NEURAL INDUCTION AND REGIONALIZATION BY DIFFERENT SUBPOPULATIONS OF CELLS IN HENSENS NODE

Cell lineage analysis has revealed that the amniote organizer, Hensen' s node, is subdivided into distinct regions, each containing a charact eristic subpopulation of cells with defined fates. Here, we address th e question of whether the inducing and regionalising ability of Hensen 's node is associated with a specific subpopulation. Quail explants fr om Hensen's node are grafted into an extraembryonic site in a host chi ck embryo allowing host- and donor-derived cells to be distinguished. Cell-type- and region-specific markers are used to assess the fates of the mesodermal and neural cells that develop. We find that neural ind ucing ability is localised in the epiblast layer and the mesendoderm ( deep portion) of the medial sector of the node. The deep portion of th e posterolateral part of the node does not have neural inducing abilit y. Neural induction also correlates with the presence of particular pr ospective cell types in our grafts: chordamesoderm (notochord/head pro cess), definitive (gut) endoderm or neural tissue. However, only graft s that include the epiblast layer of the node induce neural tissue exp ressing a complete range of anteroposterior characteristics, although prospective prechordal plate cells may also play a role in specificati on of the forebrain.