Fig. 3

Time-lapse analysis of adaxial cell behaviors. Selected frames from Supplementary Movie. Four adaxial cells expressing a membrane-targeted GFP mRNA, that will eventually become incorporated into two adjacent somites, were followed for 4.5 h. Elapsed time, in minutes, of selected images shown. (A) Adaxial cells (1–4) are initially predominantly pseudo-epithelial, with their basal membranes against the notochord. Columnar shape is lost as adaxial cells become more irregularly shaped and rounded as they begin to interleaf (B), before beginning to make transient apical constrictions (arrow, C). (D–F) Adaxial cell membranes then begin to spread out as cells begin to broaden, with cell 1 showing a broad, lamellar expansion and cell 2 dramatically rounding (E) prior to further elongating. (F, G) Cells maintain broad character and laterally directed lamellar protrusions as they further elongate and intercalate. (H) Cell 3 eventually becomes partially obscured underneath cell 4, while cell 1 becomes intercalated fully beneath cell 2 in an adjacent somite.