Differentiation of the retina and pigment epithelium in the ontogeny of the common frog. 1. The retina

Changes in the ultrastructure of the differentiating retinal cells were studied by means of electron microscopy in Rana temporaria at successive developmental stages. Common features of the onset of differentiation of the retinal cells have been shown: appearance of the granular endoplasmic reticulum elements, of the polysomes, beginning of utilization of the yolk and lipids, elimination of ovarial melanosomes. Later during the differentiation of retinal neurons the protein synthesizing machinery and Golgi complex of these cells develop markedly, the number of mitochondria increases. The differentiation of retina begins from the Müllerian cells (stage 28) which determine the direction of growth of the neuron processes. They are followed by the ganglion cells and photoreceptors (stage 29). The signs of differentiation of the inner nuclear layer neurons become apparent later, in the amacrine and horizontal cells at the same time and in the bipolars later. The main features of neuronal organization of the retina which determine the structural basis of its function of light perception are formed by stage 40.     

Synaptic transmission in the retina.

A recent highlight in the study of the retina has been the publication of evidence that the response of the ON bipolar cells is generated by a cGMP-mediated second messenger system. This GTP-binding protein mechanism is activated by the binding of glutamate, the photoreceptor neurotransmitter, to the 2-amino-4-phosphonobutyrate (APB) class of receptor.     

Peroxidases in the neural retina and pigment epithelium.

Peroxidase activity, assayed with 2 mM-H₂O₂ and suitable hydrogen donors (either p-phenyl-enediamine or diaminobenzidine), was demonstrated in homogenates of neural retina and pigment epithelium of both the dog and the cow. The enzyme is particle-associated in the native state, but is readily extractable by brief sonication or freeze-thawing. At optimum pH, which is between 4.0 and 4.5 for both sources, the specific activity is up to 40 times greater in pigment epithelial cells than in neural retina. Some catalase activity was detected in extracts from both bovine and canine neural retina, but catalase was essentially absent in pigment epithelium. Fractionation of bovine pigment epithelial cells showed that peroxidase activity is associated mainly with heavy organelles sedimenting at low centrifugal forces. Melanosomes, nuclei, melanolysosomes and plasma membranes were the principal organelles identified in these low speed sediments. It was not possible to separate them either by differential centrifugation or on discontinuous sucrose gradients. However, melanosomes were excluded as the only source of peroxidase activity by isolating separately the melanotic and amelanotic cell populations; equal peroxidase was found in both cell types. Since nuclei are not a likely source of this enzyme, it is suggested that most of the peroxidase activity in bovine pigment epithelial cells is localized in either the melanolysosomes, plasma membranes, or both.     

Localization of calcineurin in the mature and developing retina.

We studied the localization of calcineurin by immunoblotting analysis and immunohistochemistry as a first step in clarifying the role of calcineurin in the retina. Rat, bovine, and human retinal tissues were examined with subtype-nonspecific and subtype-specific antibodies for the A alpha and A beta isoforms of its catalytic subunit. In mature retinas of the three species, calcineurin was localized mainly in the cell bodies of ganglion cells and the cells in the inner nuclear layer, in which amacrine cells were distinctively positive. The calcineurin A alpha and A beta isoforms were differentially localized in the nucleus and the cytoplasm of the ganglion cell, respectively. Calcineurin was also present in developing rat retinas, in which the ganglion cells were consistently positive for it. The presence of calcineurin across mammalian species and regardless of age shown in the present study may reflect its importance in visual function and retinal development, although its function in the retina has not yet been clarified. (J Histochem Cytochem 49:187-195, 2001)     

Thyrotropin-like immunoreactivity in the developing chicken retina.

In this paper we analysed the presence and localisation of thyrotropin during retinal development in Gallus domesticus. Specific thyrotropin-like immunohistochemical staining was observed from the beginning of the second incubation week to one day post-hatching in chicken retina. Thyrotropin is a 28.3 KDa glycoprotein, synthesised by the anterior pituitary gland, and it is implicated in the stimulation of the synthesis and release of thyroid hormones. Until now, the action of thyrotropin has been established exclusively in hormonal terms. Recently, this glycoprotein has been localised in synaptic processes in the human retina by using a specific antiserum (Fdez-Trujillo et al., 1995). To the best of our knowledge this report is the first time that thyrotropin has been immunocytochemically demonstrated in the chicken retina. The pattern of thyrotropin-like immunoreactivity suggests that this glycoprotein could act as modulator of synaptic transmission, but it may also play a much broader role in regulating trophic functions.     

Local regeneration in the retina of the goldfish.

We have studied regeneration of the retina in the goldfish as a model of regenerative neurogenesis in the central nervous system. Using a transscleral surgical approach, we excised small patches of retina that were replaced over several weeks by regeneration. Lesioned retinas from three groups of animals were studied to characterize, respectively, the qualitative changes of the retina and surrounding tissues during regeneration, the concomitant cellular proliferation, and the quantitative relationship between regenerated and intact retina. The qualitative and quantitative analyses were done on retinas prepared using standard methods for light microscopy. The planimetric density of regenerated and intact retinal neurons was computed in a group of animals in which the normal planimetric density ranged from high to low. Cell proliferation was investigated by making intraocular injections of 5-bromo-2'-deoxyuridine (BUdr) at various survival times to label proliferating cells and processing retinal sections for BUdr immunocytochemistry. The qualitative analysis showed that the surgery created a gap in the existing retina that was replaced with new retina over the subsequent weeks. The BUdr-labeling experiments demonstrated that the excised retina was replaced by regeneration of new neurons. Neuroepithial-like cells clustered on the wound margin and migrated centripetally, appositionally adding new retina to the old. The quantitative analysis showed that the planimetric density of the regenerated neurons approximated that of the intact ones.     

Analysis of the retina via suprafusion electroretinography.

Electroretinographic (ERG) transient responses elicited in monkeys by abrupt changes in the periodicity of a rapidly intermittent (suprafusion) luminance stimulus were studied experimentally, and analyzed and interpreted through a theory of dynamic retinal responses. The suprafusion ERG transients are confirmed to behave in accord with theoretical expectation, as elemental responses (retinal Green's functions). By aid of the theory the ERG wave-forms can be reduced to two significant elements. One element, accounting for approximately two-thirds of the total ERG variance, is strictly linear, and correlates well with simultaneously evoked cortical (VEP) transients which were previously related to suprafusion perception in humans. The other element, comprising approximately one-third the ERG transient, is a rectification, with properties indicating that it may arise from a specific layer of retinal neurons (amacrine cells); on this assumption the theory demonstrates that high-frequency nonlinear ERG flicker can isolate activities proximal and distal to the rectifying (amacrine) layer. Thus, the hypothesis of an amacrine origin for the rectifying element entails the possibility that suprafusion ERG studies could accomplish in vivo "dissection" of the human retina.     

A duplex retina and the electroretinogram in the nocturnal Perodicticus potto.

The presence of cones in potto's retina has been proved beyond doubt although they are very restricted in number (1 cone for 300 rods). Morphologically, speaking there is no point in calling these cones "rudimentary" except for their slender outer segment. There are red sensitive elements in that retina at wavelengths beyond the spectral sensitivity of visual purple and it is tempting to assume that these elements are cones. The ERG evoked from these elements by red light differs from that in response to white and blue light. They dark-adapt faster than the receptors sensitive to blue and white flashes. However in some of their properties, for example fusion frequency, these cones behave like rods in other species. As these few cones seem to activate the bipolar cells nearly as effectively as the numerous rods, it is suggested that these cones may be responsible for day vision in the potto.     

Localization and function of dopamine in the adult vertebrate retina.

Dopamine (DA) has satisfied many of the criteria for being a major neurochemical in vertebrate retinae. It is synthesized in amacrine and/or interplexiform cells (depending on species) and released upon membrane depolarization in a calcium-dependent way. Strong evidence suggests that it is normally released within the retina during light adaptation, although flickering and not so much steady light stimuli have been found to be most effective in inducing endogenous dopamine release. DA action is not restricted to those neurones which appear to be in "direct" contact with pre-synaptic dopaminergic terminals. Neurones that are several microns away from such terminals can also be affected, presumably by short diffusion of the chemical. DA thus affects the activity of many cell types in the retina. In photoreceptors, it induces retinomotor movements, but inhibits disc shedding acting via D2 receptors, without significantly altering their electrophysiological responses. DA has two main effects upon horizontal cells: it uncouples their gap junctions and, independently, enhances the efficacy of their photoreceptor inputs, both effects involving D1 receptors. In the amphibian retina, where horizontal cells receive mixed rod and cone inputs, DA alters their balance in favour of the cone input, thus mimicking light adaptation. Light-evoked DA release also appears to be responsible for potentiating the horizontal cell-->cone negative feed-back pathway responsible for generation of multi-phasic, chromatic S-potentials. However, there is little information concerning action of DA upon bipolar and amacrine cells. DA effects upon ganglion cells have been investigated in mammalian (cat and rabbit) retinae. The results suggest that there are both synaptic and non-synaptic D1 and D2 receptors on all physiological types of ganglion cell tested. Although the available data cannot readily be integrated, the balance of evidence suggests that dopaminergic neurones are involved in the light/dark adaptation process in the mammalian retina. Studies of the DA system in vertebrate retinae have contributed greatly to our understanding of its role in vision as well as DA neurobiology generally in the central nervous system. For example, the effect of DA in uncoupling horizontal cells is one of the earliest demonstrations of the uncoupling of electrotonic junctions by a neurally released chemical. The many other, diverse actions of DA in the retina reviewed here are also likely to become model modes of neurochemical action in the nervous system.(ABSTRACT TRUNCATED AT 400 WORDS)     

Knock out of direction selectivity in the retina.

Retinal ganglion cells show direction selectivity in their responses to moving stimuli. The circuitry necessary to generate directional selectivity in these cells has been long debated. Yoshida et al. (2001) use immunotoxin-mediated cell ablation to demonstrate that the starburst amacrine cell is at the core of this computation.