Megakaryoblast precursors in rodent bone marrow: specificity of acetylcholinesterase staining

Small cells staining diffusely for acetylcholinesterase, corresponding closely to the description of Jackson (1973, 1974), have been quantitated in the bone marrow of mouse and rat. These cells fulfil the requirements of megakaryoblast precursors, showing elevation in numbers prior to megakaryocytes in response to thrombocytopenia, and carrying platelet-specific antigens demonstrated by immunofluorescence. Some show a clearly defined nucleus whilst in others the nucleus is obscured; the femoral content of both types follows the same time course after acute thrombocytopenia and the latter can be distinguished from megakaryocyte cytoplasmic fragments. Other small cells present in rodent bone marrow contain granular positivity after acetylcholinesterase staining but also granules staining with Luxol-Fast-Blue, suggesting that these cells are eosinophils. These cells are greatly increased in number with stimulation of eosinophilia by parasite infestation but show no response to acute thrombocytopenia. Peripheral blood eosinophils exhibit both acetylcholinesterase and Luxol-Fast-Blue staining. A number of properties ascribed to granular-staining small acetylcholinesterase-positive cells previously presumed to be megakaryoblast precursors will need to be reassessed.