Within the developing vertebrate head, neural crest cells (NCCs) migrate from the dorsal surface of the hindbrain into the mesenchyme adjacent to rhombomeres (r)1 plus r2, r4 and r6 in three segregated streams. NCCs do not enter the intervening mesenchyme adjacent to r3 or r5, suggesting that these regions contain a NCC-repulsive activity. We have used surgical manipulations in the chick to demonstrate that r3 neuroepithelium and its overlying surface ectoderm independently help maintain the NCC-free zone within r3 mesenchyme. In the absence of r3, subpopulations of NCCs enter r3 mesenchyme in a dorsolateral stream and an ectopic cranial nerve forms between the trigeminal and facial ganglia. The NCC-repulsive activity dissipates/degrades within 5-10 hours of r3 removal. Initially, r4 NCCs more readily enter the altered mesenchyme than r2 NCCs, irrespective of their maturational stage. Following surface ectoderm removal, mainly r4 NCCs enter r3 mesenchyme within 5 hours, but after 20 hours the proportions of r2 NCCs and r4 NCCs ectopically within r3 mesenchyme appear similar
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