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Sarcoplasmic reticulum in the conducting fibers of the dog heart.

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Sarcoplasmic Reticulum in the Conducting Fibers
of the Dog Heart
Cardiology Department, (I. S. Public Health Service Hospital, Staten Island,
New York 10304
The ultrastructure or sarcoplasmic reticulum (SR) was studied
in the conducting fibers of the dog heart. A dense network of sarcoplasmic
tubules occurred in well preserved cytoplasmic areas. Some of the tubules extended into large sacs filled with finely filamentous material. The sacs often appeared in subsarcolemmal apposition connected to the sarcolemma by electron
dense projections. The flattened cisternae of SR occurred beneath the sarcolemma in the myofibrillar region of the cell. Long flattened cisternae only partially
apposed to the sarcolemma were quite common in one specimen. Sarcoplasmic reticulum was continuous throughout the interfibrillar spaces surrounding each
myofibril. At the level of the Z-line the SR formed a quasi tubular flattened
structure surrounding the Z-line and closely adhering to it.
One of the functions attributed to the sarcoplasmic reticulum (SR) in the myocardium is
the transfer of the surface electrical impulse
to the inside of the cell and the consequent
regulation of muscle contraction and relaxation. The structures supposedly involved in
excitation-contraction coupling are flattened
cisternae (also called "junctional SR") observed beneath the sarcolemma and along
T-tubules formed by sarcolemmal invaginations Johnson and Sommer, '67; Sommer
and Johnson, '68; Fawcett and McNutt, 69;
Walker et al., '70, '71; Jewett e t al., '71, '73;
Forbes and Speralakis, '74; Saetersdal and
Myklebust, '75; Anderson et al., '76). The subsarcolemmal cisternae represent extensions
of a tubular network surrounding the myofibrils. Controversy exists as to whether this
network is differentiated into specialized Ztubules closely attached to the Z-line of the
myofibril (Simpson and Rayns, '68; Edge and
Walker, '70) or no such specialization exists
(Fawcett and McNutt, '69; McNutt and Fawcett, '74).
The basic morphologic characteristics
which distinguish conducting cells from those
of the working myocardium are the presence
of large cytoplasmic areas deprived of myofibrils and the absence of T-tubules. Information about the structures of SR in the conducting tissue of the heart is more scarce than
that concerning the working myocardium. U1ANAT. REC., 189: 237-262.
trastructural studies have shown a poor development of SR (Muir, '57; Caesar e t al., '58;
Kawamura, '61; Rhodin e t al., '61; Viragh and
Porte, '61; Bencosme et al., '69; James and
Sherf, '71; Sommer and Jewett, '71; Viragh
and Challice, '73; James e t al., '74). The
paucity of SR was supposed to be associated
with a poorly developed contractile apparatus,
although some pictures showing numerous SR
tubules in the areas lacking myofibrils have
also been presented (Viragh and Challice,
'73). In the myofibrillar region the SR commonly appears between the myofibrils. However, no SR superimposed over the myofibrils
has ever been presented in the conducting tissue, in contrast to that observed in the working myocardium (Sommer and Johnson, '68;
Fawcett and McNutt, '69; Jewett e t al., '73;
Simpson e t al., '73; McNutt and Fawcett, '74).
Structures which have attracted much attention are flattened cisternae of SR located
beneath the sarcolemma (Johnson and Sommer, '67; Sommer and Johnson, '68).
Our preliminary review of intraventricular
fibers of the conducting tissue in the dog heart
suggested that the SR system was better developed than had been assumed. The presence
of Z-tubules indicated a possible regular subReceived Jan. 19, '71 Accepted Apr 14, '17.
1 This work was supported in part by the Bureau of Medical Ser
vices, National Heart, Lung and Blwd Institute, Project HL
division of the system associated with the sarcomeric structure. Numerous sacs of ovoid
shape appeared beneath the sarcolemma.
These findings stimulated a detailed study of
the morphological arrangement of SR in the
conducting fibers.
Twelve mongrel dogs were studied, nine of
them anesthetized with intravenous Nembutal and three with Chloralose. In five animals
the intact heart was removed, transferred to
3% glutaraldehyde, and quickly opened. In
five animals the heart was perfused in vivo
through the coronary artery with 3% glutaraldehyde; and in two, perfusion was carried out
in vivo with Tyrode solution followed by 3%
glutaraldehyde. Free running strands of the
right moderator band, the anterior and posterior fascicles of the left bundle branch and peripheral Purkinje fibers were excised, cut into
small pieces and left for 2 to 24 hours in
fixative. In one dog one fiber was fixed in 6%
glutaraldehyde and one in a mixture of 6.25%
glutaraldehyde and 1%
OsO, (Hayat, ’70). Two
buffers were used with glutaraldehyde fixation: either 0.1 M phosphate buffer, pH 7.2
with CaCl,, or 0.1 M cacodylate buffer pH 7.2.
Post fixation was carried out for one hour in
1%OsO, in the same buffer as used for glutaraldehyde. One part of each sample was processed through saturated aqueous uranyl acetate for five minutes and then dehydrated
simultaneously with the remaining parts.
Graded acetone, or graded alcohols followed
either by acetone or propylene oxide were used
for dehydration. The material was embedded
either in Spurr’s low viscosity epoxy resin or
in Epon 812. Sections were cut with glass
knives on an LKB ultramicrotome, stained
with lead citrate and uranyl acetate and examined with a Zeiss EM 9 microscope.
The general morphology of the conducting
fibers is presented in figure 1. The cells contained myofibrils and large cytoplasmic areas
and they were connected to each other by
intercalcated discs. The sarcolemma appeared
either as a smooth layer or formed scalloped
indentations at the level of the Z-line (fig. 1:
arrows). In a very few cases shallow, tubular
invaginations of the sarcolemma were observed. Their shape and their association with
SR resembled T tubules specific for mammalian ventricular myocardium (figs. 2, 3).
The cytoplasmic region showed great variability in the density of sarcoplasmic reticulum from a dense and evenly distributed
network (figs. 1, 4, 5) through an irregular
arrangement of SR tubules with small “empty” spaces between them, to a complete disappearance of SR leaving a large “empty” cytoplasmic area (fig. 6). A dense network of sarcoplasmic tubules appeared to be invariably
associated with the presence of numerous microfilaments, occasional microtubules, and
deposits of glycogen remaining after en bloc
staining with uranyl acetate (figs. 4, 5). The
“empty” areas lacked microfilaments, microtubules, and glycogen; they usually contained
membranous fragments indicating remnants
of SR (fig. 6). Such structure suggested that
these cellular components were either not preserved, or destroyed by the technical procedures. The destructive factors appeared to
be highly selective as indicated by the simultaneous good preservation of other cellular
components (sarcolemma, myofibrils, mitochondria) and other cells (capillary, nerve, fibroblast) in the same sample (fig. 6).
Cross sections of the SR tubules in the cytoplasmic region showed a quite uniform diameter of about 65 nm, (fig. 5). The tubular content appeared as a moderately electron dense
material which resembled fine filaments or
less commonly, fine granules. The granular
appearance may represent cross sections of filaments. The arrangement of this material occasionally appeared as a honeycomb structure
(fig. 5: double arrow). Rough endoplasmic reticulum was usually restricted to a small
area. In addition t o ribosomes bound t o the
membranes, free ribosomes occurred in the
same areas forming small clusters or elongated helical structures (fig. 7).
The most common modification of SR tubules was irregular sacs of various sizes
reaching in some cases 750-800 nm in diameter (figs. 3, 4, 7-12a). The sacs were continuous with the tubules of SR (figs. 8, 10). Sacs
occurred in virtually all cytoplasmic regions
being located most commonly a t the cell periphery where they adhered either to the sarcolemma (figs. 8,9,12a) or to the sarcolemmal
invaginations (fig. 3). Adherence of sacs to the
undifferentiated region of the intercalated
disc was occasionally observed. Electron dense
extensions between the sarcolemma and the
adhering sacs were recognizable in several
sections (figs. 8, 9). The sacs were filled with
finely filamentous and granular material both
of which resembled the material contained in
the tubules of SR. The sacs distributed within
the cytoplasm often adhered closely to the mitochondria (figs. 10, 11).In a few cases an extremely dense accumulation of the sacs was
found in the intermitochondrial space (fig.
11).Some sacs contained the commonly observed filamentous material, whereas membranous whorls appeared in others. Sequential
numbers (1-4) in figure 11show that decrease
in the density of filamentous material in the
sacs parallels an increase in the amount of the
whorls, suggesting degenerative changes.
The rnyofibrillar region had a loose arrangement of SR in the interfibrillar space. In a
careful review of numerous longitudinal sections a t high resolution, the superimposition
of SR over the surface of the myofibril showing the en face view of both structures was
found exclusively a t the level of the Z-line occasionally extending to the I-band (fig. 13).
This indicated that a t the level of Z-line SR
adhered more closely to the myofibril than in
other regions of the sarcomere. The part of the
network superimposed exactly over the Z-line
might be regarded a s an irregular wavy “Ztubule” (fig. 13: arrows). Sometimes only the
“Z-tubule” was visible but its waviness indicated that the tubule represented an integral
part of the network, the extensions of which
run out of the plane of the section (figs. 14,
15). The circular profiles visible a t figures 13
and 14 (double arrows) might represent
branching points in the network. The dense
membrane of these profiles suggested however, that they might also represent coated
vesicles as described by Fawcett and McNutt
(’69). A review of large areas of longitudinal
sections demonstrated circular or oval profiles
of tubules commonly adhering to the margin
of the myofibril a t the level of the Z-line. A
few large spheroid profiles seemed to represent coated vesicles budding from or joining
to the SR (fig. 16: double arrows). Occasionally small profiles of tubules adhered to the Mline suggesting the presence of a n “M-tubule”
similar to the “Z-tubule” but less prominent
and possibly less regularly distributed (fig. 16:
arrowheads). In the interfibrillar space the
“Z-tubule” branched into tubules running
either transversely and connecting to the Z or
M “tubules” of the neighboring myofibril, or
vertically to form longitudinal tubules. The
distance between the latter and the myofibrils
usually exceeded the diameter of the tubules
in SR.
In cross-sections of myofibrils the tubular
and vesicular profiles surrounded the Z-disk
(fig. 17). The tubular structure usually adhered to the most electron opaque part of the
Z-disk which corresponded to the central part
of the Z-line as seen on the longitudinal sections. Vesicles surrounded the less dense part
of the Z-disk corresponding to the margin of
the Z-line, and the part of I-band adjacent to
the Z-line (fig. 17). Most probably the tubular
structure represented a cross-section of the
wavy “tubule” shown a t figures 13-15,and the
vesicles the sections of SR distant from Z-line.
The tubule adhered more tightly than the
vesicles t o the Z-disk and its diameter was
smaller than that of vesicles. The differences
in diameter between tubular and vesicular
profiles suggested that a t the point of adhesion to the Z-line the SR tubules became
flattened. The coated vesicles associated with
the Z-line were also distinguishable on crosssection (fig. 17). Tubular and vesicular profiles were occasionally found around the crosssections of M-line suggesting the arrangement of SR similar to that around Z-line (fig.
18).Cross-sections at the level of the A-band
usually showed single ovoid profiles of sarcoplasmic tubules and occasionally longitudinal
profiles occurring a t the periphery or penetrating the myofibrils (fig. 18: arrowheads).
In the subsarcolemmal part of the myofibrillar region long narrow tubular profiles of
SR were occasionally observed beneath the
sarcolemma or adhering to the sarcolemmal
invaginations (figs. 2, 12b, 19). Long narrow
profiles were seen in all planes of section
indicating that they represented sections of
flattened cisternae of SR. The cisternae usually contacted the sarcolemma over a short distance after which they approached the myofibril a t the level of Z-line or M-line (figs. 2,
One specimen demonstrated an exceptionally large number of long, flattened cisternae.
Only a small part of such cisternae was a t tached to the sarcolemma by dense conical
extensions (fig. 20). The distance between parallel cisternal walls was 34-40 nm. Dense material visible in the center of the cisterna appeared as a straight line in some cases, as a
wavy line in others or as discrete granules OCcasionally connected by a less dense line (fig.
20). The granules were separated by trans-
verse projections connecting the cisternal
The present study indicates that the conducting fibers normally contain a regular network of sarcoplasmic tubules in the cytoplasmic region. The SR in this region is continuous with the network surrounding the
myofibrils, and closely adhering to them a t
the level of the Z-line. The apparent absence
of SR in the cytoplasmic regions of some samples seems to be due to a destruction caused by
some factor in the preparation procedure, as
indicated by broken fragments of SR and simultaneous absence of microfilaments and
microtubules. Although the lack of SR is more
evident in samples treated en bloc with uranyl
acetate, it also has been observed in the untreated samples (Rybicka and Damato ’76).
The technical difficulty of preserving the SR
seems to have been encountered in other
studies of this tissue since in most published
reports the SR in the cytoplasmic regions of
conducting tissue cells is hardly recognizable
(Muir, ’57; Caesar et al., ’58; Rhodin et al.,
’61; Viragh and Porte, ’61; James and Sherf,
’71; James et al., ’74). In a few cases the
authors noted a large number of SR profiles
appearing in some cytoplasmic regions (Kawamura, ’61; Bencosme et al., ’69; Viragh and
Challice, ’73; Mochet et al., ’75). Inconsistent
preservations of SR seems to occur also in frog
myocardium which, like the conducting tissue
in mammals is composed of myofibrils interspaced by large cytoplasmic areas (Staley and
Benson, ’68; Page and Niedergerke, ’72).
Large sacs represent a prominent feature of
the SR. These structures have already been
noted in the cytoplasmic area of conducting
tissue (Bencosme et al., ’69; Hayashi, ’71;
Mochet et al., ’75) and in atrial cells with degenerating myofibrils (Thiedemann and Ferrans, ’76). Hayashi (‘71) defined them as
“structures corresponding to the so-called peripheral cistern.” The distribution of sacs
along the sarcolemma, sarcolemmal invaginations and undifferentiated region of intercalated disc confirms Hayashi’s (‘71). hypothesis.
The similarity between the sacs and flattened
cisternae (peripheral cistern) is enhanced by
the presence of dense conical projections occurring between each of these structures and
the sarcolemma, which to date have been
regarded in the myocardium as specific fea-
tures of flattened cisternae (Sommer and
Johnson, ‘68; Fawcett and McNutt, ’69; Walker et al., ’70, ‘71; Jewett et al., ‘71; Forbes and
Sperelakis, ’74; Anderson et al., ’76). The sacs
do not contain so-called “junctional granules”
which appear as a central dense line in the
sections of flattened cisternae. The “junctional granules” are specific for all flattened
cisternae of SR independently of their apposition to the sarcolemma as has been observed
in so-called “extended junctional sarcoplasmic reticulum” (Jewett et al., ’71, ’73; Sommer and Jewett, ’71; Forbes and Sperelakis,
’74; Saetersdal and Myklebust, ’75; Anderson
et ale,’76). It can be thus concluded that the
appearance of cisternal content in the form of
a dense line or “granules” is associated with
the parallel arrangement of cisternal walls
rather than with cisternal apposition to the
Another aspect of the morphology and distribution of the sacs represents their striking
similarity to microperoxisomes (Novikoff and
Novikoff, ’72). Our histochemical test for
catalase, a common indicator of peroxisomes,
has so far been unsuccessful. However, particles similar to peroxisomes and showing the
oxidative function but lacking catalase have
been already found in other organisms (Kun,
’68; Graves et al., ’71; Muller, ’73). The possibility of the absence of catalase has been included in the recent extended definition of
microperoxisomes (P. M. Novikoff, et al., ’73).
Increasing numbers of studies on these particles indicates that. they may represent ubiquitous structures in mammalian cells (A. B.
Novikoff et al., ’73). Peroxisomes have been
found in the mouse myocardium (Herzog and
Fahimi, ’74, ’76).
The SR in the myofibrillar region surrounds
each sarcomere. A part of SR which adheres to
the Z-line appears as a quasi tubular structure. The latter corresponds to “Z-tubules” occurring in skeletal muscle and observed in
myocardium (Simpson and Rayns, ’68; Edge
and Walker, ’70; Walker and Edge, ’71; Page
and Niedergerke, ’72; Simpson et al., ’73;
Walker et al., ’75). The flattening of “Ztubule” observed in cross-section and its adhesion to the Z-disk support the earlier conclusion that this is a part of the SR which forms a
close contact with the myofibril (Walker et
al., ’75). The SR covering the remaining part
of the sarcomere is separated from the myofibril by a distance usually exceeding the diam-
eter of the tubule. Since the latter equals the
thickness of the section the failure to find the
en face view of SR superimposed over the
entire length of the sarcomere becomes selfevident. Although in the working myocardium the distance between the sarcomere and
the SR is smaller it was also difficult to find
the en face view of “Z-tubules” as discussed by
Simpson et al. (’73). The only micrograph
known to the author, showing en face view of
SR along several sarcomeres in the myocardium is that presented by McNutt and
Fawcett (‘74) in figure 1.15. It suggests that a
part of SR adhering to the Z-line forms a quasi
tubular structure identical to that observed in
the present study.
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Most micrographs presented are taken from the sample perfused in vivo with 3%glutaraldehyde in phosphate buffer; figure 6,from the sample fixed by immersion in 6% glutaraldehyde in phosphate buffer and figures 9 and 20 from the sample fixed by immersion
in 3%glutaraldehyde in cacodylate buffer. All micrographs represent samples processed
through uranyl acetate before dehydration.
1 A longitudinal section of conducting fibers showing cytoplasmic (Cy) and myofibrillar
(My) regions in two cells. The nucleus of t h e cell on the left is eccentrically located
(N);arrows show scalloped indentations of t h e sarcolemma; (ID, intercalated disc). A
dense and uniformly distributed SR in the cytoplasmic region of the cell on the right
is shown at higher magnification in figures 4 and 5.Bar = 5 pm. X 4,000.
Krystyna Rybicka
Periphery of myofiber with shallow sarcolemmal invaginations (I); suhsarcolemmal
cisternae (arrows) adhere to the invaginations and to the sarcolemma (Sa) and they
join the myofiber a t the level of Z-, and M-lines (double arrows). Tubular SR appears
in the intermyofibrillar space. Note sections of “Z-tubules” (arrowheads) a t the
margin of the Z-lines. Bar = 0.5 pm. X 28,300.
3 Periphery of myofiher with sarcolemmal invaginations (I). Sacs of SR (S) adhere to
the invaginations. Bar = 1 pm. X 20,000.
Krystyna Rybicka
4 Tubules of SR and sacs (S) in the cytoplasmic region, interspaced with microfilaments
(arrows) associated with glycogen granules; Bar = 1 pm. X 23,750.
5 High magnification of SR tubules interspaced by microfilaments (arrows) associated
with glycogen granules; a microtubule is shown by arrowhead. The uniform diameter
of tubules is visible a t cross sections; the honeycomb structure of tubular content is
indicated by double arrow. Bar = 0.2 g m . X 80,000.
Krystyna Rybicka
6 Cross section of the fiber showing a conducting cell with cytoplasmic (Cy) and miyofibrillar (My1 regions, capillary (Ca), nerve (Ne),and fibroblast (F),Mitochondria (Mi),
sarcolemma (Sa).Note a good preservation of all structures except the cytoplasmic region of the conducting cell. Bar = 2 pm. X 12,300.
7 Cytoplasmic region with ribosomes (R)in clusters or in helical arrangement. Note microfilaments (arrows). microtubules (arrowheads), and a sac of SR (S). Bar = 0.5 fim.
X 38,000.
Krystyna Rybicka
8 Sacs of SR (S)adhering to the sarcolemma. Note projections between both structures (arrowheads) and continuity (arrows) of sacs and tubular SR. Bar = 0.5 Wm.
X 62,000.
9 Spheroid sacs of SR (S)apposed to sarcolemma. Arrowheads indicate projections between both structures. Bar = 0.5 pm. X 62,000.
10 SR sacs (S) of large diameter. Note continuity with SR tubules (arrows) and the contact between the sac and the mitochondrion. Bar = 0.5 pm. X 46,500.
11 High accumulation of sacs ( S ) in close contact with mitochondria. Numbers 1-4 indicate a progressive decrease in the filamentous content of the sacs and an increase in
the appearance of membranous whorls. Bar = 0.5 gm. X 30,000.
Krystyna Rybicka
12 Longitudinal sections of two parallel myofibers showing peripheral cytoplasmic
(12a) and myofibrillar (12b) regions. Subsarcolemmal SR appears as sacs (S) in the
former and as flattened cisternae (arrows) in the latter. Bar = 0.5 pm. X 36,000.
Insert ( 1 2 ~shows
the real distance between myofibers. Bar = 1 p m . X 11,000.
Krystyna Rybicka
13 Sarcoplasmic reticulum closely applied to the surface of a myofibril; Arrows indicate
the parts in the network forming a quasi tubule covering the Z-line (Z). A spheroid
profile (double arrow) represents either a branching point in the network or a coated
vesicle. Bar = 0.5 p m . X 62,000.
14 “Z-tubule” with a wavy outline (arrows) covers t h e Z-line. A spheroid section of a
tubule or coated vesicle is indicated by double arrow. Bar = 0.5p m . X 62,000.
15 The waviness (arrows) is visible in an almost straight “Z-tubule.” Bar = 0.5 p m ,
Krystyna Rybicka
16 Large myofibrillar area showing sections of the “2-tubules” (arrows) a t the margin
of every Z.line, and possibly coated vesicles (double arrows); the sections of “Mtubules” are indicated by arrowheads. Bar = 1 Fm. x 28,000.
Krystyna Rybicka
17 Cross section of a myofiber showing the Z.disk (Z)with narrow tubules (arrows)
adhering to its dense central part. Vesicular structures (arrowheads) surround a periphery of the 2-disc or the I-band. Possible coated vesicles are indicated by double
arrows. Bar = 0.5 Fm. X 37,500.
18 Cross section of a myofiber showing tubular SR (arrow) at the margin of M-band and
vesicular profiles of SR (arrowheads) in other regions of M-, A-, and I-bands. Bar =
0.5 pm. X 38,000.
Krystyna Ryhicka
19 Sac of SR (S) and flattened cisterna (arrow) apposed to the sarcolemma in the myofibrillar region. Bar = 0.2 p m . X 80,000.
20 Long flattened cisterna. Dense projections (double arrows) occur between t h e cisterna and the sarcolemma. The cisternal content forms a central wavy straight, or
granular line (arrows). Transverse connections between cisternal walls are indicated by arrowheads. Bar = 0.2 Fm. X 93,000.
Krystyna Rybicka
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fiber, reticulum, heart, sarcoplasmic, dog, conducting
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