The osteocyte: A multifunctional cell within the bone.

The knowledge of bone biology has largely changed in the last few decades. Osteocytes are multifunctional bone cells that are surrounded by mineralized bone matrix and for decades it was considered that they might be relatively inactive cells. However, nowadays it is known that osteocytes are highly active cells which are indispensable for the normal function of the skeleton, playing main roles in several physiological processes, both within and beyond the bone microenvironment. This review highlights and updates the current state of knowledge of the osteocyte and focuses on its roles in bone remodeling and mineral homeostasis, and also reviews its recently discovered endocrine function. Osteocytes secrete sclerostin (a protein that works as a negative regulator of bone mass), and FGF-23, the most important osteocyte-secreted endocrine factor, since it is able to regulate the phosphate metabolism. Moreover, osteocytes can act as mechanosensory cells, transforming the mechanical strain into chemical signaling towards the effector cells (osteoblasts and osteoclasts). Therefore, the osteocyte plays an important role in bone biology, specifically in the remodeling process, since it regulates both the osteoblast and osteoclast activity. Finally, the paper discusses the clinical application of the bone biology, updating the new therapies against bone-loss disorders.