Figure 4. Reduced internuclear commissure in Robo3 knockout mice.

(A to I) show coronal hindbrain sections at the level of the abducens nucleus, visualized by Hb9 immunostaining (in A, D, E, F, and H). (A, B, and C) illustrate the projection of abducens axons (arrowheads) across the midline (dashed line) in Robo3^+/− E13 embryos. Some GFP+ axons originate from the abducens nucleus (VI) and are immunoreactive for Robo3 (B and C). (D) The internuclear commissure (arrowhead) is also observed in P0 controls, following DiI injection at the level of the oculomotor nucleus III. (E) This commissure is almost completely absent in P0 Krox20::cre;Robo3^lox/lox mice (arrowhead). (VIIn): Genu of facial nerve. (F to I) At E15, many neurofilament+ axons cross the midline at the VI level (arrowheads) in control embryo (F and G), whereas they are rare in Krox20::cre;Robo3^lox/lox embryo (H and I). Scale bars represent 100 µm, except in (G and I), where they indicate 50 µm.