Bones are really two systems in one. The cavities inside bones are filled with spongy bone marrow, in which stem cells divide and their daughter cells differentiate into all kinds of blood cells, including large numbers of immune cells for the body's defense. The hematopoietic (literally, blood-creating) stem cells, which can give rise to any kind of blood cell, reside in special 'stem cell niches' nestled in the bones' inner walls. Inside these sheltered nurseries, the stem cells remain undifferentiated; with the help of other nearby cells, they hang on to their juvenile qualities. Only when they leave the niches do they morph into specialized blood cells, possibly becoming immune cells for fighting infection or cells for blood clotting and healing after injury. They can even respond to calls for help from organs such as the liver, migrating through the bloodstream to assist in repairing damage.
The inner walls of the bones are also sites of intensive reconstruction. While one type of cell, the osteoblast, is busy building bone, its partner, the osteoclast, breaks it down and reassimilates the material. Osteoclasts are formed when several cells (which themselves originate from hematopoietic stem cells) fuse together at a signal from the osteoblasts, and the two work together in a sort of 'urban renewal' scheme to keep the bones healthy and strong.
The Weizmann Institute team headed by Prof. Tsvee Lapidot of the Immunology'"/>
Source:American Committee for the Weizmann Institute of Science