immature blood cell, found in bone marrow, that gives rise to white blood cells of the granulocytic series (characterized by granules in the cytoplasm, as neutrophils, eosinophils, and basophils), via an intermediate stage that is called a myelocyte. The myeloblast nucleus is large and round or oval; its membrane is thin, and the contained chromatin (readily stainable nuclear material) is dispersed in fine strands or tiny granules. Several nucleoli are present; there is relatively little cytoplasm. Cells vary in size and are capable of amoeboid movement; they are difficult to distinguish in the laboratory from lymphoblasts.