Fine structure of the A-band in cryo-sections. The structure of the A-band of human skeletal muscle fibres from ultra-thin cryo-sections negatively stained

Binding and location of AMP deaminase in rabbit psoas muscle myofibrils

It is shown that an interaction exists between AMP deaminase (EC 3.5.4.6) and myofibrils that is sufficiently strong (Kd congruent to 10(-10) M) for more than 99% of the binding sites for the enzyme to be filled in vivo. The binding is not strong enough, however, to stop removal of the enzyme during the extensive washing normally used in the preparation of myofibrils. Fluorescent antibodies to the enzyme label myofibrils close to the junction of the A- and I-bands. The invariance of the position of the antibody stripes at this site, over a range of sarcomere lengths, indicates that the enzyme is attached to the A-band. The intensity of the fluorescence declines in parallel with dissociation of the enzyme. In this muscle, the number of AMP deaminase binding sites per thick filament is approximately six, suggesting that the enzyme is located at a single axial position in each half A-band. Electron microscopy of negatively stained, antibody-labelled myofibrils reveals the distance between the AMP deaminase sites at opposite ends of an A-band to be 1.69(+/- 0.02 micron). Since the length of the A-band is 1.57 micron, the binding site for the enzyme must be significantly beyond where thick filaments have previously been thought to end.

Localization of C-protein isoforms in chicken skeletal muscle: ultrastructural detection using monoclonal antibodies

Monoclonal antibodies (McAbs) specific for the fast (MF-1) and slow (ALD-66) isoforms of C-protein from chicken skeletal muscle have been produced and characterized. Using these antibodies it was possible to demonstrate that skeletal muscles of varying fiber type express different isoforms of this protein and that in the posterior latissimus dorsi muscle both isoforms are co-expressed in the same myofiber (17, 18). Since we had shown that both isoforms were present in all sarcomeres, it was feasible to test whether the two isoforms co- distributed in the same 43-nm repeat within the A-band, thereby establishing a minimum number of C-proteins per repeat in the thick filaments. Here we describe the ultrastructural localization of C- protein in myofibers from three muscle types of the chicken using these same McAbs. We observed that although C-protein was present in a 43-nm repeat along the filaments in all three muscles, there were marked differences in the absolute number and position occupied by the different isoforms. Since McAbs MF-1 and ALD-66 decorated the same 43- nm repeats in the A-bands of the posterior latissimus dorsal muscle, we suggest that at least two C-proteins can co-localize at binding sites 43 nm apart along thick filaments of this muscle.

Three-dimensional reconstruction from tilted sections of fish muscle M-band

Fish muscle provides a particularly suitable specimen for studying the organization of thick filaments in vertebrate striated muscle because the filaments are arranged with a single orientation on a well ordered hexagonal lattice. The M-band consists of sets of cross-links, which join the myosin filaments in the middle of the A-band. Previous work concluded that the fish muscle M-band had the local symmetry of the dihedral point group 32. We present here a quantitative analysis of transverse sections of the M-band, using general methods developed for crystalline layers, to combine various tilted views. The three-dimensional map computed to a resolution of about 70 A confirms previous findings; it shows new features of the thick filament structure in the M-band region and provides new information on the use of three-dimensional reconstruction from sectioned biological material. The accompanying paper describes a novel technique of three-dimensional reconstruction of the fish M-band from a single view of a slightly oblique plastic section.

A new 185,000-dalton skeletal muscle protein detected by monoclonal antibodies

The M line, which transverses the center of the thick filament region of skeletal muscle sarcomeres, appears to be a complex array of multiple structural elements. To date, two proteins have definitely been shown to be associated with the M line. They are MM-CK, localized in the M 4,4' substriations, and a 165,000-dalton (164 kd) protein, referred to as both M-protein and myomesin. Here we report the positive identification of a third M-line protein of 185 kd. In the course of making monoclonal antibodies (mAbs) against a 165-kd fraction, we also obtained mAbs that bound to the M line of isolated myofibrils as detected by indirect immunofluorescence, but recognized a protein band of 185 kd in immunoblotting experiments with either the original immunogen or low ionic strength myofibril extracts as antigenic targets. The evidence that the 185- and 165-kd proteins are distinct protein species is based on the separation of the two proteins into discrete peaks by ion exchange chromatography, the distinctive patterns of their degradation products, and non-cross-reactivity of any of seven mAbs. These mAbs recognize three unique antigenic determinants on the 185-kd molecule and at least two and probably four sites on the 165-kd molecule as determined from competitive binding and immunofluorescence experiments. To resolve the problem of multiple nomenclature for the 165-kd protein, the 185-kd protein will be referred to as myomesin and the 165-kd protein as M-protein.

Differential localization of two myosins within nematode thick filaments

The body wall muscle cells of the nematode, Caenorhabditis elegans, contain two unique types of myosin heavy chain, A and B. We have utilized an immunochemical approach to define the structural location of these two myosins within body wall muscle thick filaments. By immunofluorescence microscopy, myosin B antibodies label the thick filament-containing A-bands of body wall muscle with the exception of a thin gap at the center of each A-band, and myosin A antibodies react to form a medial fluorescent stripe within each A-band. The complexes of these monoclonal antibodies with isolated thick filaments were negatively stained and studied by electron microscopy. The myosin B antibody reacts with the polar regions of all filaments but does not react with a central 0.9 micron zone. The myosin A antibody reacts with a central 1.8 micron zone in all filaments but does not react with the polar regions.

Novel staining pattern of skeletal muscle M-lines upon incubation with antibodies against MM-creatine kinase

Incubation of chicken skeletal muscle fibers with an excess of anti-M-creatine kinase (CK) immunoglobulin G and an excess of anti-M-CK Fab fragments leads to heavy decoration of the M-line (Wallimann, T., D.C. Turner, and H.M. Eppenberger, 1977, J. Cell Biol. 75:297-317) and to removal of the electron-dense M-line structure (Walliman, T., G. W. Pelloni, D.C. Turner, and H.M. Eppenberger, 1978, Proc. Natl. Acad. Sci. USA., 75:4296-4300), respectively. On the other hand, incubation with low concentrations of monovalent anti-M-CK Fab did not extract but rather decorated the M-line, giving rise to a distinct two-line staining pattern. A similar double-line staining pattern, although less pronounced, was also observed within the M-line of paraformaldehyde-prefixed myogenic cells, which after permeabilization were incubated with low concentrations of divalent anti-M-CK antibody. In both cases, the two decorated lines appearing in the middle of the A-band were spaced axially 42-44 nm apart and correspond most likely to the two M4 and M4' m-bridge rows described by Sjöström and Squire (1977, J. Mol. Biol., 109:49-68; 1977, J. Microscopy., 111:239-278). It is concluded that the muscle-specific form of creatine kinase, MM-CK, contributes mainly to the electron density of these M4 and M4' m-bridges within the M-line structure. This specific labeling pattern is a further demonstration that CK is an integral part of the M-line.

Ultrastructural localization of M-band proteins in chicken breast muscle as revealed by combined immunocytochemistry and ultramicrotomy

Cryo-ultramicrotomy and "conventional" plastic sectioning have been used in combination with extraction and immunolabeling techniques to determine the location of the two M-band proteins characterized to date, MM-creatine kinase (MM-CK: Mr, 80,000) and M-protein "myomesin" (Mr, 165,000) within the M-region of chicken pectoralis muscle. The following main results were obtained. (1) The M-band in chicken pectoralis muscle contains five major striations (M1, M4 and M4', M6 and M6' in the terminology of Sjöström & Squire, 1977a). (2) Extraction of the bulk of the electron-dense M-band with low ionic strength removes the M-striations M1, M4 and M4' while M6 and M6' are retained. Cross-sections through the M-region of such muscles lack primary M-bridges connecting the thick myosin filaments. (3) Labeling with antibodies against MM-CK enhances the M-striations M4 and M4'; sometimes the whole region between M4 and M4' is labeled. (4) Incubation with antibodies against myomesin results in the labeling of the whole M-band from M6 to M6'; no label is found in the rest of the bare zone outside M6 and M6'. (5) Incubation of low ionic strength extracted muscle fibers with antibodies against myomesin leads to an "incomplete" labeling of the M-band between M6 and M6'; lines M6 and M6' are sometimes seen to be enhanced presumably due to antibody labeling. From these results it is concluded that MM-CK is the major protein of the M4 and M4' (and possibly also of the M1) M-bridges. Myomesin is bound within the M-band along the thick filaments from M6 to M6'. Two hypothetical models for the possible location of myomesin are discussed. According to these models myomesin would either make up the M-filaments or be directly attached to and along the central bare zone of thick myosin filaments.

Co-ordinated electron microscopy and X-ray studies of glycerinated insect flight muscle. I. X-ray diffraction monitoring during preparation for electron microscopy of muscle fibres fixed in rigor, in ATP and in AMPPNP

Synchrotron radiation was used for low-angle X-ray diffraction to monitor structural changes produced in insect flight muscle during fixation, dehydration and embedding for electron microscopy of thin sections. Fibre bundles were fixed by cold glutaraldehyde in one of three states, namely rigor, ATP or AMPPNP, followed by additional cross-linking treatment. No heavy metals were used before embedding. During fixation-embedding, all specimens lost the continuous actin layer lines of spacing 11-5 nm, shrank 18-21% in lattice spacing, shrank 0.5-2.5% in axial spacings and showed equatorial intensity changes which were similar for all three states, while the well-sampled inner layer lines (39-13 nm) were preserved with different fidelity in each state, highest for rigor and lowest for ATP. In different AMPPNP bundles, these layer lines indicated different degrees of unexplained shift (from slight to total) towards the structure of muscle fixed in ATP. Fixation in ATP caused obvious gain of intensity on 39, 19 and 13 nm layer lines, which can be interpreted as trapping of myosin crossbridge attachments to actin; this artifact was unchanged by seven variations in fixation conditions. Fixation in rigor gave no indication of crossbridge detachment nor of the presence or alteration of any significant population of non-bridging myosin heads. X-ray monitoring allowed selection of best-preserved samples for subsequent electron microscopy. The rapid pattern-recording possible with synchrotron X-ray intensity allowed us to complete and compare experiments with many fibre bundles from a single glycerinated Lethocerus muscle.

Native bare zone assemblage nucleates myosin filament assembly

Native myosin filaments from rabbit psoas muscle are always 1.5 micrometer long. The regulated assembly of these filaments is generally considered to occur by an initial antiparallel and subsequent parallel aggregation of identical myosin subunits. In this schema myosin filament length is controlled by either a self-assembly or a Vernier process. We present evidence which refines these ideas. Namely, that the intact myosin bare zone assemblage nucleates myosin filament assembly. This suggestion is based on the following experimental evidence. (1) A native bare zone assemblage about 0.3 micrometer long can be formed by dialysis of native myosin filaments to either a pH 8 or a 0.2 M-KCl solution. (2) Upon dialysis back to 0.1 M-KCl, bare zone assemblages and distal myosin molecules recombine to form 1.5 micrometer long bipolar filaments. (3) The bare zone assemblage can be separated from the distal myosin molecules by column chromatography in 0.2 M-KCl. Upon dialysis of the fractionated subsets back to 0.1 M-KCl, the bare zone assemblage retains its length of about 0.3 micrometer. However, the distal molecules reassemble to form filaments about 5 micrometers long. (4) Filaments are formed from mixes of the isolated subsets. The lengths of these filaments vary with the amount of distal myosin present. (5) When native filaments, isolated bare zone assemblages or distal myosin molecules are moved sequentially to 0.6 M-KCl and then to 0.1 M-KCl, the final filament lengths are all about 5 micrometers. The capacity of the bare zone assemblage to nucleate filament assembly may be due to the bare zone myosin molecules, the associated M band components or both.

Periodic charge distributions in the myosin rod amino acid sequence match cross-bridge spacings in muscle

The amino acid sequence of the rod portion of nematode myosin, deduced for the sequence of the unc-54 heavy chain gene of Caenorhabditis elegans, is highly repetitive and has the characteristics of an alpha-helical coiled coil. The molecular surface contains alternate clusters of positive and negative charge. Interactions between charge clusters on adjacent molecules could account for the observed spacing of the myosin cross-bridges in muscle. Calculations also suggest that the N-terminal third of the rod is only loosely associated with the thick filament backbone. Bending of the rod near the end of this region could allow the N-terminal section to act as a hinged arm during muscle contraction.

Morphometric analyses of human muscle fiber types

Fibers from the m. vastus lateralis of 10 middle-aged men were classified at ultrastructural level according to the appearance of the sarcomeric M-band. The Z-band widths had a two-peak distribution. One peak was due to type 1 fibers (mean 125 +/- 11 nm), the other to type 2 fibers. This latter could be separated into type 2A (101 +/- 9 nm) and type 2B (86 +/- 8 nm). About 83% of the fibers would have been correctly classified on the basis of the Z-band width alone. Mitochondrial volumes differed (type 1 5.6 +/- 0.8, 2A 4.0 +/- 0.8, and 2B 2.8 +/- 0.8%). However, only one third (37%) of the fibers would have been correctly classified if sorted according to this parameter. Mitochondrial volumes in the different fibers were correlated to mitochondrial enzymes, while fiber sizes and numbers were correlated to cytoplasmic variables. The correlations appeared mainly after a training program, suggesting that the relationships between structural and functional parameters are more obvious after adaptation to higher functional demands.

Muscle structure, cryo-methods and image analysis

Negatively stained cryo-sections from glutaraldehyde fixed, anti-freeze treated muscle, quench-frozen in Freon cooled by liquid nitrogen, show improved preservation of axial structure of the myofibrils compared with conventional plastic sections. Such sections are being used both to characterize the structural differences inthe M-bands of different vertebrate muscles and fibre types and also to define the axial distribution of myosin crossbridges and non-myosin proteins in the crossbridge region of the A-band. Combined with analysis of the transverse A-band structure from plastic sections, the cryo-sections are helping to reconstruct a three-dimensional picture of the molecular architecture of the A-band. This, in turn, is providing the necessary structural background with which to interpret the wealth of published X-ray diffraction data on muscle. Such data should reveal the nature of the contractile event itself. Since good X-ray diffraction patterns can be obtained from living muscles, these can be compared with optical diffraction patterns from muscle cryo-sections as a means of testing the degree of preservation in the sections. Muscle is therefore an excellent tissue with which to evaluate new cryo-techniques.

Fine structural details of human muscle fibres after fibre type specific glycogen depletion

Type 1 and Type 2 fibres of skeletal muscle (human m. vastus lateralis), selectively depleted of glycogen by sustained submaximal muscular exercise (running 30 km), were identified at light and electron microscopical level by examination of thin and ultra-thin serial sections treated particularly for visualization of glycogen. Averaged images, obtained by lateral smearing of depleted fibres (Type 1) exhibited five clearly visible cross-bridges in the M-band and had broad Z-bands. Non-depleted fibres (Type 2) showed either three central strong and two weak outer lines in the M-band and intermediate Z-bands (Type 2A), or only three central strong lines in the M-band and narrow Z-bands (Type 2B). The depleted fibres had no subsarcolemmal accumulation of glycogen particles and practically no intermyofibrillar particles. The remaining particles were small in size and seemed almost rudimentary. In non-exercised individuals, a peculiar distribution of individual glycogen particles in the I-band and A-band was found. This distribution was accounted by the structural arrangement of the myofibrillar material.

Fluorescence microscope study of the binding of added C protein to skeletal muscle myofibrils

The binding of extra C protein to rabbit skeletal muscle myofibrils has been investigated by fluorescence microscopy with fluorescein-labeled C protein or unmodified C protein plus fluorescein-labeled anti-C protein. Added C protein binds strongly to the I bands, which is consistent with its binding to F actin in solution (Moos, C., C. M. Mason, J. M. Besterman, I. M. Feng, and J. H. Dubin. 1978. J. Mol. Biol. 124:571-586). Of particular interest, the binding to the I band is calcium regulated: it requires a free calcium ion concentration comparable to that which activates the myofibrillar ATPase. This increases the likelihood that C protein-actin interaction might be physiologically significant. When I band binding is suppressed, binding in the A band becomes evident. It appears to occur particularly near the M line, and possibly at the edges of the A band as well, suggesting that those parts of the thick filaments that lack C protein in vivo may nevertheless be capable of binding added C protein.

The Mr 165,000 M-protein myomesin: a specific protein of cross-striated muscle cells

The tissue specificity of chicken 165,000 M-protein, tentatively names "myomesin", a tightly bound component of the M-line region of adult skeletal and heart myofibrils, was investigated by immunological techniques. Besides skeletal and heart muscle, only thymus (known to contain myogenic cells) was found to contain myomesin. No myomesin could however, be detected in smooth muscle or any other tissue tested. This result was confirmed in vitro on several cultured embryonic cell types. Only skeletal and heart muscle cells, but not smooth muscle or fibroblast cells, showed the presence of myomesin. When the occurrence and the distribution of myomesin during differentiation of breast muscle cells in culture were studied by the indirect immunofluorescence technique, this protein was first detected in postmitotic, nonproliferating myoblasts in a regular pattern of fluorescent cross-striations. In electron micrographs of sections through young myotubes, it could be shown to be present within the forming H-zones of nascent myofibrils. In large myotubes the typical striation pattern in the M-line region of the myofibrils was observed. Synthesis of myomesin measured by incorporation of [35S]methionine into immunoprecipitable protein of differentiating cells increased sharply after approximately 48 h in culture, i.e., at the time when the major myofibrillar proteins are accumulated. No significant amounts of myomesin were, however, found in cells prevented from undergoing normal myogenesis by 5'-bromodeoxyuridine. The results indicate that myomesin (a) is a myofibrillar protein specific for cross-striated muscle, (b) represents a highly specific marker for cross-striated muscle cell differentiation and (c) might play an important role in myofibril assembly and/or maintenance.

Changes of thick filament structure during contraction of frog striated muscle

The strongest myosin-related features in the low-angle axial x-ray diffraction pattern of resting frog sartorius muscle are the meridional reflections corresponding to axial spacings of 21.4 and 14.3 nm, and the first layer line, at a spacing 42.9 nm. During tetanus the intensities of the first layer line and the 21.4-nm meridional decrease by 62 and 80% respectively, but, when the muscle is fresh, the 14.3-nm meridional intensity rises by 13%, although it shows a decrease when the muscle is fatigued. The large change in the intensity of the 21.4-nm meridional reflection suggests that the projected myosin cross-bridge density onto the thick filament axis changes during contraction. The model proposed by Bennett (Ph.D. Thesis, University of London, 1977) in which successive cross-bridge levels are at 0,3/8, and 5/8 of the 42.9-nm axial repeat in the resting muscle, passing to 0, 1/3, and 2/3 in the contracting state, can explain why the 21.4-nm reflection decreases in intensity while the 14.3-nm increases when the muscle is activated. The model predicts a rather larger increase of the 14.3-nm reflection intensity during contraction than that observed, but the discrepancy may be removed if a small change of shape or tilt of the cross-bridges relative to the thick filament axis is introduced. The decrease of the intensity of the first layer line indicates that the cross-bridges become disordered in the plane perpendicular to the filament axis.

The structure of segments of the anisotropic band of muscle. II. Preparation and properties of A segments from vertebrate skeletal muscle

The anisotropic band of skeletal muscle is a complex structural assembly of the protein myosin and associated nonmyosin components. To study the relationships among these proteins, aggregates of thick myofilaments held together at the M line (A segments) have been prepared from fresh and glycerol extracted chicken pectoralis and rabbit psoas muscles and from fresh frog sartorius muscle. The structure of the A segments included several thick filaments, an M line, and a bare zone or pseudo-H zone, lateral to the M line. Most of the A segments exhibited a pattern of eleven periodic stripes in each half lateral to the bare zone. The A segments from fresh muscle displayed these stripes more consistently than did the A segments from glycerinated muscle. Some of the major stripes appeared to be double, and there were two subdivisions between the stripes nearest the bare zone. The more lateral of the major A band stripes, however, had one subdivision between them. The M line consisted of three prominent medial stripes and two fainter lateral stripes. In the M lines of rabbit A segments the lateral stripes were located well into the bare zone whereas the lateral stripes of M lines in chicken A segments were closer to the three medial M line stripes. Our results on the preparation and properties of A segments are compared with those of of the investigators.

Effect of C-protein on actomyosin ATPase

The effect of C-protein on the actin-activated ATPase of column-purified skeletal muscle myosin has been investigated at varied ionic strength. At ionic strengths below about 0.1, C-protein is a potent inhibitor. The inhibition is not reversed by increasing the actin concentration, showing that it is caused by C-protein bound to the myosin filaments. When the ionic strength is raised above about 0.12, on the other hand, the inhibition vanishes and C-protein becomes a mild activator of the actomyosin ATPase. Both effects appear rapidly upon addition of C-protein to pre-formed myosin filaments, so C-protein probably acts by binding to the surface of the filaments.

Biochemical and ultrastructural aspects of Mr 165,000 M-protein in cross-striated chicken muscle

To better understand the relationship between the Mr 165,000 M-line protein (M-protein) and H-zone structure in skeletal and in cardiac muscle, as well as the possible interaction of M-protein with another skeletal muscle M-line component, the homodimeric creatine kinase isoenzyme composed of two M subunits (MM-CK), we performed biochemical, immunological, and ultrastructural studies on myofibrils extracted by different procedures. In contrast to MM-CK, M-protein could not be completely removed from myofibrils by low ionic strength extraction. Fab-fragments of antibodies against M-protein could not release M-protein quantitatively from either breast or heart myofibrils but remained bound to the myofibrillar structure, whereas monovalent antibodies against MM-CK cause the specific release of MM-CK and the concomitant disappearance of the M-line from chicken skeletal muscle myofibrils. When MM-CK was removed from skeletal myofibrils by low ionic strength extraction or, more specifically, by incubation with anti-MM-CK Fab, M-protein was still not released quantitatively upon treatment with anti-M-protein Fab as judged from immunofluorescence data. In the ultrastructural investigation of low ionic strength extracted muscle fibers, M protein could be localized in two stripes on both sides of the former M-line, suggesting a reduced attachment to the residual H-zone structure, whereas the specific removal of MM-CK resulted in the same dense staining pattern for M-protein within the M-line as observed in untreated fibers. However, the binding of M-protein to the residual M-line structure seemed to be reduced, as a considerable amount of this protein could be identified in the supernate of sequentially incubated myofibrils. The results indicate a strong binding of M-protein within the H-zone structure of skeletal as well as heart myofibrils.

High-voltage electron microscopy of crossbridge interactions in striated muscle

In order to investigate the geometry of the interactions which myosin molecules make with actin filaments we have studied thick (0.2--0.5 micrometer) transverse sections of striated muscles in the 1 Me V electron microscope at Imperial College. Sections obtained from fixed relaxed frog sartorius muscle and both fixed relaxed and fixed rigor insect flight muscles, show regular electron opaque features between the thick and thin filament profiles. These are thought to be the overlapping images of the many levels of myosin heads that occur in such sections. From the appearances of these images, together with studies of thin transverse sections, it appears that of the possible interactions which one myosin molecule can make, namely that its two component heads interact with the same thin filament or with two different thin filaments, it is the former interaction (both heads on the same filament) which is predominant. Nevertheless appearances have been seen similar to those expected if an interaction of one molecule with two thin filaments occurs. It is concluded that both single filament and two filament interactions can occur depending on the steric convenience of the available actin subunits, but that the single filament interaction occurs in the majority of cases in the muscle states we have studied. Finally it is shown that the myosin filament profiles seen in thick transverse sections may be a very misleading guide to thick filament structure because of the influence which the myosin crossbridge have on the appearance of the profiles.

Immunochemistry on ultrathin frozen sections

Ultrathin frozen sections can be cut smoothly from many fixed and appropriately treated specimens. To use such sections for immunochemical localization of intracellular antigens, fixation conditions must be selected to optimize at least three variables, namely, preservation of ultrastructure, preservation of antigenicity and retention of accessibility of the antigen to the antibody. Furthermore, staining of the sections must be such that both the immunolabels and structures are clearly recognized. Our efforts to attain these goals are described in relation to their historical background. Although there are still problems to be solved and improvements to be made, we now consider that cryoultramicrotomy has reached the stage of being useful in studying many questions which will not be easily approached otherwise.

The structure of A segments from glycerinated skeletal muscle

The A band of skeletal muscle consists of an array of thick myosin-containing filaments along with non-myosin proteins such as C protein and M line protein. In order to study the arrangement of the myosin and non-myosin components, A segments which are aggregations of thick filaments held together at the M line were prepared from glycerinated chicken pectoral and rabbit psoas muscles and examined by electron microscopy. Details of the preparative technique and comparison of the morphologies of A segments and I segments are provided. The A segments from chicken pectoral muscle exhibited 11 to 12 stripes in each half lateral to the bare zone. Several less distinct bands as well as subdivisions of the individual stripes were also observed. The periodicity of the major stripes in the A segments was 424 +/- 10 A. The A segments prepared from rabbit psoas muscle had a periodicity of 432 +/- 13 A, but in contrast with chicken A segments, fewer rabbit A segments showed this periodicity. We conclude that A segments can be separated from glycerinated chicken and rabbit skeletal muscles and compare our results with those of others who prepared A segments from frog and rabbit skeletal muscles in the absence of glycerol.

Cryo-ultramicrotomy and myofibrillar fine structure: a review

In the past, the techniques of electron microscopy and X-ray diffraction have both been very informative about the ultrastructure of the muscle myofibril But X-ray diffraction patterns are difficult to interpret unambiguously and until now specimen preservation in plastic embedded muscle has been sufficiently poor to make it difficult to use electron micrographs of muscle as a means of interpreting the available X-ray diffraction evidence. The possibility of using ultrathin sections of frozen muscle, in which the disruptive steps of chemical dehydration and plastic embedding can be avoided, promises to help to bridge the information gap between present X-ray and electron microscope results. For this reason we here review the application of the cryosectioning technique to muscle, we assess the technique in terms of the improvements in preservation which have so far been obtained and which might be expected and we discuss some of the many potential advantages and uses of this technique for studies of muscle ultrastructure and function. It is concluded that this technique should be developed vigorously since it promises to play a very important role in muscle research in the future.