Fig. 5. Mouse commissural axons exhibit distinct contralateral trajectories. (A) Schematic diagram of an open-book spinal cord explant in which the placement of DiI was varied within the dorsal spinal cord (the dorsal and ventral regions of the spinal cord are indicated by brackets). The numbers in A correspond to the positions of three separate DiI applications that labeled ventrally directed axons whose trajectories are shown in panels 1-3 in B. (B) Cell bodies immediately adjacent to the roof plate (1) extend axons that project ventrally and subsequently turn rostrally on the ipsilateral side of the spinal cord for distances up to 450-500 µm. In this micrograph, the floor plate (indicated by the white lines) is shown at the top. Panels 2 and 3 show axons projecting on only the contralateral side of the spinal cord (dorsal is upwards and rostral is towards the right). Cell bodies located within successively more ventral regions (2 and 3 in (A)) give rise to axons that cross through the floor plate. A minor population of axons (2) maintained extended (>100 µm) contact with the floor plate after executing a rostral turn at the contralateral margin of this structure. An axon that is projecting diagonally away from the floor plate and along the pathway depicted in C is shown outside of the plane of focus. No axons were ever observed to recross the floor plate in these studies. A much larger population of axons (3) maintains only transient (
100 µm) contact with floor plate before assuming a more complex pathway that culminates in a rostral turn in an intermediate region of the white matter. For these analyses, 30 axon cohorts labeled by the application of DiI into each spinal cord region depicted schematically in A (see Materials and Methods for additional details) were examined. Scale bar: 100 µm. contra, contralateral; d, dorsal; ipsi, ipsilateral; v, ventral.
