Cone Myoids Research Articles

Cellular Characterization of OCT and Outer Retinal Bands Using Specific Immunohistochemistry Markers and Clinical Implications

OCT has been a technological breakthrough in the diagnosis, treatment, and follow-up of many ocular diseases, especially retinal and neuro-ophthalmologic pathologic conditions. Until now, several controversies have arisen over the specific cell types that the bands observed in the OCT represent, especially over the 4 outer retinal bands. To correlate the 4 outer hyperreflective bands observed in the OCT with the histologic structures using human retinal sections and immunocytochemistry at the fovea level. Eyes from human donors. Vertical cryosections of human retinas were immunostained with antibodies specific for cones photoreceptors, bipolar cells, mitochondria, Müller cells, and retinal pigment epithelium (RPE) cells and were visualized using confocal microscopy. Morphological correlation between histology and OCT at the fovea level. Triple immunolabeling allowed distinguishing between cells types and different cell compartments. Immunostaining with guanine nucleotide-binding protein β 3 (GNB3) and cellular retinaldehyde-binding protein (CRALBP) antibodies showed all retinal layers at the foveola, especially the separation between the outer nuclear layer and the Henle fiber layer. CRALBP and cytochrome C (Cyt C) immunolabeling revealed that hyperreflective bands 1 and 2, observed in the OCT, correspond to the outer limiting membrane and the cone ellipsoids, respectively, separated by the cone myoids. CRALBP, cytochrome C, and GNB3 showed that the RPE interdigitations extend along the entire external segment of the cones, we do not believe them to be the structure responsible for forming the third band. However, the identification of small fragments of cone outer segments within the RPE led us to characterize the third band as the cone phagosomes located in the top of the RPE. Finally, we propose that the fourth band corresponds to the accumulation of mitochondria at the basal portion of the RPE, as identified by cytochrome C immunoreactivity, and that the hyporeflective band between bands 3 and 4 corresponds to the RPE nuclei and melanosomes zone. This study proposes a new interpretation of the outer retinal bands that leads to a moreaccurate interpretation of OCT images, providing information about the health of cones and their relationshipwiththe RPE, and could help to form a better understanding of retinal disease diagnosis and prognosis.

Retinomotor Response in Larvae of Brown-marbled Grouper, Epinephelus fuscoguttatus

The brown-marbled grouper Epinephelus fuscoguttatus is an important species for aquaculture, however, there is no data about visual threshold in order to estimate the optimum light intensity for larval rearing. This study examined the retinomotor responses of 14, 28 and 42 days old larvae of E. fuscoguttatus under seven orders of magnitude of light intensities from 0 lA— to 100 lA— to determine the visual thresholds. The retinae of 14 days old larvae had a single layer of outer nuclei, of the same number as the cone cells, indicating absence of rod cells. The 28 days old larvae had more nuclei in the outer nuclei layer than cone cells, indicating the appearance of rod cells. The retinomotor response was quantified as an increase in the expansion of the pigment epithelium and a decrease in the contraction of the cone myoids. The retinomotor response was absent in 14 days old larvae and weakly evident in 28 days old larvae. The retinal pigment layer was thinner at light intensity 10 lA— in the larval rearing tanks. Full retinal function at age 42 days would be prepared for their benthic life style in next stage of 45-50 days juveniles.

Localization of nitric oxide synthase in the tree shrew retina.

Nitric oxide (NO) is a novel neuronal messenger that likely influences retinal function by activating retinal guanylyl cyclase to increase levels of cGMP. In the present study, the localization of neuronal nitric oxide synthase (nNOS, Type I NOS) in the cone-dominant tree shrew retina was studied using NADPH-d histochemistry and nNOS immunocytochemistry. Both NADPH-d and nNOS-immunoreactivity (IR) labeled the inner segments of rods and the myoids of a regular subpopulation of cones, with their corresponding nuclei outlined. The labeled cone myoids were co-localized with a marker for short-wave-sensitive (SWS) cones (S-antigen) and also displayed the regular triangular packing and density (7%) characteristic of SWS cones in tree shrew and other mammalian retinas. These measures confirmed the identity of the labeled cones as SWS cones. Photoreceptor ellipsoids of all cones were strongly labeled by NADPH-d reactivity, but lacked nNOS-IR. Another novel finding in tree shrew retina was that both NADPH-d and nNOS-IR labeled Muller cells, which have not been labeled by nNOS-IR in other mammalian retinas. Consistent with findings in rod-dominant retinas, two types of amacrine cells at the vitreal edge of the inner nuclear layer and a subpopulation of displaced amacrine cells at the scleral edge of the ganglion cell layer were labeled by both NADPH-d and nNOS-IR. Processes of these labeled cells were seen to extend into the inner plexiform layer, where dense punctate label was seen, especially in the central sublamina. These results show that localization of NOS in the cone-dominant tree shrew retina shares some common properties with rod-dominant mammalian retinas, but also shows some species-specific characteristics. The new finding of nNOS localization in tree shrew SWS cones and rods, but not in other cones, raises interesting questions about the roles of NO in the earliest level of visual processing.

Localization of guanylate cyclase-activating protein 2 in mammalian retinas

Guanylate cyclase-activating proteins (GCAP1 and GCAP2) are thought to mediate the intracellular stimulation of guanylate cyclase (GC) by Ca2+, a key event in recovery of the dark state of rod photoreceptors after exposure to light. GCAP1 has been localized to rod and cone outer segments, the sites of phototransduction, and to photoreceptor synaptic terminals and some cone somata. We used in situ hybridization and immunocytochemistry to localize GCAP2 in human, monkey, and bovine retinas. In human and monkey retinas, the most intense immunolabeling with anti-GCAP2 antibodies was in the cone inner segments, somata, and synaptic terminals and, to a lesser degree, in rod inner segments and inner retinal neurons. In bovine retina, the most intense immunolabeling was in the rod inner segments, with weaker labeling of cone myoids, somata, and synapses. By using a GCAP2-specific antibody in enzymatic assays, we confirmed that GCAP1 but not GCAP2 is the major component that stimulates GC in bovine rod outer segment homogenates. These results suggest that although GCAP1 is involved in the Ca2+-sensitive regulation of GC in rod and cone outer segments, GCAP2 may have non-phototransduction functions in photoreceptors and inner retinal neurons.

Retinomotor Movements in Isolated Teleost Retinal Cone Inner-Outer Segment Preparations (CIS-COS): Effects of Light, Dark and Dopamine

Teleost cone inner segments elongate and contract in response to light and circadian signals. Previous studies have shown that teleost cone contraction is triggered by light or dopamine, while cone elongation is triggered by darkness or experimental elevation of cAMP. We have developed procedures for isolating and purifying motile cone fragments consisting of inner and outer segments (CIS-COS) to permit more detailed analysis of light and dopamine regulation of cone retinomotor movements. When retinas are dissected from long-term dark-adapted fish. CIS-COS break off at the base of the ellipsoid and remain attached to the RPE. CIS-COS can be detached from the RPE by brief protease treatment, thereby generating a highly enriched CIS-COS suspension. CIS-COS retain normal morphology and extend new myoids when cultured in darkness or in light plus forskolin, an activator of adenylate cyclase. The microtubule and actin cytoskeletons of the new myoids resemble those of intact cone myoids in vivo. Light inhibits CIS-COS myoid elongation, suggesting that light reception by the outer segment can directly influence cone motility. In dark-cultured CIS-COS, myoid elongation is inhibited hall-maximally by nanomolar concentrations of dopamine, suggesting that dopamine effects on motility are mediated by D2-family receptors present on the cone inner and/or outer segment. After dark-induced elongation in culture, CIS-COS myoids can be induced to contract by subsequent culture in the light or with dopamine. Thus isolated cone inner and outer segments possess sufficient cytoskeletal and regulatory machinery to exhibit light- and dopamine-regulation retinomotor movement similar to that observed in intact cones in situ.

The effect of dopamine depletion on light-evoked and circadian retinomotor movements in the teleost retina.

The retinae of lower vertebrates undergo a number of structural changes during light adaptation, including the photomechanical contraction of cone myoids and the dispersion of melanin granules within the epithelial pigment. Since the application of dopamine to dark-adapted retinae is known to produce morphological changes that are characteristic of light adaptation, dopamine is accepted as a casual mechanism for such retinomotor movements. However, we report here that in the teleost fish, Aequidens pulcher, the intraocular injection of 6-hydroxydopamine (6-OHDA), a substance known to destroy dopaminergic retinal cells, has no effect on the triggering of light-adaptive retinomotor movements of the cones and epithelial pigment and only slightly depresses the final level of light adaptation reached. Furthermore, the retina continues to show circadian retinomotor changes even after 48 h in continual darkness that are similar in both control and 6-OHDA injected fish. Biochemical assay and microscopic examination showed that 6-OHDA had destroyed dopaminergic retinal cells. We conclude, therefore, that although a dopaminergic mechanism is probably involved in the control of light-induced retinomotor movements, it cannot be the only control mechanism, nor can it be the cause of circadian retinomotor migrations. Interestingly, 6-OHDA injected eyes never reached full retinomotor dark adaptation, suggesting that dopamine has a role to play in the retina's response to darkness.

A role for endogenous dopamine in circadian regulation of retinal cone movement

Cone movements in the retina of the Midas cichlid ( Cichlasoma citrinellum) take place in response both to light and endogenous circadian signals. In the normal light/dark cycle (LD) cone myoids are long at night (50–55 μm), begin to contract before expected dawn, and with light onset contract to their fully contracted positions (5 μm) which are retained throughout the day. In continuous darkness (DD) cone myoids are fully elongate at night, but undergo pre-dawn contractions to partially contracted positions which they retain throughout expected day (20–25 μm). To investigate the mechanisms by which circadian signals modulate cone myoid movements in teleost retinas, we have tested the effects on circadian cone movements of optic nerve section, intraocular injection of dopamine agonists or antagonists, and intraocular injection of melatonin. We report here that both light-induced and circadian-driven cone myoid movements can occur in the absence of efferent input from higher centres: both are retained with full amplitude after optic nerve section in vivo. Intraocular injection studies suggest that circadian regulation of cone myoid movement is mediated locally within the eye by dopamine acting via a dopaminergic D 2-receptor. Cone myoid contraction can be induced at midnight in LD or DD animals by intraocular injection of dopamine or the D 2-receptor agonist LY171555. The partially contracted cones of DD animals at expected mid-day can be induced to fully contract by intraocular injection of dopamine or the D 2-receptor agonist, or to elongate by intraocular injection of the dopamine D 2-antagonist sulpiride. Furthermore, the pre-dawn cone myoid contraction observed in both LD and DD animals in response to circadian signals can be completely blocked in DD animals by intraocular injection of the D 2-antagonist sulpiride shortly before the time of expected light onset. In contrast, circadian cone myoid movements were unaffected by intraocular injection of the D 1-receptor agonist SCH23390, or the D 1-receptor antagonist SKF38393. In addition, we report that intraocularly injected melatonin had no effect on cone position when injected in the light at mid-day, in darkness at midnight or in darkness just before expected light onset at dawn. However, both melatonin and iodomelatonin induced cone myoid contraction (the light-adaptive movement) when injected in darkness at expected mid-day in DD animals. This paradoxical result is not consistent with observations from other species in which melatonin induces dark-adaptive photoreceptor responses. Our optic nerve section observations indicate that circadian cone movement does not require efferent control. Instead they suggest that circadian cone movement is regulated locally within the eye. Our results from intraocular injection of dopamine agonists and antagonists suggest that this local circadian regulation is mediated by endogenous dopamine, acting via D 2-receptors. We find no evidence for a role for melatonin in producing dark-adaptive retinomotor movements in fish retina.

Effects of circadian phase on cone retinomotor movements in the Midas cichlid

In retinas of the Midas cichlid ( Cichlasoma citrinellum), cone myoid lengths change in response to both light and circadian signals. Cone myoids are short in day (or in the light), and long at night (or in darkness). During the normal light/dark cycle, cone myoids elongate to 58 μm after light offset at dusk, and remain long until just before dawn, at which time they begin to contract before the time of expected light onset and eventually (after light onset) assume daytime lengths of 5 μm. In continuous light, cone myoids remain fully contracted regardless of the time in the cycle. In continuous darkness, cone myoids undergo circadian movements; they elongate fully at night and contract partly during expected day to 23 μm (65% of the contraction seen at dawn in a normal light/dark cycle). To clarify further the regulation of these retinomotor movements, we have investigated the effects of circadian phase on cone movements induced by light or dark onset. Circadian phase had no significant effect on either the initial rate or the final extent of light-induced cone myoid contraction: at mid-dark cones began to contract immediately after light onset and contracted fully at 3·0±0·5 μm min −1; at mid-night cones contracted immediately and fully at 4·0±0·2 μm min −1. At all of the relatively high intensities of light tested (30–5000 lx) cones contracted at similar rates and to similar final extents. In contrast, circadian phase did affect dark-induced cone myoid elongation: both the delay before initiation and the final extent of cone elongation were strongly influenced by circadian phase. At dusk, elongation began immediately after dark onset and plateaued at 58 μm, while at dawn, elongation was delayed by 50 min and plateaued at 30 μm. At mid-day, elongation began 30 min after dark onset and plateaued at 25 μm. Our observations indicate that circadian phase has significant effects on the time of initiation and the final extent of dark-induced cone elongation, but has little or no effect on light-induced cone contraction at light intensities between 30 and 5000 lx.

Circadian rhythms in the green sunfish retina.

We investigated the occurrence of circadian rhythms in retinomotor movements and retinal sensitivity in the green sunfish, Lepomis cyanellus. When green sunfish were kept in constant darkness, cone photoreceptors exhibited circadian retinomotor movements; rod photoreceptors and retinal pigment epithelium (RPE) pigment granules did not. Cones elongated during subjective night and contracted during subjective day. These results corroborate those of Burnside and Ackland (1984. Investigative Ophthalmology and Visual Science. 25:539-545). Electroretinograms (ERGs) recorded in constant darkness in response to dim flashes (lambda = 640 nm) exhibited a greater amplitude during subjective night than during subjective day. The nighttime increase in the ERG amplitude corresponded to a 3-10-fold increase in retinal sensitivity. The rhythmic changes in the ERG amplitude continued in constant darkness with a period of approximately 24 h, which indicates that the rhythm is generated by a circadian oscillator. The spectral sensitivity of the ERG recorded in constant darkness suggests that cones contribute to retinal responses during both day and night. Thus, the elongation of cone myoids during the night does not abolish the response of the cones. To examine the role of retinal efferents in generating retinal circadian rhythms, we cut the optic nerve. This procedure did not abolish the rhythms of retinomotor movement or of the ERG amplitude, but it did reduce the magnitude of the nighttime phases of both rhythms. Our results suggest that more than one endogenous oscillator regulates the retinal circadian rhythms in green sunfish. Circadian signals controlling the rhythms may be either generated within the eye or transferred to the eye via a humoral pathway.

Regulation of reactivated elongation in lysed cell models of teleost retinal cones by cAMP and calcium.

Teleost retinal cones elongate in the dark and contract in the light. In isolated retinas of the green sunfish Lepomis cyanellus, cone myoids undergo microtubule-dependent elongation from 5 to 45 micron. We have previously shown that cone contraction can be reactivated in motile models of cones lysed with Brij-58. Reactivated contraction is both actin and ATP dependent, activated by calcium, and inhibited by cAMP. We report here that we have obtained reactivated cone elongation in lysed models prepared by the same procedures. Reactivated elongation is ATP dependent, activated by cAMP, and inhibited by calcium. The rate of reactivated elongation is proportional to the cAMP concentration between 10 microM and 0.5 mM, but is constant between 10 microM and 1.0 mM Mg-ATP. No elongation occurs if cAMP or Mg-ATP concentration is less than or equal to 5 microM. Mg-ATP is required for both cAMP-dependent and cAMP-independent processes, suggesting that Mg-ATP is required both for a regulatory process entailing cAMP-dependent phosphorylation and for a force-producing process. Free calcium concentrations greater than or equal to 10(-7) reduce the elongation rate by 78% or more, completely inhibiting elongation at 10(-5) M. This inhibition is not due to competition from calcium-activated contraction. Cytochalasin D blocks reactivated contraction, but does not abolish calcium inhibition of reactivated elongation. Thus calcium directly affects the elongation mechanism. Calcium inhibition is calmodulin dependent. The calmodulin inhibitor trifluoperazine abolishes calcium inhibition of elongation. Furthermore, calcium blocks elongation only if present during the lysis step; subsequent calcium addition has no effect. However, if calcium plus exogenous calmodulin are subsequently added, elongation is again inhibited. Thus calcium inhibition appears to require a soluble calmodulin which is lost shortly after lysis.

Circadian regulation of retinomotor movements. I. Interaction of melatonin and dopamine in the control of cone length.

In lower vertebrates, cone retinomotor movements occur in response to changes in lighting conditions and to an endogenous circadian clock. In the light, cone myoids contract, while in the dark, they elongate. In order to test the hypothesis that melatonin and dopamine may be involved in the regulation of cone movement, we have used an in vitro eyecup preparation from Xenopus laevis that sustains light- and dark-adaptive cone retinomotor movement. Melatonin mimics darkness by causing cone elongation. Dark- and melatonin-induced cone elongation are blocked by dopamine. Dopamine also stimulates cone contraction in dark-adapted eyecups. The effect of dopamine appears to be mediated specifically by a dopamine receptor, possibly of the D2 type. The dopamine agonist apomorphine and the putative D2 agonist LY171555 induced cone contraction. In contrast, the putative D1 agonist SKF38393-A and specific alpha 1-, alpha 2-, and beta-adrenergic receptor agonists were without effect. Furthermore, the dopamine antagonist spiroperidol not only blocked light-induced cone contraction, but also stimulated cone elongation in the light. These results suggest that dopamine is part of the light signal for cone contraction, and that its suppression is part of the dark signal for cone elongation. Melatonin may affect cone movement indirectly through its influence on the dopaminergic system.

Dopamine inhibits forskolin- and 3-isobutyl-1-methylxanthine-induced dark-adaptive retinomotor movements in isolated teleost retinas.

We have been investigating the mechanisms of diurnal and circadian regulation of teleost retinomotor movements. In the retinas of lower vertebrates, photoreceptors and melanin pigment granules of the retinal pigment epithelium (RPE) undergo movements at dawn and dusk. These movements continue to occur at subjective dawn and dusk in animals maintained in constant darkness. Cone myoids contract at dawn and elongate at dusk; RPE pigment disperses into the epithelial cells' long apical processes at dawn and aggregates into the cell bodies at dusk. We report here that forskolin, an adenylate cyclase activator, and 3-isobutyl-1-methylxanthine (IBMX), a phosphodiesterase inhibitor, each induces dark-adaptive cone and RPE retinomotor movements in isolated light-adapted green sunfish retinas cultured in constant light. Forskolin induces a 22-fold elevation in retinal cyclic AMP content. Forskolin- and IBMX-induced movements are inhibited approximately 65% and 95%, respectively, by 3,4-dihydroxyphenylethylamine (dopamine). However, dopamine does not inhibit dark-adaptive movements induced by dibutyryl cyclic AMP. Epinephrine is much less effective than dopamine in inhibiting forskolin-induced movements, while phenylephrine and clonidine are totally ineffective. These results are consistent with our previous findings that treatments that increase intracellular cyclic AMP content promote dark-adaptive retinomotor movement. They further suggest that dopamine inhibits adenylate cyclase activity in photoreceptors and RPE cells and thereby favors light-adaptive retinomotor movements.

N-ethylmaleimide-modified subfragment-1 and heavy meromyosin inhibit reactivated contraction in motile models of retinal cones.

The mechanism of contraction in motile models of teleost retinal cones has been examined by using N-ethylmaleimide (NEM)-modified myosin fragments (NEM-S-1 and NEM-heavy meromyosin [HMM]) to prevent access of native myosin to actin filaments during reactivation of contraction. In the diurnal light/dark cycle, retinal cones of green sunfish (Lepomis cyanellus) and bluegill (lepomis macrochirus) exhibit length changes of more than 90 mum. The motile myoid region of the cone contracts from 100 mum in the dark to 6 mum in the light. Motile models for cone contraction have been obtained by lysis of dark-adapted retinas with the non-ionic detergent, Brij-58. These cone motile models undergo Ca(++)-and ATP-dependent reactivated contraction, with morphology and rate comparable to those observed in vivo (Burnside, B.,B. Smith, M. Nagata, and K. Porrello, 1982, J. Cell Biol., 92:198-206). The cone myoids contain longitudinally oriented actin filaments which bind myosin subfragment-1 (S-1) to form characteristic "arrowhead" complexes which dissociate in the presence of MgATP (Burnside, B., 1978, J. Cell Biol., 78:227-246). Modification of S-1 or HMM with the sulfhydryl reagent, NEM, produces new species, NEM-S-1 or NEM-HMM, which still bind actin but which fail to detach in the presence of MgATP (Meeusen, R.L., and W.Z. Cande, 1979, J. Cell Biol., 82:57-65). We have used NEM-S-1 and NEM-HMM to test whether cone contraction depends on an actomyosin force- generating system. We find that reactivated contraction of cone models is inhibited by NEM-S-1 and NEM-HMM but not by the unmodified species, S-1 and HMM. Thus, reactivated cone contraction exhibits NEM-S-1 and NEM-HMM sensitivity as well as Ca(++)- and ATP- dependence. These observations are consistent with and actimyosin-mediated mechanism for force production during cone contraction.

Reactivation of contraction in detergent-lysed teleost retinal cones.

Teleost retinal cones contract in the light and elongate in the dark. In the green sunfish, Lepomis cyanellus, the necklike myoid region of the cone contracts from as much as 120 micrometers (midnight dark-adapted) to 6 micrometers in fully light-adapted state. When dark-adapted fish are exposed to light (1.4 lux), cone myoids contract with a linear rate of 1.5 +/- 0.1 micrometers/min. We report here that detergent-lysed motile models of teleost retinal cones exhibit calcium- and ATP-dependent reactivated contraction, with morphology and rate comparable to that observed in vivo. For reactivation studies isolated dark-adapted retinas were lysed with nonionic detergent Brij-58 (0.1-1.0%). In reactivation medium containing 10(-5) M free calcium and 4 mM ATP, the lysed cones contracted with normal morphology at in vivo rates (1.4 +/- 1 micrometer/min). Little contraction was observed if ATP or detergent was deleted from the medium or if free calcium levels were less than 10(-8) M. Ultrastructural examination of cone models lysed with 1% Brij-58 revealed that, in spite of extensive extraction of the cytoplasmic matrix, cytoskeletal components (thin filaments, intermediate filaments, microtubules) were still present. Thus we have produced extensively extracted motile models of teleost retinal cones which undergo calcium- and ATP-dependent reactivated contraction with normal morphology at physiological rate.

Cyclic metabolism of photoreceptors and retinal pigment epithelium in the frog

The diurnal patterns of 3H-uridine, 3H-leucine and 3H-mannose incorporation by red rods, single cones and pigment epithelial cells were evaluated utilizing quantitative autoradiography. Adult Rana pipiens, previously maintained under a 14L:10D lighting cycle for 3–4 months, were injected with 3H-labeled precursors 3 hrs prior to killing and were sampled throughout a diurnal cycle. In rods, the rate of 3H-uridine incorporation was highest during the middle of the light cycle and was lowest in the middle of the dark cycle, whereas 3H-leucine and 3H-mannose incorporation was more rapid during the latter hours of the dark cycle and lowest during the light cycle, out of phase by approximately 12 hrs from the pattern of 3H-uridine incorporation. In cones, 3H-uridine incorporation was highest during the middle of the light cycle and was lowest during the dark cycle, similar in pattern but reduced in amplitude from the changes observed in 3H-uridine incorporation in rods. 3H-leucine incorporation was high a few hours into the light cycle and just prior to the onset of the dark cycle. Relatively few grains were present over the cone myoids following 3H-mannose injection with no cyclic changes evident. In the pigment epithelium, 3H-uridine incorporation showed two peaks of rapid incorporation, both during the light portion of the diurnal cycle, whereas incorporation dropped to a low level in the middle of the dark cycle. 3H-leucine incorporation was most rapid a few hours into the light cycle, decreasing through the middle portion of the light cycle and then increasing prior to the onset of the dark cycle with this rate sustained throughout the remainder of the dark cycle. 3H-mannose incorporation showed little change throughout the diurnal cycle. In each of these three cell types, 3H-uridine (RNA) incorporation occurred most rapidly during the early part of the light cycle followed by rapid rates of 3H-leucine (protein) incorporation at later times in the day. These cyclic changes in the incorporation of precursors of RNA, protein and glycoprotein in the photoreceptor pigment epithelium complex are discussed in relationship to the times of membrane addition and loss from photoreceptors and times of phagocytosis by the pigment epithelium.

Scanning electron microscopy of the bullfrog's retina and pigment epithelium.

The retina and pigment epithelium of the bullfrog (Rana catesbiana) were studied with the scanning electron microscope. Fixed-dehydrated tissues were critical point dried with CO2, then cracked in the plane of the long axis of the photoreceptors. The cellular layers of the retina and the lateral surfaces of pigment epithelial cells were visualized. The four major types of frog photoreceptor were identified: red rod, green rod, single cone, and double cone. Cone myoids were observed to be contracted in light-adapted retinas and elongated in more dark adapted retinas.

Les reponses retinomotrices: Caracteres et mecanismes

The rods, cones and retinal epithelial pigment of teleosts, anurans, urodeles and birds undergo positional changes in response to light or darkness. The changes may take from a few minutes to several hours for completion. The phylogenetic variations observed represent an adaptive radiation of the retinomotor responses. In fishes and amphibians there is an ontogenetic parallel between the structural and functional development of the retinal elements. There is also a good correlation between retinal structure, responses, habitat and behaviour. This helps an understanding of phenomena such as phototaxis, feeding and migration. Light intensity and wavelength influence the positions of the retinal elements. Temperature and time of day do so to a lesser extent in continuous darkness and to a still lesser extent in constant light. In higher light intensities and temperatures changes from light to dark or vice versa occur faster than in lower intensities and temperatures. Retinomotor responses to wavelengths of equal energy indicate the visible spectrum of the animal. The presence of a diurnal rhythm as indicated by the position of retinal elements is not universal. Retinomotor responses occur as a result of direct photic stimulation of the eye and not of the illumination of other parts of the body. Enucleated eyes are more or less capable of retinomotor responses; this varies in animals. Unilateral illumination has a small effect on the unexposed eye indicating some neural or hormonal interactions. The action spectrum of the expansion of the retinal epithelial pigment and the contraction of cones show a direct or inverse relationship to the absorption spectrum of the scotopic visual pigment. The contraction of the cone myoids could be inhibited by Ouabain and PCMB; but neither TPA staining nor electron microscopy shows the presence of contractile elements in the myoids similar to those seen in other contractile cells like smooth muscle fibres (myosin or actin fibrils). The hypotheses of Garten, Walls and Studnitz concerning the significance of the retinomotor responses are examined in the light of our present knowledge and all three are found to be valid.

The Development of the Retina and Retinomotor Responses in the Herring

The eye of the larval herring has only single cones in the retina. They are characteristic in ultrastructure at this stage with a striated outer segment, an ellipsoid packed with mitochondria and with a connecting cilium at the junction. Cones are gradually added at the margin of the retina as the larva grows. There are no retinomotor responses in the larval eye. It has, however, adult characteristics in the postero-ventralarea temporalis(though it is undifferentiated at this stage) with its blindly ending anterior pocket and in the falciform process closing off a trough-shaped ‘ancillary’ eye.At metamorphosis (30–40 mm) a number of changes occur: theareadifferentiates, twin cones and the rods appear, the latter perhaps developing from cells originating in the bipolar layer and migrating to a position inside the external limiting membrane. Retinomotor pigment migration and change in length of the cone myoids also develop when the eye is subjected to changing light intensities, the threshold being 1.0–0.01 me (metre candles).In the adult eye the fully developed retina has anarea temporalispacked with single cones giving high acuity on the main visual axis, which is in the upperfore direction. The lens presumably gives accommodation by postero-ventral movement brought about by a postero-dorsal suspensory ‘ligament’ and a ventroanterior falciform process. This process also delimits theareaand divides off the ‘ancillary’ eye from the main retina.Over the whole range of development the cone ellipsoids grow in diameter from 2 to 12 μ and decrease in density correspondingly.

Visual Cells of the Guinea Pig

The statement of Granit1 that the guinea pig has a ‘pure rod eye’ calls for comment. Franz2, referring to a paper of his own published in 1909, says that cones are missing from the retina of the guinea pig. Ovio3 admits the presence of a few cones. Kolmer4 describes cones and rods. He makes the significant remark that observation prior to his own had been made on poorly fixed material. Rods are four to five times as numerous as cones, and he comments on the clear vacuoles at the peripheral end of the cone myoids.