Recombinant rat stem cell factor (rrSCF) administered to rats as a single intravenous injection causes a dose-dependent neutrophilia and lymphocytosis as well as the appearance of immature myeloid cells and occasional blast cells in the circulation. Neutrophilia begins at 2 hours, peaks at 4 to 6 hours, and subsides between 12 and 24 hours. Lymphocytosis occurs at 0.5 hours and has subsided by 2 hours. rrSCF-induced neutrophilia and lymphocytosis are abrogated by boiling, demonstrating that endotoxin-contamination of the rrSCF preparation is not responsible for the observed hematologic effects. The bone marrow at 6 hours after injection of rrSCF shows a left-shifted myeloid and erythroid hyperplasia as evidenced by significant increases in the absolute numbers of morphologically recognizable early myeloid and erythroid precursors. A concurrent decrease in the absolute numbers of mature marrow neutrophils is noted, suggesting that the release of marrow neutrophils contributes to the peripheral neutrophilia. After 2 weeks of daily injections of rrSCF, bone marrow smears demonstrate a remarkable mast cell hyperplasia accompanied by a decrease in total marrow cellularity and by a striking erythroid and lymphoid hypoplasia. rrSCF also causes mast cells to appear in the circulation and causes a systemic increase in embryonic connective tissue-type, but not mucosal-type, mast cells. In vitro long-term culture of lineage-depleted mouse bone marrow cells with rrSCF results in an almost pure outgrowth of mast cells.
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