Niches for Extramedullary Hematopoiesis in the Spleen

The spleen is a major site for extramedullary hematopoiesis and adult mouse and human spleens retain low numbers of hematopoietic stem cells (HSC). Hematopoiesis is however dependent on stromal microenvironments that provide appropriate molecular signals. Recent evidence now suggests that a distinct stromal cell type exists in the spleen which can function as a niche for hematopoiesis. This review considers the role of the spleen in hematopoiesis, and the possibility that the spleen can be utilized or amplified as a site for hematopoiesis during HSC transplantation. If stromal cells can be isolated and used to expand HSC in vitro, or provided as an ectopic niche in vivo for the same purpose, then the potential exists to enhance hematopoiesis in patients undergoing myeloablative treatment, HSC transplantation or involution of lymphoid tissue with ageing.