The fine structure of the pigment epithelium and photoreceptor cells of the newt,Triturus viridescens dorsalis (Rafinesque)

The morphology of the retinal pigment epithelium and photoreceptor cells has been studied in the common newt Triturus viridescens dorsalis by light, conventional transmission and scanning electron microscopy. The pigment epithelium is formed by a single layer of low rectangular cells, separated by a multilayered membrane (Bruch's membrane) from the vessels of the choriocapillaris. The scleral border of the pigment epithelium is highly infolded and each epithelial cell contains smooth endoplasmic reticulum, myeloid bodies, mitochondria, lysosomes, phagosomes and an oval nucleus. Inner, pigment laden, epithelial processes surround the photoreceptor outer and inner segments. The three retinal photoreceptor types, rods, single cones and double cones, differ in both external and internal appearance. The newt, rod, outer segments appear denser than the cones in both light and electron micrographs, due to a greater number of rod lamellae per unit distance of outer segment and to the presence of electron dense intralamellar bands. The rod outer segments possess deep incisures in the lamellae while the cone lamellae lack incisures. Both rod and cone outer segments are supported by a peripheral array of dendritic processes containing longitudinal filaments which originate in the inner segment. The inner segment mitochondria, forming the rod ellipsoid, arelong and narrow while those in the cone are spherical to oval in shape. The inner segments of all three receptor cell types also contain a glycogen-filled paraboloid and a myoid region, just outside the nucleus, rich in both rough and smooth endoplasmic reticulum. The elongate, cylindrical nuclei differ in density. The rod nuclei are denser than those of the cones, contain clumped chromatin and usually extend further vitreally. Similarly, the cytoplasm of the rod synaptic terminal is denser than its cone counterpart and contains synaptic vesicles almost twice as large as those of the cones. Photoreceptor synapses in rods and cones are established by both superficial and invaginated contacts with bipolar or horizontal cells.     

真鲷视网膜结构及其视觉特性研究：Ⅱ视网膜光感受细胞超微结构研究

对真鲷光感受细胞的超微结构进行观察,结果表明：视杆外段膜盘为游离膜盘,视锥外段膜盘则为连续的膜结构,视锥和视杆均含有连接纤毛和辅助外段。花萼状突起起源于内段。椭体内充满线粒体,无球状小体。双锥椭圆体并生膜为六层,视锥内段无鳍状突起,视锥突触带,在明适应视网膜中数量增多,在暗适应视网中数量减少,视杆突触带在这两种适应网膜中数量不变,每一杆小球只有一个突触带,而锥小足有４－６个突触带。     

Tyrosination of α‐tubulin controls the initiation of processive dynein–dynactin motility

Post‐translational modifications (PTMs) of α/β‐tubulin are believed to regulate interactions with microtubule‐binding proteins. A well‐characterized PTM involves in the removal and re‐ligation of the C‐terminal tyrosine on α‐tubulin, but the purpose of this tyrosination–detyrosination cycle remains elusive. Here, we examined the processive motility of mammalian dynein complexed with dynactin and BicD2 (DDB) on tyrosinated versus detyrosinated microtubules. Motility was decreased ~fourfold on detyrosinated microtubules, constituting the largest effect of a tubulin PTM on motor function observed to date. This preference is mediated by dynactin's microtubule‐binding p150 subunit rather than dynein itself. Interestingly, on a bipartite microtubule consisting of tyrosinated and detyrosinated segments, DDB molecules that initiated movement on tyrosinated tubulin continued moving into the segment composed of detyrosinated tubulin. This result indicates that the α‐tubulin tyrosine facilitates initial motor–tubulin encounters, but is not needed for subsequent motility. Our results reveal a strong effect of the C‐terminal α‐tubulin tyrosine on dynein–dynactin motility and suggest that the tubulin tyrosination cycle could modulate the initiation of dynein‐driven motility in cells.     

Ultrastructure of the distal retina of the adult zebrafish, Danio rerio

The organization, morphological characteristics, and synaptic structure of photoreceptors in the adult zebrafish retina were studied using light and electron microscopy. Adult photoreceptors show a typical ordered tier arrangement with rods easily distinguished from cones based on outer segment (OS) morphology. Both rods and cones contain mitochondria within the inner segments (IS), including the large, electron-dense megamitochondria previously described (Kim et al.) Four major ultrastructural differences were observed between zebrafish rods and cones: (1) the membranes of cone lamellar disks showed a wider variety of relationships to the plasma membrane than those of rods, (2) cone pedicles typically had multiple synaptic ribbons, while rod spherules had 1-2 ribbons, (3) synaptic ribbons in rod spherules were ∼2 times longer than ribbons in cone pedicles, and (4) rod spherules had a more electron-dense cytoplasm than cone pedicles. Examination of photoreceptor terminals identified four synaptic relationships at cone pedicles: (1) invaginating contacts postsynaptic to cone ribbons forming dyad, triad, and quadrad synapses, (2) presumed gap junctions connecting adjacent postsynaptic processes invaginating into cone terminals, (3) basal junctions away from synaptic ribbons, and (4) gap junctions between adjacent photoreceptor terminals. More vitread and slightly farther removed from photoreceptor terminals, extracellular microtubule-like structures were identified in association with presumed horizontal cell processes in the OPL. These findings, the first to document the ultrastructure of the distal retina in adult zebrafish, indicate that zebrafish photoreceptors have many characteristics similar to other species, further supporting the use of zebrafish as a model for the vertebrate visual system.     

The visual cells of the corsula mullet

The eye of Rhinomugil corsula has a duplex retina differentiated into dorsal and ventral halves, with the ventral retina 116·4 μm thicker than the dorsal retina. The rods of the ventral retina are relatively longer, with longer outer segments. The nuclei of the outer nuclear layer of the dorsal and ventral halves are in four and six to seven rows respectively. The rod outer segment bears a single incision. The mitochondria of cone and rod inner segments has a vitreal-scleral gradient. Single and double cones are present in both halves, with triple cones in the dorsal half only. The outer segments of double cones are equal and united. The single cones have two connecting cilia. The cone cells are arranged in a square mosaic with four double cones and five single cones to each unit in the dorsal half, and in a rhombic pattern in the ventral half.     

The accessory outer segment of rods and cones in the retina of the guppy,Poecilia reticulata P. (teleostei)

Summary The ultrastructure of the accessory outer segment (AOS) — a ciliumlike structure emanating from the inner segment and running alongside the outer segment of photoreceptors — is described. The AOS occurs in both rods and cones of Poecilia reticulata. Its ultrastructure, including the arrangement of microtubules, which originate from the ciliary stalk, is the same in rods and cones. The cone-AOS is connected with the outer segment by a thin plasmabridge, whereas the rod-AOS lies embedded within the outer segment. The outer segment of the cone, in contrast to that of the rod, is separated from the pigment epithelium by a large extracellular space. An intimate contact, however, is secured by the AOS; its membrane is closely appositioned to the pigment epithelium membrane. The functional significance of the AOS and its possible occurrence in other vertebrate classes, are discussed.     

Correlation of rhodopsin biogenesis with ultrastructural morphogenesis in the chick retina

The developing chick retina from stages 39-45 has been examined by biochemical and electron microscope techniques. The levels of rhodopsin contained in the maturing chick retina were evaluated by detergent extraction and correlated with rod outer segment formation. It was found that the appearance of rhodopsin in significant levels preceded outer segment formation by at least 2 days, thus implying that rhodopsin is synthesized in the receptor cell inner segment and translocated to the outer limb when disk membrane biogenesis occurs. The level of rhodopsin continues to rise as the rod outer segment develops. Development of both rods and cones originates and proceeds most rapidly in the fundus or central region and proceeds toward the periphery. In general, rod outer segments were noted to develop far more rapidly than cone outer segments.     

Differential Distribution of Posttranslationally Modified Microtubules in Osteoclasts

The differential distribution of microtubules in osteoclasts in culture was examined by using antibodies against acetylated, tyrosinated, or detyrosinated tubulins. Tyrosinated tubulin was found throughout the cytoplasmic microtubules in all cells examined. An expanding protrusion that contained tyrosinated tubulin but none of the detyrosinated or acetylated form was seen in the immature osteoclasts. Detyrosinated or acetylated tubulin was detectable in the peripheral cytoplasm of the mature osteoclasts displaying the loss of the expanding protrusion. Although most of the microtubules were derived from the centrosome, noncentrosomal microtubules were distributed in the expanding protrusion, which was predominantly positive for tyrosinated tubulin. By tracing single microtubules, the authors found that their growing ends were always rich in tyrosinated tubulin subunits. End binding protein 1 bound preferentially to the microtubule ends. Both acetylated and tyrosinated microtubules were shown to be closely associated with podosomes. Microtubules appeared to grow over or into the podosomes; in addition, the growing ends of single microtubules could be observed to target the podosomes. Moreover, a microtubule-associated histone deacetylase 6 was localized in the podosomes of the osteoclast. On the basis of these results, the authors conclude that posttranslational modifications of microtubules may correlate with characteristic changes in podosome dynamics in osteoclasts.     

Embryonic fissure and photoreceptor differentiation in the eye of adult Garra rufa Heckel 1843 (Cyprinidae, Teleostei)

The retina of the adult teleost Garra rufa retains a curved, open embryonic fissure indicating an asymmetrical postembryonic retinal growth. Undifferentiated, oval photoreceptors are observed on both sides of the middle of the fissure with their larger diameter running parallel to the fissure to which they may attach by desmosomes. They detach from the fissure, rotate to become perpendicular to it and begin an active process of differentiation as they slide along the temporal side of the outer half of the fissure. This process is divided into stages for simplicity. The photoreceptors develop stumpy inner segments extending into a ventricular space that appears between the retinal pigment epithelium and the photoreceptors. Calycal processes arise from the inner segments and the distal centriole of each photoreceptor forms a connecting cilium. The proximal centriole is retained for some time after the outer segment develops. The formation of rod spherules and cone pedicles takes place almost concomitantly with the outer segments. Double cones appear first as single cones before pairing. One or more of the principal cone mitochondria accumulate electron-dense material and merge to form the ellipsosome. The retinal pigment epithelium undergoes a parallel differentiation. The developmental events described in the present work conform those recorded in embryonic teleostean retinas.     

A third photoreceptor-specific GRK found in the retina of Oryzias latipes (Japanese killifish)

We previously reported that the teleost fish medaka (Oryzias latipes, Japanese killifish), possesses two kinds of G protein-coupled receptor kinases (GRKs) in the retina with different localizations: GRK7 (OlGRK-C) in cones and GRK1 (OlGRK-R1) in rods. To further clarify the diversity of teleost photoreceptor GRKs, we sought other medaka GRKs. We found an additional cDNA that encodes a second retina-specific GRK1 (OlGRK-R2). In situ hybridization experiments demonstrated that OlGRK-R2 mRNA is selectively expressed in rods. Sequence analysis of the Fugu rubripes genomic database unveiled a larger diversity of GRKs than previously expected. We also describe the light-dependent regulation of GRK1, a phenomenon that has not been found in other species. Immunocytochemical analysis indicated that OlGRK-R2 is localized in rod outer segments, independent of light condition. OlGRK-R1 is localized in the rod inner segments and synaptic termini of dark-adapted eyes, and moves to rod outer segments after light adaptation. Our studies suggest that the two medaka GRKs are not functionally redundant, and demonstrate a complicated light-dependent regulation of GRK1 in vivo.     

Differential localisation of tyrosinated, detyrosinated, and acetylated alpha-tubulins in neurites and growth cones of dorsal root ganglion neurons

The comparative distribution of tyrosinated, detyrosinated, and acetylated alpha-tubulins was examined in neurites of rat dorsal root ganglion neurones in culture using immunofluorescence microscopy. Phase contrast observations of single neurones revealed that the neurites were actively motile, and rhodamine phalloidin staining of actin filaments showed the extent of lamellopodia and microspike projections from the growth cones. From double-labelling experiments using antibodies against tyrosinated, detryrosinated, or acetylated alpha-tubulin, it was found that the three different isoforms were differentially localised in neurites and growth cones. Detyrosinated and acetylated forms of alpha-tubulin were in the main restricted to the neurites extending no further than the base of the growth cones. Tyrosinated alpha-tubulin was, however, distributed throughout the body of the growth cone and into the base of some microspikes. Following treatment with taxol to promote microtubule assembly, detyrosinated and acetylated alpha-tubulins were found to be colocalised with tyrosinated alpha-tubulins throughout the growth cones of all cells examined. These results would be consistent with axonal transport of tyrosinated alpha-tubulin followed by assembly in the growth cone and subsequent detyrosination and acetylation. In addition the presence of unmodified alpha-tubulin in the growth cone may be necessary for the provision of labile microtubules for growth cone motility and extension.     

Rapid and reversible tubulin tyrosination in human neutrophils stimulated by the chemotactic peptide,fMet-Leu-Phe

Neutrophil activation by specific stimuli, such as the oligopeptide chemotactic factor fMet-Leu-(fMLF), is associated with an increased enzymatic addition of tyrosine to tubulin α -subunits, as measured by ¹⁴C tyrosine uptake. In studies using immunoblots we have found that this increased tyrosine uptake into tubulin in activated neutrophils reflects an increase in the proportion of cellular tubulin that is tyrosinated rather than simply an increase in the turnover of tyrosinated subunits. However, the increased accumulation of tyrosinated tubulin was also found to follow an initial depletion of tyrosinated tubulin and concomitant increase in detyrosinated tubulin between 0 and 60 sec following stimulation of neutrophils with fMLF. Immunogold electron microscopy studies of intact micro tubules recovered from activated neutrophils demonstrated that these rapid changes in the relative content of tubulin isoforms in the cells were not associated with the formation or disappearance of microtubule microdomains composed of only one form of tubulin. Previously, we have shown that under conditions of fMLF-stimulated exocytosis there is an increased binding of neutrophil granules to endogenous microtubules. Since neutrophil activation by fMLF is associated with increased tyrosination of α -tubulin subunits, we speculated that rapid changes in the levels of tyrosinated tubulin in the microtubules of activated neutrophils might have a role in the regulation of granule-microtubule interactions. When the binding of purified neutrophil granules to reconstituted rat brain microtubules containing approximately 50% tyrosinated tubulin was measured by electron microscopy and compared with granule binding to microtubules that contained no detectable tyrosinated tubulin, granule-microtubule associations were found to be significantly favored by detyrosinated vs. tyrosinated tubulin. These findings indicate that interactions between cytoplasmic granules and microtubules in activated neutrophils may be modulated by rapid changes in the relative content of detyrosinated and tyrosinated tubulin in the microtubule network of the cells. © 1993 Wiley-Liss, Inc.     

Light-induced rod and cone cell death and regeneration in the adult albino zebrafish (Danio rerio) retina

Light-induced photoreceptor cell degeneration has been studied in several species, but not extensively in the teleost fish. Furthermore, the continual production of rods and cones throughout the teleost's life may result in regeneration of lost rods and cones. We exposed adult albino zebrafish to 7 days of constant darkness, followed by 7 days of constant 8000 lux light, followed by 28 days of recovery in a 14-h light:10-h dark cycle. We characterized the resulting photoreceptor layer cell death and subsequent regeneration using immunohistochemistry and light microscopy. Within the first 24 h of constant light, the zebrafish retina exhibited widespread rod and cone cell apoptosis. High levels of cell proliferation within the inner nuclear layer (INL) were observed within the first 3 days of constant light, as assessed by immunodetection of proliferating cell nuclear antigen and BrdU labeling. The proliferating cells within the INL were closely associated with the radial processes of Müller glia, similar to the pluripotent retinal stem cells observed during embryonic development. Using antibodies generated against the individual zebrafish opsins, we determined that rods and the green, blue, and ultraviolet cone cells were replaced within the 28 day recovery period. While both rods and cones were replaced, the well-ordered cone cell mosaic was not reestablished.     

Duplex Retina in the Mesopelagic Deep-Sea Teleost Lestidiops affinis (Ege, 1930)

The eye of the deep-sea teleost Lestidiops affinis has been examined primarily by light microscopy and found to possess a duplex retina consisting of two main divisions, a pure-cone and a pure-rod region, with a narrow zone of transition, possessing both cones and rods, joining the two. The pure-cone region is located in the temporal (caudal) part of the retina subserving binocular vision in the rostral direction. It has an area temporalis retinae with particularly long and densely packed single cones arranged in a regular hexagonal mosaic. Joined (double or twin) cones have not been recognized with certainty in the pure-cone region. The pure-rod region, comprising the larger part of the retina, contains rods grouped in bundles separated by retinal pigment epithelium (RPE) processes with pigmented cores. The synaptic endings of the rods are arranged in separate clusters in the outer plexiform layer, there being apparently a separate rod pedicle cluster beneath (vitread to) each rod bundle. Structural comparisons with certain other deep-sea teleosts suggest the likely presence of a retinal tapetum in L. affinis, i.e. each single cone or rod bundle is situated in a reflecting pit formed by the RPE, with a discrete reflector apposed to the tip of each cone outer segment and the tips of the outer segments of each square-cut rod bundle.     

Retention of function without normal disc morphogenesis occurs in cone but not rod photoreceptors

It is commonly assumed that photoreceptor (PR) outer segment (OS) morphogenesis is reliant upon the presence of peripherin/rds, hereafter termed Rds. In this study, we demonstrate a differential requirement of Rds during rod and cone OS morphogenesis. In the absence of this PR-specific protein, rods do not form OSs and enter apoptosis, whereas cone PRs develop atypical OSs and are viable. Such OSs consist of dysmorphic membranous structures devoid of lamellae. These tubular OSs lack any stacked lamellae and have reduced phototransduction efficiency. The loss of Rds only appears to affect the shape of the OS, as the inner segment and connecting cilium remain intact. Furthermore, these structures fail to associate with the specialized extracellular matrix that surrounds cones, suggesting that Rds itself or normal OS formation is required for this interaction. This study provides novel insight into the distinct role of Rds in the OS development of rods and cones.     

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.     

Retinal photoreceptor fine structure in the red-backed salamander (Plethodon cinereus).

The retinal photoreceptors of the red-backed salamander (Plethodon cinerus) have been studied by light and electron microscopy. Rods and single cones are present in this duplex retina in a ratio of about 25:1. The photoreceptors in this amphibian species are much larger than is reported for most vertebrates. In the light-adapted state, rods reach deep into the retinal epithelial (RPE) layer. The rod outer segment is composed of discs of uniform diameter displaying several very deep incisors. The rod inner segment displays a distal elliposid of mitochondria and a short stout myoid region. Rod nuclei are electron dense and often protrude through the external limiting membrane. Rod synaptic spherules are large and display several invaginated synaptic sites as well as superficial synapses. It is felt that the rods do not undergo retinomotor movements. The cone photoreceptors are much smaller than the rods and display a tapering outer segment, an unusual modified ellipsoid and a large parabolid of glycogen in the inner segment. Cone nuclei are less electron dense than rods and are located at all levels within the outer nuclear layer. The synaptic pedicle of the cones is larger, more electron lucent and display more synaptic sites (both invaginated and superficial) than that of rods. It is felt that cone photomechanical responses are minimal.     

Retinal photoreceptors and visual pigments in Boa constrictor imperator

The photoreceptors of Boa constrictor, a boid snake of the subfamily Boinae, were examined with scanning electron microscopy and microspectrophotometry. The retina of B. constrictor is duplex but highly dominated by rods, cones comprising 11% of the photoreceptor population. The rather tightly packed rods have relatively long outer segments with proximal ends that are somewhat tapered. There are two morphologically distinct, single cones. The most common cone by far has a large inner segment and a relatively stout outer segment. The second cone, seen only infrequently, has a substantially smaller inner segment and a finer outer segment. The visual pigments of B. constrictor are virtually identical to those of the pythonine boid, Python regius. Three different visual pigments are present, all based on vitamin A(1.) The visual pigment of the rods has a wavelength of peak absorbance (lambda(max)) at 495 +/- 2 nm. The visual pigment of the more common, large cone has a lambda(max) at 549 +/- 1 nm. The small, rare cone contains a visual pigment with lambda(max) at 357 +/- 2 nm, providing the snake with sensitivity in the ultraviolet. We suggest that B. constrictor might employ UV sensitivity to locate conspecifics and/or to improve hunting efficiency. The data indicate that wavelength discrimination above 430 nm would not be possible without some input from the rods.     

Relationship between the Golgi complex and microtubules enriched in detyrosinated or acetylated α-tubulin: studies on cells recovering from nocodazole and cells in the terminal phase of cytokinesis

Double immunofluorescence microscopy was used to study the relationship between the Golgi complex and microtubules enriched in posttranslationally modified tubulins in cultured mouse L929 fibroblasts. In interphase cells, the elements of the Golgi complex were grouped around the microtubule-organizing center. From here, tyrosinated microtubules extended to the periphery of the cells, whereas the distribution of detyrosinated and acetylated microtubules largely overlapped with that of the Golgi complex. Treatment of cells with 10 M nocodazole led to the disruption of all microtubules and dispersion of the Golgi elements. Following withdrawal of the drug, tyrosinated microtubules reformed first, followed by acetylated and then detyrosinated microtubules. In parallel, the Golgi elements moved back toward the juxtanuclear region and reestablished a close spatial relationship first with the acetylated and later also with the detyrosinated microtubules. Long-term recovery in the presence of 0.15 or 0.3 M nocodazole allowed partial reformation of tyrosinated and acetylated microtubules, whereas no or only a few detyrosinated microtubules were detected. At the same time, the Golgi elements were grouped closer together around or on one side of the nucleus in close relation to acetylated microtubules. In synchronized cells released from a mitotic block, a radiating array of tyrosinated microtubules was first formed, followed by acetylated and detyrosinated microtubules. The Golgi elements initially came together in a few groups and thereafter took an overall morphology similar to that in interphase cells. During this reunification, they showed a close spatial relationship to acetylated microtubules, whereas detyrosinated microtubules appeared only later. Microtubules enriched in acetylated and/or detyrosinated tubulin thus appear to take part in establishing and maintaining the organization of the Golgi elements within an interconnected supraorganellar system. Whether the acetylation and detyrosination of tubulin are directly involved in this process or merely represent two modifications within this subpopulation of microtubules remains unknown.On leave of absence from the Department of Histology and Embryology, Institute of Biostructure, Medical School, Warsaw, Poland     

Visual cycle: Dependence of retinol production and removal on photoproduct decay and cell morphology

The visual cycle is a chain of biochemical reactions that regenerate visual pigment following exposure to light. Initial steps, the liberation of all-trans retinal and its reduction to all-trans retinol by retinol dehydrogenase (RDH), take place in photoreceptors. We performed comparative microspectrophotometric and microfluorometric measurements on a variety of rod and cone photoreceptors isolated from salamander retinae to correlate the rates of photoproduct decay and retinol production. Metapigment decay rate was spatially uniform within outer segments and 50-70 times faster in the cells that contained cone-type pigment (SWS2 and M/LWS) compared to cells with rod-type pigment (RH1). Retinol production rate was strongly position dependent, fastest at the base of outer segments. Retinol production rate was 10-40 times faster in cones with cone pigments (SWS2 and M/LWS) than in the basal OS of rods containing rod pigment (RH1). Production rate was approximately five times faster in rods containing cone pigment (SWS2) than the rate in basal OS of rods containing the rod pigment (RH1). We show that retinol production is defined either by metapigment decay rate or RDH reaction rate, depending on cell type or outer segment region, whereas retinol removal is defined by the surface-to-volume ratio of the outer segment and the availability of retinoid binding protein (IRBP). The more rapid rates of retinol production in cones compared to rods are consistent with the more rapid operation of the visual cycle in these cells.