A method for in situ staining and scoring of eosinophil colonies and clusters in the whole culture dish has been developed for agar cultures of human marrow cells. Cultures stimulated by human placental conditioned medium were found to develop the same proportion of eosinophil, neutrophil, and monocyte colonies as cultures stimulated by conventional peripheral white cell underlayers. Eosinophil colonies and clusters always contained pure populations of eosinophils, and no mixed colonies of eosinophils with other cells were observed. Eosinophil colony formation was delayed in onset relative to neutrophil or monocyte colony formation, but by day 14 eosinophil colonies comprised 20% of all colonies. Eosinophil colonies were absent in cultures of marrow cells from patients with acute myeloid leukemia in relapse, but eosinophil colonies and clusters were found with higher than normal frequency in cultures from acute leukemic patients in remission. A high frequency of eosinophil colonies and clusters was also observed in cultures from three patients with idiopathic thrombocytopenic purpura. The new staining procedure should permit a wide range of studies on eosinophil precursor populations in the marrows of patients with various diseases.

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