Fig. 5.

VWF distribution in MKs of WT and Gata1^low mice before and after TPO treatment. Representative semithin sections of spleen from untreated a,c) and TPO-treated b,d) WT a,b) and Gata1^low c,d) mice were immunostained for VWF and analyzed by immunoelectron microscopy. Representative MKs and the corresponding cytoplasm are shown in the insets and panels, respectively. VWF staining appears in a punctuate pattern. Red arrows and arrowheads point to structures containing the protein, while the white ones point to the structure itself. In mature MKs from untreated WT mice a), VWF staining was found almost equally distributed between α-granules (arrowheads) and the cytoplasm. After TPO treatment b), only immature WT MKs could be analyzed, but the proportion of VWF in α-granules and cytoplasm remained nearly the same. Staining often was found near the DMS (whole arrows). In untreated Gata1^low mice c), only immature MKs could be studied and only a very minor portion of the VWF was bound to the α-granules. Treatment with TPO d) induced the appearance of mature MKs and in these, most of the VWF was dispersed in the cytoplasm, but also occasionally found in α-granules. Original magnifications: × 30,000 for panels; × 4,400 for insets.