CHO-GPIbβ/IX cell aggregation. CHO-GPIbβ/IX cells (10⁶ cells/mL) were exposed to rotary shaking at 6 cycles per second for 20 minutes either in the absence of vWF (A, C, and D) or in the presence of vWF (B, E, and F) to which either buffer (A and B), ristocetin (C and E), or heparin (D and F) was added. Microphotographs of cell aggregation were taken at a 10-fold magnification. vWF-dependent cell aggregation was seen in the presence of ristocetin or heparin.