512 XI. HEMOGLOBIN CATABOLISM, I 



bone marrow as reticulocytes (1206J389;2989,p.'250S) and have a constant 

 maturation time, the reticulocyte level can be used to determine the life of 

 the erythrocyte. The number of reticulocytes found in normal individuals 

 varies between and 2% {2086,2186). The mean value is frequently given 

 as 0.7 to 0.8%. Heilmeyer and Oetzel {1219) found an average of 1% reticu- 

 locytes, which would give a lifetime of 100 days. While the reticulocyte 

 count itself is not very reliable, it seems likely that the exact maturation 

 time of reticulocytes is known with even less certainty. The recent experi- 

 ments of Plum {2155; cf. Chapter XIII) indicate that the maturation of the 

 reticulocyte is under the control of reticulocyte-ripening substances, and 

 that the reticulocyte percentage is proportional to the content of these sub- 

 stances in the plasma {2157). This probably explains the results of Baar 

 and Lloyd {108). These workers found a maturation time of 11 hours for 

 reticulocytes which, taken with the average content of 0.7% in the circula- 

 tion, gives a value of 65 days for the life of a mature cell. The deduction of 

 the life of the cell from such data seems, therefore, to be relatively inaccurate 

 in comparison with other methods. 



The use of blood groups to determine the life of the mature cell, 

 was first carried out by Ashby {90,91). By transfusing group O 

 blood into recipients of group A or group B, and by subsequently 

 agglutinating the recipient's cells, the number of surviving donor 

 cells of group O may be counted. By this method, Ashby found a 

 lifetime varying from 30 to 100 days, with an average of 82 days. 

 Wearn and his co-workers {3005) found values varying from 59 to 

 113 days and Wiener (3078), from 80 to 120 days. 



2.4. Significance of the Mortality Curve 



There is thus now good agreement between reliable methods show- 

 ing that the life span of the mammalian cell is of the order of 120 

 days under normal conditions. Of the methods which we have con- 

 sidered, some, like those based on the bile pigment excretion, give 

 a figure for the average number of cells destroyed per day, as well as 

 the end point of the life of a population of new cells formed during 

 the experiment. Other methods, involving cells labeled with sulf- 

 hemoglobin, with hemin containing the N'^ isotope or with a specific 

 agglutinogen, enable an actual mortality curve to be calculated. The 

 agreement between these independent methods increases the weight 

 to be attached to the results of a recent mathematical analysis by 

 Witts and co-workers {3If.9,395) which have been derived from data 

 obtained by the third of these methods. 



Normal individuals were used, and blood was withdrawn and was 

 immediately replaced by the same amount of group O blood. In 



