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High-resolution scanning electron microscopic studies on the three-dimensional structure of the transverse-axial tubular system sarcoplasmic reticulum and intercalated disc of the rat myocardium.

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THE ANATOMICAL RECORD 228:277-287 (1990)
High-Resolution Scanning Electron Microscopic
Studies on the Three-Dimensional Structure of
the Transverse-Axial Tubular System,
Sarcoplasmic Reticulum and Intercalated Disc of
the Rat Myocardium
TAKURO OGATA AND YUICHI YAMASAKI
Department of Surgery, Kochi Medical School, Kochi, Japan
ABSTRACT
The three-dimensional structure of the transverse-axial tubular
system, sarcoplasmic reticulum (SR), and intercalated disc of the rat left ventricle
was examined by high-resolution scanning electron microscopy after removal of
the cytoplasmic matrices by the osmium-DMSO-osmium procedure. In the intermyofibrillar space, the transverse tubules (T-tubules) are accompanied by longitudinally oriented axial tubules and together form a transverse-axial system. The
junctional SR is usually small but occasionally medium or large in size and couples
with the T- or with the axial tubules. On the surface of the junctional SR facing the
T- or the axial tubule, tiny junctional processes are seen. One or two sarcotubules,
the so-called Z-tubules, frequently run parallel to the T-tubule. The sarcotubules
derived from the junctional SR or from the Z-tubule run longitudinally or obliquely
and form polygonal meshes around the myofibrils. On the surface of the SR a t the
H-band level, small fenestrations of 12-40 nm in diameter, and tiny hollows 8-20
nm in diameter are seen. Bulbous swellings of the SR, the corbular SR, are preferentially seen near the Z-band. The large and flat SR, known as the cisternal SR,
intercalates among the SR meshes. In the subsarcolemmal space, the sarcotubules
form a multilayered network (peripheral SR). The cisternal SR is frequently intercalated in these meshes and closely associated with the inner surface of the
sarcolemma. The intercalated disc appears a s a prominently undulated membrane
demarcating the border between two adjacent heart muscle cells, and occasionally
small projections 60-90 nm in diameter and 200-600 nm in length display on its
surface.
Three-dimensional models of the mammalian cardiac muscle fiber based on transmission electron microscope (TEM) observations of thin, sometimes serial,
sections have been proposed by some investigators
(Porter and Palade, 1957; Fawcett and McNutt, 1969;
Bossen et al., 1978). The three-dimensional organization of the transverse-axial tubular system (TATS) and
the sarcoplasmic reticulum (SR) has been examined in
freeze-fracture replicas (Scales, 1981; Forbes e t al.,
1984; Forbes and Van Niel, 1988), and by high-voltage
electron microscopy after labeling with horseradish
peroxidase and lanthanum (Sommer and Waugh,
1976), by osmium-ferrocyanide postfixation method
(Segretain e t al., 1981; Forbes and Van Niel, 1988) and
by the Golgi black reaction method (Scales, 1983). Using the osmium-DMSO-osmium procedure (Tanaka
and Naguro, 1981),the three-dimensional structure of
the membrane system of the myocardium of the rat
(Ohmori, 1984) and of the dog (Yoshikane et al., 1986)
were directly observed by scanning electron microscopy
(SEM). However, in these studies, the specimens were
coated with metal, and consequently the details of the
surface structure of the membrane were concealed un0 1990 WILEY-LISS, INC.
der the metal coating. In the present study, the membrane system of the rat ventricle was exposed by the
osmium-DMSO-osmium procedure and impregnated
with osmium by the tannic acid-osmium staining (Murakami, 1973), followed by osmium-hydrazine
(Kubotsu and Ueda, 1980). The specimens were examined by high-resolution SEM on a Hitachi S-900 with a
resolving power of 7 A without metal coating, and the
detailed three-dimensional structure of the TATS, SR,
and intercalated discs was elucidated.
MATERIALS AND METHODS
Male Wistar rats weighting 250-300 g were used.
The methods employed in the present study are the
same as described in the previous paper (Ogata and
Yamasaki, 1987). In short, the left ventricle was fixed
with 0.5% paraformaldehyde-1 .O% glutaraldehyde so-
Received October 23, 1989; accepted April 23, 1990.
Address reprint requests to Prof. Takuro Ogata, Department of Surgery, Kochi Medical School, Nankoku, Kochi, 783, Japan.
278
T. OGATA AND Y. YAMASAKI
lution in 0.067 M cacodylate buffer (pH 7.4) for 15 minutes by retrograde perfusion through the thoracic
aorta. The muscle was cut into small pieces and further
fixed for 15 minutes in the same fixative. The tissue
blocks were immersed in dimethyl sulfoxide (DMSO)
and frozen in liquid propane cooled with liquid nitrogen. The specimens were cracked with a single-edged
razor blade by striking it with a hammer in a freezecracking apparatus Eiko TF-1. Then they were postfixed with 1%OsO, in 0.067 M cacodylate buffer (pH
7.4) for 1 hour and left standing in 0.1% Os04 in the
same buffer at 20°C for 72-96 hours to remove the
cytoplasmic matrix (Tanaka and Naguro, 1981).Tissue
specimens prepared in this way were impregnated with
osmium by the conductive staining method (Murakami, 1973). Afterwards they were dehydrated in
ethanol and dried in a Hitachi HCP-1 critical-point
dryer. The dried specimens were mounted on a specimen stub and impregnated again with osmium by the
osmium-hydrazine method (Kubotsu and Ueda, 1980).
The observation was done under a high-resolution,
field-emission type SEM, the Hitachi S-900.
RESULTS
The osmium-DMSO-osmium method (Tanaka and
Naguro, 1981) used in the present study proved to be
very useful in disclosing the architecture of the intercellular membranous structures. Myofilaments and cytoplasmic matrices were removed by maceration in the
Os04 solution, and the membranous structures could
be disclosed without introducing any significant artifact.
The sarcolemma appeared as a fairly smooth membrane surrounding the muscle fiber, with numerous
caveolae 70-110 nm in diameter attached to its external and internal surfaces (Figs. 1, 16a). Large-diameter invaginations from the sarcolemma extended
deeply into the interior of the muscle fiber and transformed into the transverse tubules (T-tubules) (Fig. 1).
The T-tubules ran transversely to the long axis of the
muscle fiber a t each Z-band level and made relatively
frequent interconnections with the axial tubules,
which usually ran parallel to the long axis of fiber
(Figs. 2c, 6), although some were obliquely oriented
(Fig. 2a). The interconnecting point of the T- and axial
tubules was distended (Fig. 7). The diameter of the
T-tubules ranged between 70 and 100 nm. Some of
them were therefore thicker than those of the sarcotubules (Fig. 6), which ranged between 50 and 80 nm, but
others (Figs. 3,4) were of almost the same diameter as
A bbreuiations
A
C
D
H
I
J
M
S
T
Z
ZT
*
axial tubule
corbular SR
desmosome
H-band level
intercalated disc
junctional SR
mitochondria
sarcolemma
T-tubule
Z-band level
Z-tubule
cisternal SR
the sarcotubules. The diameter of the axial tubules
ranged between 90 and 120 nm (Figs. 2c, 7); their average diameter was therefore slightly thicker than that
of the T-tubules. The T- and axial tubules usually appeared as slender tubules with almost the same diameter along their length (Fig. 2a, c), but with occasional
swellings (Figs. 7, 9). In addition, bell-shaped or wartlike swellings were occasionally seen, and some coupled with the junctional SR (Fig. 4).
The SR formed lace-like networks inside the fiber
(Fig. 6). At the Z-band level, the myofibrils were encircled by sarcotubules. These sarcotubules were first observed by TEM and have been called the Z-tubules
(Simpson and Rayns, 1968; Forbes and Sperelakis,
1980). Most often, the Z-tubules lay closely adjacent to
the T-tubule (Figs. 3, 6, 8). In SEM images, the Z- and
T-tubule were arranged parallel to each other, but usually in slightly different planes (Figs. 6, 8). The T-tubule was either sandwiched between two Z-tubules
(Fig. 8) or accompanied by one Z-tubule only (Figs. 3,
6). Numerous tiny projections 10-16 nm in diameter
and 6-12 nm in height were arranged 25 nm apart
from each other on the surface of the Z-tubule facing
the T-tubule. These projections seem to correspond to
the feet (Franzini-Armstrong, 1970) or junctional processes (Sommer and Johnson, 1979) already observed
by TEM. Such arrangements were seen a t irregular
intervals and did not occur throughout the entire
length of the Z-tubule (Fig. 8). Similar tiny projections
were occasionally observed on the surface of the T-tubule (Figs. 8, 12a),but they were far fewer and lacked
regularity of arrangement.
The junctional SR that coupled with the T-tubule
was usually small and flat (Figs. 3, 9), although medium-sized (Fig. 10) or large-sized junctional SR (Fig.
11)was also seen. The coupling of the SR to the axial
tubules was also demonstrated in SEM (Fig. 2) and
Fig. 1. SEM image of the transformation of the sarcolemma into the
T-tubule. a: Large-diameter invaginations of the sarcolemma extend
deeply into the fiber at the Z-band level and transform into the Ttubules. Numerous caveolae (arrows) are attached to the external and
internal surfaces of the sarcolemma. x 21,000. b Higher magnification of part of a from a slightly different angle. The initial portion of
the T-tubule is fractured, and its inner structure is exposed (black
star). Arrows show caveolae. x 69,000.
Fig. 2. SEM images of the axial tubules. a: The axial tubules run
obliquely and connect with the T-tubules. Arrows show the coupling
of the junctional SR to the axial tubules. x 45,000. b A section of a.
Junctional SR couples with the interconnecting part of the axial tubule and the T-tubule. Arrow shows polyribosomes. x 109,000, c: Longitudinally running axial tubule. Note that the diameter is fairly
uniform throughout its course. x 34,000.
Fig. 3. SEM image of the Z-band region. The Z-tubule runs parallel
to the T-tubule. Small junctional SR couples with the T-tubule. Note
the small size of the SR mesh a t the Z-band level. x 78,000.
Fig. 4. A bell-shaped swelling of the T-tubule is coupled by junctional SR. The ovoid corbular SR attaches to the sarcotubule.
x 91,000.
Fig. 5. TEM micrograph showing the coupling of the junctional SR
to the axial tubule (arrows). x 41.000.
MEMBRANE SYSTEM OF THE HEART
Figs. 1-5.
279
280
T. OGATA AND Y. YAMASAKI
Figs. 6-10.
281
MEMBRANE SYSTEM OF THE HEART
TEM pictures (Fig. 5). SEM images of the transversely
fractured T-tubule and junctional SR complex showed
that, usually, the T-tubule was encircled by the junctional SR only partially (Fig. 12a), although occasionally completely encircled structures were also seen
(Fig. 12b).Tiny feet were also seen on the surface of the
junctional SR facing the T-tubule (Fig. 12a).
Slender sarcotubules arising from the junctional SR
or from the Z-tubule ran longitudinally or obliquely
toward adjacent sarcomeres at both sides and formed
polygonal meshes of various sizes. These meshes were
small, i.e., 0.1-0.3 pm in diameter, and oval at the
I-band (Figs. 3, 6), whereas a t the A-band they were
larger, i.e., 0.3-0.5 Fm maximal length, and long-ellipsoid shaped (Figs. 6, 15). The sarcotubules were
fairly uniform in diameter throughout their course, except for occasional distentions (Fig. 151, and presented
a relatively smooth surface.
The plate-like SR, i.e., the cisternal SR, was relatively frequently intercalated among the SR meshes
(Figs. 6,13a). This structure was preferentially located
at the I-band level, but occasionally it was also seen at
the A- and the H-band levels (Fig. 13b). Polyribosomes
were frequently attached on its surface (Fig. 13). Polyribosomes were also sporadically seen on the surface of
the junctional SR (Fig. 2b) and sarcotubules (Fig. 13b).
At the H-band level, the sarcotubules rather often
formed small meshes or fenestrations (12-40 nm in
diameter), which passed through the SR (Fig. 151, and
their surface exhibited a few tiny hollows (8-20 nm in
diameter), which did not seem to pass through the SR
(Fig. 15). These hollows were also observed on the surface of the cisternal SR, which was occasionally present
at the H-band level (Fig. 13b).
Bulbous terminal swellings of the SR 80-170 nm in
diameter, i.e., the corbular SR, were preferentially
seen near the Z-band (Figs. 4,6,14). Some appeared as
hemispheric swellings on the surface of the sarcotubules (Fig. 14a), but most were spherical or ovoid and
connected to the sarcotubules (Figs. 4,14b, c). The surface of the corbular SR was either smooth (Figs. 4,14b)
or provided with tiny granular projections about 8 nm
in diameter (Fig. 14c), which resembled the feet of the
junctional SR.
In the intermyofibrillar space, numerous spherical or
ovoid mitochondria were accumulated (Fig. 6). Around
the mitochondria, the SR was less well developed than
around the myofibrils. Simple SR networks transversely crossed the mitochondria at the Z- and H-band
levels, and a few sarcotubules originating from them
ran longitudinally at the I- and the A-band and connected the SR network of the Z-band with that of the
H-band (Fig. 6). The portion of the mitochondria underlying the SR was frequently constricted (Fig. 12a).
In the subsarcolemmal space, the peripheral (subsarcolemmal) SR formed polygonal meshes 80-200 nm in
diameter (Fig. 16). These meshes were not arranged in
a single plane, but rather formed multilayered networks (Fig. 16). Plate-like cisternal SR, polygonal in
shape and 130-400 nm in diameter, was frequently
intercalated among these meshes (Fig. 16). Most of
these subsarcolemmal cisternal SR were closely associated with the inner surface of the sarcolemma (Fig.
16).Polyribosomes were frequently attached to the surface of the subsarcolemmal junctional SR (Fig. 16b).
Numerous spherical caveolae were also seen on the
surface of the sarcolemma and frequently interspersed
in the SR meshes (Fig. 16). Very thin tubules with
beaded structures (about 20 nm in diameter) were occasionally seen among the peripheral SR network (Fig.
16b).
The sarcolemma was deeply invaginated between
two adjacent cardiac cells and transformed into the intercalated disc (Fig. 17).The intercalated disc appeared
as a markedly undulated membrane bordering two cardiac cells (Figs. 17, 18). At higher magnification, its
surface was fairly smooth (Fig. 18) with only a few
scattered caveolae (Fig. 19a, c). These caveolae were
also seen on the surface of the intercalated disc by TEM
(Fig. 20a). Slender projections about 60-90 nm in diameter and 200-600 nm in length occasionally protruded from the intercalated disc (Fig. 19b). Some were
twisted (Fig. 19b).In TEM micrographs, they were surrounded by membrane, and their inside was filled with
amorphous substance (Fig. 20b). Frequently, the SR
was closely associated with the surface of the intercalated disc (Fig. 21). Some of the SR had a cisternal-like
appearance (Fig. 21). Occasionally the summits of the
intercalated disc were surrounded by the SR (Fig. 22).
DISCUSSION
Fig. 6. SEM image of the SR network. T-tubules run transversely at
the Z-band level. The axial tubule connects with a T-tubule a t both
ends. Note that the diameter of these tubules is larger than that of the
sarcotubules. Ovoid mitochondria arrange themselves in the intermyofibrillar spaces. Around the mitochondria, the SR forms simple
meshes a t the H- and Z-band levels. At the level of the A-band, a few
longitudinally arranged sarcotubules are seen. x 30,000.
Fig. 7. Higher magnification of a part of Figure 6. Note small (small
arrow) and large (large arrow) wart-like swellings of the axial tubule.
x 106,000.
Fig. 8. The T-tubule is sandwiched between two Z-tubules. Periodically arranged tiny feet (arrowheads) are seen on the surface of the
2-tubule facing the T-tubule. Occasionally, a small projection of approximately the same size (arrow) is seen on the surface of the Ttubule. The inside of the 2-tubule is partly exposed. x 113,000.
Fig. 9. Swellings of the T-tubule (arrows). x 86,000.
Fig. 10. The medium-sized junctional SR couples with the T-tubule.
x 91,000.
In the mammalian heart, the T-tubules are often accompanied by longitudinally oriented axial tubules
and together form the transverse-axial tubular system
usually abbreviated as TATS (Sperelakis and Rubio,
1971; Forbes et al., 1984). In the present study, the
TATS was clearly demonstrated by SEM. In the mouse
ventricle, the axial tubules have been reported frequently to run obliquely (Forbes et al., 1984), and in
the present investigation a similar orientation was observed in the rat. Occasional swellings of the T-tubule
were observed, and some were coupled with SR. Similar swellings had been observed in the rat ventricle
using the ferrocyanide-osmium method (Tomita and
Ferrans, 1987).
In the dog ventricle, the SR appears as a tight network of tubules in the region of the H- and Z-bands and
forms larger meshes at the A- and I-band levels (Yoshikane et al., 1986). The present results showed that the
282
T. OGATA AND Y. YAMASAKI
Figs. 11-1 5.
MEMBRANE SYSTEM OF THE HEART
SR of the rat ventricle displays a similar distribution.
Under SEM, most of the Z-tubules were demonstrated
to run also in parallel to the T-tubule. Until now, the
relationship between the Z- and the T-tubule has not
been clearly described in the literature, probably because of the difficulty in obtaining sufficiently long
profiles of both tubules in TEM sections. In the present
SEM study, numerous tiny projections closely resembling in location, size, and periodicity the tiny feet
(Ferguson et al., 1984) or junctional processes, which
are considered to be the apparatus for coupling of the
SR to the T-tubule (Rayns et al., 1975; Forbes and Sperelakis, 1977; Sommer and Johnson, 1979), were seen on
the surface of the Z-tubule facing and T-tubule and on
the surface of the junctional SR exposed to the TATS.
These findings suggest that the excitation-contraction
coupling occurs not only between the junctional SR and
the TATS (Sommer and Johnson, 19791, but also between the Z- and T-tubule. The coupling of the Z-tubule
to the T-tubule has not been reported yet, probably
because i t is difficult to discriminate the junctional SR
from the Z-tubule by TEM. Similar tiny projections
were occasionally seen on the surface of the T-tubule,
but they are far fewer and lack regularity of their arrangement. The nature of these projections is, however,
unknown.
In the SEM images of the transversely fractured Ttubule and junctional SR complex, the junctional SR,
most frequently, encircled the T-tubule only partly, but
complete encircling of the T-tubule by the junctional
SR was occasionally seen, as already reported by TEM
(Sommer and Johnson, 1969; Forbes and Sperelakis,
1977). The coupling of the junctional SR to the axial
tubule already reported in the dog myocardium (Forbes
and Van Niel, 1988) was also observed by SEM and
TEM in the present study.
Numerous tiny fenestrations in the H-band collar
were demonstrated in the rabbit myocardium by the
Golgi black reaction method (Scales, 1983) and in the
Fig. 11. Two large junctional SR with cisterns-like appearance coupling with T-tubules. x 44,000.
Fig. 12. Transversely fractured T-tubule and junctional SR. a: The
junctional SR partly encircles the T-tubule. On the surface of the SR
facing the T-tubule, tiny feet (arrowheads) are arranged a t 25 nm
intervals. Occasionally, a solitary tiny projection (arrow) is seen on
the surface of the T-tubule. Note the constriction of the mitochondrion
beneath the sarcotubules. x 113,000. b Junctional SR completely encircling the T-tubule. x 113,000.
Fig. 13. SEM images of the cisternal SR. a: Cisternal SR at the
I-band level. Arrows show polyribosomes. X 113,000. b Cisternal SR
at the H-band level. Note tiny hollows (arrowheads) on its surface.
Arrow shows polyribosomes attached on the surface of the sarcotubules. x 117,000.
Fig. 14. SEM images of corbular SR. a: The corbular SR appears as
a hemispherical swelling. x 113,000. b: Ovoid-shaped corbular SR
with smooth surface. ~ 5 7 , 0 0 0 .c: Spherical corbular SR with tiny
projections (arrowheads) on its surface. Similar projections (arrows)
are seen on the surface of a low, round elevation of the sarcotubule.
x 113,000.
Fig. 15. Higher magnification of the SR network. Sarcotubules form
large size meshes at the level of the A-band (Ab).At the H-band level,
a small mesh (small arrow) and tiny hollows (arrow heads). are seen.
Large arrow shows distention of the sarcotubule. x 113,000.
283
guinea pig myocardium by the osmium-ferrocyanide
postfixation method (Forbes and Van Niel, 1988). An
early TEM study on Ambystoma muscle described
smaller circular patches (20 nm) in the H-band collar,
which were referred to as pores or thin places in the
membrane (Porter and Palade, 1957) and later were
thought to represent perforations in one of the two
membranes of this collar (Franzini-Armstrong, 1963).
Further TEM studies on the SR of the frog skeletal
muscle demonstrated that they were fenestrations
right through the H-band collar (Peachey, 1965). However, SEM observations on the H-band collar of the frog
muscle revealed that there are fenestrations of 20-50
nm diameter and tiny membrane depressions (15-20
nm in diameter) that seem not to pass completely
through the H-band collar and were tentatively named
H-band hollows (Ogata and Yamasaki, 1987). In the
present observations, similar fenestrations and hollows
were observed in the H-band collar of the cardiac muscle. The nature of these hollows, however, is still unknown.
Corbular SR is found attached primarily to the perimyofibrillar SR network, but not to the subsarcolemma1 SR arrays (Forbes and Van Niel, 1988), and is
preferentially located at the Z-band level. From highvoltage electron microscopic studies, the membranous
profiles of the corbular SR are always continuous with
the endoplasmic reticulum (Sommer and Waugh,
1976). The continuity of the corbular SR to the sarcotubules was also observed by SEM. The function of the
corbular SR is, however, unknown. Immunocytochemically, it was shown that calsequestrin, which has been
proposed to store Ca2+ in the lumen of the SR in resting muscle, is confined to the lumen of the junctional
SR and of the corbular SR (Jorgensen and McGuffee,
1987). It is interesting to note that tiny feet-like structures, similar to the feet on the surface of the junctional
SR facing the T-tubule, are also seen on the surface of
some corbular SR. These tiny projections of the corbular SR have also been reported in the rabbit (Dolber
and Sommer, 1984) and in the guinea pig myocardium
(Forbes and Van Niel, 1988).
The plate-like SR, i.e., the cisternal SR, is relatively
frequently intercalated among the SR meshes. On the
surface of the cisternal SR, polyribosomes are frequently seen. The presence of ribosomes on the surface
of cisternal SR has already been reported (Forbes and
Fig. 16 (overleaf).SEM image of the peripheral SR. a: This slightly
oblique view shows the sarcotubules forming multilayered polygonal
meshes. Large plate-like cisternal SR are intercalated among them.
Numerous caveolae (arrows) attach to the surface of the sarcolemma.
x 45,000. b Frontal view of the subsarcolemmal SR. The SR forms
polygonal meshes. The cisternal SR attaches to the inner surface of
the sarcolemma. Arrowhead shows polyribosomes attached on the
surface of the cisternal SR. Note that the sarcotubules form multilayered networks (large arrow). Small arrow shows the caveolae interspersed in the SR meshes. Double arrows show a very thin tubule with
beaded structure. x 77,000.
Fig. 17. Low magnification SEM image of an intercalated disc. The
intercalated disc appears as a prominently undulated membrane continuing to the sarcolemma. x 24,000.
Fig. 18. At higher magnification, the surface of the intercalated disc
is relatively smooth. x 29,000.
284
T. OGATA AND Y. YAMASAKI
Figs. 16-18.
MEMBRANE SYSTEM OF THE HEART
Figs. 19-22.
285
286
T. OGATA AND Y. YAMASAKI
Van Niel, 1988). Polyribosomes are also occasionally
observed on the surface of the junctional SR and of the
sarcotubules.
In the rat ventricle, the peripheral (subsarcolemmal)
SR is very extensively distributed on the inner surface
of the sarcolemma. Similar observations have also been
made in the dog ventricle by SEM (Yoshikane et al.,
1986). The coupling of the peripheral SR to the sarcolemma has been reported in the rat (Fawcett and McNutt, 1969), in the mouse (Forbes and Sperelakis,
1977), and in the guinea pig (Forbes and Van Niel,
1988). TEM studies showed that the mouse junctional
SR saccules (the occasionally expanded regions of the
SR containing opaque material within the distended
portion) couple with the inner surface of the sarcolemma and interlink with the subsarcolemmal tubular
meshwork of the SR network with electron-lucent lumina (Forbes and Sperelakis, 1977). The present SEM
observations revealed that the peripheral SR is composed of multilayered polygonal meshes frequently intercalated with polygonal cisternal SR and that most
cisternal SR closely attaches to the inner surface of the
sarcolemma, whereas the tubular segments are rather
apart from it. Judging from these findings, there seems
to be little doubt that the cisternal parts of the peripheral SR correspond to the peripheral junctional SR.
Fine beaded tubules are occasionally seen among the
peripheral SR. The true nature of these fine tubules is,
however, unclear.
The intercalated disc appears under SEM as a prominently undulated membrane bordering between two
cardiac cells. When observed under TEM, it contains
numerous intermediate junctions, desmosomes, and
gap junctions (Forbes and Sperelakis, 1985). In SEM
specimens prepared by the osmium-DMSO-osmium
method, the filamentous materials of these junctions
are removed by maceration, and therefore the surface
of the intercalated disc appears fairly smooth. A few
caveloae are seen scatterd on the surface of the intercalated disc, but their number is much smaller than
that on the surface of the sarcolemma. The existence of
caveolae on the surface of the intercalated disc has not
been reported yet. Tiny slender projections are occasionally seen on the surface of the intercalated disc, but
their nature awaits further study. Under SEM, the SR
was frequently seen in close association with the surface of the intercalated disc, as already reported for
TEM studies (Forbes and Sperelakis, 1977).
Fig. 19. a: Intercalated disc with a few caveolae (arrowheads) and
slender projections (arrows). x 20,000. b: Higher magnification of a
part of Fig. 19a. Note that one of the slender projections (arrow) has
a club-like appearance and the other two (arrowheads) are twisted.
x 51,000. c: Higher magnification of the caveolae (arrowheads) on the
surface of the intercalated disc. x 52,000.
Fig. 20. TEM micrograph of the intercalated disc. a: Caveolae (arrowheads) on the surface of the intercalated disc. x 46,000. b Clublike projection filled with amorphous material (arrow). x 46,000.
Fig. 21. Cisternal SR closely associated with the surface of the intercalated disc. x 65,000.
Fig. 22. The SR surrounds the summit of the intercalated disc
(arrows). x 76,000.
ACKNOWLEDGMENTS
The authors are grateful to Dr. K. Nagatani, Hitachi
Ltd., for making a n S-900 available; to Messrs. M. Yamada and T. Suzuki, Hitachi Ltd., for taking ultrahigh-resolution SEM pictures; to Ms. K. Ikeda, M.
Matsumoto, H. Okazaki, and M. Miyata for their technical assistance throughout this study.
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axial, resolution, dimensions, tubular, high, electro, three, disco, rat, system, structure, intercalated, microscopy, myocardial, reticulum, scanning, sarcoplasmic, studies, transverse
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