SLFN14-deficient MKs display reduced proplatelet formation. (A) Representative brightfield microscopy images illustrating proplatelet formation on differentiation day 5 in WT/WT, WT/K219N, and K219N/K219N cells (left). Proplatelets formed in WT/K219N and K219N/K219N cells are shorter and with a reduced ramification compared with those formed in WT/WT cells (arrows). Imaging was performed on cells at passage week 5 for WT/WT and WT/K219N, and week 3 for K219N/K219N. Quantification of proplatelet-forming cells on day 5 for 3 independent differentiations shows a significant decrease in both WT/K219N and K219N/K219N cells compared with WT/WT (right). On average, 133, 114, and 120 total number of cells were counted for WT/WT, WT/K219N, and K219N/K219N cells, respectively. Statistics performed with a pairwise Wilcoxon test. Boxplots represent the middle 50% of the data points with 25% outliers as whiskers and the mean as a black horizontal line. All individual data points were added as dots to the graph. (B) SiR-tubulin in vivo staining of day 5 differentiated MK shows the presence of shorter proplatelets with reduced ramification for WT/K219N and K219N/K219N conditions compared with WT/WT MK (representative images from a blinded assay). Scale bars, 100 μm.