Chronic platelet depletion leads to HSC exhaustion. (A) Numbers of CD41^−/+ HSCs and CD41^−/+ MPP2 in BM of mice that were injected with antiplatelet or control serum every second day for 2 weeks; BM was analyzed at day 15 (Ctrl., n = 14; Depl., n = 14). (B) Cell cycle analysis of CD41^−/+ HSCs and CD41^−/+ MPP2 after repetitive platelet depletion (Ctrl., n = 10; Depl., n = 10). (C) HSCs (300 cells) were sorted from Ubc-GFP mice after chronic platelet depletion and transplanted into lethally irradiated recipients together with 5 × 10⁵ total BM competitor cells. The frequency of donor-derived blood cells was followed up to 16 weeks (Ctrl., n = 5-11; Depl., n = 6-11). (D) Frequency of donor-derived blood cells after secondary transplantation of 300 HSCs from mice with chronic platelet depletion together with 5 × 10⁵ fresh total BM competitor cells (Ctrl., n = 6; Depl., n = 6). (E) Increased expression and localization of different growth factors and activation of their receptors consequently leads to activation of HSC/MPP2 proliferation in response to acute thrombocytopenia. Upon platelet depletion, VEGF-A is quickly released in the BM. Next, VEGF-A/VEGFR-2 activation on ECs and possibly other cells induces the production and release of PDGF-BB, which triggers the localization of SCF from the cytoplasm of Mks to their membrane (m-SCF). In turn, this potentially imposes juxtacrine signaling between Mks and HSPCs (m-SCF/c-Kit), decreases the relative distance between both cell types, and, in conjunction with VEGF-A/VEGFR-2, specifically activates HSCs and MPP2. Gray arrows represent release/production/localization, black arrows represent binding/activation, double-headed arrows represent nearing of different cells, and red color denotes changes upon platelet depletion.