Fig. 2. Endoderm fate is induced by transplantation of the cells to the margin and by activation of Nodal signalling. (A) Diagram of the experimental procedure. (B,G) In late blastula (sphere stage, lateral view), a few cells (arrowhead) expressing GFP (green) alone or combined with Tar* were transplanted to the margin (B) or the animal pole (G) of a host embryo. (C-F,H-I) Anterior to the left, dorsal to the top. At 24 hpf, GFP-expressing cells transplanted to the margin mainly become endodermal (C) and mesodermal derivatives (D) (see Table 1). By contrast, activated (Tar* + GFP) cells almost exclusively become endodermal derivatives, such as pharynx (E) and gut (F), and mesendodermal hatching gland. (H) GFP-expressing cells transplanted to the animal pole populate the eye and the forebrain, whereas (I) Tar*-expressing cells contribute progeny only to the endodermal pharynx (white arrow) and to mesendodermal derivatives (hatching gland, white arrowhead). (J-L) Transverse sections showing the expression of the endodermal markers nkx2.3 (J), insulin (K) and fkd7 (L, arrowhead) by grafted Tar*-expressing cells. In all the experiments, nls-lacZ, which encodes nuclear ß-galactosidase was coexpressed with GFP as a lineage tracer for immunohistochemistry (brown nuclei) to identify grafted cells. Dorsal to the top. no, notochord; nt, nerve tube.