Fig. 2

Defects in Endocardial Development in tal1^t21384 Mutants (A) Confocal images of the head region from embryos transgenic for fli1a:gfp at 32 hpf. Arrows indicate the location of the endocardium. In the wt endocardium, a lumen is visible, which is absent in tal1^t21384 mutants. (B) Transversal sections of wt and tal1^t21384 mutant heart ventricles at 56 hpf stained with hematoxylin/eosin. In wt embryos, the endocardium forms an epithelium attached to the myocardium, and blood cells are visible within the heart tube. In the tal1²¹³⁸⁴ mutant heart, the endocardium fails to form an epithelium and is only present in the heart ventricle. (C) Rescue of endocardial defects. Maximum z-projection of a stack of confocal images of embryos carrying the vegfr4:gfp transgene. Embryos were divided into three classes based on their endocardial phenotype. Class 1: wt length of the endocardium with normal tube formation (wt phenotype). Class 2: wt length of the endocardium but no tube formation (intermediate phenotype). Class 3: short endocardium with no tube formed (mutant phenotype). All embryos homozygous or heterozygous for the wt allele are in class 1, whereas all embryos homozygous for the tal1^t21384 allele are in class 3, showing the high penetrance of this phenotype. In 43 out of 54 embryos homozygous for the tal1^t21384 allele (83%), the endocardial phenotype can be rescued to class 1 through injection of low amounts of wt tal1 mRNA (20 pg per embryo), whereas most of the remaining (8/54, 15%) display an intermediate phenotype (class 2).