Expression of myosin heavy chain isoforms during development of domestic pigeon pectoralis muscle.

The pectoralis muscle of birds provides virtually all the power for the downstroke of the wing during flight. In adults it consists almost entirely of FOG (fast-twitch oxidative-glycolytic) and/or FG (fast-twitch glycolytic) fiber types. The aims of this study are to contrast MyHC (myosin heavy chain) transitions occurring within avian FG and FOG fibers during development, and to test the hypothesis that the pectoralis matures before the acquisition of flight. Pectoralis was obtained from pigeons (Columba livia) aged from 13 days in ovo to adult. Monoclonal antibodies generated against chicken MyHC isoforms were used with Western blots and immunocytochemistry. FG and FOG fibers were differentiated using a histochemical method demonstrating NADH (nicotinamide adenine dinucleotide), and "lesser fiber diameters" were quantified. Western blots confirm that the antibodies label pigeon MyHCs. A small number of the fibers are slow type in ovo, but these are quickly restricted in distribution and lost after hatching. In ovo fast-twitch fibers contain a ventricular isoform, and at least two embryonic-neonatal forms (designated E-N103 and E-N165). One week after hatching, fast-twitch fibers can be distinguished by NADH as FG or FOG. At fledging, four weeks after hatching, FG and FOG fibers are smaller than in older birds and E-N103 and E-N165 persist in both fibertypes. E-N103 wanes in all fibers shortly after fledging. E-N165 gradually disappears from FG fibers. Thus, despite pigeons being at adult body mass at fledging, their pectoralis is not fully mature.