Wallerian degeneration in the optic nerve of a reptile: an electron microscopic study

Functional implications of species differences in the size and morphology of the isthmo optic nucleus (ION) in birds

In birds, there is a retinofugal projection from the brain to the retina originating from the isthmo optic nucleus (ION) in the midbrain. Despite a large number of anatomical, physiological and histochemical studies, the function of this retinofugal system remains unclear. Several functions have been proposed including: gaze stabilization, pecking behavior, dark adaptation, shifting attention, and detection of aerial predators. This nucleus varies in size and organization among some species, but the relative size and morphology of the ION has not been systematically studied. Here, we present a comparison of the relative size and morphology of the ION in 81 species of birds, representing 17 different orders. Our results show that several orders of birds, besides those previously reported, have a large, well-organized ION, including: hummingbirds, woodpeckers, coots and allies, and kingfishers. At the other end of the spectrum, parrots, herons, waterfowl, owls and diurnal raptors have relatively small ION volumes. ION also appears to be absent or unrecognizable is several taxa, including one of the basal avian groups, the tinamous, which suggests that the ION may have evolved only in the more modern group of birds, Neognathae. Finally, we demonstrate that evolutionary changes in the relative size and the cytoarchitectonic organization of ION have occurred largely independent of phylogeny. The large relative size of the ION in orders with very different lifestyles and feeding behaviors suggest there is no clear association with pecking behavior or predator detection. Instead, our results suggest that the ION is more complex and enlarged in birds that have eyes that are emmetropic in some parts of the visual field and myopic in others. We therefore posit that the ION is involved in switching attention between two parts of the retina i.e. from an emmetropic to a myopic part of the retina.

The postnatal development of the optic nerve of a reptile (Vipera aspis): A quantitative ultrastructural study

The number of axons in the optic nerve of the ovoviviparous reptile Vipera aspis was estimated from electron micrographs taken during the first 5 weeks of postnatal life. One to two days after birth, the optic nerve contains about 170,000 fibres, of which about 9% are myelinated. At the end of the fifth postnatal week, the number of optic fibres has fallen to about 100,000, of which about 42% are myelinated. This fibre loss continues after the fifth postnatal week, since in the adult viper the nerve contains about 60,000 fibres, of which 85% are myelinated; overall, about 65% of the optic nerve fibres present at birth disappear before the number of axons stabilises at the adult level. This study shows, for the first time, that the mode of development of the visual axons of reptiles is not that of anamniote vertebrates but similar to that of birds and mammals.

The centrifugal visual system of vertebrates: a comparative analysis of its functional anatomical organization

The present review is a detailed survey of our present knowledge of the centrifugal visual system (CVS) of vertebrates. Over the last 20 years, the use of experimental hodological and immunocytochemical techniques has led to a considerable augmentation of this knowledge. Contrary to long-held belief, the CVS is not a unique property of birds but a constant component of the central nervous system which appears to exist in all vertebrate groups. However, it does not form a single homogeneous entity but shows a high degree of variation from one group to the next. Thus, depending on the group in question, the somata of retinopetal neurons can be located in the septo-preoptic terminal nerve complex, the ventral or dorsal thalamus, the pretectum, the optic tectum, the mesencephalic tegmentum, the dorsal isthmus, the raphé, or other rhombencephalic areas. The centrifugal visual fibers are unmyelinated or myelinated, and their number varies by a factor of 1000 (10 or fewer in man, 10,000 or more in the chicken). They generally form divergent terminals in the retina and rarely convergent ones. Their retinal targets also vary, being primarily amacrine cells with various morphological and neurochemical properties, occasionally interplexiform cells and displaced retinal ganglion cells, and more rarely orthotopic ganglion cells and bipolar cells. The neurochemical signature of the centrifugal visual neurons also varies both between and within groups: thus, several neuroactive substances used by these neurons have been identified; GABA, glutamate, aspartate, acetylcholine, serotonin, dopamine, histamine, nitric oxide, GnRH, FMRF-amide-like peptides, Substance P, NPY and met-enkephalin. In some cases, the retinopetal neurons form part of a feedback loop, relaying information from a primary visual center back to the retina, while in other, cases they do not. The evolutionary significance of this variation remains to be elucidated, and, while many attempts have been made to explain the functional role of the CVS, opinions vary as to the manner in which retinal activity is modified by this system.

Ultrastructural Study of Normal and Degenerating Nerve Fibers in the Protocerebral Tract of the Crab Ucides cordatus

Wallerian degeneration is a very well described phenomenon in the vertebrate nervous system. In arthropods, and especially in crustaceans, nerve fiber degeneration has not been described extensively. In addition, literature shows that the events do not follow the same patterns as in vertebrates. In this study we report, by qualitative and quantitative ultrastructural analyses, the features and time course of the protocerebral tract degeneration following extirpation of the optic stalk. No remarkable changes were observed seven days after lesion. After 28 days the protocerebral tracts presented apparently preserved small and large diameter axons and some degenerating medium axons, with irregular contours and empty-looking aspect of the axoplasm. Forty days after the ablation of the optic stalks, both small (type I) and medium (type II and III) axons revealed signs of partial or total degeneration, but large nerve fibers (type IV) were still intact. After 45 days, the tract showed signs of advanced stage of degeneration and, apart from large axons, normal-looking fibers were almost absent. At these 3 last time points, degenerating axons displayed different electron densities and aspects, probably correlating to different onset times of the process. In addition, cells with granules in their cytoplasm, possibly hemocytes, were quite distinct, especially at 40 and 45 days after axotomy. These cells might share with glial cells the function of phagocytosis of cellular debris during the protocerebral tract degeneration. Quantitative analysis showed that the number of degenerating fibers increased significantly from 28 to 40 days after lesion, whereas the number of normal fibers decreased accordingly. Measurements of cross-sectional areas of normal and degenerating axons showed that types II and III (medium) start to degenerate before type I (small). Type IV (large) axons do not degenerate, even after 40 days. Therefore, we can conclude that degeneration in these afferent fibers starts late after axotomy, but proceeds at a faster rate afterwards until the complete degeneration of small and medium axons.

Centrifugal visual system of Crocodylus niloticus: a hodological, histochemical, and immunocytochemical study

The retinopetal neurons of Crocodylus niloticus were visualized by retrograde transport of rhodamine beta-isothiocyanate or Fast Blue administered by intraocular injection. Approximately 6,000 in number, these neurons are distributed in seven regions extending from the mesencephalic tegmentum to the rostral rhombencephalon, approximately 70% being located contralaterally to the injected eye. None of the centrifugal neurons projects to both retinae. The retinopetal neurons are located in rostrocaudal sequence in seven regions: the formatio reticularis lateralis mesencephali, the substantia nigra, the griseum centralis tectalis, the nucleus subcoeruleus dorsalis, the nucleus isthmi parvocellularis, the locus coeruleus, and the commissura nervi trochlearis. The greatest number of cells (approximately 93%) is found in the nucleus subcoeruleus dorsalis. The majority are multipolar or bipolar in shape and resemble the ectopic centrifugal visual neurons of birds, although a small number of monopolar neurons resembling those of the avian isthmo-optic nucleus may also be observed. A few retinopetal neurons in the griseum centralis tectalis were tyrosine hydroxylase (TH) immunoreactive. Moreover, in the nuclei subcoeruleus dorsalis and isthmi parvocellularis, both ipsilaterally and contralaterally, approximately one retinopetal neuron in three (35%) was immunoreactive to nitric oxide synthase (NOS), and a slightly higher proportion (38%) of retinopetal neurons were immunoreactive for choline acetyltransferase (ChAT). Some of them contained colocalized ChAT and NOS/reduced nicotinamide adenine dinucleotide phosphate-diaphorase. Fibers immunoreactive to TH, serotonin (5-HT), neuropeptide Y (NPY), or Phe-Met-Arg-Phe-amide (FMRF-amide) were frequently observed to make intimate contact with rhodamine-labeled retinopetal neurons. These findings are discussed in relation to previous results obtained in other reptilian species and in birds.

Failure to restore vision after optic nerve regeneration in reptiles: Interspecies variation in response to axotomy

Optic nerve regeneration within the reptiles is variable. In a snake, Viper aspis, and the lizard Gallotia galloti, regeneration is slow, although some retinal ganglion cell (RGC) axons eventually reach the visual centers (Rio et al. [1989] Brain Res 479:151-156; Lang et al. [1998] Glia 23:61-74). By contrast, in a lizard, Ctenophorus ornatus, numerous RGC axons regenerate rapidly to the visual centers, but unless animals are stimulated visually, the regenerated projection lacks topography and animals remain blind via the experimental eye (Beazley et al. [2003] J. Neurotrauma 20:1263-1269). V. aspis, G. galloti, and C. ornatus belong respectively to the Serpentes, Lacertidae, and Agamidae within the Eureptilia, the major modern group of living reptiles comprising the Squamata (snakes, lizards, and geckos) and the Crocodyllia. Here we have extended the findings on Eureptilia to include two geckos (Gekkonidae), Cehyra variegata and Nephrurus stellatus. We also examined a turtle, Chelodina oblonga, the Testudines being the sole surviving representatives of the Parareptilia, the more ancient reptilian group. In all three species, visually elicited behavioral responses were absent throughout regeneration, a result supported electrophysiologically; axonal tracing revealed that only a small proportion of RGC axons crossed the lesion and none entered the contralateral optic tract. RGC axons failed to reach the chiasm in C. oblonga, and in G. variegata, and N. stellatus RGC axons entered the opposite optic nerve; a limited ipsilateral projection was seen in G. variegata. Our results support a heterogeneous response to axotomy within the reptiles, each of which is nevertheless dysfunctional.

Axonal sprouting in the optic nerve is not a prerequisite for successful regeneration

Axonal sprouting, the production of axons additional to the parent one, occurs during optic nerve regeneration in goldfish and the frog Rana pipiens, with numbers of regenerate axons exceeding normal values four- to sixfold (Murray [1982] J. Comp. Neurol. 209:352-362; Stelzner and Strauss [1986] J. Comp. Neurol. 245:83-103). To determine whether axonal sprouting is a prerequisite for regeneration, the frog Litoria moorei was examined, a species that undergoes successful optic nerve regeneration but with a different time course compared with R. pipiens. Sprouting was assessed, as in goldfish and R. pipiens, from electron microscopic counts between the lesion and chiasm. However, disconnected axons that persist after axotomy would have falsely elevated the counts. The suspected overlap of these two axon populations was confirmed by labeling regenerate axons anterogradely with DiI (1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate) and disconnected ones retrogradely with DiA (4-4-dihexadecylaminostyrl 1-N methylpyridinium iodide). Numbers of disconnected axons were estimated after preventing regeneration and subtracted from numbers in regenerate nerves. Throughout, the total number of regenerate axons was approximately one third lower than normal (P < 0.05) supporting a previous finding of minimal axonal sprouting in L. moorei (Dunlop et al. [2002] J. Comp. Neurol. 446:276-287). The validity of the subtractive electron microscopic method was confirmed by retrograde labeling to estimate numbers of retinal ganglion cells whose axons had crossed the lesion; values were approximately one third lower than normal. The data suggest that sprouting is not essential for either axon outgrowth or topographic map refinement.

Watery and dark axons in Wallerian degeneration of the opossum's optic nerve: different patterns of cytoskeletal breakdown?

In this paper we report a qualitative morphological analysis of Wallerian degeneration in a marsupial. Right optic nerves of opossums Didelphis marsupialis were crushed with a fine forceps and after 24, 48, 72, 96 and 168 hours the animals were anaesthetized and perfused with fixative. The optic nerves were immersed in fixative and processed for routine transmission electron microscopy. Among the early alterations typical of axonal degeneration, we observed nerve fibers with focal degeneration of the axoplasmic cytoskeleton, watery degeneration and dark degeneration, the latter being prevalent at 168 hours after crush. Our results point to a gradual disintegration of the axoplasmic cytoskeleton, opposed to the previous view of an "all-or-nothing" process (Griffin et al 1995). We also report that, due to an unknown mechanism, fibers show either a dark or watery pattern of axonal degeneration, as observed in axon profiles. We also observed fibers undergoing early myelin breakdown in the absence of axonal alterations.

The primary visual system of adult lizards demonstrates that neurogenesis is not obligatorily linked to central nerve regeneration but may be a prerequisite for the restoration of maps in the brain

Following optic nerve crush in the adult lizard Ctenophorus ornatus, most retinal ganglion cells regrow their axons into visual brain centres: however, the regenerated projections lack retinotopic order and the animals are blind via the experimental eye. Here we have used 3H-thymidine autoradiography to demonstrate that cell division is no longer taking place in the retina of normal adult lizards. We conclude that the optic nerve can regenerate in lizard even though cells are no longer being added to the retina. However, continued retinal neurogenesis may be linked to the ability to restore topographic maps.

Optic nerve regenerates but does not restore topographic projections in the lizard Ctenophorus ornatus

In adult fish and amphibians, the severed optic nerve regenerates and visual behaviour is restored. By contrast, optic axons do not regenerate in the more recently evolved birds and mammals. Here we have investigated optic nerve regeneration in a member of the class Reptilia, phylogenetically intermediate between the fish and amphibians and the birds and mammals. We assessed visual recovery anatomically and behaviourally one year after unilateral optic nerve crush in the adult ornate dragon lizard. Ctenophorus ornatus. Ganglion cell densities and numbers of axons in the optic nerve on either side of the crush site indicated that two-thirds of ganglion cells survived axotomy and regrew their axons. However, myelination fell from a mean of 21% in normals to 5.5% and 3%, proximal and distal to the crush, respectively. Anterograde labelling of the entire optic nerve showed that axons regenerated along essentially normal pathways and that the major projection, as in normals, was to the superficial one-third of the contralateral optic tectum. However, localised retinal injections indicated that regenerated projections lacked retinotopic order. Any one retinal region projected to the entire tectum. This feature presumably explains why the experimental lizards consistently appeared blind to stimuli via the regenerated nerve. Our findings indicate that although axons regenerate along essentially normal pathways in adult lizards, conditions within the visual centres do not allow regenerating optic axons to select appropriate central connections. In a wider context, the result suggests that the ability for regenerating central axons to form topographic maps may also have been lost in the more recently evolved vertebrate classes.

Biphasic cellular response to transection in the newt optic nerve: glial reactivity precedes axonal degeneration

Morphological interactions between axons and glia within the lesioned newt optic nerve were studied at time periods prior to the onset of Wallerian degeneration. Optic nerves were transected 0.5 mm from the eye, animals were killed at 5, 10, 20 and 30 min post-lesion, and the intracranial half of the tract was examined with light and electron microscopy. A sequence of structural changes was observed within the time interval 5-30 min post-lesion. Over the first 20 minutes these changes primarily involved the endogenous neuroglia; there was a displacement of glial nuclei from the center to the periphery of the nerve and an increase of 50-100% in glial cytoplasmic and nuclear area. Nuclei of reactive glia were euchromatic and surrounded by a high density of Golgi, vesicles, mitochondria and filaments, the last of which extended throughout the expanded glial processes. Optic axons appear intact at 20 min post-lesion except for some separation between the axolemma and myelin sheath in some of the myelinated fibres. By 30 min post-lesion both myelinated and non-myelinated fibres were found in various stages of lysis. Many of the expanded glial processes contained a population of vesicles aggregated adjacent to the glial plasmalemma. Profiles of infolded glial membranes suggested the opening of such vesicles into the extracellular space around degenerating axons. We conclude that, after optic nerve injury, there are very rapid reactive changes in glia and axons, with the changes in glia preceding the degenerative events in axons.

Neovascularization occurs in response to crush lesions of adult frog optic nerves

The capacity of the adult frog optic nerve to regenerate following a crush lesion is well established and is in contrast to the lack of regeneration of mammalian optic nerves after similar lesions. One factor which may contribute to the enhanced regenerative capacity of amphibian optic nerves is the rapid removal of cellular debris from the nerve after injury. In this study the morphology of normal and crushed frog optic nerves has been compared. Although the intraorbital region of the normal adult frog optic nerve is avascular, new intraparenchymal blood vessels appear central to the crush site 24 h after the nerve lesion. The appearance of these blood vessels is coincident with the appearance of granulocytes and macrophages in the nerve. Successful regeneration of the adult frog optic nerve may depend on this neovascularization to facilitate the rapid removal of cellular debris and to supply regenerating axons with trophic substances.

Nodal and paranodal structural changes in mouse and rat optic nerve during Wallerian degeneration

Ultrastructural changes in nodal and paranodal regions of myelinated mouse and rat optic nerve fibers were followed between 4 h and 28 days during the course of Wallerian degeneration. In the mouse, axoplasmic changes, including accumulation of organelles and segregation of microtubules, were detectable 4 h after transection, and progressed to a maximum level on day 4, at which time many axons were markedly swollen. Dense axoplasm was seen as early as 16 h and was a common feature of degenerating axoplasm at later times. Paranodal changes, which first appeared as early as 16 h after injury, included detachment of terminal loops of myelin from the axolemma, disconnection of terminal loops from compact myelin lamellae and broadening of terminal loops, or separation of the loops from each other, resulting in paranodal elongation. In freeze-fracture replicas, the E-face of the axolemma showed the normal particle distribution as late as days 3-5. By day 8, however, the nodal particles were patchy and the overall nodal particle density was reduced to approximately half normal. Some normal-looking fibers were present at all stages examined, but their number had declined to about half the total population on day 5 and to less than 10% on day 11. In the rat, the overall sequence of events and time course were comparable to those in the mouse. Thus, the morphological changes found follow approximately the same sequence as that described previously in frog nerves, but progress more rapidly in the mouse and rat.

Nodal and paranodal structure during Wallerian degeneration in frog spinal nerve

The nodal and paranodal regions of myelinated peripheral nerve fibers in frogs were examined at sequential times (1-24 days) during Wallerian degeneration. In the region up to 3 mm distal to the transection, paranodal demyelination and axoplasmic degeneration became apparent on day 4 and progressed to involve most of the nodes by day 8. The E-fracture face of the axolemma showed a patchy distribution of nodal particles and some paranodal demyelination on days 4 and 6. On day 8, nodal particles were evenly distributed at low concentration and the adjacent demyelinated paranodal regions showed a corresponding increase in particle density, suggesting redistribution of the nodal particles. The sequence of changes seen in comparable to that in Wallerian degeneration of central nervous system (CNS) fibers but progressed more rapidly in the peripheral nervous system (PNS). In addition a higher proportion of PNS fibers shows pathological changes at corresponding time periods.

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

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

Myelopathy induced by lactic acid

This study defines the conditions required to provoke myelopathic changes by dripping lactic acid onto the surgically exposed spinal cord of adult male rats. A severe necrotizing myelopathy was observed after 24 h, principally in the posterior half of the cord at the level of lactic acid (pH 1.8) application. A profound early effect on small blood vessel walls, appearing necrotic after 30 min to 2 h, was identified. Nerve fiber alterations (axonal stasis, granular axoplasm, axonal calcification, and vesicular myelin), identical to those appearing early in the myelopathies of trauma and calcium toxicity, were apparent. However, the pathogenesis of these alterations in this model remains unclear, because of the vascular events and the presumed alterations of calcium metabolism by the acid. Further studies are required to elucidate the precise mechanisms of these important reactions of myelinated axons to the injuries provoked by acid, calcium, and trauma.

Nodal and paranodal structural changes in frog optic nerve during early Wallerian degeneration

Ultrastructural changes in the nodal and paranodal regions of myelinated nerve fibres of frog optic nerves were studied during early stages of Wallerian degeneration. The earliest changes seen include retraction of paranodal loops of myelin from the axolemma and disconnection of paranodal myelin loops from myelin lamellae. These paranodal changes are asymmetric around the node and may be more advanced on either the proximal or distal side. Axoplasmic changes, including segregation of microtubules from neurofilaments, disorientation of microtubules and accumulation of abnormal organelles at nodes, appear shortly. In some axons the 'undercoating' along the widened nodal surfaces becomes patchy, and blebs appear in the nodal axolemma. In freeze-fracture replicas a mixture of particle clusters and particle-free areas appears in both E- and P-faces of the nodal axolemma. Blebs remain particle free. Initially, E-face particles remain segregated to the node and are present only at much lower concentrations in the demyelinated paranodal axolemma, suggesting that they are not freely mobile at this stage. Nodal E-face particles begin to decrease on day 5 associated with an increase in particles at the adjacent demyelinated paranode, and by day 11 the particle distribution is uniformly low over the entire extent of the nodal and demyelinated paranodal axolemma. If nodal E-face particles represent sodium channels, as has been proposed, the sequence of changes in Wallerian degeneration would be compatible with a gradual redistribution of nodal sodium channels into the demyelinated paranode.

Transganglionic degeneration in vibrissae innervating primary sensory neurons of the rat: a light and electron microscopic study

Previous studies have shown that transection of peripheral branches of primary sensory neurons leads to light microscopical degeneration argyrophilia and ultrastructural changes in the central termination areas of these neurons. This type of degeneration has been termed transganglionic degeneration (TGD). In the present experiments TGD has been studied specifically in neurons innervating the rat vibrissae at the light and electron microscopic levels. Light microscopically, small amounts of degeneration argyrophilia are observed in the magnocellular zone of the trigeminal subnucleus caudalis at 8-14 days survival. At longer survival times there are substantial amounts of degeneration in this area. At the ultrastructural level the first signs of TGD are observed at 6 days survival, when some terminals show a small increase in electron density, loss of synaptic vesicles, and mitochondrial disintegration. Terminals showing a more advanced increase in electron density become common at 8 days survival, but few of them are still left at 14 days survival. Neurofilamentous terminals appear in small numbers 8-14 days postoperatively. Various forms of degeneration in myelinated axons are observed from 8 days survival and are common also at 80 days survival. Electron-dense axons are rather unfrequent, but more or less disrupted myelin sheaths containing disintegrated axoplasmic remnants and empty areas are common as well as extremely expanded myelin sheaths. Glial cells containing axonal and myelin debris are seen from 8 days survival and become a more common finding at longer survivals. A most striking finding 8-10 days postoperatively is a complex relationship between glial cells and less darkened terminals, indicating phagocytosis before reaching an entirely darkened state. The findings clearly show that peripheral nerve transection leads to severe central alterations in a population of mechanoreceptor neurons innervating the vibrissae of the adult rat.

Differentiation between Crohn's disease and other inflammatory conditions by electron microscopy

The authors previously have demonstrated axonal necrosis of autonomic nerves in the surgically resected ilea of patients with Crohn's disease both in grossly normal ileal resection margins and in diseased areas. The present study of ileal stomal biopsies was carried out to obviate the possibility that the observed axonal damage might be related to the prolonged surgical manipulations required for ileal resection. The authors present studies of biopsies of ileal stomas and of small bowel from patients with Crohn's disease and various control disorders, including ulcerative colitis. Stomal biopsies were fixed immediately after they were obtained. Widespread, severe axonal necrosis of autonomic nerves was present in all Crohn's disease specimens, regardless of the patient's clinical status or the gross or routine microscopic evaluation of the same specimen. Controls either had no necrosis or displayed a minor degree of focal necrosis involving single axons. The authors conclude that Crohn's disease is accompanied by a severe and extensive necrosis of gut axons, and that such electron microscopic findings may serve to differentiate Crohn's disease from other inflammatory disorders.

Degenerative and regenerative changes in the trochlear nerve of goldfish

The features of unlesioned and lesioned trochlear nerves of goldfish have been examined electron microscopically. Lesioned nerves were studied between 1 and 107 days after cutting or crushing the nerve. Unlesioned nerves contained, on average, 77 myelinated axons and 19 unmyelinated axons. The latter were found in 1-2 fascicles per nerve. A basal lamina surrounded each myelinated axon and fascicle of unmyelinated axons. The numbers of myelinated axons, fascicles of unmyelinated axons and basal laminae varied by less than 5% over the intraorbital extramuscular segment of the nerve. Following interruption of the nerve, by either cutting or crushing, all of the axons and their myelin sheaths began to degenerate by 4 days in the distal nerve-stump. Both abnormally electron-dense and electron-lucent axons were observed. Both Schwann cells and macrophages appeared to phagocytose the myelin sheaths. Following a lesion, the Schwann cells and their basal laminae persisted in the distal nerve-stump. In crushed nerves, the basal laminae surrounding myelinated axons formed 97%, on average, of the Schwann tubes in the distal stump. The perimeters of the basal laminae were of similar size to those in the proximal stump, at least for the first 8 days after crush. In crushed nerves, single myelinated axons in the proximal nerve-stump gave rise to multiple sprouts, some of which reached the site of crush by 2 days, the distal stump by 4 days and the superior oblique muscle by 8 days. The regeneration of the unmyelinated axons was not examined. In both crushed and transected nerves, nearly all of the sprouts in the proximal and distal stumps were found within the basal laminae of Schwann cells, even though the spouts were disorganized in the transected region where there were no basal laminae. The growth cones of the regenerating axons were always found apposed to the inner surface of the basal laminae, which may have provided an adhesive substrate that directed their growth. Terminal sprouts from the ends of myelinated axons in the proximal stump accounted for the majority of the regenerating axons in the distal stump, as only a few collateral sprouts were found in the proximal stump, and only a small amount of axonal branching was found within the distal stump itself. The largest axons in the distal stump were remyelinated first, and the number of remyelinated axons increased progressively between 8 and 31 days after crush, at which time there were about twice as many as in unlesioned nerves.(ABSTRACT TRUNCATED AT 400 WORDS)

Myelin breakdown and elimination in the posterior funiculus of the adult cat after dorsal rhizotomy: a light and electron microscopic qualitative and quantitative study

Adult cats were subjected to unilateral dorsal L6, L7, and S1 rhizotomy. After survival times of 1-1,552 days the degeneration of axons and myelin sheaths and the elimination of degenerating myelin was studied qualitatively and quantitatively with light and electron microscopy and in Marchi-stained sections in the posterior funiculus in T12-L2. Degeneration was first observed as swollen or shrunken nerve fibers. Somewhat later there was an increased occurrence of collapsed myelin sheaths. The latter lost their myelin periods and appeared to be transformed into myelin bodies. The occurrence of myelin bodies coincided temporally with the presence of many Marchi-positive bodies. Later, an increasing number of intracellularly located lipid droplets occurred, paralleled by the occurrence of a great number of Marchi-positive granules and crystalline structures. Profiles of collapsed myelin sheaths, myelin bodies, and lipid droplets were frequently seen in the cytoplasm of microglial cells. Later, astrocytes and perivascular cells became filled with numerous lipid droplets. The findings suggest that microglial cells take up collapsed myelin sheaths and within these cells the sheaths become transformed into myelin bodies and subsequently into lipid droplets. These two products of myelin disintegration appear to correspond to the Marchi-positive structures seen during the degeneration process. The lipid droplets appear to be transported to astrocytes and perivascular cells.

An examination of intraspinal sprouting in dorsal root axons with the tracer horseradish peroxidase

Postdeafferentation reorganization in the central terminal fields of spared dorsal root axons was evaluated by examining the intraspinal distribution of horseradish peroxidase-labeled sciatic nerve afferent fibers at various intervals following the removal of several lumbar dorsal root ganglia. The sciatic projection to the spinal cord, as determined by the pattern and density of intraspinal reaction product, was remarkably stable following the ganglionectomies. For as long as 3 months later, there was no evidence that sciatic afferent fibers had formed anomalous connections either with new spinal segments or in denervated areas within normal segments of entry. These findings cast doubt upon the existence of anatomic reorganization within the spinal cord following its partial deafferentation and suggest that physiological processes other than new axonal growth underlie observations such as postdenervation alterations in the response properties of dorsal horn neurons and the recovery of behavioral function.

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

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

Development of a transient retino-retinal pathway in hooded and albino rats

Intraocular HRP injections in E16-21 embryos show that during the normal development of the central optic projections in hooded and albino rats many optic axons grow through the chiasm into the contralateral eye. This retino-retinal projection disappears shortly after birth. This suggests that an initial, imprecise guidance of growing axons is followed by a selective elimination of axons taking aberrant pathways and failing to make appropriate synapses.

Membrane specializations and cytoplasmic channels of Schwann cells in mammalian peripheral nerve as seen in freeze-fracture replicas

Mammalian Schwann cells in rat, rabbit and human fetal nerves were studied using several cryoprotective agents for electron microscopic study of freeze-fracture replicas. The findings in fixed and unfixed tissue reveal surface plasmalemma caveolar specializations and the outer layer membrane junctional complexes found in non-mammalian species. The plasmalemma also reveals a complex arrangement of contours outlining cytoplasmic channel networks distinct from the long-recognized Schmidt-Lanterman incisures and paranodal cytoplasmic loops. A specialized interconnected channel system in the outer "loose" myelin layer displays relatively uniform dimensions comparable in diameter to nodal microvilli, paranodal loops and some incisures. An adaxonal tubular channel system constituting the "axon-Schwann network" is found in the internodal region in addition to other variants of the adaxonal Schwann plasmalemma. The several forms of sequestration of Schwann cell cytoplasm presumably underlie the specialized needs of cytoplasmic continuity in a dynamic functional entity in which large domains of cytoplasm have been displaced by the formation of compact myelin.

Quantitative studies of retinal ganglion cells in a turtle, Pseudemys scripta elegans. I. Number and distribution of ganglion cells

Multiple pathways for the transmission of visual information from retina to brain have been described in reptiles, but little is known about their functional organization. These parallel channels begin at the retina, and we have therefore begun to study the functional organization of retinal ganglion cells in the turtle, Pseudemys scripta elegans. This paper describes the numbers and distribution of cells in the ganglion cell layer. To develop criteria for the identification of ganglion cells, we labelled them retrogradely by applying horseradish peroxidase (HRP) to the optic nerve. Ganglion cells were found to vary substantially in size and cytology. In low density areas of the retina, ganglion cells typically have cytoplasm with well developed Nissl substance, a distinct, pale nucleus, and a large nucleolus. In high density areas of retina, ganglion cells are small, densely staining, and gliaform. The average minimum proportion of ganglion cells in the ganglion cell layer is 75--80% of total profiles. No more than five or six percent of profiles in the ganglion cell layer are neurons which do not send an axon into the optic nerve (displaced amacrine cells or intraretinal association cells). The ganglion cell layer of P. s. elegans can be divided into a number of regions on the basis of cell density. Isodensity maps constructed from Nissl-stained, wholemounted retinas indicate that there is an elongated region of high ganglion cell density, the visual streak, which extends from nasal to temporal retina and is oriented such that its long axis follows the horizontal axis of the eye. The streak is aligned with the externally visible iris line. Seen in cross-section, the ganglion cell layer in the streak is three to four cells thick; in nonstreak retina, ganglion cells form only a monolayer of somas. Ganglion cell density drops off more rapidly above the streak than below it. The temporal arm of the streak is both shorter and broader than the nasal arm. There is a peak in ganglion cell density at the midpoint of the streak, in the approximate center of the retina. Here, ganglion cell densities exceed 20,000 cells mm-2. The total number of ganglion cells in the retina is 350,000--390,000.

An ultrastructural study of transganglionic degeneration in the main sensory trigeminal nucleus of the rat

In adult rats subjected to unilateral transection of the infraorbital nerve, the main sensory trigeminal nucleus was studied by electron microscopy. Post-operative survival times varied between 2 and 60 days. A variety of ultrastructural alterations was observed from the sixth post-operative day onwards. These changes were in many respects similar to those seen in the course of Wallerian degeneration. Neurofilamentous boutons and axons were found 6-30 days post-operatively. Various types of dark boutons were observed between 7 and 30 days and axonal changes indicative of degeneration between 7 and 60 days post-operatively. Astrocytes and microglial cells contained degenerating structures 7-60 days post-operatively. The alterations observed in the present study are interpreted as related, at least in part, to the nerve cell degeneration and the nerve cell death previously shown to occur in the trigeminal ganglion after infraorbital nerve transection.

Ultrastructural sequence of myelin degradation. I. Wallerian degeneration in the rat optic nerve

The ultrastructural events in myelin degradation in the rat optic nerve following transection have been studied. Myelin debris was found in cells similar to multipotential glia cells (Vaughn and Peters, 1968) as well as in astrocytes and in few oligodendrocytes. The different types of inclusions found during myelin degradation were described in their quantitative relations. Similarities to inclusions described in adrenoleukodystrophy adn multiple sclerosis are discussed. By comparison of the ultrastructural findings with histochemical and biochemical data available a hypothetical model of myelin degradation is presented. The process starts with the degradation of digestible proteins resulting in uniformly layered lipid inclusions. Lipid degradation leads to the formation of unstructured lipid droplets and crystals. During the late stages of Wallerian degeneration numerous polymorph inclusion typed can be found, probably representing poorly digestible lipids or lipoproteins.

Axon regeneration across the site of injury in the optic nerve of the newt Triturus pyrrhogaster

The process by which axons regenerate following a freeze injury to the optic nerve of the newt was analyzed by light and electron microscopy. Freezing destroys cellular constituents in a one millimeter segment of the nerve, leaving intact the basal lamina and the blood supply to the eye. No axons are seen at the site of injury one to seven days post lesion. This contrasts with the persistence of normal-appearing but severed unmyelinated axons within the cranial stump which thus give a false appearance of early regeneration. The first axon sprouts traverse the lesion and enter the cranial strump by ten days. The number of regenerating axons increases rapidly thereafter with no signs of random growth at the site of injury. These axon sprouts tend to be somewhat larger than normal unmyelinated axons and contain dense core vesicles and abnormal organelles similar to those in growing axons in tissue culture. The persisting basal lamina inside the optic sheath appears to provide continuity across the site of injury, to orient axon sprouts, and to favor an orderly process of axon regeneration without neuroma formation.

The early stages of Wallerian degeneration in the severed optic nerve of the newt (Triturus viridescens)

The initiation of Wallerian degeneration in the severed optic nerve of the newt (Triturus viridescens) was very rapid and intense. Significant degeneration of nonmyelinated axons was observed as early as six hours after lesion (h.a.l.) and was almost complete by 48 h.a.l. Initial degeneration of non-myelinated axons began in "extracellular digestion chambers" formed between burgeoning ependymoglial processes. The remaining fragments and debris were later phagocytized by surrounding ependymoglial processes. Many axons of myelinated fibers have degenerated as early as 6 h.a.l. However, the overall population of myelinated axons degenerates at a much slower rate than nonmyelinated ones, for many of them appear intact as late as 48 h.a.l. Some myelin sheaths show significant signs of degeneration by 6 h.a.l. Indeed, by this time a number of myelinated fibers have completely degenerated leaving only large vacuolated spaces in the nerve parenchyma. Swelling and vacuolization of the sheath are among the earliest signs of myelin degeneration. The ependymoglial cell response to optic nerve lesion is manyfold and dramatic. By 6 h.a.l. there are signs of burgeoning ependymoglial processes which begin to resemble scar formation (gliosis) by 48 h.a.l. The morphological evidence is consistent with the concept of an important phagocytic role of ependymoglial cells during the early stages of optic nerve degeneration.

An electron microscopic study of lesion-induced synaptogenesis in the dentate gyrus of the adult rat. I. Magnitude and time course of degeneration

Synapses in the rat dentate gyrus are rapidly lost after removal of the primary input from the entorhinal cortex. In this paper we describe the extent and time course of degeneration and in the subsequent paper the nature of the reinnervation processes. They synapses of entorhinal afferents are remarkably concentrated in their zone of termination. Unilateral removal of the rat entorhinal cortex results in the loss of about 86% of all synapses in the outer three-fourths of the molecular layer of the epsilateral dentate gyrus. Entorhinal synapses are all asymmetric (Gray type I) and terminate on dendritic spines. Analysis of the degeneration reaction provides a means to examine the characteristics of the loss of a relatively homogeneous afferent on a single cell type. The morphological characteristics of the the degenerating terminals showed some heterogeneity; both the electron lucent and electron dense types of degenerating terminals were identified. The electron lucent type was observed only at short survival times. The time course of the loss of degenerating terminals was resolvable into two components, each of which followed first order decay kinetics. Thus degenerating entorhinal terminals behaved as a population which disappeared randomly at a rate dependent on the fraction of terminals present at any time. The loss of degenerating terminals was accompanied by the loss of postsynaptic sites. At short survival times the majority of postsynaptic sites (defined by the presence of a postsynaptic density) had disappeared. There was also a loss of complex spines and some shrinkage of the molecular layer.

Time- and dose-dependent influence of ouabain on the ultrastructure of optic neurones

The cardiac glycoside ouabain was injected into the eye-bulb of the teleost fish, Carassius carassius. Three doses of ouabain were used: 10(-4) M, 10(-5) M, 10(-6) M. The final concentrations in the vitreous body of the eye were approximately 3-10(-5) M, 3-10-6 M and 3-10-7 M, respectively. After 8 hrs, 1, 2, 4, 6 and 8 days the ultrastructural alterations of retinal ganglion cells, the optic axons near the bulb and the terminal segments in the optic tectum were studied. The high doses of ouabain induced an early necrobiosis of the cell bodies in the retina followed by degeneration in the nerve. This is characterized as a protracted form of Wallerian degeneration. The significance of the inhibition of Na+ -K+-activated ATPase at the perikaryal level for both the integrity of axonal morphology and the axonal flow is discussed.

Degeneration of sensory and motor axons in transplanted segments of a crustacean peripheral nerve

Segments of sensory and motor axons 0.3-0.5 mm in length were taken from crayfish peripheral limb nerves and transplanted into the abdominal cavity of the same animal. Transplanted sensory axons showed relatively few ultra-structural changes after one week, many had undergone complete lysis within two weeks, and almost all degenerated within three weeks. Transplanted motor axons appeared normal after one week, except for some hypertrophy of their surrounding glial sheaths. After two weeks, glial sheaths were grossly hypertrophied around motor axons; axonal mitochondria had increased in number and many had migrated from the periphery to the centre of the axon. The axonal membranes of all motor axons were still intact after three weeks, although most were no longer continuous after four weeks. By five weeks, all axonal material had completely disintegrated. These data suggest that axonal synthetic processes in crayfish sensory (and presumably motor) axons can maintain the axons relatively intact for 7-14 days and that transfer of substances form hypertrophied glial cells to motor axons may account for the longer survival times of transplanted motor axons.

The ultrastructure of Wallerian degeneration in the severed optic nerve of the newt (Triturus viridescens)

Wallerian degeneration in the severed newt's (Triturus viridescens) optic nerve is complete between the 10-14th post operative day (p.o.d.). Consequently, the newt optic nerve displays one of the most rapid degenerative responses yet reported for the central nervous system of vertebrates. In most cases it also exhibits the speed of degenerative phenomenon in the vertebrate peripheral nervous system. The degeneration of unmyelinated axons is most rapid and is completed by 2-3 p.o.d., compared to myelinated axons, most of which degenerate between 2-10 p.o.d. Myelin ring formation (vesicular transformation) is the principal form of lamellar breakdown and occurs in a highly organized manner which can be clearly staged. The glial cell response to Wallerian degneration is two-fold: cytoplasmic hypertrophy and myelin-lytic. Glial hypertrophy subsides by the 10-14 p.o.d. with the ingrowth of numerous regenerating nerve fibers. The myelin-lytic response accounts for most of the myelin destruction. Leukocyte-like and microglia-like cells also participate in myelin breakdown but to a lesser degree.

Regeneration and remyelination of Xenopus tadpole optic nerve fibres following transection or crush

Optic nerves of stage 54-56 Xenopus laevis tadpoles were either transected or crushed, and subsequent Wallerian degeneration, regeneration, and remyelination were examined. After 4 days, normal myelinated fibres were no longer present in the distal stump, and only a few unmyelinated fibres remained. After 10-13 days, the distal nerve consisted mainly of a core of reactive astrocytes with enlarged processes and scattered oligodendrocytes which persisted throughout the degenerative period. Regenerating axons traversed the site of the lesion and extended into the distal stump within 13-15 days. As regeneration progressed, astrocytic processes extended radially from the optic nerve's central cellular core and formed longitudinal compartments for regenerating axons. Between 15-19 days, a few regenerating fibres were remyelinated and by 35 days, more axons were surrounded either by thin collars of oligodendrocyte cytoplasm or by 1-3 spiral turns of myelin membrane. By 95 days, the number of myelinated fibres had increased to about 50% of those present in control nerves. Their myelin sheaths were normal in appearance and thickness relative to their respective axon diameters. The largest axons were surrounded by compact sheaths with 4-9 lamellae.