EXPRESSION OF MYOSIN HEAVY-CHAIN ISOFORMS DURING DEVELOPMENT OF DOMESTIC PIGEON PECTORALIS-MUSCLE

The pectoralis muscle of birds provides virtually all the power for th e downstroke of the wing during flight. In adults it consists almost e ntirely of FOG (fast-twitch oxidative-glycolytic) and/ or FG (fast-twi tch glycolytic) fiber types. The aims of this study are to contrast My HC (myosin heavy chain) transitions occurring within avian FG and FOG fibers during development, and to test the hypothesis that the pectora lis matures before the acquisition of flight. Pectoralis was obtained from pigeons (Colomba livia) aged from 13 days in ovo to adult. Monocl onal antibodies generated against chicken MyHC isoforms were used with Western blots and immunocytochemistry. FG and FOG fibers were differe ntiated 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 rest ricted in distribution and lost after hatching. In ovo fast-twitch fib ers contain a ventricular isoform, and at least two embryonic-neonatal forms (designated E-N103 and E-N165). One week after hatching, fast-t witch fibers can be distinguished by NADH as FG or FOG. At fledging, f our weeks after hatching, FG and FOG fibers are smaller than in older birds and E-N103 and E-N165 persist in both fiber types. 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.