A paraxial exclusion zone creates patterned cranial neural crest cell outgrowth adjacent to rhombomeres 3 and 5.

Cranial neural crest cell migration is patterned, with neural crest cell-free zones adjacent to rhombomere (R) 3 and R5. These zones have been suggested to result from death of premigratory neural crest cells via upregulation of BMP-4 and Msx-2 in R3 and R5, consequent to R2-, R4-, and R6-derived signals. We reinvestigated this model and found that cell death detected by acridine orange staining in avian embryos varied widely numerically and in pattern, but with a tendency for an elevated zone centered at the R2/3 boundary. In situ hybridization of BMP-4 mRNA resolved to centers at R3 and R5 but Msx-2 resolved to the R2/3 border with only a faint smear from R5 to R6. Outgrowth of neural crest cells was less in isolated R3 cultures than in R1+2, R2, and R4 cultures, but R3 showed neither a decrease in outgrowth of neural crest cells nor an increase in cell death when cocultured with R1+2, R2, or R4. In addition, in serum-free culture, exogenous BMP-4 strikingly reduced neural crest cell outgrowth from R1+2 and R4 as well as R3. Thus we cannot confirm the role of intraneural cell death in patterning rhombomeric neural crest outgrowth. However, grafting quail R2 or R4 adjacent to the chick hindbrain demonstrated a neural crest cell exclusion zone next to R3 and R5. We suggest that one important pattern determinant for rhombomeric neural crest cell migration involves the microenvironment next to the neural tube.