In vitro stimulation of human neutrophil granulopoiesis by unmasking an effect of human serum. Target cell specificity.

Human bone marrow cells from normal subjects were density-separated in continuous density gradients of the Percoll medium. The distribution of cells showed a linear relationship between morphologic maturation and increasing density. The method was used to characterize the target cell of the growth-stimulating effect of ultrasonically irradiated (UI) serum. All experiments showed that a distinct cell population of higher density contained the target cells, corresponding to the distribution of early myelocytes. Furthermore, the target cell was shown not to contain the Ia like antigen and was destroyed by freezing, both facts supporting the assumption of a relatively mature target cell. It is concluded that the divisions at the myelocytic level might be under the influence of a serum factor, unmasked by UI treatment.