Microenvironmental regulation of hematopoietic stem cells.

A major challenge in hematopoietic biology is the description and understanding of the molecular mechanisms responsible for the regulation of the primitive stem cell compartment. In one sense there exists a wealth of functional and physical properties which provide insight into the biology of the stem cell and its clonal progeny. However, much of this information is descriptive and available only as a function of complex in vivo assays. In order to move beyond these limitations, in vitro systems which accurately recapitulate the self-renewal, differentiation and proliferative behaviors of stem cells are required. We have approached this issue by focusing on the in vivo stem cell microenvironment. Dissection of this microenvironment into discrete cellular entities has yielded a cell line with in vitro stem cell supportive properties consistent with those which might be expected in a stem cell niche. Studies are summarized which suggest that a single stromal cell line provides a milieu which facilitates the in vitro maintenance of transplantable stem cells as well as the generation of large populations of committed progenitors. It is anticipated that this system will allow a direct analysis of stem cell regulatory pathways.