Figure 2. Cell polarity in Ciona notochord morphogenesis.

( A– E) Schematics of the entire notochord primordium at successive stages of notochord morphogenesis. Dorsal view with anterior to the left in all cases. ( A’– E’) Zoomed in views showing just a few notochord cells with greater detail. Cell nuclei, basement membranes, polarized actin and myosin, and polarized apical and lateral markers are shown as in the key at the bottom of the figure. ( A, A’) The notochord initially forms a flat plate of cells that is relatively isodiametric in the AP (anterior-posterior) and ML (mediolateral) axes. No AP or ML polarity has been described at this stage, but there is a slight asymmetry of apical-basal markers on the ventral and dorsal sides (not seen in this dorsal view). ( B, B’) The notochord cells become mediolaterally elongated and aligned as they begin to intercalate. The notochord as a whole extends along the AP axis while narrowing across the mediolateral axis. The notochord cells are clearly polarized, with a flat edge contacting the flanking tissues and actin-rich protrusions reaching across to the other side. Cell nuclei are polarized medially. ( C, C’) At the end of intercalation, the notochord forms a single-file rod of thin, disk-shaped cells (rectangular in the section shown). A perinotochordal ECM surrounds the notochord. Nuclei are located centrally and squished between the anterior and posterior cell faces. ( D, D’) The notochord cells become progressively taller in the AP dimension while narrowing mediolaterally. A new phase of AP polarity becomes evident with nuclei localized to the posterior and myosin enriched to the anterior of each cell. ( E, E’) Late in notochord morphogenesis, a distinct circumferential belt of cortical actin forms at the equator of each cell. Extracellular lumen pockets form between each notochord cell. Apical and lateral markers become distinctly polarized between the lumen pocket domain and the remaining donut shaped ring of notochord-notochord cell contacts.