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The organization of the contractile apparatus of vertebrate smooth muscle.

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The Organization of the Contractile Apparatus of
Vertebrate Smooth Muscle '
RICHARD M. BOIS2J
Department of Anatomy, University of C a l i f o ~ n i a ,
Los Angeles, California 90024
ABSTRACT
Smooth muscle of the small intestine of the rat was fixed by
vascular perfusion employing aldehydes in a balanced salt solution, followed
by immersion fixation in aldehydes and post-osmication. In such tissue preparations thick filaments approximating 140 A in diameter are observed in virtually
all the smooth muscle cells. The thick filaments are rather uniformly distributed
among the more numerous thin filaments. The nearest neighbor distances between the thick filaments range from 400 to 700 A. The thick to thin filaments
ratio is found to apporximate 1 : 12. Only thin filaments are observed in the most
distal segment of terminal processes of muscle cells and the tips of these processes appear to be lined by attachment plaques. A clear segregation of the thick
filaments from both dense bodies and attachment plaques is seen. Distally along
the tapering extremities of muscle cells progressively more of the plasma membrane is found lined by attachment plaques. These observations are interpreted
as strong evidence that the contractile apparatus of the vertebrate smooth muscle
cell consists of interdigitating arrays of thick and thin filaments collated into
contractile units by the anchoring of the thin filaments in dense bodies and
attachment plaques.
The initial demonstrations of significant
numbers of thick filaments in vertebrate
smooth muscle prepared for thin section
electron microscopy (Nonomura, '68;
Pease, '68) were followed by numerous
ultrastructural studies of the myofilaments
of the vertebrate smooth muscle cell. Although several of these studies succeeded
in clearly demonstrating interspersed arrays of thick and thin filaments (Kelly and
Rice, '69; Rice et al., '70a,b; Devine and
Somlyo, '71; Garamvolgyi et al., '71; Cooke
and Fay, '72; Kelly and Arnold, '72; Fay
and Cooke, '73), the authenticity of the
thick filaments has been questioned (Lowy
and Small, '70; Panner and Honig, '70;
Rosenbluth, '71, '72; Small and Squire,
'72). In a few recent studies of vertebrate
smooth muscle ribbon-like structures
thought to consist largely of myosin have
been observed (Lowy and Small, '70;
Small et al., '72; Small and Squire, '72;
Sobieszek, '72). Because of a paucity of
dense bodies seen in smooth muscle
preparations exhibiting such myosin "ribbons" Lowy and Small ('70) have suggested that the dense bodies commonly
seen in vertebrate smooth muscle are artiANAT. REC., 177: 61-78.
factual and have proposed a new and unconventional model of the contractile apparatus of the vertebrate smooth muscle
cell. In this model myosin "ribbons" are
conceived to interact in series with rows
of actin filaments without intervening
dense bodies or other devices for the permanent anchoring of the arrays of actin
filaments within the sarcoplasm of the cell.
Such contractile units are assumed to be
anchored only in attachment plaques associated with the plasma membrane by
their terminal arrays of actin filaments.
The purpose of this investigation was to
examine the state of myosin aggregation
and the organization of the contractile
apparatus in vertebrate smooth muscle. It
was hypothesized that a rapid fixation by
vascular perfusion employing a balanced
physiological salt solution would more
faithfully preserve smooth muscle, revealReceived April 23, '73. Accepted May 31, '73.
1 This investigation was supported by USPHS grant
HL 01770
ZThe author wishes to thank Dr. Daniel C. Pease
for the postdoctoral fellowship which permitted the
author to conduct this investigation.
3 Present address: The Division of Anatomy, The
Department of Surgery, The University of Califorma,
San Diego, California 92037.
61
62
RICHARD M. BOIS
ing the in vivo state of myosin aggregation
and the organization of the contractile
apparatus in the vertebrate smooth muscle
cell.
MATERIALS AND METHODS
easily seen (figs. 3, 4). Golgi complexes are
found to be well preserved (fig. 1, 4 ) and
numerous microtubules can be identified
(figs. 4, 5).
The contractile appuratus
Vascular perfusion of the smooth muscle
Thick and thin filaments are clearly
of the small intestine of the living anesthe- demonstrated in these perfused smooth
tized rat was achieved by opening the muscle preparations (figs. 1-5). The thin
thoracic cavity, inserting a needle in the filaments have uniform diameters of about
descending aorta, opening the right atrium 50 A and the diameters of the thick filaand employing a perfusion pressure of ments approximate 140 A. The thick fila90 mm Hg. Modified Karnovsky ('65) for- ments are distributed rather uniformly
maldehyde-glutaraldehyde fixatives were amongst the more numerous thin filaments
employed as follows. The vasculature was (figs. 1-5). The nearest neighbor distances
first flushed with 1% paraformaldehyde between the thick filaments vary from 400
0.5% glutaraldehyde in Hanks' balanced to 700 A. The thick to thin filament ratio
salt solution (HBSS). This flushing was determined from these preparations apimmediately followed by a 15 minute per- proximates 1: 12. Comparable population
fusion with 2% paraformaldehyde
3% densities and a similar pattern of distribuglutaraldehyde in HBSS. Short segments tion of the thick and thin filaments are
of the small intestine were removed from seen in virtually all the smooth muscle
the animal and immersed in a fresh aliquot cells of these preparations (fig. 1 ) .
of the latter perfusion medium for one
Dense bodies in the sarcoplasm (figs.
hour. The tissue was then cut into strips 1-4) and attachment plaques associated
of about 1 mm width, thoroughly washed with plasma membranes (figs. 1, 4) are
with HBSS, post-fixed with 1 % osmium easily identified. Whereas the thin filatetroxide in a 1: 1 solution of 0.1 M sodium ments are commonly seen to converge and
cacodylate and HBSS for one hour and enter dense bodies and attachment plaques,
embedded in Epon (Luft, '61). All fixa- no intimate relationship between these
tives and washing solutions were adjusted structures and thick filaments is observed
to PH 7.2. Thin sections were stained with (figs. 2, 3). Distally, along the tapering
uranyl acetate and lead citrate (Reynolds, extremities of the smooth muscle cell, pro'63) and examined with a Siemens Elmi- gressively more of the plasma membrane
skop 1A.
is lined by attachment plaques. This variation in the numbers of attachment plaques
RESULTS
at different levels in the smooth muscle
General observations
cell is clearly illustrated in figure 4. It is
In these preparations no formed blood also observed that the most distal portions
elements are observed in blood vessels, of the terminal processes of smooth muscle
most of which appear dilated, indicating cells contain only thin filaments (figs.
that successful perfusion of the small in- 6-8). Preliminary examinations of serial
testine had been achieved. The smooth sections reveal that the tips of these termimuscle cells of the muscularis externa ex- nal processes are lined by an attachment
hibit very uniform preservation through- plaque. Figures 7 and 8 are sections
out both the inner circular and outer longi- through terminal processes very near their
tudinal layers. Plasma membranes and tips showing attachment plaque material
associated pinocytotic vesicles are well de- lining portions of the plasma membranes.
No structures comparable to the myosin
fined (figs. 1, 4). Mitochondria, many containing mitochondria1 granules, are well "ribbons" recently demonstrated ( L o w and
preserved and uniform in appearance (figs. Small, '70; Small et al., '72; Small and
1, 4). Elements of endoplasmic reticulum Squire, '72; Sobieszek, '72) in some concan be seen (figs. 1, 3) and free ribosomes ventionally fixed vertebrate smooth muscle
are conspicuous (figs. 1 , 3 , 4 ) . The nuclear are observed in these perfused prepaenvelope and its perinuclear cisterna are rations.
+
+
THE CONTRACTILE APPARATUS OF SMOOTH MUSCLE
DISCUSSION
Interdigitating arrays of thick and thin
filaments have been clearly demonstrated
in several studies employing conventional
immersion fixation in aldehyde(s) followed by post-fixation with osmium
tetroxide (Kelly and Rice, ’69; Rice et
al., ’70a,b; Devine and Somlyo, ’71;
Garamvolgyi et al., ‘71; Cooke and Fay,
’72; Kelly and Arnold, ’72; Fay and Cooke,
’73). Observations made by this investigator upon similarly prepared smooth muscle
indicate such immersion fixation procedures are too unreliable for the comprehensive study of the contractile apparatus
of the vertebrate smooth muscle cell. This
unreliability is confirmed by the fact that
the varying results of recent studies employing conventional immersion fixation
can be appealed to in defending any of
three currently discussed models of the
contractile apparatus of the vertebrate
smooth muscle cell. These models are : (1)
The Panner and Honig (’67) model in
which actin filaments interact with small
myosin aggregates. (2) The Lowy and
Small (’70) model in which actin filaments interact with large myosin “ribbons.”
And (3) a system of interacting arrays of
thick and thin filaments. A fourth model
based upon a hypothesized reversible aggregation of myosin during contraction and
relaxation of smooth muscle proposed by
Shoenberg et al. (’66) was not amenable
to testing in this study. However, the
results of recent experimental evahations
of this model appear to indicate that the
myosin content of the smooth msucle cell
is aggregated into filaments in both the relaxed and contracted cell (Rice et al., ’70a;
Devine and Somlyo, ’71; Garamvolgyi et
al., ’71; Cooke and Fay, ’72; Kelly and
Arnold, ’72).
To circumvent the unreliability of conventional immersion fixation vascular perfusion was employed in this study with
the thought that a more rapid fixation of
the tissue may more faithfully preserve the
in vivo organization of the contractile apparatus of the smooth muscle cell. By
employing a mixture of paraformaldehyde
and glutaraldehyde it was anticipated that
the advantage of the rapid penetration and
fixation by formaldehyde would improve
the slower, but more complete fixation ob-
63
tained with glutaraldehyde. Immersion
fixation in parafomaldehyde-glutaraldehyde followed perfusion fixation to insure
thorough aldehyde fixation of the thick filaments. This precaution was deemed advisable since earlier observations made in this
laboratory suggested that the thick filaments may not withstand subsequent
osmication and/or embedding procedures
unless they have been adequately pre-fixed
with aldehyde(s). Hanks’ balanced salt
solution was employed as the vehicle of
perfusion and immersion fixation because
earlier observations made by this investigator indicated that the use of a balanced
salt solution as the vehicle of fixation enhanced the visualization of the myofilaments of smooth muscle.
The absence of invaginations of the
plasma membranes at the sites of the
attachment plaques in the smooth muscle
cells examined in this study indicates they
were not fixed in a very highly contracted
state. The observation of convolutions in
the nuclear envelope in many nuclei also
indicates that the smooth muscle in these
preparations was not prepared under any
significant degree of stretch. Since the
smooth muscle examined in this study does
not exhibit the previously described characteristics of either contracted or stretched
smooth muscle (Lane, ’65) the organization of the thick and thin filaments seen in
this study is thought to be characteristic
of the smooth muscle cell at or near its
resting length.
The diameters, population densities and
the pattern of distribution of the thick filaments of smooth muscle presently observed
are remarkably uniform from cell to cell
and between different regions within individual cells, except for the most distal
segments of the terminal processes of cells
which have been found to contain only thin
filaments. These uniformities associated
with the thick filaments would not indicate
that they are formed during tissue preparation either by the aggregation of smaller
myosin aggregates or by the dissociation
of myosin “ribbons.” The observation of
140 A filaments in vertebrate smooth muscle is consistent with the observed similarities in the size and shape of skeletal and
smooth muscle myosin molecules (Huxley,
’63; Zobel and Carlson, ’63; BArAny et al.,
64
RICHARD M. BOIS
'66; Panner and Honig, '67, '70) and with
the demonstration that both of these
species of myosin aggregate into filaments
according to the self-assembly model proposed by HuxIey ('63) (Kaminer, '69).
The authenticity of dense bodies has recently been questioned because of their
rarity in smooth muscle preparations exhibiting great numbers of myosin "ribbons." However, the evidence indicating
that dense bodies may be fuctionally
equivalent to the Z discs of skeletal muscle
is formidable. Actin filaments have been
observed to converge upon and enter dense
bodies (Prosser et al., '60; Lane, '65; Mark,
'65). The demonstrated directionality of
the actin filaments of smooth muscle
(Panner and Honig, '67; Kristensen and
Nielsen, '71; Rostgaard et al., '72) would
appear to require that they be anchored
at one end if they are to be effective in a
contractile mechanism. Dense bodies are
the only structures observed in sufficient
numbers and having an appropriate distribution in the sarcoplasm to accomplish
this function. Both the dense bodies of
unfixed vertebrate smooth muscle and the
Z discs of unfixed skeletal muscle are lost
during glycol dehydration and subsequent
embedment in hydroxypropyl me thacrylate
(Pease, '68) suggesting that these structures are similar in composition. The dense
bodies are also lost during glycerol-extraction of smooth muscle and when glycerol-extracted smooth muscle cells are incubated with ATP the thick and thin
filaments are seen to assemble into an intricate network in the central region of
the cell (Keyserlingk, '70). Incubation in
trypsin solution also removes the dense
bodies of smooth muscle and in such
preparations a pronounced segregation of
the thick and thin filaments has been observed (Rosenbluth, '71 ). These observations on glycerol-extracted and trypsintreated smooth muscle suggest that the
dense bodies, and attachment plaques,
maintain the lattice structure of the contractile apparatus of the vertebrate smooth
muscle cell. The observed association of
the thin filaments with attachment plaques
(Pease and Molinari, '60; Lane, '65; Rosenbluth, '65; Rogers and Burnstock, '66;
Panner and Honig, '67) and dense bodies
and an apparent seggregation of these
densely staining structures from the thick
filaments found in this study all suggest
they function as anchoring devices for the
thin filaments.
The increasing concentration of attachment plaques distally in the tapering extremities and their presence in the tips of
terminal processes of smooth muscle cells
appears appropriate for their suggested
function of anchoring the longitudinally
oriented contractile apparatus to the
plasma membrane through terminal arrays
of actin filaments.
The observation of only thin filaments in
the most distal regions of the terminal
processes of smooth muscle cells in this
study is consistent with a hypothesized
sliding filament mechanism of smooth
muscle contraction which would predict
the demonstration of thick filaments in
these distal regions of terminal processes
only when the cell is in a highly contracted
state.
Tissue preparation as performed in this
study appears to be the first reliable procedure available for the clear demonstration of all the essential features of the
vertebrate smooth muscle cell consistent
with a model conceived to consist of interdigitating arrays of thick and thin filaments collated into contractile units by
dense bodies and attachment plaques. The
clear demonstration of all the essential
features of such a hypothesized contractile
apparatus in this study is strong evidence
that these elements and their organization
as seen in this report are genuine.
LITERATURE CITED
Biriny, M., K. Bgriny, E. Gaetjens and G . Bailin
1966 Chicken gizzard myosin. Arch. Biochem.
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Cooke, P. H., and F. S . Fay 1972a Correlation
between fiber length, ultrastructure and lengthtension relationship of mammalian smooth
muscle. J. Cell Biol., 52: 105-116.
1972b Thick myofilaments i n contracted and relaxed mammalian smooth muscle cells. Exp. Cell Res., 71: 265-272.
Devine, C. E., and A. P. Somlyo 1971 Thick
filaments in vascular smooth muscle. J. Cell
Biol., 49: 636-649.
Fay, F. S . , and P. H. Cooke 1973 Reversible disaggregation of myofilaments in vertebrate
smooth muscle. J. Cell Biol., 56: 399411.
Garamvolgyi, N., E. S . Vizi and J. Knoll 1971
The regular occurrence of thick filaments i n
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THE CONTRACTKE APPARATUS O F SMOOTH MUSCLE
Iluxley, H. E. 1963 Electron microscope studies
of the structure of natural and synthetic urotein filaments of striated muscle. J. Mol. Biol.,
7: 281-308.
_._
Kaminer, B. 1969 Synthetic myosin filaments
from vertebrate smooth muscle. J. Mol. Biol.,
39: 257-264.
Karnovsky, M. J. 1965 A formaldehyde-glutaraldehyde fixative of high osmolality for use in
electron microscopy. J. Cell Biol., 27: 137a.
Kelly, R. E., and J. W. Arnold 1972 Myofilaments of the pupillary muscles of the iris
fixed in situ. J. Ultrastruct. Res., 40: 532-545.
Kelly, R. E., and R. V. Rice 1969 Ultrastructural
studies on the contractile mechanism of smooth
muscle. J. Cell Biol., 42: 683-694.
Keyserlingk, D. G. 1970 Ultrastruktur glycerineextrahierter Diinndarmmuskelzellen der Ratte
vor und nach Kontraktion. Z. Zellforsch., 111:
559-571.
ICristensen, B. I., and L. E. Nielsen 1971 A
two-filament system and interaction of heavy
meromyosin (HMM) with thin filaments in
smooth muscle. Z. Zellforsch., 122: 350-356.
l,ane, B. P. 1965 Alterations in the cytologic
detail of intestinal smooth muscle cells in
various stages of contraction. J. Cell Biol., 27:
199-213.
ldowy, J., and J. V. Small 1970 The organization of myosin and actin i n vertebrate smooth
muscle. Nature, 227: 46-51.
I,uft, J. H. 1961 Improvement in epoxy embedding methods. J. Biophys. Biochem. Cytol.,
9: 409414.
Mark, J. S. T. 1956 A n electron microscope
study of uterine smooth muscle. Anat. Rec.,
125: 473493.
IJonomura, Y. 1968 Myofilaments in smooth
muscle of guinea-pig taenia coli. J. Cell Biol.,
39: 741-745.
I'anner, B. J., and C. R. Honig 1967 Filament
ultrastructure and organization in vertebrate
smooth muscle. J. Cell Biol., 35: 303-321.
-1970 Locus and state of aggregation of
myosin in tissue sections of vertebrate smooth
muscle. J. Cell Biol., 44: 52-61.
I'ease, D. C. 1968 Structural features of unfixed
mammalian smooth and striated muscle prepared by glycol dehydration. J. Ultrastruct. Res.,
23: 280-303.
I'ease, D. C., and S. Molinari 1960 Electron
microscopy of muscular arteries; pial vessels of
the cat and monkey. J. Ultrastruct. Res., 3:
447468.
l'rosser, C. L., G . Burnstock and J. Kahn 1960
Conduction in smooth muscles: Comparative
~
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structural properties. Am. J. Physiol., 199:
545-552.
Reynolds, E. S. 1963 The use of lead citrate
at high pH as a n electron-opaque stain for
electron microscopy. J. Cell Biol., 17: 208-213.
Rice, R. V., G. M. McManus, C. E. Devine and
A. P. Somlyo 1970a Regular organization of
thick filaments in mammalian smooth muscle.
Nature New Biology, 231: 2 4 s 2 4 3 .
Rice, R. V., J. A. Moses, G . M. McManus, A. C.
Brady and L. M. Blasik 1970b The organization of contractile filaments i n a mammalian
smooth muscle. J. Cell Biol., 47: 183-196.
Rogers, D. C., and G. Burnstock 1966 Multiaxonal autonomic functions i n intestinal
smooth muscle of the toad ( B u f o marinus). J.
Comp. Neurol., 126: 625-652.
Rosenbluth, J. 1965 Smooth muscle: A n ultrastructural basis for the dynamics of contraction. Science, 148: 1337-1339.
1971 Myosin-like aggregates in trypsintreated smooth muscle cells. J. Cell Biol., 48:
174-188.
1972 Unphysiological conditions favoring the aggregation of smooth muscle myosin
in situ. J. Cell Biol., 55: 220a.
Rostgaard, J., B. I. Kristensen and L. E. Nielsen
1972 Characterization of 60A filaments in
endothelial, epithelial, and smooth muscle cells
of rat by reaction with heavy meromyosin.
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Shoenberg, C. F., J. C. Ruegg, D. M. Needham,
R. H. Schirmer and W. Nemetchek-Gander
1966 A biochemical and electron microscope
study of the contractile proteins in vertebrate
smooth muscle. Biochem. Z., 345: 255-266.
Small, J. V., J. Lowry and J. M. Squire 1972 The
myosin ribbons of vertebrate smooth muscle. J.
Ultrastruct. Res., 38: 208.
Small, J. V., and J. M. Squire 1972 Structural
basis of contraction in vertebrate smooth muscle. J. Mol. Biol., 67: 117-149.
Sobieszek, A. 1972 Isolation of ribbon-shaped
elements from vertebrate smooth muscle. J.
Ultrastruct. Res., 38: 208.
Somlyo, A. P., C. E. Devine and A. V. Somlyo
1971a Thick filaments in unstretched mammalian smooth muscle. Nature New Biology,
233: 218-219.
Somlyo, A. P., A. V. Somlyo, C . E. Devine and
R. V. Rice 1971b Aggregation of thick filaments into ribbons in mammalian smooth muscle. Nature New Biology, 231: 243-246.
Zobel, C. R., and F. D. Carlson 1963 An electron microscopic investigation of myosin and
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PLATE I
EXPLANATION O F FIGURE
1
66
A transverse section of well-oriented smooth muscle cells. This micrograph illustrates the excellent and uniform preservation obtained by
employing a balanced salt solution as the vehicle for aldehyde fixation by vascular perfusion prior to immersion fixation with aldehydes
and post-fixation with osmium tetroxide. Thick filaments are seen
distributed rather uniformly throughout the sarcoplasm among the
more numerous thin filaments. The plasma membranes and associated pinocytotic vesicles are well defined. Numerous sarcoplasmic
dense bodies and attachment plaques applied to the inner surfaces of
plasma membranes are evident. Mitochondria, some containing mitochondrial granules, are well preserved and uniform in appearance.
The well-oriented cisternae of a Golgi complex is seen near a “troika
of mitochondria” in the cell containing a markedly lobated mitochondrion. All the structures present in this micrograph are more
easily seen and identified in subsequent micrographs of greater magnification. x 10,800.
THE CONTRACTILE APPARATUS OF SMOOTH MUSCLE
Richard M. Bois
PLATE 1
67
PLATE 2
EXPLANATION OF FIGURE
2
68
A longitudinal section through the tapering extremity of a smooth
muscle cell showing the rather uniform distribution of thick filaments (long arrows) oriented parallel to the more numerous thin
filaments. No intimate association between dense bodies (short arrows) and thick filaments is seen. x 40,000.
THE CONTRACTILE APPARATUS OF SMOOTH MUSCLE
Richard M. Bois
PLATE 2
69
PLATE 3
EXPLANATION O F F I G U R E
3
70
A longitudinal section through the nuclear region of a smooth muscle
cell. The absence of invaginations of the plasma membrane in the
region of the attachment plaque indicates that the cell is not i n a
highly contracted state. The involutions in the nuclear envelope also
indicates that this region of the small intestine was not distended
during tissue preparation. Thick filaments are seen distributed among
the more numerous thin filaments. The dense bodies and thick filaments appear to be segregated. Rough-surfaced endoplasmic reticulum and free ribosome are conspicuous along one margin of the cell.
Numerous free ribosomes are evident along the nuclear envelope.
X 40,000.
THE CONTRACTILE APPARATUS OF SMOOTH MUSCLE
Richard M. Bois
PLATE 3
71
PLATE 4
EXPLANATION OF FIGURE
4
72
A transverse section through the nuclear region of one smooth muscle
cell and through the tapering extremity of another smooth muscle
cell. This micrograph illustrates the distribution of attachment plaques
(arrows) on the inner surfaces of the plasma membranes of smooth
muscle cells. It is seen that distally along the tapering extremities
of the smooth muscle cell progressively more of the glasma membrane is lined by attachment plaques. In this section the distribution
of thick and thin filaments is shown to be virtually identical i n the
nuclear region and the tapering extremities of the smooth muscle
cell. The well-oriented membranes of the stacked cisternae of a Golgi
complex are conspicuous. The nuclear envelope and its enclosed perinuclear cisterna are also easily seen. x 41,200.
THE CONTRACTILE APPARATUS OF SMOOTH MUSCLE
PLATE 4
Richard M. Bois
73
PLATE 5
E X P L A N A T I O N O F FIGURE
5
74
A transverse section of smooth muscle showing the rather uniform
distribution of thick filaments (arrows) among the more numerous
thin filaments. The distances between neighbolting thick filaments
range from 400 to 700 A. A few dense bodies and microtubules are
also seen. x 100,000.
THE CONTRACTILE APPARATUS OF SMOOTH MUSCLE
Richard M. Bois
PLATE 5
75
PLATE 6
EXPLANATION OF FIGURES
76
6
A transverse section through the distal portions of three terminal
processes (arrows) of a smooth muscle cell(s). Only thin filaments
can be identified. x 80,000.
7
A transverse section through the distal portions of two terminal processes of a smooth muscle cell(s). Only thin filaments can be identified in these processes. Attachment plaque material (arrows) is seen
lining portions of the plasma membranes in these processes. Despite
their poor orientation, thick and thin filaments can be discerned i n
the cell body of a n adjacent smooth muscle cell. x 80,000.
8
A transverse section through the distal portions of two terminal
processes of a smooth muscle cell(s). Only thin filaments can be
identified in these processes. Attachment plaque material (arrows)
is seen lining portions of the plasma membranes in these processes.
Both thick and thin filaments are present in the sarcoplasm of the
cell body of a n adjacent smooth muscle cell. x 80,000.
THE CONTRACTILE APPARATUS OF SMOOTH MUSCLE
Richard M. Bois
PLATE 6
77
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