Medial LMC axons innervate the ventral flank mesenchyme. The columnar origin of neurons innervating the ventral flank mesenchyme was defined by HRP or RDA retrograde labeling at E13.5. (a) Schematic cross section at hindlimb level, showing limb and flank axonal trajectories and approximate location of the retrograde label injection. Inset: relative positions of motor columns in C-E. (b) Transcription factor combinations expressed by medial LMC (LMC(m)), lateral LMC (LMC(l)) and medial MMC (MMC(m)) neurons. (c,d,e) Representative images showing HRP colocalization with Isl1, and not Lim1 (c) or Lim3 (d), but with FoxP1 (e), in triply immunostained sections. Note that images show representative staining patterns and marker colocalization, and are not intended to be quantitative. Arrowheads: HRP+ neurons colabeled with nuclear markers. Dotted lines (c-e) delineate columnar division borders. n = 5 embryos.