Electron microscopy of severed motor fibers in the crayfish

Activity-dependent decline and recovery of synaptic transmission in central parts of surviving primary afferents after their peripheral cut in crayfish

Axons deprived of their nucleus degenerate within a few days in mammals but survive for several months in crustaceans. However, it is not known whether central synapses from sensory axons may preserve their molecular machinery in the absence of spiking activity. To assess this, we used peripheral axotomy, which removes their nuclei combined with electrophysiology techniques and electron microscopy imaging. We report the following. (1) Electron microscopy analysis confirms previous observations that glial cell nuclei present in the sensory nerve proliferate and migrate to axon tubes, where they form close contacts with surviving axons. (2) After peripheral axotomy performed in vivo on the coxo-basipodite chordotonal organ (CBCO), the sensory nerve does not convey any sensory message, but antidromic volleys are observed. (3) Central synaptic transmission from the CBCO to motoneurons (MNs) progressively declines over 200 days (90% of monosynaptic excitatory transmission is lost after 3 weeks, whereas 60% of disynaptic inhibitory transmission persists up to 6 months). After 200 days, no transmission is observed. (4) However, this total loss is apparent only because repetitive electrical stimulation of the sensory nerve in vitro progressively restores first inhibitory post-synaptic potentials and then excitatory post-synaptic potentials. (5) The loss of synaptic transmission can be prevented by in vivo chronic sensory nerve stimulation. (6) Using simulations based on the geometric arrangements of synapses of the monosynaptic excitatory transmission and disynaptic inhibitory pathways, we show that antidromic activity in the CBCO nerve could play a role in the maintenance of synaptic function of inhibitory pathways to MNs, but not monosynaptic excitatory transmission to MNs. Our study confirms the deep changes in glial nuclei observed in axons deprived of their nucleus. We further show that the machinery for spike conduction and synaptic release persists for several months, even if there is no longer any activity. Indeed, we were able to restore, with electrical activity, spike conduction and synaptic function after long silent periods (>6 months).

The effect of axotomy on firing and ultrastructure of the crayfish mechanoreceptor neurons and satellite glial cells

Neurotrauma is among main causes of human disability and death. We studied effects of axotomy on ultrastructure and neuronal activity of a simple model object - an isolated crayfish stretch receptor that consists of single mechanoreceptor neurons (MRN) enwrapped by multilayer glial envelope. After isolation, MRN regularly fired until spontaneous activity cessation. Axotomy did not change significantly MRN spike amplitude and firing rate. However, the duration of neuron activity from MRN isolation to its spontaneous cessation decreased in axotomized MRN relative to intact neuron. [Ca²⁺] in MRN axon and soma increased 3-10 min after axotomy. Ca²⁺ entry through ion channels in the axolemma accelerated axotomy-stimulated firing cessation. MRN incubation with Ca²⁺ionophore ionomycin accelerated MRN inactivation, whereas Ca²⁺-channel blocker Cd²⁺ prolonged firing. Activity duration of either intact, or axotomized MRN did not change in the presence of ryanodine or dantrolene, inhibitors of ryanodin-sensitive Ca²⁺ channels in endoplasmic reticulum. Thapsigargin, inhibitor of endoplasmic reticulum Ca²⁺-ATPase, or its activator ochratoxin were ineffective. Ultrastructural study showed that the defect in the axon transected by thin scissors is sealed by fused axolemma, glial and collagen layers. Only the 30-50 μm long segment completely lost microtubules and contained swelled mitochondria. The microtubular bundle remained undamaged at 300 μm away from the axotomy site. However, mitochondria within the 200-300 μm segment were strongly condensed and lost matrix and cristae. Glial and collagen layers exhibited greater damage. Swelling and edema of glial layers, collagen disorganization and rupture occurred within this segment. Thus, axotomy stronger damages glia/collagen envelope, axonal microtubules and mitochondria.

The curious ability of polyethylene glycol fusion technologies to restore lost behaviors after nerve severance

Traumatic injuries to PNS and CNS axons are not uncommon. Restoration of lost behaviors following severance of mammalian peripheral nerve axons (PNAs) relies on regeneration by slow outgrowths and is typically poor or nonexistent when after ablation or injuries close to the soma. Behavioral recovery after severing spinal tract axons (STAs) is poor because STAs do not naturally regenerate. Current techniques to enhance PNA and/or STA regeneration have had limited success and do not prevent the onset of Wallerian degeneration of severed distal segments. This Review describes the use of a recently developed polyethylene glycol (PEG) fusion technology combining concepts from biochemical engineering, cell biology, and clinical microsurgery. Within minutes after microsuturing carefully trimmed cut ends and applying a well-specified sequence of solutions, PEG-fused axons exhibit morphological continuity (assessed by intra-axonal dye diffusion) and electrophysiological continuity (assessed by conduction of action potentials) across the lesion site. Wallerian degeneration of PEG-fused PNAs is greatly reduced as measured by counts of sensory and/or motor axons and maintenance of axonal diameters and neuromuscular synapses. After PEG-fusion repair, cut-severed, crush-severed, or ablated PNAs or crush-severed STAs rapidly (within days to weeks), more completely, and permanently restore PNA- or STA-mediated behaviors compared with nontreated or conventionally treated animals. PEG-fusion success is enhanced or decreased by applying antioxidants or oxidants, trimming cut ends or stretching axons, and exposure to Ca(2+) -free or Ca(2+) -containing solutions, respectively. PEG-fusion technology employs surgical techniques and chemicals already used by clinicians and has the potential to produce a paradigm shift in the treatment of traumatic injuries to PNAs and STAs.

Wallerian degeneration, wld(s), and nmnat

Traditionally, researchers have believed that axons are highly dependent on their cell bodies for long-term survival. However, recent studies point to the existence of axon-autonomous mechanism(s) that regulate rapid axon degeneration after axotomy. Here, we review the cellular and molecular events that underlie this process, termed Wallerian degeneration. We describe the biphasic nature of axon degeneration after axotomy and our current understanding of how Wld(S)--an extraordinary protein formed by fusing a Ube4b sequence to Nmnat1--acts to protect severed axons. Interestingly, the neuroprotective effects of Wld(S) span all species tested, which suggests that there is an ancient, Wld(S)-sensitive axon destruction program. Recent studies with Wld(S) also reveal that Wallerian degeneration is genetically related to several dying back axonopathies, thus arguing that Wallerian degeneration can serve as a useful model to understand, and potentially treat, axon degeneration in diverse traumatic or disease contexts.

Ultrastructure of neuroglial contacts in crayfish stretch receptor

In order to explore neuroglial relationships in a simple nervous system, we have studied the ultrastructure of the crayfish stretch receptor, which consists of only two mechanoreceptor neurons enwrapped by glial cells. The glial envelope comprises 10-30 glial layers separated by collagen sheets. The intercellular space between the neuronal and glial membranes is generally less than 10-15 nm in width. This facilitates diffusion between neurons and glia but restricts neuron communication with the environment. Microtubule bundles passing from the dendrites to the axon through the neuron body limit vesicular transport between the perikaryon and the neuronal membrane. Numerous invaginations into the neuron cytoplasm strengthen glia binding to the neuron and shorten the diffusion pathway between them. Double-membrane vesicles containing fragments of glial, but not neuronal cytoplasm, represent the captured tips of invaginations. Specific triads, viz., "flat submembrane cisterns - vesicles - mitochondria", are presumably involved in the formation of the invaginations and double-membrane vesicles and in neuroglial exchange. The tubular lattice in the glial cytoplasm might transfer ions and metabolites between the glial layers. The integrity of the neuronal and glial membranes is impaired in some places. However, free neuroglial passage might be prevented or limited by the dense diffuse material accumulated in these regions. Thus, neuroglial exchange with cellular components might be mediated by transmembrane diffusion, especially in the invaginations and submembrane cisterns, by the formation of double-walled vesicles in which large glial masses are captured and by transfer through tubular lattices.

Regenerating crayfish motor axons assimilate glial cells and sprout in cultured explants

Phasic and tonic motor nerves originating from crayfish abdominal ganglia, in 2-3-day-old cultured explants, display at their transected distal ends growth zones from which axonal sprouts arise. The subcellular morphology of this regenerative response was examined with thin serial-section electron microscopy and reveals two major remodeling features. First, the external sprouts that exit the nerve are a very small part of a much more massive sprouting response by individual axons comprising several orders of internal sprouts confined to the nerve. Both internal and external sprouts have a simple construction: a cytoskeleton of microtubules and populations of mitochondria, clear synaptic vesicles, membranous sacs, and extrasynaptic active zone dense bars, features reminiscent of motor nerve terminals. Close intermingling of the sprouts of several axons give rise to a neuropil-like arbor within the nerve. Thus, extensive sprouting is an intrinsic response of crayfish motor axons to transection. Second, an equally dramatic remodeling feature is the appearance of nuclei, which resemble those of adjacent glial cells, within the motor axons. These nuclei often appear where the adjoining membranes of the axon and glial cell are disrupted and where free-standing lengths of the double membrane are present. These images signify a breakdown of the dividing membranes and assimilation of the glial cell by the axon, the nucleus being the most visible sign of such assimilation. Thus, crayfish motor axons respond to transection by assimilating glial cells that may provide regulatory and trophic support for the sprouting response.

Remodeling of the proximal segment of crayfish motor nerves following transection

Transected crustacean motor axons consist of a soma-endowed proximal segment that regenerates and a soma-less distal segment that survives for up to a year. We report on the anatomical remodeling of the proximal segment of phasic motor nerves innervating the deep flexor muscles in the abdomen of adult crayfish following transection. The intact nerve with 10 phasic axons and its two branches with subsets of 6 and 7 of these 10 axons undergo several remodeling changes. First, the transected nerve displays many more and smaller axon profiles than the 6 and 7 axons of the intact nerve, approximately 100 and 300 profiles in the two branches of a preparation transected 8 weeks previously. Serial images of the transected nerve denote that the proliferation of profiles is due to several orders of axon sprouting primary, secondary, and tertiary branches. The greater proliferation of axon sprouts, their smaller size, and the absence of intervening glia in the one nerve branch compared with the other branch denote that sprouting is more advanced in this branch. Second, the axon sprouts are regionally differentiated; thus, although in most regions the sprouts are basically axon-like, with a cytoskeleton of microtubules and peripheral mitochondria, in some regions they appear nerve terminal-like and are characterized by numerous clear synaptic vesicles, a few dense-core vesicles, and dispersed mitochondria. Both regions possess active zone dense bars with clustered synaptic vesicles found opposite other sprouts, glia, hemocytes, and connective tissue, but because the opposing membranes are not differentiated into a synaptic contact, the active zones are extrasynaptic. Third, some of the transected axons display a glial cell nucleus denoting assimilation of an adaxonal glial cell by the transected axons. Fourth, within the nerve trunk are a few myocytes and muscle fibers. These most likely originate from adjoining and intimately connected hemocytes, because such transformation occurs during muscle repair. In a crustacean nerve, however, where muscle is clearly misplaced, its presence implies an instructive role for motor nerves in muscle formation.

Electrophysiological and ultrastructural changes in severed motor axons of the crayfish

In many invertebrates, distal stumps of severed axons degenerate slowly and survive for long periods of time; this lengthy process allows the study of the physiological and structural changes involved in axonal degeneration processes. The following experiments demonstrate a reduction in EPSP amplitude, an increase in the distance between neighboring release sites, an extended duration of transmitter release, and a doubling in the average number of quanta released per stimulus at each release site. Ultrastructural examination of those stumps revealed various degrees of glial cell invasion. In the same distal stump, some axons were partially filled with glial cells, but adjacent axons could be completely filled by them. Glial cell invasion was greater at regions closer to the site of axotomy and increased as time progressed. The glia engulfing the stumps exhibited hypertrophy and changes in nuclear morphology. The nuclei of some of those glia cells were unusually close to the axonal membrane in the distal stumps. In spite of these severe morphological changes, the stumps were still capable of conducted action potentials and releasing transmitter at their synapses.

Regeneration of phasic motor axons on a crayfish tonic muscle: neuron specifies synapses

Motor neurons are matched to their target muscles, often forming separate phasic and tonic systems as in the abdomen of crayfish where they are used for rapid escape and slow postural movements, respectively. To assess the role of motor neuron and muscle fiber in forming synapses we attempted a mismatch experiment by allotransplanting a phasic nerve attached to its ganglion to a denervated tonic muscle. Regenerating motor axons sprouted 10-30 branches (typical of phasic motor neurons, as tonic ones sprout far fewer branches) to reinnervate muscle fibers and form synapses that produced large excitatory postsynaptic potentials (typical of phasic motor neurons, as tonic synapses give small potentials). Therefore motor neurons, not muscle fibers, appear to specify one of the major properties of regenerating neuromuscular synapses.

Muscle fibers in regenerating crayfish motor nerves

Single discrete muscle fibers were found in regenerating motor nerves in adult crayfish. The regenerating nerves were from native or transplanted ganglia in the third abdominal segments and consisted of several motor axons. The proximal end of these motor axons showed numerous sprouts. Muscle fibers in these regenerating nerves appeared newly developed and were innervated by excitatory nerve terminals. A likely source of these novel muscle fibers may be blood cells in the nerve or satellite cells from neighboring muscle. Contacts made by axon sprouts with other axon sprouts, glia, and muscle fiber, in the form of a dense bar with clustered clear vesicles, characterized the regenerating nerve. These contacts may provide a possible signaling pathway for axon regeneration and myogenesis.

Opiate signaling regulates microglia activities in the invertebrate nervous system

1. Evidence supporting the presence in the invertebrate nervous system of a class of glial cells resembling vertebrate microglia was obtained in the freshwater snail Planorbarius corneus. These cells are easily identified by their immunopositivity to anti-pro-opiomelanocortin (POMC)-derived peptide antibodies. 2. Invertebrate microglia, as in vertebrates, exhibit macrophage-like activity in vivo and in cell cultures. These cells respond to the trauma of ganglionic excision and their organotypic culture by leaving their location around neurons and moving to the lesion site from which they migrate in the culture dish. 3. In vitro, these microglia undergo conformational changes and show phagocytic properties in the presence of bacteria or lipopolysaccharide. The activated cells also express tumor necrosis factor-alpha-like material and an increase in nitric oxide synthase, as shown by immunocytochemistry. 4. The inhibitory effect of morphine on the mobility and phagocytic activity of invertebrate microglia provide additional functional evidence for a possible role of opiate-like compounds in downregulating immunoregulatory processes, as also observed in the circulating immunocytes.

Structure of allotransplanted ganglia and regenerated neuromuscular connections in crayfish

In adult crayfish, Procambarus clarkii, motoneurons to a denervated abdominal superficial flexor muscle regenerate long-lasting and highly specific synaptic connections as seen from recordings of excitatory postsynaptic potentials, even when they arise from the ganglion of another crayfish. To confirm the morphological origins of these physiological connections we examined the fine structure of the allotransplanted tissue that consisted of the third abdominal ganglion and the nerve to the superficial flexor muscle (the fourth ganglion and the connecting ventral nerve cord were also included). Although there is considerable degeneration, the allotransplanted ganglia display intact areas of axon tracts, neuropil, and somata. Thus in both short (6-8 weeks) and long (24-30 weeks) term transplants approximately 20 healthy somata are present and this is more than the five axons regenerated to the host muscle. The principal neurite and dendrites of these somata receive both excitatory and inhibitory synaptic inputs, and these types of synaptic contacts also occur among the dendritic profiles of the neuropil. Axon tracts in the allotransplanted ganglia and ventral nerve cord consist largely of small diameter axons; most of the large axons including the medial and lateral giant axons are lost. The transplanted ganglia have many blood vessels and blood lacunae ensuring long-term survival. The transplanted superficial flexor nerve regenerates from the ventral to the dorsal surface of the muscle where it has five axons, each consisting of many profiles rather than a single profile. This indicates sprouting of the individual axons and accounts for the enlarged size of the regenerated nerve. The regenerated axons give rise to normal-looking synaptic terminals with well-defined synaptic contacts and presynaptic dense bars or active zones. Some of these synaptic terminals lie in close proximity to degenerating terminals, suggesting that they may inhabit old sites and in this way ensure target specificity. The presence of intact somata, neuropil, and axon tracts are factors that would contribute to the spontaneous firing of the transplanted motoneurons.

Resealing of the proximal and distal cut ends of transected axons: electrophysiological and ultrastructural analysis

The fates of the proximal and distal segments of transected axons differ. Whereas the proximal segment usually recovers from injury and regenerates, the distal segment degenerates. In the present report we studied the kinetics of the recovery processes of both proximal and distal axonal segments following axotomy and its temporal relations to the alterations in the cytoarchitecture of the injured neuron. The experiments were performed on primary cultured metacerebral neurons (MCn) isolated from Aplysia. We transected axons while monitoring the changes in transmembrane potential and input resistance (Rn) by inserting intracellular microelectrodes into the soma and axon. Correlation between the electrophysiological status of the injured axon and its ultrastructure was provided by rapid fixation of the neuron at selected times postaxotomy. Axotomy leads to membrane depolarization from a mean of -55.7 S.D. 12.8 mV to -12.7 S.D. 3.3 mV and decreased Rn from tens of M omega to 1-3 M omega. The transected axons remained depolarized for a period of 10-260 s for as long as the axoplasm was in direct contact with the bathing solution. Rapid repolarization and partial recovery of Rn was associated with the formation of a membrane seal over the cut ends by the constriction and subsequent fusion of the axolema. Prior to the formation of a membraneous barrier, electron-dense deposits aggregate at the tip of the cut axon and appear to form an axoplasmic "plug." Electrophysiological analysis revealed that this "plug" does not provide resistance for current flow and that the axoplasmic resistance is homogenously distributed. The kinetics of injury and recovery processes as well as the ultrastructural changes of the proximal and distal segments are identical suggesting that the different fates of the segments cannot be attributed to differences in the immediate response of the segments to axotomy.

Functional degeneration of isolated central stumps of crayfish sensory axons

In the crayfish, Procambarus clarkii, nerve 5 carries primarily sensory axons from the tail fan to the 6th abdominal ganglion where they synaptically activate interneuron A. Since the sensory neurons have their somata located at the periphery, transection of nerve 5 part way to the ganglion allowed us to examine the fate of their soma-less central stumps. Up to 3 weeks postlesion the response to stimulation of nerve 5 consisted of a brief latency spike in interneuron A, similar to that in control animals and to stimulation of the intact nerve 4. Stimulation of the lesioned nerve 5 beyond 3 weeks failed to fire interneuron A. This loss of function was correlated to loss of axons in nerve 5 deduced by comparing the numbers in the lesioned nerve 5 to its contralateral intact counterpart. The numbers are about equal in the paired nerves but rapidly decline on the lesioned side to 50% within 1 week, 20% within 3 weeks, and less than 10% in subsequent weeks. This loss affects all size classes of axons. However, in the 3 week lesioned nerve large glial infoldings subdivided some of the larger axons and single nuclei were seen in a few of the medium-sized axons. Possibly subdivision of large axons by glial infolding may introduce glial nuclei into axons.

Long-term survival of severed crayfish giant axons is not associated with an incorporation of glial nuclei into axoplasm

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.

Long-term survival of anucleate axons and its implications for nerve regeneration

Severed distal segments of nerve axons (anucleate axons) have now been reported to survive for weeks to years in representative organisms from most phyla, including the vertebrates. Among invertebrates (especially crustaceans), such long-term survival might involve transfer of proteins from adjacent intact cells to anucleate axons. In lower vertebrates and mammals, long-term survival of anucleate axons is more likely attributed to a slow turnover of axonal proteins and/or a lack of phagocytosis by macrophages or other cell types. Invertebrate anucleate axons that exhibit long-term survival are often reactivated by neurites that have grown from proximal nucleate segments. In mammals, induction of long-term survival in anucleate axons might allow more time to use artificial mechanisms to repair nerve axons by fusing the two severed halves with polyethylene glycol, a technique recently developed to fuse severed halves of myelinated axons in earthworms.

Effect of temperature on long-term survival of anucleate giant axons in crayfish and goldfish

The effect of temperature on the electrophysiology and morphology of anucleate axons was examined following severance of crayfish medial giant axons and goldfish Mauthner axons from their respective cell bodies. Although anucleate segments of each giant axon exhibited long-term survival for weeks to months at 5-25 degrees C in crayfish and 10-30 degrees C in goldfish, the two axons differed in their survival characteristics. All measures of long-term survival in crayfish medial giant axons were independent of animal holding temperature, whereas all measures in Mauthner axons were dependent on holding temperature. Medial giant axons survived for at least 90 days in crayfish maintained at 5-25 degrees C in this and previous studies. Mauthner axons survived for over 5 months in goldfish maintained at 10 degrees C but survived for 1 month at 30 degrees C. Postoperative time had different effects on many single measures of long-term survival (axonal diameter, amplitude of action or resting potentials) in medial giant axons compared to Mauthner axons. For example, resting and action potentials in crayfish medial giant axons remained remarkably constant at all holding temperatures for 0-90 postoperative days. In contrast, resting and action potentials in goldfish Mauthner axons declined abruptly in the first 10-20 postoperative days followed by a slower decline at each holding temperature. We suggest that the mechanism of long-term survival is not necessarily the same in all anucleate axons.

Ultrastructure of an identified molluscan neuron in organ culture and cell culture following axotomy

We examined the ultrastructure of neuron 5 from the buccal ganglion of the mollusc Helisoma trivolvis after axotomy and organ culture, and after isolation of the same neuron in culture. Buccal ganglia containing axotomized neurons 5 were cultured either in host snails or in Leibovitz medium conditioned with ganglia. In addition, some neurons 5 were isolated from buccal ganglia by micro-dissection and plated into culture. Neuron 5 and its processes were identified in both whole mounts and plastic sections of buccal ganglia after intracellular injection with Lucifer Yellow or horseradish peroxidase. Five days after axotomy of neuron 5, thick sections of buccal ganglia stained with toluidine blue revealed that densely staining basophilic bodies (Nissl bodies) within the cytoplasm had dispersed, i.e., they had undergone chromatolysis. Coincident with chromatolysis was an overall increase in diffuse basophilic staining within the cytoplasm of neuron 5 when maintained in organ culture. The dispersion of Nissl bodies viewed by light microscopy correlated with a more freely arranged rough endoplasmic reticulum and associated polysomes within neuron 5 as seen by electron microscopy. Isolated neurons 5 did not possess densely staining Nissl bodies when examined after 2 days in vitro, thus indicating that chromatolysis occurred earlier in isolated neurons. Furthermore, no increase in diffuse cytoplasmic basophilia was observed within isolated neurons 5 cultured in vitro. However, isolated neurons 5 exhibited a marked increase in the number of lipid-like bodies (0.5-1.5 micron in diameter) that were particularly evident in scanning electron micrographs. Scanning and transmission electron micrographs revealed that the isolated neurons were free of associated glia, but non-neuronal cells (hemocytes) would attach themselves to the somata and neurites. Glia surrounding neuron 5 within buccal ganglia exhibited a marked hypertrophy following axotomy and organ culture. Hypertrophy of glia was absent, however, if ganglia were axotomized and left within the animal or axotomized ganglia were implanted into host animals and examined 5 days later by electron microscopy. These observations indicate that, following axotomy, a molluscan neuron may exhibit different morphological features depending on its microenvironment. In addition, the hypertrophy of glia surrounding neurons in Helisoma was not associated with axotomy per se, but with organ culture.

Ultrastructural changes in glial cells during regeneration of cockroach peripheral nerve

After peripheral nerve 5 in the cockroach Periplaneta americana was cut, changes occurring in the glial cells in the proximal stump were studied immediately after damage and during the process of nerve regeneration. During the first week haemocytes accumulated outside the nerve and morphologically similar granule-containing cells appeared inside the nerve. These cells were involved in phagocytic activity. Between the second and the fourth week, signs of regeneration were distinguishable; many small axonal sprouts were formed which were surrounded by glial processes, and the nerve stump increased in length. During this period the glial cells produced large amounts of extracellular material in which the bundles of axons and glia were embedded. The structural differences between glial and perineurial cells were lost during these stages of regeneration and there was no restriction to the penetration of the extracellular tracer lanthanum. After 8 weeks, reinnervation of the muscles had taken place, perineurial and glial cells were again distinguishable, and the perineurial cells were able to exclude lanthanum.

An epithelium-type cytoskeleton in a glial cell: astrocytes of amphibian optic nerves contain cytokeratin filaments and are connected by desmosomes

In higher vertebrates the cytoskeleton of glial cells, notably astrocytes, is characterized (a) by masses of intermediate filaments (IFs) that contain the hallmark protein of glial differentiation, the glial filament protein (GFP); and (b) by the absence of cytokeratin IFs and IF-anchoring membrane domains of the desmosome type. Here we report that in certain amphibian species (Xenopus laevis, Rana ridibunda, and Pleurodeles waltlii) the astrocytes of the optic nerve contain a completely different type of cytoskeleton. In immunofluorescence microscopy using antibodies specific for different IF and desmosomal proteins, the astrocytes of this nerve are positive for cytokeratins and desmoplakins; by electron microscopy these reactions could be correlated to IF bundles and desmosomes. By gel electrophoresis of cytoskeletal proteins, combined with immunoblotting, we demonstrate the cytokeratinous nature of the major IF proteins of these astroglial cells, comprising at least three major cytokeratins. In this tissue we have not detected a major IF protein that could correspond to GFP. In contrast, cytokeratin IFs and desmosomes have not been detected in the glial cells of brain and spinal cord or in certain peripheral nerves, such as the sciatic nerve. These results provide an example of the formation of a cytokeratin cytoskeleton in the context of a nonepithelial differentiation program. They further show that glial differentiation and functions, commonly correlated with the formation of GFP filaments, are not necessarily dependent on GFP but can also be achieved with structures typical of epithelial differentiation; i.e., cytokeratin IFs and desmosomes. We discuss the cytoskeletal differences of glial cells in different kinds of nerves in the same animal, with special emphasis on the optic nerve of lower vertebrates as a widely studied model system of glial development and nerve regeneration.

The roles of haemocytes during degeneration and regeneration of crayfish muscle fibres

Crayfish haemolymph contains three types of haemocytes with cytoplasmic granules: coagulocytes, granulocytes and amoebocytes. Muscle degeneration was induced by either a gross mechanical injury or a mild puncture injury of m. extensor carpopoditi. Granulocytes and amoebocytes were involved in the phagocytosis of disintegrating muscle fibres. Within three weeks after the gross injury the first myotubes were found. The formation of regenerated fibres started before the degenerating material was removed completely. Mild injury resulted in the formation of contraction clots, localized at the ends of a fibre and connected to a persistent external lamina in the form of an empty sheath. The external lamina sheaths were invaded by amoebocytes. They arranged themselves into a superficial layer similar to an epithelium, formed gap junctions and zonulae adherentes, and showed an increase in the number of cytoplasmic microtubules. These transformed haemocytes retained their ability to engulf material of the disintegrating fibre. In about three weeks the number of microtubules in the transformed haemocytes decreased, and newly formed contractile filaments appeared. Satellite cells are present along the normal crayfish muscle fibres. Following their activation in degenerated material, they might conceivably induce the transformation of haemocytes into myogenic cells.

Long-term persistence of GAD activity in injured crayfish CNS tissue

Crayfish CNS fibers were isolated in vivo from their cell bodies, from cellular connections in the CNS, and from peripheral sensory and effector cells. The glutamic acid decarboxylase (GAD) activity of the experimental tissues was about half of that of the sham-operated and unoperated control tissues by two weeks after surgery and remained at about that level during the ensuing six weeks. During that time, there was no significant behavioral, electrophysiological, or histological evidence of regeneration of nerve fibers across the lesion sites. The crush-isolated connectives possessed many intact axon profiles and non-neuronal cell nuclei. The long-term persistence of GAD activity in the injured CNS tissue may reflect the involvement of glial cells in maintaining neurotransmitter levels.

Developmental and other factors affecting regeneration of crayfish CNS axons

According to histological and ultrastructural criteria, nongiant CNS axons in newly hatched crayfish regenerate more rapidly and with greater frequency than do similar axons in adult crayfish. Regenerative ability is greater in one species (Procambarus clarkii) than in another species (Procambarus simulans), is greater at 20-25 degrees C than at 15-16 degrees C, and is greater in nongiant axons than in giant axons. In contrast to axonal regeneration, nerve cell bodies do not regenerate in newly hatched or adult crayfish of either species. While the ability to regenerate CNS axons differs between newly hatched and adult crayfish, the ultrastructural appearance of the CNS is very similar at any age it is examined.

Morphological and physiological survival of goldfish Mauthner axons isolated from their somata by spinal cord crush

Axon segments isolated from their somata degenerate within days or months depending on species and neuronal type. To better understand the time course of morphological and physiological changes associated with degeneration of axon segments of vertebrate central neurons, we have studied the goldfish Mauthner axon (M-axon) when it has been separated from its soma by spinal cord crush. M-axon segments survive morphologically for at least 77 days at 14 degrees C. Cross-sectional areas of isolated M-axon segments (measured 25-30 mm caudal to the wound site at postoperative days 64 and 77) were greater than those of control axons at the same level. Sheath areas did not change. Electron microscopic observations at the same spinal cord location indicated no clear changes in the configuration or number of neurofilaments or any other organelle. M-axon segments studied morphologically after 87 postoperative days had all degenerated. Mauthner axon segments were capable of conducting action potentials and eliciting ipsilateral EMG responses. Repetitive firing of the M-axon segments elicited EMG responses that fatigued more easily and remained fatigued over a longer interval than did those of control axons. The long duration of M-axon segment survival is unusual in a vertebrate and may be due to the low temperature at which the experiments were conducted (14 degrees C) and/or temperature-independent factors. The increased susceptibility to synaptic depression, which has not reported previously, may represent an early sign of the degenerative process.

Promoting functional plasticity in the damaged nervous system

Damage to the central and peripheral nervous system often produces lasting functional deficits. A major focus of neuroscience research has been to enhance functional restitution of the damaged nervous system and thereby produce recovery of behavioral or physiological processes. Promising procedures include surgical, physical, and chemical manipulations to reduce scar formation and minimize the disruption of support elements, administration of growth-stimulating substances, tissue grafts to bridge gaps in fiber pathways, and embryonic brain tissue grafts to provide new cells with the potential to generate fiber systems. Two elements are required for functional nervous system repair: (i) neurons with the capacity to extend processes must be present, and (ii) the regenerating neurites must find a continuous, unbroken pathway to appropriate targets through a supportive milieu.

Glial polypeptides transferred into the squid giant axon

The proteins synthesized by the glial sheath of an isolated segment of squid giant axon and by the cell bodies of the giant axon in the isolated stellate ganglion were labeled by incubation in the presence of [3H]leucine. The axoplasm, which contained labeled proteins transferred from the glial sheath, was separated from the sheath by mechanical extrusion. The labeled proteins in the axoplasm, the empty sheath and the stellate ganglion were analyzed and compared by one- and two-dimensional polyacrylamide gel electrophoresis. Over 80 glial polypeptides were found to be selectively transferred into the axoplasm and many of these were distinct from stellate ganglion polypeptides which presumably could be supplied to the axon via axonal transport. Three of the more highly labeled transferred glial polypeptides (TGPs) were actin, a fodrin-like polypeptide and a polypeptide we have named traversin. Our observations, considered in the context of other reports, suggest that the squid axon receives a large number of polypeptides from its surrounding glia either by phagocytozing glial cell process that project into it or via cytoplasmic channels between adaxonal glia and the axon. These TGPs may help the axon survive unfavorable conditions.

Long-term survival of centrally projecting axons in the optic nerve of the frog following destruction of the retina

A significant number of unmyelinated axons and their synaptic endings in the frog, Rana pipiens, were found to retain a normal morphology long after separation from their cell bodies. At the end of various survival periods following unilateral removal of the retina, horseradish peroxidase (HRP) was administered to the optic nerve stump by a fiber-filling method. In frogs maintained at 20 degrees C, unmyelinated optic nerve axons conducted HRP from the site of application in the orbit to layers A, C, and E of the contralateral optic tectum, even though their retinas had been removed up to 69 days earlier. Such fiber-filling was absent beyond 19 days in other frogs surviving at 35 degrees C. No labeled fibers were continuous with any intracerebral neurons. The HRP was always localized intraaxonally, and the marked axons and terminals were ultrastructurally normal. Counts of surviving axons from electron micrographs of the optic nerves showed that, at 20 degrees C, more than half of the normal complement of unmyelinated axons disappeared in the first 10 days. All the myelinated axons degenerated during the first 6 weeks survival. However, approximately 55,000 normal-appearing unmyelinated axons (12% of the unmyelinated fiber population) persisted in the optic nerve at 10 weeks following removal of the retina. The survival rate was lower at 35 degrees C. In other frogs, one eye was injected with 3H-leucine to initiate axonal transport into the retinal ganglion cell axons. That eye was removed 48 hours later. Autoradiographic analysis of brain sections of frog surviving an additional 31 to 61 days at 20 degrees C showed strong labeling of the optic tract and layers A, C, and E of the contralateral optic tectum. The absence of displaced ganglion cells that might exist within the optic nerve was verified by other observations. It is hypothesized that the potential shown by frog optic axons for long-term survival in the absence of the cell-body expresses a general property of vertebrate (and invertebrate) axons, rather than a special property of the frog optic nerve.

Long term survival of enucleated segments of glial cytoplasm in the leech Macrobdella decora

Enucleated cytoplasmic segments of the giant connective glial cell (GCGC) survive morphologically intact for at least 10 weeks in the leech Macrobdella decora. Enucleated GCGC segments isolated from regenerating nerve axons show some degenerative changes after 4 weeks compared to GCGC segments which surround intact or regenerating nerve axons. Survival of GCGC cytoplasm is associated with an increase in the number of microglia. Relatively few (10-30%) nerve axons degenerate after severance from their cell body.

Neuronal repair and avoidance behavior in the flatworm, Notoplana acticola

In Notoplana avoidance behavior is lost after bisection of the brain or removal of one of its lobes. Behavioral recovery usually occurs within 3-10 days. Recovery of individuals may be gradual or abrupt. Grouped data shows gradual linear repair of turning behavior. Most animals with all connectives between the two lobes of the brain severed recovered preoperative responses, while those with one lobe of the brain removed averaged about 60% of the preoperative level of response. Some individuals in both groups recovered completely. Histological evidence of neuronal repair was found in all animals. Where the lobes of the brain were separated, connectives between them appeared to re-form. In worms with one lobe of the brain removed, the nerves disconnected by the excision joined the remaining lobe. Action potentials are conducted across repaired tissue in both split-brain and half-brain worms in both seawater and Mg2+-rich solutions. CNS repair appears to involve functional synaptic contacts. Notoplana does not replace ganglionic tissue but does compensate adequately for CNS damage.

Ultrastructural studies of severed medial giant and other CNS axons in crayfish

The distal stumps of severed medial giant axons (MGAs) and of nongiant axons (NGAs) in the CNS of the crayfish Procambarus clarkii show long-term (5--9 months) survival associated with disorientation of mitochondria and thickening of the glial sheath. However, the morphological responses of the two axonal types differ in that neither the proximal nor the distal stump of severed MGAs ever fills with mitochondria as is observed in some severed NGAs. Furthermore, the adaxonal glial layer never completely encircles portions of MGA axoplasm as occurs in many severed NGAs; in fact, ultrastructural changes in the adaxonal layer around severed MGAs are often difficult to detect. No multiple axonal profiles are ever seen within the glial sheath of the proximal or distal stumps of severed MGAs whereas these structures are easily located within severed NGAs.

Ribosomal RNA in Mauthner axon: implications for a protein synthesizing machinery in the myelinated axon

RNA was extracted from myelin-free Mauthner axons of the goldfish on a microscale and fractionated by microelectrophoresis. Microextracts showed the presence of nominal 26 SE, 18 SE, 5 SE and 4 SE components, which co-migrated with rRNA from fish brain. In addition, a non-ribosomal 15 SE component was present in axon microextracts, but not in RNA extracts of fish brain or of myelin sheath from Mauthner axon, indicating an unusual enrichment of a putative mRNA class. Evidence was presented to support the contention that axonal rRNA was not due to contamination from the myelin sheath. Possible reasons for the lack of ultrastructural evidence for axoplasmic ribosomes are discussed.

The morphological and physiological properties of a regenerating synapse in the C.N.S. of the leech

Regeneration of an electrical synapse between particular interneurons in the medicinal leech was traced physiologically and morphologically using intracellular recording the horseradish peroxidase (HRP) injection. The synapse between S-cell interneurons lies in the connective midway between segmental ganglia, so crushing near one ganglion severs only one S-cell's axon. The severed distal stump remains connected to the adjacent uninjured S-cell and continues for weeks to conduct impulses. The injured cell regenerates, while its uninjured "target" neuron in the next ganglion does not grow. After the sprouts of the regenerating neuron cross the crush, one or a few branches grow along the surviving distal stump toward the original synapse. After about one month when the region of original synapse has been reached, regenerating neurons form electrical junctions and stop growing. Thereafter electrical coupling improves in stages. Within two months the regenerated neuron attains full caliber, the stump degenerates and function is normal. In some instances within days or weeks of crushing, the regenerating neuron forms a basket of synapses upon its severed distal stump and then continues growing to synapse with the target. When this occurs, electrical coupling and subsequent impulse transmission between S-cells rapidly resumes. These experiments indicated that the regenerating neuron is guided to its proper synaptic target by recognizing and following its severed distal stump. Sometimes the distal stump itself becomes an intermediate synaptic target.

Clinical and ultrastructural study of a sporadic case of hereditary sensory neuropathy. Morphological evidence for a slow rate fiber degeneration

A sporadic case of hereditary sensory neuropathy, with a clinical course and severe trophic and sensory alterations typical for type II, is presented. There was a severe loss of myelin in the sural nerve biopsy taken from the ankle. The most impressive microscopic feature was the number of rudimentary onion bulbs with an empty core which contained nude axons in the peripheral schwann cell layers; they were interpreted as vestigial structures left by the former myelinated fibers. Electron microscopy also revealed a definite involvement of unmyelinated fibers with attempted regeneration, which was confirmed by the overrepresentation of small axons on their frequency distribution curve. The whole ultrastructural picture suggested the protracted nature of the fiber involvement. This may be considered as agreeing with the slow course proposed for a system degeneration.

Destruction of a single cell in the central nervous system of the leech as a means of analysing its connexions and functional role

A method has been devised for killing an individual neurone in the C.N.S. of the leech by injecting it with Pronase. The technique has been used to examine the role of individual sensory and motor cells involved in producing reflex movements.1. After a neurone was injected with Pronase, either in an intact animal or an isolated ganglion, its cell body lost its resting and action potentials. Some hours later the injected cell's axons in the periphery failed to conduct impulses. In the intact animal the cell body could no longer be discerned after a few weeks.2. To test for destruction of processes within the neuropile, cells were injected first with the enzyme horseradish peroxidase (HRP) and then several hours later with Pronase. Absence of the characteristic HRP reaction product indicated that Pronase had spread throughout the arborization of the cell.3. Injection of Pronase into one cell did not produce overt electrophysiological or anatomical changes in other cells in the ganglion including neurones that were originally electrically coupled to the killed cell.4. Evidence that an individual cell was the only motoneurone supplying particular muscles was provided by destruction of that cell in otherwise intact animals, which resulted in a characteristic motor deficit in the area supplied by the killed cell. Over a period of months, functional recovery of the affected muscles occurred by way of homologous cells in adjacent ganglia.5. A further application of the technique was to trace the connexion that a particular sensory neurone makes onto two motoneurones that are electrically coupled. Normally, the sensory neurone gives rise to excitatory potentials in both post-synaptic cells. Synaptic potentials could still be recorded in one motor cell after the other had been destroyed by Pronase, indicating that synapses were made directly onto both of the motoneurones.

Histological studies of trophic dependencies in crayfish giant axons

Medial giant (MGA) and lateral giant (LGA) axons of crayfish were doubly cut in order to selectively isolate axonal segments from perikaryal and transsynaptic sources of trophic input. Isolated MGA segments remained morphologically intact for over 43 days, whereas isolated LGA segments usually degenerated within one week. The glial sheaths around isolated MGA segments had significantly increased in thickness within one week, but severed LGA segments showed no increase in sheath thickness at any time after lesioning. These data suggest that cells of the surrounding glial sheath can provide trophic support to isolated MGA segments but not to isolated LGA segments. Extent of glial hypertrophy seems dependent upon specific spatiotemporal parameters. The diameters of isolated MGA segments decreased more rapidly than the diameters of singly cut MGA segments. These data suggest that the MGA also receives some trophic support from pre- or postsynaptic sources. Conversely, some singly cut LGA segments completely degenerated within one week, whereas other singly cut LGA segments remained intact for at least 43 days after lesioning. Such results suggest that the LGA receives a significant trophic input from pre- or postsynaptic structures.

Biochemical studies of trophic dependences in crayfish giant axons

Data from previous histological studies indicate that long-term survival of crayfish medial giant axons might be due in part to trophic support from cells of the surrounding glial sheath which often hypertrophy in response to transection of the medial giants. The biochemical studies reported herein show that segments from transected ventral nerve cords (VNC) always incorporate more [3H]leucine into protein than do corresponding segments from intact VNCs. Furthermore, the relative amount of [3H]leucine incorporation in severed segments seems to be influenced by distance and direction from the lesion site as well as time after lesioning. Similar spatiotemporal parameters were previously shown to be correlated with extent of glial hypertrophy around severed medial giant axons. Quantitative autoradiography of medial giant axons after incubation in [3H]leucine revealed that the grain density of label in glial sheaths surrounding severed medial giants was over two-fold greater than in sheaths around corresponding control axons. Moreover, the grain density in the axoplasm of severed medial giants was nearly four-fold greater than the grain density in the axoplasm of control axons. Data from experiments using short or long labeling intervals suggests that labeling in the medial giant axoplasm may be due more to transfer from glial sheath cells than from inherent axonal synthetic mechanisms. In light of this and other data, we concluded that long-term survival of severed medial giant axons is probably due to the direct transfer of trophic substances from cells of the glial sheath into the axon.

Identification of locally synthesized proteins in proximal stump axons of the neurotomized hypoglossal nerve

Analysis of radioactively labeled proteins was carried out on microsamples of myelin-free axons isolated from neurotomized and normal hypoglossal nerves of the rabbit after in vitro incubation with [3H]leucine. Major radioactive peaks in axonal endings of the proximal stump 18 h after neurotomy, identified by autoradiography of microelectrophoregrams, had apparent molecular weights of 52,000d, 41,000d and 18,000d. This autoradiographic pattern differed from that of axons of untransected nerve in: (1) not having a prominent 100,000d component present in the latter; and (2) having a novel 41,000d component not present in the latter. Labeling of axonal proteins in the proximal stump in vivo resulted in no apparent retrograde transport during the 5 h studied. Autoradiographic analysis of labeled proteins released by nerves into the medium during in vitro incubation revealed a major radioactive peak with an apparent molecular weight of 77,000d. Attempts to demonstrate uptake of the 77,000d component by unlabeled axons in vitro and in vivo were unsuccessful. The hypothesis was advanced that the 52,000d protein may be neurofilament protein, and the possible significance of the 41,000d component was discussed in the contexts of the pathology of the traumatized proximal stump, and of the antecedent events leading to axonal outgrowth.

Ultrastructural basis of impulse propagation failure in a nonbranching axon

The morphological basis of intermittent conduction failure in the excitor axon innervating the crayfish opener and stretcher muscles was investigated using the electron microscope. The connective tissue component of the sheath surrounding the axon was found consistently to be thinner in the region at which blocking occurs than in control regions located one cm either proximally or distally, at which blocking does not occur. Otherwise in these regions differences in the width of the periaxonal spaces, the length or width of the mesaxons, the density of mitochondria, the width of the adaxonal glial cell layer, or the structure of lamination of the sheath are not observed. Because of the thinner connective tissue component of the sheath in the joint region, neighboring axons are distributed more densely around the excitor, and the volume of the extracellular space is reduced. The possibility that the reduced extracellular space might allow excessive accumulation of potassium during repetitive discharge, causing conduction block, is discussed. Alternative mechanisms consistent with this morphology are also considered.

Regenerating afferents establish synapses with a target neuron that lacks its cell body

When the axons of crayfish tail-fan mechanoreceptor neurons are severed, the axons regenerate into the central nervous system and after 2 to 6 weeks reestablish functional contacts with their standard interneuronal target cells. Removal of the cell body and hence the genes of the largest of these interneurons does not interfere with the successful reestablishment of synapses between it and its afferents.

Formation of hemi-desmosomes during regeneration of crayfish nerve root sheath as studied with freeze-fracture

The multilamellate glial sheath of mixed nerve roots of the sixth abdominal ganglion of crayfish contains numerous hemi-desmosomes which appear to attach glial lamellae to material in adjacent extracellular clefts. These junctions, which have been described in detail in an earlier report (Shivers and Brightman, '76), are irregular in shape, punctuate and may be as large as 1 mum in diameter. Surgical interruption of sixth ganglion nerve roots results in regeneration of motor axons and their multilamellate glial sheaths. As the glial processes grow and re-establish a highly organized axon sheath, hemi-desmosomes appear. These junctions are present at the advancing edge of glial processes as well as on their lateral margins. Developing hemi-desmosomes are characterized as a diffuse aggregation of 120-130 A intramembrane particles which are present three weeks following nerve section. As growth and reorganization of the sheath proceeds, the intramembrane particles appear to aggregate and form irregular clusters of varying dimensions. Regenerating nerves freeze-cleaved 8 to 16 weeks following surgery exhibit junctional particle aggregates similar to those in normal unoperated nerve roots. Origin of the intramembrane particles which comprise the junctional aggregated in unknown. Perhaps they are synthesized de novo by the regenerating glial cells or, they may be remnants of complexes which became dispersed following surgery. This is the first report of a freeze-fracture study of hemi-desmosome plasticity in an invertebrate nervous system.

The specificity of re-innervation by identified sensory and motor neurons in the leech

Re-innervation of skin and muscle by identified sensory and motor neurons in segmental ganglia of the leech was studied using physiological techniques. After lesions of peripheral nerves, sensory axons which re-innervated the skin always regained sensitivity to their original stimulus modality (touch, pressure or noxious stimuli). Motor neurons invariably re-innervated the appropriate type of body wall muscle, such as longitudinal or circular muscle layers. Both sensory and motor axons usually returned to the appropriate region of the body wall (dorsal, lateral, or ventral) when regenerating after a nerve crush or cut. This capacity was lost, however, when growth along old nerve branches was prevented by evulsing long segments of the nerve. Re-innervation usually occurred by way of growth of new axons all the way to the periphery, but in a few cases reconnection with the surviving distal segment of the original axon had taken place. The specificity of re-innervation can be accounted for by a combination of selective growth along appropriate nerve branches and specific interactions with target tissues.