A reliable method for demonstrating axonal degeneration shortly after axotomy

A method has been elaborated by which degenerating axons can be selectively impregnated with silver. Based on reconsideration of the physicochemical mechanisms of the degeneration methods it takes advantage of physical developers over the chemical ones. The staining procedure is applied to frozen sections of brains fixed with formol. It consists of 6 steps: (1) pretreatment with alkaline hydroxylamine, (2) washing in acetic acid, (3) impregnation in silver nitrate in the presence of ferric ions, (4) washing in citric acid, (5) physical development, and (6) washing in acetic acid. By electron microscopy silver precipitates by this method are almost entirely restricted to the cytoplasm of dense, degenerating axons, sparing mitochondria and myelin sheaths. No special expertise is required to achieve reproducible results. Large numbers of sections treated simultaneously, and large sections, can be stained uniformly. Light microscopic criteria are described which help diagnose the source of possible failures. Low background staining allows dark field illumination and television image analysis to be applied. The method works at survival times of only 3 to 5 days after axotomy. Hence, degenerating axons and axon terminals can be stained in alternating sections from the same brain using this method and another being described separately, which, using different conditions, demonstrates degenerating axon terminals.     

A Reliable Method for Demonstrating Axonal Degeneration Shortly After Axotomy

A method has been elaborated by which degenerating axons can be selectively impregnated with silver. Based on a reconsideration of the physicochemical mechanisms of the degeneration methods it takes advantage of physical developers over the chemical ones. The staining procedure is applied to frozen sections of brains fixed with formol. It consists of 6 steps: (1) pretreatment with alkaline hydroxylamine, (2) washing in acetic acid, (3) impregnation in silver nitrate in the presence of ferric ions, (4) washing in citric acid, (5) physical development, and (6) washing in acetic acid. By electron microscopy silver precipitates by this method are almost entirely restricted to the cytoplasm of dense, degenerating axons, sparing mitochondria and myelin sheaths. No special expertise is required to achieve reproducible results. Large numbers of sections treated simultaneously, and large sections, can be stained uniformly. Light microscopic criteria are described which help diagnose the source of possible failures. Low background staining allows dark field illumination and television image analysis to be applied. The method works at survival times of only 3 to 5 days after axotomy. Hence, degenerating axons and axon terminals can be stained in alternating sections from the same brain using this method and another being described separately, which, using different conditions, demonstrates degenerating axon terminals.     

Experimental studies of mechanisms involved in methods demonstrating axonal and terminal degeneration

Factors influencing the consistency and specificity of the staining of neuronal degeneration products were studied in brain sections by varying systematically the composition of solutions used in the steps which are common to the degeneration methods. The formation of nuclei of metallic silver was determined either by physical development of 110Ag, after dissolving reducible silver by acetic acid. In degenerating axons metallic silver nucleic are formed by their own reducing groups in the first (acid) and in the second (alkaline) impregnating bath. The first impregnation turned out to be sufficient to produce complete staining of degenerating axons. The reducing capacity of normal axons and myelin can be suppressed by oxidation or by lowering the pH of the impregnating solution. Degenerating axon terminals are not able to reduce silver ions in either of the impregnating baths. Rather, the metallic silver nuclei initiating their staining are formed in the Nauta reducer by interaction of its reducing agent (formol) with silver ions which had been trapped in the tissue during the impregnation. Thus the nuclei are enlarged to microscopic visibility by a nonstandardized physical developer coming about from the Nauta reducer and the silver ions transferred with the sections. In this reaction catalytic sites in degenerating terminals as well as ammonium ions and the alkali reserve of the tissue play an important role. On the basis of the present results it was possible to stabilize the conditions for staining degenerating axons and degenerating axons terminals in two separate staining procedures detailed in following papers.     

Experimental Studies of Mechanisms Involved in Methods Demonstrating Axonal and Terminal Degeneration

Factors influencing the consistency and specificity of the staining of neuronal degeneration products were studied in brain sections by varying systematically the composition of solutions used in the steps which are common to the degeneration methods. The formation of nuclei of metallic silver was determined either by physical development or ¹¹⁰Ag, after dissolving reducible silver by acetic acid. In degenerating axons metallic silver nuclei are formed by their own reducing groups in the first (acid) and in the second (alkaline) impregnating bath. The first impregnation turned out to be sufficient to produce complete staining of degenerating axons. The reducing capacity of normal axons and myelin can be suppressed by oxidation or by lowering the pH of the impregnating solution. Degenerating axon terminals are not able to reduce silver ions in either of the impregnating baths. Rather, the metallic silver nuclei initiating their staining are formed in the Nauta reducer by interaction of its reducing agent (formol) with silver ions which had been trapped in the tissue during the impregnation. Thus the nuclei are enlarged to microscopic visibility by a nonstandardized physical developer coming about from the Nauta reducer and the silver ions transferred with the sections. In this reaction catalytic sites in degenerating terminals as well as ammonium ions and the alkali reserve of the tissue play an important role. On the basis of the present results it was possible to stabilize the conditions for staining degenerating axons and degenerating axon terminals in two separate staining procedures detailed in following papers.     

Programmed cell death: Cytochemical and X-ray microanalytical characterization of calcium compartments in neuromuscular junctions during the normal breakdown of the intersegmental muscles in the giant silkmothAntheraea polyphemus

Summary Calcium stores were cytochemically demonstrated using a combined oxalate—pyroantimonate method in the neuromuscular junctions of the degenerating intersegmental muscles in the giant silkmothAntheraea polyphemus. The elemental composition of punctate precipitates of the reaction product was determined by electron probe X-ray microanalysis of unstained thin sections by energy-dispersive spectrometry and wavelength-dispersive spectrometry. The wavelength-dispersive spectra collected over terminal axons demonstrate a significant calcium signal and a trace of antimony.During the rapid lytic phase of spontaneous muscle degeneration, the calcium punctate deposits were detected in presynaptic terminals in the following sites: the synaptic vesicles and the mitochondria. Calcium precipitates were also found in the dense bodies and the mitochondria encountered in the glial convolutions. No calcium deposit was seen in the synaptic clefts and intercellular spaces of the subsynaptic reticulum of type I and type II. A comparison of calcium to antimony ratios between the terminal axons and the sarcoplasmic lysosomes revealed highly significant differences (P<0.001). Such a variability of the calcium to antimony ratio may be related to different conditions of precipitation or antimony diffusion in the different cell compartments. It was concluded that such synaptic terminals do not appear damaged in spite of the muscle degeneration and presumably continue to perform vital functions while the muscles are no longer contractile 20 h after adult ecdysis.     

Identification of phagocytic glial cells after lesion-induced anterograde degeneration using double-fluorescence labeling: combination of axonal tracing and lectin or immunostaining

 Retrograde and anterograde degeneration have been reported to be sufficient stimuli to activate glial cells, which, in turn, are involved in phagocytosis of degenerating material. Here we describe a double-fluorescence technique which allows for direct and simultaneous visualization of both labeled incorporated axonal debris and incorporating glial cells in the course of anterograde degeneration. Stereotaxic application of small crystals of biotinylated and tetramethylrhodamine (TRITC)-conjugated dextran amine Mini Ruby into the medial entorhinal cortex resulted in a stable rhodamine fluorescence confined to fibers and terminals in the middle molecular layer of the dentate gyrus, the stratum lacunosum-moleculare, and the crossed temporo-hippocampal pathway. Subsequent stereotaxic lesion of the entorhinal cortex induced transformation of rhodamine-fluorescent fibers and terminals into small granules. Incorporation of these granules by microglial cells [labeled by fluorescein isothiocyanate (FITC)-coupled Bandeiraea simplicifolia isolectin B₄] or astrocytes (labeled by FITC-coupled glial fibrillary acidic protein antibodies) resulted in phagocytosis-dependent labeling of these non-neuronal cells, which could be identified by double-fluorescence microscopy. Electron microscopical analysis revealed that, following lesion, the tracer remained confined to entorhinal axons which were found to be incorporated by glial cells. Our data show that TRITC- and biotin-conjugated dextran amines are versatile tracers leading to Phaseolus vulgaris leucoagglutinin-like axonal staining. Lesion-induced phagocytosis of anterogradely degenerating axons by immunocytochemically identified glial cells can be directly observed by this technique on the light and electron microscopical levels. Accepted: 8 January 1997     

Terminal degeneration in the mediodorsal cerebral cortex of the lizard Agama agama: Light and electron microscopy

The degeneration of axon terminals in the small-celled part of the mediodorsal cortex (sMDC) of the lizard Agama agama has been studied after lesions in the dorsal cortex at various survival periods. The Fink-Heimer stain was used to map and demonstrate terminal degeneration with the light and electron microscope. Electron microscopy was used to identify and describe degenerating boutons ultrastructurally. One sham-operated and three unoperated animals served as controls. Between 6 and 21 days postsurgically, degenerating terminals can be seen through 80% of the superficial plexiform layer, the zone adjacent to the cellular layer remaining free of degeneration. Swelling of dendrites in the outer part of the superficial plexiform layer and increased numbers of vacuolar invaginations, both present at short (24 hr–6 days; peak at 48–54 hr) survival periods, can be regarded as reaction to the surgical trauma. Degeneration of axon terminals takes three forms, all of the electron-dense type: gray boutons, degenerating bouton-dendritic spine complexes surrounded or engulfed by glia, and degeneration debris inside glial processes. Several forms of terminal degeneration occur concomitantly at any short (3–12 days) survival time. At longer survival times (15–21 days) only debris is present. From 6 days on, considerable numbers of degenerating structures are present, but the majority of degenerating boutons and debris are associated with reactive glia rather than with dendrites. From these observations it is concluded that in this lizard application of the combined degeneration-Golgi-EM technique would probably lead to little success. Electron microscopy of Fink-Heimer-stained sections suggests that degenerating bouton-dendritic spine complexes and degeneration debris accumulate silver particles, whereas gray boutons do not.     

Chemical degeneration of indolamine axons in rat brain by 5,6-dihydroxytryptamine

Summary Evidence has been obtained by electron microscopy of a direct cytotoxic effect of intraventricularly administered 5,6-dihydroxytryptamine (5,6-DHT) on unmyelinated axons in the rat brain. Ultrastructural signs of axonal damage were observed in areas rich in indolamine nerve terminals as early as 2 hrs after injection. By 6–24 hrs, characteristic and more dramatic signs of degeneration developed, involving coalescence of all axonal constituents—often in combination with a uniform osmiophilic impregnation of the axoplasm—accompanied by engulfment of the dystrophic structures by glial processes. During the next five days, the degenerating axons and axon terminals appeared to be removed by glial cell phagocytosis, whose equivalents were the inclusion of axonal residues into membrane-bound lysosome-like bodies. Concomitantly, there was a progressively increasing number of extremely large and dilated axons in all regions analysed. These axonal swellings, which have an ultramorphology similar to that of dilated stumps of mechanically severed monoamine axons, correspond most probably to proximal, dilated portions of drug-damaged axons.The present results, in combination with biochemical and fluorescence microscopical data, indicate that within a proper dose range the 5,6-DHT-induced degeneration is largely restricted to indolamine axons and axon terminals. However, unselective effects on other unmyelinated axons, on myelin, and on glial cells were observed in narrow subependymal zones close to the lateral ventricles, i.e. close to the injection cannula.Supported by grants from the Deutsche Forschungsgemeinschaft.Supported by grants from the National Institutes of Health, USPHS (NS-06701-06) and from the Swedish Medical Research Council (grants No. B72-14X-712-07B and B72-14X-56-08B).     

Ultrastructural changes in the gracile nucleus of the spontaneously diabetic BB rat.

The present study describes the structural changes in the gracile nucleus of the spontaneously diabetic BB rat. At 3-7 days post-diabetes, axons, axon terminals and dendrites showed electron-dense degeneration. Degenerating axons were characterized by swollen mitochondria, vacuolation, accumulation of glycogen granules, tubulovesicular elements, neurofilaments and dense lamellar bodies. Degenerating axon terminals consisted of an electron-dense cytoplasm containing swollen mitochondria, vacuoles and clustering of synaptic vesicles. These axon terminals made synaptic contacts with cell somata, dendrites and other axon terminals. Degenerating dendrites were postsynaptic to normal as well as degenerating axon terminals. At 1-3 months post-diabetes, degenerating electron-dense axons, axon terminals and dendrites were widely scattered in the neuropil. Macrophages containing degenerating electron-dense debris were also present. At 6 months post-diabetes, the freshly degenerating neuronal elements encountered were similar to those observed at 3-7 days. However, there were more degenerating profiles at 6 months post-diabetes compared to the earlier time intervals. Terminally degenerating axons were vacuolated and their axoplasm appeared amorphous. It is concluded that degenerative changes occur in the gracile nucleus of the spontaneously diabetic BB rat.     

An electron microscopic study of neuronal degeneration and glial cell reaction in the retina of glaucomatous rats

The present investigation was focused on the ultrastructural changes in the neurons and glial cells in the retina of rats with experimentally-induced glaucoma. An experimental glaucoma model was created by limbal-derived vein cauterization. Animals were sacrificed at 1, 3 weeks and 3 months post-operation. Retinae were dissected and processed for electron microscopy. Neuronal degeneration was observed in all the different layers of the retina at both 1 and 3 weeks post-operation. Some degenerating neurons were found in the ganglion cell layer (GCL), inner nuclear layer (INL) and outer nuclear layer (ONL). And the dying neurons presented apoptotic-like more than necrotic neurons. Many degenerating axons and axon terminals were observed between neurons in the GCL, inner plexiform layer (IPL), INL, and outer plexiform layer (OPL). Activated astrocytes and microglial cells were present in close association with degenerating neurons and axons. The Müller cells in the INL also presented longer and darker processes with more microfilaments than in normal cells. Degenerating neuronal debris, degenerating axonal profiles and electron-dense bodies were often found in the cytoplasm of macrophages. The results suggest that both microglial cells and astrocytes are activated in the process of neuronal degeneration in the retina of experimentally-induced glaucomatous rats. It is hypothesized that they may play a protective role in removing degenerating neuronal elements in the retina after the onset of glaucoma.     

Demonstration of Degenerating Nerve Fibers by a Modified Cajal Technic

Whole brains of cat were fixed in two changes of cold acetone (24 hours each) and embedded directly in paraffin. The degeneration time recommended is 5 days. Mounted sections 15-20 μ thick were deparaffined, washed in absolute alcohol and given successive treatments of 6 hours each with 1% ammoniated absolute alcohol and pure pyridine, washing well with distilled water between them and after the pyridine. Impregnation in 2% silver nitrate 12 hours at 30°C., rinsing in absolute alcohol and reducing in a 95% alcoholic solution of pyrogallol and formalin (3% and 5%) was followed by 50% alcohol, thorough washing in distilled water, toning in 1% gold chloride and intensification in 1% oxalic acid. Treatment in 10% sodium thiosulfate solution, washing, dehydrating and covering completed the procedure. Normal fibers, degenerating fibers and terminals were stained specifically.     

Distribution and ultrastructure of thalamic afferents in the cat auditory cortex

Ultrastructural features of thalamic afferent fibers were studied in the cat auditory cortex in the early stages (on the 4th day) of experimental degeneration produced by destruction of the medial geniculate body. A coordinate grid was used in conjunction with an electron micro-scope to study the topography of the degenerating elements over wide areas of sections, so that the density of degeneration could be determined quantitatively in different layers of the cortex. Degenerating axons were found in all layers. Most of the large (5–7 µ) degenerating axons are located in layer VI; their diameters were smaller in the upper layers of the cortex. Degenerative changes affecting synaptic terminals of thalamo-cortical afferents were of the "dark" type. Fibers of the geniculo-cortical tract were shown to terminate mainly in cortical layer IV. A few degenerating synapses were found in the molecular layer. Terminals with sperical synaptic vesicles are found mainly on the spines of dendrites where they form "asymmetrical" contacts. A few degenerating axo-somatic synapses were observed on stellate neurons in layer IV. The results are discussed in connection with electrophysiological investigations of the cat auditory cortex during stimulation of specific afferent fibers.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 4, No. 6, pp. 612–620, November–December, 1972.     

Demonstration of the tuberoinfundibular tract of the cat

Summary The distribution of dopaminergic nerve cells in the cat hypothalamus, particularly in the arcuate and periventricular nuclei, and the projections of their axons were studied by fluorescence and electron microscopy after electrothermic coagulation. The majority of these perikarya were located in the arcuate nucleus and the periventricular nucleus dorsocaudal to the optic chiasma. Large lesions caused a wide and diffuse depletion of dopamine fluorescence within the external layer; small lesions caused ipsilateral partial depletion of the dopamine fluorescence. Electron microscopic observations in animals with a lesioned arcuate nucleus revealed that in the external layer degenerating nerve terminals are engulfed by glial processes. In some cases nerve fibers had entirely disappeared and a heavy reactive proliferation of glial processes was observed. Persistence of the form of the median eminence in spite of the extensive degeneration of its nervous elements is considered to depend upon this glial proliferation.Dedicated to Professor W. Bargmann in honour of his 70th birthday     

The temporo-spatial course of degeneration after cutting cortico-cortical connections in adult rats

Summary Adult albino rats received callosotomies or lesions in the paracingular cortex. Between 12 h and 3 months after injury the structure and topography of the degeneration products were studied by light- and electron-microscopy. The degeneration process was quantified by television-image analysis applied to sections prepared according to a new technique that stains reliably degenerating terminals and lysosomes (Gallyas et al. 1980). All types of cortico-cortical connections show a multiphasic degeneration process: During a precursor stage a small number of dense bodies and mitochondrial granules are stained. These and the few early degenerating axon terminals are much more diffusely distributed than the large number of terminals that degenerate during the following period. The terminal degeneration shows a biphasic time course. One maximum appears at 2–7 days post operation, which corresponds to the well known direct consequence of axotomy. The second peak at 10–20 days post operation could be caused by transneuronal reorganization of the cortical connectivity. Terminal degeneration always begins along the borders between cortical regions and areas, but it may change its laminar and columnar distribution pattern during the second phase. The degeneration products that are phagocytosed by astrocytes seem to be removed by intracellular transport to their perivascular endfeet. The degeneration process ends with fiber degeneration which, especially in laminae I and VI, may form a separate peak after 20 days or more.On leave from: Department of Neurosurgery, University of Göttingen, Federal Republic of Germany;On leave from: First Department of Anatomy, Semmelweis University, Medical School, Budapest, Hungary     

MDMA神经毒性长期效应导致皮层和海马组织结构改变

通过短时间多次给药建立3,4．亚甲基二氧基甲基苯丙胺（3,4-methylenedioxymethamphetamine,MDMA）的神经毒性模型,将雄性Wistar大鼠随机分为对照组和实验组,实验组给予MDMA10mg／kg,每小时一次,共4次,即总量为40mg／kg,对照组给予等体积生理盐水。于末次给药后32周采用原位杂交检测5-HT转运体（serotonin transporter,SERT）mRNA和内源性焦虑物质苯甲二氮革结合性抑制物（diazepam binding inhibitor,DBI）的mRNA表达,免疫组织化学染色检测胶质纤维酸性蛋白（glial fibrillary acidic protein,GFAP）的表达,银染观察神经末梢变化。结果显示,短时间多次给予MDMA后,与生理盐水组比较,MDMA组大鼠海马SERTmRNA信号表达降低（P〈0．05）,大脑皮层DBImRNA的信号表达增高（P〈0．05）,GFAP表达显著升高（P〈0．05）;银染MDMA组大鼠皮层神经末梢明显减少。上述结果提示,MDMA神经毒性导致皮层和海马结构改变持续存在,进而导致脑功能的紊乱。     

Ultrastructural relation between cortical efferents and terminals containing enkephalin-like immunoreactivity in rat neostriatum

The interrelationships between cortical efferents and terminals containing enkephalin-like immunoreactivity (ELI) were examined by combining anterograde degeneration with electron microscopic immunocytochemistry in the adult rat neostriatum. Two days following unilateral removal of the cerebral cortex, the brains were fixed by aortic arch perfusion, then sectioned and processed for the immunocytochemical localization of an antiserum directed against methionine (Met5)-enkephalin. The observed relationships between the degenerating cortical efferents and immunocytochemically labeled terminals were of two types. In the first, the degenerating and ELI containing terminals converged on the same unlabeled dendrite or dendritic spine. In the second, terminal and preterminal axons of the ELI containing neurons had one surface directly apposed to the plasma membrane of a degenerating axon terminal. These findings support the concept that neurons containing opioid peptides and cortical efferents modulate the output of common recipient neurons and may also directly interact with each other through presynaptic axonal mechanisms in the rat neostriatum.     

Distribution and ultrastructure of pyramidal tract endings in the cat rhombencephalon

The distribution and ultrastructure of terminals of corticofugal fibers in the cat rhombencephalon were investigated under the optical and electron microscopes at different periods (2–6 days) of experimental degeneration evoked by destruction of the sensomotor cortex. It was shown by the Fink–Heimer method that most degenerating fibers are distributed in the reticular nuclei of the pons and medulla. Massive degeneration of corticofugal fibers also was observed in the nuclei of the dorsal columns (nuclei of Goll and Burdach). Most of the degenerating (the "pale" type of degeneration) axo-dendritic and axo-somatic synapses in the gigantocellular reticular nucleus and the nucleus of Goll retained spherical vesicles. Small endings were found on the branches of the dendrites in which degenerative changes were of the "dark" type. The topography of the degenerating elements and axo-axonal synapses was studied in large areas of sections by the coordinate grid method. The dimensions of most degenerating axons in the gigantocellular reticular nucleus were greater (1.5 µ) than those of the degenerating axons (0.5 µ) in the nucleus of Goll. Most endings of pyramidal fibers and axo-axonal synapses are located in the central part of the nucleus of Goll at a depth of 0.5–1.2 mm from the brain surface. The results are discussed in connection with electrophysiological studies of the mechanisms of cortical control over unit activity of the reticular formation of the brain stem and nuclei of the dorsal columns.     

Platelet activation and microfilament bundling

Human platelets were obtained in the fully resting state by treating discoid populations with 1.5 mM tetracaine and in the activated state by treatment with 2 microM A-23187. After gel filtration or washing, respectively, platelet suspensions were lysed with 1% Triton X-100 at pH 6.8. The precipitates from resting platelets viewed by negative staining appeared predominantly granular with a few very short microfilaments. They contained polypeptides of 250, 100, 45, 38, 36.5, and 35 Kdaltons, and three small polypeptides including one with the mobility of profilin on SDS gels. Precipitates from activated platelets lacked this low molecular weight band and contained a major band at 200 Kdaltons with the mobility of myosin; these precipitates had significant K+, Ca++ ATPase activity absent from the precipitate of resting platelets. As seen in negative staining, precipitates from activated platelets contained microfilaments arranged as nets or bundles. The granular resting precipitates were transformed in vitro into microfilament bundles by washing the precipitates in buffer at higher pH (7.6) in the presence of 5 X 10(-5) M calcium chloride.     

Essential role of the apoptotic cell engulfment genes draper and ced-6 in programmed axon pruning during Drosophila metamorphosis

Axon pruning is a common phenomenon in neural circuit development. Previous studies demonstrate that the engulfing action of glial cells is essential in this process. The underlying molecular mechanisms, however, remain unknown. We show that draper (drpr) and ced-6, which are essential for the clearance of apoptotic cells in C. elegans, function in the glial engulfment of larval axons during Drosophila metamorphosis. The drpr mutation and glia-specific knockdown of drpr and ced-6 by RNA interference suppress glial engulfment, resulting in the inhibition of axon pruning. drpr and ced-6 interact genetically in the glial action. Disruption of the microtubule cytoskeleton in the axons to be pruned occurs via ecdysone signaling, independent of glial engulfment. These findings suggest that glial cells engulf degenerating axons through drpr and ced-6. We propose that apoptotic cells and degenerating axons of living neurons are removed by a similar molecular mechanism.     

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.