An insect retina without microvilli in the male scale insect,Eriococcus sp. (Eriococcidae,Homoptera)

Summary Male scale insects of an undescribed Australian species of Eriococcus have no compound eyes but show an extraordinary arrangement of three pairs of ocelli: One pair is positioned dorsolaterally where most insects have their compound eyes. Another pair looks ventrally and is placed where insects usually have their mouthparts, and there are two small lateral ocelli. Corneal nipples and a spherical lens with an estimated F-number only 0.55 are structural adaptations considered to increase the overall light sensitivity in order to compensate for the poor quantum capture of the shallow retina whose rhabdomes are only 3 m long. The outer segment of each receptor cell consists of a central core of cytoplasm containing mitochondria and a peripheral cylinder of about 16 rhabdomeres. There is no optical separation between neighbouring outer segments. Uniquely in arthropod eyes, the light sensitive structures are not composed of cylindrical microvilli, but consist of membrane stacks whose configuration is analogous to the stacked plates of vertebrate cones. At present no conclusive answer can be given as to why the photoreceptors have plates instead of microvilli. Comparative calculations show that they do not contain more photosensitive membrane per unit volume than rhabdomeres of fly ocelli.     

Vertebrate features revealed in the rudimentary eye of the Pacific hagfish (Eptatretus stoutii)

Hagfish eyes are markedly basic compared to the eyes of other vertebrates, lacking a pigmented epithelium, a lens and a retinal architecture built of three cell layers: the photoreceptors, interneurons and ganglion cells. Concomitant with hagfish belonging to the earliest-branching vertebrate group (the jawless Agnathans), this lack of derived characters has prompted competing interpretations that hagfish eyes represent either a transitional form in the early evolution of vertebrate vision, or a regression from a previously elaborate organ. Here, we show the hagfish retina is not extensively degenerating during its ontogeny, but instead grows throughout life via a recognizable PAX6+ ciliary marginal zone. The retina has a distinct layer of photoreceptor cells that appear to homogeneously express a single opsin of the RH1 rod opsin class. The epithelium that encompasses these photoreceptors is striking because it lacks the melanin pigment that is universally associated with animal vision; notwithstanding, we suggest this epithelium is a homologue of gnathosome retinal pigment epithelium (RPE) based on its robust expression of RPE65 and its engulfment of photoreceptor outer segments. We infer that the hagfish retina is not entirely rudimentary in its wiring, despite lacking a morphologically distinct layer of interneurons: multiple populations of cells exist in the hagfish inner retina and subsets of these express markers of vertebrate retinal interneurons. Overall, these data clarify Agnathan retinal homologies, reveal characters that now appear to be ubiquitous across the eyes of vertebrates, and refine interpretations of early vertebrate visual system evolution.     

Immunohistochemical evidence for multiple photosystems in box jellyfish

Cubomedusae (box jellyfish) possess a remarkable visual system with 24 eyes distributed in four sensory structures termed rhopalia. Each rhopalium is equipped with six eyes: two pairs of pigment cup eyes and two unpaired lens eyes. Each eye type probably captures specific features of the visual environment. To investigate whether multiple types of photoreceptor cells are present in the rhopalium, and whether the different eye types possess different types of photoreceptors, we have used immunohistochemistry with a range of vertebrate opsin antibodies to label the photoreceptors, and electroretinograms (ERG) to determine their spectral sensitivity. All photoreceptor cells of the two lens eyes of the box jellyfish Tripedalia cystophora and Carybdea marsupialis displayed immunoreactivity for an antibody directed against the zebrafish ultraviolet (UV) opsin, but not against any of eight other rhodopsin or cone opsin antibodies tested. In neither of the two species were the pigment cup eyes immunoreactive for any of the opsin antibodies. ERG analysis of the Carybdea lower lens eyes demonstrated a single spectral sensitivity maximum at 485 nm suggesting the presence of a single opsin type. Our data demonstrate that the lens eyes of box jellyfish utilize a single opsin and are thus color-blind, and that there is probably a different photopigment in the pigment cup eyes. The results support our hypothesis that the lens eyes and the pigment cup eyes of box jellyfish are involved in different and specific visual tasks.     

Photoreceptor fine structure in the southern fiddler ray (Trygonorhina fasciata).

The fine structure of the retinal photoreceptors has been studied by light and electron microscopy in the southern fiddler ray or guitarfish (Trygonorhina fasciata). The duplex retina of this species contains only rods and single cones in a ratio of about 40:1. No multiple receptors (double cones), no repeating pattern or mosaic of photoreceptors and no retinomotor movements of these photoreceptors were noted. The rods are cylindrical cells with inner and outer segments of the same diameter. Cones are shorter, stouter cells with a conical outer segment and a wider inner segment. Rod outer segment discs display several irregular incisures to give a scalloped outline to the discs while cone outer segment discs have only a single incisure. In all photoreceptors a non-motile cilium joins the inner and outer segments. The inner segment is the synthetic centre of photoreceptors and in this compartment is located an accumulation of mitochondria (the ellipsoid), profiles of both rough and smooth endoplasmic reticulum, prominent Golgi zones and frequent autophagic vacuoles. The nuclei of rods and cones have much the same chromatin pattern but cone nuclei are invariably located against or particularly through the external limiting membrane (ELM). Numerous Landolt's clubs which are ciliated dendrites of bipolar cells as well as Müller cell processes project through the ELM, which is composed of a series of zonulae adherentes between these cells and the photoreceptors. The synaptic region of both rods (spherules) and cones (pedicles) display both invaginated (ribbon) synapses and superficial (conventional) synapses with cones showing more sites than the rods.     

Cross species analysis of Prominin reveals a conserved cellular role in invertebrate and vertebrate photoreceptor cells

The two fundamental types of photoreceptor cells have evolved unique structures to expand the apical membrane to accommodate the phototransduction machinery, exemplified by the cilia-based outer segment of the vertebrate photoreceptor cell and the microvilli-based rhabdomere of the invertebrate photoreceptor. The morphogenesis of these compartments is integral for photoreceptor cell integrity and function. However, little is known about the elementary cellular and molecular mechanisms required to generate these compartments. Here we investigate whether a conserved cellular mechanism exists to create the phototransduction compartments by examining the functional role of a photoreceptor protein common to both rhabdomeric and ciliated photoreceptor cells, Prominin. First and foremost we demonstrate that the physiological role of Prominin is conserved between rhabdomeric and ciliated photoreceptor cells. Human Prominin1 is not only capable of rescuing the corresponding rhabdomeric Drosophila prominin mutation but also demonstrates a conserved genetic interaction with a second photoreceptor protein Eyes Shut. Furthermore, we demonstrate the Prominin homologs in vertebrate and invertebrate photoreceptors require the same structural features and post-translational modifications for function. Moreover, expression of mutant human Prominin1, associated with autosomal dominant retinal degeneration, in rhabdomeric photoreceptor cells disrupts morphogenesis in ways paralleling retinal degeneration seen in ciliated photoreceptors. Taken together, our results suggest the existence of an ancestral Prominin-directed cellular mechanism to create and model the apical membranes of the two fundamental types of photoreceptor cells into their respective phototransduction compartments.     

Conditioned medium-mediated photoreceptor differentiation in retina from embryonic rd chickens

Retina cells from 8-day rd (retinal degenerate) chicken embryos were cultured in media supplemented with optic lobe conditioned medium (OLCM). The morphogenesis of rd photoreceptors is being described. Several differentiating photoreceptors depicted a membranous sac protruding from the apical end of the cell as revealed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Such structures were found mostly in 8-day-old cultures supplemented with OLCM and were absent in controls. They resembled rudimentary outer segments emanating from a cilium at the apical inner segments. TEM showed a few stacks of free floating disks within the membranous structure which was suggestive of a rudimentary outer segment development. The possible neurotrophic effect of OLCM on rd retina photoreceptor differentiation is being suggested.     

Histological techniques for study of photoreceptor orientation

An histological method for the study of photoreceptor orientation in primate eyes is described. To preserve photoreceptor orientation it is necessary to protect the fragile rod and cone outer segments to the maximum extent possible from mechanical deformation and from injury by solvent extraction. To prevent mechanical deformation the eyes are freeze-dried and embedded in plastic with or without prior vapor fixation. Solvent extraction from the lipidrich outer segment is limited by avoidance or restriction of organic solvents. When large segments of primate eyes are so treated, it is possible to section the plastic blocks along the visual axis, polish the block surface, and view photoreceptor orientation by epi-illumination microscopy. In such specimens a differential orientation of photoreceptors exists with the long axis of photoreceptor inner and outer segments in line with incoming light rays.     

Membrane assembly in retinal photoreceptors I. Freeze-fracture analysis of cytoplasmic vesicles in relationship to disc assembly

To study precursor-product relationships between cytoplasmic membranes of the inner segment of photoreceptors and the continually renewed outer disc membrane, we have compared the density and size distribution of intramembrane particles (IMP) in various membrane compartments of freeze-fractured photoreceptor inner and outer segments. Both rod and cone outer segments of Xenopus laevis are characterized by a relatively uniform distribution of approximately 4,400-4,700 IMP/micron2 in P-face (PF) leaflets of disc membranes. A similar distribution of IMP is found in the outer segment plasma membrane, the ciliary plasma membrane, and in the plasma membrane of the inner segment in the immediate periciliary region. In each case the size distribution of IMP can be characterized as unimodal with a mean diameter of approximately 10 nm. PF leaflets of endoplasmic reticulum, Golgi complex, and vesicles near the cilium have IMP with a size distribution like that in the cilium and outer segment, but with an average density of approximately 2,000/micron2. In contrast, IMP are smaller in average size (approximately 7.5 nm) in PF leaflets of inner segment plasma membrane, exclusive of the periciliary rgion. The similarity of size distribution of IMP in inner segment cytoplasmic membranes and those within the plasmalemma of the cilium and outer segment suggest a precursor-product relationship between the two systems. The structure of the vesicle-rich periciliary region and the segregation of IMP with different size distributions in this region suggest that components destined for incorporation into the outer segment exist as preformed membrane packages (vesicles) which fuse with the inner segment plasma membrane in the periciliary region. Subsequently, membrane components may be transferred to forming discs of the outer segment via the ciliary plasma membrane.     

Immunoreactivity of the septins SEPT4, SEPT5, and SEPT8 in the human eye.

We aimed to examine the distribution of SEPT4, SEPT5, and SEPT8 in the human eye. For each septin, five to six normal human eyes were examined by immunohistochemical staining of paraffin sections using polyclonal antibodies against SEPT4, SEPT5, and SEPT8 and an avidin biotin complex immunodetection system. SEPT4 immunoreactivity (IR) was detected primarily in the epithelium of cornea, lens, and nonpigmented ciliary epithelium; in the endothelium of cornea and vessels of iris and retina; and in the retinal nerve fiber layer, the outer plexiform layer, the outer segments of the photoreceptor cells, the inner limiting membrane of the optic nerve head, and optic nerve axons. SEPT5-IR was present in corneal endothelial cells, iris tissue, nonpigmented ciliary epithelium, and epithelial cells of the lens. SEPT8-IR almost paralleled that of SEPT4, except for a lower SEPT8-IR of the outer photoreceptor segments and a positive staining of the meningothelial cell nests in the subarachnoidal space of the bulbar segment of the orbital optic nerve. In conclusion, SEPT4, SEPT5, and SEPT8 are expressed in various ocular tissues, each revealing a distinct expression pattern. Both physiological and potential pathophysiological role of septins in the human eye deserve further investigation.     

Histamine-like immunoreactivity in photoreceptors of the compound eyes and ocelli of the flies Calliphora erythrocephala and Musca domestica

Summary Antibodies to histamine were used for immunocytochemical studies of the visual system in the flies Calliphora erythrocephala and Musca domestica. Specific immunolabeling of photoreceptors was found both in the compound eyes and ocelli of both species. In the compound eyes histamine-like immunoreactivity (HA-IR) was found in all the short visual fibers (photoreceptors R1–6) and one type of long visual fiber (photoreceptor R8). In addition, the ocellar photoreceptors also show HA-IR. In view of earlier biochemical and pharmacological/physiological findings by Elias and Evans (1983) and Hardie (1987) it thus seems likely that histamine is a neurotransmitter in insect photoreceptors. Interestingly, the second type of long visual fiber (photoreceptor R7) has recently been found to be GABA-immunoreactive (Datum et al. 1986). The two types of long visual fibers may hence use different transmitters which act on different receptors of the postsynaptic neurons in the second visual neuropil, the medulla. In addition to the photoreceptors in the retina and ocelli, we found processes of HA-IR neurons in one of the optic lobe neuropils, the lobula. This finding indicates that histamine may also be a transmitter in certain interneurons in the visual system.Abbreviations HA histamine - GABA -amino butyric acid - GAD glutamic acid decarboxylase - 5-HT 5-hydroxytryptamine (serotonin) - HA-IR histamine-like immunoreactivity - R1-R6 class of short-axoned photoreceptors - R7 and R8 long-axoned photoreceptors - LMC large monopolar neuron of lamina - HSA human serum albumin - PBS phosphate-buffered saline - DEPC diethylpyrocarbonate