Fig. 4

cas acts cell autonomously in the endodermal progenitors. A cas mutant host at 80% epiboly into which wild-type cells were transplanted contains several axial-expressing endodermal precursors in the lateral hypoblast (A). These cells all derive from the wild-type donor as they also contain the biotin dextran lineage tracer (brown stain in B); no mutant host cell was ever observed to form endoderm, as judged by axial expression. Under higher magnification (C) the presence of biotin dextran in the axial-expressing endodermal precursors is clearly seen (arrowheads indicate brown cells with purple cytoplasm); several cells not expressing axial also contain biotin dextran (arrows indicate brown cells). In 53 wild-type to wild-type control transplantations, we observed four cases in which transplanted cells formed axial-expressing endoderm (data not shown). Wild-type cells transplanted into cas mutant hosts formed axial-expressing endoderm in 5 of 77 cases (P = 0.5-0.9). cas mutant cells were never observed to form axial-expressing endoderm when transplanted into wild-type hosts (33 events; P < 0.1). (A, B) Right lateral views, anterior to the top; (C) high-magnification view of B.