Fine Structure Of Photoreceptors Research Articles

Pathologic alterations of the outer retina in streptozotocin-induced diabetes.

Neurodegeneration as an early event of diabetic retinopathy preceding clinically detectable vascular alterations is a widely proven issue today. While there is evidence for the impairment of color vision and contrast sensitivity in early diabetes, suggesting deteriorated photoreceptor function, the underlying neuropathology of these functional alterations is still unknown. The aim of the present study was to investigate the effects of early diabetes on the outer retinal cells. The retinal pigment epithelium, photopigment expression, and density and morphology of photoreceptors were studied using immunocytochemistry in streptozotocin-induced diabetes in two rat strains. The fine structure of photoreceptors and pigment epithelium was also investigated with transmission electron microscopy. Here we found that retinal thickness was unchanged in diabetic animals and that no significant increase in the number of apoptotic cells was present. Although the density of cones expressing middle (M)- and shortwave (S)-sensitive opsins was similar in diabetic and control retinas, we detected remarkable morphologic signs of degeneration in the outer segments of diabetic rods, most M-cones, and some S-cones. A decrease in thickness and RPE65 protein immunoreactivity of the pigment epithelium were evident. Furthermore, an increased number of dual cones, coexpressing both M- and S-opsins, was detected at the peripheral retina of diabetic rats. Degenerative changes of photoreceptors and pigment epithelium shown here prior to apoptotic loss of photoreceptors may contribute to functional alterations reported in diabetic human patients and different animal models, thus may serve as a potential model for testing the efficacy of neuroprotective agents in diabetes.

Diurnal changes in the fine structure of photoreceptors in an abalone, Nordotis discus.

In order to examined a diurnal (24 h) rhythm in the fine structure of photoreceptors of abalone, Nordotis discus, the animals were kept on a daily light-dark cycle (light period: 6:00 a.m.--6:00 p.m., 2--5 lx) for 10 days; subsequently their eyes were fixed at nine specified times of the day. At 5:30 a.m. the photoreceptors consisted of numerous terminal microvilli growing on the surface of a dome-shaped apical cytoplasmic process (AP). Illumination induced breakdown of the photoreceptive microvilli into many vesicles. These vesicles were densely aggregated on almost the entire surface of enlarged APs at 7:00 a.m. After 1 h they markedly decreased in number. The cell at 10:00 a.m. possessed sparse, short and irregularly-contoured microvilli. The microvilli gradually increased in number from the middle through the later half of the light period. Only within 1 h after deprivation of light shorter but characteristic terminal microvilli were observed. By midnight they became similar in fine structure to those at 5:30 a.m.

The photoreceptors and pigment epithelim of the adult Xenopus retina: morphology and outer segment renewal.

Outer segment renewal and the fine structure of photoreceptors and pigment epithelium (p. e.) were studied in the adult Xenopus retina by light microscopic autoradiography and electron microscopy. Following the injection of [ 3 H]leucine, the pattern of labelling observed in receptor outer segments was typical of that reported in other adult retinae: only diffuse labelling was found in cones, but in rods a discrete band of label accumulated at the base of the outer segment and migrated sclerally with time. The rate of band displacement and thus disk addition in Xenopus rods was 1.86 μm/day (or 78 disks/day), which is more than twice that reported for red rods in Rana under similar experimental conditions, although these species have similar metabolic rates. Average rod outer segment (r. o. s.) length did not change, demonstrating a balance between disk addition and shedding. R. o. s. renewal time was about 24 days, corresponding to the time when labelled phagosomes were first found in the p. e. Ultrastructurally, one kind of (red) rod and one kind of cone were found whose outer segments differed in membrane topology. Although microfilaments were found in the apical processes of the p. e. and its cytoplasm contained both pigment granules and myeloid bodies, pigment granules did not migrate into these processes during light adaptation. In addition to possible morphological evidence for phago­somes of cone origin, both large and small rod phagosomes were observed in the p. e. The latter appear to represent small stacks of partial disks shed from individual r. o. s. scallops.

Fine structure of photoreceptors and associated neurons in the retina of Lampetra fluviatilis (Cyclostomi)

The outer segments and the receptor bases of the visual receptors of Lampetra fluviatilis have chiefly the fine structural characteristics of rods. On the basis of these findings and earlier observations of phagocyted outer segment membranes in the pigment epithelial cells and the demonstration of visual purple in the retina, it is suggested that the receptors should be called “rodlike”. The two layers of horizontal cells are the main store of glycogen in the retina. Two types of bipolar cells were observed. No photomechanical response occurs in the lamprey retina.

Retinal differences between light-tolerant and light-avoiding slugs (Mollusca: Pulmonata)

The fine structure of photoreceptors in the eye of a light-tolerant slug, Ariolimax californicus, was compared with that of a nocturnal slug, Limax maximus. There is a marked difference in the distribution of the 800 A microvesicles (photic vesicles) in the Type I sensory cells: massed distally beneath the microvilli, the light-sensitive organelles, in A. californicus; and aggregated basally near the nuclei in L. maximus. This finding supports our hypothesis that the vesicles are involved in photoreception. Additionally, L. maximus has a unique modification of endoplasmic reticulum in the receptoral process of the Type I sensory cell that is not seen in A. californicus. And the distribution and nature of the pigment granules in the supportive cells differ in the two species of slugs. Type II sensory cells and ganglion cells in A. californicus are described briefly.

The fine structure of photoreceptors in two otoplanid species (Turbellaria, proseriata)

The fine structure of the visual clubs was studied in two species of the family Otoplanidae, Otoplana truncaspina and Notocaryoturbella bigermaria. These clubs, which are found inside the cerebral capsule, are characterized by the presence of a disk-shaped photoreceptor process containing membranous laminar or tubular whorls. These cover both faces of the disk. A thin cytoplasmic layer is found between the two faces. This continues with projections which surround the disk border. The major projection, containing the nucleus, shows Golgi apparatuses, numerous vesicles of smooth endoplasmic reticulum, multi-laminar bodies, and mitochondria localized in the region bordering the disk cytoplasm. There are no pigmented cells. The affinities with other cerebral photoreceptors are discussed.

Fine structure of photoreceptors in the owl monkey.

AbstractThe owl monkey (Aotes trivirgatus) differs from most primates in being nocturnal in habit and having primarily scotopic vision. Photoreceptor cells of the retina of this species were studied with the electron microscope and compared with corresponding cells of other primates. No striking qualitative differences were noted. Most of the receptors are typical rods, but approximately 5% are cones, based on the following features: shorter, tapering outer segments, discs with smooth rather than scalloped margins and with uniformly narrow internal spaces, the membranes of which in some cases are continuous with the plasma membrane; broad inner segments with numerous mitochondria; nuclei that lie immediately against the outer Junctional zone (outer limiting membrane) with relatively thick inner fibers leading to large pedicles in the floor of which are numerous typical “synaptic triads.” One relatively unusual feature of the rods is the prominence of the striated rootlets associated with the basal bodies of the cilium‐like connecting pieces. An apparently intimate relation of the rootlets to investing mitochondria, especially to their tubular internal cristae, is discussed with regard to the possibility of a function for the rootlet other than static anchoring of the cilium.

Fine structure of photoreceptors in larval trematodes. A comparative study.

The fine structure of photoreceptors is described in miracidia of Fasciola hepatica, Heronimus chelydrae, Allocreadium lobatum, and Spirorchis sp., and in a spirorchiid cercaria. All have in common eyespots consisting of pigment cells with chambers occupied by rhabdomeres consisting of retinular cell dendrites with numerous microvilli. Photoreceptors of the miracidia show a bilateral asymmetry which is most pronounced in H. chelydrae with a pair of well separated eyespots unequal in size. The smaller right one consists of a pigment cell and two rhabdomeres; the larger left eyespot has an anterior pigment cell with two rhabdomeres and a posterior cell containing one rhabdomere. Photoreceptors in the other species of miracidia also have five rhabdomeres but contain only two pigment cells which are closely apposed. Each contains a pair of lateral rhabdomeres and a fifth one occupies a posteromedian extension of the left pigment cell. In the number of rhabdomeres, their relationship to pigment cells and the resulting asymmetry, photoreceptors are more alike in the distantly related species of miracidia studied than they are in ocellate cercariae or even in the miracidium and cercaria of the same species or two closely related ones. From the asymmetry of photoreceptors in larvae of certain flatworms other than digenetic trematodes, it seems that eyespots of miracidia have retained an ancestral pattern whereas the diversity of photoreceptors in cercariae reflects the varied phototactic behavior of those larvae which complete their life cycles by all the means known for cercariae with a free-swimming period. In both miracidia and cercariae, photoreceptors show an anterior-posterior organization that would seem to be concerned with orientation of the larvae with respect to light.

Fine Structure of Photoreceptors in Three Types of Ocellate Cercariae

Each paired photoreceptor in cercariae of Macravestibulum eversum, Crepidostomum sp., and Skrjabinopsolus manteri consists of one or two sensory cells with microvilli, and one separate pig- ment-containing cell. But the median eyespot in M. eversum may have a single cell differentiated into a central sensory portion around which a ring of pigment forms late in development. Moreover, the zone of microvilli impinges on a separate unicellular lens which is not present in the paired eyespots of any of the three species studied. In M. eversum, the pigment cell of each paired eyespot forms a deep cup which contains an anterior and posterior sensory cell with their microvilli apposed and interdigitating. In Crepidostomum sp., the pigment cup is shallower and contains a single sensory cell with parallel micro- villi extending in opposite directions from a central portion. In S. manteri, each eyespot has two sensory cells but they are on opposite sides of the pigment cell. Thus, eyespots in cercariae differ inter se as well as from the photoreceptors of miracidia. Among the features common to both free- living and many parasitic flatworms are photo- receptors which are closely associated with the brain and consist of separate sensory and pigment-containing cells. The pigment cell forms or participates in the formation of a more or less distinct cup occupied by a part of the sensory cell whose distal or peripheral region consists of numerous microvilli. The portion of the sensory cell just external to the cup may consist of a bulbous, refractile region, which is commonly described as a lens. From it the cell narrows to become the axon which leads to the brain.

The fine structure of photoreceptors in a marine flatworm.

The ocelli or eyes of the marine polyclad turbellarian Notoplana acticola are clustered on the paired dorsal nuchal tentacles and in two longitudinal bands lateral to the cerebral ganglion. The ocelli, studied by electron microscopy, were characterized as rhabdomeric and non-ciliary in origin. There are 60 to 80 ocelli per animal each enclosed in a fibrous capsule to which muscle fibers may attach. An ocellus consists of a pigmented eyecup into which 30 to 50 photoreceptor cells send dendritic processes through interruptions in or among pigment cell projections across the eyecup opening. The dendritic processes terminate in numerous long intertwined microvilli which fill the eyecup. The nucleated cell body of each photoreceptor cell lies outside the eyecup and projects an axonal process to the cerebral mass. Within the dendritic processes are observed mitochondria, ribosomes, neurotubules, multivesicular bodies, vesicles and vacuoles. The cell body contains smaller mitochondria, endoplasmic reticulum, ribosomes, vesicles and prominent Golgi complexes.

Fine Structure of the Eyespot in the Miracidium of Philophthalmus megalurus (Cort, 1914)

Miracidia from worms reared in the eyes of chicks were prepared as sections for light and electron microscopy. The eyespot consists of two closely apposed pigment-cup cells, each occupied by two sensory cells with one anterior and the other posterior to a vertical septum formed by the cup cell. This orientation extends also to the rhabdomeres of the two sensory cells in each cup and facilitates interpretation of the phototactic behavior of the miracidium. The fine structure of photoreceptors has been described for a variety of animals, but evidently the use of electron microscopy to investigate such receptors in larval trematodes has been limited to two species of miracidia. Kiimmel (1960) described the eyespot in that larva of Fasciola hepatica, and studies dealing with the miracidium of Philophthalmus megalurus were reported in a brief abstract by the author (Isseroff, 1963). This paper presents more fully the observations included in that part of the abstract concerned with fine structure of the eyespot. In reporting the life history of this species in Indiana, West (1961) identified it as Philophthalmus gralli, which had previously been found in Indochina and Formosa. In Hawaii, Alicata (1962) also identified as P. gralli, a trematode whose life history differed in several respects from that reported by West. However, Cable and Hayes (1963) determined that West and Alicata were dealing with different species and designated the North American one as P. megalurus (Cort, 1914) because the specific name of its larva, Cercaria megalura, has priority over other names that may have been given to the same species.

Fine structure of photoreceptors in amphioxus

The cells of Hesse in the ventral wall of the neural tube of Amphioxus, long assumed to be photoreceptors, were found by electron microscopy to possess many narrow irregular tubules, or diverticula of the cell membrane, immediately adjacent to the melanocyte associated with each receptor cell. Cilium-like structures are absent. Accordingly, Hesse cells do not resemble vertebrate photoreceptors. The ependymal cells in the posterior floor of the cerebral vesicle (infundibular organ?) have cilia and long filaments, both containing strings of vesicles. These vesicles may be suggestive of the evolutionary forerunners of the flattened sacs or discs in the outer segments of vertebrate photoreceptors.