MYONUCLEAR ACCRETION IS A MAJOR DETERMINANT OF AVIAN SKELETAL-MUSCLE GROWTH

The role of satellite cells and DNA unit size in determining skeletal muscle growth was studied after mitotic activity was inhibited in the left pectoralis thoracicus of 2-wk-old tom turkeys by means of a 25-Gy dose of irradiation. Toms were killed and muscle weights were obtaine d 1 (n = 5), 4 (n = 6), 7 (n = 6), and 15 (n = 4) wk after irradiation . Satellite cell mitotic activity and DNA unit size were determined us ing enzymatically isolated myofiber segments and image analysis. Irrad iated and nonirradiated muscle weights increased (P < 0.01) between al l ages examined, but ir radiated muscle weights were significantly (P < 0.01) lower than nonirradiated muscle weights at 4, 7, and 15, wk af ter irradiation. Satellite cell mitotic activity was lower (P < 0.01) in irradiated than in nonirradiated muscles at 1 and 4 wk after irradi ation and resulted in a significant reduction (P < 0.05) in the number of myofiber nuclei per millimeter at 4 and 7 wk after irradiation. Sa tellite cell mitotic activity was higher (P < 0.05) in irradiated than in nonirradiated muscles at 7 wk after irradiation, but at 15 wk afte r irradiation it had fallen to low levels in both muscles. There was n o significant (P > 0.10) difference in DNA unit size between muscles a t any time, but there was an age-related increase (P < 0.01) for both muscles. Irradiation reduced muscle growth through a transient reducti on in myonuclear production at a critical time (3-6 wk of age) in post hatch skeletal muscle development. The age-related increase in DNA uni t size was not accelerated to compensate for the reduction in myonucle ar accretion. Thus it appears that muscle growth potential is governed mostly by myonuclear accretion and to a lesser extent by DNA unit siz e.