Fig. 6. Visceral endoderm replaces the definitive endoderm in the lateral region of the embryonic gut. (A-C) The 8.0 dpc [late headfold stage; anterior (A) and posterior (B) views of the same embryos] and 8.5 dpc (C, ventral view) mutant embryos showing the endocytotic activity of visceral endoderm revealed by the uptake of horseradish peroxidase (HRP; 2 mg/ml, brown staining reaction) for 30 minutes. The visceral endoderm in the yolk sac of the mutant embryos demonstrate normal HRP-uptake activity, but the endodermal cells displaying similar uptake activity are also found in the prospective gut region of the embryo (A,B, broken arrows; C, arrowheads). (D,E) Expression of Hnf4 in the visceral endoderm of the 8.5 dpc mutant embryos (D, lateral view; E, transverse section), reveals the expansion of the Hnf4 expression domain into the lateral region of both anterior and posterior gut in the mutant embryo (arrowheads). (F,G) Expression of Gata4 in the cardiogenic mesoderm (ht) and visceral endoderm (F, anterior view; G, transverse section) of the 8.5 dpc mutant embryos. Gata4 expression, which is normally restricted to the visceral endoderm, extends into the lateral regions of the posterior gut in the mutant embryos (G, arrowheads). (H-J) Afp expression in the visceral endoderm of the normal and mutant embryos at early neural plate (H, lateral view) and headfold stages [anterolateral (I) and posterior (J) views of the same embryos]. In the mutant embryos, Afp expression is associated with the yolk sac endoderm until the early neural plate stage (H), but by the headfold stage, expression extends into the posterior and lateral regions of the embryonic gut (I,J, broken arrows). In A,B,H-J, the anterior, lateral and posterior border between visceral and definitive endoderm is indicated by arrowheads, broken arrows and arrows, respectively. In C-E,G, arrowheads mark the border between the visceral endoderm and the definitive endoderm. The broken lines in D and F mark the plane of sectioning. fg, foregut; hg, hindgut; ht, heart. Scale bar: 100 µm.