The morphology of the Limulus visual system

An ocellus of the horseshoe crab, Limulus polyphemus, has been serially sectioned for light and electron microscopy, its sensory cells have been indexed, and the interconnections of a third of these traced. The ocellus contains 155 retinula cells and 26 arhabdomeric cells, which are secondary sensory neurons. Of these, 55 retinula cells constitute 7 quasi-ommatidial assemblages, each innervated by at least one and a total of 9 arhabdomeric cells. When known electrotonic coupling patterns are compared with gap-junctional connections, retinula cells sensitive to visible or ultraviolet light can be tentatively identified. Retinula cell axons contribute collaterals to a synaptic plexus, in which the arhabdomeric cells apparently do not participate.     

Some peculiar synaptic complexes in the first visual ganglion of the fly,Musca domestica

Summary In the lamina ganglionaris, the first optic ganglion of the fly, the inventory of cell types as well as the patterns of their connections are well known from light microscopic investigations. Even the synaptic contacts are known with relative completeness. However, the structural details visible on electron micrographs are very difficult to interpret in functional terms. This paper concentrates on two aspects: 1) the synaptic complex between a retinula cell axon and four postsynaptic elements, arranged in a constant elongated array (it is suggested that all synapses in which the retinula cell is presynaptic are of this kind), and 2) the gnarl complex in which a presynaptic specialization in one neuron is separated from another neuron by a complicated glial invagination. The participation of glia at postsynaptic sites seems to be quite common in this ganglion. Occasionally it seems that a glia cell is the only postsynaptic partner facing a presynaptic specialization within a neuron.     

Identificaton of UV,green and red receptors,and their projection to lamina in the cabbage butterfly,Pieris rapae

Summary The photoreceptors in the compound eye of a cabbage butterfly, Pieris rapae, were examined by conventional and intracellular-labeling electron microscopy by the use of the cobalt(III)-lysine complex as an ionized marker. Five types of spectral sensitivity were recorded intracellularly in electrophysiological experiments. They peaked at about 340, 380, 480, 560 and 620 nm, respectively. One of the distal retinula cells (R2) was a UV receptor, whereas the R4 distal retinula cell was a green receptor. The basal retinula cell, R9, was found to be a red receptor; it was localized near the basement membrane, having a bilobed cell body with an individual nucleus in each lobe. A small number of rhabdomere microvilli were present in a narrow cytoplasmic bridge connecting the two lobes. The axons of six retinula cells (R3–R8) in each ommatidium terminated at the cartridge in the lamina (short visual fiber), whereas those of the other three retinula cells, R1, R2 and R9, extended to the medulla (long visual fiber). The information from the UV and red receptors is therefore probably delivered directly to the medulla neurons, independent of that from the other spectral receptor types.     

The retina of the phalangid,Opilio ravennae,with particular reference to arhabdomeric cells

Summary The retina of the phalangid, Opilio ravennae, consists of retinula cells with distal rhabdomeres, arhabdomeric cells, and sheath cells. The receptive segment of retinula cells shows a clear separation into a Proximal rhabdom, organized into distinct rhabdom units formed by three or four retinula cells, and a Distal rhabdom, consisting of an uniterrupted layer of contiguous rhabdomeres. One of the cells comprising a retinula unit, the so-called distal retinula cell (DRC), has two or three branches that pass laterally alongside the rhabdom, thereby separating the two or three principal retinula cells of a unit. The two morphologically distinct layers of the receptive segment differ with respect to the cellular origin of rhabdomeral microvilli: DRC-branches contribute very few microvilli to the proximal rhabdom and develop extremely large rhabdomeres in the distal rhabdom only, causing the rhabdom units to fuse. Principal retinula cells, on the other hand, comprise the majority of microvilli of the proximal rhabdom, but their rhabdomeres diminish in the distal rhabdom. It is argued that proximal and distal rhabdoms serve different functions in relation to the intensity of incident light.In animals fixed 4 h after sunset, pigment granules retreat from the distal two thirds of the receptive segment. A comparison of retinae of day- and night-adapted animals shows that there is a slight (approximately 15%) increase in the cross-sectional area of rhabdomeral microvilli in dark-adapted animals, which in volume corresponds to the loss of pigment granules from the receptive segment. The length of the receptive segment as well as the pattern and shape of rhabdom units, however, remain unchanged.Each retinula unit is associated with one arhabdomeric cell. Their cell bodies are located close to those of retinula cells, but are much smaller and do not contain pigment granules. The most remarkable feature is a long, slender distal dendrite that extends up to the base of the fused rhabdom where it increases in diameter and develops a number of lateral processes interdigitating with microvilli of the rhabdom. The most distal dendrite portion extends through the center of the fused rhabdom and has again a smooth outline. All dendrites end in the distal third of the proximal rhabdom and are never present in the layer of the contiguous distal rhabdom. Arhabdomeric cells are of essentially the same morphology in day- and night-adapted animals. They are interpreted as photoinsensitive secondary neurons involved in visual information-processing that channel current collected from retinula cells of the proximal rhabdom along the optic nerve. A comparison is made with morphological equivalents of these cells in other chelicerate species.     

The microanatomy of the compound eye of Munida irrasa (Decapoda: Galatheidae)

The compound eye of Munida irrasa differs in several respects from the typical decapod eye. The proximal pigment is found only in retinula cells. The eccentric cell is extremely large and expanded to fill the interstices of the crystalline tract area; thus, a typical "clear-zone" is absent. Six retinula cells course distally to screen two sides of the crystalline cone. There are approximately 12,500 ommatidia in each compound eye. There are several similarities to the typical decapod eye. Each ommatidium is composed of a typical cornea, corneagenous cells, crystalline cone cells, crystalline cone, crystalline cone tract and eight retinula cells. Distal pigment cells are present and surround the crystalline cone. The distal processes of the retinula cells also contain pigment. The retinula cell processes penetrate the basement membrane as fascicles composed of processes from adjacent retinulae.     

The eyes of mesopelagic crustaceans

Summary In Streetsia challengeri left and right eyes have fused and become a single cylindrical photoreceptor, which occupies the basal half of a forward directed head projection. This unusual compound eye consists of approximately 2500 ommatidia, which are arranged in such a way that the animal has almost circumferential vision, but cannot look ahead or behind. It is thought that the eye operates on light-guide principles, and that the crystalline cones are the major dioptric component. Ommatidia in anterior-posterior rows show a greater overlap of visual fields than dorso-ventrally arranged ommatidia. Cone layer and retinula are separated by a 4 m thick screen-membrane, which contains tiny pigment granules of 0.15 m diameter. Cells of unknown function and origin, containing unusual multitubular organelles, are regularly found near the proximal ends of the crystalline cone threads. The twisted rhabdoms measure 18–20 m in diameter, and consist of microvilli 0.05 m in width, which belong to five retinula cells and which show no trace of disintegration. The position of interommatidial screening pigment, the density of retinula cell vesicles and inclusions, and the narrowness of the perirhabdomal space all suggest that the eyes have been light-adapted at the time of fixation for electron microscopy. The retinula cell nuclei lie on the proximal side of the heavily pigmented basement membrane. A tapetum or basal retinula cells are not developed. It is concluded that the eye optimally combines acuity with sensitivity, and that for distance estimation parallax may be important.Address until January 25th 1978: Scott Base, Ross Dependency, Antarctica (C/-Chief Post Office, Christchurch, New Zealand)     

棉铃虫蛾复眼的微细结构及其区域性差异

用电子显微镜观察棉铃虫蛾复眼的微细结构及其区域性差异。此复眼具有小网膜细胞柱的透明带。每个小眼包括一个外凸内平的角膜,一个晶锥,四个形成晶锥、晶束的晶锥细胞和两个围绕着晶锥的主虹膜细胞,六至八个小网膜细胞和一个基细胞。晶锥末端有一短小固定的晶束。小网膜细胞柱远侧中央有似微绒毛结构的视杆束。每个小眼被六个附色素细胞围绕。 微细结构的区域性差异:1.背方小眼视杆中段横切面近似矩形,主要由六个微绒毛平行排列的三角形视小杯组成,整个视杆包含两个互相垂直的微绒毛轴;腹方、前方、后方和侧方区域的小眼视杆中段横切面为风扇形,“V”字形视小杆内微绒毛排列不平行;2.前方区域小眼视杆中段的横切面要比后方大;3.前方、腹方区域内,有的相邻小眼的小网膜细胞柱互相连结,背方、后方区域未观察到这一现象。     

The superposition eye of Cloeon dipterum: The organization of the lamina ganglionaris

Summary The lamina ganglionaris of the superposition eye of Cloeon dipterum is composed of separate optic cartridges arranged in a hexagonal pattern. Each optic cartridge consists of one central, radially branched monopolar cell (Li) surrounded by a crown of seven retinula cell terminals and two more unilaterally branched monopolar cells (La1/La2) situated close together outside the cartridge. Projections to neighbouring cartridges have not been observed.In most cases, synaptic contacts could be seen between a presynaptic retinula cell and more than two other postsynaptic profiles, which belong to monopolar cells or sometimes to glial cells.Seven retinula cell fibers of one ommatidium pass in a bundle through the basement membrane, run into their respective cartridges without changing orientation and terminate at approximately equal levels in the lamina. Long visual fibers with endings in the medulla are not visible in the superposition eye lamina, but are present in the lateral apposition eye. The relationship between the behaviour of the animal, optic mechanisms of the superposition eye and the structure of the lamina is discussed.     

Morphogenesis in the embryo ofDrosophila melanogaster — Germ band extension

Summary Changes at the ultrastructural level during germ band extension in the embryo ofDrosophila melanogaster are described. Cytoplasmic connections between cells and the yolk sac are present during initial cellular movements. At this time, a continuous system of microfilaments is present adjacent to the membranes in the connections and at the periphery of the yolk sac. As germ band extension progresses, this system becomes discontinuous, and microfilaments are apparent only in the immediate vicinity of the connections. Cytoplasmic connections are disassembled at approximately the midpoint of extension; at the same time, extensive membrane associations develop between germ band cells and between these cells and adjacent yolk sac membranes. Positioning and orientation of cytoplasmic connections suggest that the yolk sac, via these connections, is actively involved in the cellular movements of early germ band extension.This paper is dedicated with respect and affection to Donald F. Poulson     

The structure and formation of host-parasite pit connections between the red algal alloparasiteHarveyella mirabilis and its red algal hostOdonthalia floccosa

Summary Harveyella mirabilis is a colourless red algal alloparasite which grows on and within its photosynthetic hostOdonthalia floccosa. Cells ofHarveyella establish secondary pit connections (PCs) with other parasite cells and with cells of the host. Small, uninucleate conjunctor cells are produced by parasite cells and remain connected to them by PCs. Conjunctor cells may fuse with either an adjacent host or parasite cell, with the parasite-conjunctor cell PC becoming either a host-parasite or parasite-parasite secondary PC. Occasionally the conjunctor cell does not fuse with an adjacent cell (either host or parasite) and degenerates. The secondary pit plug which forms between a parasite cell and its conjunctor cell always develops with two structurally distinct surfaces characteristic of a host-parasite pit plug. Only if the conjunctor cell fuses with another parasite cell will the structure of the pit plug be altered to that of a parasite-parasite pit plug. Fungal hyphae also invade the region of infection, andHarveyella cells respond by producing nonfunctional conjunctor cells that grow towards adjacent hyphae. Evidence suggests that secondary PCs may be induced to form mechanically, by the physical presence of another cell, rather than in direct response to a message received from an adjacent cell. The mechanism of secondary PC formation described here is similar to that reported for the closely related alloparasiteHolmsella and may be common to a number of red algal parasitic associations. Helen Margaret Quirk, B. Sc. (Hons), M. Sc. (1953–1982), student, research assistant and friend, died after a long illness on October 24, 1982.     

The organization of the lamina ganglionaris of the hemipteran insects,Notonecta glauca,Corixa punctata and Gerris lacustris

Summary Neuronal elements, i.e. first and second order neurons, of the first optic ganglion of three waterbugs, N. glauca, C. punctata and G. lacustris, are analyzed on the basis of light and electron microscopy.Eight retinula cell axons, leaving each ommatidium, disperse to different cartridges as they enter the laminar outer plexiform layer. Such a pattern of divergence is one of the conditions for neuronal superposition; it is observed for all three species of waterbugs. The manner in which the receptors of a single bundle of ommatidia split of within the lamina, whereby information from receptors up to three or five horizontal rows away can converge upon the same cartridge, differs among the species. Six of the eight axons of retinula cells R1-6, the short visual fibers end at different levels within the bilayered lamina, whereas the central pair of retinula cells R7/8, the long visual fibers, run directly through the lamina to a corresponding unit of the medulla. Four types of monopolar cells L1–L4 are classified; their branching patterns seem to be correlated to the splitting and termination of retinula cell axons. The topographical relationship and synaptic organization between retinula cell terminals and monopolar cells in the two laminar layers are identified by examination of serial ultrathin sections of single Golgi-stained neurons.An attempt is made to correlate some anatomical findings, especially the neuronal superposition, to results from physiological investigations on the hemipteran retina.     

Neurosecretory fibres in the median eyes of the scorpion,Androctonus australis L.

Summary The retina of the median eyes of the North African scorpion, Androctonus australis L., is supplied with numerous neurosecretory nerve fibres which establish synaptoid contacts on the retinula cells. The number of fibres or profiles of varicosities of fibre terminals associated with a retinular unit (five retinula cells with a fused rhabdom) varies between 10 and 20. Electron-opaque vesicles with a diameter of 80–100 nm are abundant within the axonal profiles. The synaptoid junctions are characterized by postsynaptic electron-dense material on the inner leaflet of the retinula cell membrane and, frequently, presynaptic submembranous dense material. Because of these ultrastructural features, the junctions observed here resemble typical interneuronal synaptic contacts. Hence this kind of neurosecretory junction appears to be unique among arthropods.It is suggested that the neurosecretory fibres within the retina represent the efferent pathways for the control of the circadian pigment movements within the retinula cells.Supported by the Deutsche Forschungsgemeinschaft (F1 77/7)     

Fine structural organization of the lateral ocelli in two species of Scolopendra (Chilopoda: Pleurostigmophora): an evolutionary evaluation

The lateral ocelli of Scolopendra cingulata and Scolopendra oraniensis were examined by electron microscopy. A pigmented ocellar field with four eyes arranged in a rhomboid configuration is present frontolaterally on both sides of the head. Each lateral ocellus is cup-shaped and consists of a deeply set biconvex corneal lens, which is formed by 230–2,240 cornea-secreting epithelial cells. A crystalline cone is not developed. Two kinds of photoreceptive cells are present in the retinula. 561–1,026 cylindrical retinula cells with circumapically developed microvilli form a large distal rhabdom. Arranged in 13–18 horizontal rings, the distal retinula cells display a multilayered appearance. Each cell layer forms an axial ring of maximally 75 rhabdomeres. In addition, 71–127 club-shaped proximal retinula cells make up uni- or bidirectional rhabdomeres, whose microvilli interdigitate. 150–250 sheath cells are located at the periphery of the eye. Radial sheath cell processes encompass the soma of all retinula cells. Outside the eye cup there are several thin layers of external pigment cells, which not only ensheath the ocelli but also underlie the entire ocellar field, causing its darkly pigmented. The cornea-secreting epithelial cells, sheath cells and external pigment cells form a part of the basal matrix extending around the entire eye cup. Scolopendromorph lateral ocelli differ remarkably with respect to the eyes of other chilopods. The dual type retinula in scolopendromorph eyes supports the hypothesis of its homology with scutigeromorph ommatidia. Other features (e.g. cup-shaped profile of the eye, horizontally multilayered distal retinula cells, interdigitating proximal rhabdomeres, lack of a crystalline cone, presence of external pigment and sheath cells enveloping the entire retinula) do not have any equivalents in scutigeromorph ommatidia and would, therefore, not directly support homology. In fact, most of them (except the external pigment cells) might be interpreted as autapomorphies defining the Pleurostigmophora. Certain structures (e.g. sheath cells, interdigitating proximal rhabdomeres, discontinuous layer of cornea-secreting epithelial cells) are similar to those found in some lithobiid ocelli (e.g. Lithobius). The external pigment cells in Scolopendra species, however, must presently be regarded as an autapomorphy of the Scolopendromorpha.     

The fine structure of secondary pit connection formation between the red algal alloparasiteHolmsella australis and its red algal hostGracilaria furcellata

Summary Pit connections (PCs) develop between the parasitic red algaHolmsella and its hostGracilaria. Only parasite cells initiate the formation of host-parasite pit connections. The parasite produces a small connecting cell (termed the conjunctor cell) which moves through the cell wall to fuse with either an adjacent host or parasite cell. The parasite secondary PC, which forms between the conjunctor cell and the parasite cell, is structurally different from a parasite primary PC, and has the distinct structure of a host-parasite PC. Only if the conjunctor cell fuses with another parasite cell will the former parasite-conjunctor cell PC be altered to a typical parasite-parasite PC. If the conjunctor cell fuses with an adjacent host cell the PC continues to develop as host-parasite. Occasionally a conjunctor cell fails to fuse with an adjacent cell (whether host or parasite), and the conjunctor cell and PC eventually breakdown in the cell wall. The parasite overcomes several barriers in order to infect the host, including the formation of host-parasite PCs which appear to be a necessary component of the parasiticHolmsella-Gracilaria association.     

Spike Potentials Recorded from the Insect Photoreceptor

Slow and spike potentials were recorded from single cells in the receptor layer of the compound eye of the drone of the honeybee. From electron microscopic observation of the drone ommatidium, it was concluded that the response had been recorded from the retinula cell. The following hypothesis is suggested for the initiation of spike potentials in the drone compound eye: Photic stimulation results in a decrease in the resistance of all or part of the retinula cell membrane, giving rise to the retinal action potential. The retinal action potential causes outflow of the current through the proximal process of the cell. This depolarizing current initiates spike potentials in the proximal process or axon of the retinula cell which are recorded across the soma membrane of the retinula cell.     

The fine structure of the sense organs of the cephalopod mollusc Nautilus

Summary The structure of the rhinophore, digital tentacles, post-ocular tentacles and the eye of Nautilus macromphalus are described. The rhinophore is composed of mucous cells, ciliated cells, and flask-shaped ciliated cells. The latter are probably olfactory receptors. The digital tentacles are composed of mucous cells and pigmented cells. Motor-end-plates found in the muscle layer below the epithelium of the digital tentacles are similar to those described in other cephalopods. The post-ocular tentacle contains receptor cells that bear macrocilia. These may be mechanoreceptors. The retina is composed of retinula cells and supporting cells. A complex rhabdom is formed at the distal ends of the retinula cells. The supporting cells send processes up between these rhabdoms. Both types of cells contain pigment granules but the retinula cell has a complex membranous structure in its perikaryon. No synapses were found at the bases of the retinula cells. At the side of the retina are mucous cells that are presumed to produce the jelly-like substance that fills the inside of the eye in life. The likely function of the eye is discussed and it is suggested that it is capable of simple discriminations. It is suggested that the sense organs are probably comparatively unchanged from those of fossil nautiloids. Acknowledgements. This paper is dedicated to the late Dr. Yves Merlet who collected the nautiluses used in this study.We would like to thank Prof. J. Z. Young for all his support and encouragement. The Royal Society, The Percy Sladen Memorial Fund, and University College, London, provided the financial support that enabled one of us (V.C.B.) to collect nautiluses. The Science Research Council, U.K., provided the electron microscope used in the major part of the study and a grant to one of us (V.C.B.). We would also like to thank Prof. J. B. Gilpin-Brown who provided Fig. 1, Dr. R. Catala, for aquarium facilities, Mr. M. P. Legand and the Institut Français d'Oceanie, Noumea, New Caledonia, for laboratory facilities, Dr. J.-M. Bassot and Dr. Anna Bidder for advice on catching and preserving nautiluses, Mrs. Judy Parkes and Mr. M. Barker for photographic assistance, and Miss J. Date for secretarial assistance.     

Membrane turnover in crab photoreceptors studied by high-resolution scanning electron microscopy and by a new technique of thick-section transmission electron microscopy

Summary Crab photoreceptors were examined after treatment by the osmium-DMSO-osmium method for high-resolution scanning electron microscopy. This technique of specimen preparation was also adapted for transmission electron microscopy, enabling sections up to 1 urn thick to be viewed in a conventional microscope at 75 kV. With appropriate pretreatment, some cytoskeletal elements can be visualised by both techniques. The methods were then used to investigate some of the daily changes known to occur in photoreceptor cell structure. Striking differences were found in the structure of Golgi bodies present in retinula cells during the synthesis and breakdown phases of the daily cycle of photoreceptor membrane turnover. Cyclic changes were also noticed in the mitochondria of retinula cells, and additional evidence was found for a previously proposed model of rhabdomeral microvillus formation.     

The ultrastructural organization of the visual system of the wax moth,Galleria mellonella: The retina

Summary The ultrastructural organization of ommatidial components of the retina of the moth, Galleria mellonella are described from electron microscopic observations. Each ommatidium is composed of 12 common retinula cells and one basal eccentric cell. The retinula cells are connected together by a desmosomal strip along their length. The rhabdom occupies the basal thirty percent of the ommatidium and can be divided into nine segments of parallel microvilli. Several cells may contribute to an individual rhabdomere. The rhabdomeres are arranged in a cross with single cell rhabdomeres lying between the arms of the cross. Thin sections of ommatidium absorb polarized light differentially. The total amount of plane polarized light absorbed varies with angle of rotation for an entire ommatidium but there are also differences between the amount of absorption of adjacent rhabdomeric segments. Galleria appears to be the only lepidopteran in which the possibility of the polarized light reception has been reported.     

Light-initiated responses of retinula and eccentric cells in the Limulus lateral eye

The relationship between retinula and eccentric cells in the lateral eye of Limulus polyphemus was studied using a double electrode technique which permitted simultaneous recording of light-initiated responses in two sense cells and the labeling of the cells for subsequent histological examination and identification. The following results were obtained: (a) light-initiated slow responses with and without superimposed spike potentials were recorded from retinula cells and from eccentric cells (only one eccentric cell yielded responses without superimposed spike potentials); (b) spike potentials recorded in different cells within the same ommatidium were always synchronous; (c) a complete absence of spike potentials was observed in two experiments in which no eccentric cells could be found in the ommatidia containing the labeled retinula cells; (d) the greatest differences in the characteristics of responses recorded simultaneously occurred in those recorded from retinula-eccentric combinations. The results indicate that there is only one source of spike potential activity within an ommatidium (presumably the eccentric cell) and that the light-initiated response of retinula cells may be independent of the eccentric cell response. The suggestion is advanced that the response of the retinula cell may "trigger" the eccentric cell response.     

Expression of acetylcholinesterase during visual system development in Drosophila

As in other insects acetylcholine (ACh) and acetylcholinesterase (AChE) function in synaptic transmission in the central nervous system of Drosophila. Studies on flies mutant for AChE indicate that in addition to its synaptic function of inactivating acetylcholine, this neural enzyme is required for normal development of the nervous system (J.C. Hall, S.N. Alahiotis, D.A. Strumpf, and K. White, 1980, Genetics 96, 939-965; R.J. Greenspan, J.A. Finn, and J.C. Hall, 1980, J. Comp. Neurol. 189, 741-774). In order to understand what role AChE may play in neural development, it is necessary to know, in detail, where and when the enzyme appears. The use of monoclonal antibodies to localize AChE in the developing visual system of wild type Drosophila has yielded the novel observation that AChE appears in photoreceptor (retinula) cells 4-6 hr after they differentiate and 3 to 4 days before they are functional. Three days later the staining in the cell body of these cells is reduced. Because retinula cells have no functional connections at the time when AChE is first detected, AChE can not be performing its standard synaptic function. Subsequent to the reduction of AChE in the retinula cells, midway through the pupal stage, the enzyme accumulates rapidly in the neuropils of the optic lobes of the brain. Thus, there is a biphasic accumulation of AChE in the developing visual system with the enzyme initially being expressed in the retinula cells and accumulating later in the optic lobes.