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 anterior 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 cells to examine how interactions between midbrain and hindbrain can regenerate isthmic tissue and, thereby, gain insight into the normal formation and/or maintenance of the isthmus. We show that Fgf8-expressing tissue with properties of the isthmic organiser is generated when midbrain and rhombomere 1 tissue are juxtaposed but not when midbrain contacts any other rhombomere. 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-positive cells were of midbrain origin but this appears to be the result of a local respecification to a hindbrain phenotype, a process mimicked by ectopic FGF8. Studies in vitro show that the induction of Fgf8 is the result of a direct planar interaction between the two tissues and involves a diffusible signal.
Irving
C
C
MRC Brain Development Programme, Department of Developmental Neurobiology, Guy's, King's and St. Thomas' Hospital Medical School, Hodgkin Building, Guy's Campus, London SE1 9RT, UK.
Mason
I
I
D000818:Animals; D016380:Brain Tissue Transplantation; D002642:Chick Embryo; D004625:Embryo, Nonmammalian; D051524:Fibroblast Growth Factor 8; D005346:Fibroblast Growth Factors / Q000235:genetics / Q000378:metabolism; D008636:Mesencephalon / Q000196:embryology* / Q000502:physiology* / Q000601:surgery; D009432:Neural Crest / Q000196:embryology* / Q000637:transplantation; D011784:Quail / Q000196:embryology; D012038:Regeneration; D012249:Rhombencephalon / Q000196:embryology* / Q000502:physiology* / Q000601:surgery; D015398:Signal Transduction
D051524:Fibroblast Growth Factor 8; D005346:Fibroblast Growth Factors