Actin filament organization and myosin head labelling patterns in vertebrate skeletal muscles in the rigor and weak binding states

The structures of vertebrate skeletal muscles (particularly from frog and fish) in the rigor state are analysed in terms of the concept of target areas on actin filaments. Assuming that 100% of the heads are to be attached to actin in rigor, then satisfactory qualitative low-resolution modelling of observed X-ray diffraction data is obtained if the outer ends of these myosin heads can move axially (total range about 200A) and azimuthally (total range less than 60 degrees) from their original lattice sites on the myosin filament surface to attach in defined target areas on the actin filaments. On this basis, each actin target area comprises about four actin monomers along one of the two long-pitched helical strands of the actin filament (about 200 A) or an azimuthal range of actin binding sites of about 100 degrees around the thin filament axis. If myosin heads simply label in a non-specific way the nearest actin monomers to them, as could occur with non-specific transient attachment in a 'weak binding' state, then the predicted X-ray diffraction pattern would comprise layer lines at the same axial spacings (orders of 429 A) as those seen in patterns from resting muscle. It is shown that actin target areas in vertebrate skeletal muscles are probably arranged on an approximate 62 (right-handed) helix of pitch (P) of about 720 A, subunit translation P/6 and near repeat P/2. Troponin position need not be considered in defining the labelling pattern of cross-bridges on this 62 helix of target areas; the target areas appear to be defined solely by the azimuthal position of the actin binding sites. The distribution of actin filament labelling patterns could be regular in fish muscle which has a 'crystalline' A-band, but will be irregular in higher vertebrate muscles such as frog sartorius muscle.

Three fast myosin heavy chains in adult rat skeletal muscle

A new fast myosin heavy chain isoform was electrophoretically detected in adult rat skeletal muscles. It was present at high levels in diaphragm and, therefore, designated as MHCIId. Appreciable amounts of MHCIId were detected in tongue musculature, the extraocular muscles, and in the deep red portions of various fast muscles. Its concentration in fast-twitch muscle was greatly increased by chronic stimulation.

Clonal analysis of vertebrate myogenesis. VIII. Fibroblasts growth factor (FGF)-dependent and FGF-independent muscle colony types during chick wing development

The effect of bovine fibroblast growth factor (FGF) on the in vitro differentiation of various stage-specific populations of skeletal muscle colony-forming (MCF) cells from the developing chick wing bud was examined. The results show that bovine FGF (3 ng/ml daily) delays the onset of differentiation of MCF cells obtained from Day 4-12 wing buds by about 1 day; but, in addition, the results demonstrate that a subset of colony-forming cells derived from stage 23-27 (Day 4-5) embryos require FGF for myogenic differentiation. The FGF-dependent MCF cells attach and grow in the absence of FGF, but do not differentiate unless given FGF within 1-3 days after inoculation. Thus, between stages 23 and 27 the myogenic population contains discrete subclasses that are FGF dependent and others that are FGF independent. Both subclasses are found within two of the previously classified MCF cell populations, the early and late MCF cells. FGF-dependent and independent early MCF cells are present within the wing bud until stage 25, after which only the FGF-independent early MCF subclass persists. Similarly, both FGF-dependent and -independent late MCF cells are present between stages 25 and 27, but only the FGF-independent late MCF subclass remains after stage 31. The mechanisms responsible for relative changes in the proportions of MCF cell subclasses and for the FGF requirements are not understood. In addition, while FGF is required, there is no evidence suggesting that FGF triggers skeletal muscle terminal differentiation within the FGF-dependent MCF cell subclasses.

Platelet cytoskeletal protein distributions in two triton-insoluble fractions and how they are affected by stimulants and reagents that modify cytoskeletal protein interactions

Actin filament formation during early stages of platelet activation by stimulants was analyzed by determining the distributions of cytoskeletal proteins to a low speed centrifuge pellet (TP), ultracentrifuge pellet (UP), and ultracentrifuge supernatant (US) after lysing platelets with 1% Triton X-100. TP contained actin binding protein, myosin, alpha-actinin and actin, and UP contained these cytoskeletal proteins and other proteins. During thrombin activation, total protein and actin in TP increased with time, while they decreased in UP. Aggregating agents (ADP, PMA, thrombin) and, to a small extent, colchicine and nocodazole, microtubule inhibitors, increased cytoskeletal proteins and total protein in TP, while cytochalsin B, an inhibitor of actin polymerization, had the opposite effect. When the amounts of the respective proteins in TP and UP were summed, these values were not affected by agonists and inhibitors, except in the case of thrombin stimulation. These data suggest that the actin in UP is a form intermediate between the actin filaments in TP and actin monomer in the soluble fraction, and there may be a dynamic conversion between these forms upon platelet activation. UP was also characterized by immunostaining with antibodies against fibrinogen, tubulin, and glycoprotein Ib after SDS-PAGE/electrophoretic transfer to nitrocellulose sheets.

2-D analysis of human skeletal muscle myosin light chains with immobilized pH gradients

Myosin light chains (LC) are a low molecular mass fraction non-covalently bound to the heavy chains. They are present in the myosin molecules and exhibit various degrees of polymorphism among the different species. By utilizing a highly-resolving 2-D technique, in narrow immobilized pH gradients, we have compared the LC forms of skeletal muscle in human and rabbit. Our findings: (1) both forms, LC1 and LC3, migrate in the two species with rather similar electrophoretic constants (both in terms of pI and Mr); (2) the LC2 forms of rabbit and humans exhibit the same Mr but quite different pI values, the rabbit forms being more acidic; (3) the chain LC2Sb is resolved into two spots in both rabbit and humans. In the former, the two bands have equal intensity, while in the latter the high pI component is clearly the most abundant.

Adult human masseter muscle fibers express myosin isozymes characteristic of development

Masseter muscle biopsies were obtained from nine patients undergoing orthognatic surgery or surgery for parotid tumors. A detailed enzyme-histochemical and immunocytochemical study of these muscles was performed using antibodies specific to the various isozymes of the myosin heavy chain (MHC) in order to identify the MHC isozymes that were present in the different fiber types. The contractile proteins in these same biopsies were analyzed by two-dimensional gel electrophoresis, native pyrophosphate gel electrophoresis, and by an immunopolypeptide mapping approach. These studies have shown that there is a very heterogeneous distribution of the myosin isozymes, with many fibers containing more than one myosin type. We also present evidence that in addition to adult fast and slow myosin, the human masseter muscle contains two proteins, neonatal MHC and embryonic myosin light chain, that are characteristic of developing muscle.

Changes in the distribution of slow skeletal myosin heavy chain SM1 in developing avian muscle fibres

The use of monoclonal antibodies against fast skeletal and slow skeletal myosin heavy chains (MHC) has shown the presence of significant amounts of slow skeletal type MHC in embryonic skeletal muscles of white leghorn chickens. The presence of this slow skeletal myosin heavy chain (SMHC) was not restricted to presumptive slow muscles only, as it was also observed in presumptive fast skeletal muscles. As was the case for embryonic MHC reactive with the antibody against fast skeletal myosin heavy chain (FMHC), the presence of SMHC could be detected at the earliest stages of myogenesis. It appeared to be present in most muscle cells during early embryonic development. The changes in its cellular distribution during subsequent embryonic and post-hatch period indicated its suppression in a certain proportion of the cells in both presumptive fast and slow skeletal muscles. Its time course of suppression, however, was much prolonged, not synchronized, and varied in fast and slow skeletal muscles during both embryonic and post-hatch development.

Effects of chronic sympathectomy on muscle fibre composition ISO-myosin patterns, protein synthesis and calcium content in canine gracilis muscle

We have previously reported functional and histological studies in five beagle dogs with unilateral lumbar sympathectomy. Three months later, fatiguability in the gracilis muscles was increased on the denervated sides, and this was associated with an increase in the relative distribution of FT fibres. Biochemical studies now show that these changes were associated with an increase in cytosolic protein without change in DNA content; this is consistent with an increase in cell size. There was a reduction in the proportion of slow myosin light chain isoforms from 50 +/- 7 to 34 +/- 6%. Noradrenaline levels were increased on the denervated sides but this may reflect greater vascularity. Calcium content did not correlate with fibre type but there was a positive relation with both noradrenaline content (r = 0.73; P less than 0.05) and DNA content (r = 0.84; P less than 0.05). It is concluded that sympathectomy induces several biochemical changes in skeletal muscle which constitute a change and increase in fast myosin light chain synthesis and a corresponding fibre type transformation.

Developmental changes in myosin isoforms from slow and fast latissimus dorsi muscles in the chicken

In the course of muscle differentiation, changes in fibre-type population and in myosin composition occur. In this work, the expression of native myosin isoforms in developing fast-twitch (posterior latissimus dorsi; PLD) and slow-tonic (anterior latissimus dorsi; ALD) muscles of the chick was examined using electrophoresis under nondissociating conditions. The major isomyosin of 11-day-old embryonic PLD comigrated with the adult fast myosin FM3. Two additional components indistinguishable from adult fast FM2 and FM1 isomyosins appeared successively during the embryonic development. The relative proportion of these latter isoforms increased with age, and the adult pattern was established by the end of the 1st month after hatching. Between day 11 and day 16 of embryonic development, PLD muscle fibres also contained small amounts of slow isomyosins SM1 and SM2. This synthesis of slow isoforms may be related to the presence of slow fibres within the muscle. At all embryonic and posthatch stages, ALD was composed essentially of slow isomyosins that comigrated with the two slow components SM1 and SM2 identified in adult. Several studies have reported that the SM1:SM2 ratio decreases progressively throughout embryonic and posthatching development, SM2 being predominant in the adult. In contrast, we observed a transient increase in SM1:SM2 ratio at the end of embryonic life. This could reflect a transitional neonatal stage in myosin expression. In addition, the presence in trace amounts of fast isomyosins in developing ALD muscle could be related to the presence of a population of fast fibres within this muscle.

Age-related and site-specific adaptation of the arterial endothelial cytoskeleton during atherogenesis

The authors probed the vascular endothelial cell cytoskeleton in strains of pigeons that are atherosclerosis-susceptible and disease-resistant, namely, the White Carneau and Show Racer pigeons. Endothelial cell actin and myosin were localized with the use of affinity-purified antibodies in conjunction with indirect immunofluorescence microscopy. The endothelial cell cytoskeleton was characterized in a site-specific and time-dependent manner by examination of arterial segments from each strain of pigeons. Anti-actin and anti-myosin fluorescence staining patterns of endothelial cells lining the ascending aorta, aortic arch, and thoracic aorta from the White Carneau and Show Racer pigeons sacrificed at 1 and 12 months of age were compared and analyzed. In the Show Racer, irrespective of arterial site or chronologic age, endothelial cell cytoskeletal organization is similar. Actin and myosin fluorescence is brightest at the cortex, where endothelial cells meet their neighbors. There is also an amorphous (diffuse) fluorescence throughout the cytoplasm. In addition to the diffuse and cortical cytoskeletal fluorescence in the endothelial cells of the Show Racers, the White Carneau also possess a unique cytoskeletal array of linear fluorescence, ie, the endothelial cell ridge. At 1 month of age, anti-actin staining of endothelial cell ridges averages 28.5 mu in length in the ascending aorta, 28.0 mu in the aortic arch, and 40.0 mu in the thoracic aorta. At the same time, anti-myosin fluorescence extends past both ends of the anti-actin-stained endothelial cell ridge fluorescence. In the ascending aorta, anti-myosin labeling of endothelial cell ridges is 3.5 times longer than anti-actin staining. This staining is absent in the aortic arch, whereas the thoracic aorta possesses endothelial cell ridges that extend over the entire length of the vessel segment. At 12 months of age, actin-stained endothelial cell ridges increase 1.6- and 1.4-fold in the ascending aorta and aortic arch, respectively. The thoracic aorta possesses endothelial cell ridges that cover its entire length. At 12 months of age, the length of myosin-stained endothelial cell ridges does not increase in the ascending or thoracic aorta. In contrast, the aortic arch expresses endothelial cell ridges that exceed 150 mu in length. It is proposed that the endothelial cell ridge assembles from cytoskeletal components as a focal endothelial cell response to injury, perhaps promoting endothelial cell adhesion to the underlying basal lamina through a transmembrane linkage.

Electron microscopic study on the location of 23 kDa and 50 kDa fragments in skeletal myosin head

The functional activities of myosin head are located in a 95 kilodalton (kDa) heavy chain which can be divided into three fragments of 23 kDa, 50 kDa, and 20 kDa. ATP hydrolysis sites were suggested to be located in the 23 kDa and 50 kDa fragments, and actin binding sites were in the 50 kDa and 20 kDa fragments. In this study, we obtained electron microscopic images of the myosin molecule bound with antibodies directed to the 23 kDa and 50 kDa fragments. We determined that the antigenic sites for 23 kDa fragment are located at 140-180 A from the head-rod junction of myosin, and those for 50 kDa fragment at 160 A from the junction and at the tip of the head itself. The relationship between the spatial locations and the primary structures is discussed.

Amino acid sequence of the regulatory light chain of clam foot muscle myosin

The amino acid sequence of the regulatory light chain of foot muscle myosin from surf-clam (Spisula sachalinensis) was determined by conventional methods. It was: xS-D-D-K-K-A-K-A-A-T-S-S-V-L-T-K-F-T-Q-N-Q-I-Q-E-M-K-E-A-F-T-M-I-D-Q-N-R -D-G-L- I-D-V-S-D-L-K-E-M-Y-S-N-L-G-T-A-P-Q-D-S-V-L-Q-A-M-V-K-E-A-P-Q-M-N-F-T-G- F-L-S-L- F-S-E-K-M-S-G-T-D-P-E-E-T-L-R-N-A-F-Q-M-F-D-S-D-N-T-G-Y-I-P-E-E-Y-M-K-D- L- L-E-N-M-G-D-N-F-S-K-D-E-V-R-Q-T-W-K-E-A-P-I-A-G-G-K-V-D-Y-N-A-F-V-S-K-I- K- G-K-E-Q-D-D-A. The alpha-amino group of the light chain was blocked, and a typical calcium binding structure was recognized at the 33rd to 44th residues, as in other myosin regulatory light chains. The sequences of regulatory light chains from various muscle myosins were arranged according to the well known four-domain structure, and structural homologies were obtained for each of the domains. Based on the homologies, the relationships between the structure, function, and molecular evolution were discussed.

Rat and avian myofibers having similar innervation share antigenic determinants

The antigenic relationship between extrafusal myofiber types of avian and rat muscles was investigated. Antibodies specific for myosin heavy-chain isozymes of chicken fast-twitch myofibers (anti-twitch) or chicken slow-tonic myofibers (anti-tonic) were used in the immunohistochemical examination of embryonic and adult rat muscle. Anti-twitch antibodies reacted with chicken fast-twitch myofibers, with all extrafusal myofibers of adult rat and with embryonic rat myofibers. Anti-tonic antibodies reacted with chicken slow-tonic myofibers and with embryonic rat myofibers, but not with any extrafusal myofibers of adult rat. All intrafusal myofibers of adult rat reacted with both antibodies. However, individual intrafusal myofibers reacted most strongly with one or the other of the antibodies and weakly with the remaining antibody, thereby segregating the intrafusal myofibers into two classes. These results indicate shared antigenic determinants between the myosin heavy chains of avian and rat twitch myofibers and between the myosin heavy chains of avian tonic myofibers and embryonic rat myofibers. Therefore, antigenically related myosin heavy-chain isozymes are present in avian and rat myofibers having similar patterns of innervation (twitch or tonic). This tonic vs twitch comparison presents a unique perspective for the immunochemical analysis of myosin isozymes, compared with the traditional division based on speed of contraction, slow or fast.

Two phosphorylated forms of myosin in thrombin-stimulated platelets

Three forms of 20-kDa myosin light chain (MLC), unphosphorylated, monophosphorylated, and diphosphorylated MLC (designated 20K, 20K-P, and 20K-PP) were demonstrated in thrombin-stimulated human platelets by two different gel electrophoretic methods: in the presence of glycerol urea or in two dimensions (isoelectric and sodium dodecyl sulfate). The diphosphorylation of platelet 20-kDa MLC increased, dose dependently, up to 0.4 U/ml thrombin and reached 25% of platelet 20-kDa MLC. After mono- or diphosphorylated 20-kDa MLC from thrombin-stimulated platelets was digested with trypsin, the analysis using two-dimensional peptide mapping demonstrated that two different sites were phosphorylated by MLC kinase and protein kinase C, as noted in the case of 12-O-tetradecanoylphorbol-13-acetate-stimulated platelets (M. Naka, et al. (1983) Nature (London) 306, 490-492). The more rapid monophosphorylation was catalyzed preferentially by MLC kinase while the slower and additional phosphorylation was catalyzed mainly by protein kinase C. These results suggest the importance of distinguishing multiple site phosphorylation of 20-kDa MLC in thrombin-activated human platelets.

Spectrin and calmodulin in spreading mouse blastomeres

The role of spectrin and its association with calmodulin in spreading mouse blastomeres was investigated. Embryonic spectrin binds 125I-calmodulin in a calcium-dependent fashion in the blot overlay technique. Double-labeling experiments show coordinate redistribution of spectrin and calmodulin in blastomeres preparing to undergo active spreading movement. At this stage cortical spectrin staining is lost from the region of cell-substrate contact and spectrin and calmodulin become concentrated in two structures closely associated with the contacted region: a group of spherical bodies located on the cytoplasmic side of the cortical layer and a subcortical ring that marks the perimeter of the contacted region. The localization pattern of spectrin and calmodulin is also coordinated with that of actin and myosin. The results suggest that spectrin plays a role in the spreading of blastomeres and that this function may involve linkage of spectrin, calmodulin, and the cortical contractile apparatus.

Myosin heavy chain composition of single fibres from normal human muscle

Electrophoretic analysis in the presence of 33% glycerol of purified myosin from normal human muscle shows three distinct protein bands which are identified as type 1, 2B, and 2A myosin heavy chain (MHC) isoforms by affinity-purified polyclonal antibodies. Analysis of MHC of single human muscle fibres shows that human muscles contain a large population of fibres showing the coexistence of type 2A and 2B MHC.

Changes in myosin heavy chain stoichiometry in pig tracheal smooth muscle during development

The stoichiometry of the myosin heavy chains (MHCs) has been measured in the tracheal smooth muscle of the pig after electrophoresis on SDS 4% polyacrylamide gel. The ratio of slower migrating MHC to the faster migrating MHC was 2.1 neonates, 1.5 in young and 0.95 in old pigs (P less than 0.01) showing that MHC composition changes with development. The unequal proportion of MHCs was not compatible with a heterodimeric arrangement of the MHCs in the native molecule as proposed earlier by Rovner et al. [(1986) Am. J. Physiol. 250, C861-870] and it is suggested that native molecules may be composed of homodimer heavy chains.

Nascent muscle fiber appearance in overloaded chicken slow-tonic muscle

The application of a weight overload to the humerus of chickens induces a hypertrophy of anterior latissimus dorsi (ALD) muscle fibers. This growth is accompanied by a rapid and almost complete replacement of one slow-tonic myosin isoform, SM-1, by another slow-tonic isoform, SM-2. In addition, a population of small fibers appears mainly in extrafascicular spaces and, concurrently, three additional myosin bands are detected by gel electrophoresis. Five antibodies against myosin heavy chain (MHC) isoforms were selected as immunocytochemical probes to determine the cellular location and nature of these myosins. The antibodies react with ventricular, fast skeletal muscle and either SM-1 or SM-2, or both the slow-tonic MHCs. The antifast and antiventricular antibodies react with myosin present in the 10-day embryonic ALD muscle but do not react with myosin in posthatch ALD muscle. The small fibers in overloaded muscle contain a myosin isoform characteristically expressed during the embryonic stage of ALD muscle development and therefore are named nascent myofibers. Some of the nascent myofibers do not react with the antibody to both slow-tonic MHCs, indicating the lack of the normal adult slow-tonic myosins which are expressed in 10-day embryos. In order to explore the origin of the nascent fibers, an electron microscopic study was performed. Stereological analysis of the existing fibers shows a stimulation of numbers and sizes of satellite cells. In addition, the volume occupied by nonmuscle and undifferentiated cells increases dramatically. Myotube formation with incipient myofibrils is seen in extrafascicular spaces. These data suggest that new muscle fiber formation accompanies hypertrophy in overloaded chicken ALD muscle and the process may involve satellite cell migration.

Histopathological and immunohistochemical studies on nickel sulfide-induced tumors in F344 rats

Twenty-five tumors induced in F344 male rats were examined histologically and immunohistochemically using antibodies against myoglobin, myosin, desmin and cathepsin B. Eight were from rats which had been given intramuscular (im) injection and 17 were from rats which had been given subcutaneous (sc) injection of 5 mg of Ni3S2. Among 10 rhabdomyosarcomas, myoglobin was detectable in 3, myosin in 8, and desmin in all, but cathepsin B was present in none. Out of 8 malignant fibrous histiocytomas, cathepsin B was detectable in all, but the other antigens were absent. In a leiomyosarcoma, only desmin was detected. In two fibrosarcomas, none of the markers were detected. In four undetermined tumors, one reacted only with anti-desmin antibody, two with only anti-cathepsin B antibody, and one with none of the antibodies. Of the three myogenic markers utilized in this study, anti-desmin antibody appeared to be the most sensitive. Cathepsin B was found mainly in the histiocytic cells of malignant fibrous histiocytoma. Thus, desmin appears to be particularly valuable in distinguishing immature myogenic tumors from other primitive tumors, while cathepsin B is useful in distinguishing malignant fibrous histiocytoma from other pleomorphic mesenchymal tumors.

Heart-reactive antibodies in rabbit anti-Streptococcus mutans sera fail to cross-react with Streptococcus mutans

Immunization of rabbits with Streptococcus mutans antigens results in the production of serum antibodies that bind in vitro to human, rabbit, and monkey cardiac muscle. Antibodies to heart, however, have also been reported to occur at lower titers in the sera of unimmunized rabbits. In this study, the specificities of heart-reactive antibodies (HRA) in sera of unimmunized and S. mutans-immunized rabbits were compared using indirect immunofluorescence, Western blot, and Bio-Dot immunoassays. Both groups of sera gave striational indirect immunofluorescence-staining patterns on thin sections of native human and monkey cardiac muscle. Western blot analyses revealed that antibodies in normal sera bound 9 to 20 components of human, rabbit, and monkey heart. The major bands had Mr of 205,000, 160,000, 135,000, and 70,000. Several of the normal sera did not have antibody activity to S. mutans antigens, indicating that these HRA do not cross-react with these bacteria. Although immunization of rabbits with S. mutans caused increased titers of HRA (two to three doubling dilutions), Western blot assays using anti-S. mutans sera showed banding patterns qualitatively similar to those of normal sera on heart extracts. Antibodies to skeletal muscle myosin were detected in both serum groups. Of eighteen normal rabbit sera sixteen had antimyosin titers of 10 to 40, whereas all eighteen anti-S. mutans sera had titers of 10 to 160. Affinity-purified antimyosin antibodies isolated from anti-S. mutans serum did not bind to S. mutans components. Conversely, affinity-purified antibodies to S. mutans antigens did not bind to myosin or to other cardiac muscle components. Among these were antibodies to the 185-kDa cell wall protein (also known as B, I/II, IF, Spa A, and P1) previously believed to possess antigenic mimicry. HRA were removed from anti-S. mutans sera by absorption with S. mutans but this effect was not specific, because a non-cross-reactive internal standard antibody was also absorbed to the same extent. Because previous evidence for antigenic mimicry between S. mutans and cardiac muscle was based on serum cross-absorption experiments, this immunologic relationship is not substantiated. These results indicated that naturally occurring antibodies to cardiac muscle components are present in the sera of unimmunized rabbits and that immunization with S. mutans does not stimulate production of new heart-reactive antibody, but rather serves to boost antibody production by preexisting clones of self-reactive B-lymphocytes.

Immunocytochemical and electrophoretic analyses of changes in myosin gene expression in cat posterior temporalis muscle during postnatal development

Changes in myosin gene expression during the postnatal development of the homogeneously superfast kitten posterior temporalis muscle were examined using immunocytochemical techniques supplemented by pyrophosphate gel electrophoresis and gel electrophoresis-derived enzyme linked immunosorbent assay (GEDELISA) of myosin isoforms. The antibodies used were polyclonals directed against the heavy chains of superfast and foetal myosins and monoclonals against the heavy chains of slow and fast myosins. The fibres of the posterior temporalis in the newborn kitten stained almost uniformly with the anti-foetal myosin antibody and the largest of these fibres stained strongly for superfast myosin. A subpopulation of fibres staining for superfast myosin also stained lightly for slow myosin. These slow staining fibres were evenly distributed in the centres of muscle fibre bundles, reminiscent of primary fibres in limb fast muscle. During subsequent development, slow myosin staining disappeared and superfast myosin replaced foetal myosin so that by 50 days the muscle was virtually homogeneously superfast as in the adult. Fast myosin was never expressed at any stage. It is proposed that fibres staining transiently for slow myosin are superfast primary fibres which are homologous to fast primary fibres recently described in regions of limb muscles devoid of slow fibres in the matured animal. Other jaw-closing muscles have significant populations of slow fibres in the mature animal and it is postulated that there exists in these muscles a second class of jaw primary fibres, the slow primary fibres, in which slow myosin synthesis would be sustained in the adult. It is suggested that the myogenic cells of jaw-closing and limb muscles are of two distinct types preprogrammed to express different muscle genes.

Immunocytochemical and electrophoretic analyses of changes in myosin gene expression in cat limb fast and slow muscles during postnatal development

Changes in myosin synthesis during the postnatal development of the fast extensor digitorum longus (EDL) and the slow soleus muscles of the kitten were examined using immunocytochemical techniques supplemented by pyrophosphate gel electrophoresis and gel electrophoresis-derived enzyme linked immunosorbent assay (GEDELISA) of myosin isoforms. The antibodies used were monoclonals against heavy chains of slow and fast myosins and a polyclonal against foetal/embryonic myosin. In both muscles in the newborn kitten, there was a population of more mature fibres which stained strongly for slow but weakly for foetal/embryonic myosin. These fibres were considered to be primary fibres. They formed 4.8% of EDL fibres and 26% of soleus fibres at birth, and continued to express slow myosin in adult muscles. The less mature secondary fibres stained strongly for foetal/embryonic myosin, and these could be divided into two subpopulations; fast secondaries in which foetal/embryonic myosin was replaced by fast myosin, and slow secondaries in which the myosin was replaced by slow myosin. At 50 days the EDL had a large population of fast secondaries (83% of total fibres) and a small population of slow secondaries which gradually transformed into fast fibres with maturity. The vast majority of secondary fibres in the soleus were slow secondaries, in which slow myosin synthesis persisted in adult life. There was a restricted zone of fast secondaries in the soleus, and these gradually transformed into slow fibres in adult life. It is proposed that the emergence of primary fibres and the two populations of secondary fibres is myogenically determined.

Immunohistochemical studies of DMBA-induced rat mammary tumors

In order to clarify the biological characteristics of rat mammary tumors induced by 7,12-dimethylbenz-[a]-anthracene (DMBA), histochemical and immunohistochemical studies were performed. Two types of luminal spaces were observed within the tumor. In one type, the lumen was surrounded by eosinophilic columnar cells which were strongly reactive for soybean agglutinin (SBA) but weakly stained with keratin antibodies. In the luminal spaces, substances positive for PAS, dialyzed iron ferrocyanide or alcian blue and resistant to mucopolysaccharidase were occasionally observed. Ultrastructurally, the luminal surface was characterized by the presence of microvilli and tight junctions. In the other type, the lumen was often found in highly cellular foci and surrounded by pale, polygonal or elongated cells which were weakly stained with keratin antibodies but not SBA. The luminal spaces presented a peculiar structure filled mainly with mucoid substances sensitive to hyaluronidase, chondroitinase ABC and heparitinase, and the inner surface of the spaces was surrounded by basement membrane components: laminin, fibronectin and type IV collagen. The results of the present study therefore showed that DMBA-induced mammary tumor consists, partly, of a structure resembling human adenoid cystic carcinoma.