The relationship between muscle growth and the growth of different fiber types in the chicken.

A relationship of muscle growth to fiber growth was investigated histochemically and allometrically in the chicken from 1 to 35 wk of age. Muscle fibers were classified into three types (I, IIA, and IIB) based on the reactivities for myosin adenosine triphosphatase and reduced nicotinamide adenine dinucleotide dehydrogenase. Fiber type composition widely varied in the four muscles examined in this study. Longus colli dorsalis contained all three types. The Pectoralis was composed only of IIB fibers. The caudal portion of Femorotibialis medius was made up of Type I and IIA fibers. The caudal Iliotibialis lateralis and the cranial portion of Femorotibialis medius were composed of two types, IIA and IIB. These latter two muscles showed a progressive increase of Type IIA fibers and decrease of Type IIB fibers with advancing age. Two processes controlling muscle growth were elongation and enlargement of muscle fibers. The elongation of muscle fibers stops by 15 wk of age, coincident with the cessation of bone growth. However, the enlargement of muscle fibers, the increase in diameter of muscle fibers, continued until 35 wk of age. The rate of enlargement of muscle fibers varied by muscle and fiber type. In general, greatest growth occurred in Type IIB fibers of hindlimb muscles.