Sema6A mutants have corticospinal tract (CST) axon guidance defects at the mid-hindbrain boundary (MHB) and in the caudal medulla. (a-o) Postmortem DiI placement into the motor cortex of P4 Sema6A+/- (left column) and Sema6A-/- (middle and right columns) brains (example in (c')). Levels of shown cross-sections are indicated on a schematic sagittal brain section shown in (c): 1 (a, b); 2 (d-f); 3 (g-i); 4 (j-l); 5 (m-o); white notches indicate the position of the midline in (a, b, d-l). At the MHB of Sema6A+/- mice, traced fibres at the ventrolateral surface turn and travel halfway towards the midline (a), then turn ventrally and project over the pontine nuclei (d) to finally reach the ventral surface of the medulla (g) to form the pyramids (j, m). At the MHB of Sema6A-/- mutant mice, labelled fibres took several aberrant routes: a dorsal turn at the caudal end of the cerebral peduncle (blue arrowheads in (b, e, h, f)), midline crossing (large fraction; yellow arrowheads in (b)) or dorsal turning after a correct turn towards the midline (minor fraction; green arrowhead in (b)), or, after correct reorientation ventrally, splitting into several bundles, of which some project through the pontine nuclei and/or laterally (magenta arrowheads in (e, h) or (f, i), respectively). In the medulla of Sema6A+/- mice, traced fibres travelled close to the midline (j). This could also be observed for the reduced number of labelled axons that were routed properly at the MHB of Sema6A mutants (k, l). However, at the level of the inferior olive some (o) or all (n) of these axons projected away from the midline and proceeded ipsilaterally at a ventrolateral position (white arrowheads in (n, o)), in contrast to those in Sema6A+/- mice, which remained close to the midline (m) and subsequently decussated. Scale bars 200 μm: the bar in (i) is for (a, b, d-i); the bar in (l) is for (j-l); the bar in (o) is for (m-o).