Model of action of Slit, How, and Dg during cardiac tube formation. (A) Diagram of 3D views of CBs (green) with the ectoderm (gray). Arm/DE-Cad–positive domains are visualized in red, and Dg, Slit–Robo, and Trol-positive domains in blue. (1–6) The key steps of cell behavior leading to cardiac lumen formation are represented. (B) Schematic representation of the interactions between Slit, Robo, How, and Dg in CBs to control specification and differentiation of the J and L membrane domains and to participate in lumen formation and growth. Dotted arrows, hypothetic interactions; black arrows, activation; red line, inhibition; gray, CBs; red, J domains; green, L domains.