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 mou se has produced large (QL) and small (QS) mice which differ 2-fold in their adult body weight. The purpose of this investigation was to iden tify some of the cellular mechanisms which underlie the early divergen ce 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 fou nd to be 18 and 17% greater in the biceps brachii and soleus muscles r espectively of the QL mice. It was concluded that this was not a conse quence of any alteration in the ratio of developing secondary to prima ry myofibres in either muscle. Fibre cross-sectional areas were only s ignificantly different between the QL and QS for the soleus muscle, wh ich might be explained by the relatively greater divergence in the len gth of its supporting bone (tibia) between the QL and QS compared with the humerus. Estimates of nuclear number showed that there were signi ficantly more nuclei in biceps brachii muscle of QL than in the QS mic e which could be attributed to the difference in fibre number, althoug h no such differences were found for the soleus muscle. There was no a pparent 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 grow th.