Fibre number and fibre size in a surgically overloaded muscle.

Soleus muscles of male and female mice were overloaded by surgical resection of parts of gastrocnemius and plantaris. The effects of overloading were examined histologically after 7, 55 and 208 post-operative days, and also in teased preparations. Animals studied after 7 post-operative days showed a marked increase in muscle weight, but no significant change in mean fibre diameter or fibre number. Animals studied after 55 and 208 post-operative days showed an increase in soleus muscle weight, with fibre hypertrophy (but no increase in fibre number) proximally, while distally there was an increase in the number of fibre profiles in cross sections, some being wider, some thinner than normal. The small diametered fibres seem to persist indefinitely. From the evidence, both direct and indirect, it was concluded that surgically overloaded fibres split longitudinally into unequal parts, and that this explains the increase in fibre profiles in distal cross sections as well as their variation in size. It is clear that, because of the splitting, a surgically overloaded muscle is a difficult model on which to study fibre hypertrophy.