THE FINE STRUCTURE OF STRIATED MUSCLE : A COMPARISON OF INSECT FLIGHT MUSCLE WITH VERTEBRATE AND INVERTEBRATE SKELETAL MUSCLE

The available evidence from phase contrast, polarization optical, and electron microscopic studies on vertebrate skeletal muscle, insect skeletal muscle, and dipteran flight muscle is interpreted as favoring the following general structure of striated muscle. A continuous array of filaments (actin) runs through all bands of the sarcomere. These are linked by an axially periodic system of transverse filamentous bridges. Myosin (and probably other substances) are localized in the A bands. The system of transverse bridges compensates the birefringence of actin and is thus responsible for the isotropy of the I band. Myosin is responsible for the birefringence of the A bands. On strong contraction, A band material migrates to the Z bands to form contraction bands. It is not yet certain whether this migration involves myosin or another A band component.     

BOUND WATER IN MUSCLE

1. The amount of free unfrozen water, i.e. water acting as normal solvent, in frog''s muscle at temperatures below the initial freezing-point has been calculated from the vapour pressure isotherm of the muscle. 2. Significant amounts of free water are present at –20°C. The total amount of unfrozen water at –20°C. cannot, therefore, be taken as a measure of the bound water in muscle. 3. The calculated values of free water, when compared with experimentally determined values of total unfrozen water, indicate that the amount of bound water in muscle at various temperatures is small. 4. A temperature considerably below –20°C., roughly between –40° and –60°C., is required to freeze completely the free water in muscle.     

爪蟾肌细胞的决定

爪蟾肌细胞在什么时期决定尚未有过报道,本文采用体外培养的方法,取原肠晚期到神经褶中期等五个不同时期检测肌细胞的决定。结果表明,这段期间预定肌节在离体培养条件下分化程度有明显差异,直到神经胚中期几乎所有组织块都分化为肌细胞。因此认为爪蟾肌细胞的决定在神经胚中期达到稳定的状态。     

FILAMENT LENGTHS IN STRIATED MUSCLE

Filament lengths in resting and excited frog muscles have been measured in the electron microscope, and investigations made of the changes in length that are found under different conditions, to distinguish between those changes which arise during preparation and the actual differences in the living muscles. It is concluded that all the measured differences in filament length are caused by the preparative procedures in ways that can be simply accounted for, and that the filament lengths are the same in both resting and excited muscles at all sarcomere lengths greater than 2.1 µ, viz., A filaments, 1.6 µ; I filaments, 2.05 µ. The fine periodicity visible along the I filaments also has been measured in frog, toad, and rabbit muscles and found to be 406 A.     

DISEASED MUSCLE CELLS IN CULTURE

(1) Cultures of differentiated muscle cells have been grown from diseased human, mouse and chick skeletal muscle, and from cardiac muscle of the myopathic hamster. (2) Methods of culture established for normal embryonic and adult skeletal muscle cells have proved suitable for cultures of diseased muscle cells. (3) Myoblasts obtained from dy^2J mouse muscle crushed in vivo before explanting fuse in culture and form morphologically normal myotubes. Studies of the effects of innervation by dy^2J spinal cord neurones on the differentiation of normal, dy^2J and dy myotubes have been inconclusive but it is probable that innervation does not play a part in the pathogenesis of this disorder. (4) Myoblasts prepared by trypsinization of embryonic dy muscle behave normally in culture and fuse to form myotubes that appear normal. It is not clear if myoblasts that migrate from explants of adult muscle in vitro fuse. Aggregates of non-fusing cells have been described, but under other culture conditions normal and abnormal forms of myotube have been observed. dy muscle fibres fail to regenerate even when cultured with normal spinal cord explants and dy nerves are without effect on regenerating normal muscle fibres. These tissue-culture studies suggest that the dy mouse mutation is a myopathic disorder. (5) Embryonic mdg myoblasts have a normal cell cycle in vitro and fuse to form well-differentiated myotubes with cross-striations. mdg myotubes have normal electro-physiological properties but do not contract spontaneously or on depolarization. The defect in the muscle of the mdg mutant appears to be a failure of excitation-contraction coupling. (6) Cells migrate earlier from explants of adult dystrophic chick muscle than from normal muscle but dystrophic chick myotubes appear morphologically normal. Myotubes prepared from embryonic dystrophic chick muscle become vacuolated and degenerate, changes that can be prevented by anti-proteases such as antipain. Lactic dehydrogenase isozyme subunit M4 is absent from dystrophic muscle in vivo but reappears in cultured myotubes. Dystrophic myotubes innervated in culture by either normal or dystrophic neurones exhibit bi-directional lcoupling and multiple innervation. These results suggest that there are changes in dystrophic myotubes and that chick muscular dystrophy is a myopathy. (7) Cardiac muscle cells from the cardiomyopathic hamster synthesize less actin and myosin than normal cells, and Z lines in dystrophic cells are irregularly arranged. The beat frequency of myopathic cardiac cells is lower than that of normal cells and declines more rapidly. Tissue-culture studies have not been made of hamster skeletal muscle. (8) Human dystrophic myotubes do not show degenerative changes in culture and have normal histochemical reactions. RNA synthesis appears normal in dystrophic myotubes but there may be changes in adenyl-cyclase activity and protein synthesis in dystrophic cells. Morphological and biochemical changes have been found in muscle cells cultured from a case of acid-maltase deficiency but phosphorylase activity re-appeared in myotubes cultured from biopsies of phosphorylase-deficient muscle. Innervation by normal mouse nerves does not induce degenerative changes in dystrophic myotubes. (9) Studies on the origins of myoblasts in explants of muscle fibres in culture suggest that in these conditions myoblasts are derived only from satellite cells and that this process may be the same in normal and diseased muscle.     

THE POTASSIUM EQUILIBRIUM IN MUSCLE

1. Analyses were made of the K and HCO₃ content, the irritability, and weight change of isolated frog sartorius muscles after immersion for 5 hours in Ringer''s solutions modified as to pH and potassium content. 2. At each pH a concentration of potassium in the solution was found which was in diffusion equilibrium with the potassium in the muscle. In greater concentrations potassium moved into the muscle against the concentration gradient and vice versa. 3. The greater the alkalinity of the solution the smaller the concentration of the potassium at equilibrium so that the product of the concentrations of OH and K in the solution at equilibrium tends to remain approximately constant. 4. The pH inside the muscle is approximately equal to that outside when first dissected but it tends to change during immersion so as to follow the changes in the pH of the solution. This finding is in direct conflict with the theory according to which the high potassium concentration inside should be accompanied by an equally high hydrogen ion concentration in relation to that outside. 5. The diffusion of potassium into the muscle makes its contents more alkaline but the increase in alkalinity is not always, nor usually, equivalent to the amount of potassium which has diffused and conversely, the pH inside can change in either direction according to the pH outside without there being any diffusion of potassium. Hence potassium is not the only penetrating ion. 6. The irritability of the muscles is at a maximum in concentrations of potassium which are greater than that in normal Ringer''s solution, or about 20 mg. per cent potassium. This optimum does not seem to be a function of pH and is therefore not dependent upon the direction of movement of the potassium but probably on the ratio of potassium outside to that inside. 7. Swelling of the muscles occurs in solutions which injure the muscle so as to permit both cations and anions to enter without permitting the organic protein anions to escape. Anion impermeability is necessary to prevent this same osmotic swelling under normal conditions. 8. An increase in the CO₂ tension in muscle and solution causes a greater increase in acidity in the solution than in the muscle and leads to a loss of potassium. One expects therefore a potassium shift from tissues to blood comparable to the chlorine shift from plasma to corpuscles.     

在鸡慢肌神经支配下由快肌碎片形成的新生肌肉保持快肌的某些性质

本工作使用肌肉的切碎移植的方法来作快、慢肌肉的运动神经的交叉支配。将鸡右侧前背阔肌(慢)摘除而使其神经留在原位,再将左侧后背阔肌(快)取出切碎并放在右前肌的空位(第一组),在另一组动物是将右前肌摘出切碎后仍放回原位(第二组)。数月后用电刺激慢神经并用等长杠杆记录肌肉的收缩。第一组动物的新生肌肉的单收缩显然比第二组快些,而且在高频(250/s)刺激下表现抑制现象。第二组动物的新生肌肉不但收缩较慢,而且与前背阔肌一样不产生高频抑制。上述现象证明鸡的由快肌碎片形成的新生肌肉仍保持快肌的某些性质,而且在慢肌神经支配下仍能如此。本工作也进一步提供了维金斯基抑制与神经无关而与肌肉有关的证据。本文推测肌肉很可能是通过其卫星细胞而把某些性质传给新生肌肉的。