Targeting the stem cell niche: squeezing blood from bones.

During human development, stem cells establish themselves in specific anatomic locations or niches. The niche harbors the stem cells, and regulates how stem cells proliferate. The interaction between stem cells and their niche affects stem cell function, and offers an opportunity to improve the marrow microenvironment. Osteoblasts produce hematopoietic growth factors and are activated by parathyroid hormone (PTH). A calcium sensing receptor, expressed by hematopoietic stem cells, regulates the niche and can be targeted to increase stem cell numbers. Therefore, drugs that affect osteoblast function or target calcium receptors may be useful for stem cell mobilization and engraftment. In this review, the biology of the stem cell niche and the potential therapeutic manipulations of the stem cell niche are reviewed. PTH is in clinical trials for patients who have not mobilized autologous stem cells well. The limiting cell numbers for adult cord blood transplantation increase the risk of infection, and PTH is currently in a clinical trial following cord blood transplantation in an effort to improve engraftment and immune reconstitution.