Fig. 5

ALK1 is required to restrict the size of the organizer domain. a–d To determine the function of the ALK1 receptor during early gastrulation, embryos were injected with increasing amounts of ALK1MO (3.4 and 1.7 ng/embryo). The effect of blocking this receptor on the chordin expression domain was determined by in situ hybridization. The relative (%) arc of the expression domain was determined. e–h The specificity of the ALK1MO (1.7 ng/embryo) was analyzed by co-injection with RNA encoding the mouse ALK1 receptor (2.2 ng/embryo). Changes in the domain of chordin expression were quantitated. i–p RNA encoding the truncated, dominant negative form of the mouse ALK1 receptor (tmALK1) was injected into Xenopus embryos either in the dorsal or ventrolateral regions of the embryo. The effect of the ALK1 activity was determined by in situ hybridization. Analysis of the changes in the chordin (i–l) and gsc (m–p) expression domains was performed at the onset of gastrulation. The relative (%) arc of the expression domain was determined in each case. ALK1MO amounts analysis: control, n = 18; ALK1MO 1.7, n = 18; ALK1MO 3.4, n = 15, ALK1MO rescue analysis: control, n = 83; ALK1MO, n = 62; ALK1MO + mALK1, n = 40, tmALK1 chordin analysis: control, n = 17; tmALK1 dorsal, n = 19; tmALK1 ventrolateral, n = 10, and tmALK1 gsc analysis: control, n = 34; tmALK1 dorsal, n = 15; tmALK1 ventrolateral, n = 20. p values were calculated compared to controls except in the rescue experiments that were compared to the ALK1MO sample (h). Statistical test; Dunnett’s (ANOVA) multiple comparisons test. **p < 0.01, ***p < 0.001, ****p < 0.0001, ns not significant