STUDIES ON THE ENDOPLASMIC RETICULUM : V. Its Form and Differentiation in Pigment Epithelial Cells of the Frog Retina

Pigment epithelial cells of the frog's retina have been examined by methods of electron microscopy with special attention focused on the fine structure of the endoplasmic reticulum and the myeloid bodies. These cells, as reported previously, send apical prolongations into the spaces between the rod outer segments, and within these extensions, pigment migrates in response to light stimulation. The cytoplasm of these cells is filled with a compact lattice of membrane-limited tubules, the surfaces of which are smooth or particle-free. In this respect, the endoplasmic reticulum here resembles that encountered in cells which produce lipid-rich secretions. The myeloid bodies comprise paired membranes arranged in stacks shaped like biconvex lenses. At their margins the membranes are continuous with elements of the ER and in consequence of this the myeloid body is referred to as a differentiation of the reticulum. The paired membranes resemble in their thickness and spacings those which make up the outer segments; they are therefore regarded as intracellular photoreceptors of possible significance in the activation of pigment migration and other physiologic functions of these cells. The fuscin granules are enclosed in membranes which are also continuous with those of the ER. The granules seem to move independently of the prolongations in which they are contained. The report also describes the fine structure of the terminal bar apparatus, the fibrous layer intervening between the epithelium and the choroid blood vessels, and comments on the functions of the organelles depicted.     

Biogenesis of myeloid bodies in regenerating newt (Notophthalmus viridescens) retinal pigment epithelium

Summary Myeloid bodies are believed to be differentiated areas of smooth endoplasmic reticulum membranes, and they are found within the retinal pigment epithelium in a number of lower vertebrates. Previous studies demonstrated a correlation between phagocytosis of outer segment disc membranes and myeloid body numbers in the retinal pigment epithelium of the newt. To test the hypothesis that myeloid bodies are directly involved in outer segment lipid metabolism and to further characterize the origin and functional significance of these organelles, we examined the effects on myeloid bodies of eliminating the source of outer segment membrane lipids (neural retina removal) and of the subsequent return of outer segments (retinal regeneration) in the newt Notophthalmus viridescens. Light- and electron-microscopic analysis demonstrated that myeloid bodies disappeared from the pigment epithelium within six days of neural retina removal. By week 6 of regeneration, rudimentary photoreceptor outer segments were present but myeloid bodies were still absent. However, at this time, the smooth endoplasmic reticulum in some areas of the retinal pigment epithelial cells had become flattened, giving rise to small (0.5 m long), two-to-four layer-thick lamellar units, which are myeloid body precursors. Small myeloid bodies were first observed one week later at week 7 of retinal regeneration. This study revealed that newt myeloid bodies are specialized areas of smooth endoplasmic reticulum. It also showed that a contact between functional photoreceptors and the retinal pigment epithelium is essential to the presence of myeloid bodies in the epithelial cells.     

Ultrastructural study of the retinal pigment epithelium during metamorphosis in the sea lamprey (Petromyzon marinus L.)

Summary The sequence of morphological changes in the retinal pigment epithelium during the metamorphic period of the sea lamprey Petromyzon marinus L. has been investigated using electron microscopy. At early metamorphic stages (stages I and II), photoreceptors are present in a small zone of the retina. During these stages, the lateral surface of the epithelial cells shows zonulae occludentes and adhaerentes. The degree of cell differentiation varies throughout the retinal pigment epithelium. Cells covering the differentiated photoreceptors in the central retina have phagosomes, whereas pigment granules appear only in the retinal pigment epithelium dorsal to the optic nerve head. Most epithelial cells have myeloid bodies; their morphology is more complex around the optic nerve head. At stage III, when photoreceptors develop over the whole retina, the distribution of cytoplasmic organelles is almost homogeneous in the retinal pigment epithelium. Subsequently, the basal plasma membrane of the epithelial cells becomes progressively folded and their apical processes enlarged. In addition, extensive gap junctions develop between retinal pigment cells. In late metamorphic stages, noticeable growth of myeloid bodies occurs and consequently the retinal pigment epithelium resembles that of the adult. This study also describes, for the first time, the presence of wandering phagocytes in the retinal pigment epithelium of lampreys; their role in melanosome degradation is discussed.     

Coloring cells of the cornea of trout]

Cells from the eye cornea of Hexagrammos octagrammus which are responsible for changes of the cornea colour from bright orange to colourless, depending on the light conditions, are described. It was shown that the change in cornea colour was due to a shift of red pigment from the cell body into its processes (in the light) and in the opposite direction at the dark adaptation of animals. The ultrastructural constitution of these cells has a number of characteristics. The whole cell cytoplasm is filled up with fine lipid droplets wherein carotenoid pigments giving red colour to these cells are presumably dissolved; the cytoplasmic membrane forms numerous deep and branched folds into the cell and has a lot of pinocytose visicles; the cell body and especially the process display many microtubes arranged regularly. The described cells differ greatly in their form, size and ultrastructural constitution from the known types of pigment cells (melanophores, xanthophores and erythrophores). This makes it possible to consider them as chromatophores of an independent type.     

Microscopic anatomy of the eye of the deep-diving Antarctic Weddell seal (Leptonychotes weddellii)

The microscopic anatomy of the eye of the Weddell seal was studied with various light and electron microscopic methods with a view to correlating morphological findings with the biology of this seal which is adapted to the extremes of the Antarctic environment and to extreme diving excursions into the lightless depths of the sea. In the retina an area centralis was found but no fovea centralis. The densely packed photoreceptors consist exclusively of highly differentiated rods, which in primates detect light at low intensity but have rather poor image discrimination. The ganglion cells are relatively scarce, suggesting a high degree of convergence of the light-sensitive cells on the ganglion cells. The pigment epithelium is almost devoid of pigment granules. The extensive tapetum lucidum is about 400-500 microm thick and is composed of about 30 layers of specialized cells. The cornea is 650 (center) to 800-900 (periphery) microm thick. Its structure and glycosaminoglycan histochemistry correspond to that of other mammals. The iridocorneal angle is unusually deep and pervaded by an elaborate trabecular meshwork, which together with a complex canal of Schlemm can be correlated with the ability to absorb large amounts of fluid. The ciliary muscle and its antagonist, the membrane of Bruch, are poorly developed, suggesting relatively poor abilities of accommodation. The combination of a well-developed tapetum lucidum, an unpigmented pigment epithelium, well-developed rods, and a high number of rods converging on only few ganglion cells is obviously an adaptation to an extreme light sensitivity, enabling the animals to make use of the little light available in the deep sea.     

Body colors and algal distribution in the acoel flatworm Convolutriloba longifissura: histology and ultrastructure

Convolutriloba longifissura is a red flatworm with white dots that harbors unicellular green algae within its body. The red pigment of the flatworm that is present in round cells is soluble in ethanol or acetone, whereas the white pigment contained in the crystalline (retractile) platelets of amoeboid-shaped cells is soluble in 1% NH4OH. These two types of pigment cells form the body coloration and are probably involved in light protection of the algal symbionts, as many algal cells are distributed beneath the body wall and some are in the highly vacuolated parenchyma. The ultrastructural features of these cells suggest a close relationship with Tetraselmis spp. The morphology of sagittocysts within the mantle is also described by means of scanning and transmission electron microscopy.     

Über die Ultrastruktur des Komplexauges von Daphnia pulex

Zusammenfassung Die Komplexaugen von erwachsenen Daphnien (Daphnia pulex) und deren Verankerung wurden elektronenmikroskopisch untersucht. Das Auge enthält 4 Bau-Elemente: 1. Deckzellen, 2. Interzellularsubstanz, 3. Linsenzellen und 4. pigmenthaltige Receptorzellen. Ihre Ultrastruktur wird beschrieben.Da die lichtbrechende Substanz als kompakter Körper innerhalb der Linsenzellen liegt, gehört der Linsenkörper zum euconen Typ. Er wird von aufgereihten Mitochondrion umgeben, ist nicht durch eine Membran vom Cytoplasma getrennt und zeigt eine nach innen zunehmende Elektronendichte.Die zentral gelegene, sehr dichte Zone grenzt basal an das Rhabdomer, nur durch die Zellmembran von ihm getrennt. Das Rhabdomer besteht aus Mikrovilli, die durch close contacts miteinander verbunden sind. Desmosomen-ähnliche Haftstrukturen zwischen Linsen- und Receptorzellen kommen in der Nähe der Rhabdomere vor. Aus den jeweils 8 Receptorzellen eines Ommatidiums entspringen basal 8 Neuriten, die zunächst durch eine Basalmembran, im Bereich des Hilus außerdem durch einen Gliafortsatz aus dem Ganglion opticum gebündelt werden. Das ganze Auge ist an der Körperoberfläche (Cuticularzellen und Cuticula) durch einen Halteapparat aufgehängt, der aus spezialisierten Zellen und Basalmembran-artigem Material besteht.

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On the ultrastructure of the compound eye of daphnia pulex

Summary Compound eyes of adult Daphnia pulex and their connection with the integument have been examined electron microscopically. The eye contains four elements: 1. covering cells, 2. intercellular substance, 3. lense cells and 4. receptor cells containing pigment. Their ultrastructure is described. Since the light refracting medium forms a compact lense body within the cytoplasm of lense cells it belongs to the eucone type. Though being surrounded by threaded mitochondria the lense body is not separated from the cytoplasm by a membrane. Electron density increases towards its center. At its basal part the central zone being covered only by a plasma membrane borders on the rhabdomer, the microvilli of which show close contacts between each other. There are desmosome-like contacts between lense and receptor cells at the periphery of the rhabdomer. 8 receptor cells each giving off 1 neurite build up the ommatidium. The 8 neurites of one ommatidium are bundled by a basement membrane and a glial process originating from the optic ganglion. The eye in toto is tied to the integument (cuticular cells, cuticula) by a complicated supporting apparatus, which consists of specialized cells and basement membrane-like material.

     

The photoreactivity of ocular lipofuscin.

Lipofuscin or "age pigment" is a lipid-protein complex which accumulates in a variety of postmitotic, metabolically active cells throughout the body. These complexes, which are thought to result from the incomplete degradation of oxidised substrate, have the potential for photoreactivity. This is particularly so in the retina in which the lipofuscin not only contains retinoid metabolites but is also exposed to high oxygen and fluxes of visible light all of which provide an ideal environment for the generation of reactive oxygen species (ROS). Lipofuscin is a potent photoinducible generator of ROS with the potential to damage proteins, lipids and DNA. Retinal cell dysfunction may be strongly associated with photoreactivity of lipofuscin and may contribute to age-related disease and vision loss.     

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.     

Photoreconvertible fluorophore systems in rhabdomeres,Semper cells and corneal lenses in the compound eye of the blowfly

1. The primary aim of the experiments described in this article was to localize the origin of the complex fluorescence in the compound eye of flies. The eye tissue was dissected and the fluorescence from cells and cell organelles was recorded by microspectrofluorometry. Using this technique, fluorophore systems were detected in the rhabdomeres, Semper cells and corneal lenses. The fluorophore systems are photoreconvertible by UV and blue light. 2. The fluorophore systems in the rhabdomeres and Semper cells are similar. The intensity of the fluorescence from the microvilli is enhanced up to 29 X by adaptation to UV light. The enhancement is inversely related to the rhodopsin content in the microvilli, indicating that the chromophoric group of the fluorophore is not a vitamin A derivative. 3. The enhancement of the fluorescence by UV light strongly depends on pH, suggesting that the photoreconvertible fluorophore systems in the microvilli and Semper cells are photosensitive redox pigments. These redox systems are probably located in the membranes of the microvilli in the photoreceptors, and in the endoplasmic reticulum of the Semper cells, or they are coupled to filaments in the cytoskeleton of both cell types. 4. Preliminary reaction schemes for the photoreactions based on the recorded excitation and emission spectra and photokinetics were developed. A primary pigment in the microvillous structure, AR, or in organelles in the Semper cells, AS, is converted by UV light into an excited state AR* or AS*, which either relaxes to the primary pigment by photon emission, or converts into an intermediate X, which by proton uptake changes into stable products, BR or BS. Blue illumination converts BR and BS into the excited states BR* and BS*, which either relax by photon emission to BR or BS, or convert into an intermediate Y, which after deprotonation reconverts into the primary pigment AR or AS. 5. Estimation of the molecular density showed that the concentration of the fluorophore in the microvilli presumably is almost equal to maximal rhodopsin concentration. The high density suggests that the fluorophores have a specific function in transduction or adaptation of the visual process.     

Elimination of egg pigment from developing ocular tissues in the frog Rana pipiens

The method by which egg pigment is eliminated from the developing retina, corneal epithelium and lens in Rana pipiens was studied with light and electron microscopy. The retina expells egg pigment into the space between the retina and pigment epithelium. This pigment is then engulfed by the pigment epithelial cells. The corneal epithelium eliminates egg pigment directly to the outside via the free surface of the epithelial cells. Egg pigment accumulates in a few cells in the lens. These cells probably degenerate and are extruded. These ectodermal derivatives in the eye are free of egg pigment long before ectodermal derivatives in other parts of the embryo lose their pigment. The early elimination of egg pigment from ocular tissues may related to the fact that these tissues must be transparent in order that light may pass freely to the photoreceptors.     

Action Spectra for Step-Up Photophobic Response in Blepharisma

ABSTRACT The cells of Blepharisma which possess red pigment (blepharismin) show step-up photophobic response (temporal ciliary reversal induced by a sudden increase in light intensity). Bleaching of the cells by cold shock raised a threshold light intensity for the response, Oxidation of red pigment that produced blue pigment did not raise the threshold for the response. The action spectrum for the step-up photophobic response of the cells which possess normal red pigment had peaks at about 580, 540 and 490 nm, a value which coincided with peaks of an absorption spectrum of the red pigment. The absorption spectrum of oxidized pigment (blue pigment) shifted 20 nm toward infrared light. The action spectrum for the response of the cells which possess blue pigment also shifted 20 nm toward infrared light. Results suggest that red pigment might be involved in the step-up photophobic response. Key words. Blepharismin, ciliary reversal, photoreceptors, photoresponse.     

Structure and functional aspects of the scotopic compound eye of the sugarcane borer moth

Morphology and functional aspects of the scotopic compound eye of the moth Diatraea saccharalis, studied using light and electron microscopy, is presented. An ommatidium is composed of a laminate corneal lens, four Semper cells, a refractive cone, two primary pigment cells, six screening pigment cells, a crystalline tract that functions as an optical waveguide, and six to eight sensory retinular cells. Accessory light regulators consist of screening pigment cells that, in the dark-adapted position, increase receptor sensitivity by permitting light rays to cross over to adjacent ommatidia and specialized tracheal regions that enhance sensitivity by reflecting light back toward sensory receptors.     

Relations between pigmentation of the cornea and the number of epidermal melanophores in Rana temporaria L. larvae

The dynamics of the external cornea pigmentation in Rana temporaria L. larvae at the 22d developmental stage have been studied under conditions favourable for various course of certain morphological reactions in the pigment system. The cornea together with the surrounding skin is transferred on the dorsal surface of the larva body, and the piece of the dorsal surface skin is put instead of the cornea removed. When using the reciprocal transplantation method and preserving the organism's integrity (without disturbing melanocyte-stimulating source--namely, the hypophysis, and melatonine sources--namely, the pineal gland and the lateral eyes) the corneal pigmentation is observed on the background of perfect morphological reactions in the pigment system, while the larvae are maintained on the dark and light substrates, that is at various density of the pigment cells (120 larvae have been used). The pigmentation dynamics have been studied from the 6th up to the 20th day in total preparations. The epidermal melanophores density is estimated in 4 areas of each preparation. The melanin amount is estimated by means of the electron paramagnetic resonance-spectrometry according to the contents of free radicals expressed in relative units. A direct proportional dependence between the significantly higher melanin contents (1.5-fold) and a significantly quicker (1.5-fold) process of the corneal pigmentation is revealed, that agrees with an increasing number of the pigment cells per one unit of the body surface in the larvae maintained on the dark substrate. In the larvae maintained on the light substrate, the dependence is of a reverse character. It is probable that the factors forcing the pigmented cells, at cultivation the neural crest cells in vitro to reject from each other, affect the pigmentation of the larval cornea in vivo. If it is the case, the processes specific for the embryonal period, transgress during the cornea pigmentation at the larval stages of development.     

The fine structure of the lateral ocellus of the dobsonfly larva

The lateral ocelli of the dobsonfly (Protohermes grandis, Neuroptera) larva have been examined with light and electron microscopy. The larva has six ocelli on both sides of the head, each containing a single corneal lens. A conical crystalline body, of some 10–20 cells is situated immediately posterior to the lens. From 100 to 300 elongated retinular cells are arranged perpendicular to the crystalline body except at the innermost surface of the lens, where they are absent. The distal process of each retinular cell is enclosed by a tube-like rhabdom formed by the close association of microvilli from the same and adjacent distal processes. The distal process contains many mitochondria, multivesicular bodies, microtubles and pigment granules. In the dark-adapted ocellus the pigment granules are concentrated near the nucleus which lies under the rhabdomic layer. The granules diffuse toward the rhabdomic microvilli during light adaptation. Each retinular cell has a single axon, which extends from the ocellus as an ocellar nerve fiber into the optic lobe, where it frequently synapses upon second order neurons. In addition to these afferent synapses, there are two other synaptic combinations: (1) a feedback synapse from a second order neuron to a retinular axon, and (2) a synapse between second order neurons. These results suggest that photic signals reach the more proximal part of the brain via second order neurons after some degree of integration in the optic lobe.     

Possible Involvement of Cone Opsins in Distinct Photoresponses of Intrinsically Photosensitive Dermal Chromatophores in Tilapia Oreochromis niloticus

Dermal specialized pigment cells (chromatophores) are thought to be one type of extraretinal photoreceptors responsible for a wide variety of sensory tasks, including adjusting body coloration. Unlike the well-studied image-forming function in retinal photoreceptors, direct evidence characterizing the mechanism of chromatophore photoresponses is less understood, particularly at the molecular and cellular levels. In the present study, cone opsin expression was detected in tilapia caudal fin where photosensitive chromatophores exist. Single-cell RT-PCR revealed co-existence of different cone opsins within melanophores and erythrophores. By stimulating cells with six wavelengths ranging from 380 to 580 nm, we found melanophores and erythrophores showed distinct photoresponses. After exposed to light, regardless of wavelength presentation, melanophores dispersed and maintained cell shape in an expansion stage by shuttling pigment granules. Conversely, erythrophores aggregated or dispersed pigment granules when exposed to short- or middle/long-wavelength light, respectively. These results suggest that diverse molecular mechanisms and light-detecting strategies may be employed by different types of tilapia chromatophores, which are instrumental in pigment pattern formation.     

Retinal pigment epithelial fine structure in the bobtail goanna (Tiliqua rugosa)

The retinal pigment epithelium (RPE), the choriocapillaris and Bruch's membrane (complexus basalis) have been studied by light and electron microscopy in the bobtail goanna (Tiliqua rugosa) an Australian diurnal lizard. The RPE consists of a single layer of cuboidal cells which display very deep and tortuous basal (choroidal) infoldings as well as numerous apical (vitreal) processes which interdigitate with the photoreceptor cells. The lateral cell borders are relatively smooth and joined by basally located tight junctions. Internally smooth endoplasmic reticulum is abundant while rough endoplasmic reticulum is not. The RPE cell nucleus is large and vesicular and basally located in the light-adapted state. Polysomes, mitochondria and myeloid bodies are present and widely distributed. Melanosomes are plentiful in the apical region of the epithelial cells in light-adaptation. Bruch's membrane is pentalaminate with the basal lamina of the choriocapillaris being exceptionally thick. The choriocapillaris is a single layer of large-caliber capillaries with thin but only moderately fenestrated endothelium. Numerous dense granules are always present within these endothelial cells.     

Fine structure of the segmental ocelli of Armandia brevis (Polychaeta: Opheliidae)

Summary Armandia brevis responds negatively to light during the benthic phase and positively to light during the epitokous phase of its life history. In addition to the prostomial photoreceptors this slender translucent marine worm possesses eleven pairs of ocelli arranged serially from the 7th to the 17th segments.Each ocellus is located at the inner edge of the epidermis slightly in front of the parapodium and contains a single photoreceptor cell which gives off approximately 15 sensory processes. These processes are composed of a central core of neurofibrils surrounded by a mitochondrial layer and a compact array of microvilli. The sensory processes project into and nearly fill the ocellar cavity which is lined by squamous glial cells.The pigment cup enclosing the photoreceptor is composed of about 30 cuboidal cells packed with brown granules. The pigment cells form a mesothelium, being in direct continuity with the coelom. The cup is separated from the glial cells by a basal lamina which separates the epidermal tissues from the mesodermal derivatives of the body wall. Slender muscle fibers traverse the coelom and pass between the cells of the pigment cup.The prostomial photoreceptors were re-examined and found in this material to be composed of microvilli rather than of folds containing labyrinthine tubular infoldings of the cell surface as previously reported.The author thanks Dr. Richard M. Eakin for support and criticism. This investigation was financed by a postdoctoral fellowship, number 1-F2-GM-20, and grant number GM 10292 from the National Institute of General Medical Science.     

Ultrastructural characteristics of the simple eyes of the louse Pediculus humanus corporis

The simple eye of the human louse consists of two apparatuses: dioptric and light sensitive. The dioptric apparatus contains only a biconvex lens, which represents local thickening of the cuticle. The eye lacks the crystal cone (Semper cells) and special pigment cells. The light sensitive part of the eye contains about 130 photoreceptor cells. Each photoreceptor has rhabdomere which consists of numerous microvilli. The pigment granules are located only in the photoreceptor cells.