Regeneration of isthmic tissue is the result of a specific and direct interaction between rhombomere 1 and midbrain

The midbrain-hindbrain boundary, or isthmus, is the source of signals that are responsible for regional specification of both the midbrain and anterio r hindbrain, Fibroblast growth factor 8 (Fgf8) is expressed specifically at the isthmus and there is now good evidence that it forms at least part of the patterning signal. In this study, we use Fgf8 as a marker for isthmic c ells to examine how interactions between midbrain and hindbrain can regener ate isthmic tissue and, thereby, gain insight into the normal formation and /or maintenance of the isthmus, We show that Fgf8-expressing tissue with pr operties of the isthmic organiser is generated when midbrain and rhombomere 1 tissue are juxtaposed but not when midbrain contacts any other rhombomer e. The use of chick/quail chimeras shows that the isthmic tissue is largely derived from rhombomere 1, In a few cases a small proportion of the Fgf8-p ositive cells were of midbrain origin but this appears to be the result of a local respecification to a hindbrain phenotype, a process mimicked by ect opic FGF8, Studies in vitro show that the induction of FBf8 is the result o f a direct planar interaction between the two tissues and involves a diffus ible signal.