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Fig. 7. Thalamocortical and corticofugal axons advance faster towards their
respective targets in Robo1 knockout brains. DiI was placed in the dorsal
thalamus to label thalamocortical axons (red in all panels and in schematic
diagram) and DiA was placed in the presumptive somatosensory cortex of the
same brains to label corticofugal axons (green in all panels and in schematic
diagram) at either E14.5 (A-E) or E18.5 (F-M). Robo1
knockout brains (A-C,F-I) were compared with controls (D,E,J-M) in coronal
sections of middle and caudal regions. At E14.5, thalamocortical axons in the
knockout (A,B; arrow in B indicates retrograde labelling of cortical cells)
advanced further into the cortex than in wild-type littermates (D,E) which had
not yet passed the corticostriatal notch. Similarly, corticofugal axons had
already reached the thalamus in E14.5 Robo1 knockout brains (A), but
had not yet entered the striatum in controls (D,E). (B,C) Higher power views
of the boxed regions in A (red and blue channels only). At E14.5 and E18.5, we
observed an aberrant projection of DiI-labelled axons coursing transversely
over the axons of the internal capsule in Robo1 knockout brains in a
`knot'-like structure (arrows in C and H), but not in controls (D and arrow in
L). As observed in Fig. 5, most
callosal axons (labelled here with DiA) projected aberrantly into the septum
of Robo1 knockouts (F, arrow) compared with controls (J, arrow).
(G,K) Higher power views of the boxed regions in F,J (red channel only). The
advance of thalamocortical axons persisted at E18.5 in the Robo1
knockout, where these axons projected further medially into the cortex (G,
arrow) compared with controls (K, arrow). Furthermore, back-labelled cells
appeared in greater numbers in the thalamus of mutants following placement of
dye in the cortex (compare I with M, arrow), further confirming that
thalamocortical axons had arrived earlier in these brains. Scale bars: in A,
400 µm in A,D; in B, 200 µm in B,C; in F, 400 µm in F,H,I,J,L,M; in
G, 200 µm in G,K.
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