Antibody ligation of CD9 alters the membrane structure and terminal maturation of MK cells in culture.

CD34⁺ cells were cultured for 14 days under MK conditions in the presence of CD9 mAb (A, C, D, F, G, H) or of isotype control (B, E, I) At different time-points (days 8, 12, 14), cells were harvested and analyzed by electron microscopy for ultrastructure. Isotype control cells displayed typical ultrastructural features of MKs: multilobed and indented nucleus (B), a well-developed and aligned demarcation membrane system (B, E), multivesicular bodies, numerous specific MK granules, formation (E), and shedding of proplatelets (I). CD9 mAb-treated cells also exhibited a multilobed and indented nucleus, but they showed major cytoplasmic abnormalities: demarcation membrane system heterogeneously distributed throughout the cytoplasm (A), some open cisternae containing a fibrillar material (D), numerous heterogenous multivesicular bodies (A, C, F, G), large vacuoles probably resulting from multivesicular body fusions (G), and exocytosis of the vacuolar content (H). Magnifications: × 2400 for C, E, F, I; × 3000 for A, B; × 6000 for D, G, H.