Fig. 2

osr1 is required to promote intermediate mesoderm differentiation. (A-H) Dorsal views, anterior to the left, of the posterior mesoderm at the 10-somite stage (som). (A-D) In situ hybridization shows expression of lhx1a in the IM (arrows). Compared with wild type (A), expression is narrowed in osr1 (B), widened in hand2 (C) and irregular in hand2; osr1 (D) embryos. Expression in the notochord (asterisk) is unaffected. Unlike the width of the IM, we did not observe a change in the proximal-distal length of the IM in mutant embryos. (E-H) Three-dimensional reconstructions of Pax2a immunofluorescence in the IM of wild-type (E), osr1 (F), hand2 (G) and hand2; osr1 (H) embryos. (E′-H′) Magnification of boxed 250 µm long regions used for counting Pax2a⁺ cells. White dots indicate Pax2a⁺ nuclei. (I) Quantification of Pax2a⁺ cells in a 250 µm long region of the IM in the indicated genotypes. Symbols represent individual embryos (circles, average of left and right IMs; triangles, single IM; see Materials and Methods); boxes represent interquartile range; central line marks the median; whiskers indicate maximum and minimum values. P values were calculated using non-parametric Mann–Whitney U-tests: ****P<0.0001, ***P=0.0011, **P=0.0094. Scale bars: 100 µm (A-H); 50 µm (E′-H′).