Fine structure of the retina of black bass, Micropterus salmoides (Centrarchidae, Teleostei).

The structure of light- and dark-adapted retina of the black bass, Micropterus salmoides has been studied by light and electron microscopy. This retina lacks blood vessels at all levels. The optic fiber layer is divided into fascicles by the processes of Müller cells and the ganglion cell layer is represented by a single row of voluminous cells. The inner nuclear layer consists of two layers of horizontal cells and bipolar, amacrine and interplexiform cells. In the outer plexiform layer we observed the synaptic terminals of photoreceptor cells, rod spherules and cone pedicles and terminal processes of bipolar and horizontal cells. The spherules have a single synaptic ribbon and the pedicles possess multiple synaptic ribbons. Morphologically, we have identified three types of photoreceptors: rods, single cones and equal double cones which undergo retinomotor movements in response to changes in light conditions. The cones are arranged in a square mosaic whereas the rods are dispersed between the cones.     

Light-induced structural changes of cytoskeleton in squid photoreceptor microvilli detected by rapid-freeze method

The cytoskeleton in squid photoreceptor microvilli was studied by freeze-substitution electron microscopy combined with rapid freezing using liquid helium, under dark-adapted and light-illuminated conditions. In the dark-adapted microvilli, actin filaments were regularly associated with granular structures on their surface; these granular structures were cross-linked to the rhodopsin-bearing plasma membranes through slender strands. Upon exposure to light, the granular components detached from the actin filaments, which then appeared to be fragmented and/or depolymerized. These observations have led us to conclude that light stimulation triggers the breakdown of the microvillar actin filament complex in squid photoreceptor cells. The results are discussed with special reference to the physiological role of actin filaments in photoreception.     

Immunocytochemical localization of Vitamin A in the retina and pineal organ of the frog,Rana esculenta

Summary Vitamin A immunoreactive sites were studied in the retina and pincal organ of the frog,Rana esculenta, by the peroxidase antiperoxidase, avidin-biotinperoxidase and immunogold methods. Indark-adapted material, strong immunoreaction was found in the outer and inner segments of the photoreceptor cells of both retina and pineal organ, as well as in the pigment epithelium, retinal Müller cells and pineal ependymal cells. Inlight-adapted retina, cones and green (blue-sensitive) rods were immunopositive.At the electron microscopic level, immunogold particles were found on the membranes of the photoreceptor outer segments as well as on the membranes of the endoplasmic reticulum and mitochondria. Individual retinal photoreceptor cells exhibited strong immunoreaction in the distal portion of the inner segment, the ciliary connecting piece and the electron-dense material covering the outer segment. In the pigment epithelium, the immunolabeling varied in intensity in the basal and apical cytoplasm and phagocytosed outer segments.The immunocytochemical results indicate that retinoids (retinal, retinol and possibly retinoic acid) are present not only in the photoreceptor cells of the retina but also in those of the pineal organ. The light-dependent differences in the immunoreactivity of vitamin A underlines its essential role in the visual cycle of the photopigments. Our results suggest that the pineal ependyma plays a role comparable to that of the Müller cells and pigment epithelium of the retina with regard to the transport and storage of vitamin A. The presence of a retinoid in nuclei, mitochondria and cytoplasmic membranes suggests an additional role of vitamin A in other metabolic processes.Dedicated to Professor Dr. T.H. Schiebler on the occasion of his 65th birthdaySupported by the Hungarian OTKA grant Nr. 1619 to B.V., and a grant from the Pardee Foundation to G.H.W.     

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.     

Immunocytochemical localization of vitamin A in the retina and pineal organ of the frog, Rana esculenta

Vitamin A immunoreactive sites were studied in the retina and pineal organ of the frog, Rana esculenta, by the peroxidase antiperoxidase, avidin-biotinperoxidase and immunogold methods. In dark-adapted material, strong immunoreaction was found in the outer and inner segments of the photoreceptor cells of both retina and pineal organ, as well as in the pigment epithelium, retinal Müller cells and pineal ependymal cells. In light-adapted retina, cones and green (blue-sensitive) rods were immunopositive. At the electron microscopic level, immunogold particles were found on the membranes of the photoreceptor outer segments as well as on the membranes of the endoplasmic reticulum and mitochondria. Individual retinal photorecptor cells exhibited strong immunoreaction in the distal portion of the inner segment, the ciliary connecting piece and the electron-dense material covering the outer segment. In the pigment epithelium, the immunolabeling varied in intensity in the basal and apical cytoplasm and phagocytosed outer segments. The immunocytochemical results indicate that retinoids (retinal, retinol and possibly retinoic acid) are present not only in the photoreceptor cells of the retina but also in those of the pineal organ. The light-dependent differences in the immunoreactivity of vitamin A underlines its essential role in the visual cycle of the photopigments. Our results suggest that the pineal ependyma plays a role comparable to that of the Müller cells and pigment epithelium of the retina with regard to the transport and storage of vitamin A. The presence of a retinoid in nuclei, mitochondria and cytoplasmic membranes suggests an additional role of vitamin A in other metabolic processes.     

Photoreceptor cells and neural elements with long axonal processes in the pineal organ of the lamprey,Lampetra japonica,identified by use of the horseradish peroxidase method

Summary Horseradish peroxidase (HRP) was applied to the transected end of the pineal tract of the lamprey, Lampetra japonica. Distinct reaction products of HRP were observed in 2 types of cell other than ganglion cells. The first type of cell protrudes a knob-like process into the pineal lumen. This type of cell was clearly identified by electron microscopy as a photoreceptor cell; its outer segment was connected to the ellipsoid through a sensory cilium. The other type of cell was located among photoreceptor and supporting cells. The processes of these cells were thin and slender, and they obviously did not represent photoreceptor, supporting, or conventional ganglion cells. The present results indicate that, in the lamprey, some of the photoreceptor cells of the pineal organ project their axon-like processes toward the posterior commissure, but that there is also another type of cell displaying long axonal projections. HRP-containing cells were distributed randomly over the pineal organ and were occasionally also observed in the parapineal organ.     

The human vocal cord surface

Summary The effect of phenylthiourea (PTU) on the pigment epithelium and the photoreceptor cells in the developing retina of Haplochromis burtoni was studied by electron microscopy. In the retinal pigment epithelium of 6-day old embryos, both types of melanin granule (spindle-shaped and rod-shaped) are already found. PTU inhibits the biosynthesis of melanin but does not influence the formation of premelanosomes so that in PTU-treated embryos there are no melanosomes, but an abundance of premelanosomes. The structure of the premelanosomes is described. It differs completely from that of all other vertebrates. Other changes: an increase in polysomes, retarded development of the inner segment of the photoreceptor cells and enlargement of the intercellular space between the inner and outer leaflet of the retina, may be due to a toxic effect of PTU.This investigation was supported by grants of the Deutsche Forschungsgemeinschaft     

Fine structure of the retinal photoreceptors of the ranch mink Mustela vison

The structure of the retinal photoreceptors of the ranch mink (Mustela vison) has been investigated by light and electron microscopy. In this mammalian species, the photoreceptors can be readily differentiated and adequately described by the classical terminology of rods and cones, with the rods being the more numerous. Rods are long slender cells while cones are shorter and stouter in appearance. Both rods and cones are highly differentiated and extremely polarized cells consisting of an outer segment, a non-motile connecting cilium, an inner segment, a nuclear region and a synaptic process extending to an expanded synaptic ending. Morphological differences are noted between rods and cones for most of the various regions of these cells. While rods reach to the cell body of the retinal pigment epithelial (RPE) cells, larger apical processes from the RPE extend to the shorter cone cells, so that both photoreceptor types are in intimate contact with the retinal epithelial cells.     

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.     

Taurine,amino acid transmitters,and related molecules in the retina of the Australian lungfish Neoceratodus forsteri: a light-microscopic immunocytochemical and electron-microscopic study

The morphology of the retina of the Australian lungfish Neoceratodus forsteri was investigated by means of light- and electron microscopy, whilst immunocytochemical studies were performed to determine the cellular distributions of the major amino acid neurotransmitters and other amino acids. The distributions of glycine and GABA were similar to those previously described for teleost, amphibian and mammalian retinae. Labelling was abundant in amacrine cells, whilst GABA was also present in one layer of horizontal cells and some bipolar cells. Taurine was present in both rods and cones, but, unlike the mammalian or avian retina, was absent from other cellular structures, including glial elements. Unexpectedly, the photoreceptor terminals lacked an apparent content of the excitatory amino acid transmitter glutamate. The glutamate that was present in the rods and cones occupied a crescentic arc corresponding to the location of glycogen-rich paraboloids. Asparagine was also present in rods, albeit in the modified mitochondria that formed the elipsoids of the rod inner segments. Arginine, the precursor for formation of nitric oxide, was present in glial cells, and in the paraboloids of both rods and cones.     

The renewal of protein in retinal rods and cones

The renewal of protein in retinal rods and cones has been analyzed by quantitative electron microscope radioautography in adult frogs injected with a mixture of radioactive amino acids. Protein synthesis occurs predominantly in the ergastoplasm, localized in the myoid region of the photoreceptor cells. Much of the newly formed protein next flows through the Golgi complex. In rods, a large proportion of the protein then moves past the mitochondria of the ellipsoid segment, passes through the connecting cilium into the outer segment, and is there assembled into membranous discs at the base of that structure. Discs are formed at the rate of 36 per day in red rods and 25 per day in green rods at 22.5° C ambient temperature. In cones, a small proportion of the protein is similarly displaced to the outer segment. However, no new discs are formed. Instead, the protein becomes diffusely distributed throughout the cone outer segment. Low levels of radioactivity have been detected, shortly after injection, in the mitochondria, nucleus, and synaptic bodies of rods and cones. Nevertheless, in these organelles, the renewal process also appears to involve the utilization of protein formed in the ergastoplasm of the myoid.     

Actin-dependent myoid elongation in teleost rod inner/outer segments occurs in the absence of net actin polymerization.

In the retinas of teleost fish, rod photoreceptors elongate in response to light. Light-activated elongation is mediated by the myoid of the rod inner segment and is actin-dependent. Inner segment F-actin filaments form bundles running parallel to the cell's long axis. We examined the mechanism of rod elongation using mechanically-detached rod fragments, consisting of the motile inner segment and sensory outer segment (RIS-ROS). When RIS-ROS are isolated from dark-adapted green sunfish and cultured in the light, they elongate 15 microns at 0.3-0.6 microns/min. Elongation was inhibited 65% by 0.1 microM Cytochalasin D, suggesting a requirement for actin assembly. To determine the extent of assembly during elongation, we used three approaches to measure the F-actin content in RIS-ROS: detection of pelletable actin by SDS-PAGE after detergent-extraction of RIS-ROS; quantification of fluorescein-phalloidin binding by fluorimetry, fluorescence-activated cell sorting and image analysis; estimation of total F-actin filament length by electron microscopy. All three assays indicated that no net assembly of RIS-ROS F-actin accompanied myoid elongation. An increase in F-actin content within the elongated myoid was counterbalanced by a decrease in F-actin content within the 13 microvillus-like calycal processes located at the end of the inner segment opposite to the growing myoid. O'Connor and Burnside (Journal of Cell Biology 89:517-524, 1981) showed that minus-ends of rod F-actin filaments are oriented towards the elongating myoid while plus-ends are oriented towards the shortening calycal processes. Our observations suggest that RIS-ROS elongation entails actin polymerization at the minus-ends of filaments coupled with depolymerization at the filament plus-ends.     

Photoreceptors of cubozoan jellyfish

The anatomically sophisticated visual system of the cubozoan jellyfish Carybdea marsupialis is described. Individual cubomedusae have eight complex eyes, each with a cornea, lens, and retina of ciliated photoreceptor cells, eight slit ocelli, and eight dimple ocelli. The photoreceptor cells of the complex eyes are bipolar and resemble vertebrate rod cells. Each photoreceptor has an outer cylindrical light-receptive segment that projects into a vitreous space that separates the lens and the retina, an inner segment rich in pigment granules, and a basal region housing the nucleus. The outer segment is a modified cilium with a 9 + 2 arrangement of microtubules plus stacks of membrane. These stacks of membrane form numerous discs that are oriented transversely to the long axis of the cell. The outer segment is connected to the inner segment by a slender stalk. The basal end of each photoreceptor forms an axon that projects into an underlying layer of interneurons. Each ocellus is composed of ciliated photoreceptor cells containing pigment granules. Rhodopsin-like and opsin-like proteins are found in the membrane stacks of the outer segments of the photoreceptors of the complex eyes. An ultraviolet-sensing opsin-like protein is present in the inner segments and basal regions of some of the photoreceptors of the complex eyes. Rhodopsin-like proteins are also detected in the photoreceptors of the slit ocelli. The cellular lens, composed of crystallin proteins, shows a paucity of organelles and a high concentration of homogeneous cytoplasm. Neurons expressing RFamide (Arg-Phe-amide) comprise a subset of interneurons found beneath the retinas of the complex eyes. RFamide-positive fibers extend from these neurons into the stalks of the rhopalia, eventually entering into the subumbrellar nerve ring. Vision may play a role in the navigation, feeding, and reproduction of the cubomedusae.     

The Photoreceptor Outer Segments of the River Lamprey (Lampreta fluviatilis). An Electron-, Fluorescence- and Light Microscopic Study

The outer segment of long and short photoreceptors in the retina of the river lamprey, Lampetra fluviatilis, were studied by light- and fluorescence microscopy together with some different electron microscopic methods. The outer segments show characteristics of both rods and cones and are suggested to represent intermediate kinds of photoreceptors.     

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.     

Pigment epithelial ensheathment and phagocytosis of extrafoveal cones in human retina.

The association between extrafoveal cone outer segments and pigment epithelial cells was studied by transmission electron microscopy in three human retinas; ages 5,45 and 60. The pigment epithelial apical surface from a fourth human retina, age 38,was viewed in the scanning electron microscope. Multiple villous-like apical processes protrude from the pigment epithelium into the space above each cone. Sometimes one or more of these processes is sheet-like in form and contains a wealth of intracellular organelles, including mitochondria. One or more of the villous-like procesess reaches the cone and expands to ensheath the upper one-third of the outer segment. Llike vertebrate rods, extrafoveal human cones shed their terminal disks in packets and these packets are phagocytosed by the ensheathing apical processes. The phagosomes then ascend in the processes toward the pigment epithelia soma. Digestion of phagosomes appears to begin in the apical processes.     

Photoreceptors of the larval tiger salamander retina

Six morphological types of photoreceptor were identified with electron microscopy in radial sections of the retina of the larval tiger salamander, Ambystoma tigrinum. In order of predominance these six types are: red rods, large single cones, double cones composed of principal and accessory members, small single cones, and green rods. The different types of photoreceptor can be distinguished by a number of morphological and cytological characteristics. The identification of the small single cone type now provides evidence for more than one type of single cone in an amphibian retina. The interconnections of the different types of photoreceptor by gap junctions were studied in tangential sections. Rod-rod and rod-cone gap junctions occurred in all possible combinations, but no cone-cone junctions were found even between members of double cones.     

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。     

Ultrastructure of the pineal organ of the killifish,Fundulus heteroclitus,with special reference to the secretory function

Summary The pineal organ of the killifish, Fundulus heteroclitus, was investigated by electron microscopy under experimental conditions; its general and characteristic features are discussed with respect to the photosensory and secretory function. The strongly convoluted pineal epithelium is usually composed of photoreceptor, ganglion and supporting cells. In addition to the well-differentiated photosensory apparatus, the photoreceptor cell contains presumably immature dense-cored vesicles (140–220 nm in diameter) associated with a well-developed granular endoplasmic reticulum in the perinuclear region and the basal process. These dense-cored vesicles appear rather prominent in fish subjected to darkness. The ganglion cell shows the typical features of a nerve cell; granular endoplasmic reticulum, polysomes, mitochondria and Golgi apparatus are scattered in the electron-lucent cytoplasm around the spherical or oval nucleus. The dendrites of these cells divide into smaller branches and form many sensory synapses with the photoreceptor basal processes. Lipid droplets appear exclusively in the supporting cell, which also contains well-developed granular endoplasmic reticulum and Golgi apparatus. Cytoplasmic protrusions filled with compact dense-cored vesicles (90–220 nm in diameter) are found in dark-adapted fish. The origin of these cytoplasmic protrusions, however, remains unresolved. Thus, the pineal organ of the killifish contains two types of dense-cored vesicles which appear predominantly in darkness. The ultrastructural results suggest that the pineal organ of fish functions not only as a photoreceptor but also as a secretory organ.We thank Dr. Grace Pickford for the fishes.     

Photoreceptor number and outer segment disk membrane surface area in the retina of the rat: stereological data for whole organ and average photoreceptor cell

A random sampling scheme is employed to obtain stereological estimates of disk membrane surface area in the entire retina and in the average photoreceptor cell. The scheme involves the use of vertical sections with combined light and electron microscopy at several magnification levels. Left and right retinas from six albino animals were analysed. There were no significant lateral differences. On average, the retina had a volume of 16 mm3, thickness of 200 μm and surface area of 80 mm2 (representing about 56% of the external surface of the eyeball). Photoreceptor disk membranes within outer segments amplified total retinal surface by almost 1000-fold (final surface 770 cm2 per retina). The retina contained 3×107 photoreceptors (packing density 374 000 mm-2) with an average disk membrane surface area of 2600 μm2. Mean nuclear volume in photoreceptor cells was 59 μm3 and the coefficient of variation for the distribution of nuclear volumes was 57%. The data are consistent with an average of 700 disks per photoreceptor cell, a membrane area of 4 μm2 per disk and a convergence ratio of ～260 photoreceptors per optic nerve fibre. The basic scheme could be modified for other species and for direct cell counts conducted on rods and cones separately.