Fetal-to-adult hemopoietic cell transplantation: is hemoglobin synthesis gestational age-dependent?

The most direct available approach to determine the cellular control of the switch from fetal to adult hemoglobin is to examine the pattern of hemoglobin synthesis by fetal hemopoietic cells after their transplantation to an adult animal. Such an experiment is potentially capable of distinguishing between several proposed hypotheses, and a suitable experimental animal is available, the sheep. Despite a present inability to obtain sustained hemopoietic cell engraftment in sheep, results in the first few weeks after transplantation show distinct differences in the pattern of hemoglobin synthesis by the engrafted fetal cells in the adult environment. Fetal cells at 70-75 days gestation continue to synthesize only fetal hemoglobin, whereas cells from 100-day gestation fetuses appear to rapidly switch to adult hemoglobin production after transplantation. The combined results of transplant experiments carried out throughout this age range point to the gestational age of the cells as a major factor determining the pattern of hemoglobin synthesis. However, the time at which the cells switch in the adult precedes that at which the cells would normally switch in utero by about 2-3 weeks. This may reflect the nonphysiologic "stress" imposed on the fetal cells or result from some form of induction by the adult environment.