Epcr-transduced Mpl^−/−BM cells showed increased MK numbers after transplantation. (A) Lentiviral vector architecture and transplantation scheme. lin⁻ BM cells of Mpl^−/− CD45.2 donor mice were transduced and transplanted into Mpl^−/− CD45.1 primary recipient mice or serially into secondary Mpl^−/− CD45.1 recipient mice. Mice were irradiated before transplantation. (B) Histology of BM sections of the Epcr and negative control (as indicated). Left, hematoxylin and eosin stain; original magnification ×400. Right, hematoxylin and eosin stain; ×1000 magnification. The MKs can be identified by their size and the multilobed nucleus (arrowheads). (C) MKs were counted in 3 to 9 BM sections per group and the number of MKs is expressed as MKs per field of view (mean plus or minus SD). Two persons counted 8 fields of view per section, independently. The number of MKs in the Epcr group (n = 6) is significantly increased by threefold compared with the negative control (n = 9) (2-tailed, unpaired Student t test with Welch correction [*P ≤ .05]; positive control (n = 5) vs negative control [**P ≤ .01]). PGK, human phosphoglycerate kinase promoter; R, repeat region; U3 and U5, unique region 3′ and 5′, respectively; wPRE, woodchuck hepatitis virus posttranscriptional regulatory element.