Fig. 4. Primitive macrophages develop normally in bls. Primitive macrophages develop from anterior paraxial mesoderm, and express (A) pu.1, (B) cmyb and (C) dra. Expression of pu.1 is unique to primitive macrophages, whereas cmyb and dra transcripts are also detected in the ICM at 20 somites. (A) Expression of pu.1 is not perturbed by bls, and can be detected in the anterior paraxial mesoderm at 12 somites. (B) cmyb expression in the primitive macrophage is unaffected by bls (arrowhead) but its expression in the ICM (asterisk) is absent at 20 somites. (C) Similarly, dra expression in the primitive macrophage primordia is unaffected by bls. However, dra expression in bls is significantly reduced in the lateral plate mesoderm at 12 somites (asterisk) and in the ICM at 20 somites, similar to the pattern of scl expression.