Retinula Cells Research Articles

Colour vision in butterflies

1. The retinula cells ofVanessa are similar in physiological properties, including spectral sensitivityS(λ) and negative electrical coupling, to those already described inPapilio, except that red-sensitive cells have not been found in either retina or lamina. 2. In the dark-adapted eye, with a stimulus of one colour, lamina ganglion cells yield only hyperpolarizations. 3. Some lamina ganglion cells (LMC's) have a broad flatS(λ) with angular sensitivity showing that they receive summed input from only one ommatidium. Others have narrowS(λ) and narrow field suggesting that primary receptors from single ommatidia interact on or before reaching them. Narrow peaks are near 500 or 550 nm, but not spread through the spectrum, suggesting colour specific behaviour rather than colour vision. 4. Vanessa itea, V. kershawi, Precis villida andHeteronympha merope all have optomotorS(λ) similar to the curves for green-sensitive photoreceptors, with broad peak between 500 and 550 nm. 5. Optomotor responses of butterflies fall off rapidly around 0.1 cd·m−2, whereas insects with superposition eyes are 100 times more sensitive. Calibrations suggest that the butterfly optomotor threshold is well above the photon flux that yields abundant bumps in the retinula cells. 6. There is difficulty in reconciling theS(λ) of the optomotor response with theS(λ) of any of the individual LMC's. 7. The physiological properties of receptors, LMC's and deep optic lobe units are brought together in a discussion of insect colour vision.

Effect of external level of calcium upon the photoreceptor potential of crayfish along the twenty-four hour cycle

1. 1. The influence of changes in calcium concentration of the bathing medium upon resting potential and responses to light of retinular cells of the crayfish was examined. 2. 2. The resting potential was not markedly influenced by these changes. 3. 3. In contrast, both shape and amplitude of the response to light showed a great dependence on external level of calcium. 4. 4. In both calcium-rich and calcium-free solutions the transient phase was more affected than the steady phase. 5. 5. In calcium-rich solution the response diminished and was soon depressed. 6. 6. In calcium-free solution the amplitude was initially increased, but became “square” in a few minutes. 7. 7. These changes were interpreted as evidence of the participation of calcium ions in the receptor potential. 8. 8. Measuring the receptor potential along the 24 hr cycle, we found that there were periodical changes in the recovery response to light when the retinular cells were immersed in normal Van Harreveld solution. 9. 9. Neither in a calcium-rich nor in a calcium-free medium were marked changes observed in the amplitude of the receptor potential associated with the hour of day. 10. 10. These results indicate that the isolated photoreceptor cell requires a normal saline solution to preserve its ability to oscillate.

Experimental confinement of the transient receptor potential to the peripheral retinula cells in the trp mutant Drosophila: What it tells us about the mutation and visual function

By the use of appropriate light intensities the expression of the transient nature of the receptor potential observed in the trp mutant of Drosophila melanogaster can be confined to the peripheral retinula cells in which the visual pigment can also be manipulated predictably, affording an experimental means to probe in these receptors the relationship of the visual pigment to the “electrogenic membrane”. Repeated blue light exposures cause w;trp flies to respond in a manner like cn;bw flies in which the dark-adapted rhodopsin fraction is reduced to 0.5% of the normal level by vitamin A deprivation: this comparable response behaviour, since the amount of visual pigment in w;trp flies is normal, implies that only some subfraction of the photoequilibrium value of rhodopsin may be available. Recovery of the peripheral receptors' sensitivity in ambient light conditions which would render them insensitive by expression of the phenotype is paradoxical and allows a “wavelength effectivity” curve to be constructed which identifies the involvement of the rhodopsin. Resolution of the paradox is discussed.

Spectral characteristics of isolated blowfly rhabdoms

A rapid and simple method for the isolation of rhabdoms from the eye of the blowflyCalliphora erythrocephala Meig. is described. Essentially this involves i) the excision of the retina, then its disintegration using a hypoosmotic buffer and ii) separation of the rhabdoms from other cell fragments by isopycnic centrifugation on Percoll. 1. The isolated rhabdoms consist almost exclusively of the microvillar membranes of rhabdomeres R1–8 and material of the intraommatidial matrix which adheres to them. When rhabdoms are suspended in a low-calcium buffer they lose their association with the matrix material. Subsequent extraction of the isolated rhabdoms with 2% digitonin solubilizes the microvillar membranes but not the intraommatidial matrix. 2. Spectrophotometry of such extracts reveals an absorbance peak at 486 nm and a smaller peak at about 340 nm. Both peaks are probably due to the visual pigment of retinula cells 1–6. There was no evidence of a sensitizing pigment which absorbed ultraviolet light. 3. Irradiation of isolated rhabdoms or visual pigment extracts with blue light established the same photoequilibrium between rhodopsin and acid metarhodopsin (λmax 566 nm) in each case. This indicates that the spectral characteristics of isolated rhabdoms are determined primarily by the absorption properties of rhodopsin and metarhodopsin. Irradiation of isolated rhabdoms or visual pigment extracts at pH>7 led to the formation of alkaline metarhodopsin (λmax about 380 nm). The equilibrium between acid and alkaline metarhodopsin was shifted in favour of the formation of acid metarhodopsin in isolated rhabdoms whereas the formation of alkaline metarhodopsin was favoured in extracts. 4. Metarhodopsin formed in isolated rhabdoms was found to be less stable than in the intact eye.

Electrophysiological measurement of photoreceptor membrane dichroism and polarization sensitivity in a Grapsid crab

1. Discrete depolarizations (bumps) have been recorded from retinula cells of the shore crabLeptograpsus in response to appropriate low light intensities (Fig. 1). The statistical analysis of bumps is consistent with the hypothesis that each bump represents the photoreceptor's response to an absorbed photon. The photoreceptive membrane was estimated to have a quantum capture efficiency of 0.46 (±0.12 SD;n = 6). 2. The modal polarization sensitivity value measured by bump frequency (BPS) in eleven cells was 5.0 with a range from 1.9 to 5.63 (Table 1). There is no statistically significant difference between BPS at wavelengths 621 nm and 499 nm. 3. Polarization sensitivity (PS) measured by graded potentials is equal to the polarization sensitivity measured in the same cells by bumps (Fig. 6). PS values ranged from 1.8 to 7.5 (n=28) with a modal value of 5.0 (Fig. 5). 4. Bump rates represent the direct electrophysiological measurement of the visual pigment's selective absorption to single photons having different E-vector orientations (Fig. 4), and therefore PS measured from bumps provides an accurate estimate of the physiological dichroic properties of the rhabdomere in vivo. The measurements of BPS show that the rhabdomere's dichroism accounts for the observed PS values recorded inLeptograpsus.

Mutants affecting plasticity of phototactic behaviour in Drosophila melanogaster

Eight mutants of Drosophila m. have been isolated that fail to display longlasting modification of “choice tube phototaxis”. Two different classes of stocks defective in plasticity of phototaxis ( dipp) can be discriminated: (1) Mutants which are affected in the stability of the Prolonged Depolarising Afterpotential (PDA) in the peripheral retinula cells R1–R6. (2) Mutants with normal PDA but abnormalities in fast escape phototaxis especially under conditions where only receptor cells R7–R8 serve as input for this behavioural trait.

Retinula cell responses in a moth superposition eye

The lack of photoreceptor cell movement upon stimulation in a group of moths made possible the direct electrophysiological measurement of the photoreceptor properties in a moth superposition eye. The 13–16 retinula cells of the ommatidium are usually coupled in twos or threes. There are strong antagonistic electrical interactions at the receptor level. The measured acceptance angles (Δ ρ ) of the best units are in the range 1.6-1.8°. Spectral sensitivity peaks lie near 380 and 520 nm. Some units have positive-going responses to some colours and negative-going responses to others. Polarization sensitivity is usually low, but is high if tested at a suitable wavelength in a unit showing colour opponency. The dynamic range (10-90%, length of slope) of the V /lg I curve is over 100-fold intensity range for purely ultraviolet or green-sensitive units; over 10 4 -fold for some units with responses in opposite directions to different wavelengths, and over a 10 6 -fold intensity range for units with negative-going responses at all wavelengths. The wide ranges are attributed to strong antagonistic interactons between receptors. Despite the superposition optics, the retinula cells are approximately the same sensitivity (in terms of millivolts per peak axial photon) as those of a diurnal apposition eye under the same conditions. The electrical interaction between different receptors has the effect of reducing the responses to light that is unpolarized, white, diffuse, or simultaneously applied, and of emphasizing responses to light that is polarized, coloured, localized or successively applied. Tests with a movable point source and a surround reveal several effects of the opponency between receptors by a single mechanism. Edges are emphasized by spatial interaction and differences between colours are emphasized by interaction within ommatidia. On a green background, small objects of a different colour will stand out. With certain wavelength combinations, the temporal properties are enhanced. The adaptive advantage of the diurnal superposition eye, from these measurements, is that it provides a higher intensity at the photoreceptors that in turn makes possible a greater degree of inhibitory spatial and temporal interaction at the primary photoreceptor level.

Retinal Damage and Sensitivity Loss of A Light-Sensitive Crustacean Compound Eye (Cirolana Borealis): Electron Microscopy and Electrophysiology

ABSTRACT The compound eye of the deep-water-living crustacean Cirolana borealis has been exposed to measured amounts of white light, and the effects have been analysed by electron microscopy and electrophysiology (ERG). The threshold for damage of the retinula cells lies between 117 lx (0·47Wm−2) and 1250 lx (4·9 Wm−2). With daylight exposures of more than 70 W m−2, there is severe structural derangement and the amplitude of the electroretinogram (ERG) is abolished. No recovery of the retinula cell organization or of the ERG occurs after daylight exposure and a dark period of up to 5 days. A novel type of photoreceptor membrane shedding is described for both dark-adapted and light-exposed eyes. Hence, morphologically and functionally, the Cirolana eye is strictly adapted to a dim-light environment and is destroyed by too intense illumination.

Autoradiographic localization of newly synthesized octopamine to retinal efferents in the Limulus visual system.

The biogenic amine octopamine is synthesized from both tyrosine and tyramine in the lateral, median, and ventral eyes of Limulus. The autoradiographic studies presented here were designed to locate the sites of octopamine synthesis in the ventral and lateral eyes. We found that efferent fibers, which project to ventral and lateral eyes from the central nervous system, became intensely and selectively labeled during in vitro incubations with 3H-tyramine. In the ventral eye, more than 95% of the efferent fibers were labeled. Results of biochemical analyses suggested that most of the radioactive substance within these efferent fibers was newly synthesized octopamine. The selective labeling of efferent fibers during incubation with 3H-tyramine was used as an anatomical tool to study the number and distribution of efferent fibers within the ventral eye. Light microscopic (LM) reconstructions of the distribution of label in serial longitudinal sections through ventral optic nerves together with electron microscopic (EM) autoradiographic analyses revealed between 70 and 200 efferent axons. The results of these studies and of reconstructions of efferent innervation to photoreceptor somata suggest that each ventral photoreceptor cell or each small cluster of cells is innervated by a separate efferent fiber. Both LM reconstructions and EM analyses showed that efferent fibers ramify extensively and specifically in and near the internal rhabdom of ventral photoreceptor cells. In EM autoradiographs of lateral eyes incubated with 3H- tyramine, the silver grains that were located over ommatidia were concentrated exclusively over efferent fibers. All of these efferent fibers, which lay near rhabdoms and in partitions between retinular cells, were labeled. The results of our present studies support our hypothesis that octopamine is a neurotransmitter in Limulus retinal efferent fibers. This amine may modulate the biochemistry and physiology of ventral photoreceptor cells and may mediate many of the known effects of circadian efferent innervation to the lateral eye.

Phagocytosis of rhabdomeral membrane by crab photoreceptors (Leptograpsus variegatus).

Many crabs possess fused rhabdoms which are partly broken down at dawn and re-synthesised at dusk. The cross-sectional area of the rhabdom is therefore smaller during the day than at night. The only previously described mechanism of membrane removal from the rhabdomere in Crustacea involves the formation of pinocytotic vesicles at the bases of the microvilli. The geometry of the rhabdom is such that uniform pinocytosis across the base of each rhabdomere would result in a stack of orthogonally oriented rectangles. In the process described here, microvilli from the outer edges of the rhabdomeres are engulfed by adjacent retinula cells, reducing the number as well as the length of the microvilli and maintaining the smooth longitudinal profile needed for optimal functioning of the rhabdom.

Single electrode studies on the retina of the butterflyPapilio

1. The butterfly retina exhibits strong interactions between photoreceptor responses recorded intracellularly. In general, the receptor which is locally giving the largest response suppresses the responses of neighbouring receptors that are less strongly stimulated. The effect enhances the differences between the primary photo-receptors and reduces responses to stimuli that excite all receptors together. 2. The interaction is explained in terms of a high extracellular resistance, so that receptor currents pass through other receptors in the opposite direction to those of their own responses. The result is that receptors are actively turned off by colours away from their own peak wavelength. 3. This effect applies most strongly to colour and to polarization plane when the stimulus is a point source on axis, and is therefore strong between the receptors of the same ommatidium. 4. The result is that spectral sensitivity peaks and angular sensitivity peaks are narrower, and polarisation sensitivity is greater, than expected from single retinula cells in isolation. The sensitivities measured electrophysiologically cannot be easily related to the physical properties of the visual pigments. Polarisation sensitivity (PS) can reach 50. 5. There are four types of primary photoreceptor, with peak near 380 nm, 450 nm, 550 nm and 610 nm. Cell marking usually reveals these as single retinula cells. Near the peak spectral sensitivity the responses are up to 60 mV positive-going, but away from the peak they can be negative-going. 6. Anatomically the retina ofPapilio has four distal, four proximal retinula cells, and a ninth basal cell. Narrow pigment cells and tracheoles squeeze through the substantial basement membrane along with each bundle of nine axons. 7. Two of the distal retinula cells contain red pigment grains near the rhabdom. The distal retinula cells are UV or blue sensitive. Green sensitive cells are proximal and can be coupled in opposite pairs. Red sensitive cells are proximal. 8. The UV sensitive cell with peak near 380 nm is the most sensitive of the cell types when measured by the position of theV/logI curve on the intensity axis at the spectral peak of each type. The red-sensitive cells are also sensitive. By its inhibitory effect, interaction between receptors reduces the sensitivity measurement on this scale. 9. Angular sensitivities measured with positive-going responses near the spectral peaks are narrow (Δρ-2°); when measured with negative-going responses they are wider (Δρ=3° to 5°). 10. One type of unit has only negative-going responses to −60 mV, with Δp=2° to 5°, spectral peak near 550 nm and sometimes also 380 nm or 450 nm. This type has not been marked and is regarded as a restricted channel for return current. ItsV/logI curve extends over an intensity range of 106. 11. The variety of the units suggests that their responses are not due to a simple regular network with all units connected indiscriminately to all others at all times through their terminals. There are selective channels for current flow and some retinula cells appear to be little influenced by others. 12. Theory shows that when there is a direct electrical coupling between a pair of retinula cells (not passing through the extracellular space) it is possible to balance out the negative interactions caused by current flow through their terminals. Far from degenerating the signals, direct electrical coupling can cancel the negative interaction, and this may be its normal function.

Analysis of visual system development in Drosophila melanogaster: Mutations at the Glued locus

We have analyzed several aspects of the development of flies carrying mutations at the Glued locus. Optic lobe abnormalities in individuals heterozygous for the original Glued allele were previously shown to result from an action of this mutation in the retinula cells. We have estimated when the functioning of this gene or its product is required for normal visual system development by using genetic mosaicism induced by somatic recombination and temperature shifts of a temperature-sensitive mutation at this locus. Both methods point to a period in the mid-third instar, suggesting that early events in the formation of ommatidia and/or late events in the program of retinal cells are affected. Application of a new histological stain for developing axons indicates that individuals heterozygous for Glued exhibit abnormalities in the retinula fiber projection by the late third instar. Thus, the adult phenotype is not solely the result of later cellular degeneration or rearrangement. Beneath M + Gl + clones which encompass the entire eye were found optic lobe abnormalities with features not seen in either other mosaics or Gl heterozygotes. The possibility that these abnormalities result from temporal asynchrony in the development of eye and and optic lobe in these individuals is discussed and the results of attempts to test this hypothesis are presented.

Development of Dorsal Ocelli and Ocellar Interneurons in Trichoplusia ni (Lepidoptera: Noctuidae)

The dorsal ocelli of adult Trichoplusia ni develop during the pupal stage from larval primordia. The ocellus arises from epidermal cells and is detectable in day 1 pupae. The ocellar corneagen cells and retinula cells have differentiated by day 2. By day 4 the retinula cell axons have entered the protocerebrum and large or ocellar interneurons are visible. The ocellus and interneurons assume the appearance of those in the adult by day 5, 2 days before adult emergence.

The euphausiid compound eye ? a morphological re-investigation (Crustacea: Euphausiacea)

The ommatidia of the compund eye of the krill, Meganyctiphanes norvegica, are composed of the following cell types: corneagenous cells, main cone cells, accessory cone cells, distal pigment cells, and retinula cells. The corneagenous cells secrete the cuticular lenses, below which the crystalline cones are present. The bipartite crystalline cone is formed within the main cone cells, whereas the accessory cone cells invest the distal part of the cone and proceed proximally in the retina to the basement membrane. Six distal pigment cells are arranged around each ommatidium, though shared by adjacent ommatidia. The seven retinula cells form the rhabdom, and penetrate the basement membrane as axons. The basement membrane is formed by the conjoined extended prolongations from the accessory cone cells and sheets from the basal pigment cells.

Trophospongium-like structures in an insect eye: response of retinula cells of Papilio xuthus (Lepidoptera) to irradiation with ultraviolet light.

Presumably, as a consequence of a 3-h exposure to light of 350-nm wavelength (1.5 X 10(15) photons/cm2 sec) followed by a period of 20 h of rest in the dark, the four smaller retinula cells in each ommatidium of the eye of the butterfly Papilio xuthus contain a structure in the peripheral regions of their cytoplasm that resembles a portion of a rhabdom. Evidence for and against the view that this unusual, highly ordered arrangement of membranes represents a trophospongium is presented. In view of the fact that the structure in question only occurred at the same time when rhabdomeres were in a process of disintegration or reformation, the authors conclude that the structure in question is involved in the supply or removal of substances during a period of considerable activity of the retinula cell.

Comparative Ultrastructure of the Compound Eye of the Wild, Laboratory-Reared, and Irradiated Mediterranean Fruit Fly, Ceratitis capitata (Diptera: Tephritidae)1

The compound eye of the Mediterranean fruit fly, Ceratitis capitata (Wiedemann), was found to consist of ca. 3,000 ommatidia, each composed of a dioptric apparatus overlying a cluster of eight retinula cells. A cross-section of the cell cluster reveals an open rhabdom structure with a superior R7 central cell and an inferior R8 central cell similar to that of other cyclorrhaphous Diptera. Each ommatidium has a pair of primary pigment cells and 12 shared longitudinal pigment cells. The dimensions and histology of all ommatidial components are described briefly. Various ultrastructural differences occur between wild and laboratory-reared flies. Laboratory-reared, particularly pupal-irradiated, flies have ommatidia comprised of less tightly packed retinula cells, and rhabdomeres that are consistently smaller, occasionally absent, and more peripherally distributed within the ommatidial cavity than those in wild flies. The R7 rhabdomere is sometimes expanded into a bulb at the level of the Rl-7 nuclei in laboratory-reared flies, but not in wild flies. Ommatidial cavities of laboratory-reared flies are enlarged and often contain inclusions not present in wild flies. The cytoplasmic density and concentrations of large lipid globules is reduced in the longitudinal pigment cells of laboratory-reared flies. The pattern of increasing ultrastructural deviation from the wild condition in laboratory-reared and irradiated flies is consistent with, and indeed provides a basis for, explaining a pattern of decreasing spectral sensitivity (wavelengths 350 to 750) found electroretinographically for flies obtained from the same samples.

Firm structural associations between migratory pigment granules and microtubules in crayfish retinula cells.

The morphology of associations between mobile pigment granules and microtubules of the crayfish retinula cells was examined with transmission electron microscopy. Many pigment granules were found associated with microtubules through linkages of fuzzy appearance in thin sections. The linkages were revealed as discrete strands of variable shape in rotary-shadowed replicas of freeze-fractured and deep-etched specimens. The only feature of constant morphology among these connections consisted of 2-4-nm filaments projecting laterally from the microtubules. The firmness of the pigment granule-microtubule associations was judged by their ability to hold up during cell disruption procedures of increasing disaggregation effects in a low-Ca++ stabilization buffer. The results of these tests were inspected with scanning electron microscopy and with transmission electron microscopy of negatively stained preparations. Numerous pigment granules remained associated with a stable microtubule framework after the plasma membrane had been stripped away. Moreover, granule-microtubule attachments survived breakdown of this framework into free fascicles of microtubules. The pigment granules were associated with the free microtubules either individually or as clusters entangled in a fibrous material interwoven with 10-nm filaments. These findings attest that many pigment granules are bound to microtubules through linkages that constitute effective attachments. Further, it is demonstrated that a highly cohesive substance associates the pigment granules with one another. These conclusions are discussed in terms of a pigment transport mechanism in which a network of interconnected granules would establish firm transient interactions with a supporting skeleton of microtubules.

Fine structure and convergent development of the Cirolana compound eye (Crustacea Isopoda)

Each sessile eye comprises about 60 ommatidia with large lenses (diameter 100–150 μm), spherical crystalline cones, and hypertrophied rhabdoms (diameter about 100 μm). The cones are formed by two main cone cells, and in addition two accessory cone cells are present. Seven retinula cells contribute to the rhabdom, which is fused distally and open proximally (separated rhabdomeres). A special cell type, previously designated ‘hyaline cells’ are shown to be reflecting pigment cells. These cells form a well-developed tapetal layer. Distal pigment cells screen neighbouring ommatidia. Further, the eye is delimited by two membranes homologous to the fenestrated and eye capsule membranes present in several other isopod groups. The hypertrophied rhabdoms and the elaborate tapetal layer separate the morphology of the Cirolana eye from that of other isopod eyes. These structural features makes the Cirolana eye a case of convergent development with several other deep water living crustaceans.

The compound eye of Leptodora kindtii (Cladocera). An adaptation to planktonic life.

Each of the approximately 500 ommatidia in the compound eye of the cladoceran crustacean Leptodora kindtii has a crystalline cone consisting of five cells. Five retinula cells are also present, one of which contributes to the distal 1-2 micrometer of the rhabdom only; the other four retinula cells from a continuous rhabdom. Throughout the rhabdom its cross section displays two separate halves with the axis of the microvilli in one half perpendicular to that in the other (orthogonal pattern). Interferometric analysis of the refractive index of the crystalline cone revealed an inhomogeneous system with one distal and one proximal gradient. The gradient system was found to exclude rays entering from adjacent facets, thus maintaining the optical isolation. Consequently, these optics replace distal screening pigment, which is absent in the eye. The long and unscreened crystalline cones give rise to an almost transparent eye in conformity with the overall transparency of this plank-tonic animal. The morphological characteristics of the eye of this species deviate from other cladoceran eyes, but the optical design closely resembles that of some pelagic marine amphipod crustaceans.

Giant rhabdomes in a specialized region of the compound eye of a cricket: Cycloptiloides canariensis (Insecta, Gryllidae)

The cricket Cycloptiloides canariensis (Gryllidae, Mogoplistinae) has compound eyes of the eucone apposition type. They consist of relatively few (about 160–190) ommatidia with a wide angle of divergence. Each ommatidium has two primary pigment cells, four Semper cells and eight retinula cells whose four distal rhabdomeres are arranged around the tip of the crystalline cone. The screening pigments are contained mainly in the primary pigment and retinula cells, while the secondary pigment cells are sparsely pigmented. The dorsal eye region can be distinguished by the following characteristics: lack of dioptric apparatus (flat, unfacetted cornea, absence of cones), densely packed rhabdomes in approximately parallel orientation extending distally to the cornea and possessing a markedly increased cross-sectional area in comparison with those of normal ommatidia. Screening pigments are absent. The importance of this dorsal region will be discussed.