Red blood cell life span in the ovine fetus

Red cell life span within the fetal circulation has not been reported, although erythrocyte life span has been studied in the adult and newborn. The present study quantified red cell life span in 12 chronically catheterized fetal sheep at 97-136 days gestation (term = 150 days) with the use of autologous red cells labeled with [(14)C]cyanate. Cyanate forms a permanent covalent bond with hemoglobin and acts as a permanent red cell label. In the fetuses, blood (14)C activity decreased in a curvilinear fashion with time and reached 50% of the initial activity at 16.4 +/- 1.6 (SE) days. In contrast, (14)C activity of autologous red cells in two adult ewes decreased linearly with time as expected, reached 50% of the initial (14)C activity in 59 days, and yielded life spans of 117 and 121 days. Computer modeling and parameter optimization taking into account growth and skewed life span distribution were used to analyze the (14)C disappearance curve in each fetus. The mean life span of all red cells in the fetal circulation was 63.6 +/- 5.8 days. Mean red cell life span increased linearly from 35 to 107 days as fetal age increased from 97 to 136 days (r = 0.83, P < 0.001). Life span of cells produced at the time of labeling was significantly greater than the mean life span. Fetal growth rate estimated from parameter optimization was 3.28 +/- 0.72%/day; this compared well with the rate of 3.40 +/- 0.14%/day calculated from fetal weights at autopsy. Mean corpuscular volume decreased as a function of gestational age, but the decrease was small compared with the large increase in red cell life span. We conclude the following: 1) red cell life span in the fetal circulation is short compared with the adult; 2) red cells in younger fetuses have shorter life spans than in near-term fetuses; 3) the curvilinear disappearance of labeled red cells in the fetus appears to be due primarily to an expanding blood volume with fetal growth; and 4) red blood cell life span in a growing organism will be significantly underestimated unless the expansion of blood volume with growth is taken into account.