Search

Pituitary adenomas are rare in children and adolescents and although mostly benign, they can sometimes be challenging to manage due to their locally invasive nature. In this study, we examined the clinicopathologic features of 42 pituitary adenomas in patients ≤21 years of age. The youngest patient was 8 years old (median age: 18 years), and the female-to-male ratio was 1.8:1. Five patients had recurrence after resection. There was no obvious difference between the recurrent rates in the typical (11.7%) and atypical adenomas (12.5%) based on the 2004 WHO classification. However, the recurrence rate was much higher in adenomas with an elevated proliferation index of ≥3% (20.8%) or with evidence of local invasion (18.2%). Adenomas with combination of an elevated proliferation index of ≥3% and imaging evidence of local invasion had the highest recurrence rate of 25%. In summary, pituitary adenomas are more frequent in adolescents as compared with children and are more common in girls. An elevated proliferation index of ≥3% and evidence of local invasion on imaging seem to correlate with a high probability of recurrence. Furthermore, we observe rarity of α-thalassemia/mental retardation syndrome X-linked (ATRX) protein loss (surrogate to ATRX mutation) in these tumors without any connotation on prognosis., (© 2019 American Association of Neuropathologists, Inc.)

Focal cortical dysplasia (FCD) is a common histopathologic finding in cortical specimens resected for refractory epilepsy. GABAergic neuronal abnormalities and K-Cl cotransporter type 2 (KCC2) immaturity may be contributing factors for FCD-related epilepsy. We examined surgical specimens from 12 cases diagnosed with FCD, and brain tissues without developmental abnormality obtained from 6 autopsy cases. We found that GABAergic neuronal density was abnormal in FCD with 2 distinct patterns. In 7 of 12 (58%) FCD subjects, the GABAergic neuron density in dysplastic regions and in neighboring nondysplastic regions was equally reduced, hence we call this a "broad pattern." In the remaining cases, GABAergic neuron density was decreased in dysplastic regions but not in the neighboring nondysplastic regions; we designate this "restricted pattern." The different patterns are not associated with pathologic subtypes of FCD. Intracytoplasmic retention of KCC2 is evident in dysmorphic neurons in the majority of FCD type II subjects (5/7) but not in FCD type I. Our study suggests that (1) "broad" GABAergic deficiency may reflect epileptic vulnerability outside the dysplastic area; and (2) abnormal distribution of KCC2 may contribute to seizure generation in patients with FCD type II but not in type I., (© 2019 American Association of Neuropathologists, Inc. All rights reserved.)

To determine if ventricular zone (VZ) and subventricular zone (SVZ) alterations are associated with intraventricular hemorrhage (IVH) and posthemorrhagic hydrocephalus, we compared postmortem frontal and subcortical brain samples from 12 infants with IVH and 3 nonneurological disease controls without hemorrhages or ventriculomegaly. Birth and expiration estimated gestational ages were 23.0-39.1 and 23.7-44.1 weeks, respectively; survival ranges were 0-42 days (median, 2.0 days). Routine histology and immunohistochemistry for neural stem cells (NSCs), neural progenitors (NPs), multiciliated ependymal cells (ECs), astrocytes (AS), and cell adhesion molecules were performed. Controls exhibited monociliated NSCs and multiciliated ECs lining the ventricles, abundant NPs in the SVZ, and medial vs. lateral wall differences with a complex mosaic organization in the latter. In IVH cases, normal VZ/SVZ areas were mixed with foci of NSC and EC loss, eruption of cells into the ventricle, cytoplasmic transposition of N-cadherin, subependymal rosettes, and periventricular heterotopia. Mature AS populated areas believed to be sites of VZ disruption. The cytopathology and extension of the VZ disruption correlated with developmental age but not with brain hemorrhage grade or location. These results corroborate similar findings in congenital hydrocephalus in animals and humans and indicate that VZ disruption occurs consistently in premature neonates with IVH., (© 2017 American Association of Neuropathologists, Inc. All rights reserved.)

Eosinophilic inclusions in the cytoplasm of protoplasmic astrocytes of the neocortex, usually in the clinical setting of epilepsy and/or psychomotor retardation, were first recognized and illustrated by Alois Alzheimer in 1910. Traditional special stains have failed to elucidate the specific nature of these inclusions. Ultrastructurally, the material was composed predominantly of highly electron-dense, non-membrane-bound, granular material distinct from Rosenthal fibers. Immunohistochemical examination has been informative but also sometimes inconsistent; it has recently been suggested that they may represent a filaminopathy (filamin A). We examined 5 cases with neocortical eosinophilic inclusions (3 autopsies, 2 surgical resections) using a standardized immunohistochemical protocol at a single institution. The specimens were immunostained with 32 antibodies to 30 potentially relevant proteins using several antigen retrieval protocols. We confirmed the presence of filamin A in these inclusions, but several additional proteins, particularly cytoglobin and glutamate transporter 1, were also identified. By electron microscopy in 2 cases, the granular fine structure of the inclusions was confirmed; mitochondria adjacent to, and perhaps within, the inclusions that contained many pleomorphic vesicular and membranous elements were also noted in 1 case. The pathophysiologic relevance of these proteins and the clinical significance of the hyaline inclusions are discussed.

Neuroaxonal dystrophy, a distinctive axonopathy characterized by marked enlargement of distal axons, is the hallmark pathologic alteration in aged and diabetic human prevertebral sympathetic ganglia and in corresponding rodent models. Neuroaxonal dystrophy is thought to represent the abnormal outcome of cycles of synaptic degeneration and regeneration; a systematic study of identified axon terminals in aged and diabetic prevertebral ganglia, however, has not previously been performed. We examined the initial changes that develop in presynaptic and postsynaptic elements in sympathetic ganglia of aged and diabetic mice and found numerous synaptic changes involving both presynaptic and postsynaptic elements. Early alterations in presynaptic axon terminal size, vesicle content, and morphology culminate in the development of anastomosing membranous tubulovesicular aggregates, accumulation of autophagosomes, and amorphous debris that form a continuum with progressively larger classically dystrophic swellings. Dendritic changes consist of the development of swellings composed of delicate tubulovesicular elements and mitochondriopathy characterized by increased numbers of small mitochondria and, exclusively in aged ganglia, megamitochondria. These results support the hypothesis that neuroaxonal dystrophy results from progressive changes in presynaptic axon terminals that likely involve membrane dynamics and which are accompanied by distinctive changes in postsynaptic dendritic elements.

In both humans and animal models, diabetic sympathetic autonomic neuropathy is associated with the selective development of markedly enlarged distal axons and nerve terminals (neuroaxonal dystrophy, NAD). NAD occurs in the prevertebral superior mesenteric and celiac ganglia (SMG-CG), but not in the paravertebral superior cervical ganglion (SCG). To identify molecular differences between these ganglia that may explain their selective vulnerability to NAD, we have examined global gene expression patterns in control and diabetic rat sympathetic ganglia before and after the onset of structural evidence of NAD. As predicted, major differences in transcriptional profiles exist between SCG and SMG-CG in normal young adult animals including, but not limited to, known differences in neurotransmitter-related gene expression. Gene expression patterns of diabetic SMG-CG and SCG, prior to the development of NAD lesions, also differ from their age-matched non-diabetic counterparts. However, diabetes has ganglion-specific effects on gene expression; of approximately 110 transcripts that were differentially expressed between diabetic and control sympathetic ganglia, only 5 were differentially expressed as a result of diabetes in both SCG and SMG-CG. Genes involving synapse and mitochondrial structure and function, oxidative stress, and glycolysis were highly represented in the differentially expressed gene set. Differences in the number of synapse-related gene alterations in diabetic SMG-CG (18 genes) versus SCG (2 genes) prior to the onset of NAD may also well explain the selective development of NAD in the SMG-CG. These results provide support for the specificity of diabetes-modulated gene expression for selected neuronal subpopulations of sympathetic noradrenergic neurons.

Dysfunction of the autonomic nervous system is a recognized complication of diabetes, ranging in severity from relatively minor sweating and pupillomotor abnormality to debilitating interference with cardiovascular, genitourinary, and alimentary dysfunction. Neuroaxonal dystrophy (NAD), a distinctive distal axonopathy involving terminal axons and synapses, represents the neuropathologic hallmark of diabetic sympathetic autonomic neuropathy in man and several insulinopenic experimental rodent models. Although the pathogenesis of diabetic sympathetic NAD is unknown, recent studies have suggested that loss of the neurotrophic effects of insulin and/or insulin-like growth factor-I (IGF-I) on sympathetic neurons rather than hyperglycemia per se, may be critical to its development. Therefore, in our current investigation we have compared the sympathetic neuropathology developing after 8 months of diabetes in the streptozotocin (STZ)-induced diabetic rat and BB/ Wor rat, both models of hypoinsulinemic type 1 diabetes, with the BBZDR/Wor rat, a hyperglycemic and hyperinsulinemic type 2 diabetes model. Both STZ- and BB/Wor-diabetic rats reproducibly developed NAD in nerve terminals in the prevertebral superior mesenteric sympathetic ganglia (SMG) and ileal mesenteric nerves. The BBZDR/Wor-diabetic rat, in comparison, failed to develop superior mesenteric ganglionic NAD in excess of that of age-matched controls. Similarly, NAD which developed in axons of ileal mesenteric nerves of BBZDR/Wor rats was substantially less frequent than in BB/Wor- and STZ-rats. These data, considered in the light of the results of previous experiments, argue that hyperglycemia alone is not sufficient to produce sympathetic ganglionic NAD, but rather that it may be the diabetes-induced superimposed loss of trophic support, likely of IGF-I, insulin, or C-peptide, that ultimately causes NAD.

Gliomas with hybrid oligodendroglial/astrocytic features are diagnostically problematic, and our ability to predict tumor behavior is limited. Some likely represent intermingled mixed oligoastrocytomas (MOAs), though precise diagnostic criteria and specific markers for this lesion are lacking. From the files at Washington University (1987-2000), 155 "ambiguous" glioma/intermingled MOA candidates were independently classified and graded by 5 neuropathologists, with consensus-derived pure oligodendrogliomas and astrocytomas excluded from further study. The 90 remaining cases (grades II = 29, III = 44, IV = 17) were analyzed by FISH on formalin-fixed, paraffin-embedded sections. Detectable deletions included combined 1p/19q (9%), solitary 19q (22%), PTEN/DMBT1 (26%), and p16 (32%). EGFR amplification was found in 11%. Patients were followed until death (47%) or a median of 3.3 years. Similar to prior glioma series, patient age (p < 0.0001) and tumor grade (p < 0.0001) were strongly associated with survival times. EGFR amplification (p = 0.0007) and deletions of PTEN/ DMBT1 (p = 0.016) or p16 (p = 0.014), either individually or as a group (p = 0.04), portended a shorter median survival compared with tumors lacking these alterations. We conclude that 1) distinct genetic subsets are identifiable by FISH in morphologically ambiguous gliomas, and 2) both histological grading and molecular analysis yield prognostically useful information.

We have developed an animal model of diabetic autonomic neuropathy that is characterized by neuroaxonal dystrophy (NAD) involving ileal mesenteric nerves and prevertebral sympathetic superior mesenteric ganglia (SMG) in chronic streptozotocin (STZ)-diabetic rats. Studies with the sorbitol dehydrogenase inhibitor SDI-158, which interrupts the conversion of sorbitol to fructose (and reactions dependent on the second step of the sorbitol pathway), have shown a dramatically increased frequency of NAD in ileal mesenteric nerves and SMG of SDI-treated versus untreated diabetics. Although lesions developed prematurely and in greater numbers in SDI-treated diabetics, their distinctive ultrastructural appearance was identical to that previously reported in long-term untreated diabetics. An SDI effect was first demonstrated in the SMG of rats that were diabetic for as little as 5 wk and was maintained for at least 7.5 months. As in untreated diabetic rats, rats treated with SDI i) showed involvement of lengthy ileal, but not shorter, jejunal mesenteric nerves; ii) demonstrated NAD in paravascular mesenteric nerves distributed to myenteric ganglia while sparing adjacent perivascular axons ramifying within the vascular adventitia; and, iii) failed to develop NAD in the superior cervical ganglia (SCG). After only 2 months of SDI-treatment, tyrosine hydroxylase immunolocalization demonstrated marked dilatation of postganglionic noradrenergic axons in paravascular ileal mesenteric nerves and within the gut wall versus those innervating extramural mesenteric vasculature. The effect of SDI on diabetic NAD in SMG was completely prevented by concomitant administration of the aldose reductase inhibitor Sorbinil. Treatment of diabetic rats with Sorbinil also prevented NAD in diabetic rats not treated with SDI. These findings indicate that sorbitol pathway-linked metabolic imbalances play a critical role in the development of NAD in this model of diabetic sympathetic autonomic neuropathy.

Chloroquine is a lysosomotropic agent that causes marked changes in intracellular protein processing and trafficking and extensive autophagic vacuole formation. Chloroquine may be cytotoxic and has been used as a model of lysosomal-dependent cell death. Recent studies indicate that autophagic cell death may involve Bcl-2 family members and share some features with caspase-dependent apoptotic death. To determine the molecular pathway of chloroquine-induced neuronal cell death, we examined the effects of chloroquine on primary telencephalic neuronal cultures derived from mice with targeted gene disruptions in p53, and various caspase and bcl-2 family members. In wild-type neurons, chloroquine produced concentration- and time-dependent accumulation of autophagosomes, caspase-3 activation, and cell death. Cell death was inhibited by 3-methyladenine, an inhibitor of autophagic vacuole formation, but not by Boc-Asp-FMK (BAF), a broad caspase inhibitor. Targeted gene disruptions of p53 and bax inhibited and bcl-x potentiated chloroquine-induced neuron death. Caspase-9- and caspase-3-deficient neurons were not protected from chloroquine cytotoxicity. These studies indicate that chloroquine activates a regulated cell death pathway that partially overlaps with the apoptotic cascade.

Peripheral neuropathy is a significant complication of diabetes resulting in increased patient morbidity and mortality. Deficiencies of neurotrophic substances (e.g. NGE NT-3, and IGF-I) have been proposed as pathogenetic mechanisms in the development of distal symmetrical sensory diabetic polyneuropathy, and salutary effects of exogenous NGF administration have been reported in animal models. In comparison, relatively little is known concerning the effect of NGF on experimental diabetic sympathetic autonomic neuropathy. We have developed an experimental animal model of diabetic autonomic neuropathy characterized by the regular occurrence of pathologically distinctive dystrophic axons in prevertebral sympathetic ganglia and ileal mesenteric nerves of rats with chronic streptozotocin (STZ)-induced diabetes. Treatment of STZ-diabetic rats for 2-3 months with pharmacologic doses of NGF or NT-3, neurotrophic substances with known effects on the adult sympathetic nervous system, did not normalize established neuroaxonal dystrophy (NAD) in diabetic rats in the prevertebral superior mesenteric ganglia (SMG) and ileal mesenteric nerves as had pancreatic islet transplantation and IGF-I in earlier experiments. NGF treatment of control animals actually increased the frequency of NAD in the SMG. New data suggests that, in adult sympathetic ganglia. NGF may contribute to the pathogenesis of NAD rather than its amelioration, perhaps as the result of inducing intraganglionic axonal sprouts in which dystrophic changes are superimposed. NT-3 administration did not alter the frequency of NAD in diabetic animals, although it resulted in a significant decrease in NAD in control SMG. Although deficiencies of neurotrophic substances may represent the underlying pathogenesis of a variety of experimental neuropathies, delivery of excessive levels of selected substances may produce untoward effects.

The polyol pathway and its dependent biochemical pathways are thought to play a role in the pathogenesis of diabetic neuropathy. We have developed an animal model of diabetic autonomic neuropathy characterized by neuroaxonal dystrophy involving ileal mesenteric nerves and prevertebral sympathetic superior mesenteric ganglia (SMG) in chronic streptozocin-diabetic rats. Our previous studies have shown a salutary effect of aldose reductase inhibitors on experimental autonomic neuropathy, suggesting a role for the polyol pathway in its pathogenesis. In the current studies we have examined the effect of the sorbitol dehydrogenase inhibitor (SDI) CP-166,572, which interrupts the conversion of sorbitol to fructose (and reactions dependent on the second step of the polyol pathway) resulting in markedly increased levels of sorbitol in peripheral nerve. Fourteen weeks of treatment with CP-166,572 resulted in a dramatically increased frequency of neuroaxonal dystrophy in ileal mesenteric nerves and SMG. Although lesions developed prematurely and in greater numbers in SDI-treated diabetics than untreated diabetics did, their anatomic distribution and ultrastructural appearance were identical to that previously reported in long-term untreated diabetics. CP-166,572 treatment did not produce neuroaxonal dystrophy in control animals despite the fact that sciatic nerve sorbitol levels were markedly increased, reaching the same levels as untreated diabetic animals. Treatment of diabetic rats for 14 weeks with the aldose reductase inhibitor zopolrestat resulted in a significant decrease in the frequency of neuroaxonal dystrophy compared with untreated diabetics.

Prevertebral and paravertebral sympathetic autonomic ganglia respond differently to a large number of experimental and clinical insults. The selective involvement of subpopulations of sympathetic neurons may reflect differences in their response to neurotrophic substances. To test this hypothesis, we investigated the response of prevertebral and paravertebral rat sympathetic ganglia to selected neurotrophic substances in vivo and in vitro and identified the ganglionic distribution of neurons expressing high affinity neurotrophin receptor mRNAs. Dissociated cultures of embryonic prevertebral and paravertebral ganglionic neurons showed comparable responses to NGF deprivation and only small differences in their response to rescue with other trophic substances. In situ hybridization studies of adult rat sympathetic ganglia using probes specific for the high-affinity neurotrophin receptor transcripts trks A, B, and C demonstrated that neurons in both prevertebral and paravertebral sympathetic ganglia express predominantly trkA receptors in vivo. In addition, increased tyrosine hydroxylase (TOH) activity was induced only by doses of neurotrophic substances that activate trkA and showed only small differences between neonatal prevertebral and paravertebral ganglia. Although small differences in the sensitivity of pre- and paravertebral sympathetic neurons to various neurotrophins have been identified in our studies, they are unlikely, in isolation, to explain major differences in the sensitivity of these ganglia to neuropathologic processes.

Neuroaxonal dystrophy, characterized by swollen axon terminals and, to a lesser degree, enlarged initial segments of axons or perikaryal projections, develops in human dorsal root sensory ganglia as a function of aging and diabetes. Lesions are typically located within the satellite cell capsule and are intimately applied to sensory neuronal perikarya, which are compressed and distorted but are otherwise normal. Swollen axons contain large numbers of neurofilaments that are immunoreactive with antisera to highly phosphorylated neurofilament epitopes but fail to stain with antisera directed against hypophosphorylated neurofilament epitopes. Other dystrophic swellings contain collections of tubulovesicular profiles admixed with neurotransmitter granules. Neuroaxonal dystrophy involves subpopulations of intraganglionic axons and apparent terminals, notably those containing CGRP, while apparently sparing others, including noradrenergic sympathetic axons. Diabetic subjects develop lesions prematurely and in greater numbers than in aged subjects. Individual dystrophic axons in diabetics and aged human subjects are identical in their light microscopic, immunohistochemical and ultrastructural appearance, suggesting the possibility of shared pathogenetic mechanisms.

Gracile neuroaxonal dystrophy (NAD) is a hallmark of the aging human and rodent sensory nervous systems which may represent an abnormal transganglionic response to peripheral axonal injury. To examine the structural plasticity of central dorsal root ganglia (DRG)-derived axons in the gracile nucleus, we evaluated the response of the lumbar DRG and their central projections to sciatic nerve injury in young and old rats. In uninjured rats neither the DRG nor its central projections contained histochemical immunoreactivity for neuropeptide Y (NPY). However, within 1 week of sciatic nerve crush or transection injury, NPY immunoreactivity appeared in the lumbar DRG and its central projections, reaching an apparent maximum in number and intensity of processes at 28 days. Neuropeptide Y immunoreactivity was more intense and sustained in response to transection compared to crush injury, results supported by NPY radioimmunoassay. Neuropeptide Y-immunoreactive processes in the gracile nuclei of axotomized young animals consisted of delicate axons or slightly enlarged profiles that may represent regenerative elements. Lumbar dorsal rhizotomy performed simultaneously with sciatic nerve transection prevented the transganglionic NPY response. Dystrophic axons in the gracile nucleus of non-lesioned aged animals were not NPY-immunoreactive; however, after sciatic nerve transection, NPY immunoreactivity developed in both delicate axons and markedly swollen dystrophic elements, a finding confirmed by ultrastructural immunolocalization. These results establish that despite the presence of NAD in DRG projections to aged gracile nuclei these elements remain capable of a plastic NPY response to peripheral nerve injury.

Repeated daily injections of the adrenergic blocking agent guanethidine in high doses (50 mg/kg) to either newborn or adult rats produces a permanent sympathectomy which is thought to involve an immune-mediated mechanism. Ultrastructural examination of the acute phase of guanethidine-induced neuronal degeneration in the superior cervical ganglion demonstrated a prominent lymphocytic infiltrate in the ganglionic parenchyma and perivascular spaces. Within the satelliting cell sheath there were lymphocytes with delicate processes in intimate contact with abnormal or degenerating principal sympathetic neurons. The lymphocytic infiltrate and early evidence of neuronal degeneration developed within three days of onset of treatment and reached a maximum on the 7th day. Immunocytochemical studies of the ganglion demonstrated that the mononuclear infiltrate was composed primarily of CD-8 positive, OX-19 negative lymphocytes, which may represent a natural killer (NK) or NK-like effector cell, as well as scattered monocytes and macrophages. Guanethidine showed little effect on the prevertebral (superior mesenteric and celiac) sympathetic ganglia compared to the paravertebral (superior cervical) chain ganglia of the same animals. Considerable rat strain selectivity was noted with Lewis and Sprague-Dawley strains showing a greater degree of neuronal destruction than Fischer 344 rats. Neonatal thymectomy failed to prevent neuronal degeneration and the lymphocytic infiltrate. These features of immune neuronal killing are not typical of a characterized antibody or T-lymphocyte mediated effector mechanism, but may represent a selective NK cell or NK-like cell mediated pathogenetic pathway. Guanethidine-induced neuronal destruction appears to be a unique type of drug-induced, cell-mediated, autoimmune attack on sympathetic neurons and may provide insight into autoimmune disorders and possibly into diseases characterized by selective neuronal degeneration.

Alterations in the extrinsic innervation of the alimentary tract and in various sympathetic autonomic ganglia were examined using quantitative ultrastructural and biochemical methods in streptozotocin diabetic rats maintained without treatment for 9-15 months. Ileal mesenteric nerves of 13-15 month diabetics showed the characteristic alterations of neuroaxonal dystrophy qualitatively similar to changes seen in this system in animals which were diabetic for shorter durations. Dystrophic axonopathy was not accompanied by significant axonal loss or atrophy. Dystrophic axons, presumably involving presynaptic elements, also were increased in frequency in the superior mesenteric ganglia, but not in the superior cervical ganglia, of animals diabetic for various durations. In addition, the superior mesenteric ganglia of diabetic animals contained an increased number of postsynaptic dendritic processes which were dilated by unusual tubular profiles. These ganglionic alterations were not accompanied by changes in the activity of the presynaptic cholinergic marker enzyme, choline acetyltransferase. The activity of the noradrenergic marker enzyme, dopamine-beta-hydroxylase, was unaltered in diabetic superior mesenteric ganglia compared to controls, but showed a 30-40% decrease in diabetic superior cervical ganglia, in which site it was unaccompanied by neuronal loss, atrophy, or ultrastructural alterations.

Distinctive axonal abnormalities in selected sympathetic ganglia of male and female Sprague-Dawley and male Fischer 344 rats have been characterized as a function of increasing age. Dilated intraganglionic axons contained a variety of unusual subcellular organelles, including large numbers of compact or loosely aggregated tubulovesicular profiles, layered membranous loops, and collections of normal and degenerating organelles, identical to those described in clinical and experimental neuroaxonal dystrophy. Although occasional dystrophic axons were encountered free within the ganglionic neuropil, the majority were intimately associated with the surface of major dendrites or perikarya of principal sympathetic neurons and presumably represent terminal preganglionic axons. However, neither synaptic vesicles in dystrophic axons nor the association of dystrophic axons with post-synaptic densities were demonstrated. Dystrophic axons were infrequent within the prevertebral superior mesenteric and celiac ganglia before one year of age, but increased substantially in numbers thereafter. Similar findings expressed as lesions per mm2 were approximately tenfold less frequent in the paravertebral superior cervical ganglia of the same animals.

The ultrastructural appearance of axons regenerating after crush injury was examined in rat sciatic nerves in which proximodistal growth was interrupted (frustrated regeneration) by placement of a tight ligature 1 cm distal to the original crush injury, and in nerves lacking a distal tie (unimpeded regeneration). Examination of unimpeded regenerating axons four and seven days after injury showed minute axonal sprouts as well as scattered dilated (2-10 microns) axonal profiles containing large numbers of anastomosing tubulovesicular elements and vacuoles. These dilated profiles were consistent with the appearance of growth cones, the motile tips of regenerating axons, as described in various in vivo and in vitro systems. The ultrastructural appearance of regenerating axons accumulating proximal to a frustrating tie was a function of time after arrival at the ligature. At the earliest time examined (one week) large numbers of axonal profiles accumulated at the ligature which were qualitatively similar to growth cones seen in unimpeded regeneration, although slightly larger in diameter. With time, the uniform population of growth cones proximal to the frustrating ligature was replaced by dilated axons containing large numbers of neurofilaments, dense collections of heterogeneous membranous organelles, or delicate to coarse tubulovesicular elements admixed with a variety of subcellular organelles. Rare examples of axonal dystrophy were demonstrated after chronic frustration of regeneration; however, they represented only a small percentage (less than 1%) of frustrated axons. Frustrated regeneration was accompanied by scattered examples of demyelination/remyelination of large axons proximal to the ligature as well as transperineurial growth of axons as microfascicles to escape the site of ligation.