Identification of Ca-rich dense granules in human platelets using scanning transmission X-ray microscopy

Whole mount (WM) platelet preparations followed by transmission electron microscopy (TEM) observation is the standard method currently used to assess dense granule (DG) deficiency (DGD). However, due to electron density-based contrast mechanism in TEM, other granules such as α-granules might cause false DGs detection. Herein, scanning transmission X-ray microscopy (STXM), was used to identify DGs and minimize false DGs detection of human platelets. STXM image stacks of human platelets were collected at the calcium (Ca) L2,3 absorption edge and then converted to optical density maps. Ca distribution maps obtained by subtracting the optical density map at pre-edge region from those obtained at post-edge region were used for identification of DGs based on richness of Ca. Dense granules were successfully detected by using STXM method without false detection based on Ca maps for 4 human platelets. Spectral analysis of granules in human platelets confirmed that DGs contained richer Ca content than other granules. Image analysis of Ca maps provided quantitative parameters which would be useful for developing image-based DG diagnosis models. Therefore, we would like to propose STXM as a promising approach for better DG identification and DGD diagnosis, as a complementary tool to the current WM TEM approach.