De- and regeneration of the bulbospinal serotonin neurons in the rat following 5,6-or 5,7-Dihydroxytryptamine treatment

Summary In an attempt to determine the conditions which permit central 5-HT neurons to respond to a chemical injury of their axons by sprouting and regeneration, the pattern and time-course of recovery of 5-HT concentrations and regrowth of bulbospinal 5-HT axons were evaluated in rats subjected to intraventricular treatment with either 75 g 5,6- or 150 g 5,7-DHT. While 5,6-DHT treatment is followed by a significant recovery of 5-HT concentrations in the telodiencephalon, brainstem and upper part of the spinal cord within 3 months, there is no significant restoration of the severely depleted 5-HT levels in the telodiencephalon and spinal cord, and only limited recovery in 5-HT content of the brainstem preparation after 5,7-DHT.These differences conform to the observation of widespread and effective regrowth and regeneration of the bulbospinal 5-HT neurons in the 5,6-DHT treated lower brainstem and upper spinal cord but restricted and localized sprouting efforts in the 5,7-DHT treated lower medulla oblongata. This could be explained by a cell body near lesion of the non-terminal indoleamine axons by 5,7-DHT which results in a late retrograde, irreversible degeneration of most of the indoleamine pericarya from group B1 and many of group B3.It is concluded that the preservation of a critical length of the main axon and part of its collaterals is necessary for the neuron's survival, and that the individual pattern of the neuropil architecture of brain centres which are invaded by the axonal sprouts may significantly influence their growth characteristics and thus either favour or impede their chance to reestablish connections with their original effector. Aberrant, localized, intense sprouting of drug-damaged axons may in itself reflect the need of the neuron—deprived of most of its axonal tree—to reestablish its original total axonal length by multiple branching.Supported by grants from the Deutsche Forschungsgemeinschaft. The authors are indebted to Rolf Franck for his technical assistance.Supported by grants from the Swedish Medical Research Council (No. 04 X-3874 and 04 X-56).     

Evidence for the existence of serotonin-, dopamine-, and noradrenaline-containing neurons in the gut of Lampetra fluviatilis

Summary Monoamine-containing neurons in the gut of Lampetra fluviatilis are characterized by histochemical, electron microscopical and biochemical methods. Strongly yellow fluorescent, probably serotonin-containing intrinsic neurons are found along the entire length of the intestine. Their processes aggregate to form large bundles of mainly non-terminal axons, constituting a subepithelial fibre plexus. This subepithelial, ganglion cell comprising plexus is connected to a wide-meshed subserosal plexus which has ganglion cells of different size and few varicose, single axons. Intermingled with both plexus there occur — in the anterior and middle but not in the preanal portion of the lamprey intestine — scattered green fluorescent intrinsic perikarya, emanating faintly green fluorescent, poorly varicosed axons.The formaldehyde-induced neuronal fluorophores conform to serotonin (yellow fluorescent compound), noradrenaline, and dopamine (green fluorescent substance), as revealed in microspectrofluorimetric recordings. The electron microscopical analysis of the yellow fluorescent intrinsic neurons in the terminal hindgut shows nerve cell pericarya and axons equipped with a typical population of occasional small granular and many large granular vesicles (750–1600 Å). The number and opacity of cores of the small and the osmiophilia of the cores of the large granular vesicles are significantly increased following short-term treatment with 5,6-dihydroxytryptamine. Long-term treatment with 5,6- or 5,7-dihydroxytryptamine provokes severe signs of ultrastructure impairment and eventual degeneration in the supposed serotonin-containing axons, besides indications of piling-up of organelles in the non-terminal axons due to arrest of axonal transport.Chromatography of acid extracts from the lamprey intestine, gills and kidney reveals the presence of serotonin (besides another unidentified indoleamine) and dopamine and noradrenaline in the gut, but only dopamine in the brain. The detection of serotonin, noradrenaline and dopamine in the lamprey gut is confirmed by chemical determinations.The occurrence of intrinsic serotonin-, noradrenaline- and dopamine-containing neurons in the gut of Lampetra fluviatilis deviates from the established pattern of innervation of the vertebrate intestine and is considered to be a remnant of an autonomic innervation principle common in invertebrates.Supported by grants from the Deutsche Forschungsgemeinschaft.Supported by grants from the Swedish Medical Research Council (No. 14X-712 and 14X-56.The authors are indebted to Lilan Bengtsson, Gertrude Stridsberg, Eva Svensson and Rolf Frank for skilful technical assistance.     

Innervation of iridic melanophores

Summary The nerve supply to the iridic melanophores of the rat was studied with the electron microscope. The adrenergic and cholinergic terminals were identified with the aid of 5-hydroxydopamine, which produces dense-cored 400–800 Å synaptic vesicles in adrenergic axon varicosities, whereas the synaptic vesicles of cholinergic axons remain empty. It was found that both adrenergic and cholinergic terminal axons come in close apposition (200–250 Å) with the melanophores. The appositions have the same appearance as synapses in peripheral tissues. It seems likely that the murine iridic melanophores have a double innervation, although its functional significance is obscure.This work has been supported by grants from Lunds Läkarsällskap, the Swedish Medical Research Council (Project no. B69-14X-2321-02 and B69-14X-712-04C) and NIH (06701-02).     

Effect of oculomotor and trigeminal nerve section on the ultrastructure of different myoneural junctions in the rat extraocular muscles

Summary The intramuscular nerves and myoneural junctions in the rat rectus superior, medialis and inferior muscles from 10 hours to about 10 days after section of the trigeminal and oculomotor nerves were studied with the electron microscope. Two different kinds of myoneural junctions are to be observed; one type derives from myelinated nerves and is similar to the ordinary myoneural junctions (motor end plates) of other striated skeletal muscles, while the other type derives from unmyelinated nerves, is smaller in size and has many myoneural synapses distributed along a single extrafusal muscle fibre.Section of the trigeminal nerve caused no changes in the myoneural synapses. After section of the oculomotor nerve degenerative changes occur in both the myelinated and unmyelinated nerves and in both types of myoneural junctions. In the axon terminals of both the myelinated and unmyelinated nerves the earliest changes are to be observed 10 to 15 hours after section of the nerve. First, swelling of the axoplasm, fragmentation of microtubules and microfilaments and swelling of mitochondria takes place, somewhat later agglutination of the axonal vesicles and mitochondria. The axon terminals are separated from the postsynaptic muscle membrane by hypertrophied teloglial cells about 24 hours after section of the nerve. The debris of the axon terminals is usually digested by the teloglial cells within 42 to 48 hours in both types of myoneural junction.Changes in the postsynaptic membrane are observed in the myoneural junctions of the unmyelinated nerves as disappearance of the already earlier irregular infoldings, whereas no changes take place in the infoldings of the motor end plates. The postsynaptic sarcoplasm and its ribosomal content increase somewhat.The earliest changes occur along unmyelinated axons 10 to 15 hours and along myelinated axons 15 to 24 hours after nerve section. The unmyelinated axons are usually totally digested within 48 hours, whereas the myelinated axons took between 48 hours and 4 days to disappear. The degeneration, fragmentation and digestion of the myelin sheath begin between 24 and 42 hours and still continues 10 days after the operation.The results demonstrate that in the three muscles studied structures underlying the physiologically well known double innervation of the extraoccular muscles are all part of the oculomotor system.We are grateful to Professor Antti Telkkä, M. D. Head of the Electron Microscope Laboratory, University of Helsinki, for permission to use the facilities of the laboratory.     

Effect of colchicine on axonal transport and morphology of retinal ganglion cells

Summary Intraocular injection of colchicine in doses which do not affect the protein synthesis in the retina has profound effects on the axonal transport of protein in the retinal ganglion cells of the rabbit. Rapid axonal transport in these cells is completely inhibited after treatment with relatively low amounts of colchicine. In contrast to this, a certain fraction of the slow axonal transport is resistant to colchicine treatment. Colchicine in doses which completely inhibits fast axonal transport caused discrete morphological changes in the perikaryon and in the axon of the retinal ganglion cell. No disappearance of microtubules and no general proliferation of neurofilaments was observed in the perikaryon of the retinal ganglion cells. There was a slight or moderate increase in the number of filaments in the intra-retinal part of the axons of the retinal ganglion cells.This work has been supported by grants from the Swedish Medical Research Council (B71-12X-2543-03, B71-13X-2226-05A) and the Swedish National Cancer Society (265-B70-02X).     

Fine structure and fluorescence morphology of adrenergic nerves after 6-hydroxydopamine in vivo and in vitro

Summary In parallel fine structural, fluorescence histochemical and biochemical experiments the effect of 6-OH-DA administered in vivo and in vitro on the adrenergic nerves in the mouse iris was studied. As seen in the electron microscope, in vivo administration of 6-OH-DA causes a selective, rapid degeneration of the adrenergic axon terminals similar to that found after axotomy, whereas the cholinergic nerves are unaffected at all time intervals studied. Already 1 hr after the injection of 6-OH-DA the axonal enlargements swell and the size of the dense core of the granular vesicles is strongly reduced. Since the NA stores are almost completely depleted at this time interval, the small core present may be due to a reaction between 6-OH-DA and the fixative. From 2–4 hr after the injection increasing numbers of axonal enlargements with a high electron density are observed in the Schwann cell cytoplasm, which later are digested and completely absent about 48–72 hr after the 6-OH-DA injection. During the following weeks adrenergic axons reappear. This time course of degeneration obtained is considerably faster than that seen after axotomy in other studies. After incubation in 6-OH-DA containing media similar changes were observed in the axonal enlargements, starting already after 30 min of incubation. At this time-point there is a considerable reduction of endogenous NA and a severe damage of the membrane pump uptake mechanism. Incubation with 6-OH-DA and subsequent rinsing for 2 hr caused marked changes, including partly swelling of axons and partly shrinking of the axons into electron dense bodies.The fluorescence histochemical and biochemical results are in good agreement with the ultrastructural studies demonstrating a rapid loss of NA from the adrenergic nerve terminals and main axons and a long lasting depletion of the NA, with a gradual recovery to 75% 6 weeks after the injection.The investigation has been supported by research grants from the Swedish Medical Research Council (14X-2295, 14X-2887 and 04X-3881) Karolinska Institutet, Magnus Bergvalls and Carl-Berthel Nathorst Stiftelser. For generous supplies of drugs we are indebted to the following companies: AB Hässle (6-OH-DA, through Dr H. Corrodi), Pfizer (Niamid^®), Ciba (Serpasil^®). The skilful technical assistance of Miss Bodil Flock, Mrs Waltraut Hiort and Mrs Eva Lindqvist is gratefully acknowledged.     

Soluble Axoplasm Enriched from Injured CNS Axons Reveals the Early Modulation of the Actin Cytoskeleton

Axon injury and degeneration is a common consequence of diverse neurological conditions including multiple sclerosis, traumatic brain injury and spinal cord injury. The molecular events underlying axon degeneration are poorly understood. We have developed a novel method to enrich for axoplasm from rodent optic nerve and characterised the early events in Wallerian degeneration using an unbiased proteomics screen. Our detergent-free method draws axoplasm into a dehydrated hydrogel of the polymer poly(2-hydroxyethyl methacrylate), which is then recovered using centrifugation. This technique is able to recover axonal proteins and significantly deplete glial contamination as confirmed by immunoblotting. We have used iTRAQ to compare axoplasm-enriched samples from naïve vs injured optic nerves, which has revealed a pronounced modulation of proteins associated with the actin cytoskeleton. To confirm the modulation of the actin cytoskeleton in injured axons we focused on the RhoA pathway. Western blotting revealed an augmentation of RhoA and phosphorylated cofilin in axoplasm-enriched samples from injured optic nerve. To investigate the localisation of these components of the RhoA pathway in injured axons we transected axons of primary hippocampal neurons in vitro. We observed an early modulation of filamentous actin with a concomitant redistribution of phosphorylated cofilin in injured axons. At later time-points, RhoA is found to accumulate in axonal swellings and also colocalises with filamentous actin. The actin cytoskeleton is a known sensor of cell viability across multiple eukaryotes, and our results suggest a similar role for the actin cytoskeleton following axon injury. In agreement with other reports, our data also highlights the role of the RhoA pathway in axon degeneration. These findings highlight a previously unexplored area of axon biology, which may open novel avenues to prevent axon degeneration. Our method for isolating CNS axoplasm also represents a new tool to study axon biology.     

Ibotenic acid-induced lesions of striatal target and projection neurons: ultrastructural manifestations in dopaminergic and non-dopaminergic neurons and in glia

The cytological changes elicited by central microinjections of the excitotoxin, ibotenic acid (IBO) were examined in the adult rat striatonigral system using electron microscopic immunocytochemistry. The chemical markers included tyrosine hydroxylase (TH), a biosynthetic enzyme in dopaminergic neurons, and glial fibrillary acidic protein (GFAP). Both short (1-7 day) and long (30-60 days) term effects were evaluated at the site of IBO-injections in the striatum and more distally in the substantia nigra, which both contributes afferents and receives efferents from the striatum. In the neostriatum at every survival period examined, TH-labeled axonal processes appeared equally numerous in the control and IBO-injected hemispheres. However, the TH-labeled axons in the striatum ipsilateral to the IBO-injection were slightly enlarged, and generally lacked synaptic densities. In the early period the remaining neuropil showed signs of edema and contained perikarya and dendrites with vacuolar or dense cytoplasm as well as intact, unlabeled terminals. Numerous astrocytes, and apparently demyelinated axons were more commonly seen at the 7 day period. At 30 and 60 days, bundles of myelinated axons, unlabeled axon terminals, and astrocytes containing a variety of cytosomes and other cytoplasmic inclusions were in close apposition to TH-labeled axon terminals. These results suggest that the dopaminergic terminals may serve neuromodulatory functions with respect to glia or other afferent axons remaining after IBO-injections in the striatum. In the substantia nigra, homolateral to the injection, a dense type of degeneration was seen in a few perikarya and dendrites at 7 days of survival. At this stage, electron dense anterograde degeneration also was seen in terminals contacting both TH-labeled and unlabeled dendrites. The secondary long term changes in nuclear groups located distal to the primary lesion are characteristic of certain types of progressive human neuropathological disorders.     

Ultrastructural studies on neuromuscular contacts and the formation of junctions in the flight muscle of Antheraea polyphemus (Lep.)

In the moth Antheraea polyphemed at the onset of adult development. The subsequent breakdown of the isolated motor stulongated vesicles similar in structure to channels of smooth ER, appear in large numbers in the axoplasm. Their nature as well as the functional aspects of early axonal changes are discussed. From the 7th day onward two types of axonal breakdown become prominent. The first is characterized 0y swelling axon profiles, distorted vesicles and strongly shrunken mitochondria, uhile shrinking axon profiles containing tightly packed mitochondria and unaltered vesicles are typical of the second. Both types presumably take place independently of each other in different axon terminals. Axons and the contents of at least the first type are finally removed by transformation into lamellar bodies. Glial processes obviously behave independently of degenerating terminals; they loose any contact with them and never act as phagocytes for axon remnants. During the whole period of breakdown undifferentiated contacts between nerve fibers and muscle anlagen are present but synaptic structures as in normal developing dlm have never been observed. This fact, in comparison with earlier studies, suggests a lack of trophic nervous activity on the muscle anlagen tissue. A short time after removal of the isolated stumps new nerve tracts appear between dlm-fibers (which are, of course, strongly retarded in development). They are presumably sensory wing nerves which lack a guide structure to the central target, due to axotomy. Neuromuscular contacts or even junctions formed by axons of these nerves have occasionally been detected on the dlm. Their nature is discussed. Wallerian axon degeneration is compared to the normal, metamorphic breakdown of the innervation of the larval dlm-precursor. In contrast to the former, glial processes here remain in contact with the terminals. Glia and axons first swell. Then most glial processes are transformed into lamellar bodies whereas neurites shrink and become electron-dense. Axonal organelles remain intact for a long period.     

Antidromic propagation of action potentials in branched axons: implications for the mechanisms of action of deep brain stimulation

Electrical stimulation of the central nervous system creates both orthodromically propagating action potentials, by stimulation of local cells and passing axons, and antidromically propagating action potentials, by stimulation of presynaptic axons and terminals. Our aim was to understand how antidromic action potentials navigate through complex arborizations, such as those of thalamic and basal ganglia afferents-sites of electrical activation during deep brain stimulation. We developed computational models to study the propagation of antidromic action potentials past the bifurcation in branched axons. In both unmyelinated and myelinated branched axons, when the diameters of each axon branch remained under a specific threshold (set by the antidromic geometric ratio), antidromic propagation occurred robustly; action potentials traveled both antidromically into the primary segment as well as "re-orthodromically" into the terminal secondary segment. Propagation occurred across a broad range of stimulation frequencies, axon segment geometries, and concentrations of extracellular potassium, but was strongly dependent on the geometry of the node of Ranvier at the axonal bifurcation. Thus, antidromic activation of axon terminals can, through axon collaterals, lead to widespread activation or inhibition of targets remote from the site of stimulation. These effects should be included when interpreting the results of functional imaging or evoked potential studies on the mechanisms of action of DBS.     

Morpho-functional changes in the turtle midbrain tectum after enucleation

Morpho-functional changes in the tectum mesencephali during degeneration after enucleation were studied inEmys orbicularis L. Comparison of amplitude-time characteristics of evoked potentials of the visual center with degenerative changes in axon terminals and fibers of the optic nerve in the same animals revealed a "light" type of degeneration of the terminals of unmyelinated axons and a "dark" type in terminals of myelinated axons. During "dark" degeneration (4–5 months after enucleation) the low-amplitude presynaptic component of the evoked potential, reflecting excitation of large myelinated fibers, disappeared and changes occurred in the characteristics of the first high-amplitude component, the appearance of which is connected with excitation of myelinated fibers of medium diameter. The last component disappeared 7 months after the operation, along with disappearance of the "dark" degeneration. During "light" degeneration (2.5–3.5 months) changes took place in the characteristics of the second high-amplitude component of the evoked potential, which reflects excitation of thin fibers, both myelinated and unmyelinated, whose ranges of diameters overlap. This component disappeared after 6–7 months, almost simultaneously with disappearance of the first high-amplitude component, as the result of simultaneous completion of degeneration of myelinated fibers of medium and small diameter.     

Development of axonal membrane specializations defines nodes of Ranvier and precedes Schwann cell myelin elaboration

The development of the node of Ranvier has been previously described using thin-section electron microscopy. Using freeze-fracture, we have examined the development of glial and axonal membrane specializations before and during myelination. The spinal roots of the newborn rat are composed of bundles of unmyelinated and partially myelinated axons. At this early stage of development, the axons are engulfed by Schwann cells, while certain axons are segregated into a one to one relationship with myelinating cells. Patches of uniformly shaped 150- to 300-Å particles are readily distinguished against a relatively nonparticulate axonal E face. Patches of less uniform particles are found in the axonal P face, however, they are difficult to distinguish from a particulate background. Thin processes are found closely applied to the axonal membrane on the sides of a particle patch. While engulfing the axon with one or two noncompacted windings, the Schwann cell is predominantly restricted to one side of such a particle patch. As the number of windings covering the axon increases, so does the size of the particle patch, until an annulus of particles, similar to that of an adult node, is observed. The paucity of isolated particle patches in axolemma suggests that recognition and segregation of axons by Schwann cells are followed by a rapid initiation of myelination. Throughout the early periods of myelination there is evidence of endocytotic and exocytotic events at the nodal membrane associated with the appearance of 230-Å dimeric particles in the axolemma. Despite the paucity of windings and complete absence of compaction, the fracture faces of the glial and axonal membranes show linear organizations of particles. Scalloped regions in the P face of the nodal axolemma display dimeric-particle rows oriented along the scallop. These rows adopt a more circumferential orientation when the overlying glial process is wound into a paranodal location. While the spacing of dimeric-particle rows is maintained at a constant 360 Å, the number of rows per scallop necessarily decreases with compaction of the paranodal loops until a state similar to that of the adult, in which there are approximately two rows per scallop, is reached. In regions of close apposition between axon and Schwann cell, a linear arrangement of 160- and 75-Å particles in the glial fracture faces occurs prior to the appearance of tight junctions between glial loops and prior to compaction. Though the paranodes on each side of most nodes observed developed symmetrically, some asymmetric half-nodes have been observed.     

Axonal and presynaptic RNAs are locally transcribed in glial cells.

In the last few years, the long-standing opinion that axonal and presynaptic proteins are exclusively derived from the neuron cell body has been substantially modified by the demonstration that active systems of protein synthesis are present in axons and nerve terminals. These observations have raised the issue of the cellular origin of the involved RNAs, which has been generally attributed to the neuron soma. However, data gathered in a number of model systems indicated that axonal RNAs are synthesized in the surrounding glial cells. More recent experiments on the perfused squid giant axon have definitively proved that axoplasmic RNAs are transcribed in periaxonal glia. Their delivery to the axon occurs by a modulatory mechanism based on the release of neurotransmitters from the stimulated axon and on their binding to glial receptors. In additional experiments on squid optic lobe synaptosomes, presynaptic RNA has been also shown to be synthesized locally, presumably in nearby glia. Together with a wealth of literature data, these observations indicate that axons and nerve terminals are endowed with a local system of gene expression that supports the maintenance and plasticity of these neuronal domains.     

OBSERVATIONS ON A TRANSIENT PHASE OF FOCAL SWELLING IN DEGENERATING UNMYELINATED NERVE FIBERS

This study describes the nature and time-course of a swelling phase during the degeneration of unmyelinated nerve fibers, as observed in highly organized cultures of rodent sensory ganglia. Observations were made on nerve fascicles after they were cut and during nutritional deprivation. About 12 hr after nerve transection, large, clear vacuoles appear throughout fascicles distal to the cut. These vacuoles are most numerous at 24 hr and then gradually subside; after 48 hr, only small granules mark the severed fascicles. Electron microscopy shows that the vacuoles are, in fact, massive focal dilations of unmyelinated axons. Similar focal dilations in unmyelinated axons are observed if cultures are not refed for 5–7 days; under these conditions glucose concentrations fall below 20 mg/100 ml and degenerative changes begin to appear in neuronal somas. If the gas-tight assembly is opened and the culture refed, there is rapid disappearance of axonal dilations (usually within 1 hr) and recovery of many of the damaged neurons. Cooling (4°C) prevents this reversal, suggesting that an active process is involved. It is postulated that the swellings result from the failure of active axolemmal ion-pumping mechanisms prior to loss of selective permeability in the axon membrane. The reasons for the focal nature of the swellings is unknown. A literature review indicates that a phase of focal swelling has frequently been observed during the degeneration of unmyelinated nerve fibers in vivo.     

Microtubule stability along mammalian peripheral nerves

Microtubule (MT) number, axonal area, and MT density were examined in unmyelinated axons of rat cervical vagus nerve. Study of nerve regions proximal (1-5 mm) and distal (35-40 mm) to the nodosum ganglion in controls (incubation at 37 degrees C for 1 h) showed that the number of MT per axon is significantly less in distal than in proximal nerve regions. Cooling (incubation at 0 degree C for 1 h) caused a significant reduction in the number of MT per axon in both nerve regions. The unmyelinated axons from both nerve regions showed a comparable reduction in MT number by cooling, indicating that axonal MT stability to cold was not significantly different between these two nerve regions. In these nerves no detectable changes were found in cross-axonal area of unmyelinated axons between distal and proximal nerve regions. In another experimental series, in distal nerve regions (35-40 mm from the nodosum ganglion) the number of MT was not further reduced in nerves incubated at 0 degree C by increasing the incubation time. Similar results were obtained from colchicine treated nerves (incubation at 37 degrees C, with 10 mM colchicine for 1 and 2 h). Distal nerve regions (35-40 mm from the nodosum ganglion) showed a similar reduction in the number of MT per axon when nerves were incubated at 0 degree C or with colchicine, suggesting that this drug, as well as cold, may be affecting a similar population of axonal MT, i.e., MT susceptible to anti-MT agents. These results indicate that approximately one-half of the axonal MT are stable to cold as well as to colchicine in rat unmyelinated axons.     

Axonal degeneration within the tympanal nerve of Schistocerca gregaria

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.     

Regeneration of motor axons in crayfish limbs: Distal stump activation followed by synaptic reformation

Summary Servered distal stumps of limb motor axons in the crayfish Procambarus clarkii remain ultrastructurally intact for at least 2–3 ms after being severed from their cell body. Initial regeneration of a motor axon is associated with the appearance of up to 200 small profiles (satellite axons) having no glial sheath adjacent to the large surviving stump for about 1 cm distal to the lesion at 4–5 wks postoperatively. These satellite axons are seen 2–4 cm distally at the target muscles 3–4 ms postoperatively. By 14–15 ms postoperative, the motor sheaths from the lesion site to the target muscles contain small axonal processes having thick glial sheaths. Behavioral tests show that some axons that are reconnected to the CNS at 4–5 wks may not be connected at 14–15 ms, whereas other axons not connected by 3–4 ms may be connected at 14–15 ms when the original distal stumps have degenerated.We suggest that all these data can best be explained by the view that motor axons in crayfish limbs initially regenerate via activation of the surviving distal stump by satellite axons which grow out from proximal stump. In most cases, these satellite axons continue to activate the surviving distal stump as they slowly grow to the target muscle. Eventually the satellite axons reform synapses on the target muscle and the original distal stump degenerates.This work was supported by NSF grants BNS 77-27678 and 80-22248 and an NIH RCDA 00070 to GDB. The authors would like to thank Mr. Martis Ballinger, Mr. Robert Reiss, and Mrs. Mary Raymond for their excellent technical assistance. We would also like to thank Dr. Wesley Thompson and Mr. Douglas Baxter for helpful discussions.     

Wlds protection distinguishes axon degeneration following injury from naturally occurring developmental pruning

Axon pruning by degeneration remodels exuberant axonal connections and is widely required for the development of proper circuitry in the nervous system from insects to mammals. Developmental axon degeneration morphologically resembles injury-induced Wallerian degeneration, suggesting similar underlying mechanisms. As previously reported for mice, we show that Wlds protein substantially delays Wallerian degeneration in flies. Surprisingly, Wlds has no effect on naturally occurring developmental axon degeneration in flies or mice, although it protects against injury-induced degeneration of the same axons at the same developmental age. By contrast, the ubiquitin-proteasome system is intrinsically required for both developmental and injury-induced axon degeneration. We also show that the glial cell surface receptor Draper is required for efficient clearance of axon fragments during developmental axon degeneration, similar to its function in injury-induced degeneration. Thus, mechanistically, naturally occurring developmental axon pruning by degeneration and injury-induced axon degeneration differ significantly in early steps, but may converge onto a common execution pathway.     

Electron microscopy of severed motor fibers in the crayfish

Summary Cut and crushed crayfish claw nerves were examined with the electron microscope at intervals up to 6 months after lesion. In sections 1 centimeter distal to the lesion there were no signs of degeneration among the giant motor axons even after many months. Swelling of glial wrappings was observed within 48 hours of nerve severance and was particularly notable in the innermost glial layer, the adaxonal layer. Golgi elements, rough endoplasmic reticulum, and mitochondria accumulated in the glia. These changes were perhaps indicative of a greater supportive role required by the severed axons. Regeneration from the proximal stumps of the giant axons began within one week and had proceeded across the lesion gap by 4 weeks. Axon sprouts appeared to travel toward the terminals within the glial sheaths of the distal giant axon segments. Before regeneration was complete, as determined by a simple behaviour test, the regenerating axons occupied increasing proportions of the sheath space. After regeneration was complete occasional degenerations were seen among the sprouts. These degenerations may have occurred in regenerating axons which had grown to the incorrect muscles. The original distal giant axons probably degenerated, as well, after regeneration was complete. There was no evidence of rehealing of proximal and distal segments of the axons.This work was supported by NIH postdoctoral fellowship number 1F2 NB 32, 723 N RB awarded to RHN and grants in aid from the Multiple Sclerosis Society, The American Cancer Society and The National Institutes of Health.     

Differential survival of isolated portions of crayfish axons

Summary Electron microscopic studies show that transplanted segments of sensory axons of varying lengths degenerate within 7–14 days whereas transplanted segments of crustacean motor axons survive morphologically intact for 20–30 days. The middle portion of an isolated motor axon segment degenerates less rapidly than portions of the same axon located nearer the periphery or nearer the ventral nerve cord. One week after transplantation, glial cells appear to phagocytize sensory axons whereas glial cells around motor axons appear to hypertrophy and to have more rough endoplasmic reticulum. After three weeks, motor axons also appear to be phagocytized by glial cells.These data suggest that the glia surrounding isolated motor axons can change from a supportive to a destructive function, whereas glial cells surrounding severed sensory axons primarily have a destructive function. These and other data also indicate that crustacean motor axons receive significant trophic inputs from their own perikaryon, from post-synaptic contacts, and from adjacent glial cells. The possibility that adjacent healthy cells may supply metabolically deficient cells with needed substances could be a significant adaptive advantage for the evolution of multicellular organisms.Supported in part by an NIH grant (NS-1186101) to Dr. BittnerThe authors wish to thank Mr. Martis Ballinger and Mr. Robert Riess for their valuable assistance in all stages of this research