Extravascular Phagocytosis of Proerythrocytes following 5-Fluorouracil

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Abstract
Among the perturbations induced in mouse bone marrow by 5-fluorouracil [5-FU] are the retention and phagocytosis of proerythrocytes within the extravascular compartment. Reasons for the latter response were sought using EM. Proerythrocytes underwent normal maturation after 5-FU, judged by loss of mitochondria and vesicles, but failed generally to attain a biconcave shape. Most proerythrocytes lost their organelles before phagocytosis occurred. Quantitative light microscopy showed a rise in number of proerythrocytes 1 day after 5-FU which was attributable to maturation of orthochromatic erythroblasts. Thereafter the population size declined concomitantly with increased engulfment and phagocytosis by macrophages, activity which was maximal on days 4-5. Extravascular loss of proerythrocytes was also observed when marrow was rendered hypocellular by X-irradiation or hydroxyurea. Excessive maturation of proerythrocytes in the extravascular compartment apparently leads to their phagocytosis.