The degenerative and regenerative processes after the elimination of the proliferative peripheral retina of fish

Decline in Constitutive Proliferative Activity in the Zebrafish Retina with Ageing

It is largely assumed that the teleost retina shows continuous and active proliferative and neurogenic activity throughout life. However, when delving into the teleost literature, one finds that assumptions about a highly active and continuous proliferation in the adult retina are based on studies in which proliferation was not quantified in a comparative way at the different life stages or was mainly studied in juveniles/young adults. Here, we performed a systematic and comparative study of the constitutive proliferative activity of the retina from early developing (2 days post-fertilisation) to aged (up to 3–4 years post-fertilisation) zebrafish. The mitotic activity and cell cycle progression were analysed by using immunofluorescence against pH3 and PCNA, respectively. We observed a decline in the cell proliferation in the retina with ageing despite the occurrence of a wave of secondary proliferation during sexual maturation. During this wave of secondary proliferation, the distribution of proliferating and mitotic cells changes from the inner to the outer nuclear layer in the central retina. Importantly, in aged zebrafish, there is a virtual disappearance of mitotic activity. Our results showing a decline in the proliferative activity of the zebrafish retina with ageing are of crucial importance since it is generally assumed that the fish retina has continuous proliferative activity throughout life.

Histopathological characterisation of retinal lesions associated to Diplostomum species (Platyhelminthes: Trematoda) infection in polymorphic Arctic charr Salvelinus alpinus

The eye represents an immune privileged organ where parasites can escape host reactions. This study provides the first systematic evidence of the pathology associated with Diplostomum sp. infection in the eye retina of fish (i.e. Arctic charr). Histological sections showed that the trematodes caused mechanical disengagement between the retinal pigmentary epithelium and the neurosensory retina, with damaged cones and rods in the outer segment and epithelium reduced to a single layer of pigmentary cells. The metacercariae were “floating” in possibly fluid-filled vesicles together with several round cells, mostly located in the anterio-dorsal and anterio-ventral areas of the eye near the iris. The round cells may indicate internal retinal damage repair mechanisms, without connections to the general immune system. Metacercariae intestines contained pigmented cellular debris indicating that they feed on retinal epithelium. These retinal lesions may have similar vision effects as focal retinal detachment in vertebrates. Diplostomum metacercaria alters fish visual acuity but may in a lesser degree lead to a severe or total visual impairment because of repairing mechanisms. The pathology in the retina seems thereby to be dependent on fish size, age and dose.

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Histological description of the distribution of Diplostomum in the eye of Arctic charr Salvelinus alpinus.

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Metacercaria specifically located in spaces formed between the retinal pigmentary epithelium and the neurosensory retina.

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Lesions display closer similarities with those observed in retinal detachment in other species.

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Changes in the frequency and distribution of the lesions between morphs are suggested.

Sox10 Expression in Goldfish Retina and Optic Nerve Head in Controls and after the Application of Two Different Lesion Paradigms

The mammalian central nervous system (CNS) is unable to regenerate. In contrast, the CNS of fish, including the visual system, is able to regenerate after damage. Moreover, the fish visual system grows continuously throughout the life of the animal, and it is therefore an excellent model to analyze processes of myelination and re-myelination after an injury. Here we analyze Sox10⁺ oligodendrocytes in the goldfish retina and optic nerve in controls and after two kinds of injuries: cryolesion of the peripheral growing zone and crushing of the optic nerve. We also analyze changes in a major component of myelin, myelin basic protein (MBP), as a marker for myelinated axons. Our results show that Sox10⁺ oligodendrocytes are located in the retinal nerve fiber layer and along the whole length of the optic nerve. MBP was found to occupy a similar location, although its loose appearance in the retina differed from the highly organized MBP⁺ axon bundles in the optic nerve. After optic nerve crushing, the number of Sox10⁺ cells decreased in the crushed area and in the optic nerve head. Consistent with this, myelination was highly reduced in both areas. In contrast, after cryolesion we did not find changes in the Sox10⁺ population, although we did detect some MBP^- degenerating areas. We show that these modifications in Sox10⁺ oligodendrocytes are consistent with their role in oligodendrocyte identity, maintenance and survival, and we propose the optic nerve head as an excellent area for research aimed at better understanding of de- and remyelination processes.

Chronotopographical distribution patterns of cell death and of lectin-positive macrophages/microglial cells during the visual system ontogeny of the small-spotted catshark Scyliorhinus canicula

The patterns of distribution of TUNEL-positive bodies and of lectin-positive phagocytes were investigated in the developing visual system of the small-spotted catshark Scyliorhinus canicula, from the optic vesicle stage to adulthood. During early stages of development, TUNEL-staining was mainly found in the protruding dorsal part of the optic cup and in the presumptive optic chiasm. Furthermore, TUNEL-positive bodies were also detected during detachment of the embryonic lens. Coinciding with the developmental period during which ganglion cells began to differentiate, an area of programmed cell death occurred in the distal optic stalk and in the retinal pigment epithelium that surrounds the optic nerve head. The topographical distribution of TUNEL-positive bodies in the differentiating retina recapitulated the sequence of maturation of the various layers and cell types following a vitreal-to-scleral gradient. Lectin-positive cells apparently entered the retina by the optic nerve head when the retinal layering was almost complete. As development proceeded, these labelled cells migrated parallel to the axon fascicles of the optic fiber layer and then reached more external layers by radial migration. In the mature retina, lectin-positive cells were confined to the optic fiber layer, ganglion cell layer and inner plexiform layer. No evident correlation was found between the chronotopographical pattern of distribution of TUNEL-positive bodies and the pattern of distribution of lectin-labelled macrophages/microglial cells during the shark's visual system ontogeny.

Prox1 expression in rod precursors and Müller cells

The transcription factor Prox1 acts in rodent retinogenesis, at least in promoting cell cycle withdrawal and horizontal cell production. In the mature retina, this protein is detected at the inner nuclear layer of all vertebrate groups. We have made a neurochemical characterisation of Prox1(+) cell types in two different vertebrate groups: mammals and fish. As well as Prox1(+) horizontal cells, we have observed Prox1(+)/PKC-alpha(+) rod bipolar cells in mouse and cone ON and mixed b bipolar cells in goldfish. In mouse, only some CB(+) and CR(+) amacrine cells are Prox1(+) and the TH(+) and CR(+) amacrine cells are Prox1(-). However, in goldfish all CR(+) amacrine cells and TH(+) interplexiform cells are Prox1(+) and in the GCL displaced amacrine cells are also Prox1(+). Besides its expression in different interneuron subpopulations, we demonstrate, for the first time, the presence of Prox1 in the GS(+) and CRALBP(+) Müller cells in the retina of adult mammals and in developing and mature retina of fish. The presence of Prox1 in these cells appears to be related to survival or maintenance of their phenotype. We also demonstrate that in fish, where retinal formation persists into adulthood, Prox1 is expressed in dividing PCNA(+) cells at the peripheral growing zone, in rod progenitors at the inner and outer nuclear layers as well as in early progenitors during a retinal regeneration process after cryo-lesion of the peripheral growing zone. Therefore, Prox1 functions in vertebrate retinogenesis may be more complex than previously expected.