Model showing the various cell types comprising the bone marrow hematopoietic stem cell (HSC) niche. The dormant status of HSCs is maintained by transforming growth factor beta (TGF-β) and thrombopoietin (TPO) produced by nestin+ non-myelinating Schwann cells and osteoblasts, respectively. Stem cell factor (SCF), which is essential for HSC maintenance, is mainly produced by leptin-receptor (LepR)-expressing mesenchymal stromal cells but also by nestin+ mesenchymal stem cells (MSCs) as well as sinusoidal endothelial cells (not shown). The sympathetic nervous system (SNS) negatively affects the activity of nestin+ MSCs. CXCL12 abundant reticular (CAR) cells produce the chemokine CXCL12, which facilitates lodging and engraftment as has been suggested for the high calcium concentration near the endosteal osteoblasts. The four stromal cell populations indicated in green may be somewhat overlapping and the relationship between these cell types remains to be elucidated. Osteomacs are specific macrophages that promote survival to osteoblasts and support nestin+ MSCs (not shown). Ang-1, angiopoietin-1.