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.

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