Origin and development of the zebrafish endoderm

The segregation of cells into germ layers is one of the earliest events in the establishment of cell fate in the embryo, In the zebrafish, endoderm an d mesoderm are derived from cells that involute into an internal layer, the hypoblast, whereas ectoderm is derived from cells that remain in the outer layer, the epiblast, In this study, we examine the origin of the zebrafish endoderm and its separation from the mesoderm. By labeling individual cell s located at the margin of the blastula, we demonstrate that all structures that are endodermal in origin are derived predominantly from the more dors al and lateral cells of the blastoderm margin, Frequently marginal cells gi ve rise to both endodermal and mesodermal derivatives, demonstrating that t hese two lineages have not Set separated, Cells located farther than 4 cell diameters from the margin give rise exclusively to mesoderm, and not to en doderm, Following involution, we see a variety of cellular changes indicating the d ifferentiation of the two germ layers. Endodermal cells gradually flatten a nd extend filopodial processes forming a noncontiguous inner layer of cells against the yolk. At this time, they also begin to express Forkhead-domain 2 protein. Mesodermal cells form a coherent layer of round cells separatin g the endoderm and ectoderm, In cyclops-mutant embryos that have reduced mesodermal anlage, we demonstra te that by late gastrulation not only mesodermal but also endodermal cells are fewer in number. This suggests that a common pathway initially specifie s germ layers together before a progressive sequence of determinative event s segregate endoderm and mesoderm into morphologically distinct germ layers .