Fig. 1. Zebrafish PGCs form bilateral clusters in the anterior trunk during early somitogenesis. (A,B) Dorsal views of zebrafish embryos depicting the movements (arrows) of PGCs that result in bilateral PGC cluster formation. The PGCs (red) are drawn relative to the adaxial cells, the somites and the lateral edges of the trunk mesoderm. (A) At the end of gastrulation, most PGCs have accumulated in two medial-to-lateral lines at the head-trunk border, while the rest aligns along the lateral trunk mesoderm borders in more posterior regions. During early somitogenesis, both groups of cells migrate towards the lateral mesoderm of the anterior trunk (steps IV and V). (B) At the six-somite stage, bilateral clusters of PGCs have formed in the anterior trunk, while the posterior trailing PGCs continue to migrate towards the anterior. (C-L) Fluorescent pictures taken at 16 minute intervals from a time-lapse movie showing migrating PGCs between the bud- and seven-somite stage on the right side of a wild-type embryo injected with GFP-nos1-3'UTR. Dorsal views, anterior is upwards. Medially located PGCs migrate laterally (one cell is marked by a green arrow) to join those PGCs that are already located in lateral positions. The forming cluster follows the general convergence movements medially towards the midline. A single ectopic anterior cell (red arrow) migrates posteriorly and laterally into the cluster. Note that on this side of the embryo no posterior trailing PGCs can be seen.