Abstract
BackgroundThe research field of connectomics arose just recently with the development of new three-dimensional-electron microscopy (EM) techniques and increasing computing power. So far, only a few model species (for example, mouse, the nematode Caenorhabditis elegans, and the fruit fly Drosophila melanogaster) have been studied using this approach. Here, we present a first attempt to expand this circle to include pycnogonids, which hold a key position for the understanding of arthropod evolution. The visual neuropils in Achelia langi are studied using a focused ion beam-scanning electron microscope (FIB-SEM) crossbeam-workstation, and a three-dimensional serial reconstruction of the connectome is presented.ResultsThe two eyes of each hemisphere of the sea spider’s eye tubercle are connected to a first and a second visual neuropil. The first visual neuropil is subdivided in two hemineuropils, each responsible for one eye and stratified into three layers. Six different neuron types postsynaptic to the retinula (R-cells) axons are characterized by their morphology: five types of descending unipolar neurons and one type of ascending neurons. These cell types are also identified by Golgi impregnations. Mapping of all identifiable chemical synapses indicates that the descending unipolar neurons are postsynaptic to the R-cells and, hence, are second-order neurons. The ascending neurons are predominantly presynaptic and sometimes postsynaptic to the R-cells and may play a feedback role.ConclusionsComparing these results with the compound eye visual system of crustaceans and insects – the only arthropod visual system studied so far in such detail – we found striking similarities in the morphology and synaptic organization of the different neuron types. Hence, the visual system of pycnogonids shows features of both chelicerate median and mandibulate lateral eyes.Electronic supplementary materialThe online version of this article (doi:10.1186/s12915-014-0059-3) contains supplementary material, which is available to authorized users.
Highlights
The research field of connectomics arose just recently with the development of new threedimensional-electron microscopy (EM) techniques and increasing computing power
One part of the fibers projects to the first visual neuropil located dorsolaterally in the anterior part of the protocerebrum as an ovoid region laterally embedded in the cell body rind of the brain (Figures 1, 2)
When comparing our results with the characteristics described in the compound eyes in Drosophila, we found striking similarities in the morphology and synaptic pattern of the visual neurons
Summary
The research field of connectomics arose just recently with the development of new threedimensional-electron microscopy (EM) techniques and increasing computing power. The visual neuropils in Achelia langi are studied using a focused ion beam-scanning electron microscope (FIB-SEM) crossbeam-workstation, and a three-dimensional serial reconstruction of the connectome is presented. One promising approach is (three-dimensional) reconstruction from serial section transmission electron microscopy (TEM), which is nowadays a well-established way of analyzing circuitry of neural networks [1,2,3]. In analyzing nervous systems regarding connectomics, either small animals with a small central nervous system (CNS) or a restricted region within the CNS or even within a particular neuropil are possible study subjects to obtain a comprehensive data stack
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