Retinula Cells Research Articles

ULTRASTRUCTURAL CHANGES OF RHABDOMS OF THE EYES OF OCYPODE SPECIES IN RELATION TO DIFFERENT REGIMES OF LIGHT AND DARK ADAPTATION

Abstract Ultrastructural changes of the compound eyes were compared in Ocypode species living under different daylight and annual rhythms near the equator (O. ryderi, O. ceratophthalma), in the northern (O. cursor) and in the southern hemisphere (O. quadrata). Independent of geographical latitude, the eyes of these Ocypode species undergo daily changes in morphology. Throughout the day the rhabdomeric diameter is small. A massive increase of rhabdom diameter occurs at dusk; a breakdown phase starts during the second half of the night. Before dawn the rhabdom size is near to that during daytime and seems to be prepared for the day-state. Besides the rhabdom, the size of the palisade-ER and the position of pigment granules and mitochondria within the retinula cells change. In all investigated Ocypode species the rhabdom turnover is influenced by different light and dark regimes. During extended darkness, rhabdom diameters resemble those of day-adapted animals. Extended exposures to light cause no or little ...

Daily variations in crustacean hyperglycaemic hormone and serotonin immunoreactivity during the development of crayfish.

The present study investigated changes in crustacean hyperglycaemic hormone (CHH) and serotonin (5-hydroxytryptamine, 5-HT) immunoreactivity in the retina and the X-organ/sinus gland complex (XO-SG) of the crayfish Procambarus clarkii at two developmental stages, post-embryonic stage two (PO2) and the juvenile stage, at three different times of day, under a photoperiod cycle of 12 h:12 h L:D, using qualitative and quantitative immunohistochemical methods. In the retina, CHH immunoreactivity is located in the tapetal cells, while 5-HT immunoreactivity is found in the retinular cells. In the XO-SG, CHH-immunoreactivity is localized to the CHH-producing cell perikarya and in their axons and endings in the sinus gland, while 5-HT immunoreactivity is restricted to axon endings branching into the perikarya of the CHH-producing cells. A stereological analysis demonstrates that the PO2 and juvenile stages show significant differences in the amount of the immunoreactive CHH and 5-HT material at the three selected time points, indicating daily and related changes in the levels of CHH and 5-HT in the XO-SG and the retina. Our findings therefore support the idea that daily rhythms in the secretory activity of the XO-SG complex affect the circadian sensitivity of the eye. Furthermore, the differences found between the PO2 and juvenile stages suggest that both CHH and 5-HT are key factors in the development of the circadian rhythm of retinal sensitivity.

Diurnal changes in retinula cell sensitivities and receptive fields (two-dimensional angular sensitivity functions) in the apposition eyes of Ligia exotica (Crustacea, Isopoda).

The structural organization of the retinula cells in the eye of Ligia exotica changes diurnally. At night, the microvilli elongate, losing the regular and parallel alignment characteristic of the day condition. Crystalline cones and distal rhabdom tips are not pushed into each other during the day, but at night the rhabdoms protrude into the crystalline cones by up to 5 microm. Screening pigment granules in the retinula cells disperse during the night, but migrate radially towards the vicinity of the rhabdom during the day. No such displacements of the pigment granules of either distal or proximal screening pigment cells were observed. The sensitivity of the eye, monitored by electroretinogram (ERG) recordings, changes diurnally: values at midnight are, on average, 10 times those occurring during the day. However, intracellular recordings from single retinula cells (50 during the day and 50 at night) indicate that the difference between night and day sensitivities is only 2.5-fold. Two-dimensional angular sensitivity curves, indicative of a single unit's spatial sensitivity, had considerably less regular outlines at night than during the day. If based on the 50 % sensitivity level, day and night eyes possessed receptive fields of almost identical width (approximately 2 degrees), but if sensitivities below the 50 % limit were included, then receptive fields at night were significantly more extensive. We suggest that the morphological adaptations and diurnal changes in chromophore content seen in the apposition eye of L. exotica allow this animal to improve its photon capture at night while preserving at least some of the spatial resolving power characteristic of the light-adapted state. This would explain why this animal is capable of performing complex escape behaviours in the presence of predators both in bright and in very dim light.

Wavelength-dependent polarization orientation in Daphnia.

The ability to detect and use the polarization of light for orientation is widespread among invertebrates. Among terrestrial insects, the retinula cells that are responsible for polarization detection contain a single visual pigment, either ultraviolet or short (blue) wavelength sensitive. With the exception of a few aquatic insects, the visual pigments underlying polarization sensitivity in aquatic invertebrates have yet to be determined. Here we report that polarotaxis in Daphnia pulex, a freshwater crustacean, is wavelength dependent and most likely mediated by two visual pigments with absorbance maxima in the middle (green) and long wavelength (red) parts of the spectrum. This contrasts with the response of a closely related species, D. magna, in which polarotaxis is wavelength independent and based on a single middle wavelength visual pigment. The visual systems in Daphnia are the first among crustaceans shown to utilize a middle wavelength pigment for polarization detection and, in the case of D. pulex, the first shown to use more than one visual pigment for such a purpose.

The optomotor response and spatial resolution of the visual system in male Xenos vesparum (Strepsiptera).

The Strepsiptera are an enigmatic group of parasitic insects whose phylogenetic relationships are hotly debated. Male Strepsiptera have very unusual compound eyes, in which each of a small number of ommatidia possesses a retina of at least 60 retinula cells. We analysed the optomotor response of Xenos vesparum males to determine whether spatial resolution in these eyes is limited by the interommatidial angle or by the higher resolution potentially provided by the extended array of retinula cells within each ommatidium. We find that the optomotor response in Strepsiptera has a typical bandpass characteristic in the temporal domain, with a temporal frequency optimum at 1-3 Hz. As a function of spatial wavelength, the optomotor response is zero at grating periods below 12 degrees and reaches its maximum strength at grating periods between 60 degrees and 70 degrees. To identify the combination of interommatidial angles and angular sensitivity functions that would generate such a spatial characteristic, we used motion detection theory to model the spatial tuning function of the strepsipteran optomotor response. We found the best correspondence between the measured response profile and theoretical prediction for an irregular array of sampling distances spaced around 9 degrees (half the estimated interommatidial angle) and an angular sensitivity function of approximately 50 degrees, which corresponds to the angular extent of the retina we estimated at the centre of curvature of the lens. Our behavioural data strongly suggest that, at least for the optomotor response, the resolution of the strepsipteran compound eye is limited by the ommatidial sampling array and not by the array of retinula cells within each ommatidium. We discuss the significance of these results in relation to the functional organisation of strepsipteran compound eyes, their evolution and the role of vision in these insects.

Phylogeny of the Myriapoda – Crustacea – Insecta: a new attempt using photoreceptor structure*

According to molecular sequence data Crustacea and not Myriapoda seem to be the sister-group to Insecta. This makes it necessary to reconsider how the morphology of their eyes fit with these new cladograms. Homology of facetted eye structures in Insecta (Hexapoda in the sense of Ento- and Ectognatha) and Crustacea is clearly supported by identical numbers of cells in an ommatidium (two corneageneous or primary pigment cells, four Semper cells which build the crystalline cone and primarily eight retinula cells). These cell numbers are retained even when great functional modification occurs, especially in the region of the dioptric apparatus. There are two different possibilities to explain differences in eye structure in Myriapoda depending on their phylogenetic position in the cladogram of Mandibulata. In the traditional Tracheata cladogram, eyes of Myriapoda must be secondarily modified. This modification can be explained using the different evolutionary pathways of insect facetted eyes to insect larval eyes (stemmata) as an analogous model system. Comparative morphology of larval insect eyes from all holometabolan orders shows that there are several evolutionary pathways which have led to different types of stemmata and that the process always involved the breaking up the compound eye into individual larval ommatidia. Further evolution led on many occasions to so-called fusion-stemmata that occur convergently in each holometabolic order and reveals, in part, great structural similarities to the lateral ocelli of myriapods. As myriapodan eyes cannot be regarded as typical mandibulate ommatidia, their structure can be explained as a modified complex eye evolved in a comparable way to the development to the fusion-stemmata of insect larvae. The facetted eyes of Scutigera (Myriapoda, Chilopoda) must be considered as secondarily reorganized lateral myriapodan stemmata, the so-called ‘pseudo-compound eyes’. New is a crystalline cone-like vitreous body within the dioptric apparatus. In the new cladogram with Crustacea and Insecta as sister-groups however, the facetted eyes of Scutigera can be interpreted as an old precursor of the Crustacea – Insecta facetted eye with modified ommatidia having a four-part crystalline cone, etc. as a synapomorphy. Lateral ocelli of all the other Myriapoda are then modified like insect stemmata. The precursor is then the Scutigera-Ommatidium. In addition further interpretations of evolutionary pathways of myriapodan morphological characters are discussed.

Effects of photic and thermal stress on distal and proximal rhabdomeres in the crayfish eye: why are the visual membranes of the 8th retinula cell more resilient than the others?

Visual membranes of the crayfish eye either belong to the small, distally placed rhabdomere of retinula cell R8 or are part of the much more voluminous proximal rhabdom, made up of rhabdomeres belonging to cells R1–R7. Under various conditions of environmental stress (e.g., prolonged darkness, elevated temperature, bright light with and without a concomitant rise in temperature, flickering lights) the visual membranes of R8 prove far more resistant to structural damage than those of R1–R7. Membrane damage is known to occur when dormant lipoxygenases become activated, for example through heat. Since R8 is the only type of visual cell in the crayfish retina that does not contain grains of screening pigment, the view that screening-pigment granules could “aggravate” or even “trigger” membrane damage in times of stress is strengthened. Functionally, R8's strong resistance to physical damage when exposed to flickering lights points to a role of the distal rhabdom in the movement detection system of the crayfish eye.

The rhabdom structure in the ommatidia of the Heteroptera (Insecta), and its phylogenetic significance

In its plesiomorphic state the insect ommatidium consists of eight retinula cells forming a fused rhabdom. It has long been observed that, in contrast to this pattern, Heteroptera have open rhabdoms. However, there has so far been no comprehensive and comparative study of heteropteran ommatidia. For this reason, we investigated the rhabdom structure in 36 species from all higher groups of Heteroptera, as well as from Coleorrhyncha and Auchenorrhyncha as outgroup representatives. In addition we surveyed the data of earlier authors, which brings the number of examined species to a total of more than 70. All examined Heteroptera do have open rhabdoms, with a system of six peripheral and two central rhabdomeres. Outgroup comparison shows that the open rhabdom is an autapomorphy of the Heteroptera. As for the rhabdom structure within the Heteroptera, we found further autapomorphic patterns in Corixidae (Nepomorpha), Gerromorpha, and Leptopodomorpha. Finally, the Cimicomorpha and Pentatomomorpha share a special pattern of the two central rhabdomeres, which we call V-pattern. This is a new synapomorphy of these two taxa.

Action spectra of the female's response in the firefly Photinus pyralis (Coleoptera: Lampyridae): evidence for an achromatic detection of the bioluminescent optical signal

Behavioural action spectra of the threshold of the Photinus pyralis female response to light stimuli simulating the bioluminescent optical signal of the conspecific male firefly were determined in the laboratory. The action spectra ( Fig. 1) were narrow and peaked in the yellow region of the spectrum. The females responded only to stimuli of wavelengths longer than 480 nm and not to stimuli in the blue (420–460 nm) part of the spectrum. The shape of the function corresponds with (a) the electroretinographic spectral sensitivity function in the long wavelength (520–660 nm) region of the spectrum, (b) the action spectrum of the female response ( Fig. 1), (c) the species yellow bioluminescence emission spectrum and (d) the action spectrum of the intracellular response from single retinular cells ( Fig. 2) of the compound eyes in the firefly. Such a correspondence suggests that the narrow yellow receptors of the female mediate the detection and processing of the optical signal of the conspecific male. Since the bioluminescent optical signal is processed exclusively by a single receptor class, signal detection is achromatic.

Visual adaptation modulates a potassium conductance in retinular cells of the crayfish.

Crayfish photoreceptors exhibit a voltage-dependent potassium conductance, G(K), that is generally similar to the delayed rectifier channel described in neurons and other arthropod retinular cells. G(K) activation (i.e. the apparent threshold, Vth) occurs near the resting potential and G(K) is substantially reduced by 25 mM extracellular tetraethylammonium (TEA) and by intracellular Cs+ injections. Light exposure, sufficient to reduce visual sensitivity 100-fold, increases Vth (shifts it in the depolarizing direction) by about 20 mV. The light-dependent change in Vth does not depend upon the corresponding increase (depolarization) of the steady-state membrane potential nor does it depend upon inward calcium currents. Vth is slightly influenced by fluctuations in Ko associated with the light-elicited currents. During light exposure Ko (measured with K+-sensitive electrodes) increases by 2.1 mM (equivalent to an 8 mV increase in E(K)). This increase in E(K) makes only a modest contribution to the light-dependent change in Vth as determined by perfusion with high potassium salines. Intracellular calcium injections increase Vth by 10 to 20 mV and reduce visual sensitivity by 5- to 10-fold. The results imply that during exposure to high levels of illumination, K+ currents at the steady-state membrane potential are diminished by a calcium-dependent change in G(K) gating and, to a smaller degree, by a reduced K+ concentration gradient. It is notable that Ca2+ appears to inhibit both G(K) and the light-elicited conductance from both inside and outside the plasma membrane. As a consequence of the light-dependent change in Vth, G(K) makes only modest contributions to the changes in sensitivity and speed normally associated with light adaptation. These functions are regulated by the transduction pathway and are revealed at the resting potential in the time course and magnitude of the light-elicited currents.

Two opsin genes from the vetch aphid, Megoura viciae

The cDNAs of two opsins (Megopsin1 and Megopsin2) from the vetch aphid, Megoura viciae, have been sequenced and encoded for gene products with 378 and 371 amino acid residues, respectively. Phylogenetic analysis reveals that Megopsin1 falls into the insect long-wavelength opsin group and Megopsin2 is a member of the insect UV-wavelength opsins. Both opsins share the key features of G-protein-coupled receptors and the specific motifs of photopigments. In situ hybridization demonstrated that the transcripts of Megopsin1 and Megopsin2 were expressed in the retinula cells of the compound eyes.

Changes in the structure and pigmentation of the eyes of honeybee (Apis mellifera L.) queens with the "limão" mutation

This study describes the ultrastructural differences between the compound eyes of ch li/ch li and Ch/ch li honeybee queens. Heterozygous "limão" bees had an almost normal ultrastructural organization of the ommatidia, but there were some alterations, including small vacuoles in the crystalline cones and a loss of pigment by primary pigmentary cells. In homozygous bees many ommatidia had very deformed crystalline cones and there were some bipartite rhabdoma. There was a reduction in the amount of pigment in the primary and secondary pigmentary cells and receptor cells (retinulae) of mutant eyes. However, the eyes of both heterozygous and homozygous queens had the same type of pigment granules. Certain membrane-limited structures containing pigment granules and electron-dense material appeared to be of lysosomal nature. Since these structures occurred in the retinular cells of mutant eyes, they were considered to be multivesicular bodies responsible for the reduction in rhabdom volume in the presence of light, as a type of adaptation to brightness. The reduction of pigment in the pigmentary and retinular cells and the morphological changes seen in the rhabdom of the ommatidia may originate visual deficiencies, which could explain the behavioral modifications reported for Apis mellifera queens with mutant eye color.

Photoreceptor visual fields, ommatidial array, and receptor axon projections in the polarisation-sensitive dorsal rim area of the cricket compound eye.

We made intracellular recordings from the photoreceptors of the polarisation-sensitive dorsal rim area of the cricket compound eye combined with dye marking. By measuring visual field sizes and optical axes in different parts of the dorsal rim area, we assessed the optical properties of the ommatidia. Due to the large angular sensitivities (median about 20 degrees) and the high sampling frequency (about 1 per degree), the visual fields overlap extensively, such that a given portion of the sky is viewed simultaneously by a large number of ommatidia. By comparing the dye markings in the retina and in the optic lobe, the axon projections of the retinula cells were examined. Receptors R1, R2, R5 and R6 project to the lamina, whereas R7 projects to the medulla. The microvilli orientation of the two projection types differ by 90 degrees indicating the two analyser channels that give antagonistic input to polarisation-sensitive interneurons. Using the retinal marking pattern as an indicator for the quality of the intracellular recordings, the polarisation sensitivity of the photoreceptors was re-examined. The polarisation sensitivity of recordings from dye-coupled cells was much lower (median: 4.5) than that of recordings in which only one cell was marked (median: 9.8), indicating that artefactual electrical coupling between photoreceptors can significantly deteriorate polarisation sensitivity. The physiological value of polarisation sensitivity in the cricket dorsal rim area is thus typically about 10.

Sub-cellular localisation of the white/scarlet ABC transporter to pigment granule membranes within the compound eye of Drosophila melanogaster.

The white, scarlet, and brown genes of Drosophila melanogaster encode ABC transporters involved with the uptake and storage of metabolic precursors to the red and brown eye colour pigments. It has generally been assumed that these proteins are localised in the plasma membrane and transport precursor molecules from the heamolymph into the eye pigment cells. However, the immuno-electron microscopy experiments in this study reveal that the White and Scarlet proteins are located in the membranes of pigment granules within pigment cells and retinula cells of the compound eye. No evidence of their presence in the plasma membrane was observed. This result suggests that, rather than tranporting tryptophan into the cell across the plasma membrane, the White/Scarlet complex transports a metabolic intermediate (such as 3-hydroxy kynurenine) from the cytoplasm into the pigment granules. Other functional implications of this new finding are discussed.

Synaptic organization in the lamina of the superposition eye of a skipper butterfly, Parnara guttata.

The first optic neuropil of the compound eye, the lamina, of the skipper butterfly Parnara guttata, was examined by light microscopy after Golgi-impregnation and by electron microscopy (EM) to clarify the cellular and synaptic organization. In the lamina, five different types of lamina neurons (L neurons) were characterized by using Golgi-impregnation. By EM, each cartridge was found to contain all nine receptor axons from an ommatidium, five L neurons, and a few putative centrifugal elements. Axons from photoreceptors (retinula cells) R2, R3, R4, R6, R7, and R8 terminate as short visual fibers (svfs) in the lamina cartridge. Those from R1, R5, and R9 penetrate the lamina and terminate in the medulla as long visual fibers (lvfs). In the cartridges, the synaptic contacts were formed from svfs onto L neurons, from the lvfs of R1 and/or R5 to the lvf of R9 and L neurons, and from the lvf of R9 to L neurons. The putative centrifugal fibers also make synapses to svfs and L neurons. At the most distal level of the cartridge, one of the centrifugal fibers containing dense-core vesicles makes presynaptic contacts to the putative long collaterals of the L neuron. A novel characteristic feature of this lamina is that svfs of R3 and R7 and the lvfs of R1 or R5 have long collaterals extending into neighboring cartridges. Presynaptic contacts were confirmed in such long collaterals from the svf. These results imply that receptor axons provide direct intercartridge connections as well as providing indirect connections to neighboring cartridges by way of their input upon L neurons.

Photoreceptor ultrastructure in the Antarctic mussel shrimp Acetabulastoma (Crustacea; Ostracoda), a parasite of Glyptonotus antarcticus (Crustacea; Isopoda)

The ultrastructure of the nauplius eye of the tiny Antarctic ostracode Acetabulastoma sp. is described and conclusions about its possible function are drawn. Each of the three eye-cups measures approximately 20 μm in diameter and is optically isolated from its neighbour by screening pigments, which are contained in pigment cells behind a tapetum of concentrically arranged, ca. 1-μm-long and 0.1-μm-thick, crystals. Three and sometimes four separate rhabdoms with microvilli measuring 50–60 nm in diameter project from the concave side of the tapetum up to 5 μm deep into the eye-cup interior, which is filled by the retinula cell bodies with their spherical nuclei and various organelles. Desmosomes and microtubules are seen and light-induced cell or membrane damage was minimal. The observations suggest that the Acetabulastoma eye has photoreceptors that can tolerate an exposure to bright light and it may be used to inform its owner of the approach of danger, the depth of water, and/or the season.

Microvillar orientation in the photoreceptors of the ant Cataglyphis bicolor.

Polarization sensitivity in arthropod photoreceptors is crucially dependent on the arrangement of the microvilli within the rhabdom. Here, we present an electron-microscopical study in which the degree of microvillar alignment and changes in the cross-sectional areas of the rhabdoms along their length were studied in the compound eye of the desert ant, Cataglyphis bicolor. Serial cross-sections through the retina were taken and the orientation of the microvilli was determined in the photoreceptors of individually identified ommatidia. The reconstructions of microvillar alignment were made in the three anatomically and functionally distinct regions of the Cataglyphis compound eye: the dorsal rim area (DRA), the dorsal area (DA), and the ventral area (VA). The following morphological findings are consistent with polarization sensitivities measured previously by intracellular recordings. (1) The microvilli of the DRA photoreceptors are aligned in parallel along the entire length of the cell from the distal tip of the rhabdom down to its proximal end, near the basement membrane. The microvilli of the retinular cells R1 and R5 are always parallel to each other and perfectly perpendicular, with only minor deviation, to the microvillar orientation of the remaining receptor cells. (2) In the DA and VA regions of the eye, the microvillar tufts of the small receptors R1, R3, R5, R7, and R9 change their direction repetitively every 1-4 micrometer for up to 90 degrees. In contrast, the large receptor cells R2, R4, R6, and R8 maintain their microvillar orientation rigidly. (3) In the DRA ommatidia, the cross-sectional areas of the rhabdomeres do not change along the length of the rhabdom, but substantial changes occur in the DA and VA ommatidia.

MORPHOLOGY OF THE COMPOUND EYES OF TWO ANCESTRAL PHYLLOPODS, TRIOPS CANCRIFORMIS AND LEPIDURUS APUS (NOTOSTRACA: TRIOPSIDAE)

The kidney-shaped compound eyes of adult tadpole shrimps of the genera Triops and Lepidurus are composed of about 310 and 170 ommatidia, respectively. Each ommatidium has a flattened cuticle on top, and is made of 4 cone cells that form a crystalline cone of the eucone type, 2 corneagenous cells and 8 retinula cells which form a fused rhabdom. Among the retinula cells, 6 are arranged in 3 mirror symmetrical pairs (referred to as R1/R6, R2/R5, R3/R4) on either side of the axis of the ommatidial cross section, while 2 unpaired cells (referred to as R7 and R8) are positioned one behind the other along the axis. The rhabdomere of R7 sits on top of the rhabdom, and is replaced by that of R8 more proximally. In its distal region, the rhabdom is formed like a π in Triops and is of a triangular shape in Lepidurus. The structure of the dioptric apparatus, the retinula and conecell patterns, the orientation of the rhabdomeric microvilli, the array of the ommatidia, and the arrangement of the ommatidia main axes are discussed with regard to the compound eyes of other arthropods.

Glial Cells of the Central Nervous System in the Crab Ucides cordatus

Glial cells and their processes were characterized in the fasciculated zone and in the protocerebral tract of the crab Ucides cordatus by light and electron microscopy. Thiery and PAS procedures indicate the presence of carbohydrates, particularly glycogen in cells. Immunohistochemistry was used to observe tubulin distribution in the glial cells. Our results demonstrate at least two types of glial cells in the fasciculated zone and in the protocerebral tract, separable by their location and electron density. Judging by their position, electron-lucent cells may correspond to periaxonal cells and electron-dense ones may correspond to perineurial cells. The electron-dense processes have previously been interpreted as extracellular matrix, but since they feature an enveloping membrane and contain glycogen and mitochondria (intact and with varying degrees of disruption) we consider them to be part of one type of glial cells. Additional key words: Crustacea, Malacostraca, Brachyura, glia Invertebrate glial cells have traditionally been classified according to certain relatively general morphological or functional criteria and also by their anatomical position (Haimori & Horridge 1966; Radojcic & Pentreath 1979). Because invertebrates do not have true compact myelin, no glial class equivalent to vertebrate oligodendrocytes can be identified. In addition, immunocytochemical markers for vertebrate astrocytes in most cases fail to label invertebrate glia. (One of the exceptions is glutamine synthetase, a specific marker for astrocytes in vertebrates, which has also successfully labeled non-neuronal cells in lobster olfactory regions: Linser et al. 1997.) The glial cell types of arthropods share many morphological characteristics. First, in many species, the chromatin of glial cell nuclei is clumped in the periphery. This feature is rarely seen in neuronal nuclei. Second, mitochondria, endoplasmic reticulum, and Golgi structures are generally common. Third, between neighboring glial cells and also between glial cells and neurons, diverse membrane specializations are found (see Pentreath 1987). In the nervous system of some species of higher crustaceans, i.e., Decapoda, there is good anatomical and physiological evidence for an insulating sheath with nodes and rapid saltatory conduction (Heuser & Doggenweiler 1966). Where present, the sheath coma Author for correspondence. E-mail: sallodi@chagas.biof.ufrj.br prises numerous laminae, each containing cytoplasm and forming a seam. The nodes are loosely wrapped by a characteristic type of glial cell called the nodal cell. The axons can be wrapped either simply by a single glial process or in groups, by complex, tightly packed multiple layers, resembling vertebrate myelin (Bullock et al. 1977). Important differences from vertebrate myelin are: (a) the crustacean laminae do not connect to form a spiral; (b) the nuclei of the sheath cells lie on the inside of the sheath; (c) desmosomes join adjacent laminae; (d) sometimes an adjacent extracellular matrix may occur between neighboring glial cell processes (Heuser & Doggenweiler 1966). The participation of neuroglia in the blood-brain barrier is an important function of glial cells in both vertebrates and invertebrates (Abbott et al. 1986; Abbott 1995). Unlike the situation in higher vertebrates, in which the blood-brain barrier is formed by tight junctions of endothelial cells lining capillaries of the central nervous system (see Peters et al. 1991), the barrier in crustaceans is formed by glial perineurium surrounding the ganglia. The sealing function of the barrier is thought to be due to the junctions between the perineurial cells and the extracellular matrix, which may be expanded and contain collagen-like fibrils (Abbott 1972; see Pentreath 1987). The most widely accepted function of glial cells is that they interact metabolically with neurons, providing nutrients, exchanging metabolites, and removing catabolites. Such a role is supported by glial stores of glycogen This content downloaded from 157.55.39.231 on Wed, 05 Oct 2016 04:16:00 UTC All use subject to http://about.jstor.org/terms Allodi, Silva, & Taffarel and large numbers of vesicular and granular inclusions in the cytoplasm (Pentreath 1987; Tsacopoulos & Magistretti 1996). Our goal was to characterize the organization of glial cells and their processes by light microscopy (LM) and transmission electron microscopy (TEM) and to correlate those findings with the distribution of extracellular matrix in the fasciculated zone (FZ) and in the protocerebral tract (PT) of the crab Ucides cordatus. The FZ comprises retinular cell axons that run from the retina to the lamina ganglionaris (Bell & Lightner 1988) and the PT is the tract that links the terminal medulla and the hemiellipsoid body to the anterior medial protocerebral neuropil and other areas of the brain (Sandeman et al. 1992). These neuropils were chosen because they are primarily composed of axons and glial cells.

COMPARATIVE STUDIES ON THE COMPOUND EYES OF LARVAE AND ADULTS OF AN AESCHNID DRAGONFLY,ANAX NIGROFASCIATUS NIGROFASCIATUS.(I) DORSAL PART

Abstract In the dragonfly Anax nigrofasciatus nigrofasciatus (Aeshnidae, Anisoptera), the X-tissue is the tissue which occupies the posterior corner of the larval eye and which forms the large facet region in the dorsal part of the adult compound eye after emergence. In the early period of the last larval instar, an ommatidial retinula of the X-tissue shows the same arrangement of retinular cells and crystalline cone processes as that observed in the dorsal part of the adult compound eye, although it has no rhabdom. The rhabdom starts to be formed in the middle stage of the last larval instar and the electrical responses by light can be recorded from that stage, but the chromophore retinal of visual pigment cannot be detected from the larval X-tissues. The rhabdom observed remains rudimentary till emergence. During the period of emergence, the corneal facets in the X-tissue expand to nearly twice their original size as a result of rapid enlargement of the ommatidia. On the day of emergence, the threshold ...