Time course of LMC divisional segregation. A–C, Expression in the ventral horn of a stage 23 embryo of Islet-1 and HB9 (A), Hb9 and Foxp1 (B), and FoxP1 and Islet-1 (C). Foxp1 marks the entire lateral motor column. Hb9 and Islet-1 are LMC divisional markers. Medial is to the left and is illustrated in A. LMCl cells are, at this stage, more medial to the LMCm cells. D–F, Ventral horn expression at stage 25 of the divisional markers Hb9 (D, E) and Islet-1 (D, F) and the LMC marker Foxp1 (E, F). Note that LMCl and LMCm cells are intermingled. G–I, Columnar segregation is largely complete by stage 27 as assessed by ventral horn expression of Foxp1 (H, I), Islet-1 (G, I), and Hb9 (G, H). J–L, Summary of the migration of LMCl cells through LMCm cells during motor neuron divisional segregation at st23 (J), st25 (K), and st28 (L).