Developmental changes in the function of hematopoietic stem cells.

The observation that colony-forming unit-spleen (CFU-S) cells can be found in the yolk sac, fetal liver, and various hematopoietic tissues in adult mice has contributed to the concept that "stem cells" represent a single type of immortal cell that is present at variable numbers in tissues at different stages of development. However, more recent studies have shown dramatic changes in various functional properties of purified "candidate" stem cells during ontogeny. Together with previous studies documenting developmental changes in early hematopoietic cells and observations showing a decrease in the mean telomere length of chromosomes in hematopoietic cells during ontogeny, these findings are difficult to reconcile with the concept of a stem cell as an immortal and invariable cell type. In this minireview, some developmental aspects of stem cell biology are discussed in the context of stem cell definitions and transplantation biology.