MYOD EXPRESSION MARKS THE ONSET OF SKELETAL MYOGENESIS IN MYF-5 MUTANT MICE

The expression pattern of myogenic regulatory factors and myotome-spec ific contractile proteins was studied during embryonic development of Myf-5 mutant mice by in situ hybridization and immunohistochemistry. I n contrast to somites in wild-type embryos, no expression of myogenin and Myf-6 (MRF4), or any other myotomal markers was detected in mutant animals at E9.0 and E10.0 indicating that Myf-5 plays a crucial role during this developmental period. Significantly, the onset of MyoD exp ression in rostral somites of E10.5 embryos was unaffected by the Myf- 5 mutation suggesting that the activation of the MyoD gene occurs inde pendently of Myf-5 at the correct developmental time. Immediately afte r the activation of MyoD myogenin transcripts and protein accumulated within the myotome. The first contractile proteins of the sarcomeric a pparatus appeared slightly later. By E11.5 the expression of muscle ma rkers were indistinguishable between wild-type and Myf-5 mutant mice. The migration of muscle precursor cells that leave the somites to form limb musculature was monitored in Myf-5-mutant mice by Pax-3 expressi on. Pax-3-positive cells were equally found in somites and limbs of E1 0.0 wild-type and mutant mice indicating that myogenic factor expressi on at the level of somites is not a prerequisite for determination and subsequent migration of limb precursor cells.