Figure 6.

Vegetal injections of an intracellular Wnt activator induce an ectopic organizer by diffusion into marginal cells. A, B Ventro-vegetal injections of β-catenin RNA induce ectopic expression of chordin only when the injected RNA reaches marginal cells. Embryos injected in D4 blastomeres with β-catenin (500 pg) and LacZ (1 ng) RNA were collected at stage 9 and stained by double in situ hybridization (chordin in purple, LacZ in green-blue, n=60). Ectopic chordin expression is absent when the injected RNA is confined to descendants of vegetal tier D cells (A, A1), but present when the injected RNA is found in marginal cells (B, B1, arrow). Embryos are shown whole in vegetal views (A, B, dorsal side to the left) or in sections (A1, B1, section plane indicated in A, B). Wild-type chordin expression is seen on the dorsal side (A, B1). C-E Diffusion of vegetally injected material to the marginal zone is seen in early, but not late, injected D4 cells. Fluoresceine dextran (100 ng) was injected into D4 blastomeres (C, D) immediately after the start of D/C tier separation, followed by fixation either immediately (C, n=10) or after 15 min (D, n=11), or (E) 15 min after the start of the separation (n=15). C, D, E are UV/blue channel pictures, showing the autofluorescence of sectioned embryos. C1, D1, E1 are UV/green channel pictures, showing the position of injected dextran in the same embryos. Brackets in C, D, E, and arrowheads in C and C1 indicate the limits of D4 and C4 blastomeres.