Fig. 1. Formation of the ulna and posterior forelimb digits requires Blimp1 expression. (A-D) Alcian Blue staining of E13.5 control (+/+) and mutant (m/m) forelimbs (A,B) and hindlimbs (C,D) reveals absence of the ulna and fourth and fifth digits in Blimp1 null-embryo forelimbs, whereas hindlimbs develop normally. (E) At E16.5, Alcian Blue-Alizarian Red staining shows that growth, patterning and ossification of the residual long bones and digits proceeds normally in the mutant. (F-H) Whole-mount in situ hybridisation (WISH) showing Blimp1 expression is initially confined to the posterior mesoderm at E9.5 (F) and is rapidly downregulated over the next 48 hours (G,H). At E11.5, Blimp1 expression is also detectable in the AER. (I) WISH shows that the Blimp1-Cre transgene faithfully recapitulates the pattern of endogenous Blimp1 expression. Cell-fate mapping studies in the forelimb demonstrate that Blimp1+ cells initially confined to the posterior mesenchyme at E10.5 (J), give rise to the posterior half of the limb bud by E12.5 (K,K'). (L,L') At E14.5, these mesodermal derivatives form the entire fourth and fifth posterior digits and the posterior half of the third digit, and the ulna. Blimp1⁺ cells also give rise to the posterior muscles, and connective tissues (L'). Diffuse staining in the dorsal limbs is due to the presence of lacZ-expressing endothelium. The fate map observed for Blimp1⁺ cells in the hindlimbs was identical. A, anterior; P, posterior; R, radius; U, ulna.