close

Вход

Забыли?

вход по аккаунту

?

Evidence for a structural relationship between sarcoplasmic reticulum and Z lines in dog papillary muscle.

код для вставкиСкачать
Evidence for a Structural Relationship Between
Sarcoplasmic Reticulum and Z Lines i n
Dog Papillary Muscle
MAXINE B. EDGE AND SHEPPARD M. WALKER
Department of Physiology and Biophysics, University of Louisville
School of Medicine, Louisville, Kentucky 40202
ABSTRACT
Electron micrographs of dog papillary muscle fibers show the sarcoplasmic reticulum (SR) as a n extensive three-dimensional network of diverging and
converging tubules that is continuous throughout each sarcomere and i n successive
sarcomeres. The network surrounds each myofibril and is continuous across the fiber.
Tubules of the SR network are oriented transversely and longitudinally. Interconnections of transversely and longitudinally oriented SR tubules form circular pathways
in the network. A structural relationship appears to exist between the 2 line and
longitudinally and transversely oriented tubules of the SR network a t their points
of apposition with the Z line. A rather constant space approximately 100 A wide
is seen between SR tubules apposed at the 2 line in cross-sections and in favorable
longitudinal sections of fibrils. This space is traversed by electron-opaque strands
which appear to be connections between SR tubules and the Z line. Other lines
of evidence support the view that Z lines and SR tubules are connected. One of these
is the change of position of 2 line SR tubules in relation to the change of position
of Z lines in adjacent fibrils. Another line of evidence is the continuity of SR
apposed at the Z line and SR connected to the sarcolemma.
The interfibrillar spaces of mammalian
cardiac muscle fibers contain two distinct
membranous systems, the T system and
the sarcoplasmic reticulum (SR). Tubules
of the T system traverse the muscle fiber
transversely, i.e., at right angles to the
longitudinal axis of the sarcomeres, and
are seen almost exclusively at the Z regions in heart muscle (Sommer and Johnson, '68; Simpson, '65). Occasionally
longitudinally oriented tubules of the T
system are seen between the myofibrils
(Simpson, '65). At the level of the Z line
transverse tubules of the T system are
continuous with the sarcolemma (Simpson
and Oertelis, '61). Porter and Palade ('57)
were the first to observe in cardiac muscle
fibers the network of SR tubules existing
throughout the sarcomere. Simpson and
Rayns ('68) described the SR as a threedimensional network of fine tubules with
both longitudinal and transverse orientation extending throughout the cell between
the myofibrils and forming a lacework
around the sarcomere. In early electron
microscope studies the T system was observed in apposition with the network of
SR tubules at the level of the Z line (Porter
and Palade, '57). In a more recent study
ANAT.REC., 166: 51-66.
apposition of SR at the sarcolemma is
seen in cardiac muscle fibers (Johnson
and Sommer, '67). Several investigations
have called attention to the close association of interfibrillar tubules and the Z line.
In a recent report SR tubules were found
to have a particularly constant relationship to the Z lines, each of which is surrounded by tubules of SR (Simpson and
Rayns, '68). In studies on rabbit cardiac
muscle Johnson and Sommer ('67) observed that many of the tubules associated
with Z lines traversed the interfibrillar
space, usually from Z line to Z line. In
an investigation on mammalian myocardial
cells (Simpson, '65), SR tubules which
appear to run horizontally around the myofibril at each Z line were found in crosssections of fibers. These circumferential
tubules are about 220 A in diameter and
appear to be joined by longitudinally running tubules.
A close relationship of the SR tubule
network and Z lines is found in skeletal
Received May 28, '69. Accepted July 16, '69.
1 We thank Mrs. Glenna Currier for technical assistance.
ZThis work was aided by NIH grant 5-FOI-GM38 165-02and grants from the American Heart Association and the Kentucky Heart Association.
51
52
MAXINE B . EDGE AND SHEPPARD M. WALKER
muscle fibers of fetal and newborn rats
(Walker et al., '68, '69). Studies of adult
skeletal muscle fibers failed to show distinct and separate SR tubules at the Z
line level. A typical example of findings
is illustrated in figure 3 of a previous report (Walker and Schrodt, '66). However,
a review of the literature showed that Z
lines of adult mammalian cardiac muscle
fibers are encircled by separate and distinct SR tubules. The purpose of the present study is to examine the points of contact between networks of SR tubules and
Z lines in cardiac muscle fibers of dogs and
to look for evidence of structural relationships at these points. A preliminary report
of this study has been given (Edge and
Walker, '69). The results of the study
suggest that connections exist between
SR tubules and Z lines.
METHODS
Adult dogs anesthetized with sodium
pentobarbital were usually injected with
a concentrated solution of KC1. The injection of KC1 causes the heart to stop in a
relaxed state thereby preventing fibrillation
during the removal of papillary muscle
bundles. Hearts from dogs with no KC1 injection were used in a few experiments.
Small bundles of fibers separated from
papillary muscles of the right ventricle
were tied to Plexiglas stays. The tissue was
fixed for two weeks in 3% glutaraldehyde
by the method of Sabatini et al. ('63) and
postfixed in OsOl by the Palade method
as modified by Caulfield ('57). The fibers
were dehydrated for ten minutes in 50%
alcohol and then placed overnight in 70
parts absolute alcohol and 30 parts saturated aqueous solution of uranyl acetate.
After completion of dehydration with 95%
and 100% alcohol the fibers were embedded in Maraglas and sectioned with an
LKB microtome. Thin sections were triple
stained with lead citrate, uranyl acetate,
and lead citrate. A modification of the
Reynolds method ('63) was used for lead
citrate staining. Sections exhibiting grey
or light silver interference colors were
examined with a Siemens Elmiskop IA
electron microscope.
RESULTS
The SR forms an extensive three-dimensional network of diverging and converging
tubules that extends throughout each sarcomere and in successive sarcomeres. The
network surrounds each myofibril and is
continuous across the fiber. Many of the
tubules are oriented along the longitudinal
axis of the fiber, but transversely oriented
tubules can be seen at all levels of the
sarcomere. The network can be visualized
best in sections with the plane of section
passing along and parallel to the surface of
the fibril. In such sections it is possible to
see a face-on view of the network which
shows the ramifications and interconnections of the tubules in the network (figs.
1-3). Longitudinal sections of both transversely and longitudinally oriented tubules
are visible in these sections. It can be seen
in figures 2 and 3, at points where the network shows most favorably, that the myofilaments are sparse, indicating that the
plane of section is passing along the surface of the fibril. Some interconnections of
transversely and longitudinally oriented
tubules of the SR forming circular pathways are illustrated by closed stars in
figures 1-3. Transverse orientation of tubules in the network are designated by
open stars in figures 1-3. It is noteworthy
that the SR tubules associated with Z lines
are usually transversely oriented and rather
exactly in register with the Z line (asterisks, figs. 1-3). However, some of the tubules at the Z line level appear to be longitudinally oriented as illustrated by vertical
arrows in figure 1. These longitudinally
and transversely oriented tubules at the Z
line level appear to be a part of the SR
network.
If sections are viewed with the plane of
section passing through the center of the
fibril rather than along its surface i t is
possible to see a cross-section of the SR
network rather than a face-on view (fig. 4).
Because the network is composed of longitudinally and transversely oriented tubules
a part of the tubule images will appear
as cross-sections while others appear as
longitudinal sections. Some of the crosssections of transversely oriented tubules
apposed at the Z line level are illustrated by
asterisks in figure 4. Vertical arrows (fig. 4)
designate longitudinal sections of longi-
SARCOPLASMIC RETICULUM AND Z LINES
tudinally oriented tubules closely associated with the Z line. In sections where the
limiting membranes of SR tubules are
clearly visible, a space of approximately
100 A separates the Z line from the SR
tubules (inset, fig. 4). The space between
SR tubules and the Z line is traversed by
electron-opaque strands some of which
are designated by horizontal arrows (inset,
fig. 4).
In addition to electron-opaque strands
across the space between apposed SR tubules and Z lines other lines of evidence
found in longitudinal sections of fibers support the view that Z lines and SR tubules
are closely associated. One of these is the
change of position of Z line SR tubules
in relation to the change of position of
Z lines in adjacent fibrils. In unstretched
fibers Z lines of all fibrils are aligned across
the entire fiber. In these unstretched fibers
SR tubules extending between adjacent
Z lines are always in register with the
Z line. Fibers that are stretched before
they are placed in fixative frequently show
Z lines of adjacent fibrils that are displaced from the normal transverse alignment (fig. 5). In such fibers the SR between adjacent Z lines is displaced from a
transverse orientation to precisely the same
degree as Z lines. Furthermore the SR at
its point of contact with the fibril is very
closely associated with the Z line (asterisks,
fig. 5). Figure 5 also shows the relationship of SR with T system tubules. It can be
seen that the SR associated with T system
tubules is continuous with SR tubules apposed at the Z line.
Another line of evidence that indicates
the close association of SR tubules and Z
lines is related to SR connected with the
sarcolemma. The SR connected with the
sarcolemma is continuous with SR apposed
at the Z line (fig. 6 ) . A space between the
SR tubule and the Z line can be seen. This
space is traversed by electron-opaque
strands (arrows, fig. 6 ) . The Z line is the
only point in the sarcomere that is in
close apposition with SR that also makes
connections with the sarcolemma. It is
interesting to note that SR connected with
the sarcolemma and SR connected with
T system tubules are very similar in appearance (figs. 5, 6). Both connections
show a space approximately 100 A wide
53
between the apposed membranes. Electronopaque strands connect the SR with the
sarcolemma and the T system.
In cross-sections, tubules of the SR network appear to surround the myofibril
completely at the level of the Z line. An
extensive and almost exact cross-section
of a fibril at the level of a Z line is seen
in figure 7. Although at some points the
outer wall of the encircling SR tubule is
poorly preserved long segments of intact
membrane of the inner wall apposed at the
Z line are observed. Close inspection reveals a space between the Z line and the
apposed limiting membrane of the SR tubule (upen arrow, fig. 7 ) . Although the
width of the space is variable it is usually
about 100 A. It is noteworthy in both
cross-sections (fig. 7) and longitudinal
sections (inset, fig. 4) of fibrils that, when
the characteristic space between SR tubules
and the Z line is clearly visible, electronopaque strands traversing the space are
consistently seen (arrows). It should be
noted in passing that transversely oriented
tubules of the SR network can be seen at
levels of the sarcomere other than at the
Z line (open stars, figs. 1, 2, 7).
When cross-sections are made at the Z
line level of subsarcolemmal fibrils invaginations of the sarcolemma which form
the T system are frequently seen. Figure
8 illustrates one of these invaginations
and a longitudinal section of another
T system tubule. It can be seen that
transversely oriented tubules of SR are
interposed between the T system and
the Z line (asterisk, fig. 8). Figure 8 illustrates the rather constant space between
the SR and the Z line (open arrow). At
many points electron-opaque strands traversing the space can be seen. Some of
these strands are indicated by arrows.
Electron-opaque strands are seen quite well
at a higher magnification (inset, fig. 8).
DISCUSSION
Electron microscope studies have shown
a close association of SR tubules and the
Z line in skeletal as well as cardiac muscle
fibers. The observations of Allen and Pepe
('65), showing SR tubules at the Z line
level in skeletal muscle fibers from embryonic and posthatched chicks, suggest a
close association between SR tubules and
54
MAXINE B. EDGE AND SHEPPARD M. WALKER
Z lines. Sommer and Johnson ('68) found
a component of the SR that is closely associated with the Z Line in ventricular and
Purkinje fibers from hearts of guinea pig,
rabbit, cat, goat and sheep. In a recent
study on myocardial cells of the ferret,
Simpson and Rayns ('68) described a
particularly constant element of the SR
encircling the myofibrils at the Z regions.
Shimada et al ('67) described an extensive SR network surrounding the Z and I
bands of myofibrils of embryonic chick
skeletal muscle. The SR tubular network
is equally distributed throughout the sarcomere in dog papillary muscle fibers
(figs. 1-3).
The wide variety of SR tubule profiles
at the Z line level in longitudinal sections
of papillary muscle fibers (figs. 1-4) can
be explained by comparing appropriate
sections through the fibril. The network of
SR tubules and the continuity of this network with SR which is apposed at the Z
line is conspicuous in longitudinal sections
parallel to the surface of the fibril. Crosssections at the Z line level show almost continuous encirclement of the Z line by SR
tubules. In addition a rather constant
space (approximately 100 A) between SR
tubules and the Z line is seen in crosssections. Because this space is seen consistently in cross-sections and only occasionally in longitudinal sections it seems
reasonable to suppose that favorable viewing of the space requires that the plane of
section be perpendicular to the surface
of the fibril. When the plane of section
stays within the Z line there is assurance
that the cross-section is exact and that
the plane of section is perpendicular to
the surface of the fibril (fig. 7). From the
observations on cross-sections one can
conclude that the visibility of the space
between SR tubules and the Z line in longitudinal sections is due to fortuitous sections where the plane of section passes perpendicular to the surface of the fibril
(inset, fig. 4). When this space is clearly
visible electron-opaque strands are consistently found traversing it (inset, fig. 4,
figs. 6-8).
The observations that tubules of the SR
network are found in register with the Z
line and that a rather constant space traversed by electron-opaque strands is con-
sistently seen in favorable sections suggest a structural relationship between the
network of SR tubules and Z lines. There
are other lines of evidence that support the
view that the electron-opaque strands are
connections between SR tubules and the
Z lines. The observation that Z lines are
frequently displaced from their transverse
alignment when fibers are stretched before
fixation suggests that a force or forces
are exerted on SR connecting 2 lines of
adjacent fibrils (fig. 5). Since the interfibrillar SR at the Z line level is displaced
from a transverse orientation to the same
degree as Z lines, it seems reasonable to
suppose that such a force or forces are
not sufficient to cause displacement of SR
at the point of attachment to the Z line.
Another SR tubule profile that indicates
attachment of SR tubules and Z lines is
seen in the relation between subsarcolemma1 SR and subsarcolemmal sarcomeres.
A SR tubule connected to the sarcolemma
and continuous with SR apposed at the Z
line similar to that described for fetal and
newborn rats (Walker et al., '69) can be
seen in figure 6. A space approximately
100 A wide traversed by electron-opaque
strands can be seen between the limiting
membrane of the SR tubule and the Z line.
This type of relationship seen only at the
Z line suggests that the Z line is the exclusive site of attachment of SR which is
also attached to the sarcolemma. Since
apparent connections between SR and the
Z line and SR and the sarcolemma are
frequently seen in stretched fibers, it seems
reasonable to suppose that the points of
association between SR and the Z line and
SR and the sarcolemma might resist forces
produced by the stretch.
It seems likely that the electron-opaque
strands are connections between SR tubules and the 2 line homologous to those
found in skeletal muscle fibers (Walker et
al., '68, '69). Such connections might account for the observed fact that the fibril
and the surrounding SR of a given fibril are
in register. Furthermore, interfibrillary
continuity of SR at the Z line level and
the continuity of SR connected to the
sarcolemma and that apposed at the Z
line might contribute to transverse alignment of sarcomeres in adjacent fibrils
across the fiber.
SARCOPLASMIC RETICULUM AND Z LINES
It is interesting to note that adult papillary muscle fibers are similar in many
respects to skeletal muscle fibers from
fetal and newborn animals (Walker et
al., '69). The fibrils are separated by a
rather large interfibrillar space which contains an extensive tubular network of SR
that extends throughout the fiber. Tubules
of the SR network are closely associated
with the Z line. A rather constant space of
approximately 100 A separates SR tubules
from the Z line and apparent connections
between SR tubules and the Z line have
been described. Furthermore, the part of
the SR network apposed at the Z line is
continuous with SR attached to the sarcolemma.
Allen and Pepe ('65) found Z lines associated with the interfibrillar tubular system in the 5-day chick embryo. However,
the relationship of SR and the Z line during development of the fibril is a question
that remains unanswered. An attempt will
be made to look for the onset of SR tubule
formation and the earliest association of
this tubule with Z lines during formation
of myofibrils in fetal cardiac muscle fibers.
LITERATURE CITED
Allen, E. R., and F. A. Pepe 1965 Ultrastructure of developing muscle cells in the chick
embryo. Am. J. Anat., 116: 115.
Caulfield, J. B. 1957 Effects of varying the
vehicle for OsOr in tissue fixation. J. Biophys.
Biochem. Cytol., 3: 827.
Edge, M. B., and S. M. Walker 1969 Electron
microscope study of the sarcoplasmic reticulum
at the 2 line level i n mammalian cardiac muscle. Fed. Proc., 28: 822.
55
Johnson, E. A., and J. R. Sommer 1967 A
strand of cardiac muscle. Its ultrastructure and
the electrophysical implications of its geometry. J. Cell Biol., 33: 103.
Porter, K. R., and G. E. Palade 1957 Studies on
the endoplasmic reticulum. 111. Its form and
distribution in striated muscle cells. J. Biophys.
Biochem. Cytol., 3: 269.
Reynolds, E. S. 1963 The use of lead citrate a t
high pH as a n electron-opaque stain in electron
microscopy. J. Cell Biol., 17: 208.
Sabatini, D. D.,K. Bensch and R. J. Barrnett
1963 Cytochemistry and electron microscopy.
The preservation of cellular ultrastructure and
enzymatic activity by aldehyde fixation. J. Biophys. Biochem. Cytol., 17: 19.
Shimada, Y.,D.A. Fischman and A. A. Moscona
1967 The fine structure of embryonic chick
skeletal muscle cells differentiated in witto. J.
Cell Biol., 35: 445.
Simpson, F. 0. 1965 The transverse tubular
system in mammalian myocardial cells. Am. J.
Anat., 117: 1.
Simpson, F. O., and S. J. Oertelis 1961 Relationship of the sarcoplasmic reticulum to sarcolemma in sheep cardiac muscle. Nature, 189:
758.
Simpson, F. O., and D. G. Rayns 1968 The relationship between the transverse tubular systems and other tubules a t the 2 disc levels of
myocardial cells in the ferret. Am. J. Anat.,
122: 193.
Sommer, J. R.,and E. A. Johnson 1968 Cardiac
muscle. A comparative study of Purkinje fibers
and ventricular fibers. J. Cell Biol., 36: 497.
Walker, S. M., and G. R. Schrodt 1966 Connections between the T system and sarcoplasmic
reticulum. Anat. Rec., 155: 1.
Walker, S. M., G. R. Schrodt and M. Bingham
1968 Electron microscope study of the sarcoplasmic reticulum a t the 2 line level i n skeletal muscle fibers of fetal and newborn rats. J.
Cell. Biol., 39: 469.
1969 Evidence for connections of the
sarcoplasmic reticulum with the sarcolemma
and with the 2 line in skeletal muscle fibers
of fetal and newborn rats. Am. J. Phys. Med.,
48: 63.
PLATE 1
EXPLANATION OF FIGURES
56
1
Electron micrograph showing a longitudinal section of an adult dog
papillary muscle fiber. A face-on view of the SR network is seen.
The asterisk is placed above a point showing the close association of
a transversely oriented SR tubule with Z lines. The interconnections
of transversely and longitudinally oriented tubules forming circular
pathways in the SR network are illustrated by closed stars. Vertical
arrows designate longitudinal sections of longitudinally oriented
tubules closely associated with the Z line. Some of the transversely
oriented tubules of SR at levels other than the 2 are indicated by
open stars. SR, sarcoplasmic reticulum; 2, 2 line; M, M line;
Mi, mitochondrion. x 49,000.
2
Longitudinal section of a dog papillary muscle fiber showing a transversely oriented SR tubule in register with the 2 line (asterisk). The
closed star designates a circular pathway of the SR network formed by
interconnections of transversely and longitudinally oriented SR tubules. A transverse element of the SR is indicated by the open star.
SR, sarcoplasmic reticulum; 2, Z line. X 35,000.
3
Longitudinal section of a dog papillary muscle fiber showing the
close association of SR tubules with 2 lines (asterisk). A circular
pathway of the SR network is indicated by the closed star. SR, sarcoplasmic reticulum; 2, Z line. X 35,000.
SARCOPLASMIC RETICULUM AND Z LINES
Maxine B. Edge and Sheppard M. Walker
PLATE 1
57
PLATE 2
EXPLANATION OF FIGURE
4
58
Longitudinal section of a dog papillary muscle fiber showing crosssections of transversely oriented SR tubules in apposition with Z lines
(asterisks). Vertical a r r ~ w s designate longitudinally oriented SR
tubules in close association with Z lines. SR, sarcoplasmic reticulum;
Z, Z line; M, M line; Mi, mitochondrion; T, T system tubule. x 28,000.
Inset: A higher magnification of the labeled Z line. The arrows are
directed toward electron-opaque strands traversing the space between
a longitudinally oriented SR tubule and the Z line. Electron-opaque
strands traversing the space between the cross-section of a transversely oriented SR tubule and the Z line are also seen i n the left of
the inset. x 76,000.
SARCOPLASMIC RETICULUM AND Z LINES
Maxine B. Edge and Sheppard M. Walker
PLATE 2
59
PLATE 3
EXPLANATION OF FIGURES
5 Longitudinal section of a dog papillary muscle fiber showing Z lines
of adjacent fibrils that are displaced from the normal transverse
alignment. The interfibrillar SR is displaced from a transverse
orientation and shows close association with the Z lines (asterisks).
SR, sarcoplasmic reticulum; 2, 2 line; T, T system tubule. X 102,000.
6 Longitudinal section of a dog papillary muscle fiber showing the continuity of SR connected to the sarcolemma and SR associated with
the Z line. The arrows are directed toward electron-opaque strands
traversing the space between the Z line and the SR tubule. SR, sarcoplasmic reticulum; Z, Z line; B, basement membrane; S, sarcolemma.
x 160,000.
60
SARCOPLASMIC RETICULUM AND Z LINES
Maxine B. Edge and Sheppard M. Walker
PLATE 3
61
PLATE 4
EXPLANATION O F FIGURE
7
62
Cross-section of a dog papillary muscle fiber showing a Z line
left of the figure. The plane of section passes almost exactly t
the transverse axis of the fibril. Several long segments of
tudinally sectioned SR tubules are seen i n close association w
Z line (asterisks). The open arrow designates the space ( a
mately 100 A ) between the SR tubules and the Z line. N u
electron-opaque strands traverse the space, several are indic;
arrows. Open stars, transversely oriented SR tubule a t the A ban
SR, sarcoplasmic reticulum; Z, 2 line; I, I band; A, A band; 1
ment membrane; S , sarcolemma. X 87,000.
SARCOPLASMIC RETICULUM A N D Z LINES
PLATE 4
Maxine B. Edge and Sheppard M. Walker
63
PLATE 5
EXPLANATION OF FIGURE
8
64
Cross-section of a dog papillary muscle fiber. The plane of section
passes through the Z line (2) of several fibrils and through the
I band ( I ) of others. Numerous SR tubule sections (asterisks) interposed between the T system tubule ( T ) and the Z line are seen at the
borders of the Z lines. The open arrow is directed toward the space
(approximately 100 A) between SR tubules and the Z line. The
arrows are directed toward electron-opaque strands traversing the
space. SR, sarcoplasmic reticulum; Mi, mitochondrion; S, sarcolemma;
B, basement membrane. x 63,000. Inset: A higher magnification
of the Z line and associated SR seen i n the top of the figure. The
arrows are directed toward electron-opaque strands traversing the
space between the SR tubule and the Z line. x 102,000.
SARCOPLASMIC RETICULUM AND Z LINES
Maxine B. Edge and Sheppard M. Walker
PLATE 5
65
Документ
Категория
Без категории
Просмотров
26
Размер файла
1 463 Кб
Теги
structure, muscle, papillary, evidence, reticulum, sarcoplasmic, dog, relationships, line
1/--страниц
Пожаловаться на содержимое документа