Ex vivo expansion of human hematopoietic stem cells: Implications for the modern blood bank

Pluripotent hematopoietic stem cells (PHSC) are rare cells within the marrow that are capable of self-renewal and differentiation into multiple hematopoietic lineages. Following myeloablative chemotherapy and radiation therapy and marrow transplantation, hematological reconstitution occurs after a period of 2-3 weeks. Recently, a number of laboratories have shown that both early and delayed phases of engraftment are mediated by PHSC within a graft and that engraftment can be accelerated by transplanting greater numbers of PHSC. Increasing efforts have been directed, therefore, towards developing methods to expand PHSC ex vivo. In this report, we describe an endothelial cell-based culture system to which exogenous cytokines are added which appears to permit the ex vivo expansion of PHSC. Refinement of these technologies will potentially have a major impact on the ability of blood banks to improve the quality of hematopoietic stem cell grafts.