Title: Cell mixing between the embryonic midbrain and hindbrain
Abstract: Segmentation is a mechanism that controls spatial organization along the anteroposterior axis of the neural tube and is particularly well characterized for the hindbrain region [1Lumsden A. Krumlauf R. Patterning the vertebrate neuraxis.Science. 1996; 274: 1109-1115Crossref PubMed Scopus (924) Google Scholar]. The generation of distinct and regionally specific structures from each rhombomere is achieved with the almost complete absence of cell mixing between neighboring rhombomeres [2Fraser S.E. Keynes R.J. Lumsden A. Segmentation of the chick embryo hindbrain is defined by cell lineage restrictions.Nature. 1990; 344: 421-535Crossref Scopus (549) Google Scholar, 3Lumsden A. Closing in on rhombomere boundaries.Nat Cell Biol. 1999; 1: 83-85Crossref PubMed Scopus (21) Google Scholar]. Here, we have examined cell mingling at the isthmus, where Otx2-expressing midbrain cells abut Gbx2-expressing hindbrain cells [4Simeone A. Positioning the isthmic organizer where Otx2 and Gbx2 meet.Trends Genet. 2000; 16: 237-240Abstract Full Text Full Text PDF PubMed Scopus (146) Google Scholar]. The sharp line of demarcation 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 the adhesive properties of cells derived from midbrain and anterior hindbrain and cell labeling in vivo directly to monitor cell behavior at the midbrain/hindbrain boundary. Interestingly, our data demonstrate that, in contrast to the rhombomeres, differential adhesion does not seem to operate between the midbrain and anterior hindbrain and that cells move between the two territories. We conclude that these two subdivisions are not maintained by cell lineage restriction but by cells maintaining labile fates.