Muscle at birth in mice selected for large and small body size

Selection for divergent body weight at 6 wk of age in the Q strain mouse has produced large (QL) and small (QS) mice which differ 2-fold in their adult body weight. The purpose of this investigation was to identify some of the cellular mechanisms which underlie the early divergence in size between the 2 lines. At birth, QL mice (for similar litter sizes) were 28% heavier and 6% longer than QS mice. This was reflected by measurements of longitudinal bone length which were greater in QL (tibia 6.2%, humerus 4.2%) compared with QS mice. Fibre number was found to be 18 and 17% greater in the biceps brachii and soleus muscles respectively of the QL mice. It was concluded that this was not a consequence of any alteration in the ratio of developing secondary to primary myofibres in either muscle. Fibre cross-sectional areas were only significantly different between the QL and QS for the soleus muscle, which might be explained by the relatively greater divergence in the length of its supporting bone (tibia) between the QL and QS compared with the humerus. Estimates of nuclear number showed that there were significantly more nuclei in biceps brachii muscle of QL than in the QS mice which could be attributed to the difference in fibre number, although no such differences were found for the soleus muscle. There was no apparent alteration in the proportion of nuclei found within the fibres of the biceps muscle. Overall the results indicate that selection in this situation has acted through the normal cellular processes of growth.