Ultrastructure of the flagellar apparatus of Oxyrrhis marina

Summary— Oxyrrhis marina, like all dinoflagellates, possesses one transverse and one longitudinal flagellum, which show structural differences. The transverse flagellum contains a small fibre, 20 nm in diameter, associated with doublet no.7, whereas the longitudinal flagellum is substantially by a large (200–300 nm) hollows structure closely resembling the paraflagellar rod described by several authors in kinetoplastidae and in euglenoids. This structure is made up of a hemicylindrical network of filaments which are often linked on one side to the outer doublet no. 4, and on the other side to a dense plate. Another thinner filamentous network closes this hemicyclinder. In cross-section, the wall of this structure is made up of 8 filaments 2–4 nm in diameter that show a thicker periodic structure. In longitudinal section the same filaments appear arranged in periodic rhombus meshes or a helicoidal pattern, depending on the orientation of the section relative to the axoneme.     

鲻和鲮鳃丝的扫描电镜比较观察

对鲻（Mugil cephalus）和鲮（Cirrhina molitorella）的鳃丝表面结构进行了扫描电镜比较观察,结果表明,鲻鳃丝杆状部比鲮粗．鲻鳃小片高度比鲮低;两者鳃丝表面分泌孔洞口径和密度不同;鲻和鲮细胞外被不同,鲻细胞外被稀疏,鲮的则致密复杂;鳃小片细胞和鳃丝表皮细胞的表面形态存在差异,文章还描述了鳃丝表皮形态特异的细胞。     

Cytoskeletal filaments in embryonic chick myocardial cells as revealed by the quick-freeze deep-etch method combined with immunocytochemistry

Summary The three-dimensional organization of cytoskeletal filaments associated with the myofibrils and sarcolemma of the myocardial cells of early chick embryos was studied by the rapid-freeze deep-etch method combined with immunocytochemistry. In the endoplasmic region of saponin-treated myocardial cells, 12–14 nm filaments formed a loose network surrounding nascent myofibrils. These 12–14 nm filaments attached to the myofibrils and some of them converged into Z disc regions. In the non-junctional cytocortical region thinner 8–11 nm filaments composed a dense network just beneath the sarcolemma. In myofibril terminating regions at the sarcolemma, i.e., the fascia adherens, 3–5 nm cross-bridges were observed among the thin filaments. In Triton-permeabilized and myosin subfragment 1 (S1)-treated samples, subsarcolemmal 8–11 nm filaments proved to be S1-decorated actin filaments under which there was a loose network of S1-undecorated filaments. Subsarcolemmal S1-decorated actin filaments had mixed polarity and attached to the sarcolemma at one end. A loose network of S1-undecorated filaments among myofibrils in the endoplasmic region was revealed to consist of desmin-containing intermediate filaments after immuno-gold staining for desmin. These networks connecting myofibrils with sarcolemma were assumed to play an important role in integrating and transmitting the contractile force of individual myofibrils within early embryonic myocardial cells.     

The oral apparatus of Tetrahymena pyriformis, strain WH-6. IV. Observations on the organization of microtubules and filaments in the isolated oral apparatus and the differential effect of potassium chloride on the stability of oral apparatus microtubules.

This report is an ultrastructural analysis of the organization of the isolated oral apparatus of Tetrahymena pyriformis, strain WH-6, syngen 1. Attention has been focused on the organization of microtubules and filaments in oral apparatus membranelles. Oral apparatus membranellar basal bodies were characterized with respect to structural differentiations at the distal and proximal ends. The distal region of membranellar basal bodies contains the basal plate, accessory microtubules and filaments. The proximal end contains a dense material from which emanate accessory microtubules and filaments. There are at least two possibly three different arrangements of accessory structures at the proximal end of membranellar basal bodies. All membranellar basal bodies appear to have a dense material at the proximal end from which filaments emanate. Some of these basal bodies have accessory microtubules and filaments emanating from this dense material. A possible third arrangement is represented by basal bodies which have lateral projections, from the proximal end, of accessory microtubules and filaments which constitute cross or peripheral connectives. There are at least three examples of direct associations between oral apparatus microtubules and filaments: (1) filaments which form links between basal body triplet microtubules, (2) filaments which link the material of the basal plate to internal basal body microtubules, (3) filaments which link together microtubule bundles from membranellar connectives. KCl extraction of the isolated oral apparatus resulted in the selective solubilization of oral apparatus basal bodies, remnants of ciliary axonemes and fused basal plates. Based on their response to KCl extraction two distinct sets of morphologically similar micro tubules can be identified: (a) microtubules which constitute the internal structure of basal bodies and ciliary axonemes, (b) microtubules which constitute the fiber connectives between basal bodies.     

Dense bodies and actin polarity in vertebrate smooth muscle

The arrangement of cytoplasmic dense bodies in vertebrate smooth muscle and their relationship to the thin filaments was studied in cells from rabbit vas deferens and portal vein which were made hyperpermeable (skinned) with saponin and incubated with myosin subfragment 1 (S-1). The dense bodies were obliquely oriented, elongated structures sometimes appearing as chains up to 1.5 microns in length; they were often continuous across the cell for 200 to 300 nm and were interconnected by an oblique network of 10-nm filaments. The arrowheads, formed by S-1 decoration of actins, which inserted into both the sides and ends of dense bodies, always pointed away from the dense body, similar to the polarity of the thin filaments at the Z- bands of skeletal muscle. These results show that the cytoplasmic dense bodies function as anchoring sites for the thin filaments and indicate that the thin filaments, thick filaments, and dense bodies constitute a contractile unit.     

The fine structure of the kinetochore of a mammalian cell in vitro

The chromosomes of Chinese hamster cells were examined with the electron microscope and the following observations were made concerning the structure and organization of the kinetochore. — The kinetochore consists of a dense core 200–300 Å in diameter surrounded hy a less dense zone 200–600 Å wide. The dense core consists of a pair of axial fibrils 50–80 Å in diameter which may be coiled together in a cohelical manner. The less dense zone about the axial elements is composed of numerous microfibrils which loop out at right angles to the axial fibrils. Together the structures comprise a lampbrush-like filament which extends along the surface of each chromatid. Some sections suggested that two such filaments may be present on each chromatid. The fine structure of kinetochores associated with spindle filaments was essentially the same as those free of filaments. The structure and organization of the kinetochore of these mammalian cells was compared to that of lampbrush chromosomes of certain amphibian oöcytes, dipteran polytene chromosome puffs, and the synaptinemal complex seen during meiotic prophase.The authors also wish to thank Dr. Arthur Cole of the Department of Physics for the use of his electron microscope facilities and for his helpful criticism.     

Filamentous structure of the external surface of abalone eggs revealed by quick-freeze deep-etch technique

The external surface of abalone eggs was examined by thin section and quick-freeze, deep-etch electron microscopy. In thin sections, networks of fine filaments were found interconnecting the adjacent microvilli on the surface of unfertilized eggs. Quick-freeze, deep-etch electron microscopy revealed the three-dimensional structure of these networks of filaments on the external surface of the egg. Mainly two networks of filaments were identified; one was composed of thicker (14–19 nm) filaments interconnecting with the neighboring microvilli nearly horizontally, and the other was composed of thinner (8–14 nm) branched filaments closely surrounding the microvilli surface as well as highly interconnecting neighboring microvilli in a polygonal pattern. The overall structure of the filamentous network on the egg surface showed no distinct alteration after fertilization. These networks of filaments observed on the egg surface may play a key role in sperm–egg interaction.     

The organization and fine structure of the muscles of the scolex of the cysticercoid of Hymenolepis microstoma

The organization and fine structure of the muscles of the scolex of the cysticercoid of Hymenolepis microstoma are described. The contractile apparatus consists of thick (175–325 Å diameter × 1.4 μm) and thin (60–80 Å diameter × 1 μm) filaments. The thick filaments are occasionally attached to the thin filaments by cross bridges. The thin filaments are attached to the dense bodies or to a dense zone at the sarcolemma at muscle insertions. In contracted muscle the thick filaments appear as quasi-hexagonal arrays or in lines. Each thick filament is surrounded by an orbit of up to 12 thin filaments, which in turn may be shared by adjacent thick filaments. Thin filaments may be present in quasi-rectangular or hexagonal groupings indicating some low order degree of actin lattice. The fusiform dense bodies (1,500 Å × 900 Å), consisting of up to 25 discrete substructures, are distributed uniformly throughout the myofiber and/or attached to the sarcolemma at attachment plaques. The sarcoplasmic reticulum, consisting of a presumed anastomosing network of tubules is structurally connected to the sarcolemma by periodic deposits of electron opaque material. Sarcoplasmic extensions of the myofiber(s) contain the nucleus, Golgi complexes, rough endoplasmic reticulum, ribosomes, β-glycogen, mitochondria and membrane bound electron dense structures. Upon activation of the metacestode, groups of α-glycogen and enlargement of the rough endoplasmic reticulum were observed. Microtubules which were conspicuously absent from the sarcoplasm of the unactivated worms appeared adjacent to the myofibers in activated worms.     

In the mammalian eye type VI collagen tetramers form three morphologically different aggregates.

The organization of the aggregates occurring in the stroma: (1) of the murine and human cornea after incubation in an ATP acidic solution; (2) of surgically excised epiretinal membranes (ERM); and (3) of the trabecular meshwork of monkey eyes was investigated morphologically and immunocytochemically on thin section electron microscopy. Morphology. The aggregates in the cornea appeared as cross-banded fibrils. The bands were uniformly electron dense (single banded form); they were separated from each other by interbands consisting of a bundle of filaments emerging in cross section as small areas of randomly assembled dot-like structures. In the ERM, most of the aggregates stood out as heteromorphic cross-banded bodies showing dense bands with electron denser borders (double banded form) and interbands composed of longitudinally oriented, parallel sheets or laminae of amorphous material enclosing thin, similarly oriented filaments. These extended, thinner and double in number (since interlacing with similar components of the opposite sheet), into the pale central zone of the dense band. The aggregates of the trabecular meshwork were heteromorphic, had uniformly dense bands (single banded form as in the cornea), but their interbands displayed longitudinal sheets (as the ERM aggregates). Immunocytochemistry revealed type VI collagen in the three eye aggregates with gold particles preferentially localized at the interbands. The specificity of the antibodies used was tested by Western blot analysis of type VI collagen samples extracted from human placenta and on homogenates of human cornea. In conclusion, the results indicate that the tetramers of type VI collagen may aggregate differently into structures with distinct supramolecular arrangements. These are illustrated in schematic drawings.     

OBSERVATIONS ON THE FINE STRUCTURE OF THE DEVELOPING SPERMATID IN THE DOMESTIC CHICKEN

The kinetic apparatus, the acrosome and associated structures, and the manchette of the spermatid of the domestic chicken have been studied with the electron microscope. The basic structural features of the two centrioles do not change during spermiogenesis, but there is a change in orientation and length. The proximal centriole is situated in a groove at the edge of the nucleus and oriented normal to the long axis of the nucleus and at right angles to the elongate distal centriole. The tail filaments appear to originate from the distal centriole. The plasma membrane is invaginated along the tail filaments. A dense structure which appears at the deep reflection of the plasma membrane is identified as the ring. The fine structure of the ring has no resemblance to that of a centriole and there is no evidence that it is derived from or related to the centrioles. The tail of the spermatid contains nine peripheral pairs and one central pair of tubular filaments. The two members of each pair of peripheral filaments differ in density and in shape: one is dense and circular, and the other is light and semilunar in cross-section. The dense filaments have processes. A manchette consisting of fine tubules appears in the cytoplasm of the older spermatid along the nucleus, neck region, and proximal segment of the tail. The acrosome is spherical in young spermatids and becomes crescentic and, finally, U-shaped as spermiogenesis proceeds. A dense granule is observed in the cytoplasm between acrosome and nucleus. This granule later becomes a dense rod which is interpreted as the perforatorium.     

Detection of dense intra- and perinuclear 10 nm filament systems by whole mount and embedment-free electron microscopy in several species of the green algal order Dasycladales

Summary In contrast to the immense body of evidence supporting the occurrence of intermediate filament (IF) proteins in the animal kingdom, there is only limited information on their distribution in plants. Nevertheless, a number of immunocytochemical and electron microscopical observations indicate that particularly in higher plant cells IFs contribute to the construction of the cyto- and karyoskeleton. Here we show by whole mount electron microscopy of the giant nuclei extruded together with adhering cytoplasm from the rhizoids of some species of the algal order Dasycladales that cytoplasmic 10 nm filament networks also occur in unicellular, mononucleated green organisms of early evolutionary origin. The filament systems were associated with the residual nuclear envelope which consisted of a dense arrangement of pore complexes suspended by a meshwork of short 5 to 6 nm filaments; structurally it was very similar to the nuclear envelopes obtained from mammalian cells. When the Dasycladales nuclei were processed side by side with mouse skin fibroblasts, the algal filament systems were physically almost indistinguishable from the mammalian vimentin filament network. Embedment-free thin sections of rhizoids have not only confirmed the existence of the perinculear 10 nm filaments and their seamless association with the nuclear envelope, but have demonstrated the existence of an extensive intranuclear meshwork of 10 nm filaments. The latter were morphologically indistinguishable from the perinuclear 10 nm filaments and seem to be connected to these via the nuclear envelope to form a continuum. Among a variety of antibodies directed against mammalian IF proteins, only polyclonal anti-mouse lamin B antibodies decorated the cytoplasmic filaments of the Dasycladales cells. Surprisingly, none of the antibodies decorated the thinner filaments of the nuclear envelope, which possibly represent the nuclear lamina. In accord with this observation, one anti-lamin B antibody recognized in Western blot analysis of a urea extract ofAcetabularia acetabulum rhizoids three polypeptides with M_rs of approximately 47,000, 64,000, and 76,000. The proteins did not react with the -IFA antibody. Since the Dasycladales have a fossil record of nearly 600 million years — an extant genus, Acicularia, also investigated here, evolved about 170 million years ago -, the molecular characterization of the subunit proteins of their cytoplasmic filament systems might throw further light on the evolution and biological role of IFs.Dedicated to Professor Sir Henry Harris on the occasion of his 70th birthday     

Potassium chloride-insoluble myofilaments in vertebrate smooth muscle cells

Actomyosin was extracted from avian gizzard smooth muscle. The residue was then homogenized and fractionated by differential centrifugation. Fractions of the residue that sedimented at 1 000 g and 13 000 g were examined in negatively stained and sectioned preparations with the electron microscope. The major components of both fractions were 100 Å diameter filaments and fusiform dense bodies. The filaments and dense bodies closely resembled their counterparts in sectioned preparations of unextracted smooth muscle cells from Taenia coli. The insolubility of the 100 Å diameter filaments at high ionic strength and their detailed structure suggest that they are not composed of actin and myosin. Their general features indicate that they correspond to the so-called thick filaments observed in the early studies of vertebrate smooth muscle cells.     

Rigor crossbridge structure in tilted single filament layers and flared-X formations from insect flight muscle

The averaged structure of rigor crossbridges in insect flight muscle has been studied in filtered images. Their three-dimensional structure has been deduced by relating tilt views of single filament layers in 25 nm longitudinal sections (myac layers and actin layers) to the flared-X appearance in 15 nm cross-sections showing single crossbridge levels. Tilting myac or actin layers around the filament axis makes crossbridges show one of two patterns. Beadlike densities appear either singly over thin filaments ("center-beading") or doubled and flanking thin filaments ("straddle-beading"). These express two different projections from the crossbridge-actin complexes as seen end-on in flared-X formations. Tannic acid/glutaraldehyde fixation gave improved actin preservation, showing, in 15 nm cross-sections, the long-pitch helical strands as "two-dot" profiles of consistent azimuth in the gaps between double chevrons. The azimuth in the flared-X arms was then inferred from lattice relationships, since it was not seen directly. The tangential attachment of comma-shaped crossbridges to the inferred actin dyad fits the binding geometry in recent actin-subfragment 1 complex reconstructions. However, averaged crossbridge structure differs between lead and rear members of double chevrons, unlike the uniform heads on decorated actin. In filtered images of myac layers, the lead bridges are dense and steeply angled; the rear chevron is seen as a dense bead over the thin filament with faint, less angled bars extending laterally. Actin layer images also suggest that rear and lead bridges differ in angle. Left and right flared-X arms are end-on views of lead and rear chevron bridges, respectively, and differ in shape. Improved fixation with tannic acid/glutaraldehyde allows us to distinguish three crossbridge domains in flared-X arms: (1) a dense bulb-like head merged into the thin filament; (2) a dense but thinner neck tangential to actin; and (3) a faint thin stem joining the necks to myosin filaments. Shape differences in lead and rear members between the head-neck-actin complexes are indicated by the names "L sigmoid" and "R dogleg". Within crossbridges, internal angles between the head-neck axis and the head-actin-head axis differ between sigmoid and dogleg by about 30 degrees, implying a flexible junction between bridge-head and bridge-neck. Lead and rear bridges are axially at least 13 nm apart on actin; the expected 60 degrees difference in azimuth is expressed by head-neck portions, but the head-actin-head axis rotates by only 30 degrees.(ABSTRACT TRUNCATED AT 400 WORDS)     

Actin-plasma membrane associations in mouse eggs and oocytes

Using rhodamine-phalloidin stained preparations and extracted specimens labeled with heavy meromyosin or run on polyacrylamide gels, actin-plasma membrane associations in mouse mature eggs at the second metaphase of meiosis and oocytes at meiotic prophase have been examined. Cortices of extracted oocytes possessed numerous actin filaments that emanated from the plasma membrane delimiting regions between microvilli and from microvillar apices. The membrane anchorage sites of actin filaments were marked by an electron dense material on the inner leaflet of the plasma membrane. The free ends of filaments emanating from the plasma membrane of oocytes intermeshed to form a dense, cortical layer. With meiotic maturation, changes in the organization of cortical actin were first noted approximately 3 hr after the chromosomes had become localized at the oocyte's periphery. Fewer and shorter actin filaments, which did not form a well-defined layer as in oocytes, were connected with electron-dense material to the inner leaflet of the plasma membrane of extracted egg cortices in regions other than that associated with the meiotic spindle. Cortical actin adjacent to the meiotic spindle, however, was organized into a dense, cresentic aggregation in which clusters of filaments emanated from electron-dense regions associated with both the inner and outer leaflets of the plasma membrane. These observations indicate that mouse oocyte maturation not only involves changes in the distribution of cortical actin but also local alterations in the association of actin with the plasma membrane.     

Comparison of the three-dimensional organization of unextracted and Triton-extracted human neutrophilic polymorphonuclear leukocytes

The three-dimensional organization of the cytoplasm of randomly migrating neutrophils was studied by stereo high-voltage electron microscopy. Examination of whole-mount preparations reveals with unusual clarity the structure of the cytoplasmic ground substance and cytoskeletal organization; similar clarity is not observed in conventional sections. An extensive three-dimensional network of fine filaments (microtrabeculae) approximately 7 to 17 nm in diameter extends throughout the cytoplasm and between the two cell cortices; it also comprises the membrane ruffles and filopodia. The granules are dispersed within the lattice and are surrounded by microtrabeculae. The lattice appears to include dense foci from which the microtrabeculae emerge. Triton X-100 dissolves the plasma membrane, most of the granules, and many of the microtrabecular strands and leaves as a more stable structure a cytoskeletal network composed of various filaments and microtubules. Heavy meromyosin-subfragment 1 (S1) decoration discloses actin filaments as the major filamentous component present in membrane ruffles and filopodia. Actin filaments, extending from the leading edge of the cells, are of uniform polarity, with arrowheads pointing towards the cell body. Likewise, the filaments forming the core of filopodia have the barbed end distal. End-to-side associations of actin filaments as well as fine filaments (2--3 nm) which are not decorated with S1 and link actin filaments are observed. The ventral cell cortex includes numerous substrate-associated dense foci with actin filaments radiating from the dense center. Virtually all the microtubules extend from the centrosome. An average of 35 +/- 7 microtubules originate near the pair of centrioles and radiate towards the cell periphery; microtubule fragments are rare. Intermediate filaments form an open network of single filaments in the perinuclear space. Comparison of Triton-extracted and unextracted cells suggest that many of the filamentous strands seen in unextracted cells have as a core a stable actin filament.     

An ultrastructural study of the microfilaments in rat brain by means of E-PTA staining and heavy meromyosin labeling

To identify structures involved in the translocation of the synaptic vesicles towards the presynaptic membrane, an ultrastructural study has been undertaken by means of (1) the E-PTA stain and (2) the HMM-labeling procedure. Using serial sections of E-PTA stained nervous tissue, especially those made in transversal and tangential planes, the geometric order of the presynaptic grid and of its constituents has been described in detail. It consisted of dense projections having the shape of small truncated pyramids cut parallel to their hexagonal bases which rested on the electron-lucent presynaptic membrane. The dense projections were arranged at the points of equilateral triangles. Around each dense projection, six asymmetric hexagonal holes were seen to be arrayed in an hexagonal pattern, forming thus the presynaptic sieve. From the spiny tops of the dense projections, which appeared as specialized structures of the dense material coating the inner surface of the plasma membrane at the level of the synaptic cleft, fine filaments, 40--60 A in diameter, radiated and formed a three-dimensional meshwork pervading the presynaptic bag. The dense cytoplasmic coating delineating the plasma membrane served as anchor points for these microfilaments. Upon incubation with rabbit skeletal muscle HMM the microfilaments underwent specific structural changes, consisting of: (1) a striking increase in diameter; (2) the association of periodic and polarized substructures with their surfaces. The synaptic vesicles and mitochondria were seen to be attached to the numerous HMM-decorated filaments or enmeshed in the network formed by these filaments. The actin-like filaments were anchored to the plasma membrane at many points and to the presynaptic dense projections. Following incubation in the buffer alone or in buffer HMM solutions containing Na+ pyrophosphate or ATP, no arrowheaded structures were observed. Thus, a network consisting of actin-like filaments was demonstrated in the presynaptic bag. Of particular interest was the structural relation of the actin-like filaments with the occasional, tapered myosin-like filaments. The role of the presynaptic actin-like network in the transport of synaptic vesicles towards the presynaptic membrane by a mechanism of chemomechanical transduction is discussed. In the postsynaptic dendrite or dendritic spine, a filamentous network was observed to be attached to the subsynaptic web by means of the E-PTA stain and of the HMM-labeling procedure. The occurrence of an actin-like meshwork in the postsynaptic region is suggested to produce changes in the macromolecular configuration of the postsynaptic membrane by a "mechanoenzyme" system similar to that described in the mitochondrial membrane.     

Aktomyosinartige Filamente im Teilungsmakronukleus des Ciliaten Ichthyophthirius Multifiliis

Just before nuclear division, the chromosomal elements within the large, highly polyploid macronucleus of I. multifiliis carry out rotational movements. Electron micrographs of cells fixed during the rotational movements show islets filled with microfilaments in various states of aggregation. Both thick (80–200 Å) and thin (30–80 Å) filaments occur, either as a highly dense network or as straight, in part parallel, filaments embedded in a filamentous network of lower density. Other islets of the macronucleus contain large and dense aggregates of filaments, sometimes with globular particles measuring 50–60 Å arranged along the thick filaments, occasionally forming cross-bridges with the thinner ones. — After incubation of the cells before fixation in a contractionsolution containing 0.002 M ATP, all nuclear islets show a nearly uniform appear ance of filamentous aggregates: numerous long and thick filaments are arranged in parallel with thin filaments with which they are in some parts connected by bridges. The probable myosinoid and actinoid nature of thick and thin filaments is discussed. It is suggested that the pre-divisional intranuclear rotational movement is a mechanism to avoid aneuploidy by producing a random arrangement of replicated hereditary units prior to division.     

A filamentous cytoskeleton in vertebrate smooth muscle fibers.

There are three classes of myofilaments in vertebrate smooth muscle fibers. The thin filaments correspond to actin and the thick filaments are identified with myosin. The third class of myofilaments (100 A diam) is distinguished from both the actin and the myosin on the basis of fine structure, solubility, and pattern of localization in the muscle fibers. Direct structural evidence is presented to show that the 100A filament constitute an integrated filamentous network with the dense bodies in the sarcoplasm, and that they are not connected to either the actin or myosin filaments. Examination of (a) isolated dense bodies, (b) series of consecutive sections through the dense bodies, and (c) redistributed dense bodies in stretched muscle fibers supports this conclusion. It follows that the 100-A filaments complexes constitute a structrally distinct filamentous network. Analysis of polyacrylamide gels after electrophoresis of cell fractions that are enriched with respect to the 100-A filaments shows the presence of a new muscle protein with a molecular weight of 55,000. This protein can form filamentous segments that closely resemble in structure the native, isolated 100-A filaments. The results indicate that the filamentous network has a structure and composition that distinguish it from the actin and myosin in vertebrate smooth muscle.     

Two kinds of thick filament in smooth muscle cells in the adductor of a clam, Chlamys nobilis

The ultrastructure of the opaque portion of the adductor muscle in the pecten Chlamys nobilis was investigated. The opaque portion was composed of smooth muscle cells that contained thin and thick filaments. The thick filaments were classified into two kinds, thinner and thicker, according to the statistical analysis of diameters. They were also classified as being shorter and longer, when isolated native filaments were examined. The thick filaments may consequently be classified into two kinds: thinner and shorter filaments, and thicker and longer ones. The thinner and shorter filaments were about 26.5 nm in diameter and 7.5 mum in length, and the thicker and longer ones were about 42.0 nm in diameter and 13.0 mum in length, respectively. A regular periodicity was apparent on the surface of the core after removal of myosin molecules from its surface. The periodicity seemed similar for the two kinds of thick filament.     

Polarity of myofilaments in molluscan smooth muscle

Summary Myofilaments were isolated by gently homogenizing smooth muscle cells isolated from the pedal retractor muscle (PRM) of Mytilus edulis, and observed by electron microscopy. The thick filaments isolated in the presence of ATP (10–20 mM) had projections of myosin heads except near their centre (central bare zone). After extraction of myosin, the paramyosin core of the thick filaments showed a Bear-Selby net or a striated pattern with a main periodicity of 14.5 nm. Both the Bear-Selby net and the striated patterns had a polarity that reversed at the centre of the filament where the patterns were obscured. The thin filaments were attached to dense bodies. Decoration of the thin filaments with heavy meromyosin showed that they have opposite polarity on opposing sides of the dense body. The results indicate that the thick filaments are bipolar and also that the dense bodies are functionally analogous to the Z-disk of the striated muscle.