Rod photoreceptor neurogenesis

Number and topography of cones, rods and optic nerve axons in New and Old World primates

To better understand the evolution of spatial and color vision, the number and spatial distributions of cones, rods, and optic nerve axon numbers were assessed in seven New World primates (Cebus apella, Saimiri ustius, Saguinus midas niger, Alouatta caraya, Aotus azarae, Calllithrix jacchus, and Callicebus moloch). The spatial distribution and number of rods and cones was determined from counts of retinal whole mounts. Optic axon number was determined from optic nerve sections by electron microscopy. These data were amassed with existing data on retinal cell number and distribution in Old World primates, and the scaling of relative densities and numbers with respect to retinal area, eye and brain sizes, and foveal specializations were evaluated. Regular scaling of all cell types was observed, with the exceptionally large, rod-enriched retina of the nocturnal owl monkey Aotus azarae, and the unusually high cone density of the fovea of the trichromatic howler monkey Alouatta caraya presenting interesting variations on this basic plan. Over all species, the lawful scaling of rods, cones, and retinal ganglion cell number is hypothesized to result from a conserved sequence of cell generation that defends retinal acuity and sensitivity over a large range of eye sizes.

Persistent and injury-induced neurogenesis in the vertebrate retina

The brains of all vertebrates are persistently neurogenic. However, this is not true for the neural retinas. Only three extant classes of vertebrates show significant posthatch/postnatal retinal neurogenesis: amphibians, birds and fish. The retinas of these animals contain an annulus of progenitors at the margin, from which differentiated neurons emerge. In posthatch amphibians and fish the vast majority of the adult retina is added from the margin and neurogenesis is lifelong, whereas in posthatch birds neurogenesis is limited. Unique to fish, rod photoreceptors are added in situ from stem cells within the mature retina. Strikingly, for each class of animal retinal lesions stimulate neuronal regeneration, however the cellular source differs for each: the retinal pigmented epithelium in amphibians and embryonic birds, Müller glia in posthatch birds and intrinsic stem cells in fish. The molecular events surrounding injury-induced neuronal regeneration are beginning to be identified.

Stem cells in the teleost retina: persistent neurogenesis and injury-induced regeneration

The retina of the adult teleost fish is an important model for studying persistent and injury-induced neurogenesis in the vertebrate central nervous system. All neurons, with the exception of rod photoreceptors, are continually appended to the extant retina from an annulus of progenitors at the margin. Rod photoreceptors, in contrast, are added to differentiated retina only from a lineage of progenitors dedicated to making rods. Further, when the retina is lesioned, the lineage that produces only rods ceases this activity and regenerates retinal neurons of all types. The progenitors that supply neurons at the retinal margin and rod photoreceptors and regenerated neurons in the mature tissue originate from multipotent stem cells. Recent data suggest that the growth-associated neurogenic activity in the retina is regulated as part of the growth hormone/insulin-like growth factor-I axis. This paper reviews recent evidence for the presence of stem cells in the teleost retina and the molecular regulation of neurogenesis and presents a consensus cellular model that describes persistent and injury-induced neurogenesis in the retinas of teleost fish.

Putative stem cells and the lineage of rod photoreceptors in the mature retina of the goldfish

The retinas of teleost fish grow continuously, in part, by neuronal hyperplasia and when lesioned will regenerate. Within the differentiated retina, the growth-associated hyperplasia results in the generation of new rod photoreceptors only, whereas injury-induced neurogenesis results in the regeneration of all retinal cell types. It is believed, however, that both new rod photoreceptors and regenerated neurons originate from the same populations of intrinsic progenitors. Experiments are described here that attempt to identify in the normal retina of goldfish neuronal progenitors intrinsic to the retina, particularly those which have remained cryptic because they divide infrequently. Long-term, systemic exposure to bromodeoxyuridine (BrdU) was used to label these cells. Five populations of proliferative cells were labeled: microglia, which are briefly described but not studied further; retinal progenitors in the circumferential germinal zone (CGZ); and rod precursors in the outer nuclear layer (ONL), both of which have been well characterized previously; and two populations of slowly-dividing cells in the inner nuclear layer (INL). The majority of these cells have a fusiform morphology, whereas the remaining ones are spherical. Longitudinal BrdU labeling suggests that the fusiform cells migrate to the ONL to replenish the pool of rod precursors. A subset of the spherical cells express pax6, although none are stained with markers of differentiated amacrine or bipolar cells. It is hypothesized that these rare, pax6-expressing cells are retinal stem cells, which give rise to the pax6-negative fusiform cells. Based on these data, two models are proposed: the first describes the lineage of rod photoreceptors in goldfish; the second is a consensus model of neurogenesis in the retinas of all teleosts.

Photoreceptor rescue

Photoreceptor cell death is the final, irreversible event in many blinding diseases including retinitis pigmentosa, age-related macular disease and retinal detachment. This paper examines the potential strategies for preventing photoreceptor cell death in the context of current understanding of the mechanisms of cell death. There is evidence to suggest that photoreceptor cells are inherently vulnerable, apoptosis is the final common pathway of photoreceptor cell loss, and other retinal cells play an important role in the survival of rods and cones. Furthermore, the rationale of using neurotrophic factors as therapeutic agents in retinal degeneration is discussed in detail. Photoreceptor rescue by manipulation of genes involved in apoptosis and some pharmacological agents is also described.

The embryogenesis of rod photoreceptors in the teleost fish retina, Haplochromis burtoni

Development of the retina, like that of other tissues, occurs via an orderly sequence of cell division and differentiation, producing the functional retina. In teleost fish, however, cell division and differentiation in the retina continue throughout the life of the animal in two distinct ways. Stem cells in a circumferential germinal zone at the periphery of the retina give rise to all retinal cell types and progenitor cells located throughout the retina in the outer nuclear layer (ONL) produce new rod photoreceptors. These processes in adult retina recapitulate in space the embryonic events responsible for forming the retina. Analysis of these events in an African cichlid fish, Haplochromis burtoni, confirmed that cone photoreceptors differentiate first, followed by rod photoreceptors. Correspondingly, at the margin of the eye, cone photoreceptors differentiate nearer to the margin than do rods. Control of photoreceptor production is not understood. Here we present the time of appearance and distribution pattern of GABA and vimentin which are candidates for the control of retinal cell division and differentiation. Antibody staining reveals that both GABA and vimentin exhibit unique patterns of expression during embryonic retinal development. Vimentin immunoreactivity is evident throughout the retina in a spoke-like pattern between developmental Days 4 and 7, as both cone and rod photoreceptors are being formed. GABA is expressed in horizontal cells between Days 5 and 7, corresponding to the onset of rod differentiation in time and in position within the retina. Moreover, the wave of GABAergic staining in the horizontal cells parallels the wave of rod differentiation across the embryonic retina of H. burtoni. Thus, GABA may play a role in the development of rod photoreceptors.

Insulin-like growth factor-I binds in the inner plexiform layer and circumferential germinal zone in the retina of the goldfish

Results of the previous study suggest that insulin-related peptides regulate proliferation of retinal progenitors in the adult goldfish. Because of their known roles in retinal neurogenesis, we have chosen to focus future studies on insulin-like growth factor I (IGF-I) and the IGF-I receptor. In the study described here, we characterized the spatial distribution and specificity of IGF-I binding sites in the retina of the adult goldfish by performing receptor-binding autoradiography with [125I]-IGF-I alone and with unlabeled IGF-I-related molecules (IGF-I, IGF-II, insulin, and des-[1-3]-IGF-I) as competitive inhibitors of [125I]-IGF-I binding. The results of these experiments show that IGF-I binds in two locations in the retina of the adult goldfish, within the inner plexiform layer of the differentiated retina and the circumferential germinal zone. The competition experiments suggest that [125I]-IGF-I binds at sites specific for IGF-I, and that both IGF-I receptors and IGF-I binding proteins are present in the retina.