Control of the migratory pathway of facial branchiomotor neurones

Facial branchiomotor (fbm) neurones undergo a complex migration in the segm ented mouse hindbrain, They are born in the basal plate of rhombomere (r) 4 , migrate caudally through r5, and then dorsally and radially in r6. To stu dy how migrating cells adapt to their changing environment and control thei r pathway, we have analysed this stereotyped migration in wild-type and mut ant backgrounds. We show that during their migration, fbm neurones regulate the expression of genes encoding the cell membrane proteins TAG-I, Ret and cadherin 8. Specific combinations of these markers are associated with eac h migratory phase in r4, r5 and r6, In Krox20 and kreisler mutant mouse emb ryos, both of which lack r5, fbm neurones migrate dorsally into the anterio rly positioned r6 and adopt an r6-specific expression pattern. In embryos d eficient for Ebf1, a gene normally expressed in fbm neurones, part of the f bm neurones migrate dorsally within r5, Accordingly, fbm neurones premature ly express a combination of markers characteristic of an r6 location, These data suggest that fbm neurones adapt to their changing environment by swit ching on and off specific genes, and that Ebf1 is involved in the control o f these responses. In addition, they establish a close correlation between the expression pattern of fbm neurones and their migratory behaviour, sugge sting that modifications in gene expression participate in the selection of the local migratory pathway.