Fig. 3. Heterotopic NCC grafting. Cartoons on the left summarise each experiment, and panels show dorsal views of the distribution of grafted cells 10 hours after surgery. Phase, combined phase/DiI, DiI and Hoxb1 in situ images. Broken lines mark the neuroepithelial outlines. (A-D) Unilaterally, r4 was replaced by DiI-labelled r2, and then r3 was removed. Ectopic r2 cells migrate along the normal r4 NCC pathway (arrowheads), but few cells migrate into r3* mesenchyme (arrow). Ectopic r2 did not express the r4 marker, Hoxb1 (D). (E-H) Unilaterally, r2 was replaced by DiI-labelled r4, and then r3 was removed. Ectopic r4 cells migrate along the normal r2 NCC pathway (arrowhead) and many cells migrate caudally into r3* mesenchyme (arrow), with some entering the normal r4 NCC pathway (small arrow). Ectopic r4 maintains Hoxb1 expression (H). (I-P) In similar experiments, small clusters of neuroepithelial cells were heterotopically grafted between r2 and r4. (I-L) Ectopically grafted r2 cells migrate out of r4 within the r4 NCC stream (arrowheads) and several cells migrate rostrally into r3* mesenchyme (arrow). Hoxb1 expression within r4 appears unaltered. (M-P) Ectopically grafted r4 cells migrate within the r2 NCC stream (arrowhead), but cells rarely deviate caudally into r3* mesenchyme. (P) Grafted and control sides of host hindbrain flatmount, stripped of mesenchyme to aid viewing. Within r2 neuroepithelium, very few grafted r4 cells maintain Hoxb1 expression (arrows), suggesting that in small clusters, ectopic NCCs lose their original positional identity.