Cell mixing between the embryonic midbrain and hindbrain

Segmentation is a mechanism that controls spatial organization along the an teroposterior axis of the neural tube and is particularly well characterize d for the hindbrain region [1], The generation of distinct and regionally s pecific structures from each rhombomere is achieved with the almost complet e absence of cell mixing between neighboring rhombomeres [2, 3], Here, we h ave examined cell mingling at the isthmus, where Otx2-expressing midbrain c ells abut Gbx2-expressing hindbrain cells [4], The sharp line of demarcatio n between the two expression domains suggests that this interface would be a compartment boundary, with no intermixing of cells, but this has not been directly tested. We have used short-term reaggregation assays to compare t he adhesive properties of cells derived from midbrain and anterior hindbrai n and cell labeling in vivo directly to monitor cell behavior at the midbra in/hindbrain boundary. Interestingly, our data demonstrate that, in contras t to the rhombomeres, differential adhesion does not seem to operate betwee n the midbrain and anterior hindbrain and that cells move between the two t erritories. We conclude that these two subdivisions are not maintained by c ell lineage restriction but by cells maintaining labile fates,