Neutrophil-generating regions (NGRs) contain differentiating neutrophils at all stages of maturation intimately associated with the OP9 cells. The composition of day-5 and day-7 NGRs was evaluated using fine needle aspiration to harvest loosely associated cells on the surface of these colonies. The aspirated cells were cytospun and stained with the Hema 3 staining kit (Fisher Scientific). Purified mouse bone marrow band 3 neutrophils were isolated as described in “Materials and methods.” (A) Low-power phase contrast photograph of NGRs from differentiating cells grown for 5 days in the neutrophil differentiation mix (× 180 final magnification). (B) At a higher magnification, differentiating neutrophils at multiple stages of maturation can be seen (× 400 final magnification). (C) Photograph of neutrophils harvested by fine needle aspiration from the same colony shown in panel A and stained with the Hema 3 staining kit (× 1500 final magnification). (D) Neutrophils harvested by fine needle aspiration from the surface of a day-7 NGR demonstrating polysegmented nuclei (× 1500 final magnification). (E) Hema 3–stained mouse bone marrow band 3 neutrophils (× 1500 final magnification).