650
&ahr Research, 100 (1975) 650-656 ~ Elsevier Scientific Publishing Company, Amsterdam ~- Printed in The Netherlands
Cell populations of the lateral geniculate nucleus of the cat as determined with horseradish peroxidase
LOIS K. LAEMLE Department of Anatomy, College of Medicine and Dentistry of New Jersey, Newark, N.J. 07103 (U.S.A.)
(Accepted September 9th, 1975)
Past studies of the lateral geniculate nucleus (LGN) have paid limited attention to the characteristics of its cell population, but instead, have concentrated on the laminar organization and fiber connections. Reports containing descriptions of L G N neurons indicate that morphologically the L G N of the cat is not composed of a homogeneous cell population. These studies, based on observations of Nissl stained and Golgi sections, have defined 5 'classes' or populations of L G N neurons according to size 4 11 t~ and dendritic and axonal configuration s (Table I). The neural connections and functional significance of these cell populations remain unclear. The present communication is the first of a series of investigations designed to characterize the cell populations described by others in the LGN, in terms of their axonal projections via the geniculo-cortical radiation to the cerebral cortex. This study is based primarily upon preparations of the L G N following injection of horseradish peroxidase (HRP) into focal sites in area 17. The injected H R P is picked up TABLE I CLASSIFICATION OF CELL POPULATIONS OF THE CAT
LGN
LGN neurons Investigator
Year Technique Large ( A,A1) ( #m )
Medium ( A, A1) ( l~m)
Small ( A, A1, B) ( l~m)
Telio
1904 Nissl Golgi
20-35 D 30--45 L
12-25 D 17-30 L
Thuma
Toluidine 1928 blue 30-40 D
20-25 D
Hayhow GuiUery
1958 Nissl 1966 Golgi
6-15 D 12-25 D 8-17 L 17-30 L * long axon cells ** short axon cells 10-20 D 10 D 20-25 L 10-20 D Med. size cells 10-20 D 15-30 D Class 3 Class 4 * short axon cells
30--40D 25--40 D Class 1
20-25 D 15-30 D Class 2
Medium size spindle ( lain. B only)
651 7
f
1,2
14
12,13
o
Fig. 1. Diagram of the lateral aspect of the cat brain indicatingthe sites of HRP injections.
by the axon terminals and transported to many but not necessarily all of their cell bodies of originL The HRP is revealed in these cell bodies ~,e and in some of the primary and secondary dendrites (Figs. 3-6). The present report correlates recognized cell classes with the geniculo-cortical projections. Fourteen kittens between 3 days and 3 months of age were anesthetized with Nembutal, and 0.1-0.6 #m of a freshly prepared solution of HRP (Sigma, Type VI; concentration 300 mg/#l) was injected into small loci of cortical area 17 with a 10 #1 Hamilton syringe. Each animal received only one injection. Injection sites were predominantly restricted to small loci in the caudal portions of area 17 (Fig. 1). The animals were anesthetized from 5.5 to 48 h after HRP injection and sacrificed by intracardiac perfusion with 10~ neutral formol-saline. The brains were removed and stored in fresh fixative at 4 °C overnight. In the morning the tissue was transferred to 33 ~ sucrose in neutral formol-saline where it was stored for 2 days. Frozen sections were cut at 40 #m and at 60/~m, and developed for 40-60 min at room temperature. The developing solution was prepared by dissolving 75 mg free base 3,3'-diaminobenzidine tetrahydrochloride (Sigma Chemical Co.) in 100 ml 0.2 M phosphate buffer (pH 7.4). Immediately before using the solution, 0.1 ml of 10 ~o hydrogen peroxide was added. Sections were developed individually and washed in three changes of distilled water. Sections were then floated in 1 ~o alcoholic gelatin, mounted on clean glass slides and counterstained with cresylecht violet. In a representative series of sections, the LGN was drawn with the aid of a Bausch and Lomb trisimplex projector, then carefully examined with the microscope
652
.~"~
Lamina A
28
29
30
T
0
~14"~15~17 ~18
t_ i ............: iii
J24-~
. . . . .
---
. . . . . .
. ° - o - o . - - . o - - - o
)\\
. . . .
° . . . . .
- ° - - - °
. . . . .
°°°----.4
b. 24
25
26
27
C.
Fig. 2. Sample of record obtained from microscopic observations of LGN. a: diagram of coronal section indicating level at which data were recorded, b: enlargement of L G N at level shown in a, indicating major laminae and position of H R P granule-containing cells within these laminae. Each number indicated on the lamina corresponds to the cell of the same number as illustrated in c. The position of each cell drawn with the camera lucida was indicated in this fashion. All H R P granulecontaining cells were situated within a restricted region of the laminar portion of the dorsal LGN. c: camera lucida drawings of H R P granule-containing cells. Cells were drawn using a × 40 objective and × 10 eyepieces. Cells are grouped according to laminae in which they were observed.
653 in order to identify cells which contained HRP granules. The outline of each HRP granule-containing cell was drawn with a camera lucida, measured with an ocular micrometer, and located precisely within the LGN (Fig. 2). Three hundred and fifty HRP granule-containing cells were measured and classified in laminae A and A1, and another 70 cells in lamina B. Morphologically, 4 different cell populations were noted in the present investigation. These are basically similar to observations of Tello 11, Thuma 12, and Hayhow 4. The classification of Guillery 2, based upon dendritic and axonal morphology, corresponds closely to those just noted (Table I). The cellular composition of layers A and A1 is essentially the same 8,~°, therefore they are considered together. Cells in lamina B which Guillery 3 recently subdivided into laminae C, C1, and C2 were classified according to size and shape by him as (1) large multipolar, (2) medium size multipolar, (3) small multipolar, (4) fusiform. The cells of lamina B are considered separately below. Microscopic examination of the LGN revealed that although all cells did not contain HRP granules, some cells of each category identified were included among the granule-containing cell population. These cells were restricted to the laminar portion of the ipsilateral dorsal LGN. The evidence based on HRP studies indicates that the geniculo-cortical projections are topographically organized. In animals with HRP injections placed rostrally in area 17, HRP granule-containing cells were located in rostral regions of the LGN, while for animals receiving HRP injections in caudal portions of area 17, HRP granule-containing cells were located caudally in the LGN. This is consistent with the observations of Garey and Powell 1 and Niimi and Sprague 8. Following injection of HRP in small loci of area 17, HRP labeled cells were restricted to a narrow column within the laminar portion of the LGN (Fig. 2). Within this column the great majority of the cells were labeled; however, labeled and unlabeled cells were freely intermixed. Beyond the limits of the column, no labeled cells could be found. The following is a summary, according to recognized cell class, of HRP granulecontaining cells which were observed, measured, and classified in the LGN, pars dorsalis. In laminae A and A1 50-55 ~o of the granule-containing cells were medium size cells (Fig. 4), 20-25 ~ were large cells (Fig. 3), and 20-25 ~ were small cells (Fig. 5). The HRP granule-containing cells observed in lamina B corresponded to 3 of the 4 cell types described by Guillerya: (1) fusiform; (2) medium size multipolar; (3) small multipolar. More than half of these cells were fusiform; the rest were multipolar. In the region of lamina B immediately adjacent to the optic tract, corresponding to lamina C2 (ref. 3), about 80-85 ~o of the cells containing HRP were fusiform cells (diameter 4-11 #m, Fig. 6B). The remaining 15-20~ of the HRP labeled cells in this region were medium size and small multipolar cells (Fig. 6A). In the remainder of lamina B approximately half of the HRP granule-containing cells appeared to be fusiform and half were multipolar. Approximately two-thirds of the total number of granule-containing multipolar cells were medium size; one-third were small. No large multipolar cells containing HRP granules were observed. Of the previously described cell populations in the LGN (pars dorsalis), four were identified in this study as contributing axons to the geniculo-striate projection.
||
~F
|
J~
J
C~
655 These correspond to the so-called long axon cells identified in previous investigations of the L G N in the cat 2,9,11 as (1) large or class 1 cells, (2) medium size or class 2 cells, (3) small cells, (4) fusiform or class 4 cells. Some studies have indicated that in addition to the above 4 cell classes, the L G N contains a population of intrinsic neurons or class 3 cells 2,9,11 (Table I), which are small diameter cells. The observations presented in this report raise questions regarding (1) the functional significance of morphologically determined cell classes, (2) the issue interneurons in the LGN. The present investigation indicates that in laminae A and A1, large, medium, and small cells send direct projections to the calcarine cortex. Although it seems unlikely, this does not exclude the possibility that neurons of similar morphology may not have such projections. The studies of Garey and Powell 1, using retrograde degeneration o f L G N cells, indicated that following lesions restricted to area 17 of the cortex rto change could be observed in the large cell population, although small and medium size cells appeared shrunken. Following lesions of area 18 alone, minor changes were noted in the large cells. Simultaneous ablation of area 17 and 18, however, resulted in the shrinkage of the large cells as well as the small and medium size cells. The results of Garey and Powell 1 seem to indicate that there may indeed be a functional significance or a division of labor among the morphologically determined cell classes. Further study is required, of the contribution of the cell populations of the L G N to other visual cortices before one can reach any conclusions regarding the functional significance of morphologically determined cell classes. Regarding the issue of interneurons in the LGN, morphological evidence for their existence has been derived from the Golgi studies of Guillery 2, Tello 11, and O'Leary 9. This cell has characteristically been described as a small diameter, multipolar neuron whose axon ramifies completely within the LGN. The present study confirms the existence of the small diameter, long axon cell reported by Tello 11, but not observed by Guillery 2, and demonstrates its direct projection to cortical area 17. This allows for two possibilities: either (a) two populations of small cells are present in the L G N ; one population with cortical projections as demonstrated in the present investigation, and another population of intrageniculate or intrinsic neurons as demonstrated by others noted above; or (b) only one population of small diameter cells resides in the LGN. This single population of small, multipolar cells projects its axons directly to the cortex and there exist then no true interneurons in the LGN. Resolution of this issue cannot be made on the basis of small focal injections of HRP.
Fig. 3. Large HRP granule-containing cells observed in laminae A and A1 of the LGN (pars dorsalis). Fig. 4. Medium size, HRP granule-containing cells observed in laminae A and A1. Note the presence of granules in the soma, primary dendrites, and secondary dendrites (arrow). Fig. 5. Small HRP granule-containing cells observed in laminae A and A1 of the LGN (pars dorsalis). Note the extension of granules into secondary dendrite (5B) as indicated by the arrow. Fig. 6. HRP granule-containing cells observed in lamina B of the LGN (pars dorsalis). A: a typical small multipolar neuron. B: a typical fusiform cell. This cell was situated along the ventral border of lamina B, adjacent to the optic tract. This is the region designated as lamina Cz by Guillery8.
656 F i b e r connections o f each o f the 4 cell types described in l a m i n a B by Guillery :~ have n o t been previously d e m o n s t r a t e d . Present o b s e r v a t i o n s indicate t h a t the first three o f these cell types have axons which project to the p r i m a r y visual cortex. Until m o r e d a t a are available, the absence o f H R P granules in the large m u l t i p o l a r n e u r o n s o f l a m i n a B in this study should n o t be i n t e r p r e t e d to indicate an absence o f direct c o n n e c t i o n s between these cells a n d the p r i m a r y visual cortex. In s u m m a r y , the present investigation d e m o n s t r a t e s that o f the 5 m o r p h o l o g i c a l l y defined cell classes in the L G N o f the cat, 4 are c o m p o s e d o f cells with long axons which p r o j e c t directly a n d t o p o g r a p h i c a l l y to the striate cortex. The a u t h o r wishes to t h a n k Mrs. A d a r s h A r o r a for her excellent technical assistance, Messers. M. Levine, D. K r a m e r a n d P. Christ for the illustrations, and Dr. C. R. N o b a c k for his critique o f the m a n u s c r i p t . This w o r k was s u p p o r t e d by N I H G r a n t 5R01 EY11074.
1 GAREY, L. J., AND POWELL, T. P. S., The projection of the lateral geniculate nucleus upon the cortex in the cat, Proc. roy. Soc. B, 169 (1967) 107-126. 2 GUILLERY,R. W., A study of Golgi preparations from the dorsal lateral geniculate nucleus of the adult cat, J. comp. Neurol., 128 (1966) 21-50. 3 GUILLERY,R. W., The laminar distribution of retinal fibers in the dorsal lateral geniculate nucleus of the cat: a new interpretation, J. comp. Neurol., 138 (1970) 339-368.
4 FIAYHOW,W. R., The cytoarehitecture of the lateral geniculate body in the cat in relation to the distribution of crossed and uncrossed optic fibers, J. comp. Neurol., 110 (1958) 1-64. 5 LAVAIL,J. H., AND LAVAIL, M. M., Retrograde axonai transport in the central nervous system, Science, 176 (1972) 1416-1417. 6 LAVAIL,J. H., WINSTON,K. R., AND TISH, A., A method based on retrograde intraaxonal transport of protein for identification of cell bodies of origin of axons terminating within the CNS, Brain Research, 58 (1973) 470--477. 7 NAUTA, H. J. W., PRITZ, M. B., AND LASEK, R. J., Afferents to the rat caudoputamen studied with horseradish peroxidase. An evaluation of a retrograde neuroanatomical research method, Brain Research, 67 (1974) 219-238. 8 NIIMI, K., AND SPRAGUE,J. M., Thalamo-cortical organization of the visual system in the cat, J. comp. Neurol., 138 (1970) 219-249. 90'LEARY, J. L., A structural analysis of the lateral geniculate nucleus of the cat, J. comp. Neurol., 73 (1940) 405-430. 10 PETERS,A., AND PALAY,S. L., The morphology of laminae A and Ax of the dorsal nucleus of the lateral geniculate body of the cat, J. Anat. (Lond.), 100 (1966) 451-486. 11 TELLO, F., Disposicion macroscopia y estructural del cuerpo geniculado externo, Trab. Lab. Invest. Biol. Univ. Madrid, 3 (1904) 39-62. 12 THUMA,B. D., Studies on the diencephalon of the cat. 1. Thecytoarchitecture of thecorpus geniculatum laterale, J. co~p. Neurol., 46 0928) 193-199.
&ahr Research, 100 (1975) 650-656 ~ Elsevier Scientific Publishing Company, Amsterdam ~- Printed in The Netherlands
Cell populations of the lateral geniculate nucleus of the cat as determined with horseradish peroxidase
LOIS K. LAEMLE Department of Anatomy, College of Medicine and Dentistry of New Jersey, Newark, N.J. 07103 (U.S.A.)
(Accepted September 9th, 1975)
Past studies of the lateral geniculate nucleus (LGN) have paid limited attention to the characteristics of its cell population, but instead, have concentrated on the laminar organization and fiber connections. Reports containing descriptions of L G N neurons indicate that morphologically the L G N of the cat is not composed of a homogeneous cell population. These studies, based on observations of Nissl stained and Golgi sections, have defined 5 'classes' or populations of L G N neurons according to size 4 11 t~ and dendritic and axonal configuration s (Table I). The neural connections and functional significance of these cell populations remain unclear. The present communication is the first of a series of investigations designed to characterize the cell populations described by others in the LGN, in terms of their axonal projections via the geniculo-cortical radiation to the cerebral cortex. This study is based primarily upon preparations of the L G N following injection of horseradish peroxidase (HRP) into focal sites in area 17. The injected H R P is picked up TABLE I CLASSIFICATION OF CELL POPULATIONS OF THE CAT
LGN
LGN neurons Investigator
Year Technique Large ( A,A1) ( #m )
Medium ( A, A1) ( l~m)
Small ( A, A1, B) ( l~m)
Telio
1904 Nissl Golgi
20-35 D 30--45 L
12-25 D 17-30 L
Thuma
Toluidine 1928 blue 30-40 D
20-25 D
Hayhow GuiUery
1958 Nissl 1966 Golgi
6-15 D 12-25 D 8-17 L 17-30 L * long axon cells ** short axon cells 10-20 D 10 D 20-25 L 10-20 D Med. size cells 10-20 D 15-30 D Class 3 Class 4 * short axon cells
30--40D 25--40 D Class 1
20-25 D 15-30 D Class 2
Medium size spindle ( lain. B only)
651 7
f
1,2
14
12,13
o
Fig. 1. Diagram of the lateral aspect of the cat brain indicatingthe sites of HRP injections.
by the axon terminals and transported to many but not necessarily all of their cell bodies of originL The HRP is revealed in these cell bodies ~,e and in some of the primary and secondary dendrites (Figs. 3-6). The present report correlates recognized cell classes with the geniculo-cortical projections. Fourteen kittens between 3 days and 3 months of age were anesthetized with Nembutal, and 0.1-0.6 #m of a freshly prepared solution of HRP (Sigma, Type VI; concentration 300 mg/#l) was injected into small loci of cortical area 17 with a 10 #1 Hamilton syringe. Each animal received only one injection. Injection sites were predominantly restricted to small loci in the caudal portions of area 17 (Fig. 1). The animals were anesthetized from 5.5 to 48 h after HRP injection and sacrificed by intracardiac perfusion with 10~ neutral formol-saline. The brains were removed and stored in fresh fixative at 4 °C overnight. In the morning the tissue was transferred to 33 ~ sucrose in neutral formol-saline where it was stored for 2 days. Frozen sections were cut at 40 #m and at 60/~m, and developed for 40-60 min at room temperature. The developing solution was prepared by dissolving 75 mg free base 3,3'-diaminobenzidine tetrahydrochloride (Sigma Chemical Co.) in 100 ml 0.2 M phosphate buffer (pH 7.4). Immediately before using the solution, 0.1 ml of 10 ~o hydrogen peroxide was added. Sections were developed individually and washed in three changes of distilled water. Sections were then floated in 1 ~o alcoholic gelatin, mounted on clean glass slides and counterstained with cresylecht violet. In a representative series of sections, the LGN was drawn with the aid of a Bausch and Lomb trisimplex projector, then carefully examined with the microscope
652
.~"~
Lamina A
28
29
30
T
0
~14"~15~17 ~18
t_ i ............: iii
J24-~
. . . . .
---
. . . . . .
. ° - o - o . - - . o - - - o
)\\
. . . .
° . . . . .
- ° - - - °
. . . . .
°°°----.4
b. 24
25
26
27
C.
Fig. 2. Sample of record obtained from microscopic observations of LGN. a: diagram of coronal section indicating level at which data were recorded, b: enlargement of L G N at level shown in a, indicating major laminae and position of H R P granule-containing cells within these laminae. Each number indicated on the lamina corresponds to the cell of the same number as illustrated in c. The position of each cell drawn with the camera lucida was indicated in this fashion. All H R P granulecontaining cells were situated within a restricted region of the laminar portion of the dorsal LGN. c: camera lucida drawings of H R P granule-containing cells. Cells were drawn using a × 40 objective and × 10 eyepieces. Cells are grouped according to laminae in which they were observed.
653 in order to identify cells which contained HRP granules. The outline of each HRP granule-containing cell was drawn with a camera lucida, measured with an ocular micrometer, and located precisely within the LGN (Fig. 2). Three hundred and fifty HRP granule-containing cells were measured and classified in laminae A and A1, and another 70 cells in lamina B. Morphologically, 4 different cell populations were noted in the present investigation. These are basically similar to observations of Tello 11, Thuma 12, and Hayhow 4. The classification of Guillery 2, based upon dendritic and axonal morphology, corresponds closely to those just noted (Table I). The cellular composition of layers A and A1 is essentially the same 8,~°, therefore they are considered together. Cells in lamina B which Guillery 3 recently subdivided into laminae C, C1, and C2 were classified according to size and shape by him as (1) large multipolar, (2) medium size multipolar, (3) small multipolar, (4) fusiform. The cells of lamina B are considered separately below. Microscopic examination of the LGN revealed that although all cells did not contain HRP granules, some cells of each category identified were included among the granule-containing cell population. These cells were restricted to the laminar portion of the ipsilateral dorsal LGN. The evidence based on HRP studies indicates that the geniculo-cortical projections are topographically organized. In animals with HRP injections placed rostrally in area 17, HRP granule-containing cells were located in rostral regions of the LGN, while for animals receiving HRP injections in caudal portions of area 17, HRP granule-containing cells were located caudally in the LGN. This is consistent with the observations of Garey and Powell 1 and Niimi and Sprague 8. Following injection of HRP in small loci of area 17, HRP labeled cells were restricted to a narrow column within the laminar portion of the LGN (Fig. 2). Within this column the great majority of the cells were labeled; however, labeled and unlabeled cells were freely intermixed. Beyond the limits of the column, no labeled cells could be found. The following is a summary, according to recognized cell class, of HRP granulecontaining cells which were observed, measured, and classified in the LGN, pars dorsalis. In laminae A and A1 50-55 ~o of the granule-containing cells were medium size cells (Fig. 4), 20-25 ~ were large cells (Fig. 3), and 20-25 ~ were small cells (Fig. 5). The HRP granule-containing cells observed in lamina B corresponded to 3 of the 4 cell types described by Guillerya: (1) fusiform; (2) medium size multipolar; (3) small multipolar. More than half of these cells were fusiform; the rest were multipolar. In the region of lamina B immediately adjacent to the optic tract, corresponding to lamina C2 (ref. 3), about 80-85 ~o of the cells containing HRP were fusiform cells (diameter 4-11 #m, Fig. 6B). The remaining 15-20~ of the HRP labeled cells in this region were medium size and small multipolar cells (Fig. 6A). In the remainder of lamina B approximately half of the HRP granule-containing cells appeared to be fusiform and half were multipolar. Approximately two-thirds of the total number of granule-containing multipolar cells were medium size; one-third were small. No large multipolar cells containing HRP granules were observed. Of the previously described cell populations in the LGN (pars dorsalis), four were identified in this study as contributing axons to the geniculo-striate projection.
||
~F
|
J~
J
C~
655 These correspond to the so-called long axon cells identified in previous investigations of the L G N in the cat 2,9,11 as (1) large or class 1 cells, (2) medium size or class 2 cells, (3) small cells, (4) fusiform or class 4 cells. Some studies have indicated that in addition to the above 4 cell classes, the L G N contains a population of intrinsic neurons or class 3 cells 2,9,11 (Table I), which are small diameter cells. The observations presented in this report raise questions regarding (1) the functional significance of morphologically determined cell classes, (2) the issue interneurons in the LGN. The present investigation indicates that in laminae A and A1, large, medium, and small cells send direct projections to the calcarine cortex. Although it seems unlikely, this does not exclude the possibility that neurons of similar morphology may not have such projections. The studies of Garey and Powell 1, using retrograde degeneration o f L G N cells, indicated that following lesions restricted to area 17 of the cortex rto change could be observed in the large cell population, although small and medium size cells appeared shrunken. Following lesions of area 18 alone, minor changes were noted in the large cells. Simultaneous ablation of area 17 and 18, however, resulted in the shrinkage of the large cells as well as the small and medium size cells. The results of Garey and Powell 1 seem to indicate that there may indeed be a functional significance or a division of labor among the morphologically determined cell classes. Further study is required, of the contribution of the cell populations of the L G N to other visual cortices before one can reach any conclusions regarding the functional significance of morphologically determined cell classes. Regarding the issue of interneurons in the LGN, morphological evidence for their existence has been derived from the Golgi studies of Guillery 2, Tello 11, and O'Leary 9. This cell has characteristically been described as a small diameter, multipolar neuron whose axon ramifies completely within the LGN. The present study confirms the existence of the small diameter, long axon cell reported by Tello 11, but not observed by Guillery 2, and demonstrates its direct projection to cortical area 17. This allows for two possibilities: either (a) two populations of small cells are present in the L G N ; one population with cortical projections as demonstrated in the present investigation, and another population of intrageniculate or intrinsic neurons as demonstrated by others noted above; or (b) only one population of small diameter cells resides in the LGN. This single population of small, multipolar cells projects its axons directly to the cortex and there exist then no true interneurons in the LGN. Resolution of this issue cannot be made on the basis of small focal injections of HRP.
Fig. 3. Large HRP granule-containing cells observed in laminae A and A1 of the LGN (pars dorsalis). Fig. 4. Medium size, HRP granule-containing cells observed in laminae A and A1. Note the presence of granules in the soma, primary dendrites, and secondary dendrites (arrow). Fig. 5. Small HRP granule-containing cells observed in laminae A and A1 of the LGN (pars dorsalis). Note the extension of granules into secondary dendrite (5B) as indicated by the arrow. Fig. 6. HRP granule-containing cells observed in lamina B of the LGN (pars dorsalis). A: a typical small multipolar neuron. B: a typical fusiform cell. This cell was situated along the ventral border of lamina B, adjacent to the optic tract. This is the region designated as lamina Cz by Guillery8.
656 F i b e r connections o f each o f the 4 cell types described in l a m i n a B by Guillery :~ have n o t been previously d e m o n s t r a t e d . Present o b s e r v a t i o n s indicate t h a t the first three o f these cell types have axons which project to the p r i m a r y visual cortex. Until m o r e d a t a are available, the absence o f H R P granules in the large m u l t i p o l a r n e u r o n s o f l a m i n a B in this study should n o t be i n t e r p r e t e d to indicate an absence o f direct c o n n e c t i o n s between these cells a n d the p r i m a r y visual cortex. In s u m m a r y , the present investigation d e m o n s t r a t e s that o f the 5 m o r p h o l o g i c a l l y defined cell classes in the L G N o f the cat, 4 are c o m p o s e d o f cells with long axons which p r o j e c t directly a n d t o p o g r a p h i c a l l y to the striate cortex. The a u t h o r wishes to t h a n k Mrs. A d a r s h A r o r a for her excellent technical assistance, Messers. M. Levine, D. K r a m e r a n d P. Christ for the illustrations, and Dr. C. R. N o b a c k for his critique o f the m a n u s c r i p t . This w o r k was s u p p o r t e d by N I H G r a n t 5R01 EY11074.
1 GAREY, L. J., AND POWELL, T. P. S., The projection of the lateral geniculate nucleus upon the cortex in the cat, Proc. roy. Soc. B, 169 (1967) 107-126. 2 GUILLERY,R. W., A study of Golgi preparations from the dorsal lateral geniculate nucleus of the adult cat, J. comp. Neurol., 128 (1966) 21-50. 3 GUILLERY,R. W., The laminar distribution of retinal fibers in the dorsal lateral geniculate nucleus of the cat: a new interpretation, J. comp. Neurol., 138 (1970) 339-368.
4 FIAYHOW,W. R., The cytoarehitecture of the lateral geniculate body in the cat in relation to the distribution of crossed and uncrossed optic fibers, J. comp. Neurol., 110 (1958) 1-64. 5 LAVAIL,J. H., AND LAVAIL, M. M., Retrograde axonai transport in the central nervous system, Science, 176 (1972) 1416-1417. 6 LAVAIL,J. H., WINSTON,K. R., AND TISH, A., A method based on retrograde intraaxonal transport of protein for identification of cell bodies of origin of axons terminating within the CNS, Brain Research, 58 (1973) 470--477. 7 NAUTA, H. J. W., PRITZ, M. B., AND LASEK, R. J., Afferents to the rat caudoputamen studied with horseradish peroxidase. An evaluation of a retrograde neuroanatomical research method, Brain Research, 67 (1974) 219-238. 8 NIIMI, K., AND SPRAGUE,J. M., Thalamo-cortical organization of the visual system in the cat, J. comp. Neurol., 138 (1970) 219-249. 90'LEARY, J. L., A structural analysis of the lateral geniculate nucleus of the cat, J. comp. Neurol., 73 (1940) 405-430. 10 PETERS,A., AND PALAY,S. L., The morphology of laminae A and Ax of the dorsal nucleus of the lateral geniculate body of the cat, J. Anat. (Lond.), 100 (1966) 451-486. 11 TELLO, F., Disposicion macroscopia y estructural del cuerpo geniculado externo, Trab. Lab. Invest. Biol. Univ. Madrid, 3 (1904) 39-62. 12 THUMA,B. D., Studies on the diencephalon of the cat. 1. Thecytoarchitecture of thecorpus geniculatum laterale, J. co~p. Neurol., 46 0928) 193-199.
