Light and electronmicroscope study of one of the systems of centrifugal fibers found in the lamina of muscoid flies

Chemical neuroanatomy of the fly's movement detection pathway

In Diptera, subsets of small retinotopic neurons provide a discrete channel from achromatic photoreceptors to large motion-sensitive neurons in the lobula complex. This pathway is distinguished by specific affinities of its neurons to antisera raised against glutamate, aspartate, gamma-aminobutyric acid (GABA), choline acetyltransferase (ChAT), and a N-methyl-D-aspartate type 1 receptor protein (NMDAR1). Large type 2 monopolar cells (L2) and type 1 amacrine cells, which in the external plexiform layer are postsynaptic to the achromatic photoreceptors R1-R6, express glutamate immunoreactivity as do directionally selective motion-sensitive tangential neurons of the lobula plate. L2 monopolar cells ending in the medulla are accompanied by terminals of a second efferent neuron T1, the dendrites of which match NMDAR1-immunoreactive profiles in the lamina. L2 and T1 endings visit ChAT and GABA-immunoreactive relays (transmedullary neurons) that terminate from the medulla in a special layer of the lobula containing the dendrites of directionally selective retinotopic T5 cells. T5 cells supply directionally selective wide-field neurons in the lobula plate. The present results suggest a circuit in which initial motion detection relies on interactions among amacrines and T1, and the subsequent convergence of T1 and L2 at transmedullary cell dendrites. Convergence of ChAT-immunoreactive and GABA-immunoreactive transmedullary neurons at T5 dendrites in the lobula, and the presence there of local GABA-immunoreactive interneurons, are suggested to provide excitatory and inhibitory elements for the computation of motion direction. A comparable immunocytological organization of aspartate- and glutamate-immunoreactive neurons in honeybees and cockroaches further suggests that neural arrangements providing directional motion vision in flies may have early evolutionary origins.

Synaptic organization of columnar elements in the lamina of the wild type in Drosophila melanogaster

The synaptic connections within the lamina, the first of the optic neuropiles underlying the insect's compound eye, have been little studied in Drosophila melanogaster until now, despite the genetic advantages of this animal. Here we report the reconstruction through its entire depth of one of the lamina modules, or cartridges, of a female wild-type Drosophila, for which a series of EM cross sections was analysed at levels extending from the retinal basement membrane to the first optic chiasma. A complete, comprehensive catalogue of the synaptic connections of all columnar elements has been compiled from this single series, confirmed from comparisons with less completely photographed cartridges. Combinations of the 12 types of cartridge neurons form divergent multiple-contact synapses (dyads, triads, and tetrads) throughout the lamina's depth. These 12 neuron types include 11 narrow-field elements (one class of receptor terminal, R1-R6, providing input to the cartridge; two types of long visual fiber from the ommatidium, R7 and R8; five types of monopolar cell, L1-5; and three types of medulla cell--two centrifugal neurons C2 and C3, and a third, T1) as well as a wide-field intrinsic or amacrine cell. Connections within the lamina formed by L4 from two adjacent cartridges (posterodorsal and posteroventral) contribute to the matrix of connections. In addition, connections of at least one other wide-field element have also been incorporated.

The first optic ganglion of the bee. V. Structural and functional characterization of centrifugally arranged interneurones

The organization, characterization and connectivity patterns of four different interneurone types were studied with the use of Golgi light- and electron-microscopic techniques. All four cell types originate in the outer chiasma; they have an efferent end-branch in the lamina and an afferent one terminating in the distal region of the second optic ganglion, the medulla. These interneurones are referred to as: (i) Garland-cell: The efferent fibre has on its tangential branch numerous centripetal side branches, so-called "garlands", which synapse with first- and second-order visual cells. (ii) Y-cell: The lamina branch bifurcates before entering the lamina. It innervates two neighbouring cartridges. Synaptic contacts were seen in two different laminar strata where bottle-brush-like collaterals occurred. (iii) Single bottle-brush cell: The efferent part has only one centrifugal branch, which can be compared morphologically and in terms of synaptology with those of the Y-cell. (iv) Triptych-cell: The lamina component innervates three neighbouring cartridges at three different laminar layers interconnecting different first- and second-order visual neurones. The present study provides some essential qualitative and quantitative fine-structural information, which - when compared with adequate physiological data - may lead to a better understanding of the function of the first visual information-processing centre of the bee.

Golgi and genetic mosaic analyses of visual system mutants in Drosophila melanogaster

We have used a Golgi staining procedure in Drosophila melanogaster to examine the structure of individual neurons in the visual systems of the Canton-S wild-type strain, of flies expressing mutations at the Glued, rough, glass, and uneven loci, all of which affect the organization of the visual system, and of genetic mosaics involving the Glued and uneven loci. We have found that the structure of the neurons studied in the wild type is quite similar to that reported for other diptera and that the mutants studied evidence a variety of abnormalities in neuronal morphology, each mutant being characterized by a different spectrum of aberrations. The genetic mosaic analysis of the Glued and uneven loci showed that the structure of individual neurons in the optic lobes is profoundly influenced by the genotype of the cells projecting to that region from the compound eye but that the final form attained by a neuron is not solely controlled by that factor.

Synaptogenesis in the first optic neuropile of the fly's visual system

The developmental transformation of the chief afferent class of fly photoreceptor synapse has been examined from serial electron micrographs of animals fixed at 74, 81, 94 and 100% pupal development (100% pupal development being defined by the time of normal adult eclosion). Animals were selected both by their age and the conformity of their eye coloration to standards for each stage. Two animals were analysed from each stage, one in greater detail than the other. For the first, the exact coordinates of the cartridges (the synaptic columns of the first optic neuropile) from which the analyses were made were mapped and selected to be within the same region of the eye field at all stages. From all animals a portion of one or two cartridges was analysed from series of up to 100 sections and the synapse populations (greater than 80) were analysed for their fine structure and postsynaptic composition. Adult synapses are confirmed as tetrads, with two of the four postsynaptic elements invariably from two monopolar interneurons L1 and L2, one from each. The two others are usually from alpha processes of the same amacrine cell. Synapses appear during the last half of pupal development, with no obvious asynchrony of ultrastructural maturation and in parallel with those of at least one of the other synaptic classes present (which were otherwise not studied). Many adult features of synaptic ultrastructure emerge late, only by 94% pupal development. These include adult numbers of synaptic vesicles, the complete form of the presynaptic ribbon with platform and the postsynaptic cisternae of L1/L2. Prior to 94% the synapses are smaller with postsynaptic elements having a less regular geometry and with postsynaptic densities which are subsequently lost (alpha processes) or replaced by cisternae (L1/L2). At the presynaptic sites of the younger animals (74%, 81%) dyads and triads of postsynaptic elements coexist with tetrads, those of older animals having, on average, more postsynaptic processes per synapse. It is suggested that individual synapses assemble piecemeal, element by element.

Types and arrangements of neurons in the crayfish optic lamina

The neural arrangements in the optic lamina of the crayfish Pacifastacus leniusculus Dana have been studied by light microscopy by means of silver impregnation techniques. The lamina is composed of columnar synaptic compartments (cartridges). Each cartridge is composed of seven receptor terminals distributed in two layers and second-order monopolar neurons connecting the lamina with the second synaptic region, the medulla externa. The neurons found in the lamina consist of five classes: monopolar neurons, centrifugal small-field neurons, tangential neurons, multipolar cells (possibly of a glial nature) and photoreceptor axons (fig. 13). Among the monopolar cells, five types are classified (M1-M5) according to their lamina arborizations. Two types are stratified (M3 and M5) corresponding to the photoreceptor terminal strata. On this basis, the lamina plexiform layer is subdivided into two layers (epl1 and epl2). The remaining monopolar neurons have lateral processes in both layers, two of them within one cartridge (M1 and M2) and one over several cartridges (M5). There is one type of small-field centrifugal neuron (C1) and two types of tangential medulla to lamina neurons (Tan1 and Tan2), both having processes covering a large number of cartridges. Multipolar cells with cell bodies distal (MP1) or proximal (MP2) to the plexiform layer send processes to several cartridges. The receptor axons consit of three types. One has terminals in epl1 or epl2, the second has its terminal in epl1 and a thin process to epl2, and the third (corresponding to the 8th retinular cell) bypasses the lamina and has a terminal in the medulla externa. A brief comparison is made with the neural arrangements in the lamina of the Norway lobster Nephrops norvegicus L.

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.

High voltage electron microscopy of the optic neuropile of the housefly, Musca domestica

Synaptic cartridges of the first optic neuropile (lamina ganglionaris) of the housefly were examined by high voltage electron microscopy (HVEM). Stereo pairs (from thick, i.e., 0.25 mum, sections viewed at 1,000 kV) provided a three dimensional representation of cartridge neurons and clearly revealed the lateral spread, bifurcation and some functional associations of Type I (L1, L2) monopolar interneurons. Slightly proximal to cartridge neck level, pairs of retinular (R) axons made contact with each other and it appeared that R processes projected through the cleft between the Type I interneurons. No junctional modifications were seen between contiguous R axon terminals. The speculation was made that functional contact might exist between neighboring R axons prior to their extensive synapses with principal first order interneurons. Such alleged coupling between R axons would account for several electrophysiological findings from other laboratories. Modifications in EM technique applicable for HVEM were detailed. The value of obtaining thick serial sections and the use of the HVEM in expediting three dimensional reconstructions of neuropile were demonstrated.

Branching of central neurons: intracellular cobalt injection for light and electron microscopy

Cobalt chloride can be injected into an identified nerve cell body in an insect ganglion and reacted with ammonium sulfide to stain the soma and its branches with a black precipitate. The stained cell body and its branches throughout the neuropil are visible in both the light and electron microscope. In whole mount preparations, the resolution of neurites within the neuropil is of a quality that permits the comparison of branching patterns between cells and during various functional states.