Myotonic muscle in mouse: a light and electron microscopic study in serial sections

Capillaries with fenestrations around regenerating muscle fibers in the soleus muscle of the dystrophic (dy) mouse

To investigate the morphological changes in capillaries around regenerating muscle fibers in the dystrophic (dy) mouse, we examined capillaries in the soleus muscles of 30-day-old control and dy mice by electron microscopy. In the control mice, the intramuscular capillaries were continuous. In the dystrophic mice, the muscles contained degenerative muscle fibers and many small muscle fibers regenerating after necrosis; these fibers had centrally located nuclei. The capillaries in the dy mice were mostly continuous, but some had a narrow vascular lumen around regenerating muscle fibers, possibly indicating newly formed capillaries. Moreover, about 20% of the capillaries with a narrow vascular lumen had a small number (less than five) of fenestrae bridged by a single-layered diaphragm. These findings suggest that capillary networks around muscle fibers regenerating after necrosis are remodeled by newly formed capillaries, and some of these capillaries have fenestrae in the endothelium to increase the supply of nutrients and oxygen to the regenerating muscle fibers.

Inherited muscular disorder in mutant Japanese quail (Coturnix coturnix japonica): an ultrastructural study

Ultrastructural study of muscles taken from a mutant (LWC) strain of Japanese quail with myotonia showed type 2 fibre atrophy, ring fibre formation, sarcoplasmic masses, and "moth-eaten" fibres. In these abnormal fibres, the most characteristic feature was the loss of interconnection among the myofibrils, mitochondria, and T tubules. Apparently normal muscle fibres often showed mild changes, such as proliferation of T tubules and enlarged sarcoplasmic areas with increased glycogen granules and ribosomes at the periphery of the fibres. The study suggested that one possible cause of these ultrastructural changes was a defect in cytoskeleton of muscle cells, especially in intermediate filaments.

A new inherited muscular disorder in Japanese quails (Coturnix coturnix japonica)

Thirteen adult mutant (LWC strain) Japanese quails (Coturnix coturnix japonica), between the ages of 8 and 60 weeks were examined for a progressive muscular disorder. The disorder, inherited as an autosomal dominant trait, was clinically apparent as early as 28 days of age; it was characterized by generalized myotonia, muscle stiffness, and muscle weakness. Affected birds were identified by their inability to lift their wings vertically upward and by their inability to right themselves when placed on their dorsum. Electromyographic studies in two mutant quails showed high-frequency repetitive discharges comparable to those of myotonic runs. These discharges persisted after nerve resection. The distinctive histopathologic changes in the various muscles examined were ring fibers, sarcoplasmic masses, and internal migration of sarcolemmal nuclei. A slight decrease in the size of type IIB muscle fibers and a slight increase in the size of type IIA fibers were observed in the M. pectoralis thoracicus of affected quails. In older affected birds, inter- and intrafascicular fatty infiltration with replacement of type IIB fibers by fat cells was seen in the pectoral muscles. Single fiber necrosis, nonspecific lymphorrages, and variations in the muscle fiber size and shape were also noted. The typical muscle lesions and multisystem involvement, which was manifested by testicular degeneration and atrophy in the male LWC specimens and bilateral lenticular cataracts in 6 of 13 affected mutant quails, suggest resemblance of this new inherited muscular disorder to myotonic dystrophy in man.

Skeletal muscle changes associated with equine myotonic dystrophy

A progressive neuromuscular disorder in young horses, clinically apparent as early as 1 month of age, is characterized by generalized myotonia, muscle stiffness, muscle weakness and atrophy. Myotonia is identified by percussion dimpling and myotonic EMG discharges. Changes in one case included testicular hypoplasia, cataract formation, and glucose intolerance, indicating a systemic involvement. Pathologic changes in skeletal muscles from three affected foals were examined. Sarcoplasmic masses, ringed fibers, internal positioning of sarcolemmal nuclei, and nuclear rowing were among the primary histologic changes noted. Variation in fiber diameter size, especially atrophy, and type I predominance were also prominent changes. A neurogenic involvement was indicated by type grouping changes in several muscles.

Progressive myotonia in foals resembling human dystrophia myotonica

A severe and progressive neuromuscular disorder accompanied by clinical, electrophysiological, and pathological features resembling human dystrophia myotonica was observed in three foals. This disorder was apparent as early as 1 month of age and involved progressive skeletal muscle dysfunction, initially characterized by proximal muscle hypertrophy and hypertonicity with subsequent muscle stiffness, weakness, and atrophy. Multisystem involvement was manifested in one case by testicular hypoplasia, early cataract formation, and borderline glucose intolerance. Prolonged dimpling of these large rear-limb muscles was elicited by percussion. Myotonic discharges were identified by electromyography. Percussion dimpling and the typical myotonic discharges persisted after neuromuscular blockade. Distinctive histologic muscle changes included ring fibers, sarcoplasmic mass formation, variation in fiber diameter size, and internally positioned nuclei.

Scanning electron microscopic study of the neuromuscular junction of dystrophic mice

The structure of the end-plate regions of normal and dystrophic 3-month-old mice were studied by scanning electron microscopy after the presynaptic terminals were removed by hydrochloric acid treatment. Quantitative analysis revealed that the end-plate area correlated positively with the muscle fiber diameter in both the normal and dystrophic animals. However, the motor end-plate area was significantly smaller in the dystrophic mice. The total length of the primary cleft of an end-plate correlated positively with the end-plate area and with the muscle fiber diameter in both normal and dystrophic mice. However, the total length of the primary cleft of an end-plate was significantly shorter in dystrophic mice, especially in large-diameter muscle fibers. Finally, the end-plate of dystrophic mice was characterized by shorter primary clefts with less branching points. These changes of several morphometric characteristics of the postsynaptic membrane suggest that the functional denervation of the mouse dystrophic neuromuscular junction has a postsynaptic origin.

Reduction of postjunctional fold density and depth in dystrophic mice

A total of 769 neuromuscular junction profiles (on 221 muscle fibers) from 21 normal (C57BL/6J +/+), 6 heterozygous (C57BL/6J dy2J/+) and 22 homozygous dystrophic (C57BL/6J dy2J/dy2J) gastrocnemius were photographed in electron microscopy. The fast and slow twitch muscle fibers were differentiated by the width of the Z lines. The number of secondary postjunctional folds per unit length of synaptic cleft was lower in dystrophic than in normal animals. Intermediate values were obtained in heterozygous animals. The length of the postsynaptic membrane was measured and normalized by dividing it by the length of the cross-section through the synaptic cleft. This normalized length of postjunctional membrane was significantly reduced by 45% in dystrophic animals relative to normal animals and by 41% relative to heterozygous animals. The depth of the secondary folds was also significantly reduced in dystrophic animals relative to normal animals in both fast and slow twitch fibers. These morphometric changes of the postjunctional membrane were observed on NMJs whose nerve terminals were not significantly modified by the dystrophic process and were sometimes observed on apparently normal muscle fibers.

The three-dimensional cytoarchitecture and pattern of motor innervation of branched striated myotubes

Three-dimensional reconstructions of "regenerating" myotubes in the "degeneration-regeneration" regions and in the "regenerative" foci of the extensor digitorum longus muscle of the C57BL6J/dy2J myopathic mutant mouse were made from spaced serial ultrathin section. Complex branching and recombination occurred, involving myotubes which for extensive regions along their length appeared to be independent. No accumulation of specialized organelles occurred at the branching site. Continuous branches displayed multiple discrete motor endplates.

Muscle fiber necrosis in murine dystrophy

A sampling technique based on spaced serial ultrathin sections was used to examine the ultrastructure and cytoarchitecture of myofibers, which were necrotic throughout their length, in the dystrophic mutant mouse (C57Bl6J/dy2J). Regional variations along the length of the myofibers were described. The necrotic fibers extended the full length of the muscle fascicle and did not branch. In necrotic fibers which were free of invasive cells, the only nuclei found under the basal lamina were peripherally placed, euchromatic myonuclei, or nuclear membrane remnants. In these fibers, the nuclear population was approximately 14% of the nuclear population of "healthy" fibers from the same animals. No myosatellite cells were observed to be associated with these necrotic fibers. In necrotic fibers, which had undergone foreign cell invasion, the existence of a pleomorphic population of mononucleated invasive cells, many of which had not begun to express phagocytic characteristics, caused considerable confusion, because their appearance tended to mimic that of myonuclei or satellite cells. No heterochromatic nuclei or true myosatellite cells were found associated with necrotic fibers displaying macrophage invasion.

Necrotic extrafusal muscle fibers of the dystrophic mutant mouse: the ultrastructure of the myoneural junction

At 28 days postpartum, the extensor digitorum longus muscle of the dy2J mutant mouse contains a population of myofibers which exhibit coagulation necrosis for approximately 90% of their length. Using the electron microscope, motor endplates were found on more than half of the necrotic fibers studied, occurring in mildly, moderately, and severely necrotic regions of these fibers. The ultrastructural features of the axonal terminals did not vary with the condition of the fiber segment at which the endplate occurred. No morphological criteria could be established for distinguishing between the axonal terminals of necrotic fibers and those of "healthy" fibers in the dystrophic animal. The principle morphological changes at motor endplates of necrotic fibers involved not the axonal terminal, but the muscle fiber itself. This study demonstrates that the necrotic myofibers, which are present at the onset of the first clinical symptoms of murine dystrophy, are innervated. Therefore, necrosis is not precipitated by structural denervation. Furthermore, observations of motor endplates on mildly, moderately, and severely necrotic regions of the myofibers indicate that regional changes along the necrotic fiber's length are not a function of distance from the motor endplate.

Some factors affecting spontaneous transmitter release in dystrophic mice

Neuromuscular junctions of slow- and fast-twitch skeletal muscles from dystrophic (dy2J/dy2J) and control mice of the C57BL/6J strain were used to investigate the effect of muscular dystrophy on nerve-terminal regulation of their intracellular concentration of free calcium ions. The frequency of spontaneous miniature endplate potentials (MEPPs) was taken as an indicator of the intraterminal free calcium ion concentration. Dicoumarol, 2,4-dinitrophenol, ruthenium red, and the calcium ionophore A-23187 all potentiated the MEPP frequency in dystrophic muscles at concentrations which had negligible effects on normal muscles. Dystrophic muscle preparations were also more sensitive to an increased extracellular calcium concentration. Usually, these manipulations had more effect on the nerve terminals of dystrophic slow muscle than on those of dystrophic fast muscle. We conclude that muscular dystrophy alters the nerve terminal's ability to regulate the concentration of intracellular free calcium ions.