Myeloblast The myeloblast is the first stage of the granulocytic series that is identifiable by light microscopy. It may be dificult to distinguish myeloblasts from other blasts in the peripheral blood unless one uses special stains or infers their identity from the presence of other immature cells of the same line. A myeloblast can be distinguished from a promyelocyte by its lack of cytoplamic granulation. The nucleus is composed of very fine nonaggregated chromatin that stains light blue to reddish-purple with Wright's stain. Two to five distinct nucleoli are usually present. The nucleus is often bordered at one side by a distinct perinuclear zone.
	Lymphoblast Lymphoblasts are 12-20 µm in diameter with a round to oval nucleus, sometimes eccentric in location. The nucleus to cytoplasm ratio is about 4:1 and the periphery of both the nucleus and the cell may be irregular in outline. The fine, highly dispersed nuclear chromatin stains a light reddish-purple, and one or two pale blue or colorless large nucleoli are visible. The cytoplasm is usually agranular and deeply to moderately basophilic, with marginal (peripheral) intensity a common characteristic.
	Monoblast The monoblast is the first stage of monocyte-macrophage maturation. It is about 12 to 20 µm in diameter, has a nuclear to cytoplasm ration of 4:1 to 3:1, and, like most myeloid blasts, has a round to oval nucleus with fine, lightly dispersed chromatin. From one to four nucleoli may be visible. The nucleus may be either central or eccentric and it may show evidence of indentation or folding. The cytoplasm is agranular, stains moderately to lightly basophilic, and often has an intensely stained periphery and a prominent perinuclear zone. Monoblasts never appear in the normal peripheral blood.
	Promyelocyte Promyelocytes are the second largest stage in the granulocytic series. The large, reddish-purple granules that characterize promyelocytes are nonspecific in that they are shared by the other granulocytes (eosinophils and basophils). These granules are peroxidase positive, and a lipid component reacts with Sudan black stain, providing a second cytochemical reaction for the identification of large mononuclear cells in blood and bone marrow smears. The cell size, 12 to 20 µm, is the same as that of a myeloblast, but the nucleus to cytoplasm ratio is less, usually from 3:1 to 2:1. The chromatin is still fine, but some aggregation is evident. One to three nucleoli are also visible, although these can be obscured by heavy granulation.
	Myelocyte Myelocytes have a cell diameter from 10 to 18 µm. The nucleus to cytoplasm ratio is 1:1, and the oval or round nucleus is often eccentrically located. Chromatin is finely granulated in early myelocytes and more aggregated in later cells.
	Metamyelocyte Metamyelocytes have a cell diameter from 10 to 18 µm, and the nucleus to cytoplasm ratio is 1:1. Indentation of the nucleus begins at this stage, forming an outline that varies from slightly kidney shaped, to that of a broad V shape.
	Atypical Lymphocyte Atypical, or reactive, lymphocytes are lymphocytes that, as a result of antigen stimulation, have become quite large, sometimes more than 30 µm in diameter. The cells vary greatly in size and shape. The nucleus is less clumped than that of the normal lymphocyte. The shape of the nucleus ranges from elliptic to cleft to folded. The chromatin patterns appear similar to those of a blast and faintly stained multiple nucleoli are visible. The cytoplasm may range from large, deeply basophilic, and abundant to unevenly stained and granular. Causes of reactive lymphocytosis may be: ß-Streptococcus, cytomegalovirus, drugs, Epstein-Barr virus (infectious mononucleosis), syphilis, toxoplasmosis, vaccination, and viral hepatitis.
	Plasma cell A plasma cell is a mature B lymphocyte that is specialized for antibody (immunoglobulin) production. Plasma cells are rarely found in the peripheral blood. They comprise from 0.2% to 2.8% of the bone marrow white cell count. Mature plasma cells are often oval or fan shaped, measuring 8-15 µm. The nucleus is eccentric and oval in shape. The nucleus to cytoplasm ratio is typically 2:1 to 1:1. The nucleus may be bilobed or multilobed, especially in patients with lymphoid blood dyscrasias. The perinuclear zone is very distinct, appearing white in the deeply basophilic cytoplasm. Nuclear chromatin is condensed and very patchy, appearing as dark blocks on a reddish-purple background. The cytoplasm stains deep blue to gray blue, depending on the stain and the ribosomal content of the individual cell. Plasma cells are seen in multiple myeloma, plasma cell leukemia, Waldenström's macroglobulinemia, and MGUS (monoclonal gammopathy of uncertain significance. The cells depicted in this image are from a patient with plasma cell leukemia.
	Hairy cell Hairy cells are characterized by their fine, irregular pseudopods and immature nuclear features. They are seen only in hairy cell leukemia.
	Smudge or basket cell Smudge cells or basket cells are leukocytes that have been damaged during preparation of the peripheral blood smear. This usually occurs due to the fragility of the cell. They are usually seen in chronic lymphocytic leukemia (CLL).
	Hypersegmented neutrophil Hypersegmented neutrophils are neutrophils with 5 or more nuclear lobes. They are seen in disorders of nuclear maturation, such as the megaloblastic anemias.