Regeneration and soma size changes following axotomy of the trochlear nerve

Grafting of a new target prevents synapse loss in abducens internuclear neurons induced by axotomy

The loss of afferent synaptic boutons is a prominent alteration induced by axotomy on adult central neurons. In this work we attempted to prove whether synapse loss could be reverted by reconnection with a new target. We severed the medial longitudinal fascicle of adult cats and then transplanted embryonic cerebellar primordia at the lesion site immediately after lesion. As previously shown, the transected axons from abducens internuclear neurons penetrate and reinnervate the graft [J Comp Neurol 444 (2002) 324]. By immunocytochemistry and electron microscopy we studied the synaptology of abducens internuclear neurons under three conditions: control, axotomy and transplant (2 months of survival time). Semithin sections of the abducens nucleus were immunostained against calretinin, to identify abducens internuclear neurons, and either synaptophysin (SF), to label synaptic terminals, or glial fibrillary acidic protein (GFAP) to detect the astrocytic reaction. Optical and linear density of SF and GFAP immunostaining were measured. Data revealed a significant decrease in the density of SF-labeled terminals with a parallel increase in GFAP-immunoreactive elements after axotomy. On the contrary, in the transplant group, the density of SF-labeled terminals was found similar to control, and the astrocytic reaction induced by lesion was significantly reduced. At the ultrastructural level, synaptic coverage and linear density of boutons were measured around the somata of abducens internuclear neurons. Whereas a significant reduction in both parameters was found after axotomy, cells of the transplant group received a normal density of synaptic endings. The ratio between F- and S-type boutons was found similar in the three groups. Therefore, these findings indicate that the grafting of a new target can prevent the loss of afferent synaptic boutons produced by the axotomy.

Axonal misdirection as contributing factor to aberrant reinnervation of muscles after facial nerve suture in cats

Abstract Whereas basic features of post-axotomy muscle reinnervation have been extensively studied in rats, little is known about axonal regrowth and pathfinding in cats. To address the question, adult cats were subjected to facial-facial anastomosis (FFA). First group served to establish optimal parameters for labeling of the zygomatic and buccal facial branches with 1,1'dioctadecyl-3,3,3,'3'-tetramethylindo-carbocyanine perchlorate (DiI) and Fast Blue (FB) placed onto respective transected nerves. The second group of animals underwent identical bilateral labeling 3 months after transection and suture of the right facial nerve. This group served to establish the number of motoneurons, which had branched after surgery and projected into both facial branches. On control side, DiI application onto zygomatico-orbital branch labeled 3883 +/- 598 (mean +/- S.D.) perikarya were confined to the dorsal and intermediate facial subnuclei, meanwhile an application of FB onto the buccal branch labeled 1617 +/- 552 perikarya in the lateral and ventrolateral subnuclei. There were no double-labeled cells. Three months after FFA all retrogradely labeled motoneurons were scattered throughout the entire facial nucleus. To establish the proportion of perikarya, that re-grew multiple axonal branches into both nerves, double-labeled (FB + DiI) motoneurons were counted from digital images. The zygomatico-orbital nerve contained 3311 +/- 430 DiI-labeled whereas the buccal nerve 1500 +/- 442 FB-labeled motoneurons. The occurrence of 311 +/- 103 double-labeled perikarya (DiI+FB) suggested that approximately 6% of all retrogradely labeled motoneurons branched axons into both nerves. I conclude that malfunctioning axonal pathfinding rather than deviant reinnervation contributed to poor recovery of function after FFA in the cat.

Response of abducens internuclear neurons to axotomy in the adult cat

The highly specific projection of abducens internuclear neurons on the medial rectus motoneurons of the oculomotor nucleus constitutes an optimal model for investigating the effects of axotomy in the central nervous system. We have analyzed the morphological changes induced by this lesion on both the cell bodies and the transected axons of abducens internuclear neurons in the adult cat. Axotomy was performed by the transection of the medial longitudinal fascicle. Cell counts of Nissl-stained material and calretinin-immunostained abducens internuclear neurons revealed no cell death by 3 months postaxotomy. Ultrastructural examination of these cells at 6, 14, 24, and 90 days postaxotomy showed normal cytological features. However, the surface membrane of axotomized neurons appeared contacted by very few synaptic boutons compared to controls. This change was quantified by measuring the percentage of synaptic coverage of the cell bodies and the linear density of boutons. Both parameters decreased significantly after axotomy, with the lowest values at 90 days postlesion ( approximately 70% reduction). We also explored axonal regrowth and the possibility of reinnervation of a new target by means of anterograde labeling with biocytin. At all time intervals analyzed, labeled axons were observed to be interrupted at the caudal limit of the lesion; in no case did they cross the scar tissue to reach the distal part of the tract. Nonetheless, a conspicuous axonal sprouting was present at the caudal aspect of the lesion site. Structures suggestive of axonal growth were found, such as large terminal clubs, from which short filopodium-like branches frequently emerged. Similar findings were obtained after parvalbumin and calretinin immunostaining. At the electron microscopy level, biocytin-labeled boutons originating from the sprouts appeared surrounded by either extracellular space, which was extremely dilated at the lesion site, or by glial processes. The great majority of labeled boutons examined were, thus, devoid of neuronal contact, indicating absence of reinnervation of a new target. Altogether, these data indicate that abducens internuclear neurons survive axotomy in the adult cat and show some form of axonal regrowth, even in the absence of target connection.

Trigeminal ganglion cell response to mental nerve transection and repair in the rat

PURPOSE

Animal studies have suggested that peripheral nerve transection results in substantial loss of ganglion cells and the selective survival of cells based on size. The implications are that subsequent repair of peripheral nerve injuries will be determined by the numerical density and character of the surviving cells. The purpose of this study was twofold: First, to determine the effect of mental nerve transection without repair on trigeminal ganglion cell density and morphology in adult rats, and second, to determine the variation of trigeminal ganglion cell density and morphology after immediate and delayed repair.

MATERIALS AND METHODS

In the first part of the study, 12 adult male Sprague-Dawley rats had their mental nerves exposed bilaterally (n = 24). Twelve mental nerves were then transected and prevented from regenerating, and the remaining 12 nerves were uninjured. Ninety and 180 days after transection or sham surgery, the trigeminal ganglia were serially cut into 5 microm longitudinal sections along the dorsoventral axis. The volume and volume density of the mandibular mental subdivision containing sensory cells was determined at each section level with point-counting methods. The numerical density and total number of cells was estimated on the same section, using an unbiased three-dimensional stereological probe, the dissector. Cell size and shape determinants were estimated using the dissector and computerized planimetry. In the second part of the study, six rats had the mental nerves transected bilaterally and immediately repaired by microscopic sutures. In six additional rats, the repair was delayed for 90 days. In both groups, the trigeminal ganglia were serially cut at 30, 60, and 90 days post-repair and stereologic estimates of numerical density and histomorphometry were examined using the dissector and computed planimetry.

RESULTS

In the trigeminal ganglia of the 12 sham-operated animals, the mean number of cells was 20.6 x 10(3) (+/-2.9 X 10(3)). After nerve section, the mean number of cells was 10.88 X 10(3) (+/-0.9 X 10(3)), representing a 47% reduction. The mean volume of the mandibular subdivision cells in the ganglia of the sham surgery animals was 0.3 mm3 (+/-0.05) and 0.22 mm3 (+/-0.04) in nerve-sectioned ganglia, a 38% difference. There were no ganglia cell size or shape differences between the two groups. The mean number of cells in the ganglia of immediately repaired nerves was 10.66 x 10(3)(+/-1.1 X 10(3)), and it was 12.45 x 10(3) (+/-0.9 x 10(3)) after delayed repair. The numerical density was significantly less than in the sham surgery ganglia but not different from that of the transection/no repair ganglia. The weighted mean reference volume of the mandibular subdivision after immediate and delayed repair was similar and was significantly greater than the transection/no repair group, but not different from the sham surgery group. The cell size was slightly larger in delayed-repair ganglia compared with immediate-repair ganglia, but the differences were not significant. There were no significant differences in any of the stereologic estimates when analyzed across treatment time.

CONCLUSIONS

The results of this study agree with previous reports that peripheral nerve transection produces a substantial loss of nerve cells within specified regions of sensory ganglions. However, the results conflict with evidence that cells survive transection based on size and shape. These findings also indicate that in the adult rat the substantial loss of nerve cells was unaltered by the reconnection of their severed axons. Neither immediate or delayed repair of the transected nerve altered the spectrum of surviving cells based on size or shape. The reestablishment of the mean reference volume of the mandibular subdivision after section and repair suggests that demands made on regenerating axons appear to result in the restoration of ganglionic volume normally lost after axotomy, probably the result of axo

Surgically created fourth-third cranial nerve communication: temporary success in a child with bilateral third nerve hamartomas. Case report

Shortly after birth, an otherwise healthy infant developed eye deviation and ptosis due to a hamartomatous lesion of the interpeduncular segment of the right oculomotor nerve. The left nerve became similarly involved when the child was 1.5 years of age. Direct nerve repair was not possible. Instead, the trochlear nerve was divided and its proximal end was attached to the distal end of the third nerve. Elevation of the upper eyelid and partial adduction of the eye developed gradually over the ensuing 3 to 5 months. Both functions were lost after an additional 2 months, presumably as a result of tumor recurrence or neuroma formation. This case report shows that surgically created fourth-third cranial nerve communication is feasible and may merit consideration under similar circumstances.

Expansion of the dendritic tree of motoneurons innervating neck muscles of the adult cat after permanent axotomy

The morphologic characteristics of neck motoneurons with intact axons were compared with those of neck motoneurons that had been permanently axotomized for 11 to 17 weeks. Motoneurons were identified antidromically, intracellularly stained with horseradish peroxidase (HRP) and examined after reconstructions of their entire dendritic tree. Axotomized motoneurons differed qualitatively and quantitatively from motoneurons with intact axons. The distal branches of axotomized motoneurons exhibited two novel features: some gave rise to tangled appendages that exhibited growth cone-like specializations resembling lamellipodia and filopodia; others followed a meandering path and had unusually large diameters. These branches showed a discontinuous pattern of staining that was similar to the appearance of myelinated axons stained intra-axonally with HRP. A quantitative analysis of the dendritic trees of 13 completely reconstructed dendritic trees (five axotomized motoneurons and eight motoneurons with intact axons) showed that total dendritic surface area, total dendritic length, and total number of branches increased 38, 34, and 215%, respectively, after axotomy. These measurements were confirmed by comparing the sizes of a larger number of motoneurons (16 axotomized and 21 intact), calculated on the basis of correlations between dendritic tree size and proximal dendritic diameter. We conclude, therefore, that neck motoneurons, in contrast to other types of motoneurons, expand their dendritic trees after axotomy. It is suggested that this expansion is a consequence of two mechanisms: one involves dendritic growth, possibly leading to new synaptic connections; the other causes a conversion of some dendrites into axons.

Axotomy induces a different modulation of both low-affinity nerve growth factor receptor and choline acetyltransferase between adult rat spinal and brainstem motoneurons

Adult rat spinal and brainstem motoneurons re-express low-affinity nerve growth factor receptor (p75) after their axotomy. We have previously reported and quantified the time course of this reexpression in spinal motoneurons following several types of injuries of the sciatic nerve. Other studies reported the reexpression of p75 in axotomized brainstem motoneurons. Results of these previous studies differed regarding the type of the most effective triggering injury for p75 reexpression, the relative duration of this reexpression and the decrease of choline acetyltransferase (ChAT) immunoreactivity (-IR) following a permanent axotomy of spinal or brainstem motoneurons. These differences suggest that these two populations of motoneurons respond to axotomy with a different modulation of p75 and ChAT expression. The aim of the present study was to determine whether differential modulation exists. We have analyzed and quantified the presence of p75- and ChAT-IR motoneurons in the hypoglossal nucleus following the same types of injury and the same time course we previously used for sciatic motoneurons. The results show that a nerve crush is the most effective triggering injury for p75 and that it induces similar temporal patterns of p75 and ChAT expression for sciatic and hypoglossal motoneurons. In contrast, a cut injury of the sciatic and hypoglossal nerves resulted in distinct temporal courses of both p75 and ChAT expression between these two populations of motoneurons. In fact, a permanent axotomy of the hypoglossal motoneurons induced i) a much longer maintenance phase for p75 than in sciatic motoneurons and ii) a progressive loss of ChAT-IR with a successive return to normal values in contrast to the modest decrease in the sciatic motoneurons. This evidence indicates that spinal and brainstem motoneurons respond to a permanent axotomy with a different modulation of p75 and ChAT expression. Altogether, the present data and the reported evidence of a differential post-axotomy cell death support the hypothesis that these two populations of motoneurons undergo different dynamic changes after axotomy.

Axon-glial relationships in the anterior medullary velum of the adult rat

The anterior medullary velum is a thin sheet of CNS tissue which roofs the rostral part of the IVth ventricle and contains fascicles of myelinated fibres which, in part, arise from the nucleus of the IVth cranial nerve. This study used histochemical, immunohistochemical, and intracellular dye-injection techniques to describe cellular interrelationships in the velum in whole-mounts and in sections. Rip antibody-stained whole mounts provided a unique description of both oligodendrocyte units (defined as an oligodendrocyte and the complement of myelinated internodal segments it forms), and consecutive myelin sheaths along the same axon. A broad range of unit morphologies was categorised into four arbitrary groups, according to classical criteria, which comprised small cells supporting the short, thin myelin sheaths of 15-30 small diameter axons (Type I), through intermediate types (II & III), to the largest cells forming the long, thick myelin sheaths of 1-3 large diameter axons. Rip antibody and ferric ion-ferrocyanide staining, together with intracellular dye injection, revealed oligodendrocyte process branching patterns and their mode of engagement of myelin sheaths, nodes of Ranvier, and the spatial disposition of the outer cytoplasmic rims of myelin sheaths. The latter formed a conspicuous spiral ridge on the exterior surface of myelin sheaths which connected with the paranodal loops at each heminode. Large bundles of axons decussated through the velum, the bulk of which were IVth nerve fibres which constituted the IVth nerve rootlet. The PNS/CNS transitional zone of the IVth nerve was located 0.25-0.50 mm along the root, where astrocytic end-feet defined an abrupt margin, convex towards the periphery, where the heminodes of central and peripheral myelin were apposed, and where the basal lamina tubes of the Schwann cell units were discontinued. The basal processes of ependymal cells lining the ventricular wall of the velum, passed between axon bundles before abutting on the basal lamina of the pia. Many of these processes branched and ran along the axonal bundles. A monolayer of microglia occupied a subependymal stratum in which the non-overlapping dendritic territories of each cell formed a regular mosaic throughout the velum without any obvious interaction with either axons or other glial cells. Astrocytes were also uniformly distributed; their fine processes made up a dense lattice amongst axons, often running parallel and within the fibre bundles; stouter ones had terminal end-feet which undercoated the basal lamina of both the glia limitans externa and the blood vessels in the velum.

Regenerative axonal sprouting in the cat trochlear nerve

Following peripheral trochlear nerve axotomy in the cat, the normal number of myelinated axons is restored despite significant motor neuron death, suggesting regulation of the number of myelinated axons in the regenerated nerve. In this study we used light and electron microscopy to examine the production and maintenance of axonal sprouts at different locations in the nerve and at different postoperative intervals. Despite proliferative sprouting and an overproduction of nonmyelinated axons in the regenerating trochlear nerve, the number of myelinated axons was strictly regulated. Only approximately 1,000 regenerated axons were eventually remyelinated, but many nonmyelinated axons were still present 6-8 months postaxotomy. Regenerated axons were remyelinated in a proximal-to-distal direction between 3 and 4 weeks postaxotomy. We also examined the maturation of regenerated myelinated axons by measuring axon diameter and myelin index (an expression of myelin thickness). Mean myelinated axon diameter remained significantly below normal in long-term regenerated nerves. Mean myelin index was not different from normal at 4 weeks postaxotomy but was significantly decreased at long postoperative intervals, reflecting a slightly thicker myelin sheath relative to the axon diameter. This relative increase in mean myelin thickness could serve to restore normal conduction velocity despite the decrease in mean axon diameter. We suggest that the regulation of the number of myelinated axons at the normal number despite cell death and the increase in mean myelin thickness may both be compensatory mechanisms that function to restore preoperative conditions and maximize functional recovery.

Magnetic resonance imaging of the superior oblique muscle in superior oblique myokymia

High resolution, magnetic resonance imaging was used to quantitatively study the morphometry of the superior oblique muscles of two patients with superior oblique myokymia, as well as 18 superior oblique muscles of 14 patients with normal superior oblique function. The cross sectional area of each superior oblique muscle was measured at 3-millimeter intervals along the entire muscle length. In both cases of myokymia, the affected superior oblique muscles were significantly smaller than normal (P < .05). These anatomical changes in the superior oblique muscle of patients with myokymia suggest that an antecedent injury to the trochlear nerve has occurred. This injury, even if clinically unapparent, may be the initial event which leads to subsequent development of superior oblique myokymia.

Axotomy-induced changes in cytochrome oxidase activity in the cat trochlear nucleus

Following a unilateral section of the trochlear nerve, the effects of axotomy on cytochrome oxidase levels in the trochlear nucleus were studied. Cytochrome oxidase levels in the axotomized nucleus were significantly lower than in the control nucleus. The maximal decrease was observed at 2 weeks. Following partial restoration during weeks 3 and 4, cytochrome oxidase levels stabilized at levels only slightly below normal. Since a significant number of trochlear motoneurons die following axotomy, the restoration of cytochrome oxidase levels close to normal suggests that the surviving neurons may compensate for an increased load with a permanent increase in oxidative metabolism.

Modulation of low-affinity nerve growth factor receptor in injured adult rat spinal cord motoneurons

Spinal and brainstem motoneurons of the adult rat reexpress low-affinity nerve growth factor receptor (LNGFR) and its mRNA after axotomy. We have previously reported the time courses of this reexpression after cut (no regeneration) or crush (followed by regeneration) of the sciatic nerve. We have shown that the length of the different phases of this reexpression (appearance, maintenance and disappearance) can vary according to the type of axotomy. With the present study we expand our previous data and describe and analyze the modulation the LNGFR expression in adult spinal cord motoneurons following different lesion paradigms. In one approach we have imposed three traumatic injuries that still allow regeneration of the sciatic nerve but with a different time course with respect to the crush injury (application of a silicone regeneration chamber, multiple crushes and delayed repair of ligated nerves). In a second approach, we have determined the capability of three toxic or metabolic injuries to induce LNGFR expression without any direct trauma of the nerve (experimental diabetogenesis, botulinum and alpha-bungarotoxin intoxication and 2,5-hexanedione intoxication). In a third approach, we have investigated the effect of the block of the axoplasmic transport on the LNGFR expression following different topical applications of vincristine combined with a nerve crush. The results we present are consistent with the idea that: (1) LNGFR immunoreactivity in adult motoneurons is expressed by motoneurons that are attending to an axonal outgrowth and not a generic signal of cellular damage or impairment of the motor function; (2) LNGFR expression in these motoneurons is related to and parallels the outgrowth process time frame, and (3) the signal/s that trigger and sustain this reexpression may be retrogradely transported from the periphery.

Long-term consequences of impaired regeneration on facial motoneurons in the C57BL/Ola mouse

Peripheral nerves of the C57BL/Ola mouse mutant undergo markedly slowed Wallerian degeneration following injury. This is associated with impaired regeneration of both sensory and motor axons. Following a crush lesion of the facial nerve, there was no cell loss in facial nuclei of normal (C57BL/6J) adult mice, but 40% cell loss occurred in Ola mice and the survivors increased in size during the period when functional reinnervation was established. These results are interpreted as a result, first, of prolonged deprivation of target-derived trophic factor in the slowly regenerating Ola motoneurons and second, increased peripheral field size of the survivors. Within the regenerated facial nerve, there was marked heterogeneity of myelinated fibre size in Ola mice. Some Ola axons, both proximal and distal to the lesion site, had areas over twice as great as the largest 6J axons when measured 1 year following injury. A population of small diameter fibres, not observed in 6J nerves, persisted distal to the crush site in Ola nerves, and this was associated with an increase in the total number of myelinated axons in the distal nerve: on average, each parent Ola axon retained three persistent daughter axons. The delayed Wallerian degeneration in Ola mice not only impairs immediate axon regrowth, but also results in a breakdown of the normal mechanisms which regulate axon number and size in regenerating nerve.

Experimental repair of the oculomotor nerve: the anatomical paradigms of functional regeneration

In adult guinea pigs, the oculomotor nerve was sectioned proximally (at the tentorial edge) or more distally (at the orbital fissure) and immediately repaired by reapproximation. During a 24-week postoperative period, extrinsic eye motility was assessed by analyzing the vestibulo-ocular reflexes. The regenerated oculomotor nerve was studied morphometrically on semi-thin histological sections at 16 and 24 weeks postinjury. The selectivity of muscle reinnervation was investigated by injection of both single (horseradish peroxidase) and double (fluorescent dyes) retrograde axonal tracers into the eye muscles. Following proximal repair of the oculomotor nerve, the degree of recovery of extraocular motility varied among different animals and remained stable over long-term observations. In animals with poor recovery, aberrant eye movements were always found, and the somatotopic map of the reinnervated eye muscles was greatly altered. Distortions of the central representation were also seen in those animals in which a good level of functional recovery was seen. However, in animals with good recovery, a topographic bias was re-established by about 65% of the original neuronal population, as opposed to 26% in the animals with poor recovery. Neurons located contralateral to the axotomized nucleus sprouted intra-axially and projected their axons to denervated eye muscles. The number and diameter of the regenerated axons, the number and soma diameter of the axotomized neurons, and the ratio of distal axonal branches to proximal supporting neurons were all related to the degree of functional recovery. Following repair of the oculomotor nerve at the orbital fissure, extraocular motility had recovered in all of the animals at 16 weeks without aberrant phenomena. Functional regeneration of the distally transected oculomotor nerve is thought to be the result of selective muscle reinnervation.

Progressive morphological abnormalities observed in rat spinal motor neurons at extended intervals after axonal regeneration

It is generally accepted that mammalian spinal motor neurons return to normal after axotomy if their regenerated axons successfully reinnervate appropriate peripheral targets. However, morphological abnormalities, recently observed in spinal motor neurons examined 1 year after nerve crush injury, raise the possibility that delayed perikaryal changes occur after regeneration is complete. In order to distinguish between chronic and progressive alterations in neurons with long-term regenerated axons, rat spinal motor neurons and dorsal root ganglion cells were examined at 5 and 10 months following unilateral sciatic nerve crush. Neurons with regenerated axons were identified by retrograde labelling with horseradish peroxidase. The structural properties of neurons ipsilateral to nerve injury were compared to those of neurons from the spinal cord and dorsal root ganglia on the contralateral side and from age-matched control rats. At 5 months postcrush, the morphology of motor and sensory neurons ipsilateral to injury was comparable to that of control cells. However, several features of the motor neurons with regenerated axons distinguished them from control motor neurons at 10 months postcrush. Mean perikaryal area of ipsilateral spinal motor neurons was larger than the means for control motor neurons (p less than .001). Ipsilateral spinal motor neurons also appeared clustered within the spinal cord and had thicker dendrites. Dorsal root ganglion cells with regenerated axons were slightly larger than control cells at 10 months postcrush but they exhibited no other morphological changes. The present findings indicate that spinal motor neurons are progressively altered after their regenerated axons have reestablished functional synapses with their peripheral targets.

Access to gastric tissue promotes the survival of axotomized neurons in the dorsal motor nucleus of the vagus in neonatal rats

Lesioning the vagus nerve in the neck (cervical vagotomy) results in a rapid and virtually complete loss of motoneurons in the dorsal motor nucleus of the vagus in neonatal rats. The present study sought to determine whether access to gastric target tissue will promote the survival of these motoneurons after axotomy. Quantitative analysis demonstrates that subdiaphragmatic vagotomy, which leaves the cut vagal axons in close proximity to their normal gastric targets, results in significantly less motoneuron loss than cervical vagotomy. Furthermore, the loss of motoneurons after cervical vagotomy can be significantly reduced by transplanting embryonic gastric tissue to the neck of vagotomized neonatal host rats, in the vicinity of the cut axons. The survival effect of transplanted gastric tissue appears specific because control transplants of embryonic bladder tissue fail to reduce motoneuron death after cervical vagotomy. Injections of the neural tracers Fluoro-Gold and cholera toxin-horseradish peroxidase into gastric transplants labeled surviving motoneurons in cervically vagotomized rats, whereas tracer injections into bladder transplants or into host cervical tissues did not. These results indicate that neonatal vagal motoneurons are capable of making the adjustments necessary to survive axotomy if they have access to gastric target cells. The apparent dependence of injured neonatal vagal motoneurons on gastric tissue offers a new system in which to examine in vivo the trophic interactions between neurons and their targets.

Organization of callosal connections in the visual cortex of the rabbit following neonatal enucleation, dark rearing, and strobe rearing

The organization of visual callosal projections was studied in (1) normal adult rabbits; (2) adult rabbits which had undergone monocular enucleation (ME) or binocular enucleation (BE) at birth; and (3) adult rabbits which had been deprived of normal visual experience during development by dark rearing (DR) or strobe rearing (SR). Previously published observations (Murphy and Grigonis, Behav Brain Res 30:151, 1988) on callosal organization in adult rabbits in which retinal ganglion cell activity was eliminated during development by intraocular tetrodotoxin (TTX) injections, are also summarized for comparison with these data. The tangential extent of the callosal cell zone was significantly larger than normal in DR, TTX, and ME rabbits, was unchanged in BE rabbits, and was significantly reduced in SR rabbits. An analysis of the laminar distribution of the callosal cells revealed a significant increase in the percentage of callosal cells in lamina IV in ME, DR, and TTX animals. Measurements of density of callosal cells showed a significant increase in the density of the callosal projection in ME and SR rabbits and a decrease in density in BE rabbits compared with normal. The data suggest that the mechanisms involved in the development of the tangential and laminar organization of the callosal cell zone are different. In addition, the data suggest that the mechanisms involved in the maintenance of callosal projections are different from the mechanisms involved in the elimination of callosal projections during development. The effects of these developmental manipulations on callosal organization in other mammals are reviewed and compared with the effects in rabbits. The data suggest that species differences in the degree of maturity of the visual system at birth and in the extent of callosal development at the time of eye opening, may underlie species differences in the effects of these manipulations on the organization of visual callosal projections during development.

Sequential double labelling with different fluorescent dyes coupled to dextran amines as a tool to estimate the accuracy of tracer application and of regeneration

We present a technique to estimate the accuracy of a given application procedure for neuronal tracers. In a second series of animals we used this technique for the estimation of successful regeneration of peripheral nerves. Dextran amine coupled to rhodamine was applied to the cut trochlar nerve in Xenopus tadpoles. To assess the accuracy of tracer application, experiments were done in which a second dye, dextran amine coupled to fluorescein, was applied after 1 day proximal to the first dye. More then 90% of all trochlear motoneurons were doubly labelled after this procedure. Their total numbers were not significantly different from numbers obtained after single labelling with HRP in a comparable age group. To assess success of regeneration after 5 and 8 days, the second application of fluorescein dextran amine was distal to the first application side. Statistically significant differences suggest incomplete regeneration of many neurons. After 42 days the numbers of singly and doubly labelled motoneurons was in the same proportion as before regeneration. This suggests that about 90% of the surviving motoneurons had successfully regenerated back to the periphery.