IP6K1 influences clot ultrastructure via polyP. (A) Confocal fluorescence micrographs of recalcified fibrin clots prepared from WT and Ip6k1^−/− platelet releasates mixed with autologous PPP, in the absence or presence of exogenous polyP or IP₇, as indicated. Fibrin fibers are visualized by incorporating Alexa Fluor 488–conjugated fibrinogen, and polyP is stained with DAPI. Scale bars represent 10 µm. (B) Fibrin fiber density in clots described in A was quantified using ImageJ software as described in the section Clot ultrastructure. Data are mean ± standard error (n = 5 for untreated and n = 3 for polyP containing clots). (C) PolyP content was estimated using ImageJ software by measuring relative fluorescence intensity (arbitrary units [AU]) in the DAPI channel over the entire field, averaged over 3 fields per clot. Data are mean ± standard error (n = 3). P values are from a 2-tailed Student t test (*P ≤ .05; n.s., not significant, P > .05).