Sequential immunomagnetic isolation with 2 monoclonal antibodies was used to purify and characterize an undifferentiated mast cell in adult mouse bone marrow that had not been previously recognized. This cell represents 0.02% of the cells in the bone marrow, is CD34+, CD13+, and c-kit+, and does not express FcεRI. However, by polymerase chain reaction (PCR) the cell contains message for the α and β subunits of FcεRI, mast cell-specific proteases, and carboxypeptidase A. Morphologically, this cell has a large nucleus, little cytoplasm, few cytoplasmic organelles, and no cytoplasmic granules. In vitro, in the presence of interleukin-3 (IL-3) and stem cell factor (SCF) these cells differentiate only into a granulated mast cell that now expresses CD13, c-kit, mast cell-specific gangliosides, FcεRI, and binds immunoglobulin E (IgE). When injected into lethally irradiated mice, these cells are able to reconstitute the mast cell population in the spleen.
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