Present knowledge concerning the kinetics of granulopoiesis has been reviewed and quantitative data concerning granulokinetics in normal human subjects are presented.
A. When granulocytes are labeled in vitro and returned to the circulation of the donor, the distribution of the cells in the circulation and the rate of disappearance of the cells from the circulation can be measured.
1. The total blood granulocyte pool (TBGP) consists of two compartments which are in equilibrium with each other. These pools have been designated the circulating granulocyte pool (CGP) and the marginal granulocyte pool (MGP). The size of the pools has been measured in 109 normal male subjects. The mean values, expressed as numbers of cells x 107 per Kg. of body weight were as follows: TBGP, 70; CGP, 31; and MGP, 39. The mean ratio of the CGP to the TBGP was 0.44.
2. The labeled granulocytes leave the TBGP in an exponential fashion with a mean half-time disappearance (T½) of 6.7 hours as determined in 56 normal male subjects. No evidence has been obtained for a return of granulocytes to the blood.
3. The mean value for the granulocyte turnover rate (GTR) in 56 normal male subjects was 163 x 107 granulocytes per Kg. of body weight per day. Thus, the TBGP turns over 2.3 times per day and the turnover time for the TBGP is 10.4 hours.
B. When granulocytes are labeled in vivo by the intravenous administration of DFP32, the rate of disappearance of granulocytes from the circulation and the time required for myelocytes to divide, mature and appear in the blood can be measured. In addition, the generation time of myelocytes can be approximated. From the time parameters and the GTR, the bone marrow pool sizes and turnover times can be calculated. These determinations and calculations have been made for a group of 21 normal male subjects.
1. The mean half-time disappearance (T½) of in vivo labeled granulocytes from the circulation was 7.2 hours. This value agrees well with the value of 6.7 hours obtained after the in vitro labeling of granulocytes.
2. The mean time required for myelocytes to divide, mature and appear in the blood was 11.4 days.
3. The mean generation time of myelocytes was estimated to be not more than 2.9 days.
4. The total granulocyte pool in the bone marrow (neutrophilic myelocytes, neutrophilic metamyelocytes and PMN neutrophils) was calculated to be 186 x 108 cells per Kg. of body weight with a mean turnover time of 11.4 days. The myelocyte pool was estimated to be 41 x 108 cells per Kg. with a turnover time of 2.5 days; the metamyelocyte pool consisted of about 76 x 108 cells per Kg. with a turnover time of 4.7 days; the average size of the mature marrow PMN neutrophil pool was 69 x 108 cells per Kg. of body weight with a turnover time of 4.2 days.
C. A kinetic model for granulopoiesis, based on the studies with the DFP32 label, is presented. In this model, myelocytes are depicted as approaching a self-perpetuating population of cells. Some cells enter this population from populations which are less mature but this latter source of cells is small under conditions of normal steady state kinetics. One of the daughter cells of a myelocyte division remains in the myelocyte population to divide again. The other daughter cell enters the metamyelocyte population. The metamyelocyte and PMN neutrophil population is incapable of division and cells move through this population in sequential fashion in the process of maturation. The cells then enter the blood where they equilibrate rapidly between the two blood compartments. The cells are removed from the total granulocyte pool in a random fashion. There is no appreciable pool of granulocytes in the extramedullary tissues of normal subjects and granulocytes do not return from the tissues to the blood. The entire movement of granulocytes from marrow to tissues is uni-directional.