Human MK double-labeled for the α-granule and dense granule marker P-selectin, combined with Gp Ib or Gp Iib-IIIa. (Inset) Human cultured MK are large in size and have an indented nucleus, numerous secretory granules, and a prominent demarcation membrane system. (Original magnification × 16,000.) (a) P-selectin (polyclonal anti–P-selectin/protein A 10 nm; arrowheads)/Gp Ib (polyclonal anti–Gp Ib/protein A 15 nm; arrows). P-selectin lines the α-granules (A) and the dense granule membrane (D) and is absent from the plasma membrane (pm) and demarcation membranes (dm). In contrast, Gp Ib lines the plasma membrane and demarcation membranes (dm). Gp Ib is also found within the dense granule membrane (D) colocalized with P-selectin. Mitochondria (m), as control structures, do not label. (Original magnification × 72,000.) (b) P-selectin (polyclonal anti–P-selectin/protein A 10 nm; arrowheads)/Gp IIb-IIIa (polyclonal anti–Gp IIb-IIIa/protein A 15 nm; arrows). Gp IIb-IIIa is classically located on the plasma membrane (pm) and α-granules (A). It also colocalizes with P-selectin in the α-granules and, noteworthily, within the dense granules (D). (Original magnification × 33,500.)