The relationship between osteoblastic and granulocytic cell differentiation mediated by osteoblast secretion of FGF8f. Retinoic acid (RA)-induced FGF8f directly mediates osteosarcoma U2OS cell differentiation in an autocrine manner. Simultaneously, paracrine FGF8f, secreted into osteoblastic cell-conditioned medium by U2OS cells treated with RA or transduced with phosphorylation-defective RARa or FGF8f, bypasses RA stimuli to inhibit proliferation and induce granulocytic differentiation of RA-sensitive or RA-resistant leukemic myeloblasts at the paracrine level. For details, see article by Chaudhry and colleagues on page 267.