Fig. 1. EphA4 RTK is expressed during muscle precursor migration. (A) Cross-section through stage 15 embryo at forelimb levels, hybridized with probe for EphA4 mRNA. EphA4 mRNA is present in lateral dermomyotomal cells (arrow), in the lateral plate mesoderm (*) and the developing nephric system (n). dm, dermomyotome; nt, neural tube. (B-F) Confocal z-series stacks (8 µm) from vibratome sections stained with EphA4 and Pax7 antibodies. (B) At stage 15, EphA4 appears diffusely distributed in the dermomyotome, localizing primarily to its lateral (arrow) and ventral edges, and at low levels in the lateral plate mesoderm. (C) At early stage 16, EphA4 is restricted to the lateral edge of the dermomyotome (arrow). (D) At stage 17, EphA4 is prominent in the lateral dermomyotome and encircles migrating muscle precursors in the proximal limb. Boxed area is high magnification view in inset. (E) EphA4 protein appears absent from muscle precursor cells at stage 19 but is strongly expressed in the progress zone (pz) and dorsoproximal limb mesenchyme (arrow). (F) High-magnification view of E, the lateral edge of the dermomyotome and adjacent dorsoproximal mesenchyme at stage 19.