Hematopoietic "stem cell" transplantation: are there any clouds on an expanding horizon?

Clinicians contemplating stem cell transplantation for the treatment of their patients are faced with an increasing number and complexity of options for the source of the stem cells and their manipulation prior to transplant. Many of these strategies focus on the traditional concept of the "hematopoietic stem cell" as a unitary and independent source of reconstitution. Evolving animal studies suggest that the stem cell concept is incompletely defined and that the stem cell compartment retains significant heterogeneity of function. These findings should be combined with the increasingly heterogeneous goals of stem cell transplantation to induce caution in proceeding forward with new technologies.