Fig. 6. Defective specification of primordial germ cells (PGCs) in Blimp1 mutant embryos. Endogenous alkaline phosphatase (AP) activity and SSEA1 antibody reactivity identifies PGCs. (A) Number of embryos in which AP-positive PGCs could be identified at the early headfold stage (EHF). Genotypes are indicated. Cell counts show that Blimp1 heterozygotes display less than 50% of the wild-type number of PGCs, while less than four AP-positive PGCs were observed in three out of six Blimp1 homozygous mutants. (B) AP staining in wild-type (+/+), heterozygous (+/-) and homozygous (-/-) mutants at the indicated somite (s) stage. Reduced PGC numbers were found in the heterozygotes (arrow) compared with the wild-type siblings, and no migrating PGCs were detected in homozygous mutants embryos. (C) Evaluation of individual SSEA1 cell-surface-antigen-positive cells in the hindgut of wild-type (+/+), heterozygous (+/-) and homozygous (-/-) mutants confirms the absence of migrating PGCs in Blimp1 null embryos, and reveals that significantly fewer SSEA1-positive cells are present in heterozygotes than in controls. Midline staining corresponds to SSEA1 reactivity with the hindgut lumen.