Dendritic organization of the anterior speech area

EXPERIMENTAL

NEUROLOGY

87, 109-117 (1985)

Dendritic Organization

of the Anterior

Speech Area’

A. B. SCHEIBEL, L. A. PAUL, I. FRIED, A. B. FORSYTHE, U. TOMIYASU, A. WECHSLER, A. KAo, AND J. SLOTNICK~ Departments of Anatomy and Psychiatry, and Brain Research Institute and Department Biomathematics. Division of Computer Sciences, School of Medicine, University of California, Los Angeles, California 90024; and Departments of Pathology and Neurology, Wadsworth and Brentwood Veterans Administration Hospitals, Los Angeles, California 90036 Received

August

of

2, 1984

Golgi studies revealed significant differences in dendritic patterns between neurons of the left and right opercular regions of the frontal lobe (Broca’s speech area on the dominant side) and between cells of the left and right precentral areas (the orofacial motor zones) just behind. Although total dendritic length of the basilar dendritic array seemed characteristic of an area independent of side, a larger proportion of the length on the left (dominant) side was made up of higher order (4, 5, 6) dendrite branches, and lower order (1, 2, 3) segments predominated on the right. The pattern was partially reversed in non-right-handed patients. These findings can be interpreted as indicating an early preponderance of dendrite growth in the non-speech-gifted hemisphere followed by enhanced dendrite growth in the dominant hemisphere coincident with the beginning of conceptualization and speech function. o 1985 Academic

Press. Inc.

INTRODUCTION It was recognized as early as the time of Dax (6) and Broca (2) that the two cerebral hemispheres were not functionally equivalent, and that speech was mediated primarily by the left hemisphere. Despite a number of recent studies of the relevant gross morphology (1, 7, 9- 11, 2 1, 23) and cortical cytoarchitecture (8), little is yet known about the neuronal organization Abbreviations: TDL-total dendritic length, R-right, L-left, OP-opercular gyrus, PCprecentral gyrus. ’ This work was supported in part by National Institutes of Health grant NS 1387 1. Please send reprint requests to Dr. Scheibel, Dept. of Anatomy, UCLA School of Medicine, Los Angeles, CA 90024. t Deceased. 109 0014-4886/85 $3.00 Copyright 0 1985 by Academic Press, Inc. All rigbu of reproducnon in any form reserved

110

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ET AL.

underlying this functional asymmetry. In this introductory study, we used Golgi methods to compare dendritic profiles of nerve cells in the frontal speech-related regions in the left hemisphere (areas opercularis and triangularis, abbreviated collectively as OP, also known as Broca’s area), with the homologous region on the right side. These were, in turn, compared with ipsilateral and contralateral Rolandic motor regions (areas precentralis or PC) subserving somato- and visceromotor innervation of structures concerned with speech production. For the sake of simplicity, we refer to the former cortical area as the opercular region, the latter as the precentral. We hypothesized that cells in the left opercular region should exhibit a greater degree of “branchedness” (manifested as more branches, longer branches, or both) than the homologous region on the right side as a function of processing loads imposed by speech. Further, we predicted that such a left-right difference was less likely to be obvious in the precentral region, although lateralization of motor function might impose an asymmetric load of its own. We initially tested these hypotheses by comparing lengths and numbers of dendrites of various orders in the two hemispheres. TECHNIQUES Tissue for this study was obtained at autopsy (within 5 to 28 h after death) from eight cases, all male, free of known neurological pathology and ranging from 58 to 77 years of age (mean age 65.9 SD 6.4). The subjects’ handedness was brought to our attention only at a very late stage of the study and had no influence on the choice of variables for analysis. Handedness was ascertained by subsequent communication with family members by mail or telephone. Of the six patients who were right-handed throughout adult life, one was said to have preferred the use of his left hand during infancy. Of the two non-right-handed patients, one was ambidextrous, and the other used his right hand for writing but preferred the use of his left hand for all other manual activities. Tissue blocks were sampled bilaterally (Fig. 3): from the foot of the precentral gyrus (PC) about 0.5 cm above the Sylvian fissure, and from the frontal opercular gyrus (OP), just anterior to the PC. Although the frontal operculum is made up of pars orbitalis, pars triangularis, and pars opercularis, the latter two alone constitute Brodmann’s area 44, the anterior speech zone, and were sampled in the study. Blocks of brain tissue were obtained either at autopsy or after the already removed brain had been immersion-fixed in 10% Formalin for 7 to 10 days. Tissue blocks were washed 24 h in 10% neutral buffered Formalin, refixed 72 to 80 h in an osmic acid (0.33%)-potassium dichromate (2%) solution, washed in distilled water, then placed 24 h in AgN03 (0.75%)

DENDRITES

OF

SPEECH

AREA

111

solution. Sections were mounted in sequence at 120 pm, under coverslips in a neutral synthetic resin, coded to preclude investigator bias, and dried 5 to 7 days in the dark. Six neurons from each of the four regions: left opercular (LOP), right opercular (ROP), left precentral (LPC), and right precentral (RPC) were selected according to predetermined criteria which included: (i) depth of soma (with respect to the pial surface) ranging from 400 to 1300 pm; (ii) the presence of at least three primary basilar dendrites; and (iii) cell body positioned approximately midway through the depth of the 120-pm section. All neurons selected were drawn with a camera lucida at 312.5X. Because we deliberately varied the sectioning sequence of the block (i.e., from anterior or posterior face), the ultimate cell sample was randomized throughout each block of tissue. We accordingly began with section 1, slide 1, and worked through the block until six acceptable cells had been traced. Planar coordinates of terminal and branching points (x and y values) were measured from the tracings, using a digitizing tablet controlled by a TRS80 microcomputer. Depth coordinates (i.e., values in the z plane) were also recorded by hand at each dendritic branch point and tip from the calibrated fine focus of the microscope. The status of each dendritic tip was noted as broken, cut, or a true ending. The length of each dendritic segment was then calculated using the coordinates of its origin and end points, and its branch order was recorded following a somatofugal nomenclature (i.e., the segment closest to the soma was designated “first-order,” its daughter branches “second-order,” etc.). (We report here the results of analysis on all basilar dendrites recorded.) In the course of tissue processing, the apical shafts were often broken. Thus there was a large experimentally induced variability in the length of stained apical shafts. For this reason, they were not taken into account. Statistical Methods. Because the dendritic domain of the cell might be related to its depth (with respect to the cortical surface), the latter was used as a covariate in the statistical analysis. Therefore, we report adjusted means and their within-brain standard errors. We also carried out these analyses without the covariate, and the pattern of means and significance were very similar. The statistical model used for analysis had “Location” (precentral vs. operculum), “Side” (left vs. right), and the Location X Side interaction as fixed effects. The random effects were Brain and Cells nested within Brain. For our mixed model ANOVA with unequal replication, we used the computer program BMDP3V-general Mixed Model Analysis of Variance (BMPD, Statistical Software, Univ. of California Press, 1981) pp. 413-426. Because lateralization of speech function in non-right-handed individuals may be different and even reversed compared with that in right-handed

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persons (12-15, 17), it was thought possible that left-right differences might be obscured by studying right-handed and non-right-handed cases together as one group. We therefore report the data derived from the six righthanded subjects, although results for all eight cases will be discussed individually. RESULTS There was no main effect due to Location, nor any effect due to Location X Side. These comparisons, illustrated in Table 1, showed several strong trends, however. First, fourth order dendritic segments on the left tended to be shorter than those on the right, and in two categories (second and third order segments) these differences were statistically significant (P <-0.005 and ~-0.05). Second, dendrites of higher order tended to be more numerous on the left than on the right, a difference which attained significance in fourth order segments (P < -0.01). It should be noted that the differences in dendritic number were primarily localized to the opercular region, whereas those in length were mainly the result of differences in the precentral region. As an index of the cell’s available dendritic surface, we considered total dendritic length (TDL). This was composed of all dendrites of a neuron regardless of dendritic diameter. Although TDL in the OP exceeded that in the PC by about 10% (TDL = 2712.3, 2556.1, 2215.1, and 2502.9 for left opercular, right opercular, left precentral, and right precentral, respectively; F (1,134) = 2.89, P = 0.091), there was no main effect on TDL due to Side or to Location. It was only when the proportion of TDL composed of lower order segments (first plus second plus third) was compared with the proportion composed of higher order segments (fourth plus fifth plus sixth), that robust left-right differences emerged (Fig. 1). The fraction of TDL composed of higher order dendrites was greater on the left than on the right (F (1,134) = 7.43, P = 0.007) and is also greater in the opercular region than in the precentral area (F (1,134) = 5.46, P = 0.021). This analysis indicates that, for the neuron situated in the left opercular region, higher order dendrites occupied a greater fraction of the total dendritic length than they did in any of the other three brain regions examined. The data are examined in another fashion in Fig. 2 where the number of branches of each segment order of LOP neurons was adjusted to a value of 1.0 and then compared with the same segment order from the ROP, LPC, and RPC. Viewed this way, values for early order segments in the LOP lagged all other stations, but showed progressive increments until they significantly exceeded in numbers of higher order segments, the values for

5.20 5.17 4.95 5.24

LOP ROP LPC RPC

+ 0.26 it_ 0.26 ?c 0.27 + 0.26

f 3.17 f 3.16 + 3.28 -+ 3.18

10.59 10.80 10.10 10.56

49.74 55.57 47.97 70.15

+ f f k

f f tf

2

0.56 0.56 0.58 0.56

4.82” 4.80h 5.03h 4.84h

4

f f f f

5.03” 5.01 h 5.20h 5.04h

86.4 87.50 68.43 82.24 f 2 f f

12.57 12.55 12.18 11.33

f 0.99 f 0.99 I? 1.01 f 1.00

9.52 6.89 7.72 6.36

f + f f

B. Average dendritic number by order

71.53 74.82 62.13 82.52

A. Average segment length by order

3

Order

1.38” 1.37“ 1.39” 1.37d

7.25 7.35 7.60 7.52

3.72 2.58 2.29 1.49

79.38 82.82 86.87 83.90

f + k +

+ ?I k f

5

.80 .80 .82 .81

7.90 8.21 8.68 8.50

0.74 0.42 0.09 0.23

f f -c f

0.18” 0.18d 0.18d 0.18”

w

% F

e G w Q

107.15 64.99 +k 24.53 12.68

8

3

6

88.86 tk 29.11 14.48 93.23

” All values are X f SE pm and are adjusted for depth by analysis of covariance. h The main effects were R z= L (F = 8.21, P < 0.005; F = 4.15, P < 0.05) for second and third order, respectively. ‘ Abbreviations: LOP, ROP-left, right opercular regions; LPC, RPC-left, right precentral regions. “The main effects were L > R (F = 7.31, P < 0.01) and OP > PC (F = 4.24, P < 0.05) for fourth and sixth order, respectively.

23.71 28.41 28.51 29.78

LOP‘ ROP LPC RPC

1

1

Dendrite Characteristics of Left and Right Hemispheres of Six Adult Male Human Brains”

TABLE

114

SCHEIBEL ET AL. DENDRITE

SEGMENTS 4+5+6 TDL 39.7%

1

2000

1

1

1800

1

I

1600

I

1

30.8

%

31.6

%

25.1

%

1

1400 LENGTH

IN MICROMETERS

FIG. 1. Comparison of the dendritic length and proportion of the dendritic ensemble composed of lower order (first, second, and third) and higher order (fourth, fifth, and sixth) dendritic segments in left opercular, LOP; right opercular, ROP; left precentral, LPC, and right precentral, RPC regions. To compare left vs. right the approximate standard error is 138; to compare opercular vs. precentral regions, the approximate standard error is 85. The column of figures on the extreme right shows the percentage of total dendritic length (TDL) occupied by higher order dendrites in each region.

all other areas. Summarizing the data for our six right-handed patients, dendritic analyses revealed significant degrees of difference in branching patterns among cortical regions of human brain (Fig. 3). Two left-right asymmetries were found when analyzing dendrites by order; (i) the greater length of secondary and tertiary segments on the right side was primarily a feature of the precentral cortex, whereas more numerous fourth order segments characterized the opercular region; (ii) the proportion of total dendritic length occupied by higher order dendrite segments in the left opercular region exceeded that of any other brain region in our study. When all eight cases were analyzed, the two non-right-handed individuals showed a partial reversal of the right-handed structural pattern. Only in one, the mean length of secondary and tertiary dendrites was greater in left (compared with right) precentral cortex, while in the other, the mean length of tertiary dendrites was greater on the right, but that of secondary dendrites was greater on the left. In the latter individual, as well as in one of the right-handed cases, fourth order dendrites were more numerous in the right operculum than in the left, thereby again reversing the pattern found in most right-handers.

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OF SPEECH AREA

1.3

0.20 IL

T I

2

3 BRANCHING

~~

m-7

-.---

4

5

-

6

ORDER

FIG. 2. Comparison of total basilar dendritic length at each branching order TBDL(n) relative to left opercular (LOP) values in six right handers. The value for LOP at each segment order is kept at 1.0.

FIG. 3. Somewhat schematized drawing of typical dendritic ensembles from cells of the four regions noted above. Note the increased number of higher order segments in the left operculum, LOP, compared with all other regions and the relatively greater length of second and third order branches in right operculum, ROP and right precentral region RPC. Inset drawing above shows positions from which “Bronca,” a, and “motor,” b, tissue samples were selected in opercular, op, and precentral, pc, gyri.

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DISCUSSION These differences among neocortical regions critical for the production of speech may prove to be of considerable physiologic consequence for regional cerebral function. Although dendritic diameter was not a variable in our work, it can be seen that the presence of larger numbers of fourth order segments could affect several parameters, among them (a) the resistance-impedance characteristics of the shaft (19); (b) the number and pattern of bifurcations of the dendritic ensemble (each point representing a potential locus of enhancement or suppression of local electrical activity); and (c) dendritic spine distribution. The latter represents a complex function of dendrite segment order, dendrite shaft thickness, and distance from the cell body (16) which, in turn, correlates significantly with the distribution of presynaptic terminals, i.e., type I or type II (4); facilitatory or inhibitory effects (22); and other variables. In addition, the idiosyncratic patterning we have described may reflect differential sequences of dendritic maturation (5, 20) or pruning (3) peculiar to the regions involved. Greater dendritic length of lower order branches on the right side suggests to us that portions of the right cortex may develop more actively than the left during the first year of life [when these branches are forming (5)]. During this period the infant depends extensively on sensory impressions of a highly concrete nature and is limited to relatively large scale, undifferentiated motor acts [sensory-motor period of Piaget (18)]. Somewhat later, with continuing maturation of cortical systems, symbolic operations and other “left hemisphere” tasks emerge, and motor actions become more discrete. During this period, higher order dendrite branches appear, and the left opercular cortex (Broca’s region) becomes anatomically more complex, coincident with its emerging functional dominance. This study represents the first attempt to examine dendritic structure in the human brain underlying a high order cortical function, i.e., speech. It also provides initial support for the concept of characteristic dendritic patterning as a correlate of hemispheric lateralization. It is conceivable that the dendritic differences we have described may reflect secondary responses of cortex to functional demands developed during adult life, rather than representing sequential growth patterns of the perinatal and postnatal periods. For this reason, studies are currently in progress on a graded series of brains spanning the first 6 years of life, to trace the actual growth and development of dendrite patterns in these regions. REFERENCES 1.

Anatomical asymmetries of the cerebral hemispheres. Pages l-6 in V. Ed., Interhemispheric Relations and Cerebral Dominance. Johns Hopkins Press, Baltimore.

BONIN,

G.

VON. 1962. MOUNTCASTLE,

DENDRITES

OF

SPEECH

AREA

2. BROCA, P. 1865. Du siege de la facultt du langage articule. Bull. Mem.

117 Sot. dilnthropol.

Paris, 6: 377-393.

3. BUELL, S., AND P. COLEMAN. 1981. Quantitative evidence for selective dendritic growth in normal human aging but not in senile dementia. Brain Res. 214: 23-41. 4. COLONNIER, M. 1967. The fine structural arrangement of the cortex. Arch. Neurol. 16: 65 l-657.

CONEL, J. 1939-59. The Postnatal Development of the Human Cortex. Vols. l-6. Harvard Univ. Press, Cambridge. 6. DAX, G. 1865. Lesions de la moitie gauche de l’endphale coincidant avec trouble des signes de la pens&e (lire g Montpellier en 1836). Gaz. Hibd. Med. Chir. 2nd series, 2: 5.

259. 7. 8.

9. 10. 11. 12. 13.

DIAMOND, M., G. A. DOWLING, AND R. E. JOHNSON. 1980. Morphological cerebral cortical asymmetry in male and female rats. Exp. Neurol. 71: 261-268. FAUI, G., P. PERRONE, AND L. VIGNOTO. 1982. Right-left asymmetry in anterior speech regions. Arch. Neural. 39: 239-240. GALABURDA, A., M. LEMAY, T. KEMPER, AND N. GEXHWIND. 1978. Right-left asymmetries in the brain. Science 199: 852-856. GALABURDA, A., F. SAVIDES, AND N. GESCHWIND. 1978. Human brain: cytoarchitectonic left-right asymmetries in the temporal speech region. Arch. Neurol. 35: 812-817. GESCHWIND, M., AND W. LEVITSKY. 1968. Left-right asymmetries in temporal speech region. Science. 161: 186-187. GLOMING, I., K. GLOMING, G. HAUB, AND R. QUATEMBER. 1969. Comparison of verbal behavior in right-handed and non right-handed patients with anatomically verified lesions of one hemisphere. Cortex 5: 43-52. GOODGLASS, H., AND F. QUADFESEL. 1954. Language laterally in left-handed aphasics. Brain

77: 52 I-548.

14. HECAEN, H., AND J. SAUGET. 1971. Cerebral dominance in left-handed subjects. Cortex 7: 19-48. 15. LURIA, A. 1910. Traumatic Aphasia. p. 479. Mouton, The Hague. 16. MARIN-PADILLA, M. 1967. Number and distribution of the apical dendritic spines of the layer V pyramidal cells in man. J. Comp. Nemo/. 131: 475-489. 11. PENFIELD, W., AND L. ROBERTS. 1959. Speech and Brain Mechanisms. p. 286. Princeton Univ. Press, Princeton, NJ. 18. PIAGET, J. 1954. The Construction of Reality in the Child. Basic Books, New York. 19. RALL, W. 1967. Distinguishing theoretical synaptic potentials compiled for different somadendritic distributions of synaptic input. J. Neurophysiol. 30: 1138- 1168. 20. RAM~N Y CAJAL, S. 1911. Histologie du systemme Nerveux de 1’Homme et des Vertebres. A. Maloine, Paris. 21. TESZNER, D., A. TZAVARES, J. GRUNER, AND H. HECAEN. 1972. L’asymetrie droitegauche du planum temporale; a propos de l’etude anatomique de 100 cerveaux. Rev. Neurol.

126: 444-449.

22. UCHIZONO, K. 1965. Characteristics of excitatory and inhibitory synapses in the central nervous system of the cat. Nature 207: 642-643. 23. WADA, J., R. CLARK, AND A. HAMIN. 1975. Cerebral hemispheric asymmetry in humans. Arch. Neural.

32: 239-246.

24. WITELSON, S. J., AND W. PALLIE. 1973. Left hemisphere specialization for language in the newborn. Brain 96: 64 l-646.