Osteoblast

Very large size cell, sometimes grouped in clusters, with regular round eccentric nucleus, intermediate/mildly condensed chromatin, much extended basophilic cytoplasm, poorly-defined cell outline, comet appearance, large central Golgi apparatus.

Its presence in the myelogram is a sign of the bone marrow origin of the sample. In clinical meaningful presentation (pancytopenia) it is, in case of very poor marrow, an indication for aplastic anemia rather than hemodiluted sample.