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.