Ultrastructure of striated muscle fibers in the middle third of the human esophagus

Striated muscle fibers and their spatial relationship to smooth muscle cells have been studied in the middle third of human esophagus. Biopsies were obtained from 3 patients during surgery. In both the circular and longitudinal layers, the muscle coat of this transition zone was composed of fascicles of uniform dimension (100-200 microns of diameter); some of these bundles were made up of striated muscle fibers, others were pure bundles of smooth muscle cells and some were of the mixed type. Striated muscle fibers represented three different types, which were considered as intermediate, with certain structural features characteristic of the fast fiber type. Of these, the most frequently-found fibers were most similar to the fast fiber type. Satellite cells were numerous; in mixed fascicles they were gradually replaced by smooth muscle cells. The gap between striated muscle fiber and smooth muscle cells was more than 200 nm wide. It contained the respective basal laminae and a delicate layer of amorphous connective tissue. No specialized junctions were formed between consecutive striated muscle fibers, or between striated muscle fibers and smooth muscle cells. Interstitial cells of Cajal were never situated as close to striated muscle fibers as to smooth muscle cells.     

Ultrastructure of the striated muscle tissue of the iris of birds]

Heteromorphism of the contractile elements of the iris muscular tissue in chick embryos and in chickens has been studied by means of electron microscopical investigation. The leading contractile tissue of the iris is the striated muscular tissue, which is formed as a cellular-simplastic system with its own cambium-myosatellitocytes. Some cells, containing myofilaments in their cytoplasm, are related to myofibroblastic and smooth muscle differons, which functions remain to be studied. A hypothesis is proposed on existence of two sources for development of the iris muscular elements. The first-stem cells for the striated muscular tissue; at early stages of embryonal development they are included into composition of ectomesenchyme of the neural crest and migrate into the area of the muscle anlages. The second-cells migrating from the ocular cup margins and developing into the smooth myocytes of the iris.     

Two different forms of beta myosin heavy chain are expressed in human striated muscle

Summary We have found evidence for two beta-like myosin heavy chains in humans, one cardiac and one skeletal. The cDNA sequences of the cardiac beta myosin heavy chain cDNA clone pHMC3 and the skeletal beta-like myosin heavy chain cDNA clone pSMHCZ, were compared to each other. It was found that the 3 untranslated regions as well as 482 nucleotides specifying the carboxyl coding region, were 100% homologous. Further examination revealed that the skeletal clone pSMHCZ diverges from the human cardiac beta myosin heavy chain cDNA clone pHMC3 at the 5 end. We present evidence in this report which indicates that the cardiac beta myosin heavy chain mRNA is expressed in skeletal muscle tissues. The human cardiac beta myosin heavy chain cDNA clone, pHMC3, which codes for a portion of the light meromyosin section of the myosin heavy chain, was used as a probe for S1 nuclease mapping studies with RNA derived from cardiac tissue, smooth muscle and skeletal muscle tissues consisting of fast-twitch, slow-twitch and mixed fast- and slow-twitch muscle fibres. Two probes were used to examine the expression of the mRNA. One probe (406 nucleotides) constitutes the 3 untranslated region and a portion of the coding region of the beta cardiac myosin heavy chain cDNA clone, which is 100% homologous to pSMHCZ, the skeletal cDNA clone. The other constitutes the majority of the coding region (1017 nucleotides) of the cardiac clone pHMC3 in which the first 216 nucleotides from the labelled end are 100% homologous to the skeletal clone pSMHCZ. In the soleus muscle, which is rich in slow-twitch type I muscle fibres, the expression of the cardiac beta myosin heavy chain mRNA was very prominent. In gastrocnemius muscle, a mixed fibre muscle, the expression of this mRNA was detected to a lesser degree than that for the soleus muscle. In vastus lateralis and vastus medialis, which consist of predominantly type II, fast-twitch fibres, there were trace amounts of the cardiac beta myosin heavy chain mRNA. When expression of this mRNA was tested in smooth muscle tissue none could be detected.     

Interfilament forces in striated muscle

Evaluation of the Van der Waals energy per filament suggests that molecular dispersion forces should not be very important in determining the stability of the myofilament lattice in resting muscle. In order to explain the lattice stability and other important properties of the striated muscle, it is suggested that a balance between electrostatic forces and forces developed by some interfibrillar structures is mainly responsible.     

Altered actin and troponin binding of amino-terminal variants of chicken striated muscle alpha-tropomyosin expressed in Escherichia coli

We have expressed two variants of chicken striated muscle alpha-tropomyosin in Escherichia coli: fusion tropomyosin containing 80 amino acids of a non-structural influenza virus protein (NS1) on the amino terminus and a non-fusion tropomyosin which is a variant because the amino-terminal methionine is not acetylated (unacetylated tropomyosin). From our analysis of purified proteins in vitro we suggest that the amino-terminal region, which is highly conserved in muscle tropomyosins, is crucial for all aspects of tropomyosin function. Both forms are altered in tropomyosin activity: neither shows head-to-tail polymerization, with or without troponin. Unacetylated tropomyosin binds weakly to actin, but in the presence of troponin it binds well and can regulate the actomyosin ATPase. Fusion tropomyosin binds well to actin, but binding of troponin is calcium-sensitive and it does not confer effective calcium sensitivity on the actomyosin ATPase. Our results indicate that the local charge at the amino terminus is critical for actin binding but that normal head-to-tail association is not required. The properties of fusion tropomyosin-troponin interaction are indicative of impaired troponin T binding to tropomyosin and provide evidence for its binding to the amino terminus of tropomyosin.     

Length-tension relation in Limulus striated muscle

Laser diffraction techniques coupled with simultaneous tension measurements were used to determine the length-tension relation in intact, small (0.5-mm thick, 10-mm wide, 20-25-mm long) bundles of a Limulus (horseshoe crab) striated muscle, the telson levator muscle. This muscle differs from the model vertebrate systems in that the thick filaments are not of a constant length, but shorten from 4.9 to approximately 2.0 micrometers as the sarcomeres shorten from 7 to 3 micrometers. In the Limulus muscle, the length-tension relation plateaued to an average maximum tension of 0.34 N/mm2 at a sarcomere length of 6.5 micrometers (Lo) to 8.0 micrometers. In the sarcomere length range from 3.8 to 12.5 micrometers, the muscle developed 50% or more of the maximum tension. When the sarcomere lengths are normalized (expressed as L/Lo) and the Limulus data are compared to those from frog muscle, it is apparent that Limulus muscle develops tension over a relatively greater range of sarcomere lengths.