Long-term survival of decentralized axons and incorporation of satellite cells in motor neurons of rock lobsters (original) (raw)
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Neuroscience Letters, 1991
Glial nuclei have been reported to be incorporated into the axoplasm of surviving distal stumps (anucleate axons) weeks to months after lesioning abdominal motor axons in rock lobsters. We have not observed this phenomenon in crayfish medial giant axons (MGAs) which also survive for weeks to months after lesioning. Glial nuclei were not observed within MGAs perfused with a physiological intracellular saline. However, incorporation of glial nuclei was observed after MGAs were perfused with intracellular salines containing Fast green. From these and previously published data, we confirm that glial incorporation into axoplasm can occur, but we suggest that is is not a common mechanism used by crustaceans to provide for long-term survival of anucleate axons.
Degeneration and regeneration in crustacean peripheral nerves
Journal of Comparative Physiology, 1974
and physiological ( ) data from several crustacean species kept at 19-21~ show that isolated stumps of motor axons often survive intact for 150-250 days whereas sensory axonal segments usually degenerate within 20 days. Axonal segments of both motor and sensory axons that remain connected to their cell body generally remain functionally and morphologically normal after lesioning. No evidence was found for collatecal innervation of denervated muscles from intact motor neurons supplying nearby muscle masses, although the motor nerve terminals may not have completely degenerated. Evidence is presented that motor axons specifically re-innervate their original muscle mass if such re-innervation occurs within 90 days after lesioning. Regenerating sensory and motor fibers make appropriate CNS and peripheral connections so as to re-establish correctly a peripheral reflex found in intact animals .
Ultrastructural correlates of motor nerve regeneration in crayfish
Cell and Tissue Research, 1974
Identified motor neurons innervating distal limb muscles in the crayfish claw have been examined by light and electron microscopy after surgical interruption. The functionally competent distal segments of such axons, a few weeks after the operation, show enlarged glial sheaths that contain occasional small satellite axonal profiles (cf. Nordlander and Singer, 1972); evidence is presented that such profiles can arise
Regeneration of giant axons in earthworms
Brain Research, 1976
Animals have evolved various strategies to repair neuronal tissue. For example, all regenerable axons in vertebrates11,12, z2-24 and insects 7,13,~0,25 and most sensory axons in crustaceans 2,3,19 repair axonal damage by allowing severed distal stumps to degenerate while outgrowths arise from intact proximal stumps. (In this paper, the term distal will be used to describe an axonal segment separated from its cell body whereas proximal will be used to describe an axonal process in contact with its original cell body.) These outgrowths can re-establish the original synaptic connections with varying degrees of success and specificity depending on the organism. In contrast, motor axons in crustaceans2,3,14,16 (cf. ref. l 8) and a CNS axon in an annelid 8 have been reported to regenerate by a functional reconnection or a fusion of slowly outgrowing proximal processes with the original distal stumps. Both types of repair mechanisms (regrowth of proximal stumps to target cells and axonal reconnection) are capable of restoring lost function.
Fast-axon synapses of a crab leg muscle
Journal of Neurobiology, 1978
Neuromuscular synapses of the "fast" excitatory axon supplying the main extensor muscle in the leg of the shore crab Pachygrapsus crassipes were studied with electrophysiological and electron-microscopic techniques. Electrical recording showed that many muscle fibers of the central region of the extensor muscle responded only to stimulation of the fast axon, and electron microscopy revealed many unitary subterminal axon branches. Maintained stimulation, even at a low frequency, resulted in depression of the excitatory junctional potentials (EJPs) set up by the fast axon but EJPs of different muscle fibers depressed a t different rates, indicating some physiological heterogeneity among the fast-axon synapses. Focal recording a t individual synaptic sites on the surfaces of the muscle fibers showed quantal contents ranging from 1.4 to 5.5 at different synapses; these values are relatively high in comparison with similar determinations made in the crayfish opener muscle. Synapse-bearing nerve terminals were generally relatively small in diameter and filiform, with many individual synaptic contact areas of uniform size averaging 0.6 pm2. All of the individual synapses had a presynaptic "dense body" at which synaptic vesicles clustered. If these structures represent release points for transmitter quanta, the initial high quantal content, would have an ultrastructural basis. The mitochondial content of the nerve terminals, the synaptic vesicle population, and the specialized subsynaptic sarcoplasm were all much reduced in comparison with tonic axon synaptic regions in this and other crustaceans. The latter features may be correlated with the relatively infrequent use of this axon by the animal, and with rapid fatigue.
Using fluorescence photoablation to study the regeneration of singly cut leech axons
Journal of Neurobiology, 1991
The regeneration of the axons of leech Retzius cells was compared following two different methods of axonal severing: (1) a crush of the whole connective that includes the Retzius axon; and (2) photoablation of a small segment of only the Retzius axon. The photoablation was carried out after filling the Retzius cell with Lucifer Yellow (LY). Several tests were carried out to determine whether the photoablation actually severed the axon. These included (1) using the lipophilic membrane probe DiI as an indicator of membrane severance [Fig. 4(A)]; (2) electron microscopic examination of the photoablated axon after filling it with horseradish peroxidase (HRP) (Fig. 7); and (3) filling the Retzius cell first with HRP, then photoablating, and looking for the disappearance of the HRP in the photoablated region (Figs. 5 6). These and other observations indicated that the photoablated axon was actually severed. Two differences were seen in the regeneration of the Retzius axon after crush versus after photoablation. First, the sprouting following crush was far more disorganized, and included significantly more lateral spread. Second, after photoablation, over 70% of the axons, upon refilling with LY after 3 days or more, showed the newly introduced LY suddenly extending far down the distal segment, indicating that the proximal and distal segments had become reconnected (Figs. 8 , 9). This was never seen following a crush (Fig. 1). The photuablated axons did not pass H R P into the distal segment, suggesting that the reconnection was not by fusion, but perhaps by a gap junction. The results show that axonal regeneration can take a dramatically different form than it does following a standard crush procedure if, instead, the axon is severed in a way that preserves the structural integrity of the surrounding tissue.
The Journal of Comparative Neurology, 1996
It has been postulated that phosphorylation of the carboxy terminus sidearms of neurofilaments (NFs) increases axon diameter through repulsive electrostatic forces that increase sidearm extension and interfilament spacing. To evaluate this hypothesis, the relationships among NF phosphorylation, NF spacing, and axon diameter were examined in uninjured and spinal cord-transected larval sea lampreys (Petromyzon marinus). In untransected animals, axon diameters in the spinal cord varied from 0.5 to 50 pm. Antibodies specific for highly phosphorylated NFs labeled only large axons ( > 10 pm), whereas antibodies for lightly phosphorylated NFs labeled medium-sized and small axons more darkly than large axons. For most axons in untransected animals, diameter was inversely related to NF packing density, but the interfilament distances of the largest axons were only 1.5 times those of the smallest axons. In addition, the lightly phosphorylated NFs of the small axons in the dorsal columns were widely spaced, suggesting that phosphorylation of NFs does not rigidly determine their spacing and that NF spacing does not rigidly determine axon diameter. Regenerating neurites of giant reticulospinal axons (GRAs) have diameters only 5-10% of those of their parent axons. If axon caliber is controlled by NF phosphorylation via mutual electrostatic repulsion, then NFs in the slender regenerating neurites should be lightly phosphorylated and densely packed (similar to NFs in uninjured small caliber axons), whereas NFs in the parent GRAs should be highly phosphorylated and loosely packed. However, although linear density of NFs (the number of NFs per micrometer) in these slender regenerating neurites was twice that in their parent axons, they were highly phosphorylated. Following sectioning of these same axons close to the cell body, axon-like neurites regenerated ectopically from dendritic tips. These ectopically regenerating neurites had NF linear densities 2.5 times those of uncut GRAs but were also highly phosphorylated. Thus, in the lamprey, NF phosphorylation may not control axon diameter directly through electrorepulsive charges that increase NF sidearm extension and NF spacing. It is possible that phosphorylation of NFs normally influences axon diameter through indirect mechanisms, such as the slowing of NF transport and the formation of a stationary cytoskeletal lattice, as has been proposed by others. Such a mechanism could be overridden during regeneration, when a more compact, phosphorylated NF backbone might add mechanical stiffness that promotes the advance of the neurite tip within a restricted central nervous system environment.
The Journal of Neuroscience : The Official Journal of the Society for Neuroscience
Protein maintenance and degradation are examined in the severed distal (anucleate) portions of crayfish medial giant axons (MGAs), which remain viable for over 7 months following axotomy. On polyacrylamide gels, the silver-stained protein banding pattern of anucleate MGAs severed from their cell bodies for up to 4 months remains remarkably similar to that of intact MGAs. At 7 months postseverance, some (but not all) proteins are decreased in anucleate MGAs compared to intact MGAs. To determine the half-life of axonally transported proteins, we radiolabeled MGA cell bodies and monitored the degradation of newly synthesized transported proteins. Assuming exponential decay, proteins in the fast component of axonal transport have an average half-life of 14 d in anucleate MGAs and proteins in the slow component have an average half-life of 17 d. Such half-lives are very unlikely to account for the ability of anucleate MGAs to survive for over 7 months after axotomy.
Loss of escape-related giant neurons in a spiny lobster, Panulirus argus
The Biological Bulletin, 2006
When attacked, many decapod crustaceans perform tailflips, which are triggered by a neural circuit that includes lateral giant interneurons, medial giant interneurons, and fast flexor motor giant neurons (MoGs). Slipper lobsters (Scyllaridae) lack these giant neurons, and it has been hypothesized that behavioral (e.g., digging) and morphological (e.g., flattening and armor) specializations in this group caused the loss of escape-related giant neurons. To test this hypothesis, we examined a species of spiny lobster, Panulirus argus. Spiny lobsters belong to the sister taxon of the scyllarids, but they have a more crayfish-like morphology than scyllarids and were predicted to have escaperelated giant neurons. Ventral nerve cords of P. argus were examined using paraffin-embedded sections and cobalt backfills. We found no escape-related giant neurons and no large axon profiles in the dorsal region of the nerve cord of P. argus. Cobalt backfills showed one fewer fast flexor motor neuron than in species with MoGs and none of the fast flexor motor neurons show any of the anatomical specializations of MoGs. This suggests that all palinuran species lack this giant escape circuit, and that the loss of rapid escape behavior preceded, and may have driven, alternative predator avoidance and anti-predator strategies in palinurans. Abbreviations: FAC, flexor anterior contralateral cluster; FMC, flexor medial contralateral cluster; FPI, flexor posterior ipsilateral cluster; LG, lateral giant interneuron; MG, medial giant interneuron; MoG, fast flexor motor giant neuron; N3 d , dorsal branch of nerve 3.