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Axon initial segments of the granule cell in the rat dentate gyrus: synaptic contacts on bundles of axon initial segments
Brain Research, 274 (1983) 129-134 Elsevier
129
Short Communications
Axon initial segments of the granule cell in the rat dentate gyrus: synaptic contacts on bundles of axon initial segments TOSHIO KOSAKA National Institute for Physiological Sciences, Myodaiji, Okazaki 444 (Japan)
(Accepted May 3rd, 1983) Key words: initial segment - - synapse - - granule cell - - axo-axonic cell - - chandelier cell - - basket cell - - dentate gyrus
In Golgi and electron microscopic studies, dentate granule cell axon initial segments were revealed to be fasciculated in the granule cell layer and to receive many synapses. Combined Golgi-EM studies indicated that the presynaptic elements of these synapses might be beaded string-like axon collaterals of some type of basket cell. Inhibitory synapses are usually assumed to be on neuronal somata and proximal dendrites strategically located for output control; this arrangement provides an excellent example of input segregation 2. Recently, however, several morphological investigations have revealed that a significant number of presumably inhibitory synapses are located on the initial segment (IS) of some types of neurons 5.6.8.13.15. Since the IS is regarded as the spike initiation site, such a location may be strategically more important than the soma or dendrites. In addition, a specialized type of interneuron, the chandelier cell or axo-axonic cell, is reported in some cortical areas to make synaptic contacts exclusively on pyramidal cell IS's 4,11,14-16. These observations indicate that the IS as well as the soma is as important as the site of output control in some types of neurons. This preliminary paper presents evidence that granule cell IS's are fasciculated in particular slender columns in the granule cell layer (GCL) of the rat dentate gyrus, and have many synaptic contacts on them from a type of basket cell. The animals and histological procedures used here have been described in detail in a previous paper 7. Briefly, the hippocampus was dissected out after aldehyde fixation. Two or 3 transverse slices, 0.3--0.5 mm thick, were cut from the middle portion of the hippocampus, and processed for electron microscopy 0006-8993/83/$03.00 ~) 1983 Elsevier Science Publishers B.V.
(EM). The dorsal and ventral halves of the remaining hippocampus were processed according to the rapid Golgi method. The dentate gyrus, mainly its infrapyramidal blade, was examined in the same materials used in the previous observations of CA1 and CA3 regions 7. The granule cell layer (GCL) of the dentate gyrus is about 4-7 cells deep, and about 60-80/~m thick at the middle portion of the infrapyramidal blade (Figs. 1 and 2). In Golgi materials, most granule cell axons arose from the lower pole of their somata and extended through the G C L to the polymorph layer (PL). They usually took a more or less straight course perpendicular to the G C L (Fig. 1A), although axons of the granule cells located near the stratum oriens of the CA3 region extended obliquely (Fig. 1C). In general, axons arising from neighboring granule cells extended almost parallel to each other in the GCL, and two or more axons arising from granule cells located adjacent to one another but at different levels in the G C L occasionally appeared to run in a fascicle (Fig. 1C). Several spines were frequently seen along granule cell axons in the G C L (Fig. 1A, B). Most of them were small thorn-like spines; others were pedunculated spines or rounded bulges or irregular complex protrusions. Small spines were also seen on granule cell somata.
131 In thin sections perpendicular to the G C L , 5-7 rows of round or oval profiles of granule cell somata, about 10 p m in diameter, were compactly packed (Fig. 2A). As described previously 9, the somata of the deep cells were more tightly packed than the superficial cells. A m o n g these packed granule cell somata, particular columnal areas, usually less than 5 um in width and 10-30 p m or more apart from each other, were frequently seen (Fig. 2A, B). They were parallel to each other, almost perpendicular to the GCL, and were occassionaUy seen to merge into the PL, looking like vertical striae in the G C L extending from the PL. In these columnal areas, several profiles of longitudinally or obliquely sectioned ISs, as well as many profiles of synaptic boutons and some of dendrites and myelinated fibers, were usually encountered (Fig. 2B). ISs were identified by their ultrastructural characteristics such as fascicles of microtubules, membrane undercoating and clusters of free ribosomes (Figs. 2C and 3) 10. Cisternal organelles were also frequently observed in ISs in the GCL. Almost all ISs in these columnal areas showed similar structural features and several were observed to originate from granule cell somata. Spine-like protrusions were occasionally seen to arise from these IS shafts (Fig. 2C). Many synaptic boutons with flattened or pleomorphic vesicles impinged upon these ISs and their protrusions, alone or in clusters, making symmetrical synapses (Figs. 2B, C and 3). In the PL beneath the G C L , many profiles of ISs running parallel or obliquely to the G C L were encountered, some of which were continuous with those in the GCL. Most of ISs in the PL also received symmetrical synapses.
In thin sections parallel to the G C L , small polygonal areas were scattered among tightly packed round or polygonal profiles of granule cell somata (Fig. 4A). In these areas cross-sectioned profiles of ISs, about 1 p m in diameter, were usually encountered. These polygonal areas were assumed to be cross-sectioned columnal areas described above. The number of IS profiles in a given polygonal area was about 2-3 in sections through the middle level of the G C L (Fig. 4A, B), and increased to about 10 in sections through the lower boundary of the G C L (Fig. 4C). Taking the results of the Golgi studies above into consideration, ISs in a column were assumed to arise from granule cells adjacent to this column throughout the GCL. Thus it is concluded that in the G C L granule cell ISs were fasciculated in small vertical columns, where they received many synapses of symmetrical type. What is the origin of the presynaptic elements making symmetrical synapses upon granule cell ISs? In Golgi material, groups of thin axonal collaterals with varicosities were occassionally observed to run parallel to each other, and perforate almost perpendicularly through the G C L (Figs. 5 and 6A), which closely resemble axon collaterals of Amaral's type 1 basket cell depicted as 'a curtain of beaded strings interspersed among the granule cells '1 (see also Fig. 1 in ref. 17). These beaded string-like axon collaterals ran alone (Fig. 6A) or in bundles of two or more intermingling collaterals (Fig. 5). Some appeared to terminate in the G C L , while others appeared to extend further to the PL. In several favorable cases, some of these collaterals were observed to cling to impregnated granule cell ISs (Fig. 5). In combined Golgi-EM studies, these impregnated beaded string-
Fig. 1. Light micrographs of Golgi-impregnated granule cells; 100/~m thick sections. Scale bars = 10pm. A: axon arises from the lower pole of a granule cell soma, runs perpendicularly through the GCL to the PL, where it bends acutely. Several thorn-like spines as well as a pedunculated spine (arrow) and a rounded bulge (arrowhead) are seen along this axon in the GCL. B: complex protrusions (arrows) on a granule cell axon. Arrowhead indicates another granule cell axon partially overlapping this axon. C: two axons originating from two adjacent granule cells located at different levels appear to twist around each other, Fig. 2. Electron micrographs of the GCL sectioned perpendicularly. A: low magnification electron micrograph showing compactly packed granule cell somata and a particular columnal area (outlined by the rectangle) penetrating among them, which is shown in B at higher magnification. ML, molecular layer. Scale bar = 10 ~m. B: vertical column among tightly packed granule cell somata. Profiles of ISs (asterisks) as well as those of dendrites (D) and synaptic boutons are seen in this columnal area. Two adjacent ISs (arrows) are shown in C at higher maginification. Scale bar = 5/~m. C: two obliquely sectioned ISs apposed closely with some kind of junction looking like an intermediate junction (arrow). A sessile spine with a spine apparatus (sa) arises from one of ISs, whose shaft also displays a cisternal organelle (co). Several synaptic boutons with flattened or pleomorphic vesicles impinge upon these ISs and make symmetrical synapses. Scale bar = 1/~m. Fig. 3. Longitudinally sectioned IS in the GCL, showing several synaptic boutons impinging upon it in a cluster. Scale bar = 1pm.
133 like axon collaterals were located in the particular vertical columns among granule cell somata (Fig.
exclusively on ISs, as do chandelier or axo-axonic cell
6B), where they frequently impinged u p o n granule
present observations indicate that so-called basket cells - - w h i c h have b e e n classified into two to several types based on Golgi studiesl,3,12.t7 but are tradition-
cell ISs and made synaptic contacts with the latter (Fig. 6B, C). Thus these collaterals, presumably arising from some type(s) of basket cell, may be the presynaptic elements of synapses on granule cell ISs. As the functional significance of synaptic inputs into ISs, especially from the recently discovered axoaxonic or chandelier cells, have been fully discussed in previous papers 4-6, 8,1h13-16 it may suffice here to
axon collaterals (or terminals). F u r t h e r m o r e , the
ally considered to make synapses onto granule cell somata - - should be redefined and reclassified, taking the location of their output into consideration. Third, the density of somatic synapses was reported to be different between superficial and deep granule cells9, which also have ISs with different lengths.
discuss some points specific to dentate granule cells. First, in the G C L many synaptic boutons make syn-
Quantitative analysis on the n u m b e r and density of synapses on ISs as well as somata of superficial and
apses onto several ISs originating from adjacent granule cells in a slender vertical column among
deep granule cells may be necessary to characterize them and suggest their functional heterogeneity.
granule cell somata, which thus may be considered as a specialized synaptic field. Second, beaded stringlike axon collaterals of some type(s) of basket cells were recognized to make synapses onto granule cell
able discussions, Dr. E. E. Daniel for his critical reading of the manuscript, Mr. Y. Hataguchi for his
ISs in G C L vertical columns. However, it remains to be determined whether or not they make synapses
technical assistance and Miss M. Kinoshita for her secretarial assistance.
1 Amaral, D. G., A Golgi study of cell types in the hilar region of the hippocampus in the rat, J. comp. Neurol., 182 (1978) 851-914. 2 Bullock, T. H., Introduction to Nervous System, Freeman, San Francisco, 1977. 3 Cajal, S. R., The Structure of Ammon's Horn (translated by L. M. Kraft), Charles C. Thomas, Springfield, IL, 1967. 4 Fair6n, A. and Valverde, F., A specialized type of neuron in the visual cortex of cat: A Golgi and electron microscope study of chandelier cells, J. comp. Neurol., 194 (1980) 761-779. 5 Kosaka, T.. Ruffed cell: a new type of neuron with a distinctive initial unmyelinated portion of the axon in the olfactory bulb of the goldfish (Carassius auratus). II. Fine
structure of the ruffed cell, J. comp. Neurol., 193 (1980) 11%145. Kosaka, T., The axon initial segment as a synaptic site: ultrastructure and synaptology of the initial segment of the pyramidal cell in the rat hippocampus (CA3 region), J. Neurocytol., 9 (1980) 861-882. Kosaka, T., Gap junctions between nonpyramidal cell dendrites in the rat hippocampus (CA1 and CA3 regions), Brain Research, in press. Kosaka, T. and Hama, K., Pre- and post-synaptic character of the axon initial segment of the mitral cell of the goldfish olfactory bulb, Brain Research, 169 (1979) 570--574. Lee, K. S., Gerbrandt, L. and Lynch, G., Axo-somatic synapses in the normal and X-irradiated dentate gyrus; factors
The author wishes to thank Dr. K. H a m a for valu-
6
7 8 9
Fig. 4. Electron micrographs of the GCL sectioned parallel to it. Asterisks indicate ISs. A: the GCL sectioned through its middle level. Small polygonal areas are scattered among tightly packed granule cell somata, where ISs are located. One of these polygonal areas (outlined by the rectangle) is shown in B at higher magnification. Scale bar = 5/~m. B: two cross-sectioned ISs are seen in a polygonal area surrounded by several granule cel somata. One of ISs is impinged upon by two synaptic boutons. Scale bar --- 1/~m. C: a polygonal area surrounded by several granule cell somata in a section through the lower boundary of the GCL. Note many cross-sectioned ISs as well as one longitudinally sectioned IS packed in this area, where they are impinged upon by synaptic boutons and receive synapses. Scale bar = 1~um. Fig. 5. Photomontage of Golgi-impregnated axon collaterals with varicosities in the GCL, appearing to run parallel to each other and perforate through the GCL. Some appear to run alone, while others in bundles. Arrows indicate one of these varicose collaterals coiling around an impregnated granule cell axon. Scale bar = 10/~m. Fig. 6. Combined Golgi-EM study of a group of varicose axon collaterals in the GCL, running perpendicularly to the GCL. A: light micrograph of beaded string-like collaterals in the GCL. Scale bar = 10/~m. B and C: electron micrographs of thin sectioned axon collaterals shown in A. B shows that several impregnated profiles (arrows) are located in a vertical column among granule cell somata and closely appose to ISs (asterisks). C shows one of impregnated boutons impinging upon an IS and making a synapse with it. Unimpregnated synaptic bouton (asterisk) also makes synapse on this IS. Scale bars = 1/~m.
134
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13
affecting the density of afferent innervation, Brain Research. 249 (1982) 51-56. Palay, S. L., Sotelo, C,, Peters, A. and Orkand, P. M., The axon hillock and the initial segment, J. Cell. Biol., 38 (1968) 193-201. Peters, A.. Proskauer, C. C. and Ribak, C. E., Chandelier cells in rat visual cortex. J. comp. Neurol.. 206 (1982) 397-416. Seress. L. and PokornS~, J., Structure of the granule layer of the rat dentate gyrus. A light microscopic and Golgi study, J. Anat., 133 (1981) 181-195, Sloper, J. J. and Powell, T. P. S., A study of the axon initial segment and proximal axon of neurons in the primate motor and somatic sensory cortices, Phil. Trans. B. 285 (1979) 173-197.
14 Somogyi, P., A specific "axo-axonal" interneuron in the visual cortex of the rat, Brain Research, 136 (1977) 345-350. 15 Somogyi, P., Freund, T. F. and Cowey, A., The axo-axonic interneuron in the cerebral cortex of the rat, cat and monkey, Neuroscience, 7 (1982) 2577-2607. 16 Somogyi, P., Nunzi, M. G., Gorio, A. and Smith, A. D., A new type of specific interneuron in the monkey hippocampus forming synapses exclusively with the axon initial segments of pyramidal cells, Brain Research. 259 (t983) 137-142. 17 Struble, R. G., Desmond, N. L. and Levy, W. B., Anatomical evidence for interlamellar inhibition in the fascia dentata, Brain Research, 152 (1978) 580-585.
129
Short Communications
Axon initial segments of the granule cell in the rat dentate gyrus: synaptic contacts on bundles of axon initial segments TOSHIO KOSAKA National Institute for Physiological Sciences, Myodaiji, Okazaki 444 (Japan)
(Accepted May 3rd, 1983) Key words: initial segment - - synapse - - granule cell - - axo-axonic cell - - chandelier cell - - basket cell - - dentate gyrus
In Golgi and electron microscopic studies, dentate granule cell axon initial segments were revealed to be fasciculated in the granule cell layer and to receive many synapses. Combined Golgi-EM studies indicated that the presynaptic elements of these synapses might be beaded string-like axon collaterals of some type of basket cell. Inhibitory synapses are usually assumed to be on neuronal somata and proximal dendrites strategically located for output control; this arrangement provides an excellent example of input segregation 2. Recently, however, several morphological investigations have revealed that a significant number of presumably inhibitory synapses are located on the initial segment (IS) of some types of neurons 5.6.8.13.15. Since the IS is regarded as the spike initiation site, such a location may be strategically more important than the soma or dendrites. In addition, a specialized type of interneuron, the chandelier cell or axo-axonic cell, is reported in some cortical areas to make synaptic contacts exclusively on pyramidal cell IS's 4,11,14-16. These observations indicate that the IS as well as the soma is as important as the site of output control in some types of neurons. This preliminary paper presents evidence that granule cell IS's are fasciculated in particular slender columns in the granule cell layer (GCL) of the rat dentate gyrus, and have many synaptic contacts on them from a type of basket cell. The animals and histological procedures used here have been described in detail in a previous paper 7. Briefly, the hippocampus was dissected out after aldehyde fixation. Two or 3 transverse slices, 0.3--0.5 mm thick, were cut from the middle portion of the hippocampus, and processed for electron microscopy 0006-8993/83/$03.00 ~) 1983 Elsevier Science Publishers B.V.
(EM). The dorsal and ventral halves of the remaining hippocampus were processed according to the rapid Golgi method. The dentate gyrus, mainly its infrapyramidal blade, was examined in the same materials used in the previous observations of CA1 and CA3 regions 7. The granule cell layer (GCL) of the dentate gyrus is about 4-7 cells deep, and about 60-80/~m thick at the middle portion of the infrapyramidal blade (Figs. 1 and 2). In Golgi materials, most granule cell axons arose from the lower pole of their somata and extended through the G C L to the polymorph layer (PL). They usually took a more or less straight course perpendicular to the G C L (Fig. 1A), although axons of the granule cells located near the stratum oriens of the CA3 region extended obliquely (Fig. 1C). In general, axons arising from neighboring granule cells extended almost parallel to each other in the GCL, and two or more axons arising from granule cells located adjacent to one another but at different levels in the G C L occasionally appeared to run in a fascicle (Fig. 1C). Several spines were frequently seen along granule cell axons in the G C L (Fig. 1A, B). Most of them were small thorn-like spines; others were pedunculated spines or rounded bulges or irregular complex protrusions. Small spines were also seen on granule cell somata.
131 In thin sections perpendicular to the G C L , 5-7 rows of round or oval profiles of granule cell somata, about 10 p m in diameter, were compactly packed (Fig. 2A). As described previously 9, the somata of the deep cells were more tightly packed than the superficial cells. A m o n g these packed granule cell somata, particular columnal areas, usually less than 5 um in width and 10-30 p m or more apart from each other, were frequently seen (Fig. 2A, B). They were parallel to each other, almost perpendicular to the GCL, and were occassionaUy seen to merge into the PL, looking like vertical striae in the G C L extending from the PL. In these columnal areas, several profiles of longitudinally or obliquely sectioned ISs, as well as many profiles of synaptic boutons and some of dendrites and myelinated fibers, were usually encountered (Fig. 2B). ISs were identified by their ultrastructural characteristics such as fascicles of microtubules, membrane undercoating and clusters of free ribosomes (Figs. 2C and 3) 10. Cisternal organelles were also frequently observed in ISs in the GCL. Almost all ISs in these columnal areas showed similar structural features and several were observed to originate from granule cell somata. Spine-like protrusions were occasionally seen to arise from these IS shafts (Fig. 2C). Many synaptic boutons with flattened or pleomorphic vesicles impinged upon these ISs and their protrusions, alone or in clusters, making symmetrical synapses (Figs. 2B, C and 3). In the PL beneath the G C L , many profiles of ISs running parallel or obliquely to the G C L were encountered, some of which were continuous with those in the GCL. Most of ISs in the PL also received symmetrical synapses.
In thin sections parallel to the G C L , small polygonal areas were scattered among tightly packed round or polygonal profiles of granule cell somata (Fig. 4A). In these areas cross-sectioned profiles of ISs, about 1 p m in diameter, were usually encountered. These polygonal areas were assumed to be cross-sectioned columnal areas described above. The number of IS profiles in a given polygonal area was about 2-3 in sections through the middle level of the G C L (Fig. 4A, B), and increased to about 10 in sections through the lower boundary of the G C L (Fig. 4C). Taking the results of the Golgi studies above into consideration, ISs in a column were assumed to arise from granule cells adjacent to this column throughout the GCL. Thus it is concluded that in the G C L granule cell ISs were fasciculated in small vertical columns, where they received many synapses of symmetrical type. What is the origin of the presynaptic elements making symmetrical synapses upon granule cell ISs? In Golgi material, groups of thin axonal collaterals with varicosities were occassionally observed to run parallel to each other, and perforate almost perpendicularly through the G C L (Figs. 5 and 6A), which closely resemble axon collaterals of Amaral's type 1 basket cell depicted as 'a curtain of beaded strings interspersed among the granule cells '1 (see also Fig. 1 in ref. 17). These beaded string-like axon collaterals ran alone (Fig. 6A) or in bundles of two or more intermingling collaterals (Fig. 5). Some appeared to terminate in the G C L , while others appeared to extend further to the PL. In several favorable cases, some of these collaterals were observed to cling to impregnated granule cell ISs (Fig. 5). In combined Golgi-EM studies, these impregnated beaded string-
Fig. 1. Light micrographs of Golgi-impregnated granule cells; 100/~m thick sections. Scale bars = 10pm. A: axon arises from the lower pole of a granule cell soma, runs perpendicularly through the GCL to the PL, where it bends acutely. Several thorn-like spines as well as a pedunculated spine (arrow) and a rounded bulge (arrowhead) are seen along this axon in the GCL. B: complex protrusions (arrows) on a granule cell axon. Arrowhead indicates another granule cell axon partially overlapping this axon. C: two axons originating from two adjacent granule cells located at different levels appear to twist around each other, Fig. 2. Electron micrographs of the GCL sectioned perpendicularly. A: low magnification electron micrograph showing compactly packed granule cell somata and a particular columnal area (outlined by the rectangle) penetrating among them, which is shown in B at higher magnification. ML, molecular layer. Scale bar = 10 ~m. B: vertical column among tightly packed granule cell somata. Profiles of ISs (asterisks) as well as those of dendrites (D) and synaptic boutons are seen in this columnal area. Two adjacent ISs (arrows) are shown in C at higher maginification. Scale bar = 5/~m. C: two obliquely sectioned ISs apposed closely with some kind of junction looking like an intermediate junction (arrow). A sessile spine with a spine apparatus (sa) arises from one of ISs, whose shaft also displays a cisternal organelle (co). Several synaptic boutons with flattened or pleomorphic vesicles impinge upon these ISs and make symmetrical synapses. Scale bar = 1/~m. Fig. 3. Longitudinally sectioned IS in the GCL, showing several synaptic boutons impinging upon it in a cluster. Scale bar = 1pm.
133 like axon collaterals were located in the particular vertical columns among granule cell somata (Fig.
exclusively on ISs, as do chandelier or axo-axonic cell
6B), where they frequently impinged u p o n granule
present observations indicate that so-called basket cells - - w h i c h have b e e n classified into two to several types based on Golgi studiesl,3,12.t7 but are tradition-
cell ISs and made synaptic contacts with the latter (Fig. 6B, C). Thus these collaterals, presumably arising from some type(s) of basket cell, may be the presynaptic elements of synapses on granule cell ISs. As the functional significance of synaptic inputs into ISs, especially from the recently discovered axoaxonic or chandelier cells, have been fully discussed in previous papers 4-6, 8,1h13-16 it may suffice here to
axon collaterals (or terminals). F u r t h e r m o r e , the
ally considered to make synapses onto granule cell somata - - should be redefined and reclassified, taking the location of their output into consideration. Third, the density of somatic synapses was reported to be different between superficial and deep granule cells9, which also have ISs with different lengths.
discuss some points specific to dentate granule cells. First, in the G C L many synaptic boutons make syn-
Quantitative analysis on the n u m b e r and density of synapses on ISs as well as somata of superficial and
apses onto several ISs originating from adjacent granule cells in a slender vertical column among
deep granule cells may be necessary to characterize them and suggest their functional heterogeneity.
granule cell somata, which thus may be considered as a specialized synaptic field. Second, beaded stringlike axon collaterals of some type(s) of basket cells were recognized to make synapses onto granule cell
able discussions, Dr. E. E. Daniel for his critical reading of the manuscript, Mr. Y. Hataguchi for his
ISs in G C L vertical columns. However, it remains to be determined whether or not they make synapses
technical assistance and Miss M. Kinoshita for her secretarial assistance.
1 Amaral, D. G., A Golgi study of cell types in the hilar region of the hippocampus in the rat, J. comp. Neurol., 182 (1978) 851-914. 2 Bullock, T. H., Introduction to Nervous System, Freeman, San Francisco, 1977. 3 Cajal, S. R., The Structure of Ammon's Horn (translated by L. M. Kraft), Charles C. Thomas, Springfield, IL, 1967. 4 Fair6n, A. and Valverde, F., A specialized type of neuron in the visual cortex of cat: A Golgi and electron microscope study of chandelier cells, J. comp. Neurol., 194 (1980) 761-779. 5 Kosaka, T.. Ruffed cell: a new type of neuron with a distinctive initial unmyelinated portion of the axon in the olfactory bulb of the goldfish (Carassius auratus). II. Fine
structure of the ruffed cell, J. comp. Neurol., 193 (1980) 11%145. Kosaka, T., The axon initial segment as a synaptic site: ultrastructure and synaptology of the initial segment of the pyramidal cell in the rat hippocampus (CA3 region), J. Neurocytol., 9 (1980) 861-882. Kosaka, T., Gap junctions between nonpyramidal cell dendrites in the rat hippocampus (CA1 and CA3 regions), Brain Research, in press. Kosaka, T. and Hama, K., Pre- and post-synaptic character of the axon initial segment of the mitral cell of the goldfish olfactory bulb, Brain Research, 169 (1979) 570--574. Lee, K. S., Gerbrandt, L. and Lynch, G., Axo-somatic synapses in the normal and X-irradiated dentate gyrus; factors
The author wishes to thank Dr. K. H a m a for valu-
6
7 8 9
Fig. 4. Electron micrographs of the GCL sectioned parallel to it. Asterisks indicate ISs. A: the GCL sectioned through its middle level. Small polygonal areas are scattered among tightly packed granule cell somata, where ISs are located. One of these polygonal areas (outlined by the rectangle) is shown in B at higher magnification. Scale bar = 5/~m. B: two cross-sectioned ISs are seen in a polygonal area surrounded by several granule cel somata. One of ISs is impinged upon by two synaptic boutons. Scale bar --- 1/~m. C: a polygonal area surrounded by several granule cell somata in a section through the lower boundary of the GCL. Note many cross-sectioned ISs as well as one longitudinally sectioned IS packed in this area, where they are impinged upon by synaptic boutons and receive synapses. Scale bar = 1~um. Fig. 5. Photomontage of Golgi-impregnated axon collaterals with varicosities in the GCL, appearing to run parallel to each other and perforate through the GCL. Some appear to run alone, while others in bundles. Arrows indicate one of these varicose collaterals coiling around an impregnated granule cell axon. Scale bar = 10/~m. Fig. 6. Combined Golgi-EM study of a group of varicose axon collaterals in the GCL, running perpendicularly to the GCL. A: light micrograph of beaded string-like collaterals in the GCL. Scale bar = 10/~m. B and C: electron micrographs of thin sectioned axon collaterals shown in A. B shows that several impregnated profiles (arrows) are located in a vertical column among granule cell somata and closely appose to ISs (asterisks). C shows one of impregnated boutons impinging upon an IS and making a synapse with it. Unimpregnated synaptic bouton (asterisk) also makes synapse on this IS. Scale bars = 1/~m.
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13
affecting the density of afferent innervation, Brain Research. 249 (1982) 51-56. Palay, S. L., Sotelo, C,, Peters, A. and Orkand, P. M., The axon hillock and the initial segment, J. Cell. Biol., 38 (1968) 193-201. Peters, A.. Proskauer, C. C. and Ribak, C. E., Chandelier cells in rat visual cortex. J. comp. Neurol.. 206 (1982) 397-416. Seress. L. and PokornS~, J., Structure of the granule layer of the rat dentate gyrus. A light microscopic and Golgi study, J. Anat., 133 (1981) 181-195, Sloper, J. J. and Powell, T. P. S., A study of the axon initial segment and proximal axon of neurons in the primate motor and somatic sensory cortices, Phil. Trans. B. 285 (1979) 173-197.
14 Somogyi, P., A specific "axo-axonal" interneuron in the visual cortex of the rat, Brain Research, 136 (1977) 345-350. 15 Somogyi, P., Freund, T. F. and Cowey, A., The axo-axonic interneuron in the cerebral cortex of the rat, cat and monkey, Neuroscience, 7 (1982) 2577-2607. 16 Somogyi, P., Nunzi, M. G., Gorio, A. and Smith, A. D., A new type of specific interneuron in the monkey hippocampus forming synapses exclusively with the axon initial segments of pyramidal cells, Brain Research. 259 (t983) 137-142. 17 Struble, R. G., Desmond, N. L. and Levy, W. B., Anatomical evidence for interlamellar inhibition in the fascia dentata, Brain Research, 152 (1978) 580-585.