Enrichment of murine haemopoietic stem cells: diverging roads.

The cellular elements of the peripheral blood must be constantly replenished by the process of haemopoiesis, since most blood cells have a limited life span of only days or weeks. Although the developmental lineages of haemopoietic differentiation have been depicted in textbooks for decades, the actual details of the early stages of haemopoiesis are relatively unknown due to the very low numbers of haemopoietic stem cells in bone marrow or spleen. Only by isolating these rare stem cells and developing in-vitro culture systems to maintain them can a complete understanding of the early stages of haemopoiesis be achieved. This approach has already been successfully applied to the study of the later stages of haemopoiesis. In this review, Gerald Spangrude examines several experimental approaches that have been used to enrich murine haemopoietic stem cells.