Fig. 7. T^C/T^C conceptuses exhibit defects in heart and yolk sac. (A) Equivalent sections (insets in a and b) of +/+ (a) and T^C/T^C (b) 6-s hearts show a reduction in the pericardial cavity (pc) and both myocardial (my) and endocardial (ec) layers in T^C/T^C. A few Flk1-positive cells (arrowheads) were observed in the T^C/T^C mutant endocardium. Scale bar in b: 100 µm for a,b. (B) Yolk sacs were isolated from EHF-stage conceptuses (a) and cultured for 24 hours to produce yolk sac spheres. 1 and 2 indicate the level of cuts made in the conceptus to isolate the yolk sac (see Materials and methods). (b) Histological section through a +/+ yolk sac sphere. (c-g) Higher magnification of yolk sac vasculature in +/+ (c), T^C/T^C (d,e) and T^C/+ (f,g) spheres. Flk1-positive endothelium (arrows) was discontinuous and the endoderm layer was not uniformly thick in T^C/T^C yolk sacs (compare d and e with wild type in c). Presumptive hematopoietic precursor cells (black arrowheads) were present between T^C/T^C yolk sac layers, but remained in a tight cluster and contained pyknotic cells (red arrowheads). (f,g) Two different regions of the same T^C/+ sphere reveal an intermediate phenotype (see text). Abbreviations as in Fig. 1. ht, heart; en, endoderm of foregut; se, surface ectoderm; mt, mesothelium; `x', former exocoelomic cavity of the intact conceptus. Scale bar in d: 100 µm for c-g; 400 µm for b.