Cardiac morphogenesis in zebrafish. The cardiac precursors are in the ventral margin at the onset of gastrulation in zebrafish. (A) These ventral-derived cardiac precursors migrate toward the animal axis and form bilateral heart primordia on either side of the embryo (arrowhead), flanking the prechordal plate and notochord (arrow) at the 14-somite stage. (B) The bilateral heart tubes then fuse at the midline (arrowhead), anterior to the notochord (arrow). (C) At 24 hours post fertilization (hpf), the primitive heart ‘jogs’ to the left (arrow points to the prospective atrium). The heart then gradually returns to the midline. (D) At 48 hpf, the ventricle of the midline heart loops to the right. The bilateral heart tubes and primitive heart are labeled with Nkx2.5 in A, B and C, and MF20 in D. Notochord precursors are labeled with Brachyury in A and B. (A,B) Dorsallateral view, anterior to the left. (C) Dorsal view, anterior to the bottom of the panel. (D) A ventral view. a, atrium; v, ventricle; L, embryo left.

Abnormal jogging and looping. (A) Jog reversal. Hearts that jog to the right (left panel, arrow points to the prospective atrium) always loop to the left (right panel). (B) Directionless jog. In no tail mutant embryos, the hearts fail to jog (arrowhead points to the midline) and may loop to left, right or remain straight. The heart is labeled with Nkx2.5 in 24hpf embryos and MF20 in 48hpf embryos. a, atrium; v, ventricle; L, embryo left.

Cardiac BMP4 asymmetry is disturbed in laterality mutants. (A,C) Normally, BMP4 transcripts accumulate predominantly on the left side of the heart tube at the 22-somite stage. (B) At the same stage, Nkx2.5 expression in the heart is symmetric. In those mutant embryos in which the heart fails to jog, such as floating head (flh), BMP4 is more evenly distributed in the heart (D). Mutants with a higher incidence of right-jogged heart, such as iguana (igu), have a higher incidence of right-dominant cardiac BMP4 (E). All are dorsal view, anterior to the bottom of the panel. Lines mark the midline. L, embryo left.

BMP4 asymmetry persists through the jogging phase. (A) Left-predominant BMP4 and left jog. (B) Right-predominant BMP4 and right jog. C is enlargement of the heart region of A and D is the enlargement of the heart region of B. va, visceral arches. Black arrow and white arrow head point to the heart.

Cardiac BMP4 expression pattern in shh and XBMPtR injected embryos. (A) BMP4 expression is more symmetric in the heart tube after injection of shh mRNA at the 1- to 2-cell stage. (B) Injection of the truncated BMP4 receptor, XBMP4tR mRNA, which interferes with jogging and looping, has no effect on cardiac BMP4 expression pattern. Dorsal-lateral views, anterior to the left.