Phenotype of E12.5 AML1-ETO/+ and wild-type (WT) embryos. (Top panels) AML1-ETO/+ embryos were identical in size to wild-type littermates, but were easily identifiable by the presence of fetal liver pallor and massive hemorrhages within the ventricles of the CNS and the soft tissues of the back. (Second panels) Sections showing hemorrhages within the dorsal root ganglia ofAML1-ETO/+ embryos. (Third panels) Sections of the fetal liver from AML1-ETO/+ embryos showing a complete absence of hematopoietic precursors with only rare primitive nucleated erythrocytes seen within hepatic sinusoids. By contrast, sections of the fetal liver from control littermates show numerous erythroblasts and scattered myeloblasts and megakaryocytes. (Bottom panels) Smears of peripheral blood show the absence of definitive erythrocytes and platelets in the AML1-ETO/+ embryos. By contrast, numerous enucleated definitive erythrocytes and platelets are seen in the peripheral blood from wild-type embryos.