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Some observations on the fine structure of the giant fibers of the crayfishes (Cambarus virilus and Cambarus clarkii) with special reference to the submicroscopic organization of the synapses.

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Some Observations on the Fine Structure?of the Giant
Fibers of the Crayfishes (Cambarus viri.lus and
Cambarus clarkii) with Special Reference to
the Submicroscopic Organization of the
Synapses'
KIYOSHI HAMAZ,S
Department of Anatomy, School o f Medicine, University of Washington,
Seattle 5, Washington and Department 01:Anatomy, School of Medicine,
Hiroshima University, Hiroshima, Japan
The giant fibers of the crayfish occur
on the dorsal side of the ventral nerve cord
just underneath the capsule (Krieger,
1879; Retzius, 1890; Johnson, '23). According to Johnson's description, the two
fibers lying nearest the median line extend unbranched and without interruption
through the entire length of the nerve cord.
They are called the median giant fibers. On
each side of the median fibers, in a more
lateral position, there is an additional giant
fiber which runs parallel to the median
fibers. They are called the lateral giant
fibers. Each member of the lateral pair
consists of a longitudinal chain of separate axons, resembling in some respects
the earthworm giant fibers. The abdominal portion of a lateral giant fiber is
formed by 6 such axon segments, each
joined to the next by a wedge-like contact
which will be referred to as a "segmental
septum." In each of the ventral ganglia
there exist giant motor fibers which innervate the flexor musculature. They
make synaptic contact with median and
lateral giant fibers before finally emerging
from the cord in the third root fibers of
each segment.
Johnson ('26), Furshpan and Potter
('59), Kao and Grundfest ('56) and
Wiersma ('47), have shown that the nerve
impulse traverses the segmental septa of
the lateral giant axon in either direction,
as in the segmental septum in the earthworm giant fiber (Adey, '51; Bullock, '45;
Eccles, Granit and Young, '33; Kao and
Grundfest, '57; Prosser, '52). On the other
hand, Wiersma ('47) and Furshpan and
Potter ('57, '59) have shown that the
transmission from the giants to the motor
giant is one-way. As regards the fine structure of these two types of synapses, little
has been learned except from Robertson's
early work on the synapses of the median
giant to the motor -@a& (Robertson, '53,
'54, '55).
MATER:XS AND METHODS
Adult crayfishes (Cambarus virilus and
Cambarus clar kii) were examined. Specimens were fixed in situ by injecting into
the tissue space surrounding the abdominal ganglia cold (OOC) fixative consisting
of equal parts of 5% OsOl and s-collidine
buffer, pH 7.4 (Bennett and Luft, '59).
After about 5 minutes, the abdominal
cords were removed, cut into bits and
placed in fresh fixative for one to two
hours at 0°C. The material was rapidly
dehydrated thrmgh a series of graded concentrations of ethyl alcohol and embedded
in a mixture of 90% n-butyl methacrylate
and 10% meihyl methacrylate or Epon
epoxy resin (Liift, '61). Sections were cut
with glass knikes on a Porter-Blum microtome and stained with 2% uranyl acetate
1This work was supported in part by Grant
B401 from the National Institutes of Health,
U. S. Public Health Service, Department of
Health, Education and Welfare.
* Present addrcss: Department of Anatomy,
School of Medicine, Hiroshima University, Hiroshima, Japan.
3The author wishes to express his grateful appreciation to Professor H. Stanley Bennett, who
offered him an opportunity for doing this work
and who gave many suggestions and helpful
guidance.
275
276
KIYOSHI HAMA
solution in distilled water for about two
hours. Specimens were observed with
RCA EMU 2C and Hitachi HS-6 electron
microscopes.
RESULTS
A. Axon sheath
Both median and lateral giant fibers of
the crayfish have a lamellated sheath
which consists of alternating layers of
attenuated cell cytoplasm (about 50-300
mp in thickness) ( p ) and connective tissue
layers (about 10-300 mp in thickness)
(c). The innermost layer of the lamellated
sheath is Schwann cell cytoplasm ( s )
(figs. 1, 2). The connective tissue layer
consists of regularly arranged longitudinal
and circular bundles of thin filaments,
10-12 mp in diameter. The Schwann cell
contains numerous small vesicular and
tubular components. The axon-Schwann
interface, which has a highly contorted
nature, is composed of apposing plasma
membranes of adjacent giant axon and
Schwann cell, between which is a fairly
constant gap 10-20 mp in width (figs.
1, 2). The axon membrane is somewhat
thicker and denser than that of the
Schwann cell. In the giant axon numerous
mitochondria ( m ) are frequently found
concentrated immediately below the axonSchwann interface ( i ) (fig. 2).
B . Segmental septum.
As Johnson has described ( ' 2 3 ) , the
lateral giant fiber is a segmented fiber.
Two adjacent segments join each other
like a wedge in the ganglia.
In favorable cross sections of the junctional region two cross sections of adjacent giant fiber segments may be seen
arranged side by side (figs. 3, 4). The
two segments are separated by a septum
which is composed of the sheath structures
mentioned above.
In some regions of the septum small
areas 2-5 p across are observed, which
consist of apposing plasma membranes
of adjacent nerve units. No intercalated
tissue of lamellated sheath exists between
the two plasma membranes in these areas
(arrows, figs. 3, 4, 5, 6). Sometimes two
or more of these areas can be observed
on the septum in one section (fig. 4).
Since these areas on the septum are the
only places where no sheath structure
intervenes between adjacent giant fiber
segments, they are considered to be synaptic areas between adjacent segments.
The axon membranes of each neighboring
segment continue from the axon-Schwann
interfaces to the contact area mentioned
above and come into intimate apposition
with each other with an intervening gap
of about 10 mp between (fig. 6). They
form together a pair of membranes about
23 mg wide over-all. Associated with the
apposing plasma membranes are many
vesicles with a diameter approximately
equal to that of the synaptic vesicles.
These are remarkably uniform in size (4060 mp in diameter) and shape (fig. 5 ) .
Small, circular, oval or elongated profiles
of about 20 mp in diameter, tubular endoplasmic reticulum and mitochondria are
also observed distributed near the apposing plasma membranes (fig. 6).
The small, circular, oval or elongated
profiles are considered to represent sections through a convoluted tubular system.
Thus they will be referred to as small
tubular components. Since direct continuity between these and tubular endoplasmic
reticulum can be observed, they are considered to represent a specialization of
the endoplasmic reticulum.
All of these fine structural components
are symmetrically concentrated near the
cytoplasmic surface of each of the membranes comprising a synapse. In any case,
no consistent morphological differences between the two sides of such small synapses have been observed. Thus the small
synapses which are found on the segmental septa were not recognized to be
polarized morphologically.
C. Giant motor synapses
In favorable sections small processes of
motor giant fiber (b) penetrate into the
combined sheath of the longitudinal giant
fiber and make intimate contact with the
giant axon ( a ) (fig. 7). At the area of
contact, all sheath structures, including
Schwann cells, disappear and the plasma
membranes of the process and of the
giant axon become closely applied to one
another, separated only by a narrow gap
of about 10 mp. These contact areas are
STRUCTURE O F CRAYFISH GIANT FIBERS
considered to be synapses between the
giant fiber and the motor giant fiber. Vesicles about 40-60 mv in diameter, small
tubular components about 20 my in diameter, endoplasmic reticulum and mitochondria can be observed in both axons near
the synaptic membranes.
Small cell processes several microns in
diameter embedded in the Schwann cell
cytoplasm which surround the giant axon
can frequently be observed (0) (fig. 9).
The Schwann cell covering is lacking over
a portion of the surface of these processes,
which thus present directly to the giant
axons at such sites. The plasma membranes of cell process and giant axon are
separated only by a narrow space of about
10 mv in width. Vesicles of 40-60 my in
diameter, small tubular components, endoplasmic reticulum and mitochondria
are found associated with the apposing
plasma membranes. These small cell
processes are considered to be cross sections of synaptic processes described by
Robertson (’53, ’55) comprising giant to
motor giant synapses. The amount and
distribution of the fine structural components in the synaptic area are different
in different cases (figs. 8, 9, 10, 11, 12).
Sometimes they are concentrated in postsynaptic axoplasm (i.e., synaptic process)
as Robertson has described (figs. 8, 9),
but in others they are found equally distributed in both pre- and post-synaptic
axoplasm (figs. 10, 12). In some cases
they are abundant (figs. 8, 10) and in
others they are scarce (fig. 12). In fig. 11,
there are no vesicles apparent but there
is a tubular endoplasmic reticulum associated with the pre-synaptic membrane.
No structural difference has been detected
between pre- and post-synaptic membranes in the present study.
DISCUSSION
The giant fiber of the invertebrate
was called “sheathed nerve” (Friedlander,
1889). The polarization light microscope
studies of Taylor (’40) and Bear, Schmitt
and Young (’37) revealed the lamellar nature of the invertebrate nerve fiber sheath.
Although the present study has shown
that the giant fiber of the crayfish has a
lamellated sheath, its structural detail is
different from that of the earthworm
277
giant fiber which is, in principle, similar
to the vertebrate myelin sheath (Hama,
’59). The functional significance of the
variation in str ictural detail of these two
types of giant fiber sheath has not yet
been studied.
Numerous vc?sicular and tubular components in the Schwann cell cytoplasm
and accumulation of mitochondria beneath the axor plasma membrane probably indicate high metabolic activity at
the axon-Schwann interface, as has been
suggested by Gwen and Schmitt (‘54, ’55).
Johnson (’23) claimed that the neurilemma sheaths of the lateral giant fiber
segments are absent in the region of
overlap and that only a thin membrane
intervened between the neighboring nerve
units forming ~egmentalsepta. However,
it has been dexonstrated in the present
study that in ihe segmental septum are
represented all the lamellar sheath structures of the adjacent axon segments. This
is in contrast tci the situation in the earthworm segmental septa, which consist of
two apposed plasma membranes without
inserted tissues or lamellae (Stough, ’26;
Hama, ’59).
In the septum of the crayfish lateral
giant fiber there are small synaptic areas
where the plasma membranes of the adjacent axon segments closely approach each
other without intercalated sheath structures. These synaptic areas are interpreted as being; small round areas about
2-5 v in dianeter distributed here and
there on the segmental septum. Thus the
segmental septum as a whole consists of
a cribriform combined sheath structure
studded with many small synaptic areas
a few microns in diameter. Characteristic
vesicular and t lbular structures are associated symmetiically with the two cytoplasmic surfacc s constituting the synaptic
membranes. No structural difference between apposing axon membranes has been
detected in the present study. These facts
suggest that ea:h septum furnishes many
small synapses capable of functioning in
two-way impulse transmission without any
morphological or physiological polarity.
It is supposed that this type of “cribriform” synapse may be suitable for electrical impulse i.ransmission, because it is
conceivable that the resistance to electric
278
KIYOSHI HAMA
current at the synaptic areas may be
smaller than at other parts of the septum,
and the current density at these areas
may be rather high.
Robertson ('53, '54, '55) concluded that
there are two types of morphological polarization which might be related in some
way to the physiological polarization of
the giant to motor giant synapse. This
was based upon observing the extension
of post-synaptic axoplasm toward presynaptic axoplasm and the greater density of packing of the axoplasmic filaments and vesicles in the post-synaptic
fiber. However, in the present study, the
distribution and the number of vesicles
and small tubular components were found
to be inconstant, differing in different
processes. Moreover, both pre- and postsynaptic membranes have approximately
the same thickness and density. Thus the
present study has revealed no specialized
structural feature which can be correlated
with the rectifying function of the giant
to motor giant fiber synapse.
As already mentioned, the segmental
septum of lateral giant fiber of the crayfish has been described as an electrical
non-polarized synapse (Johnson, '26;
Furshpan and Potter, '59; Prosser, '52;
Wiersma, '47) and the giants to motor
giant synapse as an electrical polarized
(Lee, rectifier) synapse (Furshpan and
Potter, '57, '59; Katz, '59).
In contrast to the variations in functional features and in anatomical relations of pre- and post-synaptic fibers, these
two types of synapses have several features in common. These include synaptic
membranes with a remarkably narrow gap
and vesicles accompanied by a highly
developed system of small tubular components.
Previous authors (De Robertis, '58; Gray,
'59; Palay, '56, '58) have postulated that
greater thickness and density of postsynaptic membranes as opposed to presynaptic might relate to the physiological properties of post-synaptic membrane.
However, no fine structural difference between pre- and post-synaptic membranes
has been observed in any of the synapses
examined in the present study.
As far as the synaptic interval is concerned, the intercellular space seen in the
crayfish synapses mentioned above are
somewhat narrower (about 10 mp) than
are the ones which have been described
in various other synapses ( 15-20 mw)
(De Robertis, '58; De Robertis and Bennett,
'55; Gray, '59; Ladman, '58; Palade, '54;
Palay, '56, '58) and in motor end plates
(50 mw or more) (Andersson-Cedergren,
'59; De Harven and Coers, '59; Reger, '59;
Robertson, '56). This finding seems to
agree well with the speculation of Furshpan and Potter ('59) to the effect that
the width of the space between pre- and
post-fiber membranes may relate to the
modes of transmission of the synapse.
Thus for maximum efficiency at an "electrical" synapse, the separation would be
small but at a "chemical" junction the
synaptic cleft would have to present a
relatively low resistance to the external
medium (Furshpan and Potter, '59).
With regard to the synaptic vesicles,
De Robertis and Bennett ('54, '55) found
the vesicles concentrated in pre-synaptic
axoplasm at invertebrate and vertebrate
synapses and speculated that the vesicles
might be associated with transmitter substance. Similar axoplasmic vesicles have
been detected in electron micrographs of
various synapses and in the motor end
plate (Anderson-Cedergren, '59; De Robertis, '56, '58; Ladman, '58; Palade, '54;
Palay, '54, '56, '58; Reger, '58; Robertson,
'55; Sjostrand, '54). From physiological
data some physiologists (Del Castillo and
Katz, '54; Fatt and Katz, '52) postulated
that the release of the chemical mediator
must be in multimolecular or quantal
units. Taking this concept, Katz ('59)
and Birks, Katz and Miledi ('59) supposed
that the synaptic vesicles might represent
intracellular bags in which the quantal
unit of acetylcholine is stored prior to its
release. On the other hand, Robertson
('53, '55) postulated that in the crayfish
median giant to motor giant synapse the
vesicles might be terminal enlargements
of the tubular endoplasmic reticulum and
they were concentrated in post-synaptic
axoplasm rather than in the pre-synaptic axoplasm. Consequently, Robertson
doubted the functional significance of the
synaptic vesicles as transmitter substance
storage devices.
STRUCTURE OF CRAYFISH GIANT FIBERS
In the present study, vesicles of 40-60
mp in diameter and small tubular components are frequently found concentrated in the synaptic areas. Since the
vesicles are remarkably uniform in shape
and size and since no greatly elongated
profiles have been detected, these structures do not seem to represent cross sections of the tubular endoplasmic reticulum, as Robertson has postulated. Rather
they seem to be differentiated as vesicles
and to correspond to the concept of synaptic vesicles described by De Robertis
and Bennett (’55). Both the first and
second components are found to be intimately associated with the synaptic
membranes and not with the axonSchwann interface. Consequently it may
be supposed that they may have a function in synaptic transmission. In addition,
as these two kinds of components are
definitely different from each other in
shape and size, it is conceivable that each
may contribute to synaptic function in a
different way, although in rare cases direct continuity between these structures
can be observed.
Further work is required to learn if the
difference in distribution of these fine
structural components relates to the physiological polarity of the synapse, and if
the differences in amounts of these components relate to the functional state of
the synapse or to the type of synapse, i.e.,
chemical or electrical synapse.
SUMMARY
Sectioned giant fibers of the crayfish
(Camburus virilus and Cumburus clarkii)
ventral nerve cord have been studied with
the electron microscope.
The giant axon is surrounded by a
lamellated sheath which consists of alternating layers of thin cell cytoplasm intervening between connective tissue layers.
The innermost cell layer of the sheath is
Schwann cell cytoplasm. Thus the structural details of the giant fiber sheath of
the crayfish are different from those of the
earthworm, which are in principle similar
to those of the vertebrate myelin sheath.
The segmental septum between lateral
giant axon segments contains representatives of each component of the sheath of
the adjacent nerve units. These sheath
279
eIements are pierced by many small synaptic areas where no sheath structure
exists and when: synaptic membranes face
each other acroEs narrow intervening gaps
of 8-12 mw. Thus the segmental septum
as a whole fornis a “cribriform synapse.”
Vesicles and small tubular structures are
found symmetrically concentrated in both
giant axons near the synaptic areas of
contact. Thus the segmental septum is
not polarized rn orphologically.
The giant fiber to motor neuron synapse
is formed by close apposition of synaptic
processes from 1he motor fiber. The membranes of the two adjacent nerve axon
units lie close tcl each other with a narrow
intervening space of about 10 mH. Synaptic vesicles and a highly developed system of small tubular components are
found to be intimately associated with
these synaptic membranes. The functional significance of the distribution and
amount of thew structures has been discussed. No cor sistent morphological evidence which might account for rectifying
function at this synapse has been detected.
LITE,RATURE CITED
Adey, W. R. 1951 The nervous system of the
earthworm Megascolex. J. Comp. Neur., 94:
57-103.
Arqdersson-Cedergr:n, E. 1959 Ultrastructure of
motor end plate and sarcoplasmic components
of mouse skeletal muscle fiber as revealed by
three-dimensiond reconstructions from serial
sections. J. Ultrsistructure Res., 1 Suppl: 1-191.
Bear, R. S., F. 0. Schmitt and J. Z. Young 1937
The sheath coinponents of the giant nerve
fiber of the squid. Proc. Roy. SOC.B, 123: 496.
Bennett, H. S. and J. H. Luft 1959 s-Collidine
as a basis for buffering fixatives. J. Biophys.
Biochem. Cytol. 6: 113-114.
Birks, R., B. Katz <mdR. Miledi 1959 Electronmicroscopic obs(:rvations on degenerating nerve
muscle junctions of the frog. J. Physiol., 146:
45P46P.
Bullock, T. H. 1945 Functional organization
of the giant fiber system of Lumbricus. J.
Neurophysiol., C : 55-71.
De Harven, E. 2nd C. Coers 1959 Electron
microscope study of the human neuromuscular
junction. J. Biaphys. Biochem. Cytol., 6: 7-10.
Del Castillo, J., and B. Katz 1954 Quantal
components of the end-plate potential. J.
Physiol., 124: 5130-573.
De Robertis, E. 1956 Submicroscopic changes
of the synapse after nerve section in the acoustic ganglion of the guinea pig. An electron
microscope study. J. Biophys. Biochem. Cytol.,
2: 503-512.
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- 1958
KIYOSHI HAMA
Submicroscopic morphology and
function of the synapse. Exp. Cell Res., Suppl.
5: 347-369.
De Robertis, E. and H. S. Bennett 1954 Submicroscopic vesicular component in the synapse. Fed. Proc., 13: 35.
1955 Some features of the submicroscopic morphology of synapses in frog and
earthworm. J. Biophys. Biochem. Cytol., I:
47-58.
Eccles, J. C., R. Granit and J. Z. Young 1933
Impulses in the giant nerve fibers of earthworms. J. Physiol., 77: 23P-25P.
Fatt, P., and B. Katz 1952 Spontaneous subthreshold activity at motor nerve endings. Ibid.,
11 7: 109-128.
Friedlander, B. 1889 Uber die markhaltigen
Nervenfasern und Neurochorde der Crustaceen
und Anneliden, Mitth. Zool. Stat. Neapel, 9:
205.
Furshpan, E. J., and D. D. Potter 1957 Mechanism of nerve-impulse transmission at a crayfish synapse. Nature, 180: 342-343.
- 1959 Transmission at the giant motor
synapses of the crayfish. J. Physiol., 145: 289325.
Geren, B. B., and F. 0. Schmitt 1954 The
structure of the Schwann cell and its relation
to the axon in certain invertebrate nerve fibers.
Proc. Nat. Acad. Sci. Wash., 40: 863-871.
1955 Electron microscope studies of
the Schwann cell and its constituents with
particular reference to their relation to the
axon. In: Fine Structure of Cells. Symposium
held at VIIIth Congress of Cell Biology, Leiden,
1954. Interscience Publishers, Inc., New York,
pp. 251-260.
Gray, E. G. 1959 Electron microscopy of synaptic contacts on dendrite spines of the cerebral cortex. Nature, 183: 1592.
Hama, K. 1959 Some observations on the fine
structure of the giant nerve fibers of the earthworm, Eisenia foetida. J. Biophys. Biochem.
Cytol., 6: 61-66.
Johnson, G. E. 1923 Giant nerve fibers in
crustaceans with special reference to Cambarus
and Palaemonetes. 3. Comp. Neur., 36: 323373.
1926 Studies on the functions of the
giant fibers of crustaceans, with special reference to Cambarus and Palaemonetes. Ibid.,
42: 19-33.
Kao, C. Y.,and H. Grundfest 1956 Conductile
and integrative functions of crayfish giant
axons. Fed. Proc., 15: 104.
- 1957 Postsynaptic electrogenesis in septate giant axons. I. Earthworm median giant
axon. J. Neurophysiol., 20: 553-573.
Katz, B. 1959 Mechanisms of synaptic transmission. In: Biophysical Science. J. L. Oncley
Ed. in Chief. John Wiley and Sons, Inc., New
York, p. 524.
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Krieger, K. R. 1879 Ueber das Centralnervensystem des Flusskrebses. 2. wiss. Zool., 33:
527-594.
Ladman, A. J. 1958 The fine structure of the
rod-bipolar cell synapse in the retina of the
albino rat. J. Biophys. Biochem. Cytol., 4:
459-466.
Luft, J. H. 1961 An improved epoxy resin embedding method. J. Biophys. Biochem. Cytol.,
9: 409414.
Palade, G. E. 1954 Electron microscope observations of interneuronal and neuromuscular
synapses. Anat. Rec., 118: 335-336.
Palay, S. L. 1954 Electron microscope study
of the cytoplasm of neurons. Ibid., 118: 336.
1956 Synapses in the central nervous
system. J. Biophys. Biochem. Cytol., 2 Suppl:
193-202.
1958 The morphology of synapses in
the central nervous system. Exp. Cell Res.,
SUPPI. 5: 275-293.
Prosser, C. L. 1952 Problems in the comparative physiology of nervous systems. In: Modern Trends in Physiology and Biochemistry.
E. S. Guzman Barron ed. Academic Press, New
York, pp. 323-336.
Reger, J. F. 1958 The fine structure of neuromuscular synapses of gastrocnemii from
mouse and frog. Anat. Rec., 130: 7-23.
1959 Studies on the fine structure of
normal and denervated neuromuscular junctions from mouse gastrocnemius. J. Ultrastructure Res., 2: 269-282.
Retzius, G. 1890 Zur Kenntniss des Nervensystems der Crustaceen. Biologische Untersuchungen, N. F., 1: 1-50.
Robertson, J. D. 1953 Ultrastructure of two
invertebrate synapses. Proc. SOC. Exp. Biol.
N.Y., 82: 219-223.
1954 Electron microscope study of an
invertebrate synapse. Fed. Proc., 13: 119.
1955 Recent electron microscope observations on the ultrastructure of the crayfish
median-to-motor giant synapse. Exp. Cell Res.,
8: 226-229.
1956 The ultrastructure of a reptilian
myoneural junction. J. Biophys. Biochem.,
Cytol., 2: 381-394.
Sjostrand, F. S. 1954 Synaptic structures of
the retina of the mammalian eye. In: Proc.
Internat. Conf. Elect. Micro., London, 1954.
R. Ross ed. Tavistock House, So., London,
pp. 428-431.
Stough, H. B. 1926 Giant nerve fibers of the
earthworm. J. Comp. Neur., 40: 409-443.
Taylor, G. W. 1940 The optical properties of
the earthworm giant fiber sheath as related to
fiber size. J. Cell. Comp. Physiol., 15: 363-371.
Wiersma, C. A. G. 1947 Giant nerve fiber system of the crayfish. A contribution to comparative physiology of synapse. J. Neurophysiol.,
10: 23-38.
PLATES
PLATE 1
EXPLANATION OF FIGURES
1 An electron micrograph of a part of the cross section of the lateral
giant fiber sheath. The giant axon ( a ) is seen at the left of the picture. The sheath consists of alternating layers of attenuated cell cytoplasm ( p ) with intervening connective tissue layers (c). The innermost layer of the lamellated sheath is Schwann cell cytoplasm (s),
which contains well developed vesicular and tubular structures. The
axon-Schwann interface ( i ) is composed of apposing plasma membranes of giant axon ( a ) and Schwann cell (s). These membranes
are highly contorted. In the connective tissue layer, one sees circular
bundles (longitudinal proflles i n the picture) and longitudinal bundles (cross section profiles in the picture) of fine &laments of 10-12
i n diameter. 72,000 x.
2
The lateral giant axon ( a ) occupies the upper half of the picture.
Many mitochondria ( m ) are found close to the axon-Schwann interface (i). The lamellated nature of the giant axon sheath is clearly
seen. 58,000 x.
Abbseviations
a, giant axon
b, motor giant axon
c, connective tissue
e, endoplasmic reticulum
i, axon-Schwann interface
m, mitochondria
282
process of the
motor giant fiber
p, attenuated cell cytoplasm
s, Schwann cell
t, small tubular components
v, synaptic vesicles
w, synaptic membranes
0, synaptic
STRUCTURE OF CRAYFISH GIANT FIBERS
Kiyoshi Hama
PLATE 1
283
PLATE 2
EXPLANATION O F FIGURES
3
A low power electron micrograph of a segmental septum separating
adjacent lateral giant fiber segments ( a ) . The lamellated sheath
of each segment continues directly into the septum. On the septum
one sees a small synaptic area (arrow), about 5 p across, where no
sheath structure intervenes between adjacent nerve units. No structural difference can be detected between the two axon segments
which face each other at the synapse. 1800 X.
4 A low power electron micrograph of a portion of the segmental septum. At two places on the septum parts of one giant axon penetrate the septum and make synaptic contact with the other giant
axon (arrows). 5200 X.
5
A picture of a synaptic area on a segmental septum. Synaptic vesicles are found to be symmetrically distributed i n the two giant axoplasms. The vesicles are intimately associated with the synaptic
membranes. 38,000 X.
Abbreviations
a, giant axon
b, motor giant axon
c, connective tissue
e, endoplasmic reticulum
i, axon-Schwann interface
m, mitochondria
284
process of the
niotor giant fiber
p, attenuated cell cytoplasm
s, Schwann cell
t, small tubular components
v, synaptic vesicles
w, synaptic membranes
0, synaptic
STRUCTURE OF CRAYFISH GIANT FIBERS
Kiyoshi H a m a
PLATE 2
285
PLATE 3
EXPLANATION O F F I G U R E
6
A high power electron micrograph of a synaptic area on a segmental
septum. The axon membranes of adjacent nerve units continue from
the axon-Schwann interface ( i ) to the synaptic area where they make
intimate contact with each other. The gap between apposing synaptic membranes is about 10 l a p . Vesicles ( v ) and small tubular
structures ( t ) are easily distinguished from each other by characteristic shapes and sizes. These two kinds of components are equally
distributed i n each giant fiber segment. 80,000 X.
Abbreviations
a, giant axon
b, motor giant axon
c, connective tissue
e, endoplasmic reticulum
i, axon-Schwann interface
m, mitochondria
286
synaptic process of the
motor giant fiber
p, attenuated cell cytoplasm
s , Schwann cell
t, small tubular components
v, synaptic vesicles
w, synaptic membranes
0,
STRUCTURE OF CRAYFISH GIANT FIBERS
Kivoshi Hama
PLATE 3
207
PLATE 4
E X P L A N A T I O N O F FIGURES
7
A low power electron micrograph showing a longitudinal giant axon
( a ) and a motor giant axon ( b ) arranged side by side. Small pseudopodium-like processes ( 0 )from the motor giant ( b ) penetrate into the
giant fiber sheath. One makes synaptic contact (arrow) with the
giant axon ( a ) . Three cross sections of synaptic processes of about
the same diameter ( 0 ) can be observed embedded in the giant fiber
sheath. The cell organelles such as mitochondria and endoplasmic
reticulum are found more abundantly distributed i n the motor giant
fiber ( b ) than i n the longitudinal giant fiber ( a ) . 3600 X.
8
A n electron micrograph of a part of the synapse between the longitudinal giant fiber ( a ) and the synaptic process ( 0 ) of a motor giant
axon. The relation between the synaptic membranes ( w ) , the axonSchwann interface ( i ) and the Schwann cell ( s ) can clearly be seen.
Synaptic vesicles ( v ) are found closely associated with the synaptic
membranes and equally distributed in both pre- and post-synaptic
fibers. On the other hand, small tubular components ( t ) are observed to be more concentrated i n the post-synaptic fiber (0) than
i n the other ( a ) . 113,000 X .
Abbreviations
a, giant axon
b, motor giant axon
c, connective tissue
e, endoplasmic reticulum
i, axon-Schwann interface
m , mitochondria
288
0, synaptic
process of the
motor giant fiber
p, attenuated cell cytoplasm
s, Schwann cell
t, small tubular components
v, synaptic vesicles
w, synaptic membranes
STRUCTURE OF CRAYFISH GIANT FIBERS
Kiyoshi Hama
PLATE 4
289
PLATE 5
E X P L A N A T I O N O F FIGURES
9
An electron micrograph showing a synaptic process of a motor giant
axon (0)cradled in a Schwann cell (s) which encloses a 1ongitudinaI
giant fiber. The synaptic vesicles are found definitely more concentrated in the synaptic process ( 0 ) which represents the post-synaptic
fiber. 31,000 X.
10
A synapse between a synaptic process (0) of a motor giant axon
and a longitudinal giant axon ( a ) . In this case, vesicles and small
tubules are found approximately equzlly distributed i n each nerve
process entering into synaptic contact. Images suggesting continuity
between endoplasmic reticulum (e ) and small tubular components
( t ) can be seen. 37,000 X.
Abbreviations
a, giant axon
b, motor giant axon
c, connective tissue
e, endoplasmic reticulum
i, axon-Schwann interface
m, mitochondria
290
o, synaptic process of the
motor giant fiber
p, attenuated cell cytoplasm
s, Schwann cell
t, small tubular components
v, synaptic vesicles
w, synaptic membranes
STRUCTURE OF CRAYFISH GIANT FIBERS
Kiyoshi Hama
PLATE 5
29 1
PLATE 6
EXPLANATION OF FIGURES
11
A high power electron micrograph of a part of a synapse between a
longitudinal giant fiber ( a ) and a synaptic process ( 0 ) of a motor
giant fiber. Two synaptic membranes ( w ) closely apply to each other.
Between them is a gap, about 8 mp in width. Tubular endoplasmic
reticulum (e) is found closely associated with the giant axon presynaptic membrane. Neither synaptic vesicles nor small tubular components are observed in the picture. 163,000 X .
12 A part of another synapse between a longitudinal giant fiber and a
synaptic process of a motor giant fiber. Vesicles ( v ) of uniform
shape and size (40-60 m p in diameter) are seen closely associated
with both synaptic membranes (w). Small tubular components ( t )
are also found near the synaptic membranes. No structural difference suggesting functional polarity can be detected in this synapse.
186,000 X .
Abbreviations
a, giant axon
b, motor giant axon
c, connective tissue
e, endoplasmic reticulum
i, axon-Schwann interface
m, mitochondria
292
0, synaptic
process of the
motor giant fiber
p, attenuated cell cytoplasm
s, Schwann cell
t, small tubular components
v, synaptic vesicles
w, synaptic membranes
STRUCTURE OF CRAYFISH GIANT FIBERS
Kiyoshi Hama
PLATE 6
293
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fiber, synapses, references, special, submicroscopic, crayfish, structure, giants, virilus, cambarus, observations, clarkii, organization, fine
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