Spatially regulated platelet spreading and α-granule secretion are impaired in gray platelet syndrome and Wiskott-Aldrich syndrome. (A) Gray platelet syndrome (GPS) patient platelets, which are deficient in α-granules, adhere and spread onto collagen microstrip patterns (blue). Healthy donor platelets extend filopodial projections (red) on protein microstrips, whereas GPS platelets remained geometrically constrained within the boundaries of the protein microstrips. (B) The number of filopodia extending beyond the collagen microstrips are significantly reduced in GPS platelets (P < .05). (C) In addition, platelet-platelet interconnections between adjacent protein microdots (of 5- and 10-μm diameters) are significantly reduced in GPS platelets (P < .0001 for both sizes of microdots). (D) Compared with healthy control platelets, Wiskott-Aldrich syndrome platelets exhibit significantly less spreading areas beyond the microdot boundaries. The scale bars = 10 μm; error bars indicate SE.