Paracrine regulation of bone remodeling

Bone remodeling tightly couples resorption and formation in a cell unit called bone modeling unit (BMU). The activity of the BMU leads to a negative balance of bone mass in adults with physiological osteopenia. The regulation of the BMU is done at a local level by paracrine and autocrine factors such as cytokines and at an endocrine level by hormones. The hormonal level has been widely discussed in previous reviews and recent studies focus on the relations between hormonal triggers and cytokine-mediated responses to their stimulations. This review is dedicated to the means of coupling osteoblasts and osteoclasts and the recruitment of early cell precursors that originate in the hematopoietic bone marrow. Bone should be considered not only as an individual organ but as integrated in the bone-bone marrow organ.