Fig. 4. Partially myosin IIB-depleted notochordal cells exhibit polarization and adhesion phenotypes. (A) Explanting the dorsal tissues of a Xenopus embryo neurula above the blastopore (Bp) and removing the endodermal archenteron roof (ar) exposes the notochord (N) and somitic mesoderm (S) for imaging during CE. (B) In such explants, confocal time-lapse imaging of a notochord mosaic for cells injected with 5 µM MHC-B MO reveals that morphant cells (labeled), in the background of a notochord expressing a membrane-bound GFP (revealing cell outlines), exhibit loss of regulation of polarized cell motility. Asterisks, NSB. (C) An epifluorescent time-lapse sequence of one partially morphant labeled cell shows that it exhibits loss of regulation of polarized cell motility and eventually is excluded from the notochord (cell outlines are normal notochordal cells in this mosaic explant). (D) Control notochordal cells at or near the NSB exhibit monopolar motile behaviors, with few protrusions toward the NSB, whereas 5 µM MO morphant cells express increased motility and improperly direct this motility towards the boundary. One unit represents 6% of the total protrusive activity directed towards that sector, and each sector is oriented relative to the NSB. Scale bars: 50 µm.