The Histology and Ultrastructure of the Adductor Muscle of the Eastern Oyster Crassostrea virginica (Gmelin)

14 Pages·1993·10.4 MB·English

by  C M Morrison

The histology and ultrastructure of the adductor muscle of the
eastern oyster Crassostrea virginica (Gmelin)
Carol M. Morrison
Department of Fisheries and Oceans, Box 550, Halifax Fisheries Research Laboratory, Halifax, Nova Scotia, B3J 2S7, Canada
Abstract. The structure ofthe translucent and opaque parts ofthe adductor muscle ofthe eastern oyster Crassostrea virginica (Gmelin) was studied
by lightandtransmissionelectron microscopy. The muscle was fixed with thevalvesheldclosedand afterthe muscle had relaxed sothatthe valves gaped.
Several fixativeswereused, somewithrecentlydevelopedmodifications. Themuscle fibresofthetranslucentandopaquepartsoftheadductormusclehave
asimilarorganisation, withcentralmyofilamentsandperipheralsarcoplasmicreticulum,mitochondriaandnucleus. Intranslucentmuscle,thethickmyofilaments
are thinner than they are in opaque muscle, and the dense bodies are sometimes obliquely oriented. Nerve endings and invaginations ofthe sarcolemma
at hemidesmosomes are shown for the first time in oyster adductor muscle. Forthe first time, presence ofparamyosin is indicated by diagonal as well as
transverseperiodicities in sectionsofthethickmyofilamentsofobliquely striatedmuscle. Thesemusclesaresimilartotheanteriorbyssus retractor muscle
ofthe blue mussel Mytilus edulis Linne, except that no nexal junctions were found between the muscle fibres.
Larval oysters develop two adductor muscles, the microscopy only. Bennett and Elliott (1981) and Elliott and
"dimyarian" condition, butfollowingattachmenttheanterior Bennett (1982) show the myofilaments of O. edulis in sec-
muscledegenerates (Galtsoff, 1964; Elston, 1980), resulting tioned material, but the other cell organelles were poorly
in the "monomyarian" condition. Adult bivalves whichbur- fixed. They also show thick myofilaments isolated from the
rowretaintwoadductors, whichareneededforthemovements translucent and opaque parts of the adductor of O. edulis,
involved in burrowing; but two adductors are not necessary and a striated appearance in tilted cross sections of thick
forthe sessile oyster. The single adductor is large and more myofilaments fromtheopaquepart. HansonandLowy (1960)
orless centrally placed, facilitating rapid closing in adverse studied the myofilaments only of an oyster.
conditionsorwhenclearingthe mantleofdetritus. It ismade These studies were focused on the myofilaments
up mainlyoftranslucent muscle, whichcancontractandclose becauseofthe interest inthe "catch" mechanism. Also, con-
the valves quickly. At one side ofthe adductor muscle is a sistently good fixation of the other organelles was difficult
smaller, crescent-shapedportionofopaque musclethat con- toachieve. However, recently modifications offixatives have
tracts more slowly, but can hold the valves shut against the been developed which seem to preserve tissues more com-
tension of the hinge ligament for several days (Millman, pletely. The present study was undertaken with the expecta-
1964), a phenomenon known as "catch". tionthat the ultrastructureofthe myofilaments andtheother
The gross structure (Galtsoff, 1964; Morrison and organellescouldbebetterpreserved, toprovide amorecom-
Odense, 1973), light microscopy and ultrastructure of the plete description of the oyster adductor muscle.
myofilaments and isolated paramyosin filaments of the ad-
ductor muscle of the eastern oyster Crassostrea virginica METHODS
(Gmelin)havebeendescribed (Philpottetal. 1960; Galtsoff,
,
1964; Morrison et al. 1970; Cohen et al. 1971; Morrison Fixatives used were 4% glutaraldehyde in seawater
, ,
and Odense, 1974). The most detailed study ofthe adductor (Morrison, 1970), Karnovsky's fixative (1% glutaraldehyde;
muscleofanoyster isthatofthetranslucentpartofthe mus- 4% formalinprepared fromparaformaldehyde, inphosphate
cleofCrassostreaangulata (Lamarck) by Hansonand Lowy buffer; Karnovsky, 1965), 1G4F(1% glutaraldehyde; 4% com-
(1961). The relationshipbetweenthickandthin myofilaments mercial formalin inphosphatebuffer, McDowell, 1978), 1G4F
andthe structureofisolatedthickmyofilamentsofC. angulata with seawater replacing someofthe distilled water (Howard
hasalsobeenstudied(Lowy andHanson, 1962; Elliott, 1964, and Smith, 1983), or 2.5% glutaraldehyde in 0.05 M cac-
1974and 1979; Hoyle, 1964; ElliottandLowy, 1970). Bowden codylatebuffer(DavidSims, pers. comm.). Generally, 1G4F
(1958) and Sala'nki and Zs-Nagy (1966) describe the ap- with sea-wateraddedgavebetter resultsthan withphosphate
pearance ofthe muscle fibres in Ostrea edulis L. using light buffer only, so only micrographs of muscle fixed with the
American Malacological Bulletin, Vol. 10(1) (1993):25-38
25
26 AMER. MALAC. BULL. 10(1) (1993)
formerfixativeare shown. All solutions wereusedatpH7.2. muscle fibres are ribbon-like, so in cross section they ap-
Toobtain fixed preparationsofadductormuscles with pear as flattened ovals about 3-4 wide and about 17 //m
the valves closed, part ofthe shell and the tissue around the long, which sometimes bifurcate (Fig. 2). The fibres occur
adductor were removedtopermit accessofthe fixative tothe ingroups, surroundedby athickerlayerofconnectivetissue,
muscle, then the whole oyster was immersed in fixative. To the perimysium.
obtainpreparationsoflengthened adductormuscle, theoyster Electronmicroscopy (Figs. 3, 4) revealsthateachmus-
was placed in4°C seawatercontaining 8% MgS04 (Galtsoff, cle fibre is composed of densely packed myofilaments and
1964) or3.75% MgCl2 (HansonandLowy, 1961), whichcause peripheral mitochondria, sarcoplasmic reticulum and a
the muscle to relax, until the valves gaped. The time taken nucleus. Thearrangementofthickandthin myofilamentsand
forthis varied fromone to several hours, andthe gape varied dense bodies is usually better shown in muscle which was
from 1-6 mm. Thegapedidnotappeartoalterwhentheoyster fixed inanextendedcondition, after relaxation with MgS04
was touched, but to make sure the valves stayed apart dental orMgCl (Fig. 5) than inmusclefromoysters fixedwiththe
2
wax was inserted between the valves. After removing part valves closed. The thin myofilaments are attachedto isolated
ofthe tissue surrounding the adductor muscle to expose the "dense bodies", which are oriented for short distances in
surface, theoysterwas placed in fixative. After 1.5 to2 hours, oblique bands.
part ofthe exposed surface ofthe adductor muscle fixed at Typically, when viewed withlightmicroscopy no stria-
both lengths was removed and small pieces were placed in tionsare visible (Fig. 1). Oblique striations areoccasionally
fresh fixative. seen, however, in fibres from preparations fixed with the
For light microscopy (LM) the specimens were valves closed where the anomalous effect known as "super-
dehydrated in methanol, then embedded in JB4 resin. For contraction" has occurred. The thick myofilaments pass in-
transmission electron microscopy (TEM) they werethen fixed to the regions at the ends ofthe sarcomeres where there are
in 1% osmium tetroxide, dehydrated inacetone then embed- normally only dense bodies and thin myofilaments, often
dedinTaabresin. JB4 resinsections werestainedwitha 1:50 becoming folded so that obliquely aligned bands are
dilutionof 1% toluidineblue in 1% sodium borate, methylene produced, which are visible using light or electron
blue/basic fuchsin, or Van Gieson stain (Dougherty, 1981). microscopy (Figs. 6, 7).
Taab resin semi-thin sections were also stained with toluidine In a cross section ofa sample from an oyster relaxed
blue for light microscopy. Sections for electron microscopy with magnesium sulphate, fields ofthick myofilaments sur-
were stained with25% uranyl acetate in methanol (Stempack rounded by thin myofilaments, and ofdense bodies among
and Ward, 1964), and lead citrate (Reynolds, 1963), and thin myofilaments, can be seen (Fig. 8). The thick
viewed inan Hitachi transmissionelectron microscope model myofilamentsareregularlyarrangedinapproximatehexagonal
MS-9. arrays.
Paramyosin paracrystals were prepared from the At higher magnifications the thick myofilaments are
opaqueandtranslucentpartoftheadductor, usingthe method cross-striated with a periodicity of about 5 nm. They also
describedby Philpottetal. (1960) andJohnsonetal. (1959), have a diagonal periodicity ofabout 35 nm and sometimes
who worked on the muscles of several bivalves including cross-linkstoadjacentthinfilamentshaveasimilarperiodicity
Crassostrea virginica. (Fig. 9). The transverse periodicity also occurs in isolated
paracrystalsofparamyosin, witheverythirdbaraccentuated
RESULTS (Fig. 10). Measurements of 100thick myofilaments in cross
section revealed a modal peak at about 32 nm (range = 18
Better general fixation was obtained than in previous to 62 nm).
studies ofoyster muscle. However, results werevariable, often Thin myofilaments are attached to the fusiform dense
within the same section. Areas of poor fixation were bodies (Fig. 11). The thick filaments are spindle-shaped, so
sometimes found near the surface as well as deeper in the they appear wider in the centre ofa sarcomere and become
tissues. 1G4F with seawater was the most dependable fix- smaller towards the dense bodies between sarcomeres (Fig.
ative. There were periodicities inthethick myofilaments, and 12). As in mammalian skeletal muscle, where thickandthin
details of other cell organelles that were not evident in our filaments overlap the thick myofilaments are surrounded by
earlier studies (Morrison, 1970; MorrisonandOdense, 1974). a ring ofthin filaments. This ring is sometimes incomplete
around the wider thick myofilaments in the centre of the
TRANSLUCENT MUSCLE sarcomere, which would be equivalent to an H zone. Cross
Thetranslucent muscleconsistsofgroupsofelongate, links can often be seen between adjacent thick and thin
thin muscle cells or fibres separated by a thin layer ofcon- myofilamentsandbetweenadjacentthinmyofilaments. Some
nectivetissue, theendomysium (Fig. 1). The elongate nuclei ofthedensebodiesareattachedtothe sarcolemma, forming
are oriented parallel to the long axis ofthe muscle cell. The hemidesmosomes (Fig. 13). Filaments from the connective
MORRISON: ADDUCTOR MUSCLES OF THE EASTERN OYSTER 27
Fig. 1. Longitudinal section (parallel to the muscle fibres ofthe adductor extending between the two valves ofthe oyster) oftranslucent part ofadductor
muscle. Specimen was fixed in2.5% glutaraldehyde incaccodylatebuffer, embedded inJB4, and stained with toluidineblue. Long, thin muscle fibres (F)
are surrounded by endomysium (E), and groups ofmuscle fibres are surrounded by perimysium (P) (scale bar = 0.1 mm). Fig. 2. Cross section (across
theadductormuscle)oftranslucentpartofadductormuscle. Specimenwasfixedin2.5% glutaraldehydeincaccodylatebuffer,embedded inJB4.andstained
with Van Gieson. The profiles ofthe muscle fibres (F) areelongateand sometimes bifurcate; each fibre is surrounded by endomysium (E). and groups of
fibres are surrounded by perimysium (P) (scale bar = 30 fim). Fig. 3. Longitudinal section oftranslucent part ofadductor muscle. TEM micrograph of
specimen fixed in 1G4F. The muscle fibres haveacentral coreofmyofilaments (M) and densebodies (DB), and hemidesmosomes (HD) are presentat the
sarcolemma.Theperipheralcytoplasmcontainselongatemitochondria(MI),sarcoplasmicreticulum(SR)andanelongatenucleus(NU). Betweenthemuscle
fibres are glial cells (GC) closely associated with nerve endings (NE) (scale bar = 2 ^m).
28 AMER. MALAC. BULL. 10(1) (1993)
Fig. 4. TEM micrograph oftranslucent part ofadductor muscle ofspecimen relaxed in Mg S04 and fixed in Karnovsky's fixative. The same features can
be recognisedas in figure 3, butthe nucleus (NU), mitochondria(ME)andsarcoplasmic reticulum(SR) are more rounded sincethis isacross section [(E)
endomysium; (GC) glial cell; (M) myofilaments] (scale bar = 2 /im) Fig. 5. TEM micrograph oflongitudinal section ofmuscle fibres oftranslucent part
ofadductormuscle. Specimen relaxed in Mg S04 and fixed in Karnovsky's fixative. Thickmyofilaments(TKM) are presentandthin myofilaments(TNM)
are continuous with dense bodies (DB), which are obliquely oriented for short distances. There are profiles ofsarcoplasmic reticulum (SR) beneath the
sarcolemma (scale bar = 1 jan).
MORRISON: ADDUCTOR MUSCLES OF THE EASTERN OYSTER 29
Fig.6. Longitudinal sectionoftranslucentpartofcontractedadductormuscleofspecimenfixedin2.5% glutaraldehydeincaccodylatebufferandembedded
for electron microscopy, 0.5 fim section stained with toluidine blue. Note oblique contraction bands (CB) in the fibres (F) (scale bar = 20 jtm).
Fig.7.TEMmicrographoflongitudinal sectionofcontractionbandsoftranslucentpartofcontractedadductormuscle. Specimenfixed in2.5% glutaraldehyde
incaccodylate buffer. Thick myofilaments (TKM) overlap and become folded in regions where there are dense bodies (DB) (scale bar = 1 fan). Fig. 8.
TEM micrographofcross sectionoftranslucent part ofadductormuscle. Specimen relaxed in Mg S04 and fixed in Karnovsky's fixative. There are fields
ofthickandthin myofilaments (TKM) andthin myofilaments (TNM) inthe muscle fibres, anddensebodies(DB) occurinthe fieldsofthin myofilaments.
Hemidesmosomes(HD)presentatthesarcolemma. Thenucleus(NU)isroundedincross-section. Vesiclesofsarcoplasmicreticulum(SR)aswellasmitochondria
(MI)occurbeneaththesarcolemma. Aglial cell (GC) withanaccompanying nerveending(NE) ispresent betweenthe musclecells (scale bar = 500nm).
30 AMER. MALAC. BULL. 10(1) (1993)
tissue stroma accumulate next to the hemidesmosome. The (Fig. 26). When the thick myofilaments are cut obliquely,
sarcolemma is often invaginated where there are they often appear to be banded (Fig. 27).
hemidesmosomes (Figs. 3, 8). The thin filaments are attached to dense bodies (Fig.
Theprofilesofsarcoplasmic reticulumaresituatedjust 28) whichdo not show any special arrangement in the mus-
beneath the sarcolemma, and dense material can be seen cle fibre (Fig. 29), so this type of muscle is classified as
between the outer membrane ofthe sarcoplasmic reticulum smooth. As inthetranslucentmusclefibre, somedensebodies
andthe sarcolemma (Fig. 14). The sarcolemmasofadjacent are attached to the sarcolemma, forming hemidesmosomes
muscle cells are often close to each other (Figs. 3-5, 8), but (Fig. 30), and filaments of connective tissue in the
no well-defined junctions were seen. Many of the mito- endomysium are closely associated with the sarcolemma at
chondria in one specimen have filamentous paracrystals and these sites. In some places the sarcoplasm surrounding the
annulated cristae (Fig. 15). myofilaments is wide, and sarcolemmal invaginations con-
Nerve endings, which contain a variety of vesicles, taining filaments of connective tissue extend to the
some small and clear, some dense-cored or ofvarying den- hemidesmosomes (Fig. 31). Glial cells and axons occur be-
sity, often occurclose to the sarcolemma (Figs. 3, 16). Glial tween the muscle fibres. Nerveendings, often accompanied
cells containing large granules, the gliosomes, are usually by glial cells, are found next to the sarcolemma, sometimes
closely associated withthese nerve endings but do not com- embedded in the sarcoplasm (Fig. 32). They contain a vari-
pletely surround them. etyofsmall clearanddense-coredvesiclesandlargervesicles
containing a varying amount of dense material.
OPAQUE MUSCLE
The muscle fibres of the opaque muscle are more DISCUSSION
rounded in cross section than those of translucent muscle,
so their diameter (10-20 /im) is similar to that ofthe length The adductor muscle ofoysters, as in most bivalved
but largerthanthe widthoftheoval profilesofthetranslucent molluscs, consists of uninucleate muscle cells of small
muscle fibres (Figs. 17, 18). Inthetransitional zonebetween diameter (3-20 /im) that are much longer than those of
the translucent andopaque muscle the muscle fibres ofeach vertebrate smooth muscle (Twarog, 1976). Each musclecell
type are intermingled (Fig. 19). As in the translucent mus- contain a single myofibril, and peripheral vesicles of sar-
cle, each muscle fibre is surrounded by endomysium, and coplasmic reticulum whichformcouplingswiththe sarcolem-
thegroupsoffibres by perimysium. The arrangement ofcell ma, as described in the adductor muscle of the scallop,
organelles isalsosimilar. Central myofilamentsaresurround- Argopecten irradians (Lamarck, 1819) (Nunzi and Franzini-
ed by vesicles ofsarcoplasmic reticulum and mitochondria, Armstrong, 1981) and the translucent part of the adductor
and the sarcolemmas ofneighbouring cells are often closely muscle of Crassostrea angulata (Hanson and Lowy, 1961).
opposed (Figs. 20, 21). The nucleus is situated in cytoplasm The invaginationsofthe sarcolemmaatthehemidesmosomes
to one side of the cell, oriented with its long axis parallel are not as well defined as the transverse tubular system of
with that ofthe muscle cell, and dense material can be seen transversely striated muscle or some unstriated muscle
betweenthe sarcolemmaand the sarcoplasmic reticulum (Fig. (Dorsett and Roberts, 1980), but may perform a similar
22). transport function.
The thick myofilaments are thicker than those ofthe The dense bodies, like the Z-line ofstriated muscles,
translucent muscle, having a modal peak at about 61 nm anchorthe actin filaments. In the translucent part ofthe ad-
(range = 18 to 123 nm). They exhibit cross striations at 5 ductor of Crassostrea virginica they form oblique bands as
nm andoftendiagonal striations like thoseoftranslucent mus- in C. angulata, in which the bands are arranged helically
cle, butthey are moreevident(Figs. 23, 24). Everythirdcross aroundtheouterpartofthefibreandbranchandanastomose
striation is accentuated. Cross links occurbetweenthe thick in the centre ofthe muscle fibre (Hanson and Lowy, 1961).
and thin myofilaments (Figs. 25, 26), and some thin These bands were not seen in normal muscle in this study,
myofilaments havecrosslinkstomorethanonethickmyofila- but can be seen using phase contrast illumination (Hanson
ment (Fig. 25). As in the thick myofilaments ofthe translu- and Lowy, 1961). The contraction bands ofsupercontracted
cent muscle fibres, the cross links seem to have a similar muscle are obvious using regular as well as phase illumina-
periodicity to that of the diagonal bands. tion (Bowden, 1958; Hansonand Lowy, 1961; Galtsoff, 1964;
Sometimes, there is a single ringofthin myofilaments Salanki and Zs-Nagy, 1966; Morrison and Odense, 1974).
surrounding each thick myofilament in the region ofoverlap, Moreregularobliquebandingoccurs in somemolluscs such
but more often there are several thin myofilaments between as theoctopus and cuttlefish, where themyofibrils surround
the thick myofilaments. Cross links can sometimes be seen acentral coreofmitochondria(Millman, 1967). Inthis study,
between adjacent thin myofilaments, and occasionally con- thedensebodies, andtheactinfilamentsenteringthem, were
nections can also be seen between thick myofilaments easierto see in specimens fixed in4% glutaraldehyde in sea
MORRISON: ADDUCTOR MUSCLES OF THE EASTERN OYSTER 31
Fig. 9. TEMmicrographoflongitudinal sectionoftranslucentpartofadductormuscleofspecimenfixedin2.5% glutaraldehydeincaccodylatebuffer. The
thickmyofilaments exhibit transversebanding (TBA) at intervalsofabout 15 nm and diagonal banding (DBA). Cross-links (CL) to the actin filaments can
be seen in some areas, at about the same intervals as the diagonal banding (scale bar = 100 nm). Fig. 10. Translucent part ofadductor muscle showing
paramyosin paracrystal with narrow transverse bands (TBA) about 5 nm apart and accentuated bands (AB) about 15 nm apart (scale bar = 10 nm).
Fig. 11. TEM micrographoflongitudinal sectionoftranslucentpartofadductor muscleofspecimen relaxed in Mg S04 and fixed in Karnovsky's fixative.
Cross links (CL) occur between the thick and thin myofilaments, as well as between the thin filaments. The latter enter dense bodies (DB) (scale bar =
200nm). Fig. 12.TEMmicrographofcrosssectionoftranslucentpartofadductormuscleofspecimenrelaxedinMgS04andfixedinKarnovsky'sfixative.
Thickmyofilaments(TKM),thinmyofilaments(TNM)anddensebodies(DB)arepresent. Eachthickmyofilamentissurroundedbyaringofthinmyofilaments,
and cross-links (CL) can often be seen between the two types ofmyofilament (scale bar = 100nm). Fig. D. TEM micrograph oflongitudinal section of
translucent partofadductormuscleofspecimen relaxed in Mg S04 and fixed in Karnovsky's fixative. There is a hemidesmosome (HD) toone sideofthe
doublemembraneformingthesarcolemma;ontheothersidethereareconnectivetissuefilaments(scalebar =200nm). Fig. 14.TEMmicrographoflongitudinal
sectionoftranslucentpartofadductormuscleofspecimenfixedin iG4F. Athinlayerofdensematerialispresentbetweenavesicleofsarcoplasmicreticulum
(SR) and the sarcolemma (scale bar = 200 nm).
32 AMER. MALAC. BULL. 10(1) (1993)
Fig. 15. TEM micrograph oftransverse section oftranslucent part ofadductor muscle ofspecimen relaxed in Mg S04 and fixed in Karnovsky's fixative.
Amitochondrioncontainsfilamentousparacrystals(FP)andannulatedcristae(AC) (scalebar = 200nm). Fig. 16. TEMmicrographoflongitudinal section
oftranslucentpartofadductormuscleofspecimenfixed in 1G4F. There isanerve-ending(NE)containingclearanddense vesicles, andanassociatedglial
cell(GC)nearthesarcolemma(scalebar = 1 /an). Fig. 17. Longitudinalsectionofopaquepartofadductormuscleofspecimenfixedin2.5% glutaraldehyde
in caccodylate buffer, embedded inJB4and stained with toluidine blue. The muscle fibres (F) that are widerthanthoseofthe translucent muscle, andare
separated by endomysium (E) and perimysium (P) (scale bar = 0.1 mm). Fig 18. Cross section ofopaque part ofadductor muscle ofspecimen fixed in
2.5% glutaraldehyde in caccodylate buffer, embedded in JB4 and stained with methylene blue/basic fuschin. The muscle fibres (F) are large and have a
rounded profile [(E) endomysium; (P) perimysium] (scale bar = 20/im).
MORRISON: ADDUCTOR MUSCLES OF THE EASTERN OYSTER 33
water used in earlier work than in more recently developed ABRM usingoptical diffractiontechniques (Sobieszek, 1973).
fixatives, presumably because less background material in Sobieszek postulates that there are sarcomeres consisting of
the cell was fixed. three to fourthick myofilaments which are 25 fim long, thin
Dense bodies also form hemidesmosomes at the sar- myofilaments which are 140 /*m long and two dense body
colemma in the anterior byssus retractor muscle (ABRM), halves. Such sarcomeres would be difficult to recognise in
where it has been suggested that they give structural stabili- sectionedmaterial, sincethey are solongandnarrow, so they
ty to the muscle by attaching the muscle cells to the closely may bepresent intheopaqueadductormuscleofCrassostrea
associated filaments of connective tissue (Twarog, 1967; virginica. Cross-bridges between the thick and thin
Twarog et al. 1973) and propagate tension to neighboring myofilaments havebeen showninothermollusc musclecells,
,
cells via the collagen of the connective tissue (Sobieszek, such as those ofthe ABRM, in varying degrees ofcontrac-
1973; Twarog, 1976). Hansonand Lowy (1961) suggested, in tion. It has been suggested that direct interaction occurs
theirstudyonthetranslucentadductormuscleofCrassostrea between the thick myofilaments (Hoyle, 1983), as indicated
angulata, thatthe sarcolemmamaybe interrupted nexttothe by the close relationship sometimes seenbetweenthem (Fig.
densebody, butitcanbeclearly seenthatthis is notthecase 26); but other workers believe that this does not play a part
in the present study (Fig. 13). in contraction (Bennett and Elliot, 1989).
In the ABRM nexal or gap junctions about 15 nm The thick myofilaments ofvertebrate striated muscle
across which may be sites ofintercellularconductivity have areveryconstant indiameter(about 16 nm)andlength(about
beendescribedbetweenthemusclecells(Twarogetal. 1973), 1.6 /mi); whereas the thick myofilaments of most mollusc
,
but these were not seen in the present study. However, the fibres, including those described in the present study, are
sarcolemmasofadjacentcellsoftenrunparallel toeachother wider and vary in width (Levine et al., 1976). This is
farther apart, but over greater distances than those found in associated with the presence of varying amounts of
nexal junctions. This has been reported in several other paramyosin (Millman, 1967). Theproportionofparamyosin
molluscs (Zs.-Nagy and Salanki, 1970; Nicaise and in transversely striated muscles such as the fast part ofthe
Amsellem, 1983). adductor ofPlacopecten magellanicus (Gmelin) is low (7%
The filamentousparacrystalsfound inthemitochondria by mass; Chantler, 1991), but is higher in obliquely striated
ofone specimen are similar to those described in the mito- muscle such as that ofthe translucent part of the adductor
chondria of the myocardial cells of Crassostrea virginica of Crassostrea angulata (16-20%; Chantler, 1983) and
(Hawkins etal. 1980). These workers also found prismatic highest in smooth muscle such as that ofthe opaque part of
,
cristae, and our specimen had annulated cristae. Their the adductor of C. angulata (22-39%; Chantler, 1983).
specimens, like ours, appeared to be otherwise normal, Paramyosin forms the core of the thick myofilament, sur-
although inotheranimals paracrystals areusually associated rounded by myosin (Szent-Gyorgyi et al. 1971) and gives a
,
with disease or states of altered metabolism. typical X-ray diffraction patternoriginally describedby Bear
Glial cells likethose found inboththetranslucentand (1944) andBearandSelby (1956), which subsequentlybecame
opaque parts ofthe adductor muscle in this study have been known as the Bear-Selby net. The transverse striations with
describedforminga "glio-interstitial network" betweentonic a periodicity ofabout 15 nm, and diagonal striations with a
musclecellsinavarietyofmolluscs, includingthetonic mus- periodicity of about 35 nm described in the thick
ABRM
cle ofthe adductor ofAnodonta, and the ofMytilus myofilaments of the translucent and opaque muscle in this
edulis (Amsellemetal. 1973; Gilloteaux, 1975; Nicaiseand study form a "checkerboard" pattern characteristic of the
,
Amsellem, 1983). They have not previously been recorded paramyosincoreof"catch" muscle(Hansonand Lowy, 1964;
inphasic muscles. Theseauthors suggestthatthesecellsmay Chantler, 1991). Thediagonal striations have notbeen shown
help to control metabolic exchanges between the interstitial previously in a translucent or phasic muscle. These
spaces and the nerves and/or muscles. periodicities were first shown by Hall et al. (1945) to cor-
The opaque muscle tissue ofthe adductor examined respond to the pattern formed by the Bear-Selby net, and
in this study is very similar in appearance and physiology result fromthe tendency ofparamyosin molecules to assem-
to the ABRM of the mussel (Lowy and Hanson, 1962; ble with a 14.5 nm intermolecular shift as a result of inter-
Millman, 1964; Twarog, 1967; Heumann and Zebe, 1968; molecular ionic interactions (Kendrick-Jones et al. 1969;
,
and Sobieszek, 1973). However, its thick myofilaments are Castellani and Cohen, 1987). The relationship between the
widerthan those ofthe ABRM, which have a peak width of patterns seen in paramyosin and myofilament preparations
about40 nm (Sobieszek, 1973). Unlike the translucent mus- from "catch" muscle usingtheelectron microscopeand the
cle, there is no obvious arrangement ofdensebodies. Lowy Bear-Selby net are considered in detail by Elliott and Lowy
and Hanson (1962) assert that some degree of order is (1970), Cohen et al. (1971), and Elliott (1979). The banding
necessary for the sliding-filament mechanism to work effi- patterns seen inthick myofilaments inoblique sectionsofthe
ciently, and such an order has been demonstrated in the opaque muscle in the present study (Fig. 27) have also been
34 AMER. MALAC. BULL. 10(1) (1993)
Fig. 19. Transversesectionofthetransitionzonebetweentheopaqueandtranslucentpartsoftheadductormuscleofspecimenfixed in2.5% glutaraldehyde
incaccodylatebuffer,embeddedinJB4andstainedwithmethyleneblue-basicfuschin. Thickopaquefibres(OF)canbeseentothebottomofthemicrograph,
andmoreelongate,thinnertranslucentones(TF)canbeseentothetop. Inthecentrefibresofbothtypesareintermingled(scalebar = 20jtm). Fig.20.TEM
micrographoflongitudinal sectionofopaquepartofadductormuscleofspecimenrelaxedinMgCl2 andfixedin2.5% glutaraldehydeincaccodylatebuffer.
Acentralcoreofmyofilaments(M)anddensebodies(DB)ispresentineachmusclefibre.Therearemitochondria(MI)andvesiclesofsarcoplasmicreticulum
(SR) next to the sarcolemma. These are not as elongate as in translucent muscle. Endomysium (E) is present between muscle fibres (scale bar = 1 /an).
Fig. 21. TEM micrograph ofcross section ofopaque part ofadductor muscle of specimen fixed in Karnovsky's fixative, showing the dense bodies (D),
endomysium (E), myofilaments (M), mitochondria (MI) and sarcoplasmic reticulum (SR) seen in longitudinal section in figure 20 (scale bar = 1 //m).
Fig. 22. TEM micrograph ofcross section ofopaque part ofadductor muscle fixed in Karnovsky's fixative. The nucleus (NU) appears rounded, and is
in the cytoplasm to one side ofthe myofilaments (M) (scale bar = 1 fim).