Invertebrate gap junctions are composed of proteins called innexins and eight innexin encoding loci have been identified in the now complete genome sequence of Drosophila melanogaster. The intercellular channels formed by these proteins are multimeric and previous studies have shown that, in a heterologous expression system, homo- and hetero-oligomeric channels can form, each combination possessing different gating characteristics. Here we demonstrate that the innexins exhibit complex overlapping expression patterns during oogenesis, embryogenesis, imaginal wing disc development and central nervous system development and show that only certain combinations of innexin oligomerization are possible in vivo. This work forms an essential basis for future studies of innexin interactions in Drosophila and outlines the potential extent of gap-junction involvement in development.
After lesion of the peripheral tympanal nerve of the adult locust (Schistocerca gregaria), sensory axons regenerate into their original target areas. We examined the individual behavior of single regenerating auditory afferents during pathway and target selection by intracellularly recording and labeling them at different times postlesion. During axotomy, spontaneous activity is not increased in either the distal or proximal part of the cells. Stimulus response properties of lesioned cells with or without regenerating axons are not influenced. Surprisingly, only 55% of sensory neurons regenerate through the lesion site and often give rise to more than one axonal fiber. Within the central nervous system, 70% of regenerated axons consistently follow an incorrect pathway to reach the correct target region. Often, one of two processes formed by a cell chooses the correct pathway, and the other the incorrect one. In the target region, regenerated axons reconstitute somatotopically ordered projections and form synapses that resemble those of intact fibers in number and structure. The regeneration process does not induce a detectable expression of antigens that are known to be expressed during neural development in these neurons. Our study clearly demonstrates that precise synaptic regeneration is possible in adult animals within a completely differentiated central nervous system, although pathfinding and formation of arborizations are disturbed in a particular and probably system‐related manner. The results strongly suggest that accurate pathfinding is unlikely to be a decisive factor in target area recognition and synaptogenesis. © 2000 John Wiley & Sons, Inc. J Neurobiol 42: 394–409, 2000
The thoracic ganglia of insects exhibit a highly ordered organization. It seems possible that the information underlying the emergence of this order during development and its maintenance throughout insect life is given via a distinct pattern of molecules distributed within the ganglion. The question we asked was whether the adult insect ganglion is subdivided by the distribution of specific carbohydrates and furthermore whether or not this distribution changes during degeneration and regeneration of neurons. In order to determine the normal carbohydrate distribution, we stained sections of the intact metathoracic ganglion of the locust Schistocerca gregaria with fluorescence-coupled lectins. We succeeded in labeling three sensory neuropil areas with either peanut agglutinin (PNA): Phaseolus vulgaris erythrolectin (PVE), soybean agglutinin, wheat germ agglutinin (WGA), or Vicia villosa agglutinin. Apart from this, PNA, WGA, and succinylated WGA also selectively labeled some neuronal cell bodies, including dorsal unpaired median neurons. Datura stramonium lectin (DSL), Griffonia simplicifolia lectin II, and Solanum tuberosum lectin (STL) bound to glial cells or glia surrounding extracellular matrix. A few lectins stained all structures within the ganglion; some showed no binding at all. In the second part of our study, we tested whether carbohydrates were differentially regulated during transient deafferentation after the axotomy of the tympanal nerve. Binding of PNA and PVE within the auditory neuropil did not change. However, binding of the two glia-associated markers, DSL and STL, clearly differed from that found in intact animals; they bound transiently (day 3-4 until day 10-20 post-surgery) to axonal tracts and neuropils of the axotomized sensory afferents.
This study describes time course and ultrastructural changes during axonal degeneration of different neurones within the tympanal nerve of the locust Schistocerca gregaria. The tympanal nerve innervates the tergit and pleurit of the first abdominal segment and contains the axons of both sensory and motor neurones. The majority of axons (approx. 97%) belong to several types of sensory neurones: mechano- and chemosensitive hair sensilla, multipolar neurones, campaniform sensilla and sensory cells of a scolopidial organ, the auditory organ. Axons of campaniform sensilla, of auditory sensory cells and of motor neurones are wrapped by glial cell processes. In contrast, the very small and numerous axons (diameter <1 microm) of multipolar neurones and hair sensilla are not separated individually by glia sheets. Distal parts of sensory and motor axons show different reactions to axotomy: 1 week after separation from their somata, distal parts of motor axons are invaded by glial cell processes. This results in fascicles of small axon bundles. In contrast, distal parts of most sensory axons degenerate rapidly after being lesioned. The time to onset of degeneration depends on distance from the lesion site and on the type of sensory neurone. In axons of auditory sensory neurones, ultrastructural signs of degeneration can be found as soon as 2 days after lesion. After complete lysis of distal parts of axons, glial cell processes invade the space formerly occupied by sensory axons. The rapid degeneration of distal auditory axon parts allows it to be excluded that they provide a structure that leads regenerating axons to their targets. Proximal parts of severed axons do not degenerate.
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