The structure of pigeon muscle and its changes due to tenotomy

Analysis of fiber types in the pigeon's metapatagialis muscle. II. Effects of denervation

The pigeon's metapatagialis muscle consists of three slips, two twitch and one tonic, and these slips are distinguishable at the gross anatomical level. Comparative studies of denervation are facilitated because the two fiber types are under the same mechanical forces, can be denervated as one muscle, and can be distinguished after denervation. Both fiber types atrophied after denervation, with the twitch fibers having a more variable response. Pathological alterations observed by light microscopy suggested that the twitch fibers were more affected by denervation than the tonus fibers. Ultrastructurally, both fiber types showed the same changes, with the twitch fibers again being more consistently altered. Proliferation of the transverse tubular system and sarcoplasmic reticulum were more marked in the tonus than twitch fibers, and the sarcoplasmic reticulum proliferated prior to the transverse tubules. Filament and fibril degeneration, peripheral and central degeneration, lysosomes and their derivatives, and satellite cell proliferation were common to both fiber types. Contracture knots were common to the denervated fibers, and were suggested to be characteristic of degenerating fibers. Degenerating motor end plates were observed, and most neurons in the fibers were naked, lacking myelin sheaths. The results are discussed in relation to the function of the neuron in maintaining the muscle, and the possibility of denervation inducing a transformation of tonic to twitch fibers.

Regeneration of pigeon fast and slow muscle fiber types after partial excision and mincing

The pigeon's metapatagialis muscles, containing fast fibers in two slips and slow fibers in another slip, were excised for a third of their length, minced, and replaced into their previous sites. After regeneration, the pattern of fiber types and their ATPase and oxidative enzymes were examined histochemically. Ultrastructural examination was carried out on the fast fibers. After 4-17 wk the muscles had regenerated into patterns histochemically similar to the controls only within the slip containing fast fibers. The slow slip was much less regenerated, and had a histochemically embryonic composition. Fiber types were characterized and their cross-sectional areas measured, and the degree of atrophy was greatest in the large fast fibers and the slow fibers. Ultrastructural studies revealed a number of alterations of the mitochondria, including dense and light areas in the matrix and an altered pattern of the cristae into parallel tubular or vesicular aggregations. Other changes included dilated sarcoplasmic reticulum, myofibril disorganization, and a compaction of filaments. The slow fibers were thought to be slower in their regeneration rates because of the pattern of multiple innervation's producing a more complex regenerative pattern.