A light and electron microscopic study of calcitonin gene-related peptide in the spinal cord of the rat

The present study localized calcitonin gene-related peptide at the light and electron microscopic levels in the lumbar spinal cord of the rat. One finding was that axons and terminals were labeled in both lamina I and IIo medially but only in lamina I laterally. The functional implications of this innervation pattern are not clear but presumably this anatomic arrangement bears on both dorsoventral and mediolateral patterns of organization of primary afferent input into the dorsal horn. We also found that although the means of labeled myelinated and unmyelinated axon diameters in the tract of Lissauer were different, there was great overlap in these populations. Furthermore, subcellular localizations indicated that immunostaining of calcitonin gene-related peptide was associated primarily with microtubules in axons and cores of large dense-core vesicles in presynaptic terminals. Finally, labeled presynaptic terminals contained relatively few large dense-core vesicles and formed the presynaptic elements of simple axodendritic contacts almost exclusively. These last findings contrast with localizations of calcitonin gene-related peptide in the monkey, which has many more large dense-core vesicles in labeled terminals and in which a much higher proportion of labeled endings form the central parts of glomeruli.

A light and electron microscopic level analysis of calcitonin gene-related peptide (CGRP) in the spinal cord of the primate: an immunohistochemical study

In the present study, the distribution of calcitonin gene-related peptide (CGRP)-stained fibers and varicosities are demonstrated in the lumbar spinal cord of the monkey at the light microscopic level. Immunostained fibers and varicosities form a dense plexus in laminae I, IIo, the reticulated region of lamina V, and in the region of the central canal. The intervaricose fibers consistently measured 1 micron or less in diameter suggesting a population of finely myelinated or unmyelinated fibers. At the electron microscopic level, two types of terminals were labeled: a glomerular type where one CGRP-labeled profile was indented by several unlabeled postsynaptic profiles and an axodendritic type with one or sometimes two postsynaptic elements. The most noteworthy cytologic feature of CGRP-labeled profiles was the presence of many relatively large vesicles with dense cores. These findings are steps towards understanding the synaptic interactions of CGRP in the monkey dorsal horn.

Primary afferent and propriospinal fibers in the rat dorsal and dorsolateral funiculi

The purpose of this study is to determine the numbers of primary afferent and propriospinal fibers in the dorsal and dorsolateral funiculi of the rat. The reason for concentrating on these areas is that they contain large numbers of unmyelinated axons. Our data are axonal numbers from the S2 segment of spinal cord in animals that had unilateral dorsal rhizotomies or spinal cord isolations. The major conclusions are 1) that 23% of the primary afferent fibers in the dorsal funiculus are unmyelinated; 2) that there are approximately 12,500 unmyelinated primary afferent fibers in the dorsolateral funiculus, which is more than the number of primary afferent fibers in the dorsal funiculus and tract of Lissauer combined, and 3) that approximately 25% of the axons in the dorsal funiculus and 44% of the axons in the dorsolateral funiculus are propriospinal. These data modify and extend previous ideas of the organization of spinal white matter.

Nerve regeneration changes with filters of different pore size

An experimental reason for placing stumps of a transected nerve in an impermeable tube is that factors and soluble substances from the nerve stumps are pooled and separated from cells and soluble substances in the body in general. Previous work showed that certain parameters of regeneration were improved, however, when the impermeable tube was made completely permeable by cutting macroscopic holes in its side. To begin exploring the reasons for these improvements, we covered the holes in the permeable tubes with filters of two different pore sizes, and found that the improvements resulted when the pore size was large enough to allow both fluid and cells to exchange but not when the pore size allowed only fluid to exchange. These findings suggest that cells from the general connective tissue should be given consideration when designing experimental procedures to maximize the regeneration potential of regenerating axons.

Persistence of anti-NGF induced dorsal root axons: possible penetration into the mammalian spinal cord

Neonatal rats were given daily injections of antisera to nerve growth factor protein (anti-NGF) for a period of 1 month and then allowed to survive 17 more months. The number of neurons in dorsal root ganglia (DRG) and axons in the dorsal root (DR) were determined in the anti-NGF rats and compared to similar numbers from untreated littermates. We found a 32% decrease in DRG neuron number and 32 and 34% increases in myelinated and unmyelinated DR fibers, respectively, in the anti-NGF rats. The sensory cell bodies in the anti-NGF rats were on the average 23% larger than in the normal rats. We conclude that in an NGF deprived environment a population of DRG neurons dies, principally the small neurons, and in response the surviving neurons emit extra processes which persist for most of the life of the rat. This suggests that the anti-NGF induced axons enter the spinal cord and synapse.

In vivo ANTI-NGF induces sprouting of sensory axons in dorsal roots

Newborn rats were given subcutaneous injections of antibodies to mouse beta -NGF (ANTI-NGF) daily for 1 month. The number of neurons in T4-T6 dorsal root ganglia (DRG) and the numbers of myelinated and unmyelinated axons in the dorsal roots of the same segments were counted in the ANTI-NGF animals and in normal littermates. The ANTI-NGF rats had 38% fewer neurons in thoracic ganglia but 17% more myelinated and 40% more unmyelinated fibers than their untreated littermates. Dorsal root ganglion cells also have a larger average size in the ANTI-NGF animals, which we interpret as a disproportionate loss of small cells. These data are interpreted as showing that some dorsal root ganglion cells, principally small ones, die when endogenous NGF is inactivated, and that the remaining cells emit more processes than normal. Thus, removal of NGF has what appears to be a paradoxical effect, a reduction in dorsal root ganglion cell numbers but an increase in dorsal root axon numbers. The relation of myelin thickness to fiber diameter is also altered, with small fibers being more thinly myelinated in the ANTI-NGF group. Thus, Schwann cell-neuronal interactions are also affected by inactivation of NGF.

Single vs multiple somatic nerve crushes in the rat

Nerve lesions modify regenerative responses to subsequent lesions. Some of the modifications might be useful. To increase our understanding of these modifications, the present study determines myelinated and unmyelinated axon numbers in the distal part of rat sciatic nerve and in 2 smaller branches, the nerve to the medial gastrocnemius muscle and the sural nerve, 8 weeks and 9 months following either single or the last of 3 crushes to the rat sciatic nerve. For myelinated axons, there is a significant and proportional increase distal to the crush in the sciatic nerve and in its smaller tributaries following both single and triple crushes. These increased axons persist. We interpret these data to indicate that some of the regenerating myelinated axons branch at the site of lesion, pass without branching into the tributary nerves, and then presumably find attachments at the periphery. If true, single or multiple crushes might be useful in conditions where it would be desirable to increase numbers of processes from surviving neurons. The major differences between single and triple crushes are that myelinated axons are increased more after triple crush and increase significantly between 8 weeks and 9 months after triple crush but not after single crush. Thus not only myelinated axon numbers, but the timing of the myelination process seems to change if regeneration following single crush is compared to similar regeneration following multiple crushes. Unmyelinated axons do not regenerate in the same way as the myelinated axons.(ABSTRACT TRUNCATED AT 250 WORDS)

Permeable tubes increase the length of the gap that regenerating axons can span

After placement of stumps of transected rat sciatic nerve in an impermeable tube, the maximum gap the axons can span is 10 mm. The present study shows that the regenerating axons cross much longer gaps if the tube is made permeable. This improvement does not require another nerve as a transplant nor the preplacement of extracellular materials in the tube. Possible mechanisms for this improvement are discussed.

Postnatal development of the rat dorsal funiculus

The present study shows that there are approximately 21,000 axons in the neonatal rat dorsal funiculus, as compared to 26,000 in 2-week-old animals. We attribute this increase primarily to arriving corticospinal fibers. In adult animals, however, there are approximately 15,000 axons. This is a decline of 58% from the 2 week level, and the decrease is proportionately similar in the corticospinal area and the dorsal funiculus proper. Thus, axon numbers decline in later postnatal development, and since the decline seems to be well past the time of the histogenetic death of the cells that give rise to these axons, we propose that the loss is not caused by the death of neurons. This is further evidence that a reduction in axon numbers not accompanied by cell death is a widespread phenomenon in mammalian postnatal neural development. We infer that the mechanism of axon loss is a reduction in axon branching and that its function is to sharpen synaptic connections.

Sciatic nerve regeneration after autologous sural nerve transplantation in the rat

The present study is concerned with the question as to whether the size of a nerve used as a transplant to bridge a gap between the stumps of transected nerves has a bearing on the number of axons and the cytological structure of the regenerate. The paradigm is rat sciatic nerve transection with 8 mm of nerve removed with the stumps placed in a silicone tube and two strands of the smaller sural nerve used as bridging transplants. The comparisons are with previously published results where the transplant, which is the removed piece of sciatic nerve, is exactly matched in size and with no transplant in the same regeneration paradigm. One surprising finding is that the size of the transplant does not seem to determine the size of the regenerated nerve. The cytological structure of the regenerated nerve is related to the size of the transplant, however, in that the proportion of axons that regenerate inside and outside the transplanted perineurial tubes differs in relation to the size of the transplant. In addition, although there is an increase in the number of blood vessels in all of these paradigms, the greatest increase is with the sural nerve transplants. The key finding in the study, however, is the similarity in numbers of regenerated axons in the gap, distal stump and tributary nerves when regeneration after sciatic nerve transplantation is compared with regeneration after sural nerve transplantation. Thus, notwithstanding the cytologic differences of the two types of regenerate, regenerated axon numbers are approximately the same. The conclusion is that the size of the transplant determines neither the size of the regenerate nor the numbers of regenerated axons in this paradigm. On the assumption that regeneration is better when axonal numbers are closer to normal, the non-matched sural nerve transplant is approximately equal to the matched sciatic nerve transplant and both are superior to the regeneration that takes place in the absence of a transplant in this paradigm.

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.

Numbers of rat dorsal root axons and ganglion cells during postnatal development

The present study demonstrates that T4 and S2 rat dorsal root axons decrease significantly from birth to adulthood with almost all of the decrease occurring in the first two weeks of life. Dorsal root ganglion cell numbers do not change during this time period. This is thus an example of postnatal axon elimination not associated with death of the cells that give rise to the axons. Presumably this regressive process is important in the formation of the normal adult nervous system. In addition, these findings raise the possibility that certain types of neonatal denervation may increase adult axon numbers by stopping a regressive process, the loss of axons, rather than initiating a progressive process, the formation of new axons.

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.

Postnatal loss of axons in normal rat sciatic nerve

Myelinated and unmyelinated axons were counted in sciatic nerves of newborn, 5-day-old, 14-day-old, and adult rats. Myelinated axons increase from essentially none at birth to approximately 8,000 in adulthood, but total axon numbers decrease steadily from 33,954 at birth to 22,872 in adulthood. Thus there is a significant postnatal loss of axons from rat sciatic nerve. This loss is, in our opinion, not associated with the death of the cells that give rise to these axons. This is thus an example of a regressive event that probably is of importance in normal neural development, namely the postnatal elimination of axons unaccompanied by death of the neurons that give rise to axons. These findings presumably imply a considerable amount of proximal peripheral axon branching, and the postnatal elimination of axons in the sciatic nerve presumably results from a reduction of this branching. Thus postnatal elimination of processes on, for example, somatic muscle cells may be at least partially the result of long axon elimination rather than local withdrawal of presynaptic processes, as is usually thought to be the case. In addition, an increased number of axons resulting from early postnatal manipulations may indicate cessation of axon loss rather than formation of new axons.

The effects of an autologous transplant on patterns of regeneration in rat sciatic nerve

Autologous transplants are often used in repair of peripheral nerve injury. Quantitative evaluation of the results of such a transplant is obviously desirable. In previous study, we determined numerical and cytologic parameters of the regeneration that followed transection of rat sciatic nerve, but no transplant was used. This work now serves as a basis for evaluating the use of an autologous transplant in the same transection paradigm. Our procedure is to remove 8 mm of sciatic nerve in the thigh. The removed segment is then put into the center of a silicone tube and the proximal and distal stumps of the severed nerve are placed into the ends of the tube. The data show: (1) a high percentage of successful regenerations; (2) a relatively large nerve in the gap; (3) a typical outer perineurium underlying the epineurium; (4) a well-developed fascicular perineurium; and (5) approximately equal numbers of myelinated and unmyelinated axons in the gap and distal stump. If a transplant is not used there are: (1) a greater number of failures of regeneration; (2) a smaller nerve in the gap; (3) a less well-developed fascicular perineurium; (4) unequal numbers of axons in the gap as compared to the distal stump; and (5) no outer perineurium forms. The presence of a typical outer perineurium after a transplant and its absence if a transplant is not used is probably the most striking cytologic difference between the two paradigms. The equal numbers of axons in the gap and distal stump following regeneration after transplantation presumably indicate that all axons in the gap enter the distal stump without branching or ending blindly, a situation that is presumably beneficial and contrasts with the findings when a transplant is not used. Both paradigms show a remarkable increase in the density of blood vessels in the regenerated nerve in the gap between the two stumps. These findings will serve as a basis for further studies on the mechanisms of peripheral nerve regeneration.

Correlation of cell body size, axon size, and signal conduction velocity for individually labelled dorsal root ganglion cells in the cat

Measurements of cell body and peripheral and central axon sizes were made for primary sensory neurons outlined by the intracellular injection of HRP. Conduction velocities were also measured on the outlined processes. The sensory neurons were then subdivided into A and C cells on the basis of the conduction velocity of the impulses carried by the processes of these cells. Central processes of both A and C cells are smaller than the peripheral processes, but the size differential is greater for the C cells. For A cells there is a linear relation between the size of the peripheral axon and the conduction velocity of the impulses carried by these axons, but the confidence limits are wide. For C cells there is a linear relation between the size of the central process and conduction velocity of the impulses carried by the processes, but for the peripheral processes two aberrant processes resulted in no correlation between process size and conduction velocity. For A cells, the size of the central and peripheral processes and the conduction velocity of the impulses carried by the peripheral processes are linearly correlated with cell body size. By contrast no such correlations can be demonstrated for C cells. This presumably implies an important difference in that the size of the cell body is correlated with axon size and impulse conduction velocity for A cells but not for C cells. A widely accepted generalization is that large sensory cells give rise to myelinated axons and small sensory cells to unmyelinated axons. In this study, myelinated and unmyelinated are defined on the basis of impulse conduction velocity. For those cells that are clearly large (greater than 50 microns in diameter), the conduction velocity of the impulses carried by their processes is always greater than 2.5 m/s, and for those cells that are clearly small (less than 35 microns in diameter), the conduction velocity is always less than 2.5 m/s. Thus for these cells the above generalization holds. For the intermediate-sized cells (35-50 microns), however, the size of the cell body bears no predictable relation to the conduction velocity of the impulses carried by those processes, and thus to whether the axons are myelinated or unmyelinated. Thus the above generalization does not hold for this intermediate group of cells, and since there are many cells in this size range, we feel that the generalization that large cells give rise to myelinated axons and small cells to unmyelinated axons is an oversimplification.

Regeneration of transected rat sciatic nerves after using isolated nerve fragments as distal inserts in silicone tubes

We determined blood vessel and perineurial fascicle densities as well as axonal numbers in regenerated rat sciatic nerves 8 weeks after the nerves had been transected, the proximal stumps placed into the proximal ends of silicone tubes, and isolated fragments of nerve placed into the distal ends of the same tubes. The data are compared with data from the normal nerve and from regeneration in a similar paradigm in which the distal stumps were used as the inserts into the distal end of the silicone tubes. A major difference between the two regeneration paradigms was that axons were discouraged from reaching the periphery when the distal insert was an isolated fragment and encouraged to reach the periphery when the distal insert was the distal stump. We found that fascicle and blood vessel densities were greater than normal but less than with the distal stump as the distal insert. Thus we concluded that the nature of the distal insert had a bearing on how many vessels and perineurial fascicles were formed during regeneration in these conditions. Myelinated axon numbers did not differ in the two conditions whereas there were more unmyelinated axons with the isolated distal stump as the distal insert. Thus at this regeneration time the numbers of myelinated axons were not as dependent on the nature of the distal insert as were the numbers of unmyelinated axons. Finally the length of the gap had a great influence on the numbers of axons that regenerated.(ABSTRACT TRUNCATED AT 250 WORDS)

Nerve regeneration through holey silicone tubes

Recent studies focus on regeneration where nerve stumps are placed in a silicone tube. Since the tube is impermeable, the fluid and cells that collect from the stumps bath the axons. This is presumably beneficial. Making the tube permeable by making holes in its walls should change the patterns of regeneration. If this is done, the major cytologic change is an increase in the fascicular perineurium. There are more individual fascicles, more cells line each fascicle and the lining cells are coated by more prominent external laminae than after similar regeneration in a regular silicone tube or in the normal untransected nerve. For axonal numbers, there are more myelinated and unmyelinated axons in the gap and more unmyelinated axons in the distal stump than after regeneration in a regular silicone tube. The numbers in the holey tube regenerate are statistically different from normal but they are closer to normal than after similar regeneration in a regular silicone tube. There are significantly fewer myelinated and unmyelinated axons than in the normal sural nerve after regeneration through a holey tube, but there are more than after regeneration through a regular tube. The numbers of axons in the nerve to the medial gastrocnemius muscle are not significantly different from normal or from the other regeneration paradigms. These data allow the suggestion that regeneration through a silicone tube with macroscopic holes in its walls may be superior in certain respects to regeneration through a regular impermeable silicone tube.

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.

Long-term patterns of axon regeneration in the sciatic nerve and its tributaries

The present study determines the numbers of axons that regenerate after sciatic nerve transection in the rat. The transections are done by removing either 4 mm or 8 mm of the nerve. The axons are counted in the gap and distal stump of the sciatic nerve and in 5 of its tributaries. Survival time is 9 months which we define as long-term to allow comparison with short-term data obtained after a much shorter survival. The first findings is that the numbers of axons in the gap and distal stump are different in the 2 transection paradigms. For the 4 mm paradigm, more axons than normal appear in the gap and only a fraction of these pass into the distal stump. For the 8 mm paradigm, the numbers of axons in the gap are normal and the numbers in the distal stump do not deviate far from these. Thus by changing only the length of the segment of removed nerve, one causes major differences in the numbers of axons that regenerate. Second the numbers of axons that regenerate in tributary nerves that innervate muscle have a different pattern than the numbers that regenerate into cutaneous nerves. Thus the factors control axonal numbers must be different in the 2 types of nerves. Finally, axons that regenerate into tributary nerves do not, by and large, regenerate in concert with those in the distal stump of the parent nerve. Thus the factors that control axonal numbers in the tributary nerves must be different from those that control the numbers in the distal stump of the parent nerve.

Unmyelinated primary afferent fibers in dorsal funiculi of cat sacral spinal cord

The present study tests the hypothesis that there are numerous unmyelinated primary afferent fibers in cat posterior funiculi. The animals have unilateral dorsal rhizotomies from L6 to Ca3. One week later the axons of both S2 dorsal funiculi are counted. The data indicate that there are approximately 22,500 myelinated and 8,500 unmyelinated axons on the unoperated side and 11,000 myelinated and 3,900 unmyelinated axons on the operated side. On this basis, we suggest that 51% of the myelinated and 54% of the unmyelinated axons in cat dorsal funiculi arise from dorsal root ganglion cells and thus are primary afferent axons. If this is correct, then 71% of the primary afferent axons in the cat dorsal funiculus are myelinated and 29% are unmyelinated. The function of this large group of previously unsuspected fine sensory axons remains to be determined.

Sexual dimorphism in the numbers of neurons in the pelvic ganglia of adult rats

The major pelvic ganglion in the adult rat differs in neuron number in the two sexes. The female pelvic ganglion has 5892 +/- 797 (mean +/- S.D.) neurons while the male contains 14,654 +/- 936. This sexual dimorphism in neuronal numbers in the autonomic nervous system is of interest because it suggests that endogenous hormone levels during development may have an influence on numbers of neurons in the adult pelvic ganglion.

Activation of dorsal horn cells by ventral root stimulation in the cat

Responses of dorsal horn cells to ventral root stimulation were determined for the L7 and S1 levels of the spinal cord of 14 anesthetized cats. Forty-six dorsal horn cells were found that were excited by stimulation of the distal stump of the cut ventral root. For maximum excitation it was necessary to use a train of stimuli. For the 34 dorsal horn cells whose peripheral receptive-field properties could be characterized, 14 were wide dynamic range cells and 19 were high-threshold cells. The other cell responded exclusively to stimulation of deep tissue. None of the cells responded exclusively to innocuous stimuli, and all responded more vigorously to noxious than to innocuous stimuli. Some cells also responded to noxious heat applied to the skin of the receptive field. Locations of 10 of the activated dorsal horn cells were identified. They were distributed throughout the dorsal horn, but most were found in laminae V and VI. In four animals, both the proximal and distal stumps of the cut S1 ventral root were stimulated while searching for dorsal horn cells. Ten dorsal horn cells were found that were excited by stimulation of the distal stump of the ventral root. No cells were found that responded to proximal stump stimulation. To prevent current spread by stimulation of the ventral root, an extra ground electrode was placed distal to the stimulating electrodes. When the ground electrode was removed, distinctive signs of current spread appeared in that a cord dorsum potential could be recorded and the dorsal horn neuronal responses changed. Dorsal horn neurons could also be excited by nonelectrical stimuli such as crushing the ventral root. If the ventral root was crushed distal to the stimulating electrodes, however, the initially excited cell could no longer be activated by ventral root stimulation. Activation of dorsal horn cells by stimulation of the distal stump of a cut ventral root was abolished when the dorsal root of the same segment was sectioned. Conduction velocities of the fibers in the ventral root that excited dorsal horn cells ranged between 0.25 and 1.78 m/s with a mean of 0.91 +/- 0.47 (SD) m/s. These results show that there are unmyelinated afferent fibers in the ventral root that enter the spinal cord through the dorsal root and excite dorsal horn cells.(ABSTRACT TRUNCATED AT 400 WORDS)