Effect of castration on muscle growth in the mouse

Male 129/Re mice were castrated at 18–24 days of age. At 20 weeks of age they were sacrificed and the body weights and the weights of the anterior tibialis, biceps brachii, extensor digitorum longus, soleus and sternomastoideus were recorded. These muscles were also examined cytologically for total fiber number and fiber diameter distributions. The differences between the muscle weights of castrates and controls were correlated with different levels of fiber growth. The level of fiber growth in the phasic anterior tibialis, biceps brachii and extensor digitorum longus (i.e. muscles bringing about body movements) was concluded as being under the indirect control of the pituitary growth hormone, in that it acts on the body as a whole thereby increasing the work load of these muscles. Whereas for the sternomastoideus there appears to be a more direct effect of testosterone.