Fig. 4. Morphogenesis and tissue patterning are largely unaffected in Blimp1-deficient embryos. Whole-mount in situ hybridization analysis of Blimp1 expression at (A) 9.5, (B,C) 10.5 and (D,E) 11.5 dpc. (A-D) Blimp1 mRNA is strongly expressed throughout the mesenchyme of the emerging forelimb bud at 9.5 dpc (A), but becomes progressively restricted to the apical ectodermal ridge (indicated by the white arrowhead in D) and underlying mesenchyme by 10.5 dpc (B). One day later (D), at 11.5 dpc, expression is confined to the posterior side of the limb bud in a region encompassing the zone of polarizing activiy (ZPA, black arrowhead). At 10.5 dpc (B,C), Blimp1 transcripts are also expressed in the myotomal compartment of the somites, and in the intersegmentary arteries (arrow in C) forming between the somites. At 11.5 dpc (D,E), expression is detected in the endothelial cells of the capillary plexus (arrow in E). Blimp1 is not expressed in the developing heart at any stage examined. (F,G) Histological analysis of wild-type (+/+) and mutant (-/-) embryos fails to reveal differences in overall morphogenesis and patterning of the heart at 10.5 dpc. (F parts 1 and 2, G parts 1 and 2) Hematoxylin-Eosin stained transverse sections of embryos shown in F and G; planes of section indicated by the red dashed lines. ba, branchial arch; he, heart; st, septum transversum; da, dorsal aorta; cv, anterior cardinal vein; uv, umbilical vein; 2baa, second branchial arch artery.