The Collaborative International Dictionary of English v.0.48:

megakaryocyte \meg`a*kar"y*o*cyte\
   (m[e^]g`[.a]*k[a^]r"[i^]*[-o]*s[imac]t) n. (Anat.)
   A large bone marrow cell not usually found in the circulating
   blood; it is regarded as the precursors of blood platelets;
   it is up to 100 [mu]m in diameter, with a polyploid nucleus.
   Also referred to as megacaryocyte, megalokaryocyte, and
   thromboblast.
   [WordNet 1.5 +PJC]

WordNet (r) 3.0 (2006):

megakaryocyte
    n 1: a large bone marrow cell; regarded as the source of blood
         platelets