Ultrastructure of chicken slow muscle after nerve cross union

Abstract

The slow anterior latissimus dorsi (ALD) of the chick was cross innervated by a predominantly fast superior brachialis nerve which had been implanted into the muscle immediately after hatching. Six to eight months after the operation, the structure of cross-innervated muscles (ALD-X) was investigated and compared with that of the control slow (ALD-C) and fast (PLD-C) muscles. Sixty to eighty percent of muscle fibers of the ALD-X were found to be innervated focally by end-plates of the fast type, the remaining fibers being supplied by multiple end-plates of the slow type. The ultrastructure of most ALD-X muscle fibers appeared to be changed toward the fast type. A sample of maximally altered ALD-X fibers was selected for quantitative evaluation. The number of triads was significantly increased in the ALD-X fibers becoming similar to the number in the control PLD-C fibers. This change is probably related to the significant shortening of the contraction time of the ALD-X, found in analogous experiments earlier. The Z lines became significantly narrower in width, but still remained considerably wider than in the PLD-C muscles. The mean content of mitochondria was reduced by 60% in ALD-X fibers in comparison to ALD-C fibers and was comparable to the mean mitochondrial content of the PLD-C; this indicates a change in the oxidative metabolism of the cross-innervated fibers. Thus, in contrast to adult birds, the ultrastructure of slow multiply innervated muscle fibers becomes altered toward the fast type if nerve cross union is performed at an early stage of development.