Sensitivity of the cerebral vasculature to induced thrombocytopenia ends within the first weeks after birth. (A) Representative flow cytometry plots of platelets in the peripheral blood of P7 mice 48 hours after treatment with IgG or anti-GP1Bα antibody. (B) Quantification of circulating platelets 48 hours after treatment with IgG or anti-GP1Bα antibody on P1, P7, or P14. P1: IgG control, n = 7; anti-GP1Bα, n = 6. P7: IgG control, n = 3; anti-GP1Bα, n = 11. P14: IgG control, n = 5; anti-GP1Bα, n = 9. Error bars represent mean ± standard deviation. ****P < .0001. (C-E) Representative images of brain and horizontal sections of the cerebellum 48 hours after injection at (C) P1, (D) P7, and (E) P14. P1: IgG control, n = 8; anti-GP1Bα, n = 8. P7: IgG control, n = 6; anti-GP1Bα, n = 14. P14: IgG control, n = 6; anti-GP1Bα, n = 15. Arrows indicate sites of ICH. Scale bars represent 1 mm. (F) Representative images of the dermal skin layer showing hemorrhage (arrows) 48 hours after injection at P1 (IgG, n = 7; anti-GP1Bα, n = 10), P7 (IgG, n = 4; anti-GP1Bα, n = 4), and P14 (IgG, n = 3; anti-GP1Bα, n = 7). (G) Frequency of hemorrhage in the cerebral cortex (CTX) and cerebellum (CB) 48 hours after antibody injection at P1 (IgG, n = 7; anti-GP1Bα, n = 10), P7 (IgG, n = 8; anti-GP1Bα, n = 8), and P14 (IgG, n = 5; anti-GP1Bα, n = 8). *P = .02; **P = .001; ***P = .0001. (H) Frequency of skin hemorrhage at P1, P7, and P14 at 48 hours after injection. (I) Summary schematics illustrating (i) identification of 3 levels of in utero thrombocytopenia that confer increasing risk of ICH and hemorrhage severity; (ii) the development of resilience to thrombocytopenia-induced ICH within the first 2 weeks after birth. Data were analyzed using the unpaired two-tail Student t test (B) or by using contingency table analysis with Fisher’s exact test (G-H). P values were adjusted for multiple testing using the Holm-Šídák method (G-H).