Abnormal distribution of organelles in Myh9^−/−MK fragments shed into the sinusoids. (A) In situ bone marrow TEM images showing MKs in the process of shedding large fragments into a sinusoid (i,iii) (arrows) or fragments already released (ii,iv) (arrowheads). (i-ii) WT bone marrow; (iii-iv) Myh9^−/− bone marrow. Note the fragment almost devoid of granules in Myh9^−/− marrow (iv, arrowhead). S, sinusoid vessel; P, platelet. (B) Scheme recapitulating the importance of organelle positioning within MKs for normal organelle distribution within platelets. An impaired granule distribution in Myh9^−/− MKs (1) leads to low-content or high-content preplatelet fragments (2) which are reorganized into low-content or high-content platelets (4), despite normal traffic along proplatelets microtubular tracks (3).