The effect of varying postnatal growth rate on skeletal muscle fiber number in the mouse.

The present study was designed to investigate the effect of increased and decreased growth rate during the early postnatal period on skeletal muscle fiber number in the mouse. Growth rate was controlled by altering litter size shortly after birth. Three litter sizes, small (4 animals), normal (8 animals), and large (12 animals) were used for the study. The animals were sacrificed at 24 days of age and the soleus muscle from one leg was excised and used for fiber number determination. Fiber number was determined by direct counts of fibers individually teased from the muscle following HNO3 fixation. Marked differences occurred in growth rates among the three groups. The small litter animals grew more rapidly than the normal and large litter animals, and the normal litter animals grew more rapidly than the large litter animals. Fiber counts of the soleus muscle revealed no difference in fiber number among animals raised in small, normal, or large litters. These data suggest that early postnatal growth rate has no effect on soleus muscle fiber number in the mouse.