Cell lineage of zebrafish blastomeres. III. Clonal analyses of the blastula and gastrula stages.

Aspects of the early lineages of blastomeres in the embryo of the zebrafish, Brachydanio rerio have been described. Because of the optical clarity of the embryo, lineages of selected cells can be followed directly by microscopy through many cell divisions. Also, it is shown here that the fluorescent molecules fluorescein-dextran and rhodamine-horseradish peroxidase can be used as cell lineage tracers, marking the clonal progeny of founding blastomeres. The labeled cells can be easily visualized in the live embryo, and utilizing a sensitive video camera to amplify fluorescence, the same clone may be examined repeatedly while the cells divide and migrate. Cells that descend from a single blastomere remain closely associated together through the end of the blastula stage. At the time when epiboly begins (early gastrula) cells in the labeled clone scatter and become dispersed among unlabeled cells. It has been observed that there is no invariant mapping of the embryo's midline (determined by the position of the embryonic shield in the gastrula) with respect to the early planes of cleavage. This finding shows that in the zebrafish the region of the embryo that a cell will occupy is not specified by the cell's early ancestory.