Fig. 3. Convergence and extension of zebrafish dorsal mesoderm. Time sequences taken from a single 4-hour 4D recording of a wild-type embryo, beginning at midgastrula stage (7.3 hpf) (Kimmel et al., 1995) (see Movies 2-5 at http://dev.biologists.org/supplemental/). The AP axis is vertical (anterior towards the top), and the ML axis horizontal, here and in Figs 5,6,7, Fig. 9, Fig. 11. The field is centered approximately on the dorsal midline. (A) Views from the recording, at 7.3 (A1), 8.8 (A2), 9.3 (A3) and 11.3 (A4) hpf (hours postfertilization). Negative confocal microscope images of the BODIPY-ceramide labeled cellular field are shown. At the first time point (A1) the blastoderm margin is evident, separating the cellular blastoderm (upper) from the yolk syncytial layer (YSL, lower). Subsequently, the blastoderm comes to cover the yolk completely by the spreading movement of epiboly. Brachet's cleft, which appears as a hazy ring (arrows, A2), separates ectoderm to the outside, and mesoderm to the inside. The notochord/somite boundaries (axial/paraxial boundaries: arrowheads, A3) appear in the mesoderm. They are barely visible in A2 and then become prominent. Convergence narrows the notochord domain to about 2 cells wide at the last time point (A4). At the lateral side of each boundary lies a distinctive row of somitic adaxial cells that will form slow muscle (Blagden et al., 1997; Devoto et al., 1996). In addition, the somite boundaries are visible and are marked by arrows. (B) About 200 cells, represented as spheres, were tracked from the recording and are shown at 7.3, 8.3, 8.8, 9.3, 10.3 and 11.3 hpf. Cells are color-coded according to their eventual fates: notochord-forming cells (green), adaxial cells (dark blue), cells that form somite 2 (yellow) and other somite-domain cells (red). Nearly all of the cells divided during course of the 4 hour recording. Almost without exception, both siblings ended up in the same domain, and we color-coded the mother cell identically to its daughters. The notochord, and somite domains are spatially separate from the outset of the recording, even though distinctive boundaries are not yet present (A1). (C) Notochord-forming cells (green) and overlying floorplate-forming cells (magenta) shear relative to one and other during extension (48 floor plate-forming cells were tracked). The two cell types were collected from separate focal planes of the original 4D recording, and a 30 µm stripe cut at a single AP level is shown at 7.3, 8.3, 9.3 and 11.3 hpf. (D) The notochord domain (green) extends more than the somite domains (red). A 30 µm horizontal stripe of mesoderm cut from the field and is shown at 7.3, 9.3 and 11.3 hpf. Scale bar: 50 µm.

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