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The number of neurons in dorsal root ganglia L4 УL6 of the rat.

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THE ANATOMICAL RECORD 219:315-322 (1987)
The Number of Neurons in Dorsal Root Ganglia
L4-L6 of the Rat
HENNING SCHMALBRUCH
Institute of Neurophysiology, University of Copenhagen, The Panum Institute,
DK-2200 Copenhagen N, Denmark
ABSTRACT
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 pm thick, or paraffin sections 5 pm thick, or
unstained 12-pm 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
pm in diameter. The nucleoli often projected into the dark cytoplasm and were
difficult to identify.
The sciatic nerve of the rat is frequently used in experimental work. Its motor and sensory fibers originate
from the spinal segments and dorsal root ganglia L4L6. The relative contribution of the individual ventral
roots to the motor axon pool of the nerve varies and the
right-left difference for individual root pairs is much
larger than that for the total number of fibers in the
three roots (Schmalbruch, 1984). Similar variations may
occur in lumbar dorsal root ganglia as well. The side
differences €or thoracic dorsal root ganglia almost vanished when rows of ganglia were compared (Ygge et al.,
1981). Because none of the lumbar ganglia exclusively
supply the sciatic nerve and because of the likely anatomical variability, experimentally induced changes of
the neuron numbers can be quantitated only when all
three segmental levels are studied.
The normal number of neurons in the ganglia L4
through L6 has been determined in two studies only
(Bondok and Sansone, 1984; Arvidsson et al., 1986).
These results, and also those of studies in which only
one of the ganglia was investigated, are inconsistent
(Table 1).The interindividual variation in all papers is
much larger than one would expect from the variability
of the number of nerve fibers in the sciatic nerve or
dorsal roots (Schmalbruch, 1986a,b). All neuron counts
appear incompatible with the observation that the sciatic
nerve contains 19,000 afferent axons (Schmalbruch,
1986a), although half of the L4 and L5 ganglion cells
0 1987 ALAN R. LISS, INC.
have axons in nerves other than the sciatic nerve (Devor
et al., 1985).
The aim of the present study was to determine the
number of neurons in the three ganglia by means of
different histological methods, and to elucidate why the
neuron counts vary.
MATERIALS AND METHODS
Twenty Wistar rats of both sexes and 23-96 weeks old
were used. The ganglia were taken from rats in which
the contralateral sciatic nerve had been cut at birth or
at 4 weeks to study induced neuron death (Schmalbruch,
1987).Three rats were used as unoperated controls. The
animals were anesthetized with halothane. Twelve rats
were perfused transcardially with 2.5%glutaraldehyde
in Ringer solution. The lumbar and sacral spinal cord
together with roots and ganglia were excised, and the
ganglia were identified with the aid of the adherent
ventral roots. The ventral root L6 was distinctly thinner
than the ventral root L5, which was equal to, or slightly
thicker than, the L4 ventral root. There was no reliable
criterion to distinguish excised ganglia or dorsal roots
directly. The ganglia were postfixed with osmium tetroxide and embedded in epoxy resin (Embed 812). Eight
rats were perfused with isotonic buffered 4% paraformaldehyde, and the sacral spinal cord with adherent lumbar ganglia was embedded en bloc in paraffin.
Received February 5, 1987; accepted June 1,1987.
316
H. SCHMALBRUCH
Complete series of 3-pm sections were cut from the
ganglia embedded in resin. The sections were stained
individually with p-phenylenediamine and mounted 10
per slide. A typical series for one ganglion comprised
400-600 sections. The paraffin blocks were cut into series of 5-pm cross sections. Sections containing the ganglia L4-L6 were mounted and stained with acidic cresyl
violet. A series of 8-pm sections was prepared but the
results were discarded (see Sources of Error). A Spencer
rotary microtome (American Optic, Buffalo, N.Y.) with
glass knives (resin sections) or steel knives (paraffin
sections) was used.
One set of ganglia was prepared following Hatai’s
(1902) procedure. A rat was killed by an overdose of
chloroform; the ganglia were excised, fixed for 24 hr
with 1%osmium tetroxide in water, and embedded in
paraffin, and serial cross sections 12 pm thick were
mounted without additional staining.
Counts and Measurements
use any of the correction formulas for split nucleoli that
are based on average nucleolus size and section thickness (see Discussion). Therefore, the empirical method
of Coggeshall et al. (1984)was applied. A given area was
photographed in 10-20 consecutive sections. In one section, all neurons with nucleolar profiles were identified,
and in flanking sections, the number of additional nucleolar profiles contained in the same nuclei was determined. If a nucleus in one section contained more than
one nucleolar profile, these were counted as one. Correction factors were computed from the number of cells and
nucleolar profiles. The correction factors with which the
raw data were multiplied to obtain the number of neurons were 0.84 for 5-pm paraffin sections and 0.61 (L4
and L5) and 0.57 66)for 3-pm resin sections (1,218cells
studied in resin sections). The nucleolus counts in 12-pm
sections were not corrected.
Sources of Error
Criteria for counting the nucleoli and their reliability
The number of neurons was determined by counting were evaluated on consecutive serial sections. The minucleolar profiles in 10% of the sections. One of the 10 crographs of paraffin sections infrequently showed mulsections on each slide was randomly selected to elimi- tiple nucleoli, but rather often no nucleolus at all could
nate the effect of systematic variations in section thick- be seen in adjacent sections. This was frequently so in
ness. Raw data for the number of nucleoli (i.e., cells) 8-pm sections and these results were discarded. The
were obtained by multiplying the results by 10. Phase identification of nucleoli in resin sections was without
contrast (objectiveLeitz Plan 25, N.A. 0.45) was used for problems and at least one nucleolar profile was visible
resin sections and brightfield (objective Leitz Apo 24, in each neuron. Two nucleolar profiles in consecutive
N.A. 0.65) was used for paraffin sections. The microscope sections of a small neuron were never due to splitting
was equipped with an eyepiece graticule to prevent dou- but to the presence of more than one nucleolus (Fig. 1).
Displacement of nucleoli by the edge of the knife (Camble counting.
Ganglion cells were photographed and the negatives mermeyer, 1967)did not occur in resin-embedded tissue,
were projected onto a digitizer tablet (MOP 11, Kontron, but in paraffin sections, large-cell nucleoli were often so
FRG) to a final magnification of 800-1,000. The perime- displaced. Tangentially sectioned large-cell nucleoli in
ters of all neurons containing nucleoli were measured resin sections sometimes appeared with low contrast,
and divided by 3.14 to obtain “idealized” diameters in- and could not be distinguished from low-contrast condependent of the irregular shape of the neuron. The densations of chromatin (Fig. 2). Hence, low-contrast
sections to be photographed were at regular intervals structures were not counted as nucleoli. It was essential
selected from the series passing through a ganglion to that counting of nucleoli and determining the correction
reduce bias by the nonhomogeneous distribution of large factors be done by the same person.
and small neurons.
R ESULTS
The size of nucleoli was determined in the microscope
Five sets of ganglia L4-L6 embedded in resin con(objective Zeiss Plan Apo 100 oil, N.A. 1.3)with the aid
of a screw micrometer eyepiece. The thickness of the 3- tained an average of 41,000 neurons with a standard
pm resin sections was controlled by focusing the 1 0 0 ~deviation of 8% of the mean value. The ganglia L4, L5,
oil immersion objective onto the upper and lower sur- and L6 contained 12,000, 15,000, and 14,000 neurons,
faces of the section. The accuracy of the microscope drive respectively, with standard deviations for the individual
was better than 1 pm (Leitz Ortholux 51.2). Individual ganglia of 7-17% of the mean.
1 pm. The mag- Ten complete sets of ganglia embedded in paraffin
section thicknesses varied less than
nification of all micrographs was calibrated against mi- were investigated and the mean value for the total numcrographs of a stage micrometer. All counts and ber of neurons was 32,000, with a standard deviation of
8% of the mean value. The ganglia L4-L6 contained
measurements were performed by the author.
10,000, 12,000, and 11,000neurons, with standard deviaCorrection Factors
tions for the individual ganglia of 11-16% (Table 2). The
Study of the resin sections, in particular when consec- deficit for the total number of neurons in the paraffin as
utive sections were evaluated, disclosed that almost all compared to the resin series was 23%.
The neuron counts in unstained 12-pm sections after
small “dark” neurons contained 2-3 nucleoli that were
close to the nuclear membrane. Large Yight” neurons, osmium fixation were comparable to those in stained 5however, contained only one centrally placed nucleolus pm sections. The preservation of the cells after osmium
(Figs. 1,Z). The diameter of the nucleolar profiles after fixation was better than after formalin fixation, and the
resin embedding was 1.1-2.4 pm in small neurons and black myelin sheaths facilitated counting, but it was
4.5-5.5 pm in large neurons. The nucleoli in paraffin necessary to focus through the section for each neuron
sections measured 1.6-2.6 pm in small neurons and 3.2- because the section thickness by far exceeded the focal
depth of the objective and it was also larger than the
4.0 pm in large neurons.
In view of the varying numbers of nucleoli and the diameter of the nuclei of the small ganglion cells. A
differences in their size, it did not seem appropriate to series of 12-pmcross sections for one ganglion comprised
NUMBER OF NEURONS IN RAT DRG L4-L6
Fig. 1. Two series of consecutive sections of a spinal ganglion L6;
resin sections 3 pm thick, phase contrast. A-E: Large neuron. The
nucleus contains one large nucleolus, which is centrally placed; it is
split and appears in B and C (arrows).The nucleus is almost 20 pm in
diameter but only 3 distinct (B-D) and 2 indistinct (A, E) nuclear
317
profiles are seen (see Fig. 4). F-I: Small neurons. The nucleoli are
peripheral and may project into the cytoplasm (F, left). The nuclei
measure slightly more than 10 pm in diameter but occur in only 2-3
successive sections. One cell contains only one nucleolus (bent arrows).
Bar:20 pm.
318
H. SCHMALBRUCH
TABLE 1. Comparison of neuron counts in spinal ganglia L4-L6 in the rat
L4
L5
L4-L6
8,400
(5,80011,300)
Langford and Coggeshall
(1979)
Bondok and Sansone
(1984)
L6
5,300
(SD 240)
5,400
(SD 310)
4,100
(SD 330)
14,800
n
Technique
6
Paraffin,
8cLm
10
Paraffin/
frozen,
10 pm
Yip et al. (1984)
7,000
(SD 1,150)
11
Paraffin,
8cLm
Feringa et al. (1985)
2,000
(SD 230)
14
Paraffin,
7pm
16
Paraffin,
10 pm
Frozen,
lOpm
Devor and GovrinLippmann (1985)
7,100
(5,000-9,800)
7,300
(5,200-9,800)
Arvidsson et al. (1986)
5,900
(4,600-9,100)
8,900
(5,10011,400)
6,000
(3,30010,900)
20,800
(13,00029,200)
12
Schmalbruch (this study)
12,500
(10,70016,000)
14,800
(13,20017,200)
14,100
(13,40015,100)
41,400
(38,50046,600
5
EPOXY,
3 P
9,600
(7,00011.900)
11,700
(8,20013.800)
10,600
(9,30012.800)
32,000
(28,50035.500)
10
Paraffin,
5pm
The figures are mean values, either given by the authors or calculated from the data for individual ganglia. The range is given in
brackets; if this was not possible the standard deviation (SD) was calculated from the standard error of mean and the number of
observations. n: number of ganglia assessed; technique: embedding medium and thickness of the serial sections. In all papers listed,
the number of nucleoli was counted in serial sections and different correction methods for split nucleoli were applied to determine
the number of neurons.
Fig. 2. Three consecutive sections of a large neuron from a ganglion cap of the nucleolus is seen, but its contrast is low and this structure
L5. Resin sections 3 pm thick; phase contrast. The origin of the axon is indistinguishable from other densities within the nucleoplasm. C:
is seen in B (arrow), the dark granules in A and B are lipofuscin. A The section passes through the periphery of the nucleus but still
The section passes through the middle of the nucleus and contains the contains a complete slice (not a cap) of the nucleus. The delineation is
nucleolus. The nucleoplasm is smoothly cut, and the nucleolus is not poor because the nuclear membrane is cut obliquely.The nuclear slice
displaced. The nuclear membrane is distinct. B: The section is still has a large and a small face; the contour of the small face, which has
close to the center of the nucleus; its size is about the same as in A. A a diameter of 5-6 pm, is visible. Bar: 10 pm.
3 19
NUMBER OF NEURONS IN RAT DRG L4-L6
TABLE 2. The number of neurons in rat spinal ganglia L4-L6
Age o r
weieht
Resin, 3 pm
23 weeks
38 weeks
39 weeks
43 weeks
96 weeks
Mean
Standard
deviation
%
L4
L5
L6
Total
11,490
12,810
10,690
16,010
11,330
12,470
13,190
14,800
14,130
17,210
14,760
14,820
15,050
13,690
13,690
13,400
14,570
14,080
39,730
41,300
38,510
46,620
40,660
41,360
2,130
17
1,490
12
700
5
3,120
8
6,980
9,890
11,920
11,070
9,160
10,310
9,840
7,400
10,050
11,110
8,120
9,620
13,760
12,040
12,050
12,750
8,190
30,580
32,250
35,280
35,260
29,680
12,890
12,350
9,350
12,180
11,080
11.660
9,840
10,320
11,310
11,440
12,330
9,480
12,780
10,040
10,620
9,680
9,260
10.650
1,570
16
1,700
15
1,170
11
2,650
8
8,460
10,070
11,240
29,770
Paraffin, 5 pm
300 gm
300 gm
300 gm
300 gm
300 gm
300 gm
72 weeks
72 weeks
77 weeks
77 weeks
89 weeks
Mean
Standard
deviation
%
Osmium-paraffin, 12 pm
300 gm
lost
-
35,510
29,790
30,020
32,970
28,460
31,980
The nucleoli were counted in epoxy-resin sections 3 pm thick, in paraffin sections 5 pm thick,
and in paraffin sections 12 pm thick. The counts were corrected for multiple and split nucleoli by
the empirical method of Coggeshall et al. (1984). In rats of known age, the contralateral sciatic
nerve had been cut at birth or at age 4 weeks. 300 gm: unoperated male rats of 250-350 gm
weight.
PARAFFIN
EPOXY RESIN
%
8
4
0
b
i0
40
Fig. 3.Histograms of the diameter of the perikarya of L5 dorsal root
ganglion cells embedded in epoxy resin and paraffin. Note pronounced
shrinkage of the paraffin-embedded cells.
200-300 sections of which 20-30 were analyzed. The
results differed by less than 2% when the counts from
each 20th section (i.e., from only 10-15 sections) were
used to calculate the number of neurons.
The tissue embedded in paraffin showed more severe
shrinkage than after resin embedding, and the large
neurons always were detached from their surrounding
satellite cells. The size histograms confirmed that
shrinkage in paraffin was excessive. No neurons larger
than 40 pm were found, whereas 20-25% of the resinembedded neurons were of this size. Very small neurons
with diameters of less than 26 pm accounted for 20-25%
of the resin-embedded neurons, but in paraffin sections
more than 80%of the cells measured less than 26 pm in
diameter. The histograms were skewed and probably
had two peaks after both embedding methods (Fig. 3).
The nuclei of large and of small neurons measured
roughly 20 pm and 10 pm in diameter. The large nuclei
in the 3-pm series occurred in five consecutive sections;
small nuclei were seen in three, sometimes in only two
sections (Fig. 1).This was unexpected, considering the
diameter of the nuclei and the thickness of the sections.
Several series were evaluated to exclude the possibility
that the “deficit” of nuclear profiles was due to loss of
320
H. SCHMALBRUCH
I
H
Fig. 4. Schema illustrating the relation between section thickness,
orientation of nuclei and nucleoli, and their appearance in sections. I:
Nucleus of a large neuron with one nucleolus; 3-pm sections. The slices
2-6 are seen in the microscope, the caps 1 and 7 are lost because of the
overlying dark cytoplasm and the poor delineation by the obliquely
sectioned nuclear membrane. Note that the delineation is poor for the
first and last complete slices (2,6) a s well. The nucleolus may be split
and appear in sections 3 and 4 (A), or it may appear in section 4 only
because both caps are lost (B). (cf. Figs. 1, 2C.) 11: Nucleus of a small
neuron with two nucleoli, 3-pm sections. The nucleus may be cut into
5 (A) or 4 (B, C) slices; the slices 1 and 5 (A), or 1@), or 1and 4 (C), are
probably invisible and the nucleus appears in only 3 (A, B), or 2 (C)
consecutive sections. Both nucleoli may be in the same (A) or in different sections @, C). The lower nucleolus in B, and both in C, project
into the cytoplasm (cf. Fig. 1).111: Nucleus of a small neuron with two
nucleoli; 5-pm sections. The nucleus is cut into 3 (A, B, D) or 2 (C) slices
of which 1 or 2 are seen in the microscope. Both nucleoli are visible in
A and C; the lower one in B may be masked by the dark cytoplasm.
The nuclear caps 1 and 3 in D are lost together with both nucleoli.
Counting neurons in spinal ganglia is usually desections, or that the sections were mounted in the wrong
order. The thickness of these sections was controlled as scribed as routine procedure, and it is stated (or assumed) that each cell contains one nucleolus such that
well.
one only has to calculate a correction factor for split
DISCUSSION
nucleoli. Bondok and Sansone (1984) believe that this
The ganglia L4-L6 contained 41,000 neurons when problem is eliminated by counting spaced sections-that
series of 3-pm resin sections were used for counting. is, their data in reality are raw counts. Only Hulsebosch
There were 23% fewer cells counted in series of 5-pm et al. (1986) encountered difficulties. The ganglion cells
paraffin sections, but even the counts in paraffin sec- of their rats, as in the present study, had multiple nutions were considerably higher than previously reported cleoli and the counts in T4 and S2 ganglia were unredata. The mean data for the L5 ganglion now range producible when different investigators were involved.
from 2,000 (Feringa et al., 1985) to 15,000 cells (this As the identification of nucleolar profiles entailed an
article) (Table 1). Most of the present data are from element of arbitrariness, empirical correction factors
operated rats, but it is unlikely that contralateral nerve were determined for each investigator (Coggeshall et al.,
section increases the number of ganglion cells and the 1984). Hulsebosch et al. (19861, however, did not control
counts are the same in unoperated rats (Table 2). The whether, in consecutive sections, at least one nucleolus
only explanation is that the variation in counts is due was recognized in each cell. Counting nucleolar profiles
to different histological methods. The main problem en- in resin sections is unambiguous but the use of 3-pm
countered after paraffin embedding in this study was sections may lead to an overestimate because the section
the identification of nucleoli in the smallest cells. Their thickness is close to the size of the nucleoli. The correcnucleoli were situated in the periphery of nuclei that tion factors actually represent the main source of error.
measured only twice the thickness of a 5-pm sectiort, On the other hand, even the largest possible error in
and many projected into the intensely stained cytoplasm. determining a factor does not explain the variation in
The thickness of the sections in previous studies of rat neuron counts reported.
The prerequisites for applying correction formulas for
lumbar ganglia was 7-10 pm. In this study, stained
sections thicker than 5 pm were found t o be unsuitable split nucleoli (Abercrombie, 1946; Konigsmark, 1970)
for counting nucleoli of small cells. Nevertheless, thick are 1)that each cell have only one nucleolus, 2) that the
sections are not the only cause of low neuron counts. nucleoli be cut rather than pushed by the knife, and 3)
The number of visible nucleoli may also depend on the that all fragments, or at least those larger than 1 pm
preservation of the tissue. Paraffin-embedded specimens (Konigsmark, 19701, be identified. None of these condiin comparison with resin-embedded specimens in this tions was fulfilled in the present study. Nucleoli in parstudy were technically poor, but probably not below the affin sections may be pushed by the knife edge
common standard. Feringa et al. (1985) embedded and (Cammermeyer, 1967)and the calculated factors are too
cut the entire vertebral column after decalcification with small. The visibility of a structure depends on its density
acid. This might have been rather harmful to the struc- in relation to the background and on its delineation. The
relative density of a nucleolus projecting into the densely
ture of the neurons (Table 1).
NUMBER OF NEURONS IN RAT DRG L4-L6
321
stained cytoplasm is low, and the delineation of a nu- presumably myelinated. The number of neurons with
cleolar cap is poor because its surface projects obliquely. unmyelinated axons is probably larger in L6 ganglia
The effect of poor delineation is obvious for caps of large and the number of ganglion cells at L4-L6 may be
nucleoli, but caps of small nucleoli are invisible and the roughly three times that of myelinated dorsal root finucleoli of small neurons never appear split (Figs. 1,4). bers. Nevertheless, the large and nonsymmetric anastoThis indicates that the error due to split nucleoli has moses between adjacent dorsal roots (Jacob and Weddell,
previously been overrated, whereas the identification 1975; Schmalbruch, 1987) may distort this relation for
problem has practically been neglected.
individual ganglia.
The fact that even large fragments are lost is illus- The assumed 1:l relation of ganglion cells and nerve
trated by the appearance of nuclei in consecutive sec- fibers in dorsal roots or peripheral nerves refers to the
tions. Counting nuclei instead of nucleoli to determine established view that each ganglion cell has one central
the number of neurons (Smolen et al., 1983) appears and one peripheral process (Dogiel, 1896; Ram6n y Caattractive because nuclei are large and easily seen and jal, 1909). This has been contested because the number
because there is only one per cell. The correction factors of central or peripheral axons exceeded the neuronal
obtained by using Abercrombie’s (1946) formula for 10- counts in the corresponding ganglia by 30-120% (Langpm and 20-pm nuclei, respectively, for 3-pm sections are ford and Coggeshall, 1979,1981;Hulsebosch et al., 1986).
0.23 and 0.13. This means that a small nucleus should The number of neurons was determined from series of
appear in 4-5 sections and a large one in 7-8 sections. paraffin sections and therefore might have been
In fact, the number of nuclear profiles that were visible underestimated.
in consecutive sections was even smaller than the num- The results presented in this paper indicate that neuber obtained by simply dividing section thickness into ronal counts in rat spinal ganglia embedded in paraffin
nucleus diameter (Figs. 1, 4). Smolen et al. (19831, in a tend to be too low because the nucleoli of some small
study of the superior cervical ganglion of the rat, found neurons are not detectable, and that this error increases
that slices of less than a 6-pm diameter from 9-pm nuclei with the thickness of the sections and with poor tissue
were not identifiable in 1.5-pm resin sections. Neuron preservation. Applying calculated correction factors excounts for this ganglion range from 13,000 to 45,000 (for acerbates the problem. Thin paraffin sections may be
references see Smolen et al., 1983), and it appears as if suitable for comparing pairs of ganglia after unilateral
the discrepant findings in lumbar ganglia are not an experimental procedures, but probably not for comparisolated phenomenon. Neuron counts in other systems ing different populations of rats. Cutting and staining
complete series of 3-pm resin series of 6 ganglia for each
might need confirmation as well.
The counts in unstained 12-pm paraftin sections were rat is tedious and time-consuming and hardly suited for
also underestimates but they were larger than previous studies using many animals. Whether other staining
counts (Tables 1, 2). The method was used by Hatai methods will provide more satisfying solutions remains
(1902), who found 9,000 cells in the L2 ganglion, which to be seen. Thick unstained sections of osmicated tissue
has about the same size as the L4 ganglion, and 70% (Hatai, 1902)represent a relatively convenient means t o
more cells in the T4 ganglion than Hulsebosch et al. estimate the number of neurons and this method might
(1986) found in stained 10-pm sections. Because the in- be improvable by prefixation with aldehyde or the use
fluence of split and multiple nucleoli is negligible in 12- of embedding media other than paraffin.
pm sections, this difference must be due to the fact that
ACKNOWLEDGMENTS
the nucleoli are easier to recognize after osmication.
Reducing the sample size to half in my counts had very The author thanks Mrs. M. Bjarg for unfailing technilittle effect on the results. This excludes nonsystematic cal assistance and for meticulously cutting the many
counting errors.
serial sections. The Danish Medical Research Council,
The claim that the number of neurons in the lumbar and the Foundations for Experimental Research in Neuganglia supplying the rat sciatic nerve has hitherto been rology and for Progress in Medicine (A.P. Mdller), prounderestimated is supported by circumstantial evi- vided financial support. I am grateful to Dr. Roy J. Levin
dence. The nerve at midthigh contains 1,600 motor and for correcting my English.
19,000 sensory axons (Schmalbruch, 1986a).The ventral
roots L4-L6 together contain 4,000 myelinated fibers
(Schmalbruch, 1984). The sensory-motor fiber ratio in
LITERATURE CITED
the nerve suggests that the corresponding spinal ganglia rather contain 40,000 than 20,000 or even fewer Abercrombie, M. (1946) Estimation of nuclear population from microtome sections. Anat. Rec., 94:239-247.
neurons. The L6 dorsal root contains 10,000-14,000 axons (Langford and Coggeshall, 1979). The dorsal roots Arvidsson, J., J. Ygge, and G. Grant (1986) Cell loss in lumbar dorsal
root ganglia and transganglionic degeneration after sciatic nerve
L4 and L5 each contain more than 4,000 myelinated
resection in the rat. Brain Res., 373:15-21.
fibers (Schmalbruch, 1986b);unfortunately data for un- Bondok, A.A., and F.M. Sansone (1984) Retrograde and transganglionic degeneration of sensory neurons after a peripheral nerve
myelinated fibers are not available. The myelinated filesion at birth. Exp. Neurol., 86:322-330.
bers in thoracic and sacral dorsal roots represent only
J. (1967) Artifactual displacement of neuronal nucleoli
23-33% of all fibers (Langford and Coggeshall, 1979, Cammermeyer,
in paraffin sections. J. Hirnforsch. 9:209-224.
1981; Hulsebosch and Coggeshall, 1983; Hulsebosch et Coggeshall, R.E., K. Chung, D. Greenwood, and C.E. Hulsebosch (1984)
An empirical method for converting nucleolar counts to neuronal
al., 1986);hence, the dorsal roots L4 and L5 may contain
numbers. J. Neurosci. Methods, 12:125-132.
between 13,000 and 19,000 axons. These figures comM., and R. Govrin-Lippmann (1985) Neurogenesis in adult rat
pare well with the neuron counts in this study. Harper Devor,
dorsal root ganglia. Neurosci. Lett., 61:189-194.
and Lawson (19851, in a physiological study of L4 gan- Devor, M., R. Govrin-Lippmann, I. Frank, and P. Raber (1985) Proliferation of primary sensory neurons in adult rat dorsal root ganglia, found 34-45% neurons with fast-conducting axons,
322
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