Left-sided Pitx2 expression is conserved between mouse, Xenopus and zebrafish. (A-D) Pitx2 expression in the mouse from E7.0 to E9.0. (A) Earliest localized expression of Pitx2 is seen in two patches in the prospective head mesenchyme. Anterior view of an E7.0 embryo. (B) Pitx2 is asymmetrically expressed in the left LPM and in the left myocardium of the linear heart tube. Ventral view of a E8.0 embryo (6 somites). hf, headfolds; h, heart; l, left; r, right. (C,D) Left-sided expression of Pitx2 in the tubular gut at E9.0. Plane of section in D is indicated by the line in C. bw, body wall; g, gut; lsp, left splanchnopleura; rsp, right splanchnopleura; va, vitelline artery. (E-H) Pitx2 expression in Xenopus from stage 12 to stage 26. (E-G) From stage 12 to 18 sickle-shaped expression is seen in the cement gland anlage (E-G) and in two patches of head mesenchyme (G). (H) At stage 26 strong expression persists in the cement gland. Asymmetric expression is seen in the left (top) but not in the right (bottom) lateral plate mesoderm. l, left; r, right. (I-L) Pitx2 expression in the zebrafish embryo from 90% epiboly to 23 hour in the polster (I-K) and left LPM (L). (I) Animal pole view of an embryo at 90% epiboly (9 hours). (J) Animal pole view of a 3 somite embryo (10 hours). (K) Animal pole view of a 11 somite embryo (14.5 hours). (L) Dorsal view of a 23 hour embryo. Note that the staining is confined to the left LPM. l, left; r, right.