Morphological and optical properties of human immature platelet-enriched population produced in immunodeficient mice

Ultrastructure analysis of immature platelets is difficult because of the lack of a suitable marker and their relatively low concentration in total platelets. We investigated the morphological and optical properties of human immature platelets produced and enriched in immunodeficient mice via human CD34-positive cell administration. Immunodeficient mice were injected with human CD34-positive cells and administered eltrombopag orally for 14 days (eltro-mice). Some of these mice were maintained for 2-3 months (steady-state-mice). Platelets were double-stained with a human CD41 antibody and a nuclear staining dye (Sysmex hematology analyzer XN series reagent), and then analyzed by flowcytometry FCM to identify human immature platelets. Human CD41-positive cells were isolated from citrated blood by magnetic cell sorting with human CD41 antibody, and examined using electron microscopy. Flow cytometric analysis with the XN reagent demonstrated that peripheral blood from eltro-mice had a higher percentage of immature platelet fraction in human platelets than that from steady-state-mice. The geometric mean of XN reagent fluorescence for human platelets, divided with that for mouse platelets, revealed that the ratios in eltro-mice were significantly higher than those in steady-state-mice, thus indicating that immature platelets were highly enriched in eltro-mice. Scanning and transmission electron microscopy revealed that human citrated platelets isolated from eltro-mice tended to be larger (n = 15, p = 0.276) and contained more mitochondria than those isolated from steady-state-mice (n = 10, p = 0.0002). Therefore, an increased number of mitochondria, rather than platelet size, is a distinctive feature of immature platelets.