Detection of dense granules in immature MKs at day 8 of culture by double labeling for GPIb and granulophysin.

Arrowheads indicate GPIb (15-nm gold particles), and arrows indicate granulophysin (10-nm gold particles). This immature MK, seen on a frozen thin section, displays a nucleus (N) with abundant euchromatin and a prominent nucleolus (n) and appears morphologically undifferentiated. It can only be identified by GPIb-specific expression on its plasma membrane (pm). Granulophysin labels the membrane of vacuolar structures, which can be identified as dense granules (d) because of the presence of this specific marker (original magnification ×41 800; inset, ×3920).