Structural changes in Z-disks of skeletal muscle myofibrils during growth of chicken

Postnatal structural changes in Z-disks of skeletal muscles of chicken from 2 to 35 wk after hatching were examined to elucidate how Z-disks develop from the embryonic to the mature stage. The mechanical strength of Z-disks of breast muscle (i.e. the ratio of the number of myofibrils to the total number of myofibrils and myofibrillar fragments composed of 1-4 sarcomeres, which were formed under the mechanical forces exerted during homogenization) increased from 66% at 2 wk of age to 75% at 10 wk, and leveled off at 76% at 25 wk. The Z-disks of leg muscle were stronger than those of breast muscle throughout growth. Measurements by electron microscope showed that the width of Z-disks increased during growth from 26 to 33 nm, 48 to 53 nm, and 89 to 101 nm, in white, intermediate, and red muscle fibers, respectively. It was proved that the configuration of Z-filaments, the structural backbone of Z-disks, takes its final shape within 2 wk after hatching; the immature Z-disks are then reinforced by the accumulation of amorphous matrix materials. These results confirm that the maturation of Z-disks is brought about by physical motion in muscle tissue and the accompanying development of tension.