Stromal cell CD9 and the differentiation of hematopoietic stem/progenitor cells

CD9 belongs to the tetraspan family of proteins that facilitates the regula tion of cell proliferation, motility, and adhesion. In mouse hematopoietic organs; CD9 is expressed by myeloid and stromal cells. Although the precise mechanisms are not clear, antibody ligation of CD9 on stromal cells regula tes the adhesion between stromal cells and hematopoietic stem cells, the pr oduction of myeloid cells in long term bone marrow cultures and the differe ntiation of hematopoietic stem cells. A 100 kD protein co-precipitated with CD9 is distinct from several previously reported CD9-associated molecules with respect to size and distribution. Identification and analysis of this interesting protein may clarify the molecular mechanisms through which CD9 bearing stromal cells control the differentiation of hematopoietic stem cel ls and/or allow them to maintain their vital self-renewal capacity.