Establishment of the epaxial–hypaxial boundary in the avian myotome

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Abstract
Trunk skeletal muscles are segregated into dorsomedial epaxial and ventrolateral hypaxial muscles, separated by a myoseptum. In amniotes, they are generated from a transient structure, the dermomyotome, which lays down muscle, namely the myotome underneath. However, the dermomyotome and myotome are dorsoventrally continuous, with no morphologically defined epaxial–hypaxial boundary. The transcription factors En1 and Sim1 have been shown to molecularly subdivide the amniote dermomyotome, with En1 labeling the epaxial dermomyotome and Sim1 the hypaxial counterpart. Here, we demonstrate thatEn1andSim1expression persists in cells leaving the dermomyotome, superimposing the expression boundary onto muscle and skin.En1‐expressing cells colonize the myotome initially from the rostral and caudal lips, and slightly later, directly from the de‐epithelializing dermomyotomal center.En1expression in the myotome is concomitant with the appearance ofFgfr4/Pax7‐expressing mitotically active myoblasts. This finding suggests thatFgfr4+/Pax7+/En1+cells carry their expression with them when entering the myotome. Furthermore, it suggests that the epaxial–hypaxial boundary of the myotome is established through the late arising, mitotically active myoblasts. Developmental Dynamics 235:1884–1894, 2006.