Regeneration of a tonic avian muscle: transformation to a twitch morphology

Properties of standard avian slow muscle grafts following long-term regeneration

The tonic anterior latissimus dorsi (ALD) of adult pigeons was orthotopically homografted and evaluated after 11 months of regeneration for histological, histochemical, electromyographic (EMG), and mechanical properties. The resting EMG activity of the grafts was lower in amplitude than that of the controls, but showed the tonic pattern typical for these tonic muscles. The control and grafted muscles had a histochemically homogeneous population of fibers with moderate myofibrillar adenosine triphosphatase activity. Succinic dehydrogenase activity was moderate for the control muscles, but low for the grafts. The regenerated muscles had fewer and smaller fibers and had much larger intersynaptic distances. Both the regenerated and the contralateral control muscles were slow contracting and maintained tetanic tension for prolonged periods with direct electrical stimulation. The relaxation was slower in the grafted muscle than in the control. The grafts produced 40% of the maximum tension of the control muscles, but the rate of tension development was similar between the two groups. The results indicate that the tonic properties were regenerated, but the innervation pattern was altered and the grafted muscles did not have normal mature fibers even after long-term regeneration.

The development of fibre types in grafts of a slow tonic avian muscle, the dorsocutaneous latissimus dorsi

The dorsocutaneous (DLD) and anterior (ALD) latissimus dorsii are both homogeneous slow tonic muscles. Autografts of mature DLD were attached onto the ALD of chickens to study regeneration of slow tonic muscle fibres innervated exclusively by slow tonic nerves. Fifty-three grafts were examined from 3 to 231 days after implantation for myosin ATPase, and for heavy chains of fast myosin. New muscle fibres in grafts were initially type 1 (slow) or type 2 (fast twitch). Tonic type 3 fibres were slow to differentiate and were not seen within 59 days. From 105 days many fibres were type 3A and type 1 were no longer apparent. However, type 2 fibres persisted and appeared to be present instead of type 3B fibres even after 8 months.

Conversion of muscle fiber types in regenerating chicken muscles following cross-reinnervation

Slow-tonic anterior latissimus dorsi (ALD) and fast-twitch posterior latissimus dorsi (PLD) muscles of 7 to 10-day-old White Leghorn chickens were crushed and allowed to be reinnervated by their own nerve, or crushed and transplanted to the other side and allowed to be reinnervated by the nerve of the side to which they were transplanted. Following transplantation, changes in the weight of the muscle, fiber-type composition and innervation pattern during regeneration were investigated. Normal growth rate of PLD was about twice that of ALD. Regenerating PLD, however, atrophied rapidly after crushing and denervation whether innervated by its own nerve or the other nerve type, whereas ALD reinnervated by its own nerve showed marked hypertrophy. PLD fibers transformed rapidly to fast-twitch alpha or slow-tonic (ST) fibers when they were reinnervated by PLD or ALD nerve, respectively. When ALD fibers were reinnervated by their own nerve, they differentiated into ST fibers that were surrounded by smaller immature fibers. ALD fibers were, however, resistant to complete control by fast-twitch PLD nerve and contained a large number of slow fibers (ST and beta) long after transplantation. Slow fibers in regenerates were initially multiply innervated, but later transformed into fast-twitch alpha fibers that were focally innervated. The mode of differentiation and innervation pattern of different muscle fiber types in regenerating muscles are discussed.

Reconstruction of the contractile apparatus of striated muscle. I. Muscle maintained in extension

The ultrastructure of the contractile apparatus was observed in muscles maintained in excessive extension, i.e. in conditions in which an increase takes place in the number of sarcomeres. Rat leg muscles (soleus, extensor digitorum longus and gastrocnemius) were studied, at variable time intervals in the range 3-7 days. Several irregularities were found in the contractile structure. The most frequent were the variability of sarcomere length, the appearance of 'extra' sarcomeres, irregularities of the Z-line (including Z-band 'streaming') and A-bands of abnormal length. The character of these irregularities depended on the muscle fibre type. Variations of the Z-line were seen mostly within continuously working fibres, especially slow ones, while anomalies in the size of the A-band and variability of the sarcomere length were more pronounced in fast fibres. All these irregularities appearing in the muscles maintained in excessive extension were also occasionally found in control muscles. The reasons for these contractile structure irregularities, and their possible significance for contractile structure reorganization, are discussed.

Enhanced reappearance of fast fibers in regenerating crayfish claw closer muscles

In the pristine claws of adult crayfish the muscle fibers of the closer are all of slow type as judged by sarcomere lengths of greater than 6 micron, and a uniform degree of myofibrillar ATPase activity. In regenerating claws of mature and immature crayfish, the muscle has a central band of fast type fibers as characterized by shorter sarcomeres (less than 6 micron) and a higher degree of ATPase activity than the surrounding slow fibers. During primary development, the closer muscle has a fiber composition similar to that of the regenerating muscle except for a smaller proportion of fast fibers. Thus the reappearance of fast fibers during regeneration recapitulates ontogeny while their enhanced proportions may reflect epigenetic influences such as restriction of nerve-mediated muscle activity in the limb bud.

Skeletal muscle-smooth muscle interaction: an unusual myoelastic system

The serratus superficialis metapatagialis (SSM) of pigeons is a skeletal muscle with unusual properties. It lies between the ribs and the trailing edge of the wing, where it is attached to the skin by a system of smooth muscles having elastic tendons. Wing movements during flight induce marked changes in this muscle's length. The SSM inserts onto the deep fascia, and at its termination the skeletal muscle contains large numbers of microtubules. Many myofibrils attach to leptomeric organelles, which then attach to the terminal end of the skeletal muscle fiber. The deep fascia next connects to the dermis of the skin by bundles of smooth muscles that have elastic tendons at both ends. This system allows large movements of the muscle while preventing its fibers from overstretching. The movements and presumed forces acting at this muscle make the presence of sensory receptors such as muscle spindles unlikely. Spindles are absent in this muscle.

Patterns of avian muscle fiber type regeneration. Evidence for a myotrophic influence

Serratus superficialis metapatagialis (SSM) and posterior biventer cervicis (BVC) muscles were free-grafted into the site of the slow tonic anterior latissimus dorsi (ALD) of adult pigeons. Grafts were prepared for histochemical examination at 1, 3, 7, 12 and 15 days; at monthly intervals from 1 to 4 months; and at various intervals up until 23 months after surgery. Few original muscle fibers survived the trauma of grafting and contributed to the regenerated muscle. The arrangement of fast and slow fibers and the percentage of slow fibers were recorded for each graft at least one month of age. Young grafts of the SSM (one to four months of age) regenerated a segregated pattern of fast and slow fibers that resembled control SSM muscles, whereas early grafts of the BVC regenerated a mosaic arrangement characteristic of normal BVC muscles. The percentages of slow fibers in both SSM and BVC grafts up to 4 months after surgery did not significantly differ from control muscles. Both SSM and BVC grafts older than 4 months of age had a significantly higher percentage of slow fibers than control muscles. These data indicate that the donor muscle influences the pattern of fiber types that is originally manifested and this pattern is maintained relatively intact for 4 months. Eventually the nerve at the host site modifies this pattern, and the fibers of the graft assume the histochemical characteristics of the muscle removed from the host site. In addition, the time required for histochemical stabilization of muscle fibers of avian heterotopic autografts is longer than that for whole muscle grafts in rats or avian minced muscle preparations, and is comparable to tenotomized avian muscle.

Tenotomy of the avian anterior latissimus dorsi muscle. I. Effect of age on fiber-type transformation and regeneration from the stump in chicks

The chick's anterior latissimus dorsi muscle (ALD) was tenotomized at its origin at either 1 day or 4 weeks of age, and investigated histochemically and ultrastructurally at intervals thereafter to determine whether muscle fiber-type transformation from a slow to a twitch type is greater in young birds than older birds. No transformation of fiber type occurred in either procedure, but a new muscular connection regenerated between the scar tissue at the end of the original tenotomized stump and the former origin. This regenerated muscle had a mosaic pattern of fiber types, as demonstrated by myofibrillar ATPase activity, and contained predominantly fast fibers, as contrasted with controls or the tenotomized portion, which contained predominantly slow tonic muscle fibers. The regenerated portion contained muscle spindles. The original portion of the tenotomized muscle was indistinguishable from the control muscle. These responses of the chick ALD to tenotomy are quite different from those in the pigeon, which are reported in the following study.

Tenotomy of the avian anterior latissimus dorsi muscle. II. Can regeneration from the stump occur in the pigeon?

Because the chick's anterior latissimus dorsi muscle (ALD) regenerates a fast-twitch muscular connection after tenotomy, the pigeon's ALD was tenotomized, either at the origin or through the muscle 0.5 cm from the origin, to determine whether this muscle behaves similarly to the chick muscle. These procedures were compared in pigeons operated upon at 7 weeks, versus 5 to 9 months of age. The pigeon's ALD did not regenerate a new connection, and other differences were observed between the pigeon and chick ALD. The pigeon ALD has only a single slow muscle-fiber type, has fewer fast fibers, and transforms to a fast-twitch muscle more readily than the chick ALD after tenotomy. The transformation of muscle fiber types occurred more readily in the older pigeons than those tenotomized at 7 weeks of age. Tenotomy induced morphological alterations of the muscle fiber structure in all of the pigeons, which is in contrast to the absence of changes in the tenotomized chick ALD. Therefore the pigeon and chick ALD respond completely differently to tenotomy.

Cellular responses to free grafting of the extensor digitorum longus muscle of the rat

The cellular and subcellular responses related to the survival or destruction and subsequent regeneration of muscle fibers within the freely grafted extensor digitorum longus muscle of the rat were examined by light and electron microscopy. A small number of fibers at the periphery of the grafts survived the initial ischemia but underwent denervation changes and accumulated lipid deposits. The majority of fibers in the grafts, however, became ischemic and underwent an intrinsic degeneration within 4 hours. Cell-mediated destruction of the degenerating fibers occurred as the grafts became revascularized. The basal laminae and some of the satellite cells were the only elements of the original fibers that persisted. Regeneration began at the periphery of the graft within three days after grafting and reached the center about three days later. After phagocytosis of the original fibers, presumptive myoblasts within the grafts differentiated into myoblasts and myotubes. The formation of myotubes followed a biphasic pattern of development comparable to that of normal fetal muscle. Although most of the myotubes were formed within the basal lamina remaining from the original fiber, there was also evidence for regeneration outside the basal lamina. Myotubes matured into muscle fibers which were essentially normal in apperance when examined up to 180 days after grafting. Some fibers, however, were atrophic, presumably due to a failure to become innervated, and some fibers were joined by myo-myous junctions. Pre-denervated grafts and Marcaine-treated grafts were also examined. There were more surviving fibers in pre-denervated grafts, and cell-mediated destruction of degenerating fibers proceeded more rapidly than in normal grafts. No surviving fibers were found in Marcaine-treated grafts. The changes in these grafts were otherwise similar to normal grafts. A schematic model of the spatial and temporal sequence of degeneration and regeneration within a free muscle graft is presented.