendrocytes of the regenerating goldfish optic nerve/tract may, despite striking differences, represent the equivalent to mammalian optic nerve oligodendrocytes.

Time-course of ultrastructural changes in regenerated optic fiber terminals of goldfish

The ultrastructure of regenerated optic fiber terminals differs from normal terminals during the first 12 months following optic nerve crush. The area of the regenerated terminals occupied by axoplasm initially increases (1 month postcrush, mpc), then declines to a below normal level (8-12 mpc) and eventually returns to the normal level (16 mpc). The density of vesicles within the regenerated terminals remains initially the same (1 mpc), then increases (4-12 mpc) and finally returns to normal values by 16 mpc. The multiplicity of reestablished retino-tectal synapses gradually increased from an initially lower value at 1 mpc to the normal value by 4 mpc whereas the length of their synaptic contacts decreased from an initial elongation (1 mpc) to the normal length (4 mpc).

Stages of axonal regeneration following optic nerve crush in goldfish: contrasting effects of conditioning nerve lesions and intraocular acetoxycycloheximide injections

The progress of axonal outgrowth after a crush lesion of the goldfish optic nerve can be determined by examining longitudinal silver-stained sections at selected intervals. The outgrowth of leading axons proceeded at 0.46 mm/day after an initial delay of 4.2 days. Outgrowth can be rapidly characterized by differentiating among a series of qualitatively different stages. In the sprouting (S) phase of regeneration, stage S1 is defined by the presence of isolated axonal sprouts reaching into the crush zone, and stage S2 by bundles of sprouts in the crush zone. In the elongation (E) phase of regeneration, stage E1 is defined by bundles that bridge the crush zone, stage E2 by bundles that reach the optic chiasm, and stage E3 by bundles that reach the level of the hypothalamus. During normal regeneration, stage E2 was attained 7-9 days after the crush (testing lesion), and stage E3 at 11 days. However, if the testing lesion had been preceded by a similar (conditioning) lesion 2 weeks earlier, stage E2 was reached at 3 days and stage E3 at 5 days. Conversely, when a protein synthesis inhibitor (acetoxycycloheximide) was injected into the right eye daily from the 5th through 9th day after a testing lesion, the injected side lagged 1-2 stages behind the contralateral control side in nerves examined on the 10th day.

The re-establishment of synaptic transmission by regenerating optic axons in goldfish: time course and effects of blocking activity by intraocular injection of tetrodotoxin

Intraocular injections of tetrodotoxin were used to block activity for 27 days in normal fish and for the first 27 or 31 days of regeneration in fish with one optic nerve crushed. Synaptic activity was then assessed by a current source-density analysis of field potentials evoked by optic nerve shock at different times following the TTX treatment. In normal fish, the lack of activity for 4 weeks had no significant effect on the maintenance of synaptic strength. Likewise, in fish with nerve crush, lack of activity did not prevent the regenerating optic fibers from forming synapses that were nearly as effective as those formed in controls injected with the citrate buffer vehicle. The earliest synapses were formed at the rostromedial corner of the tectum (where the tract enters) at 20 days after nerve crush, when fibers had not yet reached the caudal areas. By 28 days synaptic potentials could be recorded everywhere on the surface of the tectum in both controls and TTX injected fish. However, the latency of the responses with TTX were longer, suggesting a smaller caliber of fiber, which is consistent with an earlier finding of decreased axonal transport in TTX fish. Maturation of the regenerating fibers proceeded slowly in both TTX and control fish. After more than 5 months, the projections were nearly normal but still not completely normal.

Effect of a conditioning lesion on optic nerve regeneration in goldfish

Following a 'test lesion' (crush) of the optic nerve in goldfish, histological study of axons in silver-stained sections showed that outgrowth of the leading axons began after an initial delay of 4.3 days and proceeded at 0.34 +/- 0.03 mm/day. When a 'conditioning lesion' (crush at the same site) preceded the testing lesion by 2 weeks, the initial delay was 2.5 days and the outgrowth rate was 0.74 +/- 0.13 mm/day (P less than 0.01). Two additional methods, utilizing intraocular injections of tritiated proline or fucose to label axonally transported proteins, were used to examine the outgrowth of leading optic axons. (a) Measurement of the distances reached by labeled axons in the nerve at 6 and 10 days after a testing lesion alone yielded an initial delay of 4.6 days and an outgrowth rate of 0.41 +/- 0.04 mm/day. However, when a conditioning lesion preceded the testing lesion, labeled optic axons were already found to have reached the optic tectum by 10 days after the testing lesion, indicating an outgrowth rate in excess of 0.64 mm/day. (b) Determination of the times at which labeled axons arrived at the optic tectum showed that the outgrowth rate after a testing lesion along was 0.40 mm/day whereas when the testing lesion was preceded by a conditioning lesion it was 0.74 mm/day. Thus, as a result of a conditioning lesion the initial delay was reduced by nearly half and the outgrowth rate was nearly doubled.

Early stages of axonal regeneration in the goldfish optic tract: an electron microscopic study

Two hours after the goldfish optic tract was cut, the severed axons in the retinal stump of the tract showed ballooning of the axoplasm and myelin sheath in the region of the cut, with accumulation in the swollen axon of various organelles, including dense cored vesicles. By day 1 the myelin sheath had degenerated back to a node of Ranvier and the tip of the severed axon had formed a myelin-free terminal bulb with a well-organized core of 9-10 nm filaments. By 2 days, such terminal bulbs were often seen to be extended on a neck of cytoplasm a few micrometers in length, presumably indicating axonal outgrowth. In addition, occasional small bundles of axon sprouts were first seen at this time. The sprouts had a diameter of about 2 micrometers and contained a central core of 9-10 nm filaments surrounded by a mantle of cell organelles (smooth endoplasmic reticulum, mitochondria and diverse vesicles), with few if any microtubules. Sprouts within a bundle were separated by fairly uniform 10-15 nm spaces. Beginning at 3 days, significant numbers of microtubules appeared in the sprouts, and there was an increasing proportion of small diameter (greater than or equal to 0.3 micrometer) sprouts. Thus it was not until 3 days that the sprouts took on the appearance usually considered to be typical of regenerating axons. By 6 days a dense layer of glial cells or macrophages formed a cap over the cut surface of the tract. Penetrating this layer were bundles containing up to 20-30 axon sprouts and also single axons which may have been serving as 'pioneering' fibres to which later-emerging axons would attach. There was no evidence that the regenerating axons were guided by the glial cells. At 6 days astroglia began to separate individual axons within the bundles but oligodendrocytes were still inactive at this time.