Fig. 5. R4 transplants can induce expression of val and krox-20. (A) Diagrams of the transplantations. Putative r4 cells (positioned midway between the first somite boundary and the tip of the notochord) are excised from fluorescein-labeled donor embryos at early 1-somite stage and transplanted into either the r4 region of early 1-somite-stage hosts (left arrow) or ventral ectoderm of shield-stage hosts (right arrow). (B-B'') Host embryo (16 s) showing donor cells (B), krox-20 expression (B'), and merge of the two labels (B''). Most donor cells are in r4, some are in r5. (C) Host embryo (10 s) with r4 transplant in lateral ectoderm (arrow) stained for val (blue), krox-20 (red). (D-E'') Magnified view of transplant in C. (D) val (blue) and krox-20 (red) expression largely overlap (the krox-20-expressing cells express val, but the blue val is partially obscured). Donor cells (E) and krox-20 expression (E’) partially overlap (arrowhead) in a merge (E''), showing induction of krox-20 (and val, compare with D) in the host (arrow). 12/12 transplants showed some val and krox-20 expression in donor cells; three out of 12 showed some val and krox-20 expression in host cells. (F-G'') R4 transplant (in the ventral ectoderm of a 10 s host embryo) showing hoxb1a (blue) and krox-20 (red) (F), donor cells (G), krox-20 (G') and merge (G''). hoxb1a expression is found in donor cells (arrows) and in host cells. krox-20 expression is found in donor cells and host cells (arrowheads). Six out of eight transplants showed some hoxb1a and krox-20 expression in donor cells; two of these cases also showed hoxb1a and krox-20 expression in host cells; two out of transplants showed no hoxb1a or krox-20 expression associated with the transplant. (H-H'') R6 transplant (in the ventral ectoderm of a 10 s host embryo) showing donor cells (H) and val (H'). An overlay shows val expression within the transplant (H''). Anterior is towards the left. Scale bars: in B, 50 µm for B-B''; in C, 100 µm for C; in D, 50 µm for D-H''.