Fetal Development of the Double Muscled Condition in Cattle

Abstract

Four bovine fetuses, homozygous for the double muscled condition (DM) were compared with normal fetuses of similar crown-rump (CR) length. DM fetuses had externally visible signs of muscle enlargement, increased dry weights of muscles and many more myofibers in midlength transverse sections of their sartorius muscles. In sartorius muscles of DM fetuses, myofibers were not increased in diameter. Midlength transverse sections of an extraocular muscle (anterior rectus) showed that this muscle did not have extra myofibers in DM fetuses. In older fetuses (normal and DM), the number of myofibers in midlength transverse sections of the sartorius continued to increase even when there was no equivalent morphological evidence for the formation of new myofibers. This was thought to be due to growth in length of intrafascicularly terminating myofibers which had not previously reached the muscle midlength. The increase in number of myofibers at the muscle midlength, presumed to be due to this process, was greater in DM fetuses. However, the relative rate of increase (increase per single myofiber present at 26 cm CR length) was very similar in both normal and DM fetuses. It was concluded, therefore, that DM was due to real myofiber hyperplasia during the early development of muscles as suggested by other researchers.