THE ULTRASTRUCTURE OF THE NEUROMUSCULAR JUNCTIONS OF MAMMALIAN RED, WHITE, AND INTERMEDIATE SKELETAL MUSCLE FIBERS

Distinct ultrastructural differences exist at the neuromuscular junctions of red, white, and intermediate fibers of a mammalian twitch skeletal muscle (albino rat diaphragm). The primary criteria for recognizing the three fiber types are differences in fiber diameter, mitochondrial content, and width of the Z line. In the red fiber the neuromuscular relationship presents the least sarcoplasmic and axoplasmic surface at each contact. Points of contact are relatively discrete and separate, and axonal terminals are small and elliptical. The junctional folds are relatively shallow, sparse, and irregular in arrangement. Axoplasmic vesicles are moderate in number, and sarcoplasmic vesicles are sparse. In the white fiber long, flat axonal terminals present considerable axoplasmic surface. Vast sarcoplasmic surface area is created by long, branching, closely spaced junctional folds that may merge with folds at adjacent contacts to occupy a more continuous and widespread area. Axoplasmic and sarcoplasmic vesicles are numerous. Both axoplasmic and sarcoplasmic mitochondria of the white fiber usually contain intramitochondrial granules. The intermediate fiber has large axonal terminals that are associated with the most widely spaced and deepest junctional folds. In all three fiber types, the junctional sarcoplasm is rich in free ribosomes, cisternae of granular endoplasmic reticulum, and randomly distributed microtubules.     

THE ULTRASTRUCTURE OF MAMMALIAN CARDIAC MUSCLE

Papillary muscles of rat and dog hearts were fixed in such a way as to prevent excessive shortening during the procedure. The material was embedded in either araldite or methacrylate and was stained in various ways. The filamentous fine structure of mammalian cardiac muscle is similar to that previously described for striated skeletal muscle. The sarcomeres are composed of a set of thick and thin filaments which interdigitate in the A band proper. The filament ratios and the filamentous array are in accord with those found in skeletal muscle. The functional significance of this twofold array of filaments is not entirely clear. Various other structural aspects of cardiac cells such as surface membranes, mitochondria, nuclei, and cytoplasmic granules are described. The sarcoplasmic reticulum is discussed in detail as are the various structural components forming the intercalated discs. Fairly frequent deep invaginations of the sarcolemma with basement membrane are observed in addition to the intercalated discs. These surface membrane invaginations probably explain the branching appearance of cardiac fibers as seen under the light microscope.     

CYTOCHEMICAL STUDIES OF ADENOSINE TRIPHOSPHATASES IN SKELETAL MUSCLE FIBERS

Mitochondrial ATPase and myosin ATPase have been localized in the muscle fibers of the rat diaphragm. The principal fiber type possesses a structure favorable for making this cytochemical separation with the light microscope. This small red fiber has numerous large, nearly spherical, mitochondria (ca. 1.5 µ) which are aggregated beneath the sarcolemma. In the interior of the fiber, smaller paired filamentous mitochondria (ca. 0.2 µ diameter) are aligned with the I band. Distribution of mitochondria was determined by sudanophilia, succinic dehydrogenase activity, and by direct examination with the electron microscope. ATPase activity at pH 7.2 is located in the large peripheral mitochondria and in the smaller mitochondria associated with the I band. The alignment of the small mitochondria results in a discrete cross-striated appearance in fibers stained for this enzymic activity. This mitochondrial ATPase does not cleave adenosine diphosphate or adenosine monophosphate; it is not sulfhydryl dependent and, in fact, is enhanced by the mercurial, p-hydroxymercuribenzoate. It requires magnesium ion and is stimulated by dinitrophenol. It is inhibited after formol-calcium fixation, but the residual activity is demonstrable by lengthening the incubation time. At pH 9.4 the ATPase is myofibrillar in origin and is located in the A bands. This myosin ATPase activity is sulfhydryl-dependent. Mercurial at this high pH has an interesting dual effect: it suppresses myosin ATPase but evokes mitochondrial ATPase activity. A third type of ATPase activity can be demonstrated, especially in the large white fibers. This activity occurs at pH 7.2 in the presence of cysteine. Its position is manifested cytochemically as a fine reticular pattern which surrounds individual myofibrils. The distribution suggests that it may originate in the sarcoplasmic reticulum.     

THE ULTRASTRUCTURE OF THE Z DISC IN SKELETAL MUSCLE

This electron microscopic study deals with the structure of the Z disc of frog's skeletal muscle, with special regard to the I filaments—whether they pass through the Z disc or terminate at it. In most longitudinal sections the I filaments terminate as rod-like projections on either side of the Z disc, one I filament on one side lying between two I filaments on the opposite side. This indicates that the I filaments are not continuous through the Z disc. The rod-like projections are often seen to consist of filaments (denoted as Z filaments) which meet at an angle. In cross-sections through the Z region the I filaments and Z filaments form tetragonal patterns. The I filaments are situated in the corners of the squares; the oblique Z filaments form the sides of squares. The tetragonal pattern formed by the Z filaments is rotated 45 degrees with respect to the tetragons formed by the I filaments on both sides of Z. This structural arrangement is interpreted to indicate that each I filament on one side of the Z disc faces the center of the space between four I filaments on the opposite side of Z and that the interconnection is formed by four Z filaments.     

THE ULTRASTRUCTURE OF STRIATED MUSCLE AT VARIOUS SARCOMERE LENGTHS

1. Rest and equilibrium length muscle sarcomeres are composed of thin filaments (actin) which traverse the sarcomeres from the Z membranes up to the H band; at this level the filaments are considerably thicker and less numerous. 2. Shortening of muscle is associated with a transformation of thin into thick filaments in the A band. 3. These observations are discussed in terms of interaction of actin and myosin to form a supercoiled structure as the basis of contraction.     

CONTRACTILITY AND ULTRASTRUCTURE IN GLYCEROL-EXTRACTED MUSCLE FIBERS : I. The Relationship of Contractility to Sarcomere Length

This study was undertaken to determine whether glycerol-extracted rabbit psoas muscle fibers can develop tension and shorten after being stretched to such a length that the primary and secondary filaments no longer overlap. A method was devised to measure the initial sarcomere length and the ATP-induced isotonic shortening in prestretched isolated fibers subjected to a small preload (0.02 to 0.15 P₀). At all degrees of stretch, the fiber was able to shorten (60 to 75 per cent): to a sarcomere length of 0.7 µ when the initial length was 3.7 µ or less, and to an increasing length of 0.9 to 1.8 µ with increasing initial sarcomere length (3.8 to 4.4 µ). At sarcomere lengths of 3.8 to 4.5 µ, overlap of filaments was lost, as verified by electron microscopy. The variation in sarcomere length within individual fibers has been assessed by both light and electron microscopic measurements. In fibers up to 10 mm in length the stretch was evenly distributed along the fiber, and with sarcomere spacings greater than 4 µ there was only a slight chance of finding sarcomeres with filament overlap. These observations are in apparent contradiction to the assumption that an overlap of A and I filaments is necessary for tension generation and shortening.     

FINE STRUCTURE OF RAT INTRAFUSAL MUSCLE FIBERS : The Equatorial Region

An ultrastructural study has been undertaken on the equatorial (sensory) region of the rat muscle spindle. Two kinds of intrafusal muscle fibers, a nuclear bag fiber and a nuclear chain fiber, have been identified in this region on the basis of fiber diameter, nuclear disposition, and M-band appearance. The large-diameter nuclear bag fiber contains an aggregation of tightly packed vesicular nuclei, while the small-diameter nuclear chain fiber contains a single row of elongated, well-separated nuclei. Both muscle fibers contain an attenuated peripheral cylinder of myofilaments surrounding a central core of sarcoplasm. Elements of the sarcotubular system, dilatations of the sarcoplasmic reticulum, and the presence of other sarcoplasmic organelles and inclusions are considerably more abundant in the nuclear chain fiber than in the nuclear bag fiber. Leptomeric organelles and membrane-bounded sarcoplasmic granules are present in both intrafusal fiber types and may be situated between the myofibrils or in intimate association with the sarcolemma. The functional significance of some of these structural findings is discussed.     

中国大鲵卵母细胞发育的显微和超微结构

用光镜和电镜观察了大鲵卵母细胞在发育过程中的显微和超微结构变化，着重对类核周体结构和线粒体与卵黄前颗粒的关系进行了详尽观察。贴近卵核的类核周体由核仁样体和线粒体群构成，远离卵核的类核周体仅由线粒体群构成。线粒体群是线粒增殖区，其中有多种形态的原线粒体，有些处于增殖状态，它们未形成明显的线粒体嵴。散在于卵质中的线粒体是成的线料体，有明显的嵴，其中许多线粒体内沉积着致密物质，一些致密物质从线粒体中向外     

THE ULTRASTRUCTURE AND DISPOSITION OF VESICULATED NERVE PROCESSES IN SMOOTH MUSCLE

The walls of the gastrointestinal tract and urinary bladder of rats were fixed in osmium tetroxide, embedded in methacrylate, and sectioned for electron microscopy. The examination of sections of smooth muscle tissue with the electron microscope reveals the presence of bundles of unmyelinated nerve fibers within the intercellular spaces. In addition, vesiculated nerve processes, bounded on their outer surfaces by delicate plasma membranes and typically containing varying quantities of synaptic vesicles and mitochondria, make intimate contact with the surface of smooth muscle cells. These nerve processes are similar in structure and disposition to nerve endings previously described in skeletal muscle, in the central nervous system, in peripheral ganglia, in receptors, and in glands. It is concluded that the relationships existing between vesiculated nerve processes and the surface of smooth muscle cells constitute neuromuscular junctions. Profiles of protrusions of smooth muscle cells are often seen protruding into the intercellular spaces. Here they occur singly or in groups, originating from one or more cells. Because of the plane of section the protrusions may sometimes appear as individual entities between the muscle cells. In such cases care must be exercised in their identification because they have characteristics similar to sectioned nerve processes which also occur in the intercellular spaces.     

ULTRASTRUCTURAL ORGANIZATION OF OBLIQUELY STRIATED MUSCLE FIBERS IN ASCARIS LUMBRICOIDES

The somatic musculature of the nematode, Ascaris, is currently thought to consist of smooth muscle fibers, which contain intracellular supporting fibrils arranged in a regular pattern. Electron microscopic examination shows that the muscle fibers are, in fact, comparable to the striated muscles of vertebrates in that they contain interdigitating arrays of thick and thin myofilaments which form H, A, and I bands. In the A bands each thick filament is surrounded by about 10 to 12 thin filaments. The earlier confusion about the classification of this muscle probably arose from the fact that in one longitudinal plane the myofilaments are markedly staggered and, as a result, the striations in that plane of section are not transverse but oblique, forming an angle of only about 6° with the filament axis. The apparent direction of the striations changes with the plane of the section and may vary all the way from radial to longitudinal. A three-dimensional model is proposed which accounts for the appearance of this muscle in various planes. Z lines as such are absent but are replaced by smaller, less orderly, counterpart "Z bundles" to which thin filaments attach. These bundles are closely associated with fibrillar dense bodies and with deep infoldings of the plasma membrane. The invaginations of the plasma membrane together with intracellular, flattened, membranous cisternae form dyads and triads. It is suggested that these complexes, which also occur at the cell surface, may constitute strategically located, low-impedance patches through which local currents are channeled selectively.     

家兔晶状体纤维的超微结构扫描电镜和冰冻断裂的研究

关于脊椎动物眼睛晶状体纤维细胞的研究,已有不少报道,但涉及到其细胞表面结构的有关工作还不多。至于不同部位细胞的结构,特别是晶状体核纤维细胞的构形,报道也不完全一致。本文主要介绍用常规扫描电镜以及冰冻断裂扫描电镜技术,研究家兔晶状体纤维细胞的三维结构。Hansson(1970);Sakuragawa和Kuwabara(1975);Nelson和Rafferty(1976)均以小自鼠为材料。Farnsworth(1974)以及Sakuragawa(1975)分别用大白鼠为研究对象。他们先后开展了晶状体的扫描电镜研究,并对该课题的发展作出了一定贡献。Dickson和Crock(1972;1975)将冰冻断裂扫描电镜技术引入对猴晶状体     

ULTRASTRUCTURE AND CALCIUM TRANSPORT IN CRUSTACEAN MUSCLE MICROSOMES

Fragmented sarcoplasmic reticulum (FSR) from crustacean muscle was examined following preparation by a variety of electron microscopic techniques. The 30–40 A particles which appeared on the outer surface of FSR vesicles following negative staining were not observed following preparation by freeze-drying, freeze-etching, thin sectioning, or critical-point drying. Crustacean FSR exhibited high values of calcium uptake and extensive nodular formation in the presence of oxalate. 80–90 A diameter membrane particles were seen in freeze-etch preparations of both intact lobster muscle and FSR vesicles. Thin sections of FSR vesicles revealed a membrane thickness of 60–70 A. The membrane appeared to be triple layered, each layer having a thickness of 20–25 A.     

THE STRUCTURE OF INTERSEGMENTAL MUSCLE FIBERS IN AN INSECT, PERIPLANETA AMERICANA L

The organization of intersegmental muscle fibers associated with the dorsal abdominal sclerites of the cockroach is described. These fibers correspond closely, in the disposition and derivation of the membranes of the transverse tubular system and sarcoplasmic reticulum cisternae, with insect synchronous flight muscle fibers, but differ markedly from these in their fibrillar architecture and mitochondrial content. The mitochondria are small and generally aligned alongside the prominent I bands of the sarcomere, and, in the best-oriented profiles of the A bands, thick filaments are associated with orbitals of twelve thin filaments, a configuration that has also been observed in striated fibers of insect visceral muscle. These structural features of insect muscles are compared and discussed in terms of possible variations in the control of contraction and relaxation, and in the nature of their mechanical role.     

去神经对快,慢肌纤维肌球蛋白ATPase影响的组织化学观察

本文用组织化学方法观察了豚鼠比目鱼肌(SOL)和腓骨第三肌(PT)在去神经后其快、慢纤维肌球蛋白ATPase特性的变化。在正常肌肉中Ⅰ型(慢)纤维和Ⅱ型(快)纤维分别具有酸和碱稳定ATPase活性。慢纤维在去神经后出现了碱稳定ATPase活性,而快纤维则无明显变化。结果表明,只有慢纤维的肌球蛋白ATPase特性才与神经支配有关。