Fig. 4. Mouse commissural axons extend along the major contralateral pathway followed by their chick counterparts. (A-C) Commissural axons were anterogradely labeled with DiI in open-book spinal cord explants obtained from E11-E13 mouse embryos as described in Material and Methods. (A) At E11, leading commissural axons extend ventrally and are found within, or just on the contralateral side of, the floor plate (region between the white lines). This preparation was visualized under both fluorescence and phase-contrast optics and shows a single growth cone emerging from the floor plate and executing a rostral turn (as well as other axons that have just entered the floor plate). (B,C) Decussated commissural axons projecting on only the contralateral side of the floor plate between E12 and E13 (dorsal is up and rostral is to the right). (B) As observed in the chick spinal cord, most crossed commissural axons extend alongside the floor plate for approximately 100 µm before turning diagonally into more dorsal regions of the white matter. (C) By E13, most commissural axons have executed a final rostral turn into the longitudinal axis within an intermediate region of the marginal zone. Scale bars: in A, 100 µm; in C, 100 µm for B,C.
