"Slow" myosins in vertebrate skeletal muscle. An immunofluorescence study.

Specific antisera were raised in rabbits against column-purified myosins from a slow avian muscle, the chicken anterior latissimus dorsi (ALD), and a slow-twitch mammalian muscle, the guinea pig soleus (SOL). The antisera were labeled with fluorescein and applied to sections of muscles from various vertebrae species. Two distinct categories of the slow fibers were identified on the basis of their differential reactivity with the two antisera. Fibers stained by anti-ALD appear to correspond in distribution and histochemical properties to physiologically slow-tonic fibers, i.e., fibers that display multiple innervation and respond to stimulation with prolonged contractures. In mammals, only a minority of fibers in extraocular muscles and the nuclear bag fibers of muscle spindles were brightly labeled by this antiserum. In contrast, fibers labeled by anti-SOL in mammalian muscle appear to correspond in distribution and histochemical properties to physiologically slow-twitch fibers. Anti-SOL was also found to stain a population of fibers in reptiles, amphibians, and fishes that did not react, or reacted poorly, with anti-ALD; in avian muscle, only a minor proportion of the slow fibers were labeled by anti-Sol. these findings point to the existence of two antigenically distinct, though partly cross-reacting, types of "slow" myosin in vertebrate muscle.

Abnormal myomuscular junctions and AChE in a congenital neuromuscular disease.

We studied a patient with a congenital neuromuscular disease clinically characterized by ophthalmoplegia, slight limb muscle weakness, and normal electromyography. In the muscle biopsy, there were frequent examples of interdigitation of areas of muscle and extracellular spaces giving a fragmented appearance to muscle fibers. These structures resemble myomuscular junctions; acetylcholinesterase activity was present in the vicinity of these structures and on the muscle cell surface.

Immunohistochemical identification of slow-tonic fibers in human extrinsic eye muscles.

A fluorescent antiserum against myosin from chicken anterior latissimus dorsi muscle, which stains specifically the multiply innervated slow fibers of birds and amphibians, was applied to frozen sections of human extraocular muscles. A proportion of fibers in oculorotatory muscles were labeled by the antiserum. In contrast, no labeled extrafusal fiber was present in the levator palpebrae or in other body muscles. Serial study of sections stained for acetylcholinesterase and myosin ATPase showed that the labeled fibers in human oculorotatory muscles are multiply innervated and display acid-stable myosin ATPase activity.

Myomuscular junctions in re-innervated rat skeletal muscle.

Re-innervated extensor digitorum longus, soleus and plantaris muscles of the rat were studied after denervation performed at various postnatal ages. The muscle fibres, which normally run from tendon to tendon as independent units, were found to be very frequently connected by myomuscular junctions, both in the form of terminal insertions of one fibre into another and of lateral bridges which may join two or more muscle fibres at one or more levels. Positive reaction for AChE activity was demonstrated at the level of the junctions. Incubation for myosin ATPase activity showed that myomuscular junctions are only found between fibres of the same histochemical type, which in re-innervated muscles are usually aggregated in 'type groupings'. Ultrastructural features were similar in both forms of myomuscular junctions. The appearance is that of an interdigitation of muscle projections from neighbouring fibres, each projection being covered by a basement membrane with attached collagen fibrils. The finger-like projections at their endings contain vesicles and elongated cisternae filled with granular dense material. It is postulated that the synchronous activity of neighbouring fibres within the compact motor units of reinnervated muscles is a causal factor initiating the formation of myomuscular junctions.

The fate of newly formed satellite cells during compensatory muscle hypertrophy.

Compensatory hypertrophy was induced in the rat extensor digitorum longus muscle by extirpation of the synergistic tibialis anterior. Injections of 3H-thymidine (2 muCi/g body weight) was given at 24, 48, and 72 h after the operation. In animals killed immediately after the last injection both satellite cell nuclei and true muscle nuclei were labeled, whereas in animals killed 4 weeks later only true muscle nuclei were labeled. The results are interpreted as indicating incorporation of newly formed satellite cells into the fibers during compensatory hypertrophy.