Cell division tracking and expansion of hematopoietic long-term repopulating cells.

The combined use of rigorous assays for quantitating transplantable stem cell numbers and precise cell labeling and tracking procedures have provided definitive evidence that stem cell self-renewal divisions can occur in vitro in the absence of stromal feeder layers. These findings set the stage for defining conditions that may alter the ability of these cells to maintain their primitive status when mitogenically activated.