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Prefrontal granular cortex of the rhesus monkey. II. Interhemispheric cortical afferents
Brain Research, 132 (1977) 235-246 (~) Elsevier/North-Holland Biomedical Press
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P R E F R O N T A L G R A N U L A R C O R T E X OF T H E RHESUS M O N K E Y . 1I. I N T E R H E M I S P H E R I C C O R T I C A L A F F E R E N T S
STANLEY JACOBSON and JOHN Q. TROJANOWSK1 Anatomy Department, Tufts University School of Medicbm, Boston, Mass. 02111 (U.S.A.)
(Accepted December 17th, 1976)
SUMMARY In 6 adolescent rhesus monkeys, horseradish peroxidase (HRP) was injected into 6 regions of the dorsolateral convexity of the prefrontal granular cortex. The commissural connections originated in both homotopical and heterotopical zones of the hemisphere contralateral to the injection site. The areas affected by the injections, i.e. areas 46, 45, 10, 9, 12 and 8a, received extensive homotopical interhemispheric input. HRP-labeled neurons were less extensive in heterotopical as opposed to homotopical cortex but they were seen in all 6 cases and were most common in prefrontal areas and less common in cingulate areas, areas 21 and 22 in the superior temporal sulcus and in insular cortex. The cells, whether of heterotopical or homotopical origin, were located primarily in layer 111. The most common distribution pattern was a horizontal band of HRP-labeled neurons which waxed and waned in cell density especially in homotopical cortex or patches and clusters of labeled cells especially in heterotopical cortex. This waxing and waning and grouping of neurons in patches and clusters may well represent a vertical type of organization to the neurons which give rise to the interhemispheric cortical afferents to prefrontal granular cortex in the monkey.
INTRODUCTION The development of the horseradish peroxidase (HRP) technique has permitted investigators to analyze more effectively than hitherto possible the cells of origin of fiber systems within the central nervous system of various experimental animals1.5, t7. Ira the study reported in the preceding paper, we have described the intrahemispheric cortical afferents to the prefrontal granular cortex in rhesus monkey based on the H R P technique 9. It was observed that these afferents arise in diverse parts of cortex, i.e. areas within frontal, temporal, parietal, occipital and limbic lobes and that the cells
236 of origin of these afferents are small to medium pyramids which are located primarily in layer III and may be organized in a vertical manner. The present report, based on the same material, describes the interhemispheric cortical afferents to prefrontal granular cortex identifying the areas in homotopical and heterotopical regions in which the cells of origin of these afferents arise as well as describing the morphology, laminar distribution and organization of these callosal neurons. MATERIALS AND METHODS The 6 adolescent animals used in the study of the commissural connections of the prefrontal granular cortex were the same ones used in the study of the intrahemispheric cortical afferents to the prefrontal granular cortex reported in the preceding paperL All procedures were performed using sodium pentobarbital as the general anesthetic (35 mg/kg body weight). The prefrontal granular cortex on the dorsolateral surface o f frontal lobe was divided into 6 zones as noted in the previous p a p e r All animals received unilateral multiple injections of a 10",~ aqueous H R P solution with each single injection consisting of 0.6/~1 volume of the marker solution. The animals survived 65-80 h and were perfused, fixed and cut in the stereotaxic plane of Otszewski 2°. F o r details of the fixation, sectioning and incubation to demonstrate the HRP-positive neurons, the preceding paper should be consulted. Every 10th coronal section was examined with the aid of an X - Y plotter and where necessary more sections were examined. All observations were made in unstained material and examined under dark-field illumination. The secuons were then counterstained to identify the areas and layers in which the HRP-positive callosal cells were located. The cortical areas in the prefrontal cortex are parcellated after Walker 2s. the remaining areas are parcellated after Brodmann ~. Roberts and Akert 24 and Seltzer and Pandya 2'~. RESULTS The presentation of our data concerning the interhemispheric cortical afferents to prefrontal granular cortex will begin with the H R P injections into frontal polar cortex, continue caudally through area 46 above and below the principal sulcus and conclude with the data from injections into cortex bounded by the arcuate sulcus.
Interhemispheric cortical afferents to prefrontal granular cortex (A) Frontalpole. In case 1 (Fig. 1), 15 injections were made into the dorsotateral convexity and medial surface of the hemisphere including areas 9 and 10. The bulk ofthe interhemispheric cortical afferents to this injection site arose in homotopicat cortex in the contralateral hemisphere where large numbers of HRP.labeled neurons were found. Heterotopical projections were seen to arise in areas 12 and 46 in frontal lobe as well as in cingulate area 25. In addition, some cells were also noted in area 22 in the dorsal bank of the superior temporal sutcus (STS) and adjacent s u ~ r i o r temp0ral gyrus, f r o m its rostral pole to nearly as far posterior as its caudal end. These labeled cells were only seen in layer II I and 2-4 of them were noted in every section we examined through this region.
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Fig. 1. Case 1. Injection site involving areas 10 and adjacent area 9. The bulk of the callosal projections are from homotopical areas 9 and 10. Some heterotopical projections are also seen from areas 22, 23 and 45. In this and the following figures, the hatching in the brain at the upper right of the figure indicates the injection site. Below it the medial and lateral aspect of the contralateral hemisphere is depicted. Coronal sections are likewise through the contra[ateral hemisphere at levels indicated by the corresponding numbers. Stippling indicates HRP-labeled neurons. Arrows indicate boundaries between numbered areas. See list of abbreviations and text for further details.
(B) Cortex above the principal sulcus. I n case 2 (Fig. 2), 15 injections were confined to area 46 in the dorsal b a n k and on the dorsolateral convexity above the principal sulcus. The bulk of the interhemispheric afferents arose from the homotopical cortex in the opposite hemisphere, however, a few labeled cells were also seen in cingulate areas 23 a n d 24 a n d temporal area 22 in the dorsal b a n k of the STS. (C) Cortex below the principal sulcus, in case 3 (Fig. 3), 14 injections were made in area 46 in the lower b a n k of the principal sulcus and on the dorsolateral convexity ventral to principal sulcus with some involvement of adjacent area 12. The n u m b e r of H R P - l a b e l e d n e u r o n s f o u n d in homotopical cortex was far fewer in n u m b e r in this case t h a n in a n y o f the other cases a n d they were f o u n d in area 46 in a n d a r o u n d the principal sulcus. Heterotopical interhemispheric afferents were seen to arise from area
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Fig. 2. Case 2. Injection site in area 46 above the principal sulcus. The bulk of the callosa[ projections are from homotopical area 46. Heterotopical projections are seen from areas 22. 23 and 24.
23 a n d the insula as well as the parietal operculum. These l a b e l e d cells were few in n u m b e r (2-6 per section) but were seen in all the sections e x a m i n e d in these regions. ( D ) Cortex in the dorsal portion of the concavity of the arcuate sulcus. In case 4 (Fig. 4), 12 injections were m a d e which involved areas 46 a n d 8a. There was a s t r o n g p r o j e c t i o n f r o m the h o m o t o p i c a l areas 46 a n d 8a. M a n y cells were also seen in h e t e r o topical cortex m areas 45 a n d 9. A few cells (2-7) per section were also seen in h e t e r o topical a r e a 22 in the d o r s a l b a n k o f the STS a n d in cingulate a r e a 23. (E) Cortex in the ventral portion of the concavity of the areuate suh'us. In case 5 (Fig. 5), 10 injections were m a d e in areas 45 a n d 46. M a n y labeled cells were seen in h o m o t o p i c a l cortex in areas 45 and 46. A few cells (2-6 per section) were seen in h e t e r o t o p i e a l cortex in a r e a 23 a n d m t e m p o r a l a r e a 21 in the lower b a n k o f the STS. (F) Entire region within the concavity of the arcuate suleus. I n case 6 (Fig. 6), 25 injections were m a d e t h r o u g h o u t this region a n d i n c l u d e d areas 8a, 45 a n d 46. T h e areas covered were similar to those in cases 4 a n d 5 t a k e n together. T h e b u l k o f the H R P labeled cells was seen in h o m o t o p i c a l areas 8a. 45 a n d 46. L a b e l e d cells were also seen in h e t e r o t o p i c a l areas 46, 8b a n d p r e m o t o r a r e a 6 as well as in areas 23 a n d 24 and m t e m p o r a l a r e a 21 in the l o w e r b a n k o f STS a n d in the insula o f STS.
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Fig. 3. Case 3. Injection site in area 46 below the principal sulcus and adjacent area 12. Only a small number of homotopica[ labeled cells are found in area 46. Heterotopical connections are seen from area 23, insula and parietal operculum.
Laminar distribution and morphology of interhemispheric afferent neurons to prefrontal granular cortex In all 6 cases the b u l k o f the H R P - l a b e l e d cells was in layer I l l b . A few labeled cells were also noted in II, lIIa, V a n d VI. The h o m o t y p i c a l H R P - p o s i t i v e neurons were distributed as a h o r i z o n t a l b a n d primarily within layer III. Some waxing and waning in the density o f the H R P positive neurons within the horizontal band was observed (Fig. 7A). In some cases in h o m o t y p i c a l cortex the labeled neurons were distributed in clumps or patches. In the heterotypical cortex in frontal lobe, both d i s t r i b u t i o n patterns for the labeled neurons were seen. In the r e m a i n i n g heterotypical cortical areas the labeled neurons were distributed in clumps and patches or were t o o few in n u m b e r to discern any such organization. The neurons giving rise to interhemispheric cortical
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3 Fig. 4. Case 4. Injection site in areas 46 and 8A. Many labeled cells are found in homotopical areas 46 and 8A. Heterotopical cells are found in areas 45, 9, 22, 23.
afferents to prefrontal granular cortex from heterotypical areas were medium to large pyramids (Fig. 8) whereas labeled cells in homotypical cortex were predominantly small pyramids (Fig. 7). DISCUSSION
Previous studies of the callosal connections of the prefrontal granular cortex have been based on anterograde degeneration techniques subsequent to transection of the corpus callosum or lesions in this region 4-6,13,zl,z3. The focus has been on the site and mode of termination ofcallosal terminals rather than on the cells of origin of callosal projections, Complete transection of the corpus callosum results in a varying density of callosal terminals in prefrontal granular cortex. In the region of theprincipal sulcus, the terminals are sparse and patchy, The remainder of this region has a moderate amount of degenerating terminals with the exception of cortex in the dorsal portion of the region bounded by the arcuate sulcus. In the present study, we have used a technique based on retrograde axoplasmic
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2 Fig. 5. Case 5. Injection site in area 45 and 46. M a n y labeled cells are found in h o m o t o p i c a l areas 45 a n d 46. Heterotopical cells are found in areas 23 a n d 21.
transport, i.e. the H R P technique. With the exception of area 46 below the principal sulcus, homotopical cortex was found to contain a large number of neurons with callosal projections. These were areas 45, 8a, 9, 10 and 46 above the principal sulcus. Just as anterograde degeneration techniques have indicated heterotypical callosal projections in addition to homotypical ones, so too, we have found evidence of converging interhemispheric cortical afferents to prefrontal granular cortex from heterotypical cortex. Outside ofhomotypical prefrontal granular cortex, the areas with heterotypical callosal afferents to this region were areas 21, 22, 23, 24 and 12 as well as the insula. On the basis of our data and that of previous studies using anterograde degeneration techniques, it would appear that the callosal connections of the prefrontal granular cortex, especially in homotypical regions, are reciprocal. Callosal projections to prefrontal granular cortex from temporal and cingulate cortex had not previously been demonstrated. Studies using anterograde degeneration techniques subsequent to transection of the corpus callosum were complicated by possible damage to cingulate cortex. The HRP method overcomes this difficulty. The callosal projections demonstrated here from cingulate and temporal cortex to prefrontal granular cortex interestingly parallel similar intrahemispheric projections to
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Fig. 6. Case 6. Injection site in areas 45, 46 and 8A. The m~jorltyof the labeled cells are found in homotopical areas 45, 46 and 8A. A few cells are found in heterotopical areas 46. 8B, 6, 23, 24 and 21.
the same region as seen in the intrahemispheric portion of the present study and in previous studies4, lx,16,22,30. In this study of prefrontal granular cortex in the monkey, the cells that give rise to the callosal terminals in prefrontal granular cortex are found primarily in layer llI although very few cells are found also in layers V. V1 and II. The findings that primarily layer III gives rise to these callosal connections is in agreement with reports for somatosensory region in the rhesus monkey H and the occipital region of the cat 2 and human 2v. I n other studies of the callosat system ofsomatosensory 11,26, visual 2 and motor cortex ~a. a columnar organization to the terminals and cells of origin of this system has been noted. Such a columnar organization was not unequivocally present in the material presented here. Rather a waxing and waning in the density of HRP-labeled neurons with callosal projections to prefrontal granular cortex was observed. This waxing and waning in cell density may be an expression of some type of vertical organization in this region. In our earlier studies of the callosal system in the rat 5,7 and in the study of Wise2% the cells in layers III and V gave origin to callosal connections. Very few caltosal cells were seen to originate in layer V of the monkey. In the rat 4,5,7,31, as in the cat and monkey2,3,8,19, subcortieal projections from cortex originate in layers V a n d VI.
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Fig. 7. Dark-field photomicrograph of HRP-labeled neurons in homotopical area 46 from case 2. The labeled cells are in layer Ill primarily and their density can be seen to wax and wane. A: :,~ 45, B: 100. The s u p r a g r a n u l a r layers in the rat receive callosal input while in the cat a n d rhesus m o n k e y the g r a n u l a r and s u p r a g r a n u l a r layers receive callosal terminals4,6,10,18. Data are currently lacking concerning the layers of origin of the rodents' intrahemispheric connections. Clarification of this point may help to decide if the difference in the layers of origin of the callosal system in the rat versus cat and m o n k e y represents some basic difference in the organization of the cortical laminae in the rat. ABBREVIATIONS C CA CI I IA IO IP IPa
central sulcus calcarine sulcus cingulate sulcus insula inferior arcuate sulcus inferior occipital sulcus intraparietal sulcus insula of superior temporal sulcus
L L MO OT SA SP ST
~ lateral fissure (in temporal lobe) - lunate sulcus (in occipital lobe) medial orbital sulcus : occipito-temporal sulcus superior arcuate sulcus - subparietal sulcus - superior temporal sulcus
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245 ACKNOWLEDGEMENTS
The authors wish to thank Dr. H. G. J. M. Kuypers for the use of some of his material. For expert technical assistance, Ms. M. Heung, Mr. J. Ginzberg and Ms. N. Tovsky are to be thanked. This research was supported by USPHS Grant NS 07666 and the Charlton Fund of Tufts University.
REFERENCES I Brodmann, K., Vergleichende Lokalisationslehre der Grosshirnrhule in ihren Prinzipielt dargestellt al~['Grund des Zellenbaus, Barth, Leipzig, 1909. 2 Gilbert, C. D. and Kelly, J. P., The projections of cells in different layers of the cat's visual cortex, J. comp. Neurol., [63 (1975) 81 106. 3 Hollfinder, H., On the origin of the corticotectal projections of the cat, Exp. Brain Res., 21 (1974) 433 440. 4 Jacobson, S., Distribution of commissural axon terminals in the rat neocortex, Exp. Neurol., 28 (1970) 193 205. 5 Jacobson, S., lntralarninar, interlaminar, cal[osal and thalamocortical connections in frontal and parietal areas of the albino rat, J. comp. Neurol., 124 (1965) 131-146. 6 Jacobson, S. and Marcus, E. M., The laminar distribution of fibers of the corpus callosum: a comparative study in the rat, cat, rhesus monkey and chimpanzee, Brain Research, 24 (1970) 517 520. 7 Jacobson, S. and Trojanowski, J. Q., The cells of origin of the corpus callosum in rat, cat and rhesus monkey, Brain Research, 74 (1974) 149-155. 8 Jacobson, S. and Trojanowski, J. Q., Corticothalamic neurons and thalamocortical terminal fields: an investigation in rat using HRP and autoradiography, Brain Research, 85 (1975) 385 501. 9 Jacobson, S. and Trojanowski, J. Q., Prefrontal granular cortex of the rhesus monkey. 1. lntrahemispheric cortical afferents, Brain Research, 132 (1977) 209 233. l0 Jones, E. G., Lamination and differential distribution of thalamic afferents within the sensory motor cortex of the squirrel monkey, J. comp. Neurol., 160 (1975) 167 204. I I Jones, E. G., Burton, H. and Porter, R., Commissural and cortico-cortical "column~" in the somatic sensory cortex of primates, Science, 190 (1975) 572 574. 12 Jones, E. G. and Powe[l, T. P. S., An anatomical study of converging sensory pathway~ within the cerebral cortex of the monkey, Brain, 93 (1970) 7910) 793-820. 13 Karol, E. A. and Pandya, D. N., The distribution of the corpus callosum in the rhesus monkey, Brain, 94 ( 1971 ) 471-486. 14 KOnzle, H., Alternating afferent zones of high and low axonal density within the macaque motor cortex, Brain Research, 103 (1976) 365 370. 15 Kuypers, H. G. J. M., Kievit, J. and Groen-Klevant, A. C., Retrograde axonal transport of HRP in rat's forebrain, Brai~z Research, 67 (1974) 211 218. 16 Kuypers, H. G. J. M., Szwarcbart, M. K., Mishkin, M. and Rosvold, H. E., Occipitotemporal corticocortical connections in the rhesus monkey, Exp. Neurol., 11 (1965) 245 262. 17 LaVail, J. H. and LaVail, M. M., Retrograde axonal transport in the C.N.S., Scienee, 176 (1972) 1416 1417. 18 Lorente de N6, R., Cerebral cortex: architectonics, intracortical connections. In J. F. Fulton (Ed.), Physiology o f the Nervous System, 3rd ed. Oxford University Press, New York, 1949, pp. 275-315. 19 Lund, J. S., Lund, R. D., Hendrickson, A. E., Bunt, A. H. and Fuchs, A. F., The origin of efferent pathways from the primary visual cortex, area 17, of the macaque monkey as sho,an by retrograde transport of HRP, J. comp. Neurol., 164 (1975) 287 304. 20 Olszewski, J., The Thalamus o f the Macaca nmlatta : an AtlasJor Use with the Stereotaxie hlstrument, Karger, Basel, 1952. 21 Pandya, D. N., Karol, E. A. and Heilbronn, D., The topographical distribution of interhemispheric projections in the corpus callosum of the rhesus monkey, Brain Research, 32 (1971) 31 43.
246 22 Pandya, D. N. and Kuypers, H. G. J. M., Cortico-cortical connections in the rhesus monkey. Brain Research, 13 (1969) 13-36. 23 Pandya, D. N. and Vignola, L. A., lntra- and interhemispheric projections of the precentral, premotor and arcuate areas in the rhesus monkey, Brain Research, 26 (1971) 217-233. 24 Roberts, T. S. and Akert, K., lnsular and opercular cortex and its thalamic projection in Macaca mulatta, Schweiz. Arch. Neurol. Neurochir. Psychiat.. 92 11963) 1-43. 25 Seltzer, B. and Pandya, D. N., The associational connections onto the superior temporal sulcus. Soe. Neurosci. 6th Meeting. (1975) 166. 26 Shanks, M. E., Rockel, A. J. and Powell, T. P. S., The commissural fiber connections of the primary sensory cortex, Brain Research, 98 (1976) 166-17 I. 27 Shomura, K., Ando, T. and Kato, K., Structural organization of callosal OBg in human corpus callosum agenesis, Brain Research, 93 (1975) 241 252. 28 Walker, A. E., A cytoarchitectural study of the prefrontal area of the macaque monkey, J. comp. Neurol., 73 (1940) 59-86. 29 Wise, S. P., The laminar organization of certain afferent and efferent fiber systems in the rat somatosensory cortex, Brain Research, 90 (1975) 140-143. 30 Zeki, S. M., Convergent input from the striate cortex (area 17) to the cortex of the superior tempora sulcus in the rhesus monkey, Brain Research, 28 (1971~ 338-340.
23 5
P R E F R O N T A L G R A N U L A R C O R T E X OF T H E RHESUS M O N K E Y . 1I. I N T E R H E M I S P H E R I C C O R T I C A L A F F E R E N T S
STANLEY JACOBSON and JOHN Q. TROJANOWSK1 Anatomy Department, Tufts University School of Medicbm, Boston, Mass. 02111 (U.S.A.)
(Accepted December 17th, 1976)
SUMMARY In 6 adolescent rhesus monkeys, horseradish peroxidase (HRP) was injected into 6 regions of the dorsolateral convexity of the prefrontal granular cortex. The commissural connections originated in both homotopical and heterotopical zones of the hemisphere contralateral to the injection site. The areas affected by the injections, i.e. areas 46, 45, 10, 9, 12 and 8a, received extensive homotopical interhemispheric input. HRP-labeled neurons were less extensive in heterotopical as opposed to homotopical cortex but they were seen in all 6 cases and were most common in prefrontal areas and less common in cingulate areas, areas 21 and 22 in the superior temporal sulcus and in insular cortex. The cells, whether of heterotopical or homotopical origin, were located primarily in layer 111. The most common distribution pattern was a horizontal band of HRP-labeled neurons which waxed and waned in cell density especially in homotopical cortex or patches and clusters of labeled cells especially in heterotopical cortex. This waxing and waning and grouping of neurons in patches and clusters may well represent a vertical type of organization to the neurons which give rise to the interhemispheric cortical afferents to prefrontal granular cortex in the monkey.
INTRODUCTION The development of the horseradish peroxidase (HRP) technique has permitted investigators to analyze more effectively than hitherto possible the cells of origin of fiber systems within the central nervous system of various experimental animals1.5, t7. Ira the study reported in the preceding paper, we have described the intrahemispheric cortical afferents to the prefrontal granular cortex in rhesus monkey based on the H R P technique 9. It was observed that these afferents arise in diverse parts of cortex, i.e. areas within frontal, temporal, parietal, occipital and limbic lobes and that the cells
236 of origin of these afferents are small to medium pyramids which are located primarily in layer III and may be organized in a vertical manner. The present report, based on the same material, describes the interhemispheric cortical afferents to prefrontal granular cortex identifying the areas in homotopical and heterotopical regions in which the cells of origin of these afferents arise as well as describing the morphology, laminar distribution and organization of these callosal neurons. MATERIALS AND METHODS The 6 adolescent animals used in the study of the commissural connections of the prefrontal granular cortex were the same ones used in the study of the intrahemispheric cortical afferents to the prefrontal granular cortex reported in the preceding paperL All procedures were performed using sodium pentobarbital as the general anesthetic (35 mg/kg body weight). The prefrontal granular cortex on the dorsolateral surface o f frontal lobe was divided into 6 zones as noted in the previous p a p e r All animals received unilateral multiple injections of a 10",~ aqueous H R P solution with each single injection consisting of 0.6/~1 volume of the marker solution. The animals survived 65-80 h and were perfused, fixed and cut in the stereotaxic plane of Otszewski 2°. F o r details of the fixation, sectioning and incubation to demonstrate the HRP-positive neurons, the preceding paper should be consulted. Every 10th coronal section was examined with the aid of an X - Y plotter and where necessary more sections were examined. All observations were made in unstained material and examined under dark-field illumination. The secuons were then counterstained to identify the areas and layers in which the HRP-positive callosal cells were located. The cortical areas in the prefrontal cortex are parcellated after Walker 2s. the remaining areas are parcellated after Brodmann ~. Roberts and Akert 24 and Seltzer and Pandya 2'~. RESULTS The presentation of our data concerning the interhemispheric cortical afferents to prefrontal granular cortex will begin with the H R P injections into frontal polar cortex, continue caudally through area 46 above and below the principal sulcus and conclude with the data from injections into cortex bounded by the arcuate sulcus.
Interhemispheric cortical afferents to prefrontal granular cortex (A) Frontalpole. In case 1 (Fig. 1), 15 injections were made into the dorsotateral convexity and medial surface of the hemisphere including areas 9 and 10. The bulk ofthe interhemispheric cortical afferents to this injection site arose in homotopicat cortex in the contralateral hemisphere where large numbers of HRP.labeled neurons were found. Heterotopical projections were seen to arise in areas 12 and 46 in frontal lobe as well as in cingulate area 25. In addition, some cells were also noted in area 22 in the dorsal bank of the superior temporal sutcus (STS) and adjacent s u ~ r i o r temp0ral gyrus, f r o m its rostral pole to nearly as far posterior as its caudal end. These labeled cells were only seen in layer II I and 2-4 of them were noted in every section we examined through this region.
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Fig. 1. Case 1. Injection site involving areas 10 and adjacent area 9. The bulk of the callosal projections are from homotopical areas 9 and 10. Some heterotopical projections are also seen from areas 22, 23 and 45. In this and the following figures, the hatching in the brain at the upper right of the figure indicates the injection site. Below it the medial and lateral aspect of the contralateral hemisphere is depicted. Coronal sections are likewise through the contra[ateral hemisphere at levels indicated by the corresponding numbers. Stippling indicates HRP-labeled neurons. Arrows indicate boundaries between numbered areas. See list of abbreviations and text for further details.
(B) Cortex above the principal sulcus. I n case 2 (Fig. 2), 15 injections were confined to area 46 in the dorsal b a n k and on the dorsolateral convexity above the principal sulcus. The bulk of the interhemispheric afferents arose from the homotopical cortex in the opposite hemisphere, however, a few labeled cells were also seen in cingulate areas 23 a n d 24 a n d temporal area 22 in the dorsal b a n k of the STS. (C) Cortex below the principal sulcus, in case 3 (Fig. 3), 14 injections were made in area 46 in the lower b a n k of the principal sulcus and on the dorsolateral convexity ventral to principal sulcus with some involvement of adjacent area 12. The n u m b e r of H R P - l a b e l e d n e u r o n s f o u n d in homotopical cortex was far fewer in n u m b e r in this case t h a n in a n y o f the other cases a n d they were f o u n d in area 46 in a n d a r o u n d the principal sulcus. Heterotopical interhemispheric afferents were seen to arise from area
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Fig. 2. Case 2. Injection site in area 46 above the principal sulcus. The bulk of the callosa[ projections are from homotopical area 46. Heterotopical projections are seen from areas 22. 23 and 24.
23 a n d the insula as well as the parietal operculum. These l a b e l e d cells were few in n u m b e r (2-6 per section) but were seen in all the sections e x a m i n e d in these regions. ( D ) Cortex in the dorsal portion of the concavity of the arcuate sulcus. In case 4 (Fig. 4), 12 injections were m a d e which involved areas 46 a n d 8a. There was a s t r o n g p r o j e c t i o n f r o m the h o m o t o p i c a l areas 46 a n d 8a. M a n y cells were also seen in h e t e r o topical cortex m areas 45 a n d 9. A few cells (2-7) per section were also seen in h e t e r o topical a r e a 22 in the d o r s a l b a n k o f the STS a n d in cingulate a r e a 23. (E) Cortex in the ventral portion of the concavity of the areuate suh'us. In case 5 (Fig. 5), 10 injections were m a d e in areas 45 a n d 46. M a n y labeled cells were seen in h o m o t o p i c a l cortex in areas 45 and 46. A few cells (2-6 per section) were seen in h e t e r o t o p i e a l cortex in a r e a 23 a n d m t e m p o r a l a r e a 21 in the lower b a n k o f the STS. (F) Entire region within the concavity of the arcuate suleus. I n case 6 (Fig. 6), 25 injections were m a d e t h r o u g h o u t this region a n d i n c l u d e d areas 8a, 45 a n d 46. T h e areas covered were similar to those in cases 4 a n d 5 t a k e n together. T h e b u l k o f the H R P labeled cells was seen in h o m o t o p i c a l areas 8a. 45 a n d 46. L a b e l e d cells were also seen in h e t e r o t o p i c a l areas 46, 8b a n d p r e m o t o r a r e a 6 as well as in areas 23 a n d 24 and m t e m p o r a l a r e a 21 in the l o w e r b a n k o f STS a n d in the insula o f STS.
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Fig. 3. Case 3. Injection site in area 46 below the principal sulcus and adjacent area 12. Only a small number of homotopica[ labeled cells are found in area 46. Heterotopical connections are seen from area 23, insula and parietal operculum.
Laminar distribution and morphology of interhemispheric afferent neurons to prefrontal granular cortex In all 6 cases the b u l k o f the H R P - l a b e l e d cells was in layer I l l b . A few labeled cells were also noted in II, lIIa, V a n d VI. The h o m o t y p i c a l H R P - p o s i t i v e neurons were distributed as a h o r i z o n t a l b a n d primarily within layer III. Some waxing and waning in the density o f the H R P positive neurons within the horizontal band was observed (Fig. 7A). In some cases in h o m o t y p i c a l cortex the labeled neurons were distributed in clumps or patches. In the heterotypical cortex in frontal lobe, both d i s t r i b u t i o n patterns for the labeled neurons were seen. In the r e m a i n i n g heterotypical cortical areas the labeled neurons were distributed in clumps and patches or were t o o few in n u m b e r to discern any such organization. The neurons giving rise to interhemispheric cortical
240
8B
2;
ST
1
3 Fig. 4. Case 4. Injection site in areas 46 and 8A. Many labeled cells are found in homotopical areas 46 and 8A. Heterotopical cells are found in areas 45, 9, 22, 23.
afferents to prefrontal granular cortex from heterotypical areas were medium to large pyramids (Fig. 8) whereas labeled cells in homotypical cortex were predominantly small pyramids (Fig. 7). DISCUSSION
Previous studies of the callosal connections of the prefrontal granular cortex have been based on anterograde degeneration techniques subsequent to transection of the corpus callosum or lesions in this region 4-6,13,zl,z3. The focus has been on the site and mode of termination ofcallosal terminals rather than on the cells of origin of callosal projections, Complete transection of the corpus callosum results in a varying density of callosal terminals in prefrontal granular cortex. In the region of theprincipal sulcus, the terminals are sparse and patchy, The remainder of this region has a moderate amount of degenerating terminals with the exception of cortex in the dorsal portion of the region bounded by the arcuate sulcus. In the present study, we have used a technique based on retrograde axoplasmic
241
SA
1
2 Fig. 5. Case 5. Injection site in area 45 and 46. M a n y labeled cells are found in h o m o t o p i c a l areas 45 a n d 46. Heterotopical cells are found in areas 23 a n d 21.
transport, i.e. the H R P technique. With the exception of area 46 below the principal sulcus, homotopical cortex was found to contain a large number of neurons with callosal projections. These were areas 45, 8a, 9, 10 and 46 above the principal sulcus. Just as anterograde degeneration techniques have indicated heterotypical callosal projections in addition to homotypical ones, so too, we have found evidence of converging interhemispheric cortical afferents to prefrontal granular cortex from heterotypical cortex. Outside ofhomotypical prefrontal granular cortex, the areas with heterotypical callosal afferents to this region were areas 21, 22, 23, 24 and 12 as well as the insula. On the basis of our data and that of previous studies using anterograde degeneration techniques, it would appear that the callosal connections of the prefrontal granular cortex, especially in homotypical regions, are reciprocal. Callosal projections to prefrontal granular cortex from temporal and cingulate cortex had not previously been demonstrated. Studies using anterograde degeneration techniques subsequent to transection of the corpus callosum were complicated by possible damage to cingulate cortex. The HRP method overcomes this difficulty. The callosal projections demonstrated here from cingulate and temporal cortex to prefrontal granular cortex interestingly parallel similar intrahemispheric projections to
242
4
Fig. 6. Case 6. Injection site in areas 45, 46 and 8A. The m~jorltyof the labeled cells are found in homotopical areas 45, 46 and 8A. A few cells are found in heterotopical areas 46. 8B, 6, 23, 24 and 21.
the same region as seen in the intrahemispheric portion of the present study and in previous studies4, lx,16,22,30. In this study of prefrontal granular cortex in the monkey, the cells that give rise to the callosal terminals in prefrontal granular cortex are found primarily in layer llI although very few cells are found also in layers V. V1 and II. The findings that primarily layer III gives rise to these callosal connections is in agreement with reports for somatosensory region in the rhesus monkey H and the occipital region of the cat 2 and human 2v. I n other studies of the callosat system ofsomatosensory 11,26, visual 2 and motor cortex ~a. a columnar organization to the terminals and cells of origin of this system has been noted. Such a columnar organization was not unequivocally present in the material presented here. Rather a waxing and waning in the density of HRP-labeled neurons with callosal projections to prefrontal granular cortex was observed. This waxing and waning in cell density may be an expression of some type of vertical organization in this region. In our earlier studies of the callosal system in the rat 5,7 and in the study of Wise2% the cells in layers III and V gave origin to callosal connections. Very few caltosal cells were seen to originate in layer V of the monkey. In the rat 4,5,7,31, as in the cat and monkey2,3,8,19, subcortieal projections from cortex originate in layers V a n d VI.
243
Fig. 7. Dark-field photomicrograph of HRP-labeled neurons in homotopical area 46 from case 2. The labeled cells are in layer Ill primarily and their density can be seen to wax and wane. A: :,~ 45, B: 100. The s u p r a g r a n u l a r layers in the rat receive callosal input while in the cat a n d rhesus m o n k e y the g r a n u l a r and s u p r a g r a n u l a r layers receive callosal terminals4,6,10,18. Data are currently lacking concerning the layers of origin of the rodents' intrahemispheric connections. Clarification of this point may help to decide if the difference in the layers of origin of the callosal system in the rat versus cat and m o n k e y represents some basic difference in the organization of the cortical laminae in the rat. ABBREVIATIONS C CA CI I IA IO IP IPa
central sulcus calcarine sulcus cingulate sulcus insula inferior arcuate sulcus inferior occipital sulcus intraparietal sulcus insula of superior temporal sulcus
L L MO OT SA SP ST
~ lateral fissure (in temporal lobe) - lunate sulcus (in occipital lobe) medial orbital sulcus : occipito-temporal sulcus superior arcuate sulcus - subparietal sulcus - superior temporal sulcus
3
Y
r~
°.
0 C~
245 ACKNOWLEDGEMENTS
The authors wish to thank Dr. H. G. J. M. Kuypers for the use of some of his material. For expert technical assistance, Ms. M. Heung, Mr. J. Ginzberg and Ms. N. Tovsky are to be thanked. This research was supported by USPHS Grant NS 07666 and the Charlton Fund of Tufts University.
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