Hierarchy of the stromal niche cells. MSPC populations are found in both bone and bone marrow compartments and contribute to the generation of mesenchymal lineage cells that generate bone, fat, and cartilage tissues. The relationship between bone-derived MSPCs and marrow MSPCs remains unclear. Stromal cells are also thought to derive from MSPCs, but the nature or origin of most stromal cells is still obscure. Self-renewing mesenchymal cells and their progeny can be prospectively isolated by using specific surface markers and lineage tracing approaches with transgenic labeling to decipher the origin of niche cells. Arrows with solid lines indicate that one cell population that can give rise to another cell population; arrows with dashed lines indicate a relationship between cell populations that remains to be clarified. Mx-1, Myxovirus resistance-1; Prx-1, paired-related homeobox-1.