Structural changes in kittens' ventral and dorsal roots L7 after early postnatal sciatic nerve transection

EXPERIMENTAL

NEUROLOGY

67, 265-279

(1980)

Structural Changes in Kittens’ Ventral and Dorsal Roots L7 after Early Postnatal Sciatic Nerve Transection M. RISLING,~. Drprrrrment

REMAHL,~.

qf Ancrtom)‘.

KtrrolinsXrr

AND H. ALDSKOGIUS'

HILDEBRAND, Instirrrtrt.

S-IV4

01 Sroc~kholm

60.

Swcdrn

The number and size distribution of myelinated and unmyelinated axons were studied in spinal roots L7 of 19 kittens, 8 to 200 days after early postnatal left sciatic nerve transection. Ventral and dorsal roots on the side of transection were compared with corresponding contralateral roots. Three normal kittens were used as additional controls. On the control side the proportion of unmyelinated ventral root axons increased from about 15 to 30% between 3 and 7 months postnatally. In the ventral roots on the lesion side there was a loss of myelinated axons of all sizes (total loss I5 to 25%). The loss seemed to be somewhat greater in the gamma population. The number of unmyelinated ventral root axons increased markedly through sprouting. This increase was similar at different root levels. The persistence of such axonal sprouts in the proximal stump after ventral root division in one kitten indicated that they originate proximally in the ventral root or within the central nervous system. The dorsal roots on the lesion side showed a 3Oc/rmdeficit of both myelinated and unmyelinated axons. Signs of axonal sprouting were not observed. Both in ventral and dorsal roots the size spectra of myelinated axons were markedly shifted to the left on the lesion side due to a growth retardation of larger axons. With respect to the unmyelinated axons the size distribution was expanded toward larger sizes in the ventral roots and remained largely unaltered in the dorsal roots.

INTRODUCTION It is well established that subsequent to axotomy, the injured neurons show a retrograde response of varying character and eventually either Abbreviations: NVR (NDR)-ventral (dorsal) root L7 of normal kittens. aged 90 days. CVR (CDR)-ventral (dorsal) root L7 on the control side (right) of operated kittens, EVR (EDR)-ventral (dorsal) root L7 on the experimental side (left) of operated kittens. I This work was supported by grants from the Swedish Medical Research Council (Project No. 03761 and 05420) and from Karolinska Institutet. Technical assistance was given by Mrs. Pippi Lindqvist. Mrs. Maria Meier. Mrs. Britt Meijer. Mrs. Gunvor Pettersson. and Mrs. Katrin Stighall. 265 00 l4-4886/80/020265-

15$02.00/O

Copyright 0 1980 by Academic Press. Inc. All right5 of reproduction m any form reserved

266

RISLING

ET AL.

degenerate and die or regress into an atrophic state or recover (11,21,22, 35). Sectioning of a mixed peripheral nerve may thus result in a loss of both motor and sensory neurons (5, 10, 12, 16, 23, 24, 26, 27) particularly in young individuals [see (22)]. With respect to the question does this loss preferentially affect certain neuronal types little information is available (22). In addition to a possible preferential loss the surviving neurons might react in different ways at the axonal level. Such factors would conceivably influence the axonal content of the proximal stumps and thereby the potential for functional recovery. In the present study we tried to deal with this issue by examining the ventral and dorsal roots L7 at various intervals after sciatic neurectomy in kittens. MATERIAL

AND METHODS

Nineteen 6- to 9-day-old kittens (El-E19, Table 1) were anesthetized with Mebumal(30 mglkg, i.p.) or by halothane inhalation. The left sciatic nerve was exposed and the segment between the hamstring branch and the popliteal fossa was removed. Fascia and skin were sutured in layers. No postoperative complications occurred. At 8 to 200 days after operation the animals were perfused with 5% glutaraldehyde in phosphate buffer, as described elsewhere (3, 18). The ventral and dorsal roots L7 were removed bilaterally and postfixed, osmicated, dehydrated in acetone, and embedded in Vestopal W (3, 30). In one additional neurectomized kitten (E20) a laminectomy was made after 49 days survival and the left ventral root L7 was divided. This kitten was perfused 3 days later. For light microscopy whole semithin transverse sections were cut from each root, halfway between the proximal and distal ends, and stained with toluidine blue. Total cross-sectional areas were determined planimetrically on low magnification light micrographs. The average number of myelinated axons per unit cross-sectional area was obtained from countings in three different regions, considering about 3000 axons in each root. The average axon density and total cross-sectional area were used for estimating the total content of myelinated axons. External diameter spectra of about 1000 myelinated axons in the central part of each cross section were obtained from light micrographs (X 1000) using a Zeiss TGZ-3 particle size analyzer. (4, 29). For electron microscopy thin transverse sections were cut from the central part of each root, contrasted with uranyl acetate and lead citrate and examined in a Philips EM 300 or 301 electron microscope. The relative proportion of unmyelinated axons was estimated as described by Coggeshall et al. (7). In some animals the occurrence of unmyelinated axons was also examined at proximal and distal levels in the left ventral

STRUCTURAL

CHANGES

267

IN SPINAL ROOTS

root L7. In most animals the first 15 to 30 bundles of unmyelinated axons encountered were photographed. Prints for size measurements were prepared at a total linear magnification of ~20,000. In kitten E20 the occurrence of unmyelinated axons was examined in thin sections from the proximal and distal stumps of the divided ventral root and compared with the corresponding intact root. In addition to the roots on the right side of the operated animals the spinal roots L7 of three normal 90-day-old kittens (Nl-3) were prepared as described above and used as controls. RESULTS Ventral Roots. Comparisons between the NVR and the CVR of age-matched kittens showed an excellent agreement with respect to the TABLE

1

Ventral Roots: Cross-Sectional Areas. Number of Myelinated Axons. Proportion of Unmyelinated Axons and Calculated Total Number of Axons in Ventral Roots L7 of Experimental (EVR) and Control (CVR) Sides of Operated Kittens and in Ventral Roots L7 of Normal Control Kittens

Animal No. (age at operation in days)

Cross-sectional (xlOjpmZl

area

Calculated number myelmated

total of axons

PWXntage unmyelinated U.O”S ____ CVR EVR

surviva1 time (days)

CVR

EVR

Ratio E:C

CVR

EVR

Rat10 E:C

17) (7) (7) (7) 16) 16) 17) (71 (61 (81 19) 19) 19) (6) (81 (8) (9) 16) (6)

8 8 I? I? 24 24 49 49 80 80 80 80 80 80 80 80 80 2Oil 200

1.52 1.61 2.77 1.78 2.84 2.56 3.37 3.61 4.51 4.85 5.35 4.43 4.87 4.95 9.16 5.38 5.50 7.50 5.59

1.43 1.53 2.18 1.63 2.41 1.89 2.87 2.53 3.50 3.25 3.87 3.04 3 34 4.28 6.45 3.76 4. I4 5.90 4.06

0.94 0.95 0.79 0.92 0.85 0.74 0.85 0.70 0.78 0.67 0.72 0.69 0.69 0.86 0.70 0.80 0.75 0.79 0.73

4861 4939 6352 4597 6399 5963 5337 4994 5845 5296 5376 4755 4926 5661 11284 57 IO 5706 6016 5112

4342 4837 5080 4556 5077 4324 4702 4209 4514 4310 3908 3407 3397 4938 8843 4481 4355 495 I 4615

0 89 0.98 0.84 0.99 0.79 0.73 0.88 0.84 0.77 0.81 0.73 0.72 0.69 0.87 0.78 0.78 0.76 0.82 0.90

12.3 15.0 14.3 13.8 9.9 14.6 10.6 14.6 I2 5 14.1 136 9.3 13.1 12.5 16.3 17.0 18.4 30.5 28.8

Normal control kittens

Age (days)

Right

Left

Ratio L:R

Right

Left

Ratio L:R

90 90 90

5.28 5.65 5.45

5.25 5.47 5.28

0.99 0.97 0.97

55w 5698 5294

1.02 0.91 0.97

El8 El9 El2 El3 El0 Eli El4 El5 El E? E3 E4 ES E6 E7 E8 E9 El6 El7

NI N? N3

5481 627 I 5484

Calculated total number of axons

CVR

EVR

Ratio E:C

20.6 20.5 24.5 10.4 24.6 23.5 81.7 63.7 53.3 54 0 80.7 77.3 50.1 56.2 605 42.4 34.9 54.8 68.0

5543 5811 7412 5333 7102 6982 5970 5848 6680 6165 6222 5243 5669 6470 13482 6880 6993 8656 7180

6097 6729 5085 6733 5652 25694 11595 9645 9370 20249 I5009 6808 I1274 22387 7780 6690 10954 I4422

0.99 I .a5 0.91 0.95 0.95 0.81 4.30 1.98 1.44 I .52 3.25 2.86 1.20 1.74 1.66 I.13 O.% 1.27 2.01

Right

Left

Right

Left

Ratio L:R

16.1 16.7 11.6

18.0 12.9 14.5

6533 7528 6204

6817 6542 6192

1.04 0.87 1.00

268

RISLING

ET AL.

examined parameters. Also, the left-right variation was very small in the NVR (Table 1). This justifies the use of the CVR as controls. Both the total cross-sectional area and the number of myelinated axons decreased in the EVR relative to the CVR during the first postoperative weeks, the latter being about 75% of the control level at 24 days. At later stages this figure varied between 70 and 90% (Table 1). The occurrence of myelinated axons per unit cross-sectional area remained largely similar on the two sides. Direct signs of myelinated axon degeneration were occasionally observed in the EVR at 8 days, were more frequent at 12 to 49 days, and very rare or absent at later stages. Schwann cell profiles without associated axons were most common at survival times of 12 to 24 days and largely absent by 80 to 200 days. Although the surviving myelinated axons generally presented a similar size range in the CVR-EVR pairs, the size distribution was markedly affected (Fig. 1). Eight and twelve days after operation the EVR size 8 days po 30 lx,

E I9 CVR

3 o-%

E 12 CVR

E 19 EVR

301%

I2 days PO E I2 EVR

30-%

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5

7

9

P

I

3

5

7

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9

24 days p.0. 30%

E II CVR

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ho

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5

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9

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3

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7

9

49 days po. 30%

E I4 CVR

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30%

5

7

9

II

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304,

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FIG. 1. Histograms illustrating caliber spectra of myelinated axons in the ventral roots L7 of control (CVR) and experimental (EVR) sides at various survival times after sciatic neurectomy.

STRUCTURAL

CHANGES

IN SPINAL ROOTS

269

FIGS. 2,3. Electron micrographs showing the general appearance of the ventral roots of the Intro1 (CVR) (2a) and the experimental (EVR) (2b) sides and of the dorsal roots of the control ZDR) (3a) and experimental (EDR) (3b) sides of kitten E9 (80 days survival). x 1200.

270

RISLING

ET AL.

distribution showed a moderate shift to the left in comparison with the control side. At 24 to 49 days well-defined alpha and gamma peaks were seen in the CVR but in the EVR an alpha peak was lacking, the gamma peak was elevated, and the proportion of medium size axons was increased [(cf. (15)]. At later stages the EVR size spectra appeared highly flattened in comparison with the CVR (Fig. 1; see also Fig. 2). Using the percentage histograms and the calculated total number of myelinated axons, the number of axons in different size ranges, defined by the CVR histograms, was estimated and expressed as percentages of corresponding CVR values (see Table 2). This was made in the animals surviving for 24 days or more, in which well-defined alpha and gamma peaks had appeared in the CVR. As seen from Table 2 the EVR gamma and alpha groups showed a similar deficit at 24 days survival. At later stages the number of axons in the gamma group had increased (to 90 to 150%) whereas the alpha size range axons tended to decrease further in number (to 60 to 70%). Considering instead TABLE

2

Number of Myelinated Axons Ventral Root L7 on Experimental in Different Size Groups

Side

Survival time (days)

Gamma

Alpha

I

II

III

Total

24 49 80 200

17" 149 88 102

77 58 12 76

65 129 65 67

87 87 88 102

-0 56 63 Sl

76 86 77 86

3op,

E 2EVR

n Percentage of corresponding control values. The figures are average values for each included survival time. b At 24 days survival, group III contained so few axons that no calculations were made.

FIG. 4. Electron micrographs showing representative examples of unmyelinated axons in the ventral roots of the control (CVR) (a) and the experimental (EVR) (b) sides of kitten E4 (80 days survival). Note the presence of bundles with numerous unmyelinated axons showing a wide range of size in the EVR. In the CVR each bundle contained few individual axons and the size range was limited. x20.800 (a) and x 12,500 (b). 271

272

RISLING

ET AL.

the groups of small, medium size, and large axons (see Table 2) it is seen that at all survival times, except the 49-day stage, the loss of myelinated EVR axons was more pronounced with respect to small and large axons both in comparison with the intermediate size range and with the total loss. At the 49-day stage the number of small axons is transiently greatly increased. Turning now to the unmyelinated ventral root axons, the findings showed that they constituted about 14% (range 9 to 18%) of the total axonal population in normal and control ventral roots 2 weeks to 3 months after birth (Table 1). However, in the two kittens surviving for 200 days, 30% unmyelinated axons were found in the CVR. The bundles of unmyelinated axons occurred in association with groups of small myelinated axons and each bundle usually contained one to five individual axons (Fig. 4a). On the operated side, the EVR presented a markedly different picture at all survival times. As seen from Table 1 the proportion of unmyelinated axons increased relative to the control side, from about 20 to 60 to 80% between 8 and 49 days and thereafter remained elevated at a somewhat lower proportion. Examination of EVR from 49- and 200-day-survival kittens showed that the proportion of unmyelinated axons was similarly elevated at proximal, middle, and distal EVR levels (Table 3). In addition the unmyelinated EVR bundles often consisted of 10 to 20 individual axons (Fig. 4b). Single large unmyelinated axons invested by Schwann cells and early myelinating axons also occurred at longer survival times. Thus, the size distribution of the unmyelinated axons was found to be shifted to the right in EVR from kittens surviving for 49 days or more (Fig. 5). In the rhizotomized kitten E20 numerous bundles of intact unmyelinated axons were found in the proximal EVR stump (Fig. 6). In the distal stump intact unmyelinated axons were scarce. Dorm1 Roots. As in the ventral roots. the NDR and CDR of TABLE Proportion

3

of Unmyelinated Axons at Proximal, Middle, L7 on the Experimental Side from Four

Animal No. (survival time in days) El4 El5 El6 El7

(49) (49) (200) (200)

and Distal Levels Operated Kittens

in Ventral

Proximal

Middle

Distal

75.6 59.5 49.6 62.5

81.7 63.7 54.8 68.0

77.0 71.0 53.4 67.2

Root

STRUCTURAL

CHANGES

IN SPINAL ROOTS

273

FIG. 5. Histograms showing caliber spectra of unmyelinated axons in ventral root L7 in control (CVR) and experimental (EVR) sides (kitten E141.

age-matched, operated kittens were very similar and the left-right differences were small (Table 4). The CDR may thus serve as controls. Between postoperative days 8 and 49 the total cross-sectional area as well as the number of myelinated axons in the EDR decreased relative to the CDR. Thus by 49 days the transverse EDR area was barely half that of the CDR and the myelinated axon content showed a 30% reduction compared with the CDR (Table 4). Concomitantly, the number of myelinated axons per unit area increased markedly. These deficits

FIG. 6. Electron micrograph from the proximal stump of ventral root L7 on the experimental side that was transected 49 days after sciatic neurectomy (kitten E20). Three days after the rhizotomy numerous bundles of unmyelinated axons remained intact. x 10.200.

274

RISLING ET AL. TABLE

4

Dorsal Roots: Cross-Sectional Areas, Number of Myelinated Axons, Proportion of Unmyelinated Axons, and Calculated Total Number of Axons in Dorsal Roots L7 of Experimental (EDR) and Control (CDR) Sides of Operated Kittens and in Dorsal Root L7 of Normal (NDR) Control Kittens Calculated number myelinated

total of axons

CDR

EDR

Ratio E:C

CDR

EDR

Ratio E:C

Percentage unmyelinated axons ___ CDR EDR

(8) (8) (12) (12) (24) (24) (491 (49) (80) (80 (80) (801 W (80) W WJ) E9 (80) El6 (200) El7 (200)

2.67 2.76 4.12 3.30 4.56 3.85 4.80 5.70 5.97 6.45 8.46 6.50 7.60 8.01 6.68 7.47 7.31 8.78 8.40

2.10 2.46 3.40 2.57 2.67 2.40 2.35 2.69 4.08 2.75 3.08 2.59 2.69 3.82 3.00 3.82 3.21 4.32 3.44

0.79 0.89 0.83 0.78 0.59 0.62 0.49 0.47 0.68 0.43 0.36 0.40 0.35 0.48 0.45 0.51 0.44 0.49 0.41

8785 8279 9860 9571 13078 11136 9605 12204 9954 II427 II952 9%1 II259 13231 13179 11322 13300 II322 11679

7337 8364 9463 8213 10627 8738 6562 8752 12561 9102 8082 76% 7412 7882 10340 8987 8774 8915 7564

0.84 I.01 0.96 0.86 0.81 0.78 0.68 0.72 1.26 0.80 0.68 0.77 0.66 0.60 0.78 0.79 0.66 0.79 0.65

74.3 70.0 64.1 69.2 68.2 61.8 66.9 64.3 66.7 70.6 63.7 62.9 64.1 61.3 61.7 62.7 71.9 63.3 59.5

67.8 71.0 62.4 11.3 64.3 64.0 65.1 61.6 58. I 56.3 59.0 61.8 63.1 65.3 56.1 68.3 54.6 60.4 60.5

34183 27597 27466 3 1094 41126 29152 29018 34185 29892 38867 32926 26849 3 1362 34189 34410 30354 4733 I 30850 28837

22786 28841 25168 36181 29768 24292 18802 22792 29979 20828 19712 20147 20087 22715 23554 28350 19326 22513 19149

0.67 I .05 0.92 1.16 0.72 0.83 0.65 0.67 1.00 0.54 0.60 0.75 0.64 0.66 0.68 0.93 0.41 0.73 0.66

Normal control kittens; Age (days)

Righl

Left

Ratio L:R

hght

Left

Ratio L:R

Right

Left

Right

Left

Ratio L:R

NI (90) N2 190) N3 (‘W

7.w 7.59 7.83

7.40 8.22 7.47

I.06 1.08 0.95

10763 10694 11844

11716 12434 II640

1.09 1.16 0.98

63.7 64.7 61.2

60.5 63.8 63.5

2%50 30295 30526

2%6l 34348 31890

1.00 I.13 1.04

Antmal No. (survival time in days) El8 El9 El2 El3 El0 El1 El4 El5 El” E2 E3 E4 E5 E6 E7

I33

L1The EDR

Cross-sectional area , x 1tY pm?)

I included

one intersegmental

connection

with the Sl dorsal

Calculated total number of axons

CDR

EDR

Ratio E:C

root [cf. (17)].

remained largely unaltered throughout the rest of the postoperative period. Direct signs of myelinated axon degeneration were observed in the EDR 8 to 49 days postoperatively. Both Schwann cells and basement membrane profiles lacking associated axons occurred 12 to 200 days after operation. In most animals the myelinated EDR axons showed a narrower size range compared to the CDR (Fig. 7). With respect to size distribution, the EDR presented a clearcut shift to the left relative to the CDR already by 12 days (Fig. 7). This difference became more marked with increasing survival time until 49 days postoperatively and thereafter persisted largely unaltered (Fig. 7; see also Fig. 3). The proportion of unmyelinated axons was about 2:3 in the CDR except in the youngest animals where a somewhat higher

STRUCTURAL

CHANGES

24 day:

IN SPINAL ROOTS

275

PO

FIG. 7. Histograms illustrating caliber spectra of myelinated axons in the dorsal roots L7 on the control (CDR) and experimental (EDR) sides at various survival times after sciatic neurectomy.

figure was found (Table 4). It varied considerably in the EDR, but the average pattern suggested that this proportion was essentially unchanged on the operated side releative to the control side (Table 4). Comparisons of size spectra of unmyelinated EDR and CDR axons likewise did not reveal any clearcut differences (Fig. 8).

FIG. 8. Histograms showing caliber spectra of unmyelinated axons in dorsal root L7 of control (CDR) and experimental (EDR) sides (kitten E14).

276

RISLING

ET AL.

DISCUSSION Considering first some observations on the control side, it was found that the number of myelinated axons in ventral root L7 did not change in a clearcut manner with development during the period examined, but varied between 5000 and 6000. This clearly exceeds the number (about 4000) found by Coggeshall et al. (6) in the adult cat. The unmyelinated ventral root axons constituted about 14% of the total number of CVR axons between 15 days and 3 months after birth and the normal 3-month-old kittens similarly showed 15% unmyelinated axons bilaterally in the NVR. However in the two kittens surviving for 200 days, 30% unmyelinated axons were found, in agreement with observations in adult cats (6-8). This indicates that between 3 and 7 months postnatally the content of unmyelinated axons increases 100% in the feline ventral root L7. In the dorsal root the number of myelinated axons increased from about 8500 at an age of 15 days to about 12,000 from 1 month and thereafter. The proportion of unmyelinated dorsal root axons remained relatively stable at 2:3 after a slight decrease during the 1st month. Thus whereas the total axonal content tended to increase with development in the CVR it seemed to remain constant in the CDR. On the experimental side both ventral and dorsal roots L7 lost some myelinated axons as a result of sciatic nerve transection. This loss, which was already initiated 8 days postoperatively and was completed between 24 and 49 days after operation, should result from retrograde neuron death, which was shown to occur both in the ventral horn (12, 24, 28) and in sensory ganglia (2, 5, 16, 26, 27) subsequent to axotomy. At shorter survival times the loss of myelinated axons was similar in the EVR and the EDR but at survival times of 49 days or more it was larger in the EDR. The late divergence should, at least partly, be related to the appearance of new axons in the EVR but not in the EDR. Analysis of the loss of axons in different size ranges in the EVR showed that both alpha and gamma axons disappear after peripheral nerve injury. At later survivals the deficit was greater in the alpha than in the gamma size range. This could result from an addition both of retarded alpha axons and of axonal sprouts to the gamma size range. The finding that large alpha axons showed a greater loss than the total population may be explained by growth retardation whereas the greater than average loss of small gamma axons seen at some stages seems to be real. With respect to the unmyelinated axons a markedly increased relative and absolute number was found in the EVR but not in the EDR. The increase was discernible 8 to 12 days after operation in some animals and was a constant finding at later survival times. It reached a maximum 49 days

STRUCTURAL

CHANGES

IN SPINAL

ROOTS

277

after operation and then seemed to stabilize at a lower value. This change appears to result from an early massive axonal sprouting, followed by a late degeneration of some of the new axons. The finding of a rich production of unmyelinated axons in a ventral root, at a site 40 mm proximal to the sciatic lesion (at operation), was unexpected and seems to represent a new observation. The fact that the size spectra of the unmyelinated EVR axons were shifted toward larger sizes and that some of them became myelinated after long survivals suggests that the EVR axonal sprouts derive from myelinated stem axons. The most probable source is the large and medium size axons of alpha neurons (25). which seem to possess a greater regenerative capacity than gamma axons (32. 33). The presence of approximately the same proportion of unmyelinated axons at different EVR proximodistal levels suggests that the new axons originate peripheral to the dorsal root ganglion level or central to the proximal end of the EVR specimens. The findings in reoperated kitten E20 support the latter alternative. In the dorsal root on the other hand, the unmyelinated EDR axons decreased in number pari passu with the myelinated axons, so that the relative proportions remained largely unchanged. This contrasts with previous observations in adult rat dorsal root ganglia, according to which small sensory neurons were more apt to degenerate and die than large ones after axotomy (5, 16, 27). Both in the EVR and the EDR the size spectra of the myelinated axons were shifted toward smaller sizes compared with the controls. This change was most marked in the dorsal root and seemed mainly to result from an arrest or retardation of axonal development. In the EVR the maldevelopment primarily affected the large alpha axons and the gamma population did not show any obvious signs of axonal atrophy. In the unmyelinated size range the newly produced EVR axons distorted the size spectra toward larger sizes and obscured whatever maldevelopment or atrophy that might have taken place in the original population. In the EDR the large myelinated axons likewise showed signs of severe growth retardation whereas the population of small myelinated axons and the unmyelinated axons did not. A retrograde atrophy of mature myelinated axons (as well as of neuronal perikarya) after neurotomy has been described in various peripheral nerves (32, 34). The atrophic state and an associated decrease in conduction velocity (1, 9, 13, 14. 3 1) appears to be permanent when regeneration is prevented and transient in cases of eventual successful regeneration (20). In the present study regeneration was prevented by removal of a substantial portion of the sciatic nerve and abnormal size spectra persisted throughout the experimental period. With respect to the sensory neurons the axonal growth disturbance probably affects the whole central process.

278

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ET AL.

Thus, in neonatal rats sciatic nerve injury inhibits axonal growth ipsilaterally in the dorsal column (19). In addition the terminals of surviving but retarded or atrophied EDR axons might possibly undergo a structural remodelling. As a result of axon loss and growth retardation of large axons the transverse growth of the whole EVR and EDR was retarded. At the end of the experimental period the EDR cross-sectional area was less than half that of the CDR and the EVR area was three quarters of the CVR value. The difference should be related to the more pronounced axonal changes and the markedly increased myelinated axon density in the dorsal root, as well as to the appearance of new axons in the ventral root. The generally greater regressive effects of the sciatic lesion on the dorsal root axons may be partly explained by the fact that the lesion was situated closer to the dorsal root ganglion neurons than to the cells in the ventral horn (30 to 32 mm versus 44 to 45 mm at operation) (22). REFERENCES 1. AITKEN, J. T., AND P. K. THOMAS. 1962. Retrograde changes in fibre size following nerve section. J. Am?. 96: 121-129. 2. ALDSKOGIUS, H., AND J. ARVIDSSON. 1978. Nerve cell degeneration and death in the trigeminal ganglion of the adult rat following peripheral nerve transection. J. Neurocytol.

7: 229-250.

3. BERTHOLD, C. -H. 1968. A study on the fixation of large mature feline myelinated ventral lumbar spinal-root fibres. Acta Sot. Med. Upsal. 73(Suppl. 9): l-36. 4. BERTHOLD, C. -H.. AND T. CARLSTEDT. 1973. Fixation and numerical estimation of myelinated nerve fibers during initial myelination in the cat. Neurobiology 3: 1- 18. 5. CAVANAUGH, M. W. 1951. Quantitative effects of the peripheral innervation area on nerves and spinal ganglion cells. J. Camp. Neural. 94: 181-219. 6. COGGESHALL, R. E., J. D. COULTER, AND W. D. WILLIS, JR. 1973. Unmyelinatedfibersin the ventral root. Brain Res. 57: 229-233. 7. COGGESHALL, R. E., J. D. COULTER, AND W. D. WILLIS, JR. 1974. Unmyelinated axons in the ventral roots of the cat lumbosacral enlargement. J. Comp. Neural. 153: 39-58. 8. COGGESHALL, R. E., AND H. ITO. 1977. Sensory fibres in ventral roots L7 and SI in the cat. J. Physiol. (London) 267: 215-235. 9. CRAGG, B. G.. AND P. K. THOMAS. 1961. Changes in conduction velocity and fibre size proximal to peripheral nerve lesions. J. Physiol. (London) 157: 315-327. 10. FRIEDLANDER, C.. AND F. KRAUSE. 1886. Ueber Veranderungen der Nerven und des Riickenmarks nach Amputationen. Forrschr. Med. 4: 749-764. 11. GRAFSTEIN, B., AND I. G. MCQUARRIE. 1978. Role of the nerve cell body in axonal regeneration. Pages 155-195 in C. W. CO.TMAN, Ed., Neuronal Plasticity. Raven Press, New York. 12. GRANT, G. 1968. Silver impregnation of degenerating dendrites, cells and axons central to axonal transection. II. A Nauta study on spinal motor neurones in kittens. Exp. Bruin Res. 6: 284-293.

13. GUTMANN, E., AND F. K. SANDERS. 1943. Recovery of fibre numbers and diameters in the regeneration of peripheral nerves. J. Physiol. (London) 101: 489-518. 14. GUTMANN, E.. AND J. HOLUBAR. 1951. Atrophy of nerve fibres in the central stump

STRUCTURAL

15.

16. 17. 18.

19. 20.

21. 22.

23.

24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34.

35.

CHANGES

IN SPINAL

ROOTS

279

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