Unique ultrastructural characteristics of pigeon anterior latissimus dorsi muscle.

The pale anterior latissimus dorsi (ALD) muscle of the adult pigeon was studied ultrastructurally. There were embryonic heart and skeletal muscle features present. These included loosely packed myofibrillar apparatus, subsarcolemmal pads of sarcoplasm, abundant pinocytotic vesicles, and tubular aggregates. The latter were focal, smooth surfaced, and bilaminar. An increased accumulation of Z-disk material was widespread in the muscle fibers. There were distinct sarcolemmal evaginations which possess knob-like projections. Myocellular extensions were evident at muscle fiber ends and these produced bud-like configurations in transverse section. These features in the normal adult ALD rules out the assumption that they represent a denervation phenomenon, as suggested by others.