CELL MOVEMENTS, NEURONAL ORGANIZATION AND GENE-EXPRESSION IN HINDBRAINS LACKING MORPHOLOGICAL BOUNDARIES

Rhombomeres are segmental units of the hindbrain that are separated fr om each other by a specialised zone of boundary cells. Retinoic acid a pplication to a recently segmented hindbrain leads to disappearance of posterior rhombomere boundaries. Boundary loss is preceded by changes in segmental expression of Krox-20 and Cek-8 and followed by alterati ons in Hox gene expression. The characteristic morphology of boundary cells, their expression of follistatin and the periodic accumulation o f axons normally associated with boundaries are all lost. In the absen ce of boundaries, we detect no change in anteroposterior dispersal of precursor cells and, in most cases, no substantial cell mixing between former rhombomeric units. This is consistent with the idea that linea ge restriction can be maintained by processes other than a mechanical barrier composed of boundary cells. Much of the early organisation of the motor nuclei appears normal despite the loss of boundaries and alt ered Hox expression.