Wiktionary
megakaryoblast
n. (context cytology English) A precursor cell in hematopoiesis which gives rise to the promegakaryocyte, which in turn gives rise to the megakaryocyte.
Wikipedia
Megakaryoblast
A megakaryoblast is a precursor cell to a promegakaryocyte, which in turn becomes a megakaryocyte during haematopoiesis. It is the beginning of the thrombocytic series.