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Recent electron microscope observations on the ultrastructure of the crayfish median-to-motor giant synapse
Experimental
226
BRIEF RECENT ELECTRON ULTRASTRUCTURE
Cell Research, 8, 226-229 (1955)
NOTES
MICROSCOPE OBSERVATIONS ON THE OF THE CRAYFISH MEDIAN-TO-MOTOR GIANT SYNAPSE l J. D. ROBERTSON
Department
of Pathology and Oncology, University Kansas
City, Kansas,
of Kansas Medical School,
U.S.A.
Received August 5, 1954
CEIWAINbasic features of the crayfish median-to-motor giant synapse were described previously (6). This synapse is formed in each abdominal ganglion by the side-to-side apposition of two giant axons. It is physiologically polarized since impulses pass only from the median giant fibers to the third motor root giant fibers forming the synapse. The motor fibers extend numerous pseudopodium-like axoplasmic processes through the combined sheaths of the synapsing giant fibers. These processes measure up to 15 p in diameter and end in median giant Schwann cytoplasm. They are completely invested by the motor fiber axon-Schwann membrane (axolemma) which measures about 300 A in thickness by shadowing and is structurally like its median giant counterpart. Axoplasmic filaments are more concentrated in motor axoplasm than in median giant axoplasm. It is the purpose of this brief preliminary communication to present a few new observations on this synapse. Recent electron microscope observations of very thin (c. 200-300 A) sections through the giant synapse have revealed some principal new features which are listed below, numerically. 1) The axon-Schwann membranes in cross section consist of two dense edges with a light central zone of about 100-200 A thickness. The complete dense edged (“double contoured”) membranes measure about 200-300 A in overall thickness. Although previously not observed (7), we now know that the motor fiber axon-Schwann membrane, which envelopes the synaptic processes of motor axoplasm fuses with the median giant axon-Schwann membrane to form a single membrane of structure apparently identical with that of either of the fusing membranes (Fig. 1).8 The dense edges of the fusing membranes are continuous with one another as shown in r This work was supported by a research grant from the National Institutes of Health, U.S. Public Health Service, from the Kansas Division of the American Cancer Society and by an institutional research grant from the American Cancer Society. * Palade (4) and De Robertis and Bennett (2) reported, at the same time that our results were reported, observations on vertebrate and other invertebrate synapses similar in some respects to those described here. Experimental
Cell Research 8
Ultrastructure of crayfish giant synapse
227
Fig. 1. That these membranes are high in lipid content is suggested by the role which they and similar membranes seem to play in the metatropic reaction (3). If this role is correctly assigned, then the lipid molecules should be arranged transversely in the membranes probably in double layers whose exact location must await further evidence. 2) Concentrations of small (c. 0.1-0.8 p), closely aggregated, concentrically lamellated bodies have been seen in motor axoplasm, particularly in the synaptic processes (Figs. l-5). These bodies sometimes assume unusual branched shapes (Fig. 3), and the concentric lamellae of which they are composed appear, in some preparations, to consist of dense edged membranes resembling the axon-Schwann membranes (Fig. 5-6). Similar bodies appear in axoplasm in various crayfish nerve fibers and have been observed in the fibers of the clrcumesophageal commissures away from synapses (Fig. 6). Therefore, they are not peculiar to the synapse but merely are concentrated in great numbers in the motor axoplasm of the synaptic processes. These bodies resemble to some extent those observed by Beams and Tahmisian (1) in snail spermatozoa and by Rhodin (5) in renal epithelial cells. In the absence of specific chemical evidence these bodies cannot yet be positively identified with known cytoplasmic inclusions such as mitochondria. 3) The axoplasm of the synaptic processes is also remarkable for numerous small vesicular bodies measuring from about 250-1000 A in diameter. These are frequently attached to a tenuous filamentous tail around 100 A in diameter and of indefinite length (Fig. 3). Similar vesicles are seen in median giant axoplasm and in other crayfish nerve fibers far removed from synaptic regions. However, they, like axoplasmic filaments, are greatly concentrated in the synaptic processes. Indeed, the structural relations suggest that these vesicles may represent localized swellings of axoplasmic filaments. Palade (4) and De Robertis and Bennett (2) have observed somewhat similar axoplasmic vesicles at vertebrate and other invertebrate synapses but they found them concentrated in presynaptic rather than postsynaptic axoplasm as we have observed. Palade (4) and the author (8) found similar vesicles concentrated in the terminal axoplasm at vertebrate myoneural junctions. However, in none of these cases has a direct association with axoplasmic filaments been noted as yet. 4) Although space does not permit its demonstration here, the synaptic processes of motor axoplasm appear to be completely surrounded at all times by a layer of Schwann cytoplasm except for the regions of fusion described above. Thus it appears that motor fiber Schwann cytoplasm fuses with median giant Schwann cytoplasm at the synapse. The relations of the outer Schwann cell membranes are not yet clear. 5) The motor fibers synapse with one another during their decussation as the median-to-motor giant synapse begins (Fig. 2). In this case all of the above synaptic features are identical. The lamellated bodies again accumulate in great numbers in the axoplasm of the processes. Although it is not shown in Fig. 2, fusion of the axon-Schwann membranes occurs in the same fashion as above. In this case, however, both .synapsing giant fibers extend synaptic processes toward one another, unlike the polarized synapse with the median giant fibers in which case only the motor fibers extend such processes. It thus appears that the ability to extend axoplasmic synaptic processes is a property of the motor fibers not possessed by the median giant fibers. Experimental
Cell Research 8
228
Experimental
J. D. Robertson
Cell Resenrch 8
Ultrastructure
of crayfish giant synapse
229
The author wishes to acknowledge with gratitude the constant support and constructive criticism of Prof. F. 0. Schmitt under whose guidance this work was begun as a Ph. D. Thesis topic. REFERENCES 1. BEAMS, H. W., and TAHMISIAN, T. N., Expll. Cell Research, 6, 87 (1954). 2. DEROBERTIS, E. D. P., and BENNETT, H. S., Federation Proc., 13, 35 (1954). 3. GEREN, B. B., and SCHMITT, F. O., Proc. Natl. Aead. Sci., 40, 863 (1954). 4. PALADE, G. E., Anat. Rec., 118, 335 (1954). 5. RHODIN, J., Correlation of ultrastructural organization and function in normal
mentally changed proximal convoluted tubule cells of the mouse kidney, Godvil, Stockholm, 1954. 6. ROBERTSON, J. D., Proc. Sot. Exptl. Biol. Med., 82, 219 (1953). 7. __ Federation Proc., 13, 119 (1954). 8. -Anat. Rec., 118, 346 (1954).
and experiAktiebolaget
Fig. 1. A motor axoplasm synaptic process (Pr:) is shown in median giant Schwann cytoplasm (SC). Median giant axoplasm (MG), lies above. The motor fiber axon-Schwann membrane (a) fuses with the median giant axon-Schwann membrane (b) to produce the axon-axon synaptic membrane (c). The motor fiber axon-Schwann membrane also appears to fuse with a Schwann cell intra-cytoplasmic membrane at (d). Note the large dense bodies concentrated in the motor axoplasm of the process. These are considered to be poorly preserved concentrically lamellated bodies such as those in Figs. 3-6. The inset shows an enlargement of the dotted square. Mag. 29,000 x . Inset mag. 59,000 x . Fig. 2. A motor fiber synaptic process (Pr.) containing many dense bodies is seen lying in the Schwann cytoplasm (SC) of the adjacent motor fiber at the level of motor fiber decussation. Motor fiber axoplasm (MF) lies to the upper left. The dense bodies in the process are considered analogous to those seen in the synaptic process of Fig. 1. The motor fiber axon-Schwann membranes are not fused in this locus as they are in others. An aggregated granular component of Schwann cytoplasm is seen at (G). Mag. 8,800 x . Fig. 3. This is an enlargement of the axoplasm of a synaptic process. Note the large (c. 0.1-0.8 p) irregularly shaped bodies with concentric lamellation (C). A second smaller (250-1000 A) vesiculaz component (v) is observed. Note the delicate filamentous strands (f) with which the latter are continuous. The inset is an enlargement of the dotted area. It shows a vesicle (v) attached to a filament (f). Mag. 20,800 x . Inset mag. 60,500 x. Fig. 4. One of the concentrically lamellated bodies of a synaptic process is shown. The component membranes are poorly preserved in this case. Mag. 76,500 x . Fig. 5. One of the concentrically lamellated bodies of synaptic process axoplasm similar to that shown in Fig. 4 but from a different preparation is shown. The arrows designate the dense edges of one of its component concentric membranes. Only two such membranes are seen in this body. Mag. 170,000 x . Fig. 6. A concentrically lamellated body observed in a small nerve fiber in a circumesophageal commissure is shown. Note the resemblance of the component membranes designated by the arrows to the axon-Schwann membranes. Mag. 53,000 x . 15 - 643701
Experimental
Cell Research 8
226
BRIEF RECENT ELECTRON ULTRASTRUCTURE
Cell Research, 8, 226-229 (1955)
NOTES
MICROSCOPE OBSERVATIONS ON THE OF THE CRAYFISH MEDIAN-TO-MOTOR GIANT SYNAPSE l J. D. ROBERTSON
Department
of Pathology and Oncology, University Kansas
City, Kansas,
of Kansas Medical School,
U.S.A.
Received August 5, 1954
CEIWAINbasic features of the crayfish median-to-motor giant synapse were described previously (6). This synapse is formed in each abdominal ganglion by the side-to-side apposition of two giant axons. It is physiologically polarized since impulses pass only from the median giant fibers to the third motor root giant fibers forming the synapse. The motor fibers extend numerous pseudopodium-like axoplasmic processes through the combined sheaths of the synapsing giant fibers. These processes measure up to 15 p in diameter and end in median giant Schwann cytoplasm. They are completely invested by the motor fiber axon-Schwann membrane (axolemma) which measures about 300 A in thickness by shadowing and is structurally like its median giant counterpart. Axoplasmic filaments are more concentrated in motor axoplasm than in median giant axoplasm. It is the purpose of this brief preliminary communication to present a few new observations on this synapse. Recent electron microscope observations of very thin (c. 200-300 A) sections through the giant synapse have revealed some principal new features which are listed below, numerically. 1) The axon-Schwann membranes in cross section consist of two dense edges with a light central zone of about 100-200 A thickness. The complete dense edged (“double contoured”) membranes measure about 200-300 A in overall thickness. Although previously not observed (7), we now know that the motor fiber axon-Schwann membrane, which envelopes the synaptic processes of motor axoplasm fuses with the median giant axon-Schwann membrane to form a single membrane of structure apparently identical with that of either of the fusing membranes (Fig. 1).8 The dense edges of the fusing membranes are continuous with one another as shown in r This work was supported by a research grant from the National Institutes of Health, U.S. Public Health Service, from the Kansas Division of the American Cancer Society and by an institutional research grant from the American Cancer Society. * Palade (4) and De Robertis and Bennett (2) reported, at the same time that our results were reported, observations on vertebrate and other invertebrate synapses similar in some respects to those described here. Experimental
Cell Research 8
Ultrastructure of crayfish giant synapse
227
Fig. 1. That these membranes are high in lipid content is suggested by the role which they and similar membranes seem to play in the metatropic reaction (3). If this role is correctly assigned, then the lipid molecules should be arranged transversely in the membranes probably in double layers whose exact location must await further evidence. 2) Concentrations of small (c. 0.1-0.8 p), closely aggregated, concentrically lamellated bodies have been seen in motor axoplasm, particularly in the synaptic processes (Figs. l-5). These bodies sometimes assume unusual branched shapes (Fig. 3), and the concentric lamellae of which they are composed appear, in some preparations, to consist of dense edged membranes resembling the axon-Schwann membranes (Fig. 5-6). Similar bodies appear in axoplasm in various crayfish nerve fibers and have been observed in the fibers of the clrcumesophageal commissures away from synapses (Fig. 6). Therefore, they are not peculiar to the synapse but merely are concentrated in great numbers in the motor axoplasm of the synaptic processes. These bodies resemble to some extent those observed by Beams and Tahmisian (1) in snail spermatozoa and by Rhodin (5) in renal epithelial cells. In the absence of specific chemical evidence these bodies cannot yet be positively identified with known cytoplasmic inclusions such as mitochondria. 3) The axoplasm of the synaptic processes is also remarkable for numerous small vesicular bodies measuring from about 250-1000 A in diameter. These are frequently attached to a tenuous filamentous tail around 100 A in diameter and of indefinite length (Fig. 3). Similar vesicles are seen in median giant axoplasm and in other crayfish nerve fibers far removed from synaptic regions. However, they, like axoplasmic filaments, are greatly concentrated in the synaptic processes. Indeed, the structural relations suggest that these vesicles may represent localized swellings of axoplasmic filaments. Palade (4) and De Robertis and Bennett (2) have observed somewhat similar axoplasmic vesicles at vertebrate and other invertebrate synapses but they found them concentrated in presynaptic rather than postsynaptic axoplasm as we have observed. Palade (4) and the author (8) found similar vesicles concentrated in the terminal axoplasm at vertebrate myoneural junctions. However, in none of these cases has a direct association with axoplasmic filaments been noted as yet. 4) Although space does not permit its demonstration here, the synaptic processes of motor axoplasm appear to be completely surrounded at all times by a layer of Schwann cytoplasm except for the regions of fusion described above. Thus it appears that motor fiber Schwann cytoplasm fuses with median giant Schwann cytoplasm at the synapse. The relations of the outer Schwann cell membranes are not yet clear. 5) The motor fibers synapse with one another during their decussation as the median-to-motor giant synapse begins (Fig. 2). In this case all of the above synaptic features are identical. The lamellated bodies again accumulate in great numbers in the axoplasm of the processes. Although it is not shown in Fig. 2, fusion of the axon-Schwann membranes occurs in the same fashion as above. In this case, however, both .synapsing giant fibers extend synaptic processes toward one another, unlike the polarized synapse with the median giant fibers in which case only the motor fibers extend such processes. It thus appears that the ability to extend axoplasmic synaptic processes is a property of the motor fibers not possessed by the median giant fibers. Experimental
Cell Research 8
228
Experimental
J. D. Robertson
Cell Resenrch 8
Ultrastructure
of crayfish giant synapse
229
The author wishes to acknowledge with gratitude the constant support and constructive criticism of Prof. F. 0. Schmitt under whose guidance this work was begun as a Ph. D. Thesis topic. REFERENCES 1. BEAMS, H. W., and TAHMISIAN, T. N., Expll. Cell Research, 6, 87 (1954). 2. DEROBERTIS, E. D. P., and BENNETT, H. S., Federation Proc., 13, 35 (1954). 3. GEREN, B. B., and SCHMITT, F. O., Proc. Natl. Aead. Sci., 40, 863 (1954). 4. PALADE, G. E., Anat. Rec., 118, 335 (1954). 5. RHODIN, J., Correlation of ultrastructural organization and function in normal
mentally changed proximal convoluted tubule cells of the mouse kidney, Godvil, Stockholm, 1954. 6. ROBERTSON, J. D., Proc. Sot. Exptl. Biol. Med., 82, 219 (1953). 7. __ Federation Proc., 13, 119 (1954). 8. -Anat. Rec., 118, 346 (1954).
and experiAktiebolaget
Fig. 1. A motor axoplasm synaptic process (Pr:) is shown in median giant Schwann cytoplasm (SC). Median giant axoplasm (MG), lies above. The motor fiber axon-Schwann membrane (a) fuses with the median giant axon-Schwann membrane (b) to produce the axon-axon synaptic membrane (c). The motor fiber axon-Schwann membrane also appears to fuse with a Schwann cell intra-cytoplasmic membrane at (d). Note the large dense bodies concentrated in the motor axoplasm of the process. These are considered to be poorly preserved concentrically lamellated bodies such as those in Figs. 3-6. The inset shows an enlargement of the dotted square. Mag. 29,000 x . Inset mag. 59,000 x . Fig. 2. A motor fiber synaptic process (Pr.) containing many dense bodies is seen lying in the Schwann cytoplasm (SC) of the adjacent motor fiber at the level of motor fiber decussation. Motor fiber axoplasm (MF) lies to the upper left. The dense bodies in the process are considered analogous to those seen in the synaptic process of Fig. 1. The motor fiber axon-Schwann membranes are not fused in this locus as they are in others. An aggregated granular component of Schwann cytoplasm is seen at (G). Mag. 8,800 x . Fig. 3. This is an enlargement of the axoplasm of a synaptic process. Note the large (c. 0.1-0.8 p) irregularly shaped bodies with concentric lamellation (C). A second smaller (250-1000 A) vesiculaz component (v) is observed. Note the delicate filamentous strands (f) with which the latter are continuous. The inset is an enlargement of the dotted area. It shows a vesicle (v) attached to a filament (f). Mag. 20,800 x . Inset mag. 60,500 x. Fig. 4. One of the concentrically lamellated bodies of a synaptic process is shown. The component membranes are poorly preserved in this case. Mag. 76,500 x . Fig. 5. One of the concentrically lamellated bodies of synaptic process axoplasm similar to that shown in Fig. 4 but from a different preparation is shown. The arrows designate the dense edges of one of its component concentric membranes. Only two such membranes are seen in this body. Mag. 170,000 x . Fig. 6. A concentrically lamellated body observed in a small nerve fiber in a circumesophageal commissure is shown. Note the resemblance of the component membranes designated by the arrows to the axon-Schwann membranes. Mag. 53,000 x . 15 - 643701
Experimental
Cell Research 8