The organization of perpendicular fibre pathways in the insect optic lobe.

High resolution serial photomicrography has been used to plot the axonal projection patterns between retina, lamina and medulla in the optic lobes of various insects with differing ommatidial receptor arrangements. Observations are reported on the cabbage white and skipper butterflies, the bee, locust, fly, backswimmer and waterbug. The patterns of these fibre pathways have previously eluded non-rigorous analyses primarily because of their physical dimensions but are revealed in this study to have striking precision and uniformity between species when examined at the level of individually identifiable cells. Axon bundles of the tracts between retina and lamina or lamina and medulla project between a single ommatidium and its corresponding lamina cartridge or between corresponding lamina and medulla cartridges. Lateral interweaving of axons between adjacent bundles is absent. The bundles preserve the retinotopic order within their total array, so transferring the pattern of retinulae directly upon the lamina and thence after horizontal inversion in the chiasma upon the medulla. Within the lamina neuropile on the other hand the trajectories of the individual terminals from a bundle have patterns which are species-specific, sometimes involving lateral divergences. In species with open-rhabdomere ommatidia the terminals distribute to a group of lamina cartidges with a pattern which resembles the receptor pattern in the overlying ommatidium. In species with fused-rhabdome ommatidia the terminals of a single retinula behave less interestingly and all enter the same cartridge, within which, again, each occupies a position related to its cell body position within the retinula. Long visual fibres in both eye types penetrate the lamina and terminate in the particular medulla cartridge that connects with the lamina cartridge underlying their ommatidium. The perpendicular fibre pathways therefore project the visual field exactly upon the medulla in all species while the lack of interweaving between adjacent fibre bundles precludes their involvement in lateral interactions between pathways with differing visual axes. Uniformity of these projection patterns between cell layers and species differences in retinular terminal locations in the lamina can be correlated with different modes of axon growth between and within neuropile layers during optic lobe neurogenesis. Further discussion surrounds the question of which particular receptors give rise to which type of axon, for which no clear generalization has yet emerged.     

The first optic ganglion of the bee

Summary The arrangement of first and second order neurons in an optic cartridge and the topographical relationships of the second order neurons within a cartridge and to groups of surrounding cartridges have been analyzed in the visual system of the bee, Apis mellifera, from light and electron microscope studies on Golgi preparations. At the level of the monopolar cell body layer, the nine retinula cell fibres of each ommatidium, the six short visual fibres arranged in a circle surrounding the three long visual fibres, become cartridges as a consequence of the appearance of the second order neurons (L-fibres) which join the R-fibre bundles. Two of the four different L-fibre types, L-1 and L-2, remain together in the centre of the cartridge throughout the lamina. The axons of the L-3 and L-4 fibres, however, have their position integrated into the circle formed by the endings of the short visual fibres. On the basis of further examination of light and especially electron microscopical Golgi material, the different L-fibres can be classified into four types which appear in each cartridge. The clear stratification in the first synaptic region (A, B and C) seems to be the best criterion for a morphological classification since such a classification necessarily also includes a functional basis. According to a naming system based on the position of the lateral processes, L-fibres with side branches in strata A, B and C are called L-1 fibres. Fibres with lateral processes in strata A and B are L-2 fibres; monopolar cell fibres with branches only in the second stratum B are L-fibres of type 3; and all monopolar cells with branches only in stratum C are called L-4 fibres. In addition to the branching pattern covering only the parent cartridge, two of the four fibre types (L-2 and L-4) have long collaterals reaching neighbouring cartridges: L-2 in stratum A and L-4 in stratum C. These collaterals presumably form a substrate for lateral interactions.     

The first optic ganglion of the bee

Summary The synaptic relationships between and within receptor-cell axons (RCAs), first-order interneurones (L-fibres) and accessory fibres (acc) in the first optic ganglion (the lamina) of the worker bee were studied in serial sections with Golgi-EM and routine transmission electron microscopy. The ommatidium contains nine retinular (photoreceptor) cells all of which project as RCAs to a single optical cartridge in the lamina. Six of the RCAs end as short visual fibres (svf) in the lamina, while the remaining three, the so-called long visual fibres (lvf), pass the lamina and end in the second optic ganglion, the medulla. In addition to the RCAs and an unknown number of accessory fibres, the cartridge also contains four L-fibres (L 1–4). The spatial arrangement of the RCAs and L-fibres within a cartridge is constant throughout the depth of the lamina. Serial sections reveal a great number of chemical synapses interconnecting RCAs, L-and acc fibres. Double T-shaped presynaptic dense projections are surrounded and in close association with either spherical or flattened synaptic vesicles. The finding of gap junctions between and within identified RCAs and L-fibres suggest that these axons may be electronically coupled. A model for information processing in the lamina of the bee is suggested from observations of synaptic connectivity between and within fibres of one cartridge.     

Anatomical identification of spectral receptor types in the retina and lamina of the Australian orchard butterfly,Papilio aegeus aegeus D.

Summary The nine receptor cells examined in each ommatidium of the butterfly Papilio aegeus aegeus can be named according to their positional orientation across the fused rhabdom. Six of them end as short visual fibres (svf) in the second stratum of the lamina, whereas the remaining three retinula cells (lvf) pass together with the lamina fibres (L-fibres) the first optic ganglion and the outer chiasma to end in the three most distal layers of the second optic ganglion, the medulla. The organization of the retinula-cell axons within the pseudocartridge and the cartridge remains almost uniform throughout the first optic ganglion. Five L-fibres, which have their origin in the fenestrated layer (FL), join each laminar cartridge before entering the neuropil of the first optic region. Four of these L-fibres (L-1, L-2, L-3 and L-4) could be definitely located and characterized using Golgi-stained light- and electron-microscopic techniques. Whereas L-1 and L-3 show a lateral branching pattern reaching only fibres of the same cartridge, L-2 and L-4 have long collaterals interconnecting several neighbouring cartridges in a characteristic pattern. Serial sections of silver-impregnated retinula-cell axons as well as L-fibres were investigated for their synaptic connectivity patterns between and within these fibres. These cellular interactions and possible information processing are discussed.     

The first optic ganglion of the bee

Each visual unit (ommatidium) of the compound eye of the honey bee contains nine retinula cells, six of which end as axons in the first synaptic ganglion, the lamina, and three in the second optic ganglion, the medulla. A technique allowing light- and electron microscopy to be performed on the same silver-impregnated sections has made it possible to follow all types of retinula axons of one ommatidium to their terminals in order to study the shape of the terminal branches with their position in the cartridge. 1. The axons of retinula cells 1-6 (numbered according to Menzel and Snyder, 1974) end as three different types of short visual fibres (svf) in the lamina; the axons of retinula cells 7-9 run through the lamina to terminate in the medulla and are known as long visual fibres (lvf). Retinula cells of each type are identified by the location of their cell bodies and by the direction of their microvilli. The retinula cells 1 and 4 (group I according to Gribakin, 1967) end as svf type 1 with three tassel-like branches in stratum B of the first synaptic region. The pair of cells 3, 6 and the pair 2, 5 (group II) end in the first synaptic region in stratum A. Cells 3 and 6 have forked endings, svf type 2, whereas cells 2 and 5 have tapered endings, svf type 3. The remaining retinula cells 7, 8 and 9 have long fibres. Nos. 7 and 8 (group III) have tapered endings and are termed lvf types 1 and 2, respectively. The 9th cell is the lvf type 3 with a highly branched ending. 2. The nine axons in the bundle from one ommatidium have relative positions which do not change from the proximal retina to the monopolar cell body layer. 3. By following silver-stained retinula cells and their corresponding axons, it is possible to describe mirror-image arrangements of fibres in the axon bundles in different parts of the eye. This correlation of numbered retinula cells with specific axon types, together with the highly organized pattern in an axon bundle, allows the correlation between histological and physiological findings on polarization and colour perception.     

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.     

The Organization of the lamina ganglionaris of the prawn,Pandalus borealis (Kröyer)

The Lamina ganglionaris (first optic neuropile) of the decapod crustacean Pandalus borealis has its optic cartridges (synaptic compartments) arranged in horizontal rows. Each optic cartridge contains seven receptor axon terminals and the branching axis fibres of five monopolar second order neurons. Four types of monopolar neurons are classified. Their cell bodies are arranged in two layers. The inner layer contains the cell bodies of exclusively one of these types, and each cartridge is invaded by two neurons of this neuron type (type M 1:a and M 1:b). The outer layer contains the cell bodies of the remaining three types (M 2, M3 and M4). One gives rise to a large radially branched axis fibre in the centre of the cartridge. The other two have wide branches which may make inter-cartridge contacts, one proximally and the other distally in the plexiform layer, which is clearly bistratified. The receptor axons terminate in two levels corresponding to these strata. Two sets of tangenital fibres form networks in the proximal and the mid-portion of the lamina. Both networks have fibres with primary branches in the vertical plane and secondary branches in the horizontal plane. The fibres of the networks are derived from axons that pass from the second optic neuropile, the medulla externa.     

The Fine Structure of Photoreceptor Terminals in the Compound Eye of Pandalus borealis (Crustacea)

Seven of the photoreceptor axons of each ommatidium in the compound eye of the prawn Pandalus borealis end in two layers in the optic lamina. They have expanded terminals in the optic cartridges; four distally and three proximally in each cartridge. All seven receptor terminals are presynaptic to one lamina monopolar neuron (M₂) of the cartridge. This monopolar neuron is situated centrally in the cartridge and has a thick axis fibre with radially arranged branches, and its axon has a terminal in medulla externa. At the synapses, an arrowlike presynaptic bar is found facing three postsynaptic profiles. The receptor terminals have several characteristics. Their cytoplasm is filled with empty and coated vesicles, and contains numeorus large mitochondria and clusters of tubular elements. There is a longitudinally arranged fascicle of filaments partly surrounded by electron-dense amorphous material in the terminals. Centrally towards M₂, numerous neural spines invaginate into the terminal. Along the entire terminal periphery, there are invaginations from the glial cells. The terminals also form small knoblike protrusions extending into the surrounding glial cells.     

The retina-lamina projection in the visual system of the bee,Apis mellifera

Single Golgi impregnated visual cells and their axons were treated from the retina to the first synaptic layer (lamina) in serial electron microscopic sections. This analysis of the retina-lamina projection was undertaken in the upper dorso-median eye region which is known to be involved in the perception of polarized light. For identification of individual visual cells and their fibres a numbering system was used which relates the number of each of the nine visual cells within one retinula to the transverse axis of the rhabdom (TRA) (Fig. 1). Because of the twist of the retinula along its course to the basement membrane (Fig. 6), individual visual cells change their position relative to any eye-constant co-ordinate system. Each axon bundle originating from one 9-celled retinula performs a 180 degrees-rotation before entering the lamina (Fig. 2). The direction of rotation (clockwise or counter-clockwise), which may differ even between adjacent bundles, is related to the two mirror-image types of rhabdoms in the corresponding retinulae and is opposite to the direction of rhabdom twist. Thus, even in small groups of the in total 5500 ommatidia in the eye of the bee, two types of retinulae exist which can be characterized by the geometry of the rhabdoms as well as by the direction of rotation of the retinulae and the axon bundles (Fig. 1). Visual cell numbers 1, 2, and 9, the microvilli of which are oriented in the direction of TRA, form three long visual fibres terminating in the second synaptic layer (medulla). In cross sections of laminar pseudocartridges they appear as the smallest fibre profiles arranged in a symmetrical line of the pseudocartridge bundle (=the transverse axis of the pseudocartridge; TPA) (Fig. 4). The remaining six fibres (cell numbers 3-8) only project to the lamina (short visual fibres; svf's). Two of them (cell numbers 5 and 6), which are the largest cells in the proximal retinula and have their microvilli perpendicularly arranged to TRA (Fig. 1), give rise to the two thickest axons of the underlaying pseudocartridge. In cross sections, t he connecting line of these two axons is orthogonally oriented to TPA (Fig. 5). A model was developed, in which all long visual fibres originate from ultraviolet receptors and in which the polarization sensitivity of the basal ninth cell is enhanced by the twist of the rhabdom. Finally, this model is discussed in light of behavioral experiments revealing the ultraviolet receptors as the only cells involved in the detection of polarized light.     

Gogo Receptor Contributes to Retinotopic Map Formation and Prevents R1-6 Photoreceptor Axon Bundling

Background

Topographic maps form the basis of neural processing in sensory systems of both vertebrate and invertebrate species. In the Drosophila visual system, neighboring R1–R6 photoreceptor axons innervate adjacent positions in the first optic ganglion, the lamina, and thereby represent visual space as a continuous map in the brain. The mechanisms responsible for the establishment of retinotopic maps remain incompletely understood.

Results

Here, we show that the receptor Golden goal (Gogo) is required for R axon lamina targeting and cartridge elongation in a partially redundant fashion with local guidance cues provided by neighboring axons. Loss of function of Gogo in large clones of R axons results in aberrant R1–R6 fascicle spacing. Gogo affects target cartridge selection only indirectly as a consequence of the disordered lamina map. Interestingly, small clones of gogo deficient R axons perfectly integrate into a proper retinotopic map suggesting that surrounding R axons of the same or neighboring fascicles provide complementary spatial guidance. Using single photoreceptor type rescue, we show that Gogo expression exclusively in R8 cells is sufficient to mediate targeting of all photoreceptor types in the lamina. Upon lamina targeting and cartridge selection, R axons elongate within their individual cartridges. Interestingly, here Gogo prevents bundling of extending R1-6 axons.

Conclusion

Taken together, we propose that Gogo contributes to retinotopic map formation in the Drosophila lamina by controlling the distribution of R1–R6 axon fascicles. In a later developmental step, the regular position of R1–R6 axons along the lamina plexus is crucial for target cartridge selection. During cartridge elongation, Gogo allows R1–R6 axons to extend centrally in the lamina cartridge.     

Daily rhythmic changes of cell size and shape in the first optic neuropil in Drosophila melanogaster

Daily rhythms of changes in axon size and shape are seen in two types of monopolar cell—L1 and L2—that are unique cells within each of the modules or cartridges of the first optic neuropil or lamina in the fly's optic lobe. In the fruit fly Drosophila, L1 and L2's axons swell at the beginning of both day and night, with larger size increases occurring at the beginning of night. Later, they shrink during the day and night, respectively. Simultaneously, they change shape from an inverted conical form during the day to a cylindrical one at night. This is because the axonal cross section of L1 increases during the night, especially at proximal depths of the lamina, closest to the brain, whereas the axon of L2 increases in size at distal lamina depths. The cross‐sectional areas of the L1 cell and of an individual cartridge both change under constant darkness (DD), indicating the circadian origin of changes observed under day/night (LD) conditions. We sought to see whether such changes impart a net change to the entire lamina's volume or shape that is visible by light microscopy, but oscillations in the volume or the curvature of the whole lamina neuropil are found neither in LD nor in DD. These size changes are discussed in relation to previous findings in the housefly Musca, with respect to differences in L1 and L2 between the two species, and to differences in the time course of their circadian changes. © 1999 John Wiley & Sons, Inc. J Neurobiol 40: 77–88, 1999     

Temperature-induced modifications in size and pattern of microtubular organelles in a ciliate,Dileptus II. Formation and spatial arrangement of the microtubular skeleton of the oral parts

Summary Microtubular organelles formed during continuous exposure to high or low temperature were studied. Neither heat nor cold prevents formation of the microtubular skeleton in the oral parts of the ciliateDileptus. Elevated temperature causes the formation of short microtubular fibres, while the size of the oral structure is not diminished. This leads to failure in sculpturing of the cytostome. Heat-treatment may also alter the localization of the anchor site of fibres around the circumference of the basal bodies, and the orientation of the fibres. Cold-treatment evokes the formation of small mouthparts containing a lower number of organelles, although these are properly shaped and there are no deviations in the position or orientation of fibres. It seems that low temperature may suppress the rate of formation of microtubular organelles, while elevated temperature affects their patterning.Abbreviations MTOC microtubule organizing centre - T transverse fibres - B basal body - Cy cytostome - K kinetodesma - P postciliary fibre - C compound fibre - L lamina - F filamentous bundle - M monokinetid     

Ultrastructure and organization of the epineural canal and the nerve cord in sea urchins (Echinodermata,Echinoida)

Summary The radial nerve cord ofMespilia globulus has been examined as an example of echinoid nerve cords. In the radius of echinoids only the ectoneural component of the nerve cord is present which is a derivative of the ectoderm. The nerve cord runs in the interior of the body and is accompanied by the epineural canal. In echinoids, the neuroepithelium makes up the upper and side walls of the epineural canal. Each lateral branch of the nerve cord forms a sort of neural tube. It encloses a branch of the epineural canal which represents an open connection with the sea water. Thus, the epineural canal exhibits numerous openings which probably allow sea water to flow back and forth. This organization is unique in echinoderms. — The neuroepithelium exhibits the organization of an epidermis with well-developed nervous elements. Glial cells are not present. The support cells are the true epithelial cells. Their monociliated cell bodies border the lumen and, by means of cytoplasmic stems that contain a bundle of filaments, they reach up to the basal lamina. The nerve cells and their trunk of nerve fibres fill the spaces between the support cells. — Three types of nerve cells can be distinguished according to their polarity: (1) Primary sensory cells that project a cilium into the epineural canal, the axon hillock region is at the opposite pole. (2) Subluminal cells whose cilium originates in the axon hillock region. (3) Neurones that lie within the trunk of nerve fibres. They are highly stretched in the direction of the nerve cord and are also provided with a cilium. Types 2 and 3 may be homologized with the basal nerve cells of the epidermis. They are possibly multipolar. — The lateral nerve cords make contact with the ampulla and pass the ambulacral plate parallel to the channel that connects the ampulla and the tube foot. The activity of the tube foot-ampulla system is possibly controlled by means of transmitter substances that diffuse through the connective tissue layer between the nerve cord and the myoepithelia of the ampulla and the tube foot respectively.     

Neural organisation in the first optic ganglion of the nocturnal bee <Emphasis Type="Italic">Megalopta genalis</Emphasis>

Each neural unit (cartridge) in the first optic ganglion (lamina) of the nocturnal bee Megalopta genalis contains nine receptor cell axons (6 short and 3 long visual fibres), and four different types of first-order interneurons, also known as L-fibres (L1 to L4) or lamina monopolar cells. The short visual fibres terminate within the lamina as three different types (svf 1, 2, 3). The three long visual fibres pass through the lamina without forming characteristic branching patterns and terminate in the second optic ganglion, the medulla. The lateral branching pattern of svf 2 into adjacent cartridges is unique for hymenopterans. In addition, all four types of L-fibres show dorso-ventrally arranged, wide, lateral branching in this nocturnal bee. This is in contrast to the diurnal bees Apis mellifera and Lasioglossum leucozonium, where only two out of four L-fibre types (L2 and L4) reach neighbouring cartridges. In M. genalis, L1 forms two sub-types, viz. L1-a and L1-b; L1-b in particular has the potential to contact several neighbouring cartridges. L2 and L4 in the nocturnal bee are similar to L2 and L4 in the diurnal bees but have dorso-ventral arborisations that are twice as wide. A new type of laterally spreading L3 has been discovered in the nocturnal bee. The extensive neural branching pattern of L-fibres in M. genalis indicates a potential role for these neurons in the spatial summation of photons from large groups of ommatidia. This specific adaptation in the nocturnal bee could significantly improve reliability of vision in dim light. B.G. is grateful for travel awards from the Royal Physiographic Society, the Per Westlings Fond, the Foundation of Dagny and Eilert Ekvall and the Royal Swedish Academy of Sciences. E.J.W. acknowledges the receipt of a Smithsonian Short-Term Research Fellowship and thanks the Swedish Research Council, the Crafoord Foundation, the Wenner–Gren Foundation and the Royal Physiographic Society of Lund for their ongoing support. W.T.W. was supported by general research funds from the Smithonian Tropical Research Institute     

The first optic ganglion of the bee

The nine receptor cells in each ommatidium of the worker bee end as six short visual fibres in the lamina and as three long visual fibres in the medulla. Behavioural and physiological evidence for regional variation in spectral sensitivity prompted observations on the morphology of the visual units. The distribution, branching pattern, diameter and the arrangement of axonal protusions of the characteristic receptor-cell axons were studied in various regions of the lamina. The six short visual fibres and two of the long visual fibres in each laminar cartridge are uniform over the total eye surface. Only the receptor axons of the ninth cell a UV and polarised light-sensitive cell, show obvious regional variation. In view of the regional constancy in morphology of eight of the nine receptor-cell axons, the regional variations in spectral sensitivity demand either functional subdivision of morphologically indistinguishable photoreceptors (e.g., content of different visual pigments) or a highly complex connectivity pattern of their axons in the first optic ganglion.     

Immunocytochemical evidence for the presence of “mammalian” neurohormonal peptides in neurones of the tapeworm Diphyllobothrium dendriticum

Summary In the nervous system of the obligatory endoparasite Diphyllobothrium dendriticum immunoreactivity (IR) to growth hormone-releasing factor (GRF), peptide histidine isoleucine (PHI), bovine pancreatic polypeptide (BPP), gastrin, gastrin-releasing peptide (GRP), oxytocin, FMRF-amide (FMRF) and serotonin (5HT) was demonstrated by immunocytochemical methods. A very strong GRF-IR was observed in the CNS and PNS of larvae and of the constantly growing adult worms. GRF-IR axon terminals occur beneath the basal lamina of the tegument along the inside of the bothridia, the holdfast organ of the worm. GRF-IR fibres surround the yolk producing vitelline glands and occur in the wall of the vagina. PHI-IR was observed in the CNS and PNS of larvae and adult worms. PHI-IR terminals occur beneath the basal lamina of the tegument along the strobila, the nutrient absorbing surface of the worm. PHI-IR fibres seem to innervate the testicular follicles. FMRF-IR fibres and perikarya occur close to the vitelline glands and the uterine pore and in the male copulatory organ. Numerous large 5HT-IR perikarya with long varicose fibres were observed in the nervous system of the worm. 5HT-IR perikarya occur close to the genital atrium. D. dendriticum is the phylogenetically lowest organism in which IR to PHI has been demonstrated.     

Neural organization of the lamina neuropil of the larva of the tiger beetle (Cicindela chinensis)

Six neural elements, viz., retinular axons, a giant monopolar axon, straight descending processes (type I), lamina monopolar axons (type II), processes containing clusters of dense-core vesicles (type III), and processes coursing in various directions with varicosities (type IV), have been identified at the ultrastructural level in the lamina neuropil of the larval tiger beetle Cicindela chinensis. Retinular axons make presynaptic contact with all other types of processes. Type I and II processes possess many pre-and postsynaptic loci. Type II processes presumably constitute retinotopic afferent pathways. It remains uncertain whether type I processes are lamina monopolar axons or long retinular axons extending to the medullar neuropil. Type III processes may be efferent neurons or branches of afferent neurons contributing to local circuits. A giant monopolar axon extends many branches throughout the lamina neuropil; these branches are postsynaptic to retinular axons, and may be nonretinotopic and afferent. Type IV processes course obliquely in the neuropil, being postsynaptic to retinular axons, and presynaptic to type I processes.     

The organisation of the lamina ganglionaris of the crabs Scylla serrata and Leptograpsus variegatus

Summary The gross structure and neuronal elements of the first optic ganglion of two crabs, Scylla serrata and Leptograpsus variegatus, are described on the basis of Golgi (selective silver) and reduced silver preparations. Of the eight retinula cells of each ommatidium, seven end within the lamina, while the eighth cell sends a long fibre to the external medulla. Five types of monopolar neurons are described, three types of large tangential fibres, and one fibre which may be centrifugal. The marked stratification of the lamina is produced by several features. The main synaptic region, the plexiform layer, is divided by a band of tangential fibres; the short retinula fibres end at two levels in the plexiform layer; and two types of monopolar cells have arborisations confined to the distal or proximal parts of the plexiform layer. The information presently available concerning the retina-lamina projection in Crustacea is examined. Some of the implications of retina and lamina structure are discussed in conjunction with what is known about their electrophysiology.     

Cholinergic neurons in the lamina ganglionaris of the blowfly Calliphora erythrocephala

Summary The distribution of putative cholinergic neurons in the lamina of the blowfly Calliphora erythrocephala was studied by immunocytochemical and histochemical methods. Three different antibodies directed against the AChsynthesizing enzyme, choline acetyltransferase (ChAT), revealed a cholinergic population of fibres running parallel to the laminar cartridges, which have branch-like structures at the distal lamina border. Cell bodies in the chiasma next to the lamina border were also labelled by the anti-ChAT antibodies. Monopolar cell bodies in the nuclear layer were faintly labelled. The distribution of the acetylcholine hydrolyzing enzyme, acetylcholine esterase (AChE), was revealed by histochemical staining and was similar to the ChAT immunocytochemistry. The arrangement of ChAT positive fibres in transverse and longitudinal sections and the distribution of AChE stained fibres indicate that the amacrine cells of the lamina are cholinergic cells.We dedicate this work to Prof. F. Zettler who passed away in fall 1988: K.-H. Datum, I. Rambold     

Neuronal connectivity patterns in the compound eyes of Artemia salina and Daphnia magna (Crustacea: Branchiopoda)

Summary The neuronal types and patterns in the visual system of the species Artemia salina and Daphina magna have been studied with the Golgi method and electron microscopy. The lamina contains five classes of neurons: photoreceptor axons, monopolar, centrifugal, tangential and amacrine neurons. The terminals of the receptor axons are distributed in two (A. salina) or three (D. magna) layers. The dilated terminals have an extensive and wide array of fine branches. One axon from each ommatidium bypasses the lamina and terminates in the medulla in A. salina. A. salina has four types of monopolar neurons, two of which are stratified, whereas in D. magna only two types are found, one of which is bistratified. Tangential T-neurons connect the lamina with the protocerebrum. D. magna has in addition one tangential T-neuron connecting both the lamina and the medulla with the protocerebrum. In both species monopolar-type centrifugal neurons connect the medulla and the lamina, whereas that of A. salina has a wide laminar distribution. Both species also have amacrine cells in the lamina. The medulla contains, besides those shared with the lamina, transmedullary neurons (two types in A. salina), amacrine cells and neurons originating in the protocerebrum.Cartridge-type synaptic compartments are lacking in the investigated species, although a periodic arrangement is discernible in the distal portion of the lamina of A. salina. The receptors from three types of specialized contacts in Artemia, one of which involves a dyad. D. magna has only one-to-one synapses. Neurosecretory fibres are absent in A. salina.The investigation was supported by the Swedish Natural Science Research Council (Grant No. 2760-009)