Definition: An immature cell (10–18 mcm in diameter) in the granulocytic series, occurring normally in bone marrow, but not in the circulating blood (except in certain diseases). When stained with the usual dyes, the cytoplasm is light blue, nongranular, and variable in amount, sometimes being only a thin rim around the nucleus; the latter is deep purple-blue with finely divided, punctate, threadlike chromatin that is somewhat condensed at the periphery. A few light blue nucleoli are usually present in the nucleus, and these generally disappear as the myeloblast matures into a promyelocyte and then a myelocyte. Myeloblasts ordinarily yield a negative reaction with peroxidase.
[myelo- + G. blastos, germ]