[Structure and function of bone cells].

Bone modeling and remodeling as well as systemic calcium homeostasis are modulated predominantly by bone cells such as mesencymal-derived osteoblasts and osteocytes and haematopoietic-derived osteoclasts. Osteoblasts are classified into preosteoblasts, mature osteoblasts and bone lining cells (BLC) based on their cell structures and biological functions. Preosteoblasts appear to regulate the activities of mature osteoblasts and osteoclasts as well as to differentiate into mature osteoblasts. Cuboidal mature osteoblasts show well-developed Golgi apparatus and rough endoplasmic reticulum, and synthesize actively major bone matrix proteins including type I collagen, osteopontin, osteonectin and osteocalcin. Unlike mature osteoblasts, BLCs display flattened cell bodies and few cell organelle, therefore, indicating less activities. These osteoblasts communicate through gap junctions and adheres junctions with each other or with osteocytes, consequently forming three-dimensional cellular network in the bone tissues. In contrast, osteocytes, compacted cell type embedded in the bone matrix, provide a cellular environment in the bone matrix by means of their numerous cytoplasmic processes. Moreover, it is likely that they participate in calcium transport from bone matrix to tissue fluid, that is "osteocytic osteolysis". Osteoclasts, multinucleated giant cells with ruffled borders and clear zones, are responsible for bone resorption, although probably they are controlled by surrounding osteoblast phenotype. Active osteoclasts secrete H+ and proteolytic enzyme such as cathepsin and ACPase towards resorption pits through ruffled borders. These bone cells play an important role in bone modeling and remodeling.