Retrograde and transganglionic degeneration of sensory neurons after a peripheral nerve lesion at birth

Effacement and regeneration of tactile lamellar corpuscles of rat after postnatal nerve crush

The development of Meissner-like lamellar corpuscles was studied in rat toe pads under normal conditions and after crushing the sciatic nerve in 1- to 15-day-old animals. During normal development, rat lamellar corpuscles begin to differentiate first by postnatal day 8. By this time, sensory axons have grown up to the apex of dermal papillae and form axon terminals beneath epidermis. The terminals are ensheathed by lamellar cells derived from Schwann cells. First thin lamellae are formed around the terminals 8-12 days after birth, and the number of lamellar layers increases until the corpuscles become structurally mature by 20 days after birth. A mature corpuscle consists of two or more terminals, each surrounded by approximately 10 lamellae, all components being enclosed by an incomplete capsule. No lamellar corpuscles develop in toe pads after crushing the sciatic nerve in newborn rats, and only occasional corpuscles regenerate after nerve crush at 5 days of age. The corpuscles fail to develop because dermal papillae remain permanently denervated after crushing the nerve early postnatally. After nerve crush in 10-day-old rats, lamellar corpuscles regenerate by 1 month after the operation, but they remain underdeveloped: their number and size are smaller than normal even 1 year after injury, and their terminals are encircled only by 1-3 lamellar layers. After nerve crush in 15-day-old rats, the corpuscles recover upon reinnervation and their size and lamellation become almost normal.

Fiber counts at multiple sites along the rat ventral root after neonatal peripheral neurectomy or dorsal rhizotomy

We hypothesized that the afferent fibers in the ventral root of the rat are the third branches of dorsal root ganglion cells; these afferent processes in the ventral root are of varying length and end bluntly along the length of the root. In the case of an injury at either the central or the peripheral processes of the dorsal root ganglion cells in the neonatal stage, these fibers sprout at the blunt endings along the length of the ventral root. We cut either the sciatic nerve or the dorsal root on one side in neonatal rats. After the rats were fully grown, the number of both myelinated and unmyelinated fibers was counted in electron photomicrographs at multiple sites along the length of the ventral root. We observed a greatly increased number of unmyelinated fibers in the ventral root after the sciatic nerve had been cut at the neonatal stage. The magnitude of increase was more at the distal than at the proximal portion of the ventral root, suggesting that added fibers originated from the distal side. Neonatal dorsal rhizotomy, however, did not produce the same result. These results are consistent with our hypothesis that peripheral nerve injury at the neonatal stage triggers sprouting of the third branches of the dorsal root ganglion cells which end bluntly along the length of the ventral root in the normal animal.

Structure-function relationships in rat brainstem subnucleus interpolaris: VII. Primary afferent central terminal arbors in adults subjected to infraorbital nerve section at birth

Prior studies in this series have clarified the normal organization of subnucleus interpolaris and the response of higher-order neurons to neonatal deafferentation. The present report describes the response of individual rat trigeminal primary afferents to transection of the infraorbital (IO) nerve on the day of birth. Physiologically characterized afferents in adult animals were labeled by intraaxonal injection of horseradish peroxidase (HRP). Qualitative and quantitative examination of the interpolaris collaterals of 62 recovered neurons revealed: 1) an increase in the transverse area of vibrissa afferent terminal arbors, 2) a decrease in the number of boutons per collateral of vibrissa afferents, 3) a decrease in the bouton density of both vibrissa and guard hair primary afferents, 4) a decrease in the circularity of guard hair afferent arbors, 5) an increase in the number of collaterals given off by nociceptive fibers, and 6) abnormal primary afferent topography. The data support the hypothesis that vibrissa afferents respond to neonatal axotomy by central arbor expansion, but not by sprouting. Arbor expansion provides a morphological substrate for the abnormal histochemical staining patterns seen in animals subjected to IO damage in the early postnatal period.

Electrophysiological evidence for an increase in the number of ventral root afferent fibers after neonatal peripheral neurectomy in the rat

Our recent study has shown that many afferent fibers in the ventral root are third branches of dorsal root ganglion cells in addition to their processes in the peripheral nerve and the dorsal root. From results of this study, we hypothesized that most of the afferent fibers in the normal ventral root are extra processes of certain dorsal root ganglion cells. To accommodate experimental findings by others, we formulated several working hypotheses in the present study as an extension of our previous hypothesis: these afferent processes in the ventral root are of varying length; they end bluntly along the length of the root; and in an event such as peripheral neurectomy in the neonatal stage, these fibers sprout at the blunt endings along the length of the ventral root. We tested the above hypotheses using electrophysiological methods. The sciatic nerve on one side in neonatal rats was cut. After the rat was fully grown, volleys of neural activity were recorded along the length of the ventral root while stimulating the dorsal root of the same segment. There was a great increase in the size of compound action potentials in the ventral root on the sciatic nerve-lesioned side. Various lines of evidence suggest that this enhancement of the evoked potentials is likely to be due to an increase in the number of afferent fibers in the ventral root in response to neonatal peripheral nerve injury. The results are consistent with our hypotheses.

Deafferentation-induced expansion of saphenous terminal field labelling in the adult rat dorsal horn following pronase injection of the sciatic nerve

We have examined the effect of the degeneration of sciatic nerve afferents on the distribution of saphenous terminals in the adult rat dorsal horn. Deafferentation was produced by injection into the sciatic nerve of pronase, a combination of proteolytic enzymes, which causes death of ganglion cells and degeneration of their terminal fields. The saphenous terminal fields were labelled by exposing the cut nerve to a combination of horseradish peroxidase (HRP) and wheat germ agglutinin-horseradish peroxidase (WGA-HRP). Terminals were mainly found in the superficial dorsal horn, indicating that small-diameter afferents were heavily labelled. In one group of control animals, the normal sciatic and normal saphenous terminal fields were shown to be bilaterally symmetrical. In the experimental group, the initial injection of one sciatic nerve with pronase was followed 4 months later by bilateral HRP/WGA-HRP labelling of both saphenous nerves. In each animal, the terminal field of the saphenous nerve on the lesioned side was expanded in the medial, lateral, and caudal directions. Medially and laterally, the expanded terminal field overlapped more of the sciatic territory than normal; caudally, saphenous terminals were found in the rostral portion of the L5 segment, in an area normally filled by sciatic terminals and devoid of saphenous terminals. The expansion resulted in a total saphenous area 26% larger than the control side. Electron microscopy demonstrated that the label in both the normal and expanded territories was primarily contained in axons and terminals, with minor transneuronal labelling. Labelled terminals in the expanded areas were both simple terminals with round, clear vesicles, and glomerular terminals with multiple synaptic contacts; these terminal types resemble those previously described for primary afferents in the superficial dorsal horn. Although the preexistence of "silent" synaptic terminals in the expanded areas cannot be disproven, the data support the hypothesis that primary afferents in the adult have the potential to sprout and establish synapses when the conditions of the deafferentation are favorable.

Number of dorsal root ganglion neurons and axons in cats of different ages

Neurons and axons were counted in normal L7 dorsal root ganglia (DRG) and dorsal roots of cats from 2 months to 11 years of age. Over this time period no change in neuronal but a slight increase in axonal numbers were observed. These findings indicate that in contrast to previous reports on male rat lumbar ganglia, neurogenesis in cat DRG does not continue into adulthood.

Morphological and physiological properties of neurons after long-term axonal regeneration: observations on chronic and delayed sequelae of peripheral nerve injury

Axonal regeneration has been the focus of extensive investigation of mechanisms which mediate structural and functional recovery after injury to mammalian peripheral nerves and has proven to be a valuable model for development and plasticity in the nervous system. Although details of the acute morphological and physiological responses to nerve injury are well-described, less information is available to nerve injury are well-described, less information is available about long-term alterations which persist or develop after regenerated axons have established connections with their targets. The present paper briefly discusses the mammalian neuron's initial response to peripheral nerve injury and subsequent events which occur during regeneration. Morphological and physiological alterations observed in neurons after long-term axonal regeneration are described and are considered in the context of their potential implications for clinical recovery after nerve injury, as well as their potential contribution to the appearance of delayed neurological dysfunction. Selective responses to neuronal injury during development and in different fiber populations are discussed.

Function and structure of atypical muscle spindles after neonatal nerve crush in rats

During the early postnatal period, the differentiation and maturation of muscle spindles in the rat is still dependent on their sensory innervation. When a nerve is crushed during this period, most spindles in the denervated muscles degenerate and after reinnervation only occasional spindles of atypical structure are to be found in these muscles. We determined the basic functional properties of these atypical spindles in adult rats and attempted to correlate them with their structural characteristics. The discharge rates of 13 afferent units from the soleus or lateral gastrocnemius muscles were evaluated in response to stretch. These units were capable of a slowly adapting response to 2-4 mm stretches. Their mean discharge frequencies at any point of the ramp-and-hold stretch were, however, on an average 50% lower than normal values. The conduction velocities of afferents from the atypical spindles were in the range of 10-40 m/s. Histological examinations revealed that 90% of the atypical muscle spindles found in the soleus or lateral gastrocnemius muscles had only 1 or 2 intrafusal fibres without any nuclear accumulations as compared to four intrafusal fibres in normal muscle spindles in the rat. The proportional decrease of the discharge rate in both the dynamic and static part of the response of these atypical spindles could be due to the decreased synaptic area between the sensory terminals and the intrafusal fibres and/or to altered structural properties of the intrafusal fibres.

Neonatal transection alters the percentage of substance-P-positive trigeminal ganglion cells that contribute axons to the regenerate infraorbital nerve

Neonatal transection results in a marked reduction of the number of trigeminal (V) ganglion cells that contribute axons to the regenerate infraorbital nerve (ION; Jacquin and Rhoades, 1985; Chiaia et al., 1987). Such lesions also produce a profound deafferentation of the V brain stem complex that appears to spare the innervation of layers I and II of subnucleus caudalis (SpC) by substance-P-positive (SP-positive) primary afferents (Jacquin and Rhoades, 1985; Rhoades et al., 1988). In the present study, we combined retrograde tracing with immunocytochemistry to determine whether neonatal transection of the ION alters the percentage of SP-positive V ganglion cells that contribute axons to this V branch upon regeneration. In V ganglia ipsilateral to the intact ION (n = 8), 11.6% +/- 3.2% of the cells labeled after application of true blue (TB) to the ION were also SP-positive. In ganglia ipsilateral to the neonatally damaged nerve (n = 8), 18.6% +/- 4.7% of the cells labeled after application of TB to the regenerate ION were also SP-positive (p less than 0.001). We also compared the SP content of intact ganglia (n = 10) with that of ganglia ipsilateral to the damaged nerve (n = 10) by means of radioimmunoassay. The normal V ganglia contained (mean +/- SD) 3496 +/- 774 pg SP/mg protein. The value for the ganglia ipsilateral to the damaged nerve was 5533 +/- 1746 pg SP/mg protein (p less than 0.01). There was no significant difference between SP levels on the control and partially deafferented sides of the brain stem in neonatally nerve-damaged adult rats. In one additional experiment, we injected TB into both vibrissa pads of seven rats on the day of birth prior to transection of the ION. After an 8-hr delay, the nerve on one side was then cut and allowed to regenerate, and both V ganglia were then processed for immunocytochemistry. On the nerve-damage side, 25.8% of the TB-labeled cells were SP-positive. The value for the intact side was 12.0% (p less than 0.000001). This result demonstrated that the lesion-induced change in the percentage of SP-positive ION cells was not the result of either late-growing axons from SP-positive ganglion cells that may have been missed by our nerve cuts or collateral sprouting into the regenerate ION by undamaged SP-positive ganglion cells.

Cortical organization after treatment of a peripheral nerve with ricin: an evaluation of the relationship between sensory neuron death and cortical adjustments after nerve injury

The present study was designed to assess whether cortical changes after peripheral nerve damage are related to the degree of death of primary sensory neurons in the damaged nerve. The cytotoxin ricin was injected into the sciatic nerves of adult rats to kill primary sensory neurons with axons through the injection site. Following periods of 6-101 days, the S-I hindpaw map was evaluated with neurophysiological techniques and compared with the hindpaw maps of previously studied normal adult rats and adult rats that had undergone adult or neonatal sciatic section at a comparable level of the nerve. These comparisons allowed evaluation of cortical functional organization following different degrees of sensory neuron loss after sciatic nerve injury. There were three main results. 1) The comparison of ricin-treated and normal adult rats indicated that ricin treatment interrupted inputs from the sciatic skin territory on the hindpaw and caused a limited increase in the size of the cortical area that was activated by stimulation of hindpaw skin innervated by the remaining saphenous nerve. 2) The cortical maps of rats that had undergone adult ricin treatment (relatively large primary neuron loss) or section during adulthood (small to moderate primary neuron loss) were similar. In both groups, only the saphenous hindpaw skin was represented in cortex, and the cortical area that was activated by stimulation of the saphenous hindpaw skin had undergone a comparable limited enlargement. 3) The comparison of ricin-treated adult rats (relatively large primary neuron loss) and adult rats that had undergone neonatal section (relatively large primary neuron loss) indicated that cortical organization differed after these treatments. In particular, after ricin treatment the cortical area that was activated by stimulation of the saphenous hindpaw skin was larger than the comparable area in neonatal denervates, and the topographical progressions between the hindpaw and adjacent body representations were not as variable as after neonatal section. These findings indicate that cortical maps are altered after injection of ricin into a nerve. The similarity in cortical organization after ricin treatment (relatively large sensory neuron loss) and nerve section in adults (relatively small sensory neuron loss) and the differences in cortical organization after ricin treatment and nerve section in neonates (both relatively large sensory neuron loss) indicate cortical changes do not covary as a simple function of the degree of peripheral neuron death.

The effect of neonatal peripheral nerve section on the somadendritic growth of sensory projection cells in the rat spinal cord

Sciatic nerve section and ligation on the day of birth results in marked growth retardation of the rat dorsal horn. This transneuronal effect was examined in spinal cord cells that project to the brain by retrograde labelling with HRP from contralateral dorso- and ventrolateral tracts in the thoracic white matter. HRP-impregnated gel pellets were implanted in the tracts for 48-72 h to allow intense somadendritic staining of the projection cells. The results show that cells in rats whose sciatic nerve has been sectioned at birth have a mean somal area that is 40% smaller than controls. Primary dendrites are reduced from a mean of 4.1 per cell to 3.1 per cell and secondary branching is reduced by 75%. The results suggest that there was no actual cell death, only growth retardation. An intact primary afferent input apparently has a strong transneuronal trophic influence on spinal cord sensory cells projecting to the brain.

Effect of age and maturation on sudomotor nerve regeneration in mice

This study demonstrates that regeneration of unmyelinated sudomotor axons in mice becomes progressively slower during aging. Identical lesions were made in mice aged 0, 2, 4, 6, 7, 24 and 60 weeks. The peroneal, sural and saphenous nerves were cut and tied to prevent regeneration. The sciatic nerve was then frozen at the thigh, leaving the hind paw completely denervated. By 7 days, sweat glands (SGs) of the paw had ceased sweating after pilocarpine injection. Subsequent regeneration of sudomotor axons was judged by the rate of return of pilocarpine sensitivity. SGs in the hind paws of normal newborn mice did not sweat at birth. Cholinergic stimulation first activated sweating at 13 days of life. The number of responsive SGs increased progressively to reach the adult level by 30 days. In one-week-old mice, whose sciatic nerve had been sectioned, the SG response to cholinergic stimulation was very delayed in time and reduced in number. Sweat glands of young mice, between 2 and 4 weeks of age, regained cholinergic sensitivity at a faster rate than mature animals and attained normal SG counts. Throughout a broad intermediate range of age in adulthood (7-24 weeks), the rate of sudomotor nerve regeneration was the same, but in older mice (60 weeks) it was slower and less complete.

Anatomical consequences of neonatal infraorbital nerve transection upon the trigeminal ganglion and vibrissa follicle nerves in the adult rat

A large body of experimental literature has demonstrated that neonatal infraorbital nerve damage in rodents produces anatomical and/or functional alterations of the normal whisker representation in central trigeminal structures. Less is known about the organization of primary afferent components of the trigeminal system following this manipulation. Such information provides an important basis for interpreting the central changes observed following damage of infraorbital nerve fibers at birth. We have therefore examined the composition and order of peripheral innervation in the pathway from the trigeminal ganglion to the vibrissa follicles in adult rats subjected to unilateral neonatal infraorbital nerve transection. Electron microscopy was used to determine the number and diameter of myelinated and unmyelinated fibers in vibrissa follicle nerves of these animals. Wheat germ agglutinin-horseradish peroxidase and fluorescent retrograde tracers were employed to examine the number and diameter, as well as the topographic organization and branching, of ganglion cells innervating the vibrissae in these rats. The data presented below indicate that neonatal infraorbital nerve transection has the following consequences within the adult trigeminal nerve and ganglion: 1) an alteration of the gross morphology of vibrissal nerves, 2) a significant reduction in the average number (85.4%) and diameter (32.6%) of myelinated, but not unmyelinated, follicle nerve axons, 3) a significant decrease in the average number (36.8%) of trigeminal ganglion cells innervating vibrissa follicles, 4) no significant change in the distribution of ganglion cell diameters, 5) an increase in peripheral branching (1.8-fold) of these ganglion cell axons, and 6) an alteration of somatotopic order within the trigeminal ganglion. Taken together, these data indicate that neonatal infraorbital nerve transection produces a profound reorganization of the primary afferent component of the trigeminal neuraxis.

Preventing regeneration of infraorbital axons does not alter the ganglionic or transganglionic consequences of neonatal transection of this trigeminal branch

Retrograde and transganglionic tracing with a combination of horseradish peroxidase (HRP) and wheatgerm agglutinin (WGA)-conjugated HRP (WGA-HRP) was employed to determine whether transection of the infraorbital (IO) nerve on the day of birth and prevention of regeneration by retransecting it at weekly intervals until the time of a terminal anatomical experiment had effects upon ganglion cell survival and innervation of the brainstem by this trigeminal (V) branch that differed from those which followed a single transection of the same nerve on the day of birth without any attempt to prevent peripheral regeneration of the cut axons. Counts of labelled ganglion cells and examination of the brainstem labelling produced by application of HRP and WGA-HRP to the IO nerve proximal to the point of transection(s) at 6 weeks of age demonstrated no differential effects of preventing regeneration of the cut nerve. In animals subjected to a single transection of the nerve (n = 9), we counted an average of 5001.2 (S.D. = 1286.9) labelled ganglion cells and these had an average diameter of 22.7 micron (S.D. = 6.3). In the rats (n = 9) that sustained multiple nerve cuts, the average number of labelled ganglion cells was 4447.8 (S.D. = 1060.9). The mean diameter for these primary afferent neurons was 21.5 micron (S.D. = 6.6). Neither of these values were significantly different from those from the rats subjected to a single nerve cut. The cell counts from both of these groups were significantly lower than those obtained after application of HRP and WGA-HRP to the IO nerve in normal rats (n = 3, X = 12,553.3, S.D. = 1454.8), but the average cell diameter in the normals (X = 23.2, S.D. = 6.6) was not significantly greater than that in the nerve-damaged animals. The pattern of brainstem labelling observed in the rats subjected to multiple nerve cuts was the same as that in the rats which sustained a single transection of the IO nerve on the day of birth. Very little terminal labelling was observed in nucleus principalis, subnucleus oralis, subnucleus interpolaris or the magnocellular portion of caudalis. There was, however, very heavy labelling in laminae I and II of the latter nucleus.

The number of neurons in dorsal root ganglia L4-L6 of the rat

The number of neurons in the dorsal root ganglia L4-L6 of the rat was determined because published data are inconsistent and in general incompatible with the number of afferent axons in the sciatic nerve. Nucleoli were counted in serial sections; epoxy-resin sections 3 microns thick, or paraffin sections 5 microns thick, or unstained 12-microns paraffin sections of osmicated tissue were used. Correction factors for split and multiple nucleoli were obtained by counting nucleolar profiles in consecutive sections of identified cells. Dividing the number of nucleolar profiles into the number of cells gave the factor by which the counts of nucleolar profiles had to be multiplied to obtain the number of neurons. The ganglia L4, L5, and L6 contained about 12,000, 15,000 and 14,000 neurons, respectively, when resin sections were used. The standard deviation for the average of 41,000 neurons in the three ganglia was 8% of the mean value. The results compare well with the number of dorsal root fibers, and with the fact that the sciatic nerve at midthigh, to which less than half of the neurons connect, contains 19,000 afferent axons. The data obtained from the paraffin series were 23% smaller, but still considerably higher and less variable than all previously reported data. The main problem with stained paraffin sections was that most small neurons had multiple nucleoli attached to the membrane of the nuclei, which only measured 10 microns in diameter. The nucleoli often projected into the dark cytoplasm and were difficult to identify.

Loss of sensory neurons after sciatic nerve section in the rat

In this study, the loss of sensory neurons in the rat was assessed after sciatic nerve section at birth and at 4 weeks of age. The neuronal deficit in ganglia L4-L6, 39-89 weeks after neonatal denervation, was 10,000-17,000. The nerve contains about 19,000 afferent axons, so some axotomized neurons survived. Degenerating perikarya were absent, probably because all surviving neurons had reestablished target contacts. Sectioning the nerve at age 4 weeks, in five rats, after 19-92 weeks had caused the death of 7,000-11,500 neurons. Whether the nerve regenerated or not in these rats apparently did not influence the extent of neuron death. Nevertheless, no deficit was observed in a sixth rat in which muscle reinnervation was very good. Therefore, the results are inconclusive with respect to the effect of axonal regeneration. Ganglia of rats operated at age 4 weeks regularly contained perikarya with axonal reaction; this supports the notion that some mature neurons are able to permanently survive without target contact. There was no evidence for selective loss of large or small neurons after nerve section at birth or at age 4 weeks. The extent of cell loss in individual ganglia varied, indicating varying contributions of the three ganglia to the nerve. Hence, it is not possible to quantify the effect of experimental conditions on the number of sensory neurons when only one of the several ganglia contributing to the nerve is investigated.

Numerical patterns of axon regeneration that follow sciatic nerve crush in the neonatal rat

Different proportions of axons regenerate in cutaneous nerves compared with muscle nerves after sciatic nerve crush in the rat. The questions here are whether or not these differences reflect the proportions of axons that regenerate in the different nerves from which these nerves arise and do the differences persist. The findings indicate that the numbers of axons that regenerate in the muscle nerves do not reflect the numbers in the nerve from which the muscle nerves originate and that these differences persist. This leads to the conclusion that some factors determining numbers of axons in the muscle nerves must operate distal to the lesion. Thus when one is considering mechanisms that control regenerated axon numbers after neonatal nerve crush, one must consider not only the lesion site but also branching and axon diversion far distally, presumably at the origin of the muscle nerves.

Glycogen accumulation in synaptic boutons in Clarke's nucleus neuropil after sciatic nerve crush at birth. An electron microscopic study

Glycogen accumulation in the Clarke's nucleus neuropil of young adult rats whose sciatic nerves were crushed in the first postnatal day was investigated with the electron microscope. Glycogen was observed in synaptic boutons and in small myelinated axons. In some terminals, glycogen accumulated in membrane-bound structures resembling mitochondria and formed large multigranular bodies which were entirely separated from the axoplasm. The multigranular body reached the size of 1.3 micron. Glycogen was present as single beta particles of about 25-40 nm in diameter and in aggregations of large alpha clusters. The astrocytic glycogen distribution was almost similar to that of the control specimens. Glycogen was not observed in other glial cells. It is probable that glycogen accumulation in synaptic terminals of partially deafferentiated Clarke's nucleus may result from impaired glycolysis due to deficient resupply of the distal axon with glycolytic enzymes caused by a defect in axoplasmic transport from the hypoplastic sensory neuronal perikarya.

Axon and neuron numbers after forelimb amputation in neonatal rats

It seems a paradox that more primary sensory neurons are lost but recovery is better after peripheral nerve injury in neonates as compared to adult mammals. A possible explanation is that surviving neurons sprout in the neonate. To test this, forelimbs in neonatal rats were amputated, which caused the death of many primary sensory neurons. The number of neurons in the dorsal root ganglia, and the number of myelinated and unmyelinated fibers in the dorsal and ventral roots were determined on the amputated and contralateral normal sides. On the amputated side, soma loss in the ganglia was 30%, and the fiber numbers were decreased by 16% in the dorsal root and increased by 20% in the ventral root. These data are compatible with the hypothesis that there is axonal branching or sprouting from surviving sensory neurons. In addition, morphometric analyses showed a changed myelin-axon relationship for central processes of sensory cells whose distal processes have been cut.

Differences in horseradish peroxidase labeling of sensory, motor and sympathetic neurons following chronic axotomy of the rat sural nerve

In an attempt to clarify the ultimate fate of permanently axotomized adult primary neurons, horseradish peroxidase (HRP) was used as a cell marker to label the motor, sensory and postganglionic sympathetic neurons of rat sural nerves which had been sectioned at the ankle and prevented from regenerating for periods of up to 80 weeks. Axotomy did not affect sympathetic neurons, but resulted 4 weeks later in a sudden reduction in the number of labeled sensory and motor cells which persisted to the end of the study. The missing neuronal population amounted to 44.4% and 45.9% respectively of the normal sensory and motor contingent and included most of the large afferent and efferent neurons. However, examination of sural nerves at the thigh, 30 mm proximal to the neuroma, revealed marked axonal atrophy but no change in the number of myelinated and unmyelinated fibers up to 52 weeks after axotomy. Such prolonged survival of the peripheral processes is indirect evidence that axotomized neurons can endure long-term detachment from their end organs and suggests that the lack of HRP labeling in certain sensory and motor neurons does not imply their degeneration, but expresses one of many retrograde dysfunctions triggered by axotomy.

Numbers of regenerated axons in tributary nerves following neonatal sciatic nerve crush in rat

We determined numbers of regenerated axons in 5 tributary nerves 8 weeks after the complete axonal loss that follows crush of newborn rat sciatic nerve. The major findings are: (1) that proportionately many more axons are lost in cutaneous than in muscle nerves, (2) that myelinated axons are greatly increased over normal in two of the muscle nerves, and (3) that unmyelinated axons are normal or close to normal in muscle nerves. The major conclusions are: (1) there is a different pattern of regeneration in cutaneous as compared to muscle nerves after neonatal sciatic nerve crush, and (2) the responses after neonatal crushes are different than after adult nerve crushes.

The reaction of primary sensory neurons to peripheral nerve injury with particular emphasis on transganglionic changes

This paper reviews light- and electron microscopic, histochemical and physiological evidence which demonstrate that peripheral nerve injury in mammals is followed by profound structural and functional changes in the central terminals of the affected primary sensory neurons. Available evidence indicates that at least some of these so-called transganglionic changes are the result of ganglion cell degeneration and death, although other mechanisms are probably in effect as well. Existing data suggest that this ganglion cell death does not effect all types of ganglion cells equally, but do not permit a clearcut answer to the question of which kinds of ganglion cells are affected more than others. Results from studies with microtubule inhibitors and antibodies to nerve growth factor are compatible with the notion that depletion of retrogradely transported trophic factors is involved in the production of certain transganglionic changes. This issue needs further examination, however. Physiological studies indicate marked alterations in certain primary afferent synaptic connections after peripheral nerve lesions. So far, these changes have not been satisfactorily correlated with the structural changes induced by similar lesions. Further studies on the structural and functional response of primary sensory neurons to peripheral nerve injury are likely to contribute to the understanding of the frequent failure to regain normal sensory functions after peripheral nerve lesions in man, as well as of the basic aspects of lesion-induced changes in general in the peripheral and central nervous system.

Proliferation of primary sensory neurons in adult rat dorsal root ganglion and the kinetics of retrograde cell loss after sciatic nerve section

This study was aimed at measuring the kinetics of retrograde death among primary sensory neurons axotomized by transection of the ipsilateral sciatic nerve in adult rats. Using electrophysiological and retrograde transport methods, we first determined that most sciatic afferents enter the spinal cord along the L4 and L5 dorsal roots (DRs), and that about 54% of the cells in the L4 and L5 dorsal root ganglia (DRGs) project an axon into the sciatic nerve. Knowing this value, we could then calculate the rate of loss of axotomized neurons from the overall rate of neuron loss in the DRGs at different times after the lesion. Following unilateral sciatic neurectomy, we found a steady falloff in the ratio of DRG neurons on the operated versus the intact control sides in cresyl-violet-stained serial paraffin sections. We were surprised to note, however, that on the control side there was a steady increase in the cell count with age. Counts done on a series of unoperated rats of various ages confirmed this natural increase. Overall, new neurons accrete at an average rate of 18.1 cells per day to the combined L4 and L5 DRGs, nearly doubling their numbers during the adult life of the animal. The new cells add mostly to the small-diameter neuronal compartment. Evidence from neonatally operated rats indicates that the decline in the ratio of neurons in operated versus control DRGs following sciatic nerve section in the adult results more from a halt in the accretion of new neurons to the sciatic compartment than from frank cell death. From our data, we calculate that the loss of axotomized neurons occurs at a rate of only about 8% per 100 postoperative days.

Quantitative ultrastructural stereology of synapses in nucleus dorsalis after a peripheral nerve injury at birth

Utilizing recent techniques in quantitative stereology, this investigation studied the synaptology of nucleus dorsalis (Clarke's column) in 12-week-old rats whose sciatic nerves were crushed in the 1st postnatal day. Four morphometric variables were analyzed at the levels of L1 and L3 spinal cord segments: total surface area of synaptic contact zones per unit volume (SV), total length of synaptic contact zones per unit area (LA), average length of synaptic membrane (L), and numerical density of synapses per unit volume (NV). The original raw data were corrected for Holmes's effect. The results indicated that peripheral nerve crush at birth induced a transganglionic change in central sensory terminals with a loss of numerous synapses. A significant loss (P less than 0.001) of about 32% in the SV and LA and a significant loss (P less than 0.001) of about 36% in the NV were observed on the experimental side. There was no preferential loss of synapses in either segment. The mean synaptic membrane length showed no significant difference between the control and experimental sides. The control values of the four morphometric variables calculated for L3 were lower than those calculated for L1. The loss of synapses after a peripheral nerve lesion was probably due to the loss of sensory neurons and their central processes, but there were other possibilities.