Chapter 6 – The Osteocyte

Publisher Summary This chapter focuses on osteocytes, which is the most abundant cell type of bones. There are approximately 10 times as many osteocytes as osteoblasts in adult human bone and the number of osteoclasts is only a fraction of the number of osteoblasts. Mature osteocytes are stellate shaped or dendritic cells enclosed within the lacunocanalicular network of bones. The lacunae contain the cell bodies and from these cell bodies, long, slender cytoplasmic processes radiate in all directions, with the highest density perpendicular to the bone surface. The more mature osteocytes are connected by these cell processes to neighboring osteocytes, and the most recently incorporated osteocytes to neighboring osteocytes and to the cells lining the bone surface. Osteocytes have to remain in contact with other cells, and ultimately, with the bone surface to ensure the access of oxygen and nutrients. Osteogenic cells arise from multipotential mesenchymal stem cells, which have the capacity to also differentiate into other lineages, including those of chondroblasts, fibroblasts, adipocytes, and myoblasts. A newly formed osteocyte starts to produce an osteoblast inhibitory signal when its cytoplasmic processes connecting the cell with the osteoblast layer reach their maximal length. The osteoid production of the most adjacent, most intimately connected osteoblast are relatively more inhibited by that signal than that of its neighbors. The life span of osteocytes is probably largely determined by bone turnover, when osteoclasts resorb bone and liberate osteocytes. Osteocytes may have a half-life of decade if the particular bone they reside in has a slow turnover rate.