Mks generated from MkMP coculture display characteristic PPT structures and synthesize both α- and dense-granules. CD34⁺ cells were cocultured with MkMPs starting at day 0. (A) At day 11, cells were stained for β1 tubulin (TUBB1 [i]), VWF [ii], and serotonin (5-HT [iii-iv]) to visualize PPT structures, α-granules, and dense-granules, respectively. Panels (iii) and (iv) displaying serotonin staining of both cells with a nucleus (panel iii) and anuclear cellular fragments (PPTs; panel iv) to demonstrate the early development of dense-granules in cells prior to fragmentation. Scale bar: 50 µm (panels i-ii) and 20 µm (panels iii-iv). (B) TEM thin section of an Mk from day 11 of the coculture. G, granules; IMS, invaginated membrane system; N, nucleus.