EFFECTS OF PITUITARY DWARFISM IN THE MOUSE ON FAST AND SLOW SKELETAL-MUSCLES

The Snell dwarf mouse exhibits impaired growth of the anterior pituita ry resulting in reduced levels of growth hormone and thyroid stimulati ng hormone. Ten dwarf mice and 10 phenotypically normal littermates we re killed at 33 days of age. M. biceps brachii (a predominantly fast m uscle) and m. soleus (a relatively slow muscle) were removed from each animal and complete frozen transverse sections obtained. Serial secti ons were reacted for various enzyme activities in order to identify mu scle fibre types. There was no difference in the total number of muscl e fibres in m. biceps brachii but a small difference in m. soleus betw een normal and dwarf mice. There were marked differences in the size o f all fibre types between normal and dwarf mice with the largest diffe rences in m. soleus. The percentage of slow oxidative fibres was simil ar (about 32%) in both groups of mice for m. soleus but there was a ma rked difference for this fibre type in m. biceps brachii being about 1 .5% in normal mice and 8.0% in dwarf mice. This may be related to a di fference in levels of thyroid hormone. Nuclear density was very signif icantly greater in dwarf muscles although total nuclear numbers were l ess than in normal muscles. These differences are most likely due to g rowth hormone levels. Differences in nuclear content were much greater in m. soleus than in m. biceps brachii.