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Three dimensional arrangement of mitochondria and endoplasmic reticulum in the heart muscle fiber of the rat.

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THE ANATOMICAL RECORD 200:139-151 (1981)
Three Dimensional Arrangement of Mitochondria and
Endoplasmic Reticulum in the Heart Muscle Fiber of the Rat
D. SEGRETAIN, A. RAMBOURG, AND Y.CLERMONT
Departement de bwbgie, Centre &Etudes N w h i r e s de Saclny, 91190 Gif-sur-Yvette,
France
ABSTRACT
The three-dimensional arrangement of mitochondria and endoplasmic reticulum was studied in thick sections of the heart left ventricle fixed
in glutaraldehyde and impregnated with the Ur-Pb-Cu technique and in thin
sections of glutaraldehyde-fixed tissue post-fixed in potassium ferrocyanide-reduced osmium. Squarish flattened mitochondria, approximately the size of a sarcomere, were arranged in longitudinal columns in the clefts between the myofibrils. At the periphery of the fiber, the endoplasmic reticulum took the appearance
of a subsarcolemmal network of plate-like and tubular cisternae running parallel
to the cell surface. Between the myofibrils, the ER network formed longitudinally
oriented repetitive units whose structure varied according to their position in
relation to the A- or I- bands of the myofibrils. In front of the A-band, the endoplasmic reticulum appeared as a single layered network of anastomotic tubules
compressed between the adjacent myofibrils. In front of the I-band, it formed a
multilayered network the three-dimensional arrangement of which was dependent upon the presence or absence of the T-tubule. In the absence of the T-tubule,
the ER cisternae were loosely anastomosed and occasionally displayed bulbous
terminal swellings. In the presence of T-tubules, tubular ER cisternae were seen
running parallel on both sides of the T-tubules and were continuous with sheetlike cisternae sandwiched between the distended T-tubule and adjacent extremities of longitudinally arranged mitochondria. These tubular or flattened cisternae were connected to each other by numerous bridging cisternae around the Ttubules.
Since the pioneer study of Porter and Palade
('57) the ultrastructure of the cardiac muscle
fiber has been investigated extensively (reviews by Fawcett, '61; James and Sherf, '68;
Sommer and Jewett, '71; Simpson et al., '73;
McNutt and Fawcett, '74; Sommer and Waugh,
'76; Katz, '77). Three-dimensional models
based on electron microscope observations of
thin, sometimes serial, sections have been proposed by Porter and Palade ('57), Nelson and
Benson ('63), Fawcett and McNutt ('69), Katz
('75), and Bossen et al. ('78). High voltage electron microscopy of thick sections combined
with stereoscopic viewing has permitted a
more direct appreciation of the three-dimentional configuration of selectively stained organelles. Thus, in cardiac muscle, the threedimensional organization of the T-system or
the endoplasmic reticulum has been examined
in the electron microscope after labeling the
T-system with horseradish peroxidase and of
the endoplasmic reticulum with lanthanum
0003-276)3/81/2002-0139$04.00
0 1981 ALAN R. LISS, INC.
(Sommer and Waugh, '76). However, the low
contrast obtained with such a microscope due
to the acceleration of the electrons prevents
the analysis of the fine three-dimensional details of the organelles. Staining techniques
which impregnate mitochondria, cisternae of
the endoplasmic reticulum, and elements of
the Golgi apparatus have been utilized recently on thick sections to visualize with a n
electron microscope a t 100 kV the three-dimensional architecture of the red muscle fiber
in the rat diaphragm (Rambourg and Segretain, '80). It is the purpose of the present study
to examine with the same staining methods
the three-dimensional configuration of the rat
heart muscle fiber with special emphasis on
the structure of the endoplasmic reticulum and
mitochondria as well as on the spatial relationship of these two organelles.
Y. C.'s current addrees: Department of Anatomy, McGill University, Montreal, Quebec, Canada.
Received August 13, 1980; accepted Oetober 27,1980.
140
D. SEGRETAIN, A. RAMBOURG, AND Y. CLERMONT
MATERIALS AND METHODS
Ten adult male rats were fixed by perfusion
through the left ventricle with a 3.5% glutaraldehyde solution in 0.1M sodium cacodylate.
After 15 minutes of perfusion, a portion of the
left ventricle was excised and cut into small
pieces that were immersed in the same fixative
for an additional hour. They were then processed according to two different techniques.
According to the first technique, the tissue
was rinsed overnight a t 4°C in sodium cacodylate and then immersed for one hour a t room
temperature in a 1:l mixture of 2% aqueous
osmium tetroxide and 3% aqueous potassium
ferrocyanide (Karnovsky, '71). This technique
will be referred to as the reduced osmium technique.
According to the second technique, the tissue
was impregnated as described by Thi6ry and
Rambourg ('76).After a quick rinse in distilled
water, the tissue was immersed for one hour
at 40°C in a 5% uranyl acetate aqueous solution, rinsed in distilled water, and post-stained
for one hour at 40°C in a double lead and copper citrate solution prepared as indicated by
Thi6ry and Rambourg ('76). Following several
rinses in distilled water, the tissue was postfixed overnight a t 4°C in 1% unbuffered
aqueous osmium tetroxide. This technique will
be referred to as the Ur-Pb-Cu technique.
Following these treatments, the tissue
blocks were dehydrated in ethanol and embedded in Epon. Thin and thick (0.5-1 pm) sections were prepared with a MT1 Porter-Blum
ultramicrotome, mounted on formvar coated
grids and examined with the electron microscope. Thin sections of tissue postfixed in reduced osmium were counterstained for two
minutes with lead citrate and examined at 60
kV in a Philips 301 (or 400) electron microscope. Thick sections of Ur-Pb-Cu impregnated
material were examined without counterstaining at 80 or 100 kV.
For stereoscopy, thick sections were placed
on the goniometric stage of the electron microscope, and two photographs of the same
field were taken after tilting the specimen a t
-6" and +6" from the original 0" position of
the goniometric stage. A three-dimensional
view of the impregnated structure was obtained by looking a t properly adjusted pairs
of such pictures with a stereoscopic binocular
lens.
RESULTS
The mitochondria were well demonstrated
with both staining techniques. In cross or
slightly oblique sections (Figs. 3, 7, 121, the
mitochondrial profiles located between the
myofibrils varied considerably in size and
shape. The longitudinal sections (Figs. 4, 5,
13), they formed longitudinal or slightly
oblique columns. Individual members of these
mitochondrial columns were frequently rectangular, flattened, and slightly curved elements. They were in register with a myofibrillar zone demarcated by two Z lines and
were usually narrower at their equator in the
region facing the H band (Figs. 10, 13). Their
extremities frequently showed transverse expansions that ran parallel to each other on
each side of the H band (Fig. 15). In the perinuclear cytoplasm spheroidal mitochondria
accumulated in clusters.
In thick sections of heart muscle fibers impregnated with Ur-Pb-Cu, the endoplasmic reticulum showed a repetitive pattern along the
myofibrils (Fig. 5, 6). In register with the Abands, the cisternae, generally tubular in appearance, were mostly oriented parallel to the
myofibrils and showed numerous short arched
lateral connections (Fig. 2). In front of the H
bands, these tubular cisternae formed a tight
anastornotic network which did not show any
definite orientation in relation to the long axis
of the fiber (Figs. 2, 5, 6). In several places
along this ER network, but mostly facing the
junctions of the A and I bands, the tubular
cisternae merged to form plate-like cisternae
(Figs. 2, 6). In stereopairs of thick longitudinally or obliquely cut fibers, the ER network
facing the A bands appeared as curtain-like
structures with the cisternae aligned in a single plane (Figs. 12, 13). In reduced osmiumstained thin sections of longitudinally cut fibers, the overall organization of the ER network remained similar to the one previously
described,although the cisternae were slightly
more distended than in sections stained with
Ur-Pb-Cu (Fig. 4). In cross sections of muscle
fibers stained with reduced osmium, the linear
rows of cisternal profiles, a t the level of the Abands, clearly indicated that the cisternae
were distributed within a single plane a t the
interface of myofibrillar territories (Fig. 7).
The arrangement and distribution of ER cisternae around mitochondria was visualized, in
Ur-Pb-Cu stained preparations, even though
the mitochondria were heavily impregnated
with the metallic precipitate (Figs. 5, 10, 12,
15). The ER network surrounding mitochondria a t the level of the A-band was formed of
longitudinally running tubular cisternae
MITOCHONDRIA AND ER IN HEART MUSCLE FIBERS
141
A
\-
I
I-
Fig. 1. Diagrammatic representation of the tridimensional organization of the endoplasmic reticulum as seen in
a heart muscle fiber stained by the Ur-PbCu impregnation
procedUre.
A t the H-band level (H), middle of the A band (A), the
ER takes the appearance of a tight network of tubules. From
this network, cisternae of ER are roughly parallel to the
long axis of the myofibrils and extend in both directions
toward the I-band level where they merge in large meshes
over the A-I junction. Sometimes thew longitudinal cisternae join together to give rise to plate-like or flattened cisternae.
Over the mitochondria (M), the longitudinal cisternae are
more widely dieperaed and a tight network is seen to face
the depression of the mitochondria.
At the I-band level (I), cistemae of ER form a 3-Dnetwork
made up of large polygonal meshes a n a s t o m 4 in all directions of space. In the absence of the T-tubule, this meahwork simply extended across the I-band and showed some
terminal bulbous swellings (S).In the preeence of the Ttubule (t), ER were parallel on one or both sides of it. Over
or below the T-tubule, some #hod tubular or Aatbned
bridges of ER ensure longitudinal continuity.
In the space between the tipa of aajacent mitochondria
and facing the I-band, in absence of the T-tubule (upper
right), the ER form a loose three-dimensional network of
tubules. In presence of the T-tubule (lower right), ER cisternae are arranged in a casing around the distended Ttubule (t) and are in continuity with the longitudinally and
tranavemlly running ER cisternae.
Fig. 2. Endoplasmie reticulum and T-tubule in a 0.5-pm
thick longitudinal section of a cardiac muscle fiber impreg
nated with the Ur-Pb-Cutechnique. At the I-band level,
tubular cisternae (arrows)are running parallel on each side
ofthe T-tubule (TI. They are interconnected by longitudinal
tubular or plate-like elements (P)that bridge in front and
behind the T-tubules and the ER network (ER)extending
in front of two adjacent sarcomeres. A tight network of tubules is observed at the H-band level (H).
In the bottom of
the picture, the contours of an unreactive T-tubule are indicated by a broken line. 2: 2 band. X 117,000.
Fig. 3 Thin croea section through a cardiac muscle fiber
stained with ferrocyanide-reduced osmium. Mitochondria
(M) and sheets of endoplasmic reticulum (ER)delimit myofibrillar areas of various sizes and shapes. (m0. A region
of close contact between mitochondria and endoplasmic reticulum is indicated by two vertical arrows. N: nucleus.
x 15,200.
Fig. 4 Thin longitudinal section through a cardiac muscle fiber stained with ferrocyanide-reducedosmium. Mitochondria (M) approximately the size of a m o m e r e are arranged in longitudinal columns betweem the myofibrils. A t
the 2 band level (74, the lumen of transversely oriented Ttubule (T)becomes wider in the region comprised between
the tips of two adjacent mitochondria. At the H-band level,
the endoplasmic reticulum appears aa a fenestrated sheet
G: glycogen. x 33,000.
(arr~ws).
144
D. SEGRETAIN. A. RAMBOURG, AND Y.CLERMONT
which were more dispersed than those seen in
the adjacent areas (Fig. 12). Facing the depression seen at the equator of the mitochondria
which was in register with the H band, the
tubular cisternae formed a tighter net. The ER
network just described was usually seen along
one or two surfaces of the mitochondria but
frequently was also found to completely surround these organelles (Fig. 12). In thin sections stained with reduced osmium the tubular
cisternae seen along the mitochondria were
readily identified. They were occasionally
found in longitudinal grazing sections of the
mitochondria1 surface (Fig. 9) or appeared, in
cross sections, as membranous profiles either
isolated or in linear arrays closely associated
with the surface of mitochondria (Figs. 3, 7).
Continuous with the ER cisternae seen in
register with the A-band, the cisternae facing
the I-band formed a three-dimensional multilayered anastomotic network with large polygonal meshes (Fig. 2). This network was particularly evident in Ur-Pb-Cu stained thick
sections viewed in stereopairs (Figs. 13, 14).
In the absence of the T-tubule, this meshwork,
formed of tubular or plate-like cisternae, simply extended across the I-band (Figs. 5, 6, 13).
Some of these tubular cisternae terminated
with bulbous swellings which were heavily impregnated with Ur-Pb-Cu (Figs. 5,6,13). Such
terminal swellings were usually seen in small
clusters and were connected to the lattice of
anastomotic ER tubules (Figs. 6, 13). In the
presence of the T-tubule, some ER cisternae
formed tubular structures that ran parallel, on
one or both sides, of the T-tubule (Figs. 2, 14).
Such ER cisternae were connected to each
other by short bridging tubular cisternae running above or below the T-tubule (Figs. 2,14).
Other ER cisternae assumed a flattened and
curved shape with their concave face applied
to the surface of the T-tubule. Such incomplete
sleeve-like structures continuous with the rest
of the ER network were seen along the distended or non-distended segments of the T-tubule (Figs. 2, 14).
In the space seen between the tips of adjacent mitochondria, which were facing the I
bands, the ER cisternae formed, in the absence
of the T-tubule, a loose three-dimensional network of anastomotic tubules. In presence of the
T-tubule, which was usually distended in this
location, the ER cisternae formed a casing
around it made up of curved, flattened cisternae connected to each other by short anastomosing tubular cisternae (Fig. 15). The flattened cisternae were lodged in the narrow
spaces located between the extremities of the
mitochondria and the T-tubule (Fig. 15). This
system of ER cisternae was continuous with
the cisternae running longitudinally and parallel to the surface of mitochondria described
above. In thin sections stained with reduced
osmium, the multilayered arrangement of the
ER cisternae facing the I bands along the myofibrils was readily detected under the form of
superimposed layers of membranous profiles
(Fig. 8). Similarly, the close association of flattened ER cisternae with the surface of the distended T-tubule located in the intermitochondrial spaces was clearly and frequently
identified in such a material (Fig. 4).
In the subsarcolemmal cytoplasm, flattened
cisternae were interconnected by tubular elements (Fig. 11) and formed a well-developed
network of endoplasmic reticulum running
parallel to the cell surface.
DISCUSSION
Recent reviews on the ultrastructure of the
heart muscle fiber (McNutt and Fawcett, '74;
Katz, '77) have emphasized the pleomorphism
of mitochondria which, in living cells, vary
constantly in shape and distribution. In the
present study, the examination of the threedimensional distribution of mitochondria has
confirmed earlier observations (Moore and
Fig. 5 Overall architecture of the endoplasmic reticulum
as seen in 9.5-pm thick section of a cardiac muscle fiber
impregnated with the Ur-Pb-Cu technique. The endoplasmic reticulum forms longitudinally oriented repetitive
units the structure of which varies according to their position in front of the A or the I bands. In h n t of the Aband, numerous elongated flattened cisternae run parallel
to the main axis of the fiber (horizontal arrows). Towards
the mid portion of the A-band, they merge with a tight
In
network of tubules located in front of the H-band (H).
front of the I-band and Z line (Z), the tubular cistemae are
more loosely anastomosed and display bulbous Swellings
(arrowhead). Plate-like cisternae, in the vicinity of the AI junction are labeled P. The strongly impregnated mitochondria (MI are arranged in longitudinal columns. x
35,000.
Fig. 6. Endoplasmic reticulum as seen in a 0.5-pm thick
longitudinal section of a cardiac muscle fiber. Ur-Pb-Cu
technique. Fmm the tight network of tubules located in
front of the H-band (H), elongated flattened cisternae run
parallel to the long axis of the fiber towards the A-I junction
where they are interconnected by numerous lateral arched
connections. At the Z-band level (Z), in the middle part of
the picture, the loosely anastomoaed cisternae are terminated by bulbous endings (arrows). In the lower part of the
picture, a plate-like cisternae (P) is seen to bridge transversely oriented ER elements over an empty space that corresponds to an unstained T-tubule (T)the contours of which
are indicated by a dotted line. Other plate-like cisternae
(arrowheads) are seen at the A-I junctions. x 90,OOO.
MITOCHONDRIA AND ER IN HEART MUSCLE FIBERS
145
Fig. 2. Endoplasmie reticulum and T-tubule in a 0.5-pm
thick longitudinal section of a cardiac muscle fiber impreg
nated with the Ur-Pb-Cutechnique. At the I-band level,
tubular cisternae (arrows)are running parallel on each side
ofthe T-tubule (TI. They are interconnected by longitudinal
tubular or plate-like elements (P)that bridge in front and
behind the T-tubules and the ER network (ER)extending
in front of two adjacent sarcomeres. A tight network of tubules is observed at the H-band level (H).
In the bottom of
the picture, the contours of an unreactive T-tubule are indicated by a broken line. 2: 2 band. X 117,000.
Fig. 9. Thin longitudinal section through a cardiac muscle fiber stained with fermcyanide-reducedosmium. At the
H-band level (H),
a transversely oriented membranous profile (ER) is seen to leave at right angles to a longitudinally
oriented cisternae of the endoplasmic reticulum (arrowheads). G glycogen; M: mitochondrion. x 105,000.
Fig. 10. Thick (1 km) longitudinal section through a
cardiac muscle fiber impregnated with the Ur-Pb-Cu technique. Longitudinally oriented ER cisternae (ER) are seen
on each side of the squarish mitochondria (M)at the A-band
level (A). In the upper part of the picture, plate-like cisternae (P) are sandwiched between the distended T-tubdm
(T)and the tips of the mitochondria. X 95,000.
Fig. 11. Thick section (0.5 pm) parallel to the surface
of a cardiac muscle fiber impregnated with the Ur-Pb-Cu
technique. The subsarcolemmal network of endoplasmic r e
ticulum is made up of platelike cisternae (arrows) interconnected by tubular elements. M: mitochondria; mf myofibril. x 33,000.
148
D. SEGRETAIN, A. RAMBOURG, AND Y.CLERMONT
Figures 12-15 are stereopairs. The two membera of each
pair of photographs of the same field are taken at different
angles ( f7")h m the original 0" position of the goniometric
stage. A single stereoscopic image of the impregnated organelles may be obtained by using a stereoscopic binocular
lens, the distance between the central axes of the two lenses
being adjusted to 65-70 mm. All figures were taken from
1-pm thick sections of cardiac muscle fibers impregnated
by the Ur-Pb-Cu technique.
Fig. 12. Overall distribution of endoplasmic reticulum
and mitochondria as seen in an oblique section. The endoplasmic reticulum at the A-band level (A) forma a single
sheet of densely packed elements which is more loosely anaatomosed and displays bulbous terminal endings at the Iband level (I). Note the close relationship between mitochondria (M)and endoplasmic reticulum (arrowhead). x
11.700.
Fig. 13. Longitudinal section showing the difference in
the three-dimensional arrangement of the endoplasmic reticulum at the I- and A-band levels. Whereas the H-band
network and the longitudinal cisternae at the A-band level
are mainly located in a single plane, the tubular cisternae
at the I-band level are oriented and anastomosed in all
directions of space (arrows). Bulbous terminal swelling are
also seen in this location. Between adjacent extremities of
two longitudinally arranged mitochondria (MIan unstained
T-tubule (TI is surrounded by heavily stained flattened cistemae. x 25,000.
Fig. 14. Three-dimensional distribution of the endoplaamic reticulum and T-tubule at the I-band level. Transversely oriented tubular cisternae run on each side of the
T-tubule (T) located over one half of the I-band on the left
of a Z line (2). They are interconnected by small bridges
(arrow) over and below the T-tubule. A flattened and
slightly curved cistema may be seen in the lower half of the
field (arrowhead). x 43,000.
Fig. 15. Three-dimensionalarrangement of endoplasmic
reticulum and mitochondria at the T-tubule level. In this
longitudinal thick section, the mitochondria (M)give rise
to short transverse expansions which are surrounded by a
network of tubular ER cisternae. At left, the two flattened
ER cisternae running parallel to the transverse expansions
of two adjacent mitochondria (arrows) are interconnected
by a loose network of tubular elements making up a caselike structure around the unstained T-tubule. x 19,500.
MITOCHONDRIA AND ER IN HEART MUSCLE FIBERS
149
150
D. SEGRETAIN, A. RAMBOURG, AND Y. CLERMONT
Ruska, '57; Porter and Palade, '57; Muir, '67;
Fawcett and McNutt, '69) and shown that
squarish flattened mitochondria, approximately the size of a sarcomere, are arranged
in longitudinal columns in clefts between the
myofibrils.
Earlier studies on the structural organization of the endoplasmic reticulum in cardiac
muscle fiber have demonstrated clearly the
transverse and longitudinal continuity of this
intracytoplasmic
membranous
system
throughout the muscle fibers (Porter and Palade, '57; Fawcett and McNutt, '69). This morphological feature was amply verified in the
present study on thick sections stained with
Ur-Pb-Cu. Furthermore, the variability of its
configuration in various animal species was
stressed (McNutt and Fawcett, '74; Sommer
and Waugh, '76). Thus, for example, the existance of a network of tubular cisternae, in
front of the H band, described in the heart
muscle of hamster and mouse (Sommer and
Waugh, '76) and in the rat, as shown in the
present study, was not observed in the cat
myocardium (Fawcett nd McNutt, '69). These
various studies have not, however, stressed the
differences in the three-dimensional organization of the ER cisternae at the level of the
A- and I-bands or presented detailed descriptions of the three-dimensional relationships of
the ER cisternae with the T-tubules or mitochondria.
When stereopairs of thick sections stained
with Ur-Pb-Cu are examined, the ER a t the
A-band level appears as a single sheet of anastomotic tubular or plate-like cisternae,
whereas a t the I-band level it forms a multilayered network (Fig. 1).In the absence of Ttubules this loose network of ER tubular cisternae in front of the Z line is often associated
with clusters of bulbous terminal swellings
(Fig. 1).These terminal bulbs seemingly correspond to the coated vesicles observed in thin
sections by Fawcett and McNutt ('69) and
thought to bud or fuse with the endoplasmic
reticulum. Sommer and Waugh ('76) working
with the high voltage EM on 1-pm thick sections observed, as in the present investigation,
that such membranous profiles were always
continuous with the endoplasmic reticulum.
The functional role of these clusters of terminal ER swellings remains to be discovered.
In the presence of T-tubules the ER cisternae
were seen to assume a variable configuration.
Frequently, however, tubular cisternae, continuous with the rest of the ER network, were
found to run transversely, parallel, and on
each side of the T-tubules. Simpson ('65) and
Simpson and Rayns ('68) have emphasized the
existence of such transverse tubules following
the Z line and called them "Z-tubules." McNutt
and Fawcett ('74) indicated that such tubules
did not differ from ER cisternae. The present
description is in accord with this view and further demonstrates the existence of numerous
short tubules bridging these transversal tubules over and below the interposed T-tubule
(Fig. 1).
In models prepared from thin sections (Perter and Palade, '57; Nelson and Benson, '63;
Fawcett and McNutt, '69; Katz, '75) areas of
close contact between ER and the T-tubule
were usually shown as blind-end, foot-like expansions of the cisternae applied to the surface
of the T-tubule. In Ur-Pb-Cu stained thick sections such foot-like ER cisternae were not seen.
While the T-tubule-associated ER cisternae
assumed a flattened and curved shape closely
apposed to the surface of the T-tubule, as demonstrated by Simpson and Rayns ('68), Farbes
et al. ('771, and Forbes and Sperelakis ('771,
they were shown in the present study to be
continuous by means of numerous branches
with the rest of the ER network (Fig. 1).Furthermore, the analysis of thick sectons also
revealed that zones of close contact between
endoplasmic reticulum and distended segments of the T-tubule were found preferentially in the sarcoplasm located between the
extremities of mitochondria and the interposed
T-tubule (Fig. 1).The functional significance
of
this
association:
T-tubules-ER
cisternae-mitochondria remains to be clarified,
Rambourg and Segretain ('80) in their threedimensional study of the ER of red muscle fibers of the rat diaphragm speculated that the
bidimensional and three-dimensional configuration of the ER network facing the A- and
I-bands, respectively, represented an adaption
of this organelle t o the selective shortening of
the myofibrils at the I-band level during contraction. In the cardiac muscle fibers some of
the tubular ER cisternae were shown to be
attached to the Z-line by a cytoplasmic filamentous material (Simpson and Rayns, '68;
Edge and Walker, '70; Jewett et al., '71). During contraction the ER cisternae would be stabilized at that point and be prevented from
moving out of the I bands, but, simultaneously,
would form a t this level a compact multilayered network. Furthermore, since the mitochondria are likely to be maintained in position by the endoplasmic reticulum, the
MITOCHONDRIA AND ER IN HEART MUSCLE FIBERS
relationship between mitochondria, endoplasmic reticulum, and T-tubule would remain
constant during the cycle of contraction and
relaxation.
ACKNOWLEDGMENTS
This work was supported by a France-Quebec Exchange program. The work done a t
McGill University was supported by the Medical Research Council of Canada. The help of
Dr.L. Hermo in the preparation of the manuscript is acknowledged.The drawing was prepared by Margo Oeltzschner, McGill University.
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fiber, dimensions, muscle, endoplasmic, arrangement, reticulum, heart, three, rat, mitochondria
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