**9. Conclusion**

In the last ten years a body of evidence has accumulated on the hematopoietic stem cell niches and the mechanisms by which they regulate HSCs homeostasis. However many questions remain to be addressed. What we can speculate with today data is that in the endosteal niche dissimilar stages of differentiating osteoblasts provides diverse signals. These signals induce a quiescent or a self-renewal fate. However after leaving quiescence HSCs are prone to accumulate mutations that will lead to senescence. To prevent HSCs exhaustion mechanisms to enhance survival are also induced. Several of these additional signals come from the osteoblasts but also from other cells from microenvironment as stromal cells and endothelial cells. This is specially visualized in the decision of a symmetric or asymmetric division. From the control of all these interconnected pathways depends a normal hematopoiesis.
