Localization of cyclin B1 in endomitotic MKs. Cultures were performed as described in Fig 1 and cells were stained as described in Fig 3 by an anti-cyclin B1 polyclonal antibody (b and d; TRITC), an anti-vWF MoAb (e; FITC), and the Hoechst dye (a and c). They were examined by conventional fluorescent microscopy (original magnification × 500). In this polyploid MK in endomitosis (a), cyclin B1 staining draws the mitotic spindle with its multiple asters. In two MKs in metaphase, cyclin B1 is detectable (arrows on the left; c and d). In contrast, in a polyploid MK with an anaphase figure (arrow on the right; c and d) as shown by segregation of the chromatids, cyclin B1 is undetectable. In this endomitotic MK (f) expressing the vWF (e), cyclin B1 colocalized with the asters, as shown by the arrows.