Download Cardiac Fine Structure in Selected Arthropods and Molluscs

AMER. ZOOL., 19:9-27 (1979).
Cardiac Fine Structure in Selected Arthropods and Molluscs
JOSEPH W. SANGER
Pennsylvania Muscle Institute and the Department of Anatomy, School of Medicine,
University of Pennsylvania, Philadelphia, Pennsylvania 19104 and The Bermuda
Biological Station, St. George's West, Bermuda
SYNOPSIS. The ultrastructure of the single-chambered hearts of selected arthropods is
compared with that of the multi-chambered hearts of three molluscs. I used the following
four systems to make the comparison: (1) contractile apparatus, (2) sarcoplasmic reticulum
and surface invaginations, (3) cell to cell junctions, and (4) nerves. The contractile apparatus is composed of thin and thick filaments. While the thin filaments have the same
diameter, the diameter of the thick filaments differs from one heart to another. Evidence is
presented to indicate that this is due to varying amounts of paramyosin in the thick
filaments. The arthropod cardiac cells have an extensive system of sarcoplasmic reticulum,
the terminal vesicles of which are coupled to the plasmalemma and to the invaginations of
the plasmalemma, the T-system. The molluscan cardiac cells lack a typical T-system,
which is presumably due to their small cell size (about 10 ftm). They possess, however,
an elaborate system of sarcoplasmic reticulum which extends from just under the
plasmalemma to the middle of the cell. In addition to elaborate sarcoplasmic reticulum, the
heart of the whelk (Busycon canaltculatum) possess many small invaginations of the
plasmalemma, called sarcolemmic tubules. These invaginations of the cell surface are not
found in the hearts of the few bivalves examined. All arthropod and molluscan hearts have
intercalated discs which can be seen in the light microscope. Two types ofjunctions can be
distinguished in the electron microscope. The mechanical junction is at the level of the
terminal sarcomere where the thin filaments are embedded in the cell wall and dense
granular material appears to cause the two adjacent cells to adhere to each other. The
electrical junction is found along the lateral borders of cells of both the molluscan and
arthropod hearts. Finally, while nerves appear to be absent in the myogenic moth heart,
they are abundant in the myogenic cockroach heart and in the neurogenic lobster heart.
Furthermore, two types of nerves appear very prominently in the myogenic molluscan hearts.
A heart is a group of muscular cells
which are usually electrically coupled so
that the cells beat as if they were a syncytium (e.g., in contrast, amphibian lymph
hearts are not electrically coupled). Nerves
may be present among these cells to serve
as excitatory or inhibitory agents. In some
hearts the muscular cells themselves pro"ThiT^rklhas been done during many stays at the
Bermuda Biological Station for Research and the
Marine Biological Laboratory. I would like to thank
Dr. Wolfgang Sterrer, the Director of the B.B.S. and
his staff for the hospitality given me over the years. 1
am also indebted to Dr. Robert Summers, who went
to great lengths to set up a functional electron microscope lab at the B.B.s. This paper is a contribution
from the B.B.S. (Contribution #736). Once again I
am indebted to Dr. Jean M. Sanger for her construe-
vide t n e stimulus for contraction and thus
these organs are called myogenic hearts.
Conversely, in other cardiac organs, nerves
from ganglia provide the stimulus and
these organs are called neurogenic. A
heart is a pump —a repeating p u m p that
by its contraction or compression, propels
a volume of fluid o u t of its muscular
chamber. A heart can be subdivided into
four anatomical systems: 1) contractile apP ^ t U S , 2) sarcoplasmic reticulum and
surface invaginations, 5) junctions, 4)
nerves. Since the heart is composed of
muscle cells it must possess a contractile
a p p a r a t u s composed of actin and myosin
•
•i
•
• •
•
together with tropomyosin, actinin and
perhaps troponin (Lehman, 1976). There
must also be a sarcoplasmic reticulum Syst e m t o r e gulate the contractile apparatus
tive and critical review of this paper. The original
•
INTRODUCTION
°
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j
i
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i •
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work was supported by Grants from the National ty sequestering and releasing calcium ions.
Science Foundation and from the National Institutes Vesicles ot the Sarcoplasmic reticulum are
of Health.
coupled to t h e cell surface or plas-
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JOSEPH W. SANGER
malemma. The third component of the
heart, the cell membrane or plasmalemma
is bound to the contractile apparatus via
the thin actin filaments embedded in its
walls. Adjacent cardiac muscle cells are
linked to each other by mechanical junctions and electrical junctions between regions of their membranes. The fourth
component of most hearts is a system of
excitatory and inhibitory nerves. These
nerves release neurohumors which alter
the electrical properties of the membranes.
Ultimately this nerve-membrane effect, via
the sarcoplasmic reticulum, induces the
activation or inhibition and contraction of
the myofibrillar units.
I have focused in this paper on two
phyla, Arthropoda and Mollusca, because
arthropods have tubular or sac-like hearts
while molluscs have chambered hearts.
Moreover, most of the very few published
papers on ultrastructure of invertebrate
hearts have been done on animals from
these two phyla. 1 would like to give you a
vivid idea ofjust how few papers have been
published on invertebrate hearts. In a
fairly recent review, Page and Fozzard in
1973 reviewed the ultrastructure of heart
muscle (1960-1972) and out of approximately three hundred papers only
twenty-five dealt with invertebrate hearts.
Only four types of insects were studied in
this period — a moth, a fruit Hy, a cockroach and two types of grasshoppers. Four
different crustaceans had been studied—a
crayfish, a mantis shrimp, a sand flea and
two types of lobsters. Two different types
of horseshoe crabs were studied to round
out the arthropods. Among the molluscs
the studies were just as few — two types of
cuttlefish, one clam and six different types
of snails. True these reviewers missed a
few papers, but very few, and in the years
since this review there have been very few
new papers published. The number of invertebrate hearts examined represents
such a small percentage of the total number, that it is quite possible that further
studies will reveal unusual hearts which
will help our understanding of the fundamental properties of cardiac function.
animal. The tubular myogenic heart of the
moth, Hyalophora cecropia is composed of a
single layer of cross-striated muscle (Fig. 1)
(Sanger and McCann, 1968a), arranged in
a spiral around the lumen of the vessel as is
true for all other insect hearts which have
been studied (Edwards and Challice, 1960;
Baccetti and Bigliardi, 1969; Burch et al.
(1970). As can be observed in Figure 1,
the muscle is folded and contracted upon
itself to yield an accordion-like image. The
insect heart is generally described as possessing only one layer of cells —the cardiac
muscle cells. This has to be modified to
consider the two coatings of basement
membrane on the inside and outside of
the heart. The inner basement membrane
material is probably made and secreted
by the muscle cells. Embedded in the polysaccharide matrix of the outer basement
membrane are processes of trachea cells
as well as a set of alary muscles and pericardial cells. The insertion of another set
of muscles, the alary muscle fibers, directly
onto the heart (Fig. 2) (Sanger and
McCann, 1968/?) is an unusual feature of
this heart. These muscles are believed to
facilitate the re-expansion phase of the
heart by pulling the heart wall out
(McCann and Sanger, 1969). They can be
viewed easily in the scanning electron microscope (Fig. 3), and in light micrographs
of saggital sections cut through the heart
wall and through the layer of alary muscles which run ventrally along the long
axis of the tubular heart (Fig. 4). As is
apparent from an examination of Figures
2 and 4, there is a dramatic difference in
the cardiac and alary muscle systems. In
the neglected field of invertebrate cardiology, alary muscle cells have been completely neglected since we first demonstrated unequivocally that they were indeed muscle cells and discovered that they
formed a myomuscular junction with the
muscle cells of the myocardium (Sanger
and McCann, 19686). The function and
relationship of these alary muscles to the
cardiac cells should be the subject of a
physiological investigation.
The arthropod heart is a tubular vessel
which runs along the dorsal side of the
americanus and Panulirus <irgii\), although
The neurogenic lobster heart (Homarus
tubular, is much more massive than the
CARDIAC FINE STRUCTURE
II
FIG. 1. Transverse section through the heart. The
arrow indicates the attachment of the heart to the
dorsal integument. Note the irregular lumen (L) of
the heart formed by the accordion-like contractions
of the heart musculature. An alary (A) muscle fiber is
associated with the ventral roof of the heart. Scale =
100 fjLm.
FIG. 2. A section along the long axis of the tubular
heart illustrating the insertion of an alary muscle fiber
(A) on to the cardiac muscle wall (M). The arrows
indicate the presence of two intercalated discs marking the boundaries of one cell. Scale = 50 fim.
JOSEPH W. SANGER
FIG. 3. A scanning electron micrograph of a section
of a moth heart. The straight line indicates the long
axis of the heart as well as the pericardial cells which
give a cobbled appearance to the heart wall. The alary
muscle fibers (A) can be observed to insert into the
ventral roof of the heart.
moth myocardium. A heart I isolated from a much thinner walled atrium. The vena four pound lobster (Homarus) weighed tricular wall is about half a millimeter
3.1 g. This is more than the total weight of thick and composed of many layers of
a single moth (about 2.5 g). The thickness muscle cells in the cuttlefish, Sepia officinof the lobster heart wall is due to many alis, (Schipp and Schafer, 1969) and in
layers of muscle cells (about fifty layers) Busycon canaliculatum. In contrast, in BusySmith 1963; see also, Baccetti and Big- con the atrial walls are only one to three
liardi, 1969, on Panulirus vulgaris). Two cell layers thick. Covering the muscle layers
other crustacean hearts, although very on the epicardial surface is a single layer
small in size, are also layered. The sant of epithelial cells. There is no endocardial
sand Hea, Daphnia pulex, has one to five lining in the molluscan heart (Hill and
layers of muscle forming a wall two to fifty Welsh, 1966). Presumably, smaller mol/xm thick (Stein et al., 1966), and the man- luscs would have fewer layers of cells in
tis shrimp, Squilla oratona, has two layers their heart walls. Nevertheless, hearts from
of muscle cells forming a wall about 12 both these phyla, different as they are in
/xm thick. (Irisawa and Hama, 1965). All shape and weight, share a common phymolluscan hearts are myogenic and have siological property: they all undergo rhytwo chambers: a thick walled ventricle and thmical contractions. In this paper, 1 will
CARDIAC FINE STRUCTURE
FIG. 4. A grazing section passing through the muscle
(M) of the ventral heart roof and into the layer of
alary muscle cells which runs along the long axis of
the heart. Note the difference in sarcomere lengths
between the myocardial and alary muscle cells. Scale
= 50 /i.m.
compare the ultrastructure of several arthropod and molluscan hearts in terms of
their, 1) contractile apparatus, 2) sarcoplasmic reticulum and surface invaginations, 3) junctions, and 4) nerves.
ferent maximum diameters. The maximum diameters of the thick filaments of
the various hearts I have studied are:
Busycon, 300 A; Spondylus, (americanus, the
thorny oyster), 410 A; Hyalophora, 200 A;
and Panuliris, (argus, the spiny lobster), 200
A. The maximum diameter of all verteContractile apparatus
brate thick filaments in cardiac muscle is
No matter what kind of cardiac cell is about 180 A (Page and Fozzard, 1973).
examined in the electron microscope, thin This disparity in size from vertebrate
and thick filaments are present. Figure 5 is hearts suggests the presence of a protein in
a cross section through the ventricular wall addition to myosin in the invertebrate
of a molluscan heart (the whelk, Busycon thick filaments. I thought this extra procanaliculatum) while Figure 6 is a cross tein might be paramyosin.
section through the wall of an arthropod
Paramyosin was first found in molluscan
(the moth Hyophora cecropia). Both figures adductor muscles, especially in the "catch"
demonstrate the two types of filaments. muscles (Bailey, 1956). More recently
Measurements we have made on these two workers have discovered it in several nonhearts, as well as others, indicate that the catch muscles {e.g., insect flight muscle,
thin filaments have the same diameter, 60 Bullard et ai, 1973; Winkelman, 1976). I
A, characteristic of actin filaments. The extracted hearts of Biisycon and Lobster
thick filaments have a greater variety in (Panuliris and Honuirm) with a high salt
their diameter. This is not simply due to solution (0.6 M KC1, 0.04 M Tris, O.01 or
sectioning at different regions along the EDTA pH 7.0) using a method designed to
tapering thick filament, but results also isolate paramyosin (Stafford and Yphantis,
from the fact that the filaments have dif- 1972). This muscle extraction was mixed
JOSEPH W. SANGER
*$ .* *-J.
g
FIG. 5. Transverse section through a ventricular
and thick filaments as well as a gap junction (g) can be
detected. Scale = 0.1 /xm.
muscle cell of a Busycnn heart. The presence of thin
15
CARDIAC FINE STRUCTURE
ft*
sr
FIG. 6. Transverse section through a moth cardiac
muscle cell. The presence of flattened transverse
tubules (t) is indicated. Sarcoplasmic reticulum (sr)
can be observed to associate with the plasmalemma
and with transverse tubules (t). The sarcoplasmic reticulum labeled with a small arrow at one o'clock has
-t
septae or surface couplings that bind the terminal
cisternae to the transverse tubular system. In other
areas the sarcoplasmic reticulum (sr) has been sectioned through the longitudinal tubular elements
which are not associated with the transverse tubular
system. Scale = 1 /*m.
16
JOSEPH W. SANGER
with an equal volume of 95% ethanol to a
precipitate which was collected by centrifugation. The pellet was extracted with
high salt and dialyzed against low salt (0.01
M PO4 pH 7.0) overnight. Birefringent
crystals formed in the presence of low salt.
The crystals were collected by centrifugation, redissolved in high salt and again
dialyzed against low salt to reform crystals.
The recrystallization was performed three
times to purify the protein. When these
crystals were run on SDS acrylamide gels
only one band was observed. Its molecular
weight was about 105,000 daltons, similar
to that of paramyosin. An examination of
the crystals in the electron microscope revealed a periodicity identical to that of
adductor paramyosin (Fig. 7). I have also
recently isolated paramyosin from the pulsating blood vessel of the sea cucumber
hostichopus badionotus (Selenka). This work
indicates then, that paramyosin is a common component of hearts from three different phyla (echinoderm, molluscan, and
arthropod). This protein is presumably
located in the core of the thick filaments
(Szent-Gyorgyi et al., 1971) of the cardiac
muscles and, may thus account for the
larger diameter of these cardiac thick filaments as compared to vertebrate muscle
thick filaments. This is consistent with the
observations that the thick filaments of
Busycon are 310 A in diameter while those
of lobster are 200 A and much more
paramyosin can be isolated from the Busycon heart on a per gram basis than from
the lobster heart (Sanger, unpublished
results).
Longitudinal sections of lobster and
moth hearts (Fig. 2) indicate that these
muscles are cross-striated (Smith, 1963;
Sanger and McCann, 1968). Longitudinal
sections of the ventricle of the whelk,
Busycon, show that this molluscan heart
also possesses striated muscle (Fig. 8). This
has been confirmed by using polarized and
phase microscopy on homogenized glycerinated ventricular heart cells isolated
from Busycon. While crustacean and horseshoe crab cardiac muscles have a solid Zband, insect hearts possess a Z-band made
up of several discontinuous, dense bodied
structures similar to those dense bodies observed in vertebrate smooth muscles (Devine and Somlyo, 1971) and in molluscan
smooth muscles (Sanger and Hill, 1973).
Of the molluscs, only scallop (Sanger, unpublished) is known to have solid Z-bands
in cardiac muscle (Helix aspersa, North,
1963; Archachatina marginata Nisbet and
Plummer, 1969; and in the cuttlefish Sepia
officincdis by Schipp and Schafer, 1969). A
Z-band composed of dense bodies may
allow the cardiac thick filaments to slide
past them as happens in smooth molluscan
muscle (Sanger and Hill, 1973). The dense
bodies in both the moth hearts (and in
other insect hearts) and the whelk heart
FIG. 7. A paracrystal of paramyosin isolated and
purified from the hearts of the whelk, Busycon.
Periodicity is a 145 A repeat.
CARDIAC FINE STRUCTURE
17
-fi
i ••
sr
I
sr
FIG. 8. Longitudinal section of a ventricular muscle reticulum (sr) system can be detected. Elements of the
cell from the heart of Busycon. Note that the Z-band is sarcoplasmic reticulum can also be observed ascomposed of aligned discrete dense bodies. In among sociated with the plasmalemma. Scale =
the myofilaments, the interconnected sarcoplasmic
18
JOSEPH W. SANGER
(and other molluscan hearts) are attached
to the plasmalemma at the periphery of
the cell. Thin filaments extend from these
membrane-bound dense bodies to interact
with thick filaments. In the interior of the
cell the dense bodies are attached only to
thin filaments which in turn interact with
thick filaments and other thin filaments.
Thus, the membrane of the cell and the
contractile apparatus are linked to each
other via the dense bodies.
Sarcoplasmic reticulum and surface
imaginations
Not surprisingly, both molluscan and
arthropod hearts possess sarcoplasmic reticulum (Figs. 6, 8). In both the arthropods
(Fig. 6) and in the molluscs (Fig. 8) the
sarcoplasmic reticulum is attached to the
plasmalemma (and in the arthropods to
invagination of the plasmalemma as well)
via septae or "feet." The nature and function of these surface couplers located between the cell surface and the wall of the
sarcoplasmic reticulum is unknown
(Franzini-Armstrong, 1973). Presumably
this is the site where the electrical impulse
induces the sarcoplasmic reticulum to release its calcium stores. The sarcoplasmic
reticulum of both arthropod and molluscan cardiac tissues is similar to vertebrate
cardiac sarcoplasmic reticulum, consisting
of tubular channels aligned along the long
axis of the cell enlarging at either end into
vesicles of varying sizes known in vertebrates as terminal cisternae (Peachey,
1965; Franzini-Armstrong, 1973). It is
only the terminal cisternae which are
linked to the plasmalemma via the surface
couplings or septae.
Molluscan cardiac cells are much smaller
than arthropod heart cells. Cardiac cells of
Busycon and Spondylus range in diameter
from a few microns to about 10 /urn; moth
heart cells, from 10 to 25 /xm in diameter;
lobster cardiac cells, somewhat larger; and
horseshoe crab cardiac muscle (Limulus
polyphemus), 6.4 to 28 /xm (Sperelakis,
1971). The smaller molluscan cardiac
cells lack the system of deeply invaginating surface membrane systems (Figs. 5,
13), which is clearly observed in arthropod
muscle (Fig. 6) (Sanger and McCann,
1968; Leyton and Sonnenblick, 1971;
Sperelakis, 1971). Presumably the larger
arthropod cell requires an invaginating
membrane system to transmit the electrical impulse to the interior of these cells.
In the smaller molluscan cells surface
coupled sarcoplasmic reticulum is able to
release calcium ions and activate the contractile apparatus in the middle of the
cell without a transverse tubular system
(Sanger, 1971). It is of interest to note that
for some unexplained reason the transverse tubular system of arthropods is composed of Battened tubules rather than the
circular profiles observed in vertebrate
material (Peachey, 1965)
The molluscan cardiac sarcoplasmic reticulum appears to possess an unusual
compensation to make up for the absence
of T-tubules. The cardiac sarcoplasmic reticulum (Fig. 9) unlike that of molluscan
adductor muscle cells (Sanger, 1971) is not
limited to the area under the plasma membrane, but consists of a system of interconnected tubules which extend throughout
the cell (Figs. 9, 13, 15). This would seem
to indicate that the stimulus to release
calcium ions is initiated at the plasma
membrane and propagated along the sarcoplasmic reticulum to the interior of the
cell. This should be investigated, for it
would indicate that sarcoplasmic reticulum
transmits impulses in the way attributed to
T-tubules (Mandrino, 1977).
Before I leave this topic I would like to
indicate some differences in the membrane
systems of gastropods and bivalves. I have
stated that molluscan caidiac cells do not
possess a transverse tubular system. However, in the heart of the gastropod, Bmycon, I have found what we call sarcolemmic tubules (Fig. 9). These structures were
originally reported in the radular protractor muscle of Busycon (Sanger and Hill,
1972). In the heart, these tubules are about
600 A in diameter, penetrate only half a
micrometer from the surface of the cell
and interdigitate with the sarcoplasmic reticulum associated with the plasmalemma.
There is a periodic substructure associated
with the leaflet of the tubular membrane
bordering the extracellular space. While
CARDIAC FINE STRUCTURE
19
• * % . . -
FIG. 9. Sarcoplasmic reticulum (sr) associating with
the plasmalemma via septae (small arrow). Note the
fenestrated interconnected sarcoplasmic reticulum
extending into the interior of the cell (sr, large
arrow). A sarcolemmic tubule (st) is also labeled in this
Busycon ventricular cell. Scale = 0.5 /xm.
the sarcolemmic tubules are very abundant
(in the radular protractor, they increase the
surface area of the cell by a factor of two;
Sanger and Hill, 1972), we have no idea of
their function. Figure 9 illustrates clearly
that in the Busycon heart the sarcoplasmic
reticulum is coupled to the uninvaginated
plasmalemma. Thus, there is no need for
impulses to be transmitted via sarcolemmic
invaginations to the sarcoplasmic reticulum. Neither the thorny oyster nor the
scallop, both bivalves, possess any surface
tubules. It will be of interest to see if these
sarcolemmic tubules are observed in other
molluscan hearts, especially in classes of
molluscs other than gastropods or in
members of the bivalves other than scallop
and oyster.
(Irisawa and Homa, 1965). Longitudinal
and transverse sections through the intercalated disc illustrate the tortuous nature
of these junctions (Sanger and McCann,
1968a). In studying the structure of the
molluscan heart I was surprised to see that
the Busycon cardiac muscle cells were also
held together by intercalated discs (Fig.
10), since these discs were not observed in
the snail heart (Helix aspersa, North, 1963).
My observations were confirmed in the
polarized microscope where these discs can
be detected readily. The discs are only a
few micrometers wide unlike the broad intercalated discs of the moth heart (Fig. 2).
This is a direct consequence of the smaller
size of the Busycon cardiac cells (10 /xm in
maximum diameter, in contrast to the
larger width of the moth cardiac cell, up to
35 fj,m). These intercalated discs are areas
of adjacent cell membranes to which the
thin filaments of sarcomeres attach, and
between which is granular material presumably acting like glue to hold the cells
together.
Molluscan cardiac muscle cells are also
closely associated by series of gap junctions
believed to serve as sites of electrical
Junctions
Both the lobster and moth hearts are
composed of cells joined to each other by
intercalated discs (Fig. 2) (Smith, 1963;
Sanger and McCann, 1968a). These discs
have also been reported in hearts of the
fruit fly (Burch etal., 1970), horseshoe crab
(Sperelakis, 1971) and mantis shrimp
20
JOSEPH W. SANGER
FIG. 10. A longitudinal section through the interca- tortuous nature. Scale = I /i.m.
lated disc of two Busycon ventricular cells indicating its
transmission (Fig. 5, Busycon ventricle;
Figs. 12 and 13, Spondylus cardiac muscle
cells). Figure 13 is a demonstration of a
network of electrically linked cells with
shared gap junctions which are very easy to
demonstrate in molluscs because of their
small cell size. Thus, this group of cells is in
intimate electrical and mechanical contact
allowing separate cells to contract together
as if they were a syncytium. Gap junctions
have also been described in the mantis
shrimp (Irisawa and Hama, 1965) and
septate junctions in the moth heart (Sanger
and McCann, 1968a).
Nerves
While moth heart cells appear free of
nerves (Sanger and McCann, 1968), cockroach cardiac cells have them in abun-
dance (Miller and Thomason, 1968). In
the cockroach heart two types of
neuromuscular synapses were observed.
One type was characterized by the presence of small electron dense granules
about 150-300 A in diameter and the second type by electron opaque granules
about 400 A in diameter. The identity of
the material in these vesicles has yet to be
determined. Despite the presence of these
cardiac nerves, the cockroach heart is
myogenic (Miller, 1974), as is the moth
heart (McCann and Sanger, 1969). We
were never able to see any nerves in the
moth heart wall or among the alary muscles near the heart (Sanger and McCann,
1968a,ft; McCann and Sanger, 1969).
However, I never looked at the ends of the
alary muscle near their lateral body attachment. It might be possible for the alary
CARDIAC FINE STRUCTURE
•+ —
FIG. 11. Longitudinal section of the fast adductor of
disc in this non-cardiac muscle.
the scallop, Aequipecten irridians. Note the intercalated
muscles to be innervated and these alary
fibers then convey impulses to the heart.
Clearly, much more work is needed to
understand the innervation of insect
hearts. (Other papers in this symposium
should be consulted for the presence and
role of cardiac nerves in crustacean
hearts.)
In molluscan hearts that I have examined (thorny oyster, Fig. 13, and whelk,
Figs. 14-16), nerve endings with two types
of vesicles have been observed closely associated with the cardiac muscle (Figs. 13,
14). About half of these nerve endings
contain clear "empty" vesicles about 600 A
in diameter .vhile the other endings possess dense granular vesicles up to 1,200 A
in diameter. These types of nerves are
similar to those we (Hill and Sanger, 1974)
have reported in the liusycon radular protractor. Acetylcholine is known to inhibit
the liusycon heart while serotonin stimulates it, but no work has been done to
indicate whether acetylcholine is in the
opaque vesicles and serotonin in the dense
granules of the Busycon heart, as has been
shown for other muscles.
Molhiscan cardiac and non-cardiac muscle
What distinguishes cardiac muscle from
non-cardiac muscle cells? The straightforward reply is that cardiac muscle cells are
found in the heart and non-cardiac muscles are not! From an ultrastructural point
of view, this question is not so readily
answered. I would like to illustrate this
problem with two examples. The first
example is found in the whelk, B.
•canalkulatum, when the ventricular muscle
is compared with the radular protractor
muscle (RPM). The RPM is a smooth,
non-catch muscle functioning in the return
stroke of the radula. The thick filaments of
the RPM are very large (380 A diameter)
as are those of the ventricular muscle (300
A). I have been able to isolate paramyosin
from the radular muscle bundle as well as
from the heart and presume that the
thickness of these filaments is due to the
presence of paramyosin. The thick filaments are aligned in the heart, as would be
expected of striated muscle, but are not
aligned in sarcomeres in the "smooth"
RPM. The disposition of the sarcoplasmic
reticulum beneath the sarcolemma and
running into the interior of the cell is a
characteristic of both the RPM (Sanger and
Hill, 1972), and the ventricle. Both muscles
also have sarcolemmic tubules.
An additional similarity in the two muscles is the presence of nerves among the
muscular tissues. There are no specialized
motor neuromuscular junctions in either
muscle type, but within the nerve endings
there are two types of synaptic vesicles:
agranular (clear) and granular (dense).
Another important similarity between the
cardiac muscle and the radular protractor
is the presence in both of gap junctions
between muscle cells. However, there are
no intercalated discs in the radular muscle
as there are in the heart. In view of the
similarity in structure of the heart and
Tl
JOSEPH W. SANGER
FIG. 12. Transverse section through two ventricular
muscle cells of the thorny oyster. A gap junction (g)
connects the two cells. Marker = 0.5 /
radula, it is not surprising that spontaneous rhythmical contraction can occur in
the radular protractor muscle (Hill et «/.,
1970). Moreover, these rhythmical contractions can be induced experimentally.
Perhaps the radular protractor muscle is
also a pump, but one which produces a
sawing motion rather than a pump which
propels fluid.
A comparison of the fast adductors of
scallop with the scallop ventricular muscles
also revealed some ultraslructural
similarities. The cross-striated swimming
adductor has all of its sarcoplasmic reticulum localized just under the sarcolemma (Sanger, 1971). In contrast the
sarcoplasmic reticulum of scallop ventricular cells is found not only under the
plasmalemma but in the interior of the cell
as in Busycon ventricular and radular muscle cells. (In contrast to whelk muscular
tissue, scallop muscles have no sarcolem-
CARDIAC FINE STRUCTURE
FIC». 13. Transverse section through a field of ventricular oyster cells illustrating the interconnected
network formed by the connecting gap junctions (arrows). A nerve is labeled (n). Scale = 1 /im.
24
JOSEPH W. SANGER
mic tubules. This is a puzzling observation
to account for in trying to decipher the
role of these structures!) The scallop heart
has intercalated discs as expected of cardiac muscle. In fact, intercalated discs have
been thought to be present exclusively in
cardiac tissue. A study of longitudinal sections of scallop cross-striated adductor,
however, revealed intercalated discs here
as well (Fig. 11). Intercalated discs are
found in the swimming muscle, because
these muscle cells are not tapered but are
ribbon-shaped and do not run the whole
length of the adductor. Where they join
end to end intercalated discs are formed.
We are unable to find gap junctions in the
adductor muscles as we did in the heart.
Thus, although they are coupled mechanically via intercalated discs, adductor cells
FIG. 14. A ventricular muscle (m) cell of the Busycon heart is in intimate association with a nerve.
lack the electrical couplings which are
found between the heart cells.
CONCLUSION
A heart is an organ composed of muscle
cells linked together to form a functional
system. Microscopy has enabled us to learn
much of the appearance of the contractile
elements, sarcoplasmic reticulum, membrane junctions, and nerves. Biochemistry
has given us much information on the
properties of the contractile elements and
sarcoplasmic reticulum. We need to know
more of the molecular parameters involved in the interactions between plasmalemma and sarcoplasm reticulum, and
between cell membrane and cell membrane. Arthropod and molluscan hearts
Scale = 1
CARDIAC FINE STRUCTURE
25
KIG. 15. A nerve ending containing clear vesicles is interconnected tubular sarcoplasmic reticulum (sr) in
lying next to a ventricular Busycon heart cell. Note the this cardiac muscle cell. Scale = 0.5 /u.m.
are much simpler in their construction
than vertebrate hearts and should serve as
models for the study of fundamental
physiological mechanisms.
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Hill, R. B., M. J. Greenberg, H. Irisawa, and H.
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NOTE ADDED IN PROOF
VVasserthal and Wasserthal (1977) re- crop, stomach and pericardium oi Aplysia.
ported the presence of a multiterminal in- Although smaller than the sarcolemmic tunervation of the heart and alary muscles in bules of Busycon (Sanger and Hill, 1973),
Sphinx hgn.stri and Ephestru kuehmella using these caveolae also result from imagina-
scanning and transmission electron micros- tions of the cell surface.
copy. The muscles were innervated by
branches of the transverse segmental
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