The ocular morphology of the southern hemisphere lamprey Geotria australis gray, with special reference to optical specialisations and the characterisation and phylogeny of photoreceptor types

This paper describes the ocular morphology of young adults of the southern hemisphere lamprey Geotria australis, the sole representative of the Geotriidae, and makes comparisons with those of holarctic lampreys (Petromyzontidae). As previously reported for the holarctic lamprey Ichthyomyzon unicuspis [Collin and Fritzsch, 1993], the lens of G. australis is non-spherical and possesses a cone-shaped posterior that may be capable of mediating variable focus. The avascular retina of G. australis is well differentiated, containing three retinal ganglion cell populations, three layers of horizontal cells and three photoreceptor types. In contrast to petromyzontids that contain only two photoreceptor types (short and long), G. australis possesses one rod-like (R1) and two cone-like (C1 and C2) photoreceptors. Although the rodlike receptor in G. australis may be homologous with the short receptors of holarctic lampreys, the two cone-like receptors have morphological characteristics that differ markedly from those of the long receptors of their holarctic counterparts. The features which distinguish the two cone-like receptors from those of the long receptor type in holarctic lampreys are the characteristics of the mitochondria and the presence of large amounts of two different types of stored secretory material in the endoplasmic reticulum of the myoid (refractile bodies). The endoplasmic reticulum of each receptor type has a different shape and staining profile and is polymorphic, each showing a continuum of distension. It is proposed that the presence of two cone-like photoreceptors with different characteristics would increase the spectral range of G. australis and thus be of value during the parasitic phase, when this lamprey lives in the surface marine waters. The irideal flap, present in G. australis but not petromyzontids, would assist in reducing intraocular flare during life in surface waters. The results of this study, which are discussed in the context of the proposed evolution of lampreys, emphasise that it is important to take into account the characteristics of the eyes of southern hemisphere lampreys when making generalizations about the eyes of lampreys as a whole

Retinal pigment epithelium and photoreceptor maturation in a wallaby, the Quokka

Cell generation and the early stages of maturation of the retinal pigment epithelium (RPE) and photoreceptors were examined in a marsupial, the quokka, Setonix brachyurus. Results are presented for animals aged up to postnatal day (P)250. RPE cell generation was studied by analysis of cell number from wholemounted retinae and by tritiated thymidine (3HThy) autoradiography in sectioned material. For 3HThy autoradiography, quokkas aged P1–P200 were injected with 3HThy and killed either 6–20 hours later (pulse-kill) or at P100 or P250 (pulse-leave). The extent of pigmentation of the RPE sheet was examined from sections of embryonic and early postnatal stages. Retinae from animals aged P5 to P160 were also examined at the electron microscope. By P100, RPE cell number is within the range found in adults. New RPE cells are generated in a peripheral band which moves outwards as cells leave the cell cycle in more central locations. RPE cells thus complete their last cell division in a centre-to-periphery wave centred about the optic nerve head. At any given retinal location, RPE cells complete their last cell division earlier than the overlying layers of the neural retina. Cells of the RPE rapidly develop a mature morphology. For example, melanin granules are observed at P5 and Verhoeff's membrane (the terminal bar complex) is evident by P25. By contrast, photoreceptor development in this species is protracted; cone inner segments are observed by P40, whilst the first rod inner segments are observed at P60. Despite being generated earlier, morphological maturation of the cones appears retarded and prolonged compared with that of the rods. The last stages of RPE cell maturation occur late in development, in synchrony with the generation of rods

Fine structure of the retinal pigment epithelium and cones of Antarctic fish Notohenia coriiceps Richardson in light and dark-conditions

The Antarctic fish Notothenia coriiceps Richardson, 1844 lives in an environment of daily and annual photic variation and retina cells have to adjust morphologically to environmental luminosity. After seven day dark or seven day light acclimation of two groups of fish, retinas were extracted and processed for light and transmission electron microscopy. In seven day dark adapted, retina pigment epithelium melanin granules were aggregated at the basal region of cells, and macrophages were seen adjacent to the apical microvilli, between the photoreceptors. In seven day light adapted epithelium, melanin granules were inside the apical microvilli of epithelial cells and macrophages were absent. The supranuclear region of cones adapted to seven day light had less electron dense cytoplasm, and an endoplasmic reticulum with broad tubules. The mitochondria in the internal segment of cones adapted to seven day light were larger, and less electron dense. The differences in the morphology of cones and pigment epithelial cells indicate that N. coriiceps has retinal structural adjustments presumably optimizing vision in different light conditions.<br>O peixe Antártico Notothenia coriiceps Richardson, 1844 habita meios com variações fóticas diária e anual e as células da retina se adaptam morfologicamente a esta luminosidade ambiental. Dois grupos de peixes foram aclimatados durante sete dias à luz constante ou ao escuro constante. Após secção medular, as retinas foram extraídas e processadas para microscopia de luz e microscopia eletrônica de transmissão. No epitélio pigmentar da retina adaptado sete dias ao escuro, os pigmentos de melanina agregam-se na base coroidal das células epiteliais pigmentares e macrófagos são encontrados no interior do processos apicais entre as células fotorreceptoras. No epitélio adaptado sete dias à luz os pigmentos de melanina se dispõem ao longo das projeções apicais das células epiteliais pigmentares e os macrófagos são ausentes. A região supranuclear dos cones adaptados sete dias à luz possui um citoplasma menos elétron denso devido a presença de canais do retículo endoplasmático que se dilatam aumentando o seu lúmen. As mitocôndrias existentes no segmento interno dos cones adaptados à luz são maiores, e menos elétron densas. As diferenças na morfologia dos cones e nas células epiteliais pigmentares indicam que N. coriiceps possui mecanismos celulares de ajustes na retina provavelmente otimizando a capacidade visual em diferentes situações de luz