Granulocyte Transit from Bone Marrow to Blood

The release of granulocytes from marrow to blood was studied in the dog by hourly sampling of the peripheral blood from 48 to 143 hours after injection of tritiated thymidine. Labeled granulocytes were classified by grain count and cells with 30 or more grains were considered to represent the progeny of a single generation. The blood curve for the most heavily labeled cells was corrected for exponential cell loss with a T½ of 6 hours. It was possible in this way to determine that granulocyte release from bone marrow was normally distributed with a mean time from labeling as a myelocyte to appearance in blood of 102 ± 13.8 hours. The mean transit time of segmented granulocytes in marrow is about 50 hours, which is indicative of a much larger pool of mature cells than formerly thought.