This chapter contains section titled: Patterns in the Retina Development of the Retina Axon Projection Patterns Between the Retina and First Two Optic Neuropiles Proliferation of Optic Anlagen Differentiation of Lamina Ganglion Cells Patterns of Fibre Growth Growth of Individual Axon Bundles Establishment of Retinotopic Pattern Between Axon Bundles The Formation of Connections Within the Lamina Acknowledgements References Discussion References
Unlike other monoamine neurotransmitters, the mechanism by which the brain's histamine content is regulated remains unclear. In mammals, vesicular monoamine transporters (VMATs) are expressed exclusively in neurons and mediate the storage of histamine and other monoamines. We have studied the visual system of Drosophila melanogaster in which histamine is the primary neurotransmitter released from photoreceptor cells. We report here that a novel mRNA splice variant of Drosophila VMAT (DVMAT-B) is expressed not in neurons but rather in a small subset of glia in the lamina of the fly's optic lobe. Histamine contents are reduced by mutation of dVMAT, but can be partially restored by specifically expressing DVMAT-B in glia. Our results suggest a novel role for a monoamine transporter in glia that may be relevant to histamine homeostasis in other systems.
Abstract Deriving the detailed synaptic connections of the entire nervous system has been a long term but unrealized goal of the nascent field of connectomics. For Drosophila , in particular, three sample preparation problems must be solved before the requisite imaging and analysis can even begin. The first is dissecting the brain, connectives, and ventral nerve cord (roughly comparable to the brain, neck, and spinal cord of vertebrates) as a single contiguous unit. Second is fixing and staining the resulting specimen, too large for previous techniques such as flash freezing, so as to permit the necessary automated segmentation of neuron membranes. Finally the contrast must be sufficient to support synapse detection at imaging speeds that enable the entire connectome to be collected. To address these issues, we report three major novel methods to dissect, fix, dehydrate and stain this tiny but complex nervous system in its entirety; together they enable us to uncover a Focused Ion-Beam Scanning Electron Microscopy (FIB-SEM) connectome of the entire Drosophila brain. They reliably recover fixed neurons as round profiles with darkly stained synapses, suitable for machine segmentation and automatic synapse detection, for which only minimal human intervention is required. Our advanced procedures use: a custom-made jig to microdissect both regions of the central nervous system, dorsal and ventral, with their connectives; fixation and Durcupan embedment, followed by a special hot-knife slicing protocol to reduce the brain to dimensions suited to FIB; contrast enhancement by heavy metals; together with a progressive lowering of temperature protocol for dehydration. Collectively these optimize the brain’s morphological preservation, imaging it at a usual resolution of 8nm per voxel while simultaneously speeding the formerly slow rate of FIB-SEM. With these methods we could recently obtain a FIB-SEM image stack of the Drosophila brain eight times faster than hitherto, at approximately the same rate as, but without the requirement to cut, nor imperfections in, EM serial sections.
A comparative ultrastructural study of photoreceptor synapses formed upon homologous postsynaptic neurones in insects has been made by using serial-section electron microscopy in representative Diptera from a monophyletic series of 14 families. At all of the synaptic contacts there is a presynaptic dense bar, surmounted in phylogenetically more recent families by a presynaptic platform. Opposite the bar lies a pair of postsynaptic elements that invariably originate one each from two unique monopolar neurones L1 and L2. Both elements contain increasingly elaborate cisternae in more recent flies. Within the phylogenetic series, the postsynaptic ensemble itself changes from the original dyad to a tetradic configuration in more recent Muscomorpha by the addition of two new postsynaptic elements from an amacrine cell. This transition occurs once only in the series, which, gauged by the fossil record, covers divergences from the stem line extending back greater than 200 million years.
