PDGF-BB directly activates m-SCF but stimulates HSC/MPP2 proliferation indirectly. (A) PDGF-BB in plasma 30 minutes (Ctrl., n = 5; Depl., n = 6) and 2 hours (Ctrl., n = 5; Depl., n = 4) after platelet depletion. Ctrl. in blue, Depl. in red. (B) PDGF-BB in BM 30 minutes (Ctrl., n = 7; Depl., n = 7) and 2 hours (Ctrl., n = 10; Depl., n = 9) after depletion. (C) Quantitative PCR analysis of Pdgf-b expression in ECs (Ctrl., n = 11; Depl., n = 11) and Mks (Ctrl., n = 3; Depl., n = 3) at 2 hours after platelet depletion. (D) PDGF-BB in the BM of Pf4-cre⁺; iDTR mice at 2 hours after depletion (Ctrl., n = 4; Depl., n = 5). (E) Total SCF in the BM after PDGFR inhibitor treatment and platelet depletion at 2 hours (Ctrl., n = 5; Depl., n = 5). (F) Cell cycle analysis of CD41⁻ HSCs and CD41⁻ MPP2 after PDGFR signaling inhibition and induction of thrombocytopenia (Ctrl., n = 5; Depl., n = 7; Ctrl.+blocking, n = 7; Depl.+blocking, n = 7). (G) Representative graphs of pStat5 and pErk1/2 in HSCs, with and without PDGFR inhibition, 2 hours after platelet depletion (data combined from 2 independent experiments, n = 4). (H) Total SCF in the BM after PDGFRb blocking antibody treatment and platelet depletion at 2 hours (Ctrl., n = 5; Depl., n = 5). (I) Frequency of m-SCF⁺ Mks after PDGFRb blocking antibody treatment and platelet depletion at 2 hours, as analyzed by fluorescence-activated cell sorting (Ctrl., n = 5; Depl., n = 5).