Specification of regenerating retinal ganglion cells in the adult newt, Triturus cristatus

Tissue interaction between the retinal pigment epithelium and the choroid triggers retinal regeneration of the newt Cynops pyrrhogaster

Complete retinal regeneration in adult animals occurs only in certain urodele amphibians, in which the retinal pigmented epithelial cells (RPE) undergo transdifferentiation to produce all cell types constituting the neural retina. A similar mechanism also appears to be involved in retinal regeneration in the embryonic stage of some other species, but the nature of this mechanism has not yet been elucidated. The organ culture model of retinal regeneration is a useful experimental system and we previously reported RPE transdifferentiation of the newt under this condition. Here, we show that cultured RPE cells proliferate and differentiate into neurons when cultured with the choroid attached to the RPE, but they did not exhibit any morphological changes when cultured alone following removal of the choroid. This finding indicates that the tissue interactions between the RPE and the choroid are essential for the former to proliferate. This tissue interaction appears to be mediated by diffusible factors, because the choroid could affect RPE cells even when the two tissues were separated by a membrane filter. RPE transdifferentiation under the organotypic culture condition was abolished by a MEK (ERK kinase) inhibitor, U0126, but was partially suppressed by an FGF receptor inhibitor, SU5402, suggesting that FGF signaling pathway has a central role in the transdifferentiation. While IGF-1 alone had no effect on isolated RPE, combination of FGF-2 and IGF-1 stimulated RPE cell transdifferentiation similar to the results obtained in organ-cultured RPE and choroid. RT-PCR revealed that gene expression of both FGF-2 and IGF-1 is up-regulated following removal of the retina. Thus, we show for the first time that the choroid plays an essential role in newt retinal regeneration, opening a new avenue for understanding the molecular mechanisms underlying retinal regeneration.

Neural cell differentiation from retinal pigment epithelial cells of the newt: an organ culture model for the urodele retinal regeneration

Transdifferentiation from retinal pigment epithelium (RPE) to neural retina (NR) was studied under a new culture system as an experimental model for newt retinal regeneration. Adult newt RPEs were organ cultured with surrounding connective tissues, such as the choroid and sclera, on a filter membrane. Around day 7 in vitro, lightly pigmented "neuron-like cells" with neuritic processes were found migrating out from the explant onto the filter membrane. Their number gradually increased day by day. BrdU-labeling study showed that RPE cells initiated to proliferate under the culture condition on day 4 in vitro, temporally correlating to the time course of retinal regeneration in vivo. Histological observations of cultured explants showed that proliferating RPE cells did not form the stratified structure typically observed in the NR but they rather migrated out from the explants. Neuronal differentiation was examined by immunohistochemical detection of various neuron-specific proteins; HPC-1 (syntaxin), GABA, serotonin, rhodopsin, and acetylated tubulin. Immunoreactive cells for these proteins always possessed fine and long neurite-like processes. Numerous lightly pigmented cells with neuron-like morphology showed HPC-1 immunoreactivity. Fibroblast growth factor-2 (FGF-2), known as a potent factor for the transdifferentiation of ocular tissues in various vertebrates, substantially increased the numbers of both neuron-like cells and HPC-1-like immunoreactive cells in a dose-dependent manner. These results indicate that our culture method ensures neural differentiation of newt RPE cells in vitro and provides, for the first time, a suitable in vitro experimental model system for studying tissue-intrinsic factors responsible for newt retinal regeneration.

Gliosis during optic fiber regeneration in the goldfish: an immunohistochemical study

Antisera directed against the 48 kDa and 50 kDa cytoskeletal antigens were used to examine changes in the astroglial fabric of the goldfish visual pathways following optic nerve crush. Several major observations are described. First, an optic nerve crush lesion in these animals appears to be devoid of glial cells for at least the first month after surgery. As a corollary, regenerating axons that grow across the lesion may do so over an aglial substrate. Once the axons cross the lesion, their growth is confined to the astroglial domains of the proximal nerve stump. In the optic nerve, gliosis comprises hypertrophy of astrocytic processes such that the open framework characterizing the normal nerve is obscured. In addition, during regeneration, optic nerve glia express large amounts of the 50 kDa cytoskeletal protein, which they ordinarily express at only minimal levels. In the optic tract, gliosis is reflected in a markedly increased expression of the 50 kDa protein as well as an apparent increase in the number and complexity of glial processes. In addition, optic tract glia begin to express the 48 kDa antigen during regeneration. This protein is ordinarily confined for the most part to the optic nerve and is not seen in the tract glia. Finally, no obvious changes were seen in the glia of the optic tectum. These results demonstrate many points of similarity between gliosis in the goldfish and in mammals. However, in some particulars the two responses differ, and it is possible that these differences are related to the differing ability of central axons to regenerate in the two groups of organisms.

Regeneration of the retina in the adult newt, Triturus cristatus, following surgical division of the eye by a post-limbal incision

An autoradiographic and histological study of retinal regeneration following a post-limbal ocular incision, in Triturus cristatus, is described. In these cases there is a major contribution to the regenerating retina from both the anterior complex (pars ciliaris retinae: ora serrata) and the retinal pigment epithelium. The tissue derived from the anterior complex can independently differentiate into retinal tissue if the anterior and posterior parts of the regenerate do not join. This contrasts with the situation following a limbal incision to the eye where the regenerative response of the anterior complex is inhibited. The significance of labelling patterns in the regenerating retina, following administration of 3H-thymidine, is discussed.

The distribution of intercellular gap junctions in the developing retina and pigment epithelium of Xenopus laevis

The distribution of gap junctions in the developing retina and pigment epithelium of Xenopus has been examined from optic vesicle to photoreceptor outer segment stages. In the retina and pigment epithelium walls of the optic vesicle gap junctions are found between ventricular cells in the apical contact zone and close to the outside wall; a neural tube type of distribution. Optic cup formation brings the retina and pigment epithelium into contact and gap junctions form between them. After the stage of retinal specification gap junctions, previously more numerous dorsally, are more often found in the ventral retina and pigment epithelium. They continue to be found in the retinal centre between ventricular cell processes in the apical contact zone and near the vitreal basement lamella above the ganglion cell layer until stage 35. Gap junctions are occasionally made by ganglion cells and their axons, providing possible pathways initiating cell differentiation. After stage 35 gap junctions seem to be confined to the peripheral rim of the eye cup where new cells are formed throughout larval life. The observed distribution suggests that gap junctions may provide intercytoplasmic pathways for retinal specification intrinsic to the retina and pigment epithelium.

Regeneration of the retina in the adult newt, Triturus cristatus, following surgical division of the eye by a limbal incision

The regeneration of a retina in adult Triturus cristatus, following surgical division of the eye by a limbal incision, was studied. In agreement with recent reports, it was found that the regenerating retina is dervied from two sources; the retinal pigment epithelium and the pars ciliaris retinae. However, following a limbal incision, most of the retina appears to be derived from the retinal pigment epithelium in the posterior part of the eye. An unexpected finding of this study was that large cystlike spaces form in the fundal regions of the eye, between the regenerating retina and the retinalpigment epithelium. These spaces appear between five and eight days post-operative and persist long enough (25 to 30 days postoperative) to disrupt the fundal portion of the rengenerating retina and to cause it to lag behind the rest of the regenerate, in its development. The relationship of these observations to the genesis of positional markers in the regenerating retina is discussed.