Download the organization and myofilament array of insect visceral muscles

J. Cell Sci. i, 49-57 (1966)
Printed in Great Britain
THE ORGANIZATION AND
MYOFILAMENT ARRAY OF INSECT
VISCERAL MUSCLES
D. S. SMITH, B. L. GUPTA AND UNA SMITH
Department of Biology, University of Virginia, Charlottesville, Virginia, U.S.A.
SUMMARY
The cytological organization of three insect visceral muscles has been examined in the
electron microscope. In each instance, the fibres were found to be striated, and the striation
pattern has been shown to reflect the distribution along the sarcomere of two sets of myofilaments. In transverse sections of the fibre at the level of the A band, these muscles have been
found to exhibit an unusual myofilament array in which each thick (myosin) filament is surrounded by twelve thin (actin) filaments rather than six, as in insect flight muscle and vertebrate
skeletal muscle. The distribution of T-system tubules and cisternae of the sarcoplasmic reticulum in these visceral fibres is described, and compared with the corresponding membrane
systems in other striated muscles.
INTRODUCTION
In anatomical considerations of the insect body, the muscular systems are commonly
divided into two general categories: 'skeletal' and 'visceral' fibres. The former act
upon the articulated exoskeleton, while the latter invest the various regions of the
intestinal tract and other internal organs within the body cavity. Whereas many of the
skeletal muscles, notably those concerned with flight and locomotion, may contract
rapidly and often at high frequency, the visceral fibres, like their analogues in the
vertebrate body, generally exhibit a slower peristaltic or irregular activity.
Hitherto, investigations on the cytological organization of insect muscles have
mainly been focused upon the fibres involved in the flight mechanism, and these
studies have revealed a consistent deviation in the arrangement of the myofilaments
of the contractile system from the pattern occurring in vertebrate skeletal muscles.
In the latter (Huxley, 1957; Huxley & Hanson, i960) the thick and thin filaments of
the sarcomere, respectively containing myosin and actin, are arranged in a double
hexagonal array in the A bands, with the actin components situated in the trigonal
position with respect to the myosin filaments. In the indirect flight muscles of the
blowfly Calliphora, on the other hand, it was found (Huxley & Hanson, 1957) that
the thin filaments are relatively more numerous and are situated opposite to, and
midway between, adjoining myosin filaments in the hexagonal array. In the former
instance, each thin filament is 'shared' by three thick filaments and, in the latter, by
two thick filaments.
The configuration of myofilaments reported in the asynchronous flight muscle of
4
Cell Sci. 1
50
D. S. Smith, B. L. Gupta and U. Smith
Calliphora also occurs in the flight muscle fibres of Odonata (Anisoptera and Zygoptera) (D. S. Smith, unpublished), which have synchronous contraction properties,
and may be characteristic of insect flight muscles in general. An identical disposition
occurs in the tymbal muscle of the cicada Tibicen (D. S. Smith, unpublished), but,
in the absence of detailed studies on other insect skeletal muscle fibres, it is not possible
to generalize further on the organization of the myofilament array in this category of
muscle cells.
This report is concerned with the structure of visceral muscle from three locations
in the insect body. It has been found that in each instance the contractile material is
represented by a double array of thick and thin filaments, occurring in a configuration
strikingly different from that of insect flight muscle fibres. In the muscular sheath
investing the midgut of Ephestia larvae (Lepidoptera), in that of the spermatheca of
Periplaneta (Orthoptera) and of the seminal vesicle of Carausius (Orthoptera), it was
found that, while the thick filaments within the A band maintain an hexagonal array,
each of these filaments is surrounded by a ring of twelve thin filaments. These visceral
muscle fibres were found to be similar not only with respect to their fibrillar organization, but also in their general structure, and, while this account concerns primarily the
last of the above examples, the cytological features described are for the most part
common to each of these visceral fibres.
MATERIALS AND METHODS
An adult male specimen of the stick-insect Carausius morosus was found in a laboratory culture of this parthenogenetic species, and was selected for a study of the
organization of the seminal vesicle. This paper is concerned mainly with the muscular
sheath investing this region of the reproductive tract. The spermatheca was fixed for
2 h in ice-cold 2-5 % glutaraldehyde, maintained at pH 7-4 in 0-05 M cacodylate buffer
containing o-i5M sucrose. After overnight washing in cold cacodylate-buffered
0-3 M sucrose, the material was treated with veronal-acetate-buffered 1% osmium
tetroxide at the same pH, for 1 h, dehydrated in an ethanol series, and embedded in
Araldite. Sections were cut on a Huxley microtome and examined in a Philips
EM 200 electron microscope. Contrast in the sections was enhanced by double
'staining', initially with saturated uranyl acetate in 50% ethanol (30 mm) and subsequently with lead citrate (Reynolds, 1963) for 3-5 min.
Visceral muscle fibres associated with the midgut of larvae of Ephestia kiihniella,
and with the spermatheca of adult Periplaneta americana were also prepared according
to the above schedule.
RESULTS
General organization of the muscle fibres
As is frequently the case in organs within the insect body cavity, and also in the
body wall of certain invertebrates which lack an exoskeleton (e.g. platyhelminths;
annelids) the muscle fibres surrounding the seminal vesicle of Carausius are disposed
Insect visceral muscles
51
in both longitudinal and circular fashion, perpendicular to one another, over the
surface of the organ. This double investment is, however, incomplete, and many
fields include fibres oriented in only one direction.
Each of these muscle fibres is elongated in transverse section, varying between
about 1 and 2-5 fi in width (Figs. 2, 3 and 8), and the contractile material, except in the
vicinity of the nucleus, almost fills the cell. The fibrillar system is not divided into
lamellar or cylindrical fibrils as in insect skeletal fibres (Tiegs, 1955; Pringle, 1957;
Smith, 1962). The nuclei are generally situated laterally in the muscle cell, and, while
skeletal fibres are typically multinucleate, these visceral fibres appear to contain only
a single nucleus. The sarcoplasm extending from the poles of the nucleus is extensive
and free from myofilaments, a circumstance similar to that occurring in vertebrate
smooth muscle cells. This region of sarcoplasm contains mitochondria and (Fig. 9)
ribosomes, sparse cisternae of the rough-surfaced endoplasmic reticulum and welldefined Golgi complexes. The fibres are linked at frequent intervals by adhesion
plates or desmosomes (Fig. 6), exhibiting a layer of dense extracellular material. In
skeletal fibres of insects such desmosomes do not occur, and it is interesting to note
that in visceral fibres these structures differ from the ' septate desmosomes' generally
interpolated in regions of close contact between insect cells (Locke, 1965).
Each fibre is surrounded externally by a basement membrane or sarcolemma
(Fig. 2) in which, as in other insect muscles (Smith, 1961a), no collagen-like fibrils
have been resolved.
The cell membrane
Each fibre is limited by a typical plasma membrane, underlying the sarcolemma and
bounding the contractile material. At intervals, this membrane is inflected or invaginated into the fibre to form blindly ending tubes, generally about 100-200 A in
diameter (Figs. 7, 8) but sometimes (Fig. 11) dilating to form larger cavities, up to
700 A in diameter. In the Carausius and Ephestia material, these invaginations are
irregularly disposed, but in the muscle investing the spermatheca of Periplaneta, in
which the fibres are larger (up to 15/i in diameter), these derivatives of the cell
membrane extend across the radius of the fibre, and are arranged in a more precisely
radial pattern. These tubules appear to correspond to the transverse tubular system
(T-system) elements which form an array precisely oriented with respect to the myofibrillar striations in insect flight and leg muscle (Smith, 1961 a, b, 1962) and vertebrate
skeletal muscle (Porter & Palade, 1957; Andersson-Cedergren, 1959; Fawcett & Revel,
1961; Revel, 1962; Franzini-Armstrong & Porter, 1964; Peachey, 1965), where they
are believed to play an important part in contraction, by providing the pathway,
electrically coupled with the surface cell membrane, along which excitation may be
distributed throughout the fibre. In the visceral muscles so far examined, the sarcoplasmic reticulum cisternae are very reduced in extent, and are apparently represented
by flattened vesicles, closely adjoining the invaginated T-system tubules. The close
juxtaposition of these two membrane components, in the dyad configuration (corresponding to the 'triads' of vertebrate muscle, see references above), is illustrated in
Fig. 10.
4-2
52
D. S. Smith, B. L. Gupta and U. Smith
The contractile system
As has been described above, the contractile material of each visceral muscle fibre
corresponds anatomically to a single fibril. This situation has been described in other
invertebrate muscles; for example, in the striated fibres of an ostracod (Fahrenbach,
1964) and a coelenterate (Chapman, Pantin & Robson, 1962). The striated nature of
this visceral muscle is readily seen in longitudinal sections of the fibre (Figs. 2,4 and 5).
Well-defined Z bands traverse the contractile system, following an irregular course,
the sarcomere length defined by adjacent Z bands being about 7 to 8 ft. Each Z band
is flanked by similarly irregular I bands, about 0-7 to 1 -o fi in width. It should be noted
that, in these longitudinal sections, the light H band, occurring in the central region
of the sarcomere in insect and vertebrate skeletal muscles, is not clearly defined. The
longitudinal sections (Figs. 4, 5) indicate that both thick (myosin) and thin (actin)
filaments occur in the A band, while the former are excluded from the I bands, the
situation that obtains in vertebrate skeletal muscle (Huxley & Hanson, i960; Huxley,
i960). Further details of the disposition of these myofilaments in visceral fibres are
revealed in transverse sections.
In transverse profiles of the A band of Carausius visceral muscle (Fig. 7) the double
array of thick and thin myofilaments is distinct. At this relatively low magnification,
the latter are resolved as rings, surrounding the myosin filaments; the unusual nature
of the geometrical relationship between these two sets of filaments becomes apparent
at higher magnification. The familiar pattern of six thin filaments surrounding each
W
(b)
W
Fig. 1. D i a g r a m m a t i c representation of t h e distribution of thick (myosin) a n d thin
(actin)filamentsin the A-band region of: (a) vertebrate skeletal muscle (Huxley, 1957;
Huxley, i960; Huxley & Hanson, i960); (6) insect flight muscle fibres (Huxley &
Hanson, 1957; Hanson & Lowy, i960; Smith, 1962); (c) insect visceral muscles,
described in this paper (compare Fig. 12).
thick filament, as in the insect and vertebrate muscles so far described, does not occur;
instead, the thin filaments form twelve-membered 'orbitals' around the thick filaments. Very slight distortion in the spacing of this double array is sufficient to obscure
the detailed geometry of the double lattice, and the diagram of this configuration
shown in Fig. 1 c (and compared with other striated muscles, Fig. 1 a, b) accords with
regions of transverse sections of visceral muscle fibres where distortion of the lattice
appears to be minimal (Fig. 12).
In insect flight muscle and vertebrate skeletal muscle each myosin filament is sur-
Insect visceral muscles
53
rounded by six actin filaments but, as Huxley & Hanson (1957) and Smith (1962)
noted, the trigonal situation of the actin filaments in vertebrate fibres reduces the
ratio of actin to myosin filaments, compared with the flight muscle fibres, in which
each actin filament is shared by only two myosin filaments (Fig. 1 a, b). In insect
visceral muscles, each thin filament is shared, as in flight muscle, by two thick filaments, but the number of thin filaments is exactly doubled: in flight muscle a myosin
filament and its six neighbours are associated with thirty actinfilaments(Fig. 1 b), while
in visceral muscle (Fig. 1 c) sixty similar myofilaments occur within the same area.
In glutaraldehyde-fixed visceral muscle of Carausius, the diameter of the thick and
thin myofilaments is respectively about 160-180 A, and about 40-50 A. In osmiumfixed vertebrate muscles, the corresponding values have been measured as n o and
50 A (Huxley & Hanson, i960). An increase in the apparent diameter of the myosin
filaments after glutaraldehydefixation,to about 150 A, has been described by FranziniArmstrong & Porter (1964) in fish skeletal muscle. The centre-to-centre spacing
of the thick filaments in the A-band array of Carausius visceral muscle is about
440-460 A; somewhat greater, that is, than the 370-A spacing encountered in osmiumfixed asynchronous insect flight muscle (Smith, 1962).
In transverse sections, as in the longitudinal plane, the region of junction between
the A and I bands of visceral muscle is clearly demarcated. Fig. 11 illustrates a region
of junction between these sarcomere regions in Carausius visceral muscle. An A-band
profile, containing both myofilament populations, adjoins a well-defined though
irregularly disposed area containing only thin I-band filaments. In the I band, however, the orbital arrangement of the I filaments is lost, and these filaments appear to
be closely and irregularly packed. The irregularity of the margin between these bands
in precisely transverse sections conforms with their zigzag disposition, viewed in
longitudinal section. In longitudinal sections of Carausius seminal vesicle muscle, the
Z bands appear as well-defined regions of increased density, in the middle of the span
of I filaments, paralleling the contours of the A/I junction. In transverse sections of
this muscle (Fig. n ) the Z bands are less well defined; however, in the midgut muscle
of Ephestia, the I-filament region is divided by an area containing dense structures,
perhaps representing aggregates of Z-band material (Fig. 8).
The apparent absence of an H zone is a striking feature of longitudinal sections of
Carausius visceral muscle. In vertebrate skeletal muscle (Huxley & Hanson, i960)
this region of the sarcomere in a relaxed fibre is defined- by the inner extremities of
the actinfilaments,and is traversed by the mid-region of the myosinfilaments.Although
longitudinal sections of insect visceral muscle appear to lack H bands, this region of
the sarcomere is more clearly defined in transverse profiles of the fibre, as is illustrated
in Fig. 13. At the margin of this band, the thin filaments terminate more or less
abruptly, and, as is seen in the case of the A/I junction of these fibres (Figs. 2, 4), the
A/H junction appears to be similarly irregular.
In their detailed construction, then, these visceral muscles exhibit several points of
resemblance with other striated fibres of insects and vertebrates; though, as regards
their sarcomere organization, the striation repeat in visceral muscles has been achieved
by a hitherto undescribed myofilament configuration.
54
D. S. Smith, B. L. Gupta and U. Smith
DISCUSSION
The sliding-filament model of striated muscle contraction and relaxation, as proposed by Huxley & Hanson (i960), has been most amply substantiated in the skeletal
fibres of vertebrates. However, as Hanson & Lowy (i960) point out, a similar mechanism may operate not only in invertebrate striated fibres, but perhaps also in certain
'smooth' muscles in which two sets of cross-linked myofilaments are present, though
not aligned in the sarcomere register characteristic of striated fibres.
In vertebrate skeletal muscle fibres it is clear (Huxley & Hanson, i960; Page &
Huxley, 1963) that the length of the A and I filaments (the myosin and actin components)
remains constant during the activity cycle; furthermore, the length of these filaments is
more or less constant in different vertebrate skeletal muscles. In invertebrates, on the
other hand, the relaxed sarcomere length, and hence presumably the lengths of the
constituent myofilaments, may vary considerably from one muscle to another. The
question remains whether invertebrate muscles operate by the same sliding-filament
mechanism as occurs in vertebrate fibres, or whether change in filament length is an
alternative or additional method of changing sarcomere length. There is no evidence
that the length of the A band changes during contraction in insect flight muscle fibres
(Hanson, 1956), but such change has been reported elsewhere in invertebrates, as in
Limulus skeletal muscle (de Villafranca, 1961) and in femoral muscle of Locusta
(Gilmour & Robinson, 1964). While no observations have as yet been made on the
changes in band pattern in insect visceral muscle fibres, an important preliminary to
such studies is the recognition of the dimensions and distribution of their constituent
myofilaments.
The Z bands of Carausius visceral muscle, although not precisely aligned transversely to the long axis of the fibre, define a sarcomere length of about 7-8 ju,, that is
between three and four times that of insect flight muscle or vertebrate skeletal muscle.
In the present material, although no assessment has been made of the degree of contraction or relaxation of the fibres examined, the I-band width is in the range, of
0-7-1-o p. Considerably greater sarcomere lengths have been recorded in invertebrate
striated muscles; Haswell (1889) described fibres in the pharynx of syllids (Annelida)
with a sarcomere length of 33 /i.
The apparent absence of an H band in longitudinal sections of visceral muscle fibres
deserves further mention. In the relaxed sarcomere of vertebrate muscle, this region
is traversed by the medial portions of the myosin filaments, and is delimited by the
inner ends of the actin filaments. It has recently been reported (Franzini-Armstrong &
Porter, 1964) that in glutaraldehyde-fixed vertebrate muscle the H band may also be
traversed by narrow connecting strands linking the inner extremities of the actin
filaments. H bands are clearly demarcated in transverse sections of insect visceral
fibres, and some indication of similar linking strands traversing this region is evident,
and it is possible that this feature, together with the irregularity of this narrow region,
serves to obscure this portion of the sarcomere in longitudinal sections.
In summary, this preliminary study suggests that the slowly contracting visceral
muscles of insects contain contractile material constructed on essentially the same
Insect visceral muscles
55
plan as that of vertebrate fibres in that it consists of two morphologically distinct sets
of myofilaments which, though differing from those of vertebrate striated muscle in
their geometrical arrangement about the long axis, are nevertheless distributed in a
comparable fashion along the sarcomere bands.
The electron microscope, coupled with biochemical and physiological studies, has
contributed much to a more complete understanding of the role of the membrane
systems of striated muscle fibres; notably, the function of membranes within the
fibre in controlling the phases of the activity cycle. In insect flight muscles, it is known
that the plasma membrane at the surface of the fibre is continuous with open tubular
invaginations passing radially into the fibre at regular intervals, at a level either midway between the Z and H bands (synchronous fibres) or at other levels (asynchronous
fibres) (Smith, 1961a, b, 1962). These tubules correspond to the T-system invaginations that occur in vertebrate striated muscle fibres (H. E. Huxley, 1964; Page, 1964;
Franzini-Armstrong & Porter, 1964). Micro-depolarization experiments (A. F. Huxley,
1959, 1964; Huxley & Peachey, 1964) on vertebrate and crab muscle have suggested
that these invaginations may be responsible for the triggering of myofibrillar activation
by acting as the pathway along which surface excitation is passively conducted into the
fibre. In rapidly contracting synchronous muscles of insects and vertebrates, these
T-system tubules are flanked by, and in close association with, cisternae of the sarcoplasmic reticulum in triad or dyad configurations (Smith, 1961a, b, 1962; Porter &
Palade, 1957; Andersson-Cedergren, 1959; Fawcett & Revel, 1961; Revel, 1962;
Franzini-Armstrong & Porter, 1964; Peachey, 1965). Peachey & Porter (1959) compared the disposition of internal membrane systems in striated and smooth muscle
fibres, in the context of the speed of contraction of these muscles, and pointed out that,
if some derivative of the cell membrane (now identified as the T-system tubules) is
responsible for internal conduction of excitation in striated muscle fibres, then it is
possible that the absence of such invaginations in smooth muscle fibres of vertebrates
may be correlated with the slowness of their contractile response.
Insect visceral muscle fibres appear to represent an intermediate condition between
smooth muscle and striated muscles with a highly developed system of transverse
tubules (T-system tubules) and associated sarcoplasmic reticulum cisternae. The
fibres have a diameter below that of most vertebrate smooth muscle fibres; short
T-system invaginations are present, associated in dyad configurations with cisternae
of the sarcoplasmic reticulum, which are reduced to small flattened vesicles. The
distance across which diffusion of an ' activating substance' must take place to trigger
contraction of the fibrillar material in these visceral muscles is of the order of 1 {i;
that is, the distance between the cell membrane and tubular derivatives, and the centre
of the contractile apparatus. This distance is similar to that separating the centre of
the sarcomere from the T-system tubules in the fast-acting fibres of insects and
vertebrates, and the speed of contraction of visceral fibres falls well within the limits
imposed by the length of the excitation-contraction coupling pathway (compare
Peachey & Porter, 1959). Furthermore, if the activating substance in insect visceral
muscles represents an influx of calcium ions to the contractile system (Porter, 1961;
Ebashi, 1961; Hasselbach, 1964) then it is possible that the slow relaxation of these
56
D. S. Smith, B. L. Gupta and U. Smith
fibres is mediated by active sequestration of calcium ions within the reduced cisternae
of the sarcoplasmic reticulum, perhaps augmented by a calcium-pump mechanism
at the cell membrane.
The most unusual feature of these visceral muscles is the disposition of the myofilaments within the sarcomere. It should be pointed out that, while comparison has
been drawn between the filament array in vertebrate fibres and insect visceral fibres,
there is at present no evidence available on the chemical composition of the thick and
thin filaments in the latter, and any analogy must therefore be tentative. Nevertheless,
the doubling of the number of thin filaments associated with the thick filaments
in the A-band region of these muscles divides them sharply from other insect fibres
hitherto examined. The extent of occurrence of this pattern of myofilament distribution in insects and other animals, and its physiological significance, remain to be
determined.
One of us (D. S. S.) gratefully acknowledges support from the National Science Foundation
(Grant number GB-1291).
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ANDERSSON-CEDERGREN,
Fig. 2. A low-power electron micrograph of the muscle investing the seminal vesicle
in a male Carausius morosus. The fibre is associated with a layer of extracellular
basement-membrane material (bm) constituting the sarcolemma. Note the obliquely
situated Z bands {Z)flankedby I bands (/) and the apparent absence, in this plane of
section, of a mid-sarcomere H band. The contractile material in this muscle consists
of a single fibril (fi), bordered on one side by a layer of sarcoplasm (sp). Note the
small mitochondria (m). x 25000.
Fig. 3. A low-power transverse section of Carausius seminal vesicle muscle. Note that
in this section the contractile material virtually fills the muscle cell, and that the
plasma membrane is invaginated into the fibre at irregular intervals, to form T-system
tubules (T). x 45000.
journal of Cell Science, Vol. i, No. i
bm
wmm
D. S. SMITH, B. L. GUPTA AND U. SMITH
{Facing p. 56)
Fig. 4. Longitudinal section of Carausius visceral muscle, including a Z band and the
adjoining regions of the sarcomere. Note the irregular disposition of this band (Z), and
the adjoining I bands containing only thin filaments (about 50 A in diameter) (/). From
the edge of the I bands extend the A bands of the sarcomere, in which thick and thin
filaments interdigitate (A). The dense particles present in the I-band region probably
represent glycogen deposits (g). x 75000.
Journal of Cell Science, Vol. i, No. i
D. S. SMITH, B. L. GUPTA AND U. SMITH
Fig. 5. Longitudinal section of Caraushis visceral muscle, including a Z band (Z)
flanked by A regions (A). Note that the I region extending from the Z band contains
only thin filaments (2) but that the neighbouring A band contains both thick (1) and
thin (2) myofilaments. x 140000.
Fig. 6. The intercellular linkage between muscle fibres investing the seminal vesicle
of Caraushis. In these regions of close apposition the cell membranes are separated
by an intercellular gap containing a layer of dense material (arrows), a situation
resembling that occurring in the 'desmosomes' or adhesion plates between many
epithelial cells. Note that a layer of dense material occurs within the muscle cells in
the desmosome region, and that I filaments (/) appear to terminate beside the cell
membrane in this region. It is possible that the dense sarcoplasm bordering the desmosome represents Z-band material, and that the Z bands traversing the fibre may sometimes terminate at the cell surface, x 60000.
Journal of Cell Science, Vol. i, No. i
D. S. SMITH, B. L. GUPTA AND U. SMITH
Fig. 7. Transverse section of visceral muscle of Carausius, through an A-band region.
Note the two sets of myofilaments, and the invaginated T-system tubules (T); also
the microtubules (mf) situated near the surface of the fibre, x 80000.
Journal of Cell Science, Vol. i, No. i
D. S. SMITH, B. L. GUPTA AND U. SMITH
Fig. 8. Transverse section of visceral muscle investing the midgut of larval Ephestia.
Note the A-band profiles (A) containing thick and thin myofilaments, the I bands
containing only thin filaments (/) and the aggregations of dense material in the middle
of the I bands, apparently representing Z-band material (Z). As in Carausius visceral
muscle, the plasma membrane of these fibres is invaginated into the cell at irregular
intervals, to form T-system tubules (T). x 75000.
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D. S. SMITH, B. L. GUPTA AND U. SMITH
Fig. 9. Longitudinal section of Carausius visceral muscle, including a portion of the
nucleus («) and the adjoining sarcoplasm. The latter contains sparsely distributed
cisternae of the rough-surfaced endoplasmic reticulum (er) and well-developed
smooth-membraned Golgi complexes (go), x 56000.
Fig. 10. Longitudinal section of Carausius visceral muscle, illustrating the disposition
of membrane systems within the fibre. A profile of the plasma membrane is included
at left (pm). Within the fibre, a portion of a T-system tubule (T) invaginated from the
surface plasma membrane (compare Fig. 7) lies alongside the fibrillar material (fi),
and is closely associated with a flattened vesicle or cisterna of the sarcoplasmic
reticulum (sr) containing electron-dense material, x 105000.
Journal of Cell Science, Vol. i, No. i
go
10
D. S. SMITH, B. L. GUPTA AND U. SMITH
Fig. I I . The distribution of myofilaments at the junction of A and I bands in Caraitsius
visceral muscle. In the A bands (.-?) the thick filaments are surrounded by orbitals of
thin filaments, while in the I bands (/) only thin filaments occur. Dense material, probably representing the Z band (Z) is also included in this section. Note the invaginated
tubules of the T-system (T): the mouth of the upper tubule contains a layer of extracellular material (indicated by an asterisk), bordered by regions of increased density
(arrows) within the sarcoplasm, possibly confluent with the Z bands (compare Fig. 6).
x noooo.
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z
*
•»
D. S. SMITH, 13. L. GUPTA AND U. SMITH
- » * *
•
Fig. 12. The disposition of myofilaments in the A band of Caraiisius visceral muscle.
The thick filaments form a more or less regular hexagonal array (particularly within
the marked rectangle), and each thick filament is surrounded by a ring of twelve thin
filaments. The geometry of the disposition of these myofilaments is illustrated in
diagrammatic form in Fig. i c. x 180000.
Fig. 13. The disposition of myofilaments at the A/H junction in Caraiisius visceral
muscle. In the A band (.4) thick and thin filaments occur together, while in the
H band (H) only the former are present. There is some evidence that fine strands traverse the H band (arrows). Note the small mitochondrion (m) and transverse profiles
of intrafibrillar microtubules (mi), x 120000.
Journal of Cell Science, Vol. i, No. i
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D. S. SMITH, 13. L. GUPTA AND U. SMITH
Insect visceral muscles
57
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(Received 23 May 1965)