Fig. 6. Expression of rib mRNA. In situ hybridization of wild-type embryos with rib RNA probe at embryo stages indicated. (A) Stage 1 (0-15 minutes after egg lay). No transcript is detected. (B) Stage 5 (3 hours). (C) Stage 7 (3.5 hours). Note expression in cells invaginating to form anterior and posterior midgut (amg, pmg). (D) Dorsal view of same embryo. rib transcript is not detected in cells invaginating to form cephalic furrow (cf) and posterior transverse fold (ptf). (E) Stage 8/9 (4 hours). Expression has turned off in the pmg and initiated in the proctodeum (pr) as it invaginates and in mesoderm as cells take on mesenchymal character and begin migration. (F) Stage 10 (5 hours). rib expression has initiated throughout the epidermis and is enriched in segmental patches (arrowhead). (G) Stage 11 (7 hours), surface view. Note rib expression in tracheal cells as they invaginate to form tracheal pits (tp). (H) Same as G, internal focal plane. Note rib transcript in the amg, pmg, foregut (fg) and hindgut (hg) regions, and mesodermal clusters (ms). (I) Stage 13 (10 hours). rib is expressed throughout the epidermis (ep) as it spreads during dorsal closure. Transcript is also seen in the amg and pmg deep to the focal plane. (J) Stage 13, ventral view. rib transcript is present broadly throughout the embryo including the amg and pmg, as the cells migrate towards each other and form the central gut tube, and the hindgut as it elongates. Broad expression continues in stage 14 and 15 (not shown). Scale bar: 30 µm.