Axonal transport, degeneration, and regeneration in the visual system of the goldfish

Ganglion cell regeneration following whole-retina destruction in zebrafish

The retinas of adult teleost fish can regenerate neurons following injury. The current study provides the first documentation of functional whole retina regeneration in the zebrafish, Danio rerio, following intraocular injection of the cytotoxin, ouabain. Loss and replacement of laminated retinal tissue was monitored by analysis of cell death and cell proliferation, and by analysis of retina-specific gene expression patterns. The spatiotemporal process of retinal ganglion cell (RGC) regeneration was followed through the use of selective markers, and was found to largely recapitulate the spatiotemporal process of embryonic ganglion cell neurogenesis, over a more protracted time frame. However, the re-expression of some ganglion cell markers was not observed. The growth and pathfinding of ganglion cell axons was evaluated by measurement of the optic nerve head (ONH), and the restoration of normal ONH size was found to correspond to the time of recovery of two visually-mediated behaviors. However, some abnormalities were noted, including overproduction of RGCs, and progressive and excessive growth of the ONH at longer recovery times. This model system for whole-retina regeneration has provided an informative view of the regenerative process.

Control of vertebrate retinal cell production

Regeneration of vertebrate sensory cells can be seen as an extension and elaboration of the process of cellular repair and to understand repair requires knowledge of how cell division and cell fate are determined. To approach these problems, we have developed a slice culture for the teleost retina. Cells continue to divide in the same pattern in this slice culture as they do in vivo as demonstrated with [3H]thymidine labeling. Moreover, cells which divided in culture became retinal cell phenotypes as identified with monoclonal antibodies. Some presumptive rod progenitors in the outer nuclear layer in the center of the retina were also labeled cone-specific, possibly as a regeneration response. These data add to the evidence that cell fate is determined by the environment. This slice preparation will be a useful model system for analyzing putative environmental cues responsible for guiding cell proliferation and differentiation in the fish retina.

In vitro protein synthesis in the goldfish retinotectal pathway during regeneration: evidence for specific axonal proteins of retinal origin in the optic nerve

Four proteins with molecular weights of 58,000 can be separated as a linear array by two-dimensional gel electrophoresis. They are highly concentrated in the goldfish optic nerve and are designated as ON1, ON2, ON3, and ON4. Proteins ON1 and ON2 are undetectable in the optic nerve after disconnection and their concentration is gradually restored during regeneration. In vitro incubations of retinas, optic nerves, or tecta in the presence of [35S]methionine indicate that proteins ON1 and ON2 are of retinal origin. The labeling rate of these proteins in the retina increases fourfold after optic nerve crush whereas the overall labeling rate in the retina remains largely constant. Their synthesis cannot be detected in tissues devoid of retinal ganglion cells. This is consistent with the view that ON1 and ON2 are synthesized by retinal ganglion cells and are consequently of neuronal origin in the optic nerve. In contrast, similar experiments indicate that ON3 and ON4 are of nonneuronal origin. They are synthesized in the optic nerve in the absence of retinal ganglion cells.

Myelinated axons of ganglion cells in the retinae of teleosts

In the retina of the goldfish and the rainbow trout, the axons of ganglion cells belong to the unmyelinated or the myelinated types. The unmyelinated fibers are either arranged in bundles in direct contact with neighboring fibers or they are separated by intervening lamellae of oligodendroglial cytoplasm. The myelin sheaths of the myelinated fibers differ greatly in thickness. Most fibers show 3 to 5 myelin layers; single fiber elements, however, show 10 or even more layers.