Ultrastructure of muscle fibres in head and axial muscles of the perch (Perca fluviatilis L.)

Summary White, pink, red and deep red fibres, selected from a head muscle and from axial muscles of the perch, show significant differences in actin filament length, Z line thickness, Z line lattice space, myofibril girth, the percentages volume occupied by T system and terminal cisternae of the SR, and in the degree of T system SR contact per sarcomere. In both muscles the degree of T system SR contact decreases in the order: white, pink, red, deep red, which suggests a decrease of contraction velocity in the same order.The position of the T system (at the Z line or at the AI junction) is related to the actin filament length. The actin filaments in the red fibres are appreciably longer than in the white, which suggests that the sarcomeres of the red fibres have a broader length-tension curve. The Z line thickness is positively correlated with the actin filament length and, in the white and the red fibres, negatively with the degree of sarcomere shortening. Thicker Z lines are suggested to allow greater sarcomere sizes (length or girth).The percentage volume occupied by mitochondria varies independently of the extent of membrane systems.The ultrastructural characteristics of the fibre types are in agreement with the functional roles as reported in literature.     

A COMPARISON OF THE FINE STRUCTURES OF FROG SLOW AND TWITCH MUSCLE FIBRES

The organisation of the myofibrils and the sarcoplasmic reticulum in frog slow muscle fibres has been compared with that in twitch fibres. It has been found that the filaments have the same length in the two types of fibre, but that there are differences in their packing: (a) in contrast to the regular arrangement of the I filaments near the Z line in twitch fibres, those in slow fibres are irregularly packed right up to their insertion into the Z line; (b) the Z line itself shows no ordered structure in slow fibres; (c) the fine cross-links seen between the A filaments at the M line level in twitch fibres are not present in slow fibres. The sarcoplasmic reticulum in slow fibres consists of two separate networks of tubules. One set of tubules (diameter about 500 to 800 A) is oriented mainly in a longitudinal direction. The tubules of the other network (diameter about 300 A) are oriented either transversely at approximately Z line level or longitudinally, connecting the transverse tubules. Triads are very rarely found, occurring at only every 5th or 6th Z line of each fibril. The central element of these triads is continuous with the thin tubules. Slow fibres from muscles soaked in ferritin-containing solutions contain ferritin particles in the network of thin tubules, the rest of the sarcoplasm remaining free of ferritin.     

The Florey Lecture, 1982. Discovery: accident or design?

The lecturer reviews the extent to which his own experiments on muscle have followed the course intended when they were planned. His observations on changes in the striation pattern were designed to reinvestigate the formation of 'contraction bands', repeatedly observed in the 19th century but neglected more recently. This phenomenon was indeed seen during active shortening, but the most important outcome consisted of two quite unexpected observations which suggested the existence of a sliding-filament system. Experiments on local activation were planned on the hypothesis that activation was conducted inward from the surface membrane along the Z line. This was apparently confirmed in the first experiments, on fibres from frog muscle, but experiments on muscle fibres from other animals, together with improvements in electron microscope technique, showed that this was a coincidence and that the Z line as such is not involved. Investigation of the transient changes of tension when a stimulated muscle fibre is suddenly shortened required a series of exploratory measurements before a useful hypothesis could be formulated. Some personal factors that have motivated scientists, including Lord Florey himself, are discussed.     

The fine structure of red and white myotomal muscle fibres of the coalfish (Gadus virens)

Summary The fine structure of the red and white myotomal muscles of a marine teleost, the coalfish Gadus virens, has been examined and ultrastructural measurements and analyses carried out. The sarcomere lengths of the red and white fibres were found to be 1.60 minimum, 1.82 maximum and 1.70 minimum, 1.85 maximum, respectively. No significant difference was found between the red and white fibres in their percentage of sarcoplasmic reticulum and T system. Both were found to have regularly occurring triads at the Z disk level, to have distinctive M lines and to be multiply innervated. Ultrastructurally the two fibres can be distinguished by the thicker Z line and more abundant mitochondria of the red fibre, and by the ribbon-shaped peripheral myofibrils of the white fibres. The structure of the fibres in these two types of muscle is discussed in relation to their possible role in swimming.This work was supported by a research grant from the National Environmental Research Council.     

Ultrastructural transformation of fast chicken muscle fibres induced by nerve cross-union

Summary The fast posterior latissimus dorsi (PLD) muscle of newly hatched chickens was transposed and cross-innervated by the slow-type nerve originally innervating the anterior latissimus dorsi (ALD) muscle. The innervation and the ultrastructure of the cross-innervated posterior latissimus dorsi (PLD-X) muscle was investigated from one week up to 18 months after the operation and compared with that of the control fast (PLD-C) and control slow (ALD-C) muscles. All nerve terminals in the PLD-X muscle were of the slow type. Yet the degree of ultrastructural transformation differed from fibre to fibre. Only about 30% of PLD-X fibres had transformed ultrastructure closely resembling the control slow fibres. In this group of maximally altered fibres, the myofibrils had large diameters, wide Z lines and indistinct M lines as the control slow fibres. The amount of mitochondria was increased to levels found in control slow fibres. The mean percentage of triads was also comparable to that of control slow fibres, being approximately by two thirds lower than in control fast fibres.The differences in the degree of ultrastructural transformation are presumably due to different plasticity of muscle cells at the time of cross-innervation. In the transposed PLD-X muscles large areas undergo degeneration and regeneration. It is suggested that an almost complete changeover of the fibre type is only brought about after cross-innervation of newly differentiating muscle cells, whereas partial alteration occurs after reinnervation of young myofibres.The skillful technical assistance of Dr. Z. Liková, Mrs. M. Sobotková, Ing. M. Doubek and Mr. H. Kunz is gratefully acknowledged.     

Mechanism of myofibril growth and proliferation in fish muscle.

The mechanisms of myofibril growth proliferation were investigated in the red and white muscles of fish. In both types of muscle the ratio of lattice filament spacings between the Z disk and M line was found to be greater than that required for perfect transformation of a square into a hexagonal lattice. This mismatch was considered to result in the thin filaments being pulled obliquely instead of at right angles to the Z disk. The angle of pull of the thin filaments was measured in longitudinal sections. The splitting process was found to decrease the degree of pull. Splitting was also observed in transverse sections of the peripheral myofibrils. In both red and white fibres these myofibrils were found to commence splitting when they reached a size of approximately 1-2 mum diameter. Evidence from ultrastructural and autoradiographical studies suggested that growth of the myofibrils within the fibres is centrifugal. The outermost myofibrils appear to be the ones which are being built up and which split. The data indicated that in fish muscle a considerable number of filaments may be added to the daughter regions whilst splitting of the myofibril is still continuing.     

The ultrastructure of oesophageal striated muscle in the guinea-pig and marmoset

The ultrastructural characteristics of oesophageal striated muscle from guinea-pig and marmoset have been examined using transmission electron microscopy and compared with ultrastructural features of skeletal muscle. The findings show that, although similar to skeletal muscle, oesophageal striated muscle exhibits important differences in the arrangement of its muscle fibres and their motor innervation. It was also found impossible to classify oesophageal striated muscle according to the usual ultrastructural criteria since its motor endplates bore a greater resemblance to those of intrafusal skeletal muscle fibres. The consideration of these differences in conjunction with characteristics revealed elsewhere by other techniques confirms the unique nature of oesophageal striated muscle.     

Slowly contracting muscles power the rapid jumping of planthopper insects (Hemiptera, Issidae)

The planthopper insect Issus produces one of the fastest and most powerful jumps of any insect. The jump is powered by large muscles that are found in its thorax and that, in other insects, contribute to both flying and walking movements. These muscles were therefore analysed by transmission electron microscopy to determine whether they have the properties of fast-acting muscle used in flying or those of more slowly acting muscle used in walking. The muscle fibres are arranged in a parallel bundle that inserts onto an umbrella-shaped tendon. The individual fibres have a diameter of about 70 μm and are subdivided into myofibrils a few micrometres in diameter. No variation in ultrastructure was observed in various fibres taken from different parts of the muscle. The sarcomeres are about 15 μm long and the A bands about 10 μm long. The Z lines are poorly aligned within a myofibril. Mitochondrial profiles are sparse and are close to the Z lines. Each thick filament is surrounded by 10–12 thin filaments and the registration of these arrays of filaments is irregular. Synaptic boutons from the two excitatory motor neurons to the muscle fibres are characterised by accumulations of ~60 translucent 40-nm-diameter vesicle profiles per section, corresponding to an estimated 220 vesicles, within a 0.5-μm hemisphere at a presynaptic density. All ultrastructural features conform to those of slow muscle and thus suggest that the muscle is capable of slow sustained contractions in keeping with its known actions during jumping. A fast and powerful movement is thus generated by a slow muscle.     

Histochemical and ultrastructural properties of myoid cells in the thymus of the frog

Summary Histochemical and ultrastructural properties of myoid cells in the thymus of the frog were investigated and compared with properties of skeletal muscle fibres. The histochemical reactions of phospholipids, phosphorylase, succinic dehydrogenase and adenosine triphosphatase activities in myoid cells were characterized by considerable variability. Individual myoid cells apparently possess different enzyme activities which correspond to different stages of development, maturity and degeneration of these cells. The mature mononucleated myoid cells have similar enzymatic properties to the fast muscle fibres of the frog. This finding has been extended by ultrastructural observations. Features, typical of fast muscle fibres of the frog, e.g. the presence of the M-line, straight and narrow Z-line and well developed triads were found in the majority of mature myoid cells.     

The three-dimensional structure of the Z disc in insect supercontracting muscles

Two types of Z disc structure have been reported in insect supercontracting muscle fibres: (i) a perforated Z disc where Z material forms a reticulum and (ii) a fragmented Z disc composed of separate, discrete Z bodies. The use of thick (I μm) sections in conjunction with high voltage electron microscopy can distinguish between these two types while conventional thin sections may lead to misinterpretation of structure. It is shown that in one insect, the crane-fly Tipula, the larval body-wall muscles, for which a fragmented Z disc has been proposed, do in fact have a perforated disc. In the wax moth Galleria, homologous muscle fibres have a similar type of Z disc, a finding which indicates the need for re-examination of other lepidopteran muscles claimed to have fragmented discs. A redefinition of supercontraction is proposed which includes reference to the perforated type of Z disc.     

The lateral musculature of the common bully, Gobiomorphus cotidianus, a freshwater fish from New Zealand

A histochemical and ultrastructural study has shown that the myotome of the common bully, Gobiomorphus cotidianus , is composed of three muscle fibre types: white, pink and small diameter fibres. There are no red fibres. Both white and pink fibres have characteristics similar to these fibres found in other teleosts. The small diameter fibres are located in the position usually occupied by red fibres and are identified by their small size and poor staining characteristics. At the ultrastructural level these small fibres are seen to have few mitochondria and a poorly developed sarcoplasmic reticulum. It is suggested that the small diameter fibres are a type of tonic muscle used for positioning the trunk.     

Ultrastructural identification of muscle fiber types by immunocytochemistry

In a fast-twitch muscle, three types of fibers (red, intermediate, and white) can be distinguished on the basis of mitochondrial content. Red fibers, identified by abundant mitochondria, can be further differentiated on the basis of a positive or negative response to antibodies specific for white ("fast") myosin. Because there is also a difference in Z-line width among fibers of the same muscle, the possibility existed that the two red fibers, which differ in type of myosin, might also differ in dimensions of the Z line. We therefore examined, with the electron microscope, fibers which had been exposed to antibody against white myosin. In those fibers which react with the antibody, an electron-opaque band is evident in the H-band region, thereby distinguishing reactive from unreactive fibers. The red fiber can now be subdivided on the basis of a positive or negative response to anti-white myosin visualized directly with the electron microscope. Both categories of red fibers ("fast" and "slow") have wide Z lines, and thus are distinguished from white and intermediate fibers, which react with the antibody but which have narrow Z lines. On the basis of combined immunocytochemical and ultrastructural characteristics, four types of fibers can be recognized in a single muscle. Moreover, it is demonstrated here that a wide Z line does not necessarily imply a slow speed of contraction.     

Ultrastructural features of cardiac muscle cells in a tarantula spider

The ultrastructural features of cardiac muscle cells and their innervation were examined in the tarantula spider Eurypelma marxi Simon. The cells are transversely striated and have an A band length of about three microns. H zones are indistinct and M lines are absent. Thick and thin myofilament diameters are approximately 200 and 70 Å respectively. Eight to 12 thin filaments usually surround each thick one. Accumulations of thick and thin myofilaments occur perpendicular to the bulk of the myofilaments in some cells. The Z line is discontinuous and thick filaments may pass through the spaces in the Z line. Extensive systems of sarcoplasmic reticulum and transverse tubules are present; these form numerous dyadic junctions in both A and I band regions. Sarcolemmal invaginations form Z line tubules; lateral extensions of the plasma membrane portion of these invaginations form dyads. Nerve branches of the cardiac ganglion make multiple neuromuscular synapses with at least some of the cardiac muscle cells. Both large granular and small agranular vesicles are present in the presynaptic terminals. Intercalated discs similar to those present in other arthropod hearts occur between the ends of adjacent cardiac muscle cells.     

Myofibril breakdown during spreading damage. II. A calcium-free medium]

Peculiarities of Zenker's degeneration (ZD) have been investigated in fast muscle fibres of the frog incubated in a Ringer solution free of Ca++ (R--Ca) with a normal or increased (by 100 mM) concentration of KCl. ZD in these solutions is distinguished by a 10--90 minutes delay of the appearance of the primary contraction knot and cessation of ZD development in the majority of fibres after formation of several (1--5) contraction knots. In the presence of 0.5 mM EDTA in R--Ca, after a few typical contraction knots are formed, fibres commonly fall into large fragments that retain cross-striation. Contracted or super-contracted state of sarcomeres in detached contraction knots and at the necrosis boundary, as well as an increasing lysis of contactile material and proliferation of fibre membrane structures in the region of ZD arrested boundary, are characteristic of ultrastructural changes during ZD in calcium-free solutions.     

Quantitative studies on the distribution of myofilaments in intrafusal muscle fibres

Summary Muscle spindles contain two types of intrafusal muscle fibre, nuclear bag fibres and nuclear chain fibres. The intrafusal fibres of rabbit and guinea pig spindles have been studied using quantitative stereological techniques at the ultrastructural level. The crosssectional areas occupied by myofilaments have been measured in the polar and equatorial regions of both types of intrafusal fibre. There are considerably fewer myofilaments in the equatorial regions of both types of fibre compared with their polar regions.This work was carried out with the aid of grants from the Medical Research Council and the Science Research Council of Great Britain.     

Cytological differentiation of human fetal skeletal muscle.

The ultrastructural differentiation of several different muscles was investigated in human fetuses ranging in age from 13 weeks to neonatal. At approximately 16 weeks of gestation cell cluster containing both myotubes and satellite cells lie enclosed by a newly formed basal lamina and show evidence of fusion. The development of organelles is evident in myoblasts, proceeds as the cells transform into myofibers, and continues in the neonate. Filament synthesis occurs primarily in the cell periphery where thin filaments appear to align themselves in relations to parallel arrays of ribosome-studded thick filaments: Z line formation follows the appearance of thin filaments. Intermediate filaments, approximately 10-12 nm thick, were also consistently observed in perinuclear regions and distal to filament assembly. Although sarcoplasmic reticulum (SR) development is closely related to fibril formation, connections between Z lines and SR are not consistent, thus supporting the conclusion that SR does not evoke the formation of the Z line. Bristlecoated vesicles appear to be the precursors of elements of the SR, possibly the lateral sacs. Development of the transverse tubules, as invaginations of the sarcolemma, is closely associated with the formation of lateral sacs since the latter occur along the sarcolemma as soon as transverse tubules appear. Cytological differentiation is similar, though not identical, in several different muscles. During the last trimester muscle fibers show some evidence of diversity mainly of variation in Z line width. In gerneral the results suggest that the sequence and stages of human myogenesis are similar to those of other species.     

Ultrastructural alterations in extrinsic eye muscles induced by vincristine.

A comparative morphological analysis of the effects of vincristine on particular types of muscle fibres of the eye and selected trunk muscles of the mouse was performed. Great resistance of the mouse organism to the action of sublethal doses of vincristine has been found. Degenerative changes of great intensity (atrophy of myofibrils, disturbances of the Z line) and the appearance of new changes, not mentioned hitherto among the vincristine myopathies (megamitochondria, intermembranous inclusions, glycogen in mitochondria and very large vacuoles) were observed in the trunk muscles of mice. The eye muscles are seem to be more resistant to the action of vincristine. The intensity of changes in the eye muscles was connected with the types of muscle fibres. Red fibres, rich in mitochondria, underwent relatively greatest changes, whereas the smallest changes were found in tonic fibres, poor in mitochondria and sarcotubular system, i.e. these structures from which spheromembranous bodies, most characteristic of the pathogenic effects of vincristine arise.     

Development of immunohistochemical characteristics of intrafusal fibres in normal and de-efferented rat muscle spindles

Intrafusal muscle fibres in adult muscle spindles differ in their myosin composition. After selective motor denervation intrafusal muscle fibres develop mature ultrastructural characteristics. In order to evaluate the role of fusimotor innervation on the maturation of the myosin composition of intrafusal muscle fibres we have examined with immunohistochemical techniques i) the postnatal development of muscle spindles in new-born rats and in 7-21 day old rats; ii) muscle spindles in the EDL of 21-day-old rats de-efferented at birth. For the characterization of myosins in intrafusal fibres we used three myosin antisera: antipectoral myosin, antiheart myosin and antiheart myosin adsorbed with muscle powder from the soleus muscle of guinea pig. We show in this study that during development intrafusal fibres change immunoreactivity and that in the absence of motor innervation bag fibres do not fully develop the myosin characteristics of control spindles. We conclude that the maturation of bag1 and bag2 fibres apparently requires next to the inductive influence of sensory axon terminals the presence and activity of fusimotor axons.     

Development of immunohistochemical characteristics of intrafusal fibres in normal and de-efferented rat muscle spindles

Summary Intrafusal muscle fibres in adult muscle spindles differ in their myosin composition. After selective motor denervation intrafusal muscle fibres develop mature ultrastructural characteristics. In order to evaluate the role of fusimotor innervation on the maturation of the myosin composition of intrafusal muscle fibres we have examined with immunohistochemical techniques i) the postnatal development of muscle spindles in new-born rats and in 7–21 day old rats; ii) muscle spindles in the EDL of 21-day-old rats de-efferented at birth. For the characterization of myosins in intrafusal fibres we used three myosin antisera: antipectoral myosin, antiheart myosin and antiheart myosin adsorbed with muscle powder from the soleus muscle of guinea pig. We show in this study that during development intrafusal fibres change immunoreactivity and that in the absence of motor innervation bag fibres do not fully develop the myosin characteristics of control spindles. We conclude that the maturation of bag₁ and bag₂ fibres apparently requires next to the inductive influence of sensory axon terminals the presence and activity of fusimotor axons.     

In vitro differentiation in the absence of nerve of avian myoblasts derived from slow and fast muscle rudiments

Slow anterior latissimus dorsi (ALD) and fast posterior latissimus dorsi (PLD) muscles of 9-day-old quail embryos were cultured in vitro without neurons for 1 to 12 weeks. Several differences could be observed between ALD- and PLD-derived cells. PLD cultures proliferated less rapidly than ALD cultures. ALD-derived muscle fibres exhibited wide Z lines, numerous mitochondria, and a poorly developed sarcotubular system, while PLD-derived muscle fibres exhibited narrow Z lines, few mitochondria, and an abundant sarcotubular system. Staining for myofibrillar ATPase revealed that all well-differentiated ALD-derived muscle fibres were of the beta' type, while PLD-derived fibres were of beta and beta R types. These results show that myoblasts from slow and fast muscle rudiments can express in vitro some of the characteristic features of slow and fast muscle fibres, independently of motor innervation.