MKs with distinct expression signatures are spatially organized in the BM niche, and CD41⁺ARNTL⁺ MKs are the dominant population in proplatelet formation. (A) Representative image of an Lin⁻CD48⁻CD41⁻CD150⁺ HSC (violet; indicated by arrowheads) and an MYLK4⁺ MK stained with MYLK4 (green) and CD41 (red; indicated by arrows). The images represent 3 independent experiments. (B) Quantification of the frequency of MKs (CD41⁺) in contact with HSCs: MYLK4⁺CD41⁺(21 cells), LSP1⁺CD41⁺ (18 cells), and ARNTL⁺CD41⁺ (20 cells). (C) Representative images of CD105⁺ blood vessel (violet) and ARNTL⁺ MK stained with ARNTL (green) and CD41 (red). The images represent 3 independent experiments . (D) Quantification of the distance between cells and blood vessels: MYLK4⁺CD41⁺ (58 cells), LSP1⁺CD41⁺ (49 cells), and ARNTL⁺CD41⁺ (185 cells). (E) Representative images of proplatelet-bearing MKs in the whole BM section using CD41⁺ staining. Proplatelet-bearing MKs were costained with ARNTL⁺, but these proplatelet-bearing MKs could not be clearly co-stained with MYLK4 or LSP1. The images represent at least 3 independent experiments. Arrows indicate proplatelet-bearing MKs; arrowheads indicate non–proplatelet-bearing MKs costained by MYLK4 and CD41. (F) Quantification of the frequency of MYLK4⁺CD41⁺ (50 cells), LSP1⁺CD41⁺ (21 cells), or ARNTL⁺CD41⁺ (47 cells) in proplatelet-bearing MKs. Scale bars, 20 μm. Error bars indicate SEM. Data are from one-way ANOVA. ****P ≤ .0001.