The spatial and temporal expression pattern of the muscle regulatory gene Myf-6 (MRF4/herculin) has been investigated by in situ hybridization during embryonic and fetal mouse development. Here, we report that the Myf-6 gene shows a biphasic pattern of expression. Myf-6 transcripts are first detected in the most rostral somites of the mouse embryo at 9 d of gestation and accumulate progressively in myotomal cells along the rostro-caudal axis. This expression is transient and Myf-6 mRNA can no longer be detected in myotomal cells after day 12 post coitum (p.c.). In contrast to other muscle determination genes (MyoD1, myogenin, Myf-5), Myf-6 mRNA is not detected in limb buds or visceral arches and skeletal muscle of the mouse embryo (day 8-15 p.c.). In fetal mice, Myf-6 transcripts appear at day 16 p.c. in all skeletal muscles, and the gene continues to be expressed at a high level after birth. These results suggest that early Myf-6 expression may be restricted to a population of myogenic cells that does not contribute to the embryonic muscle masses in limb buds and visceral arches. The reappearance of Myf-6 mRNA in fetal skeletal muscle coincides approximately with secondary muscle fiber formation and the onset of innervation.

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