Fig. 9. A model for the regulation of the final steps of zebrafish PGC migration. Arrows indicate the direction of cell movement. (A) During early somitogenesis the lateral mesoderm of the anterior trunk (blue), which is marked by expression of the wt1 gene, produces signals that attract PGCs. (B) At the six-somite stage PGCs have formed clusters in the attracting region, while in some embryos posterior trailing cells are still migrating anteriorly (step V). (C) At about the 10-somite stage, the attraction center stops to function as such (light blue) or the PGCs no longer respond. The clusters of PGCs and remaining trailing PGCs, which have not yet reached the clusters, start to migrate (downward arrows) towards their final target, located in the lateral mesoderm around somite levels 8 to 10 (green). It is possible that the final target, which presumably gives rise to the somatic tissues of the gonad, also attracts PGCs. (D) At 24 hpf, all PGCs have reached the final target (green), while the cells of the intermediate attraction center (light blue) contribute to formation of the pronephros.
