andDifferentiation of Bene Cells

Bone piays a pivotaE role in storing calciurri and phosphate in vertebrates. This tissue is maintained by the balance of bene formatien and bene resorption. Osteoblast-tineage c lls, consisting of osteoblasts, esteocytes and bone lining cells, are engaged in bone formation. Bene resorption is mediated by osteoclasts. Recent research revealed that receptor activator ofNF-KB (RANK)-RANK ligand (RANKL) mechanisrn is essential for the differentiating and activating osteoelasts. Osteoblast-1ineage cells regulate bone resorption via the expression ofRANKL and osteoprotegerin (OPG), a decoy reeeptor for RANKL. Additionally, osteoblastlineage cells participate in degradatien efbone rnatrix by secreting MMP-13. Thus, bone remodeling is achieved by the harmonized orchestration ofosteoblast-lineage cells and osteoclast-lineage cells. Key words/ Bone, Cell-celHnteraction, Osteoblasts,Osteoclasts Illtrodllction Bone is not inert issue but dynamically metabelized cennective tissue throughout life Old bone matrices are always replaced by newly formed matrices. This continual process, named bene remedeling, is important for maintaining bene volume and slrength. Bone votume is maintained by the balance of bone resorption and bone formation. Bene cells consist of osteoblast]ineage cells 3' 'O and osteeelast-tineage cells'S). Their differentiatien and function are regulated by osteotrepic hormones and cytokines, Recent research has revealed that osteeblast-lineage cells are not enly involved in bone formation but also in bone resorption via supporting differentiat{on and activation efosteoclasts fi). Hence, we need to re-consider the fttnctional and morphological varieties of osteoblast-lineage cells. This review describes morphologica[ eharacteristics ef osteoblast-lineage and osteoclast-]ineage cells and also discusses their function and differentiation. Mbrphology andfltnetion qfosteoblasts Osteoblasts are engaged in bone formation. They are generally round in shape and line on the bone surfaces (Fig.IA). Li ltrastructural propert>, of osteoblasts shows typical secretory characteristics, possessing wet[-develeped rough endoplasmic reticulum with dilated cisternai' A large Golgi complex comprises multiple Gelgi stacks, vesicles and vacuo]cs containing fibrillar st uctures which are considered to rcpresent pro-collagen and proteeg]ycans (Fig. IA), Newly tbrmed bone matrix is net calcified immediately. Therefore, uncalcified matrix, named CorrespondencetoDr.HiroakiNakemnura,DcpartmentofOralHistology, "tTatsumoto Dental University, l780 Gobara HiTooka, Shiojiri, Nagano 399-07gl,JapanTe]./+8t-263-51-2042;Fax/+81-263-53-3456.E-mail address/nakam@po,mdu.ac,jp. 15 osteoid, exists under the regulation of active bone'forming osteoblasts. Quantity ofosteoid is elosely related with bone-forming aetivity of osteoblasts. Much osteoid are seen under actively bone-forming osteoblasts. After active bone formatien, some osteoblasts become osteocytes buried in bone matrix. Others exist on quieseent bone surfaces and are called as bone 1ining cells. Bone lining cells show ftattened shape and contain a few cell organelles. These morphological characteristics indicate that bone Eining cells are hardEy engaged in bone formation. 1n fact, little osteoid is seen under the bone lining ce]ls. Osteoeytes are considered to be the terminal differentiation stage of osteoblasts. They are embedded in osteocytic lacunae and are most abundant eells in bene tissue, Osteocytic lacunae areconnectedbycanaliculicontainingtheircytoplasmicprocesses, Tliese canalicuti serve a$ pathway to supply nutrients and ox}・gen fi'em bloed eapi11ai", to osteoc>,tes. Osteocytes possess extremely large surface areu because ofnumerous c},toplasmic processes. Additienally, these processes contain wetL-dcvelopcd bundles of uctin filaments receivable ofmechanical stress. It is conceivable that osteecytes are involved in bone metabotism by receiving and transducing meehanical stress. tn fact, recent research revealed that osteocytes express stretch activated channel :' ) and shear-stressrespensive etement S). However, mechanism ofsignal transductgon and genes regulated by mechanical stress are not cEarified y¢ t. Bone consists of 70V6 inorganic component, 20% organic component, and 1e% water. Approximately 90% oforganic eontent is type 1 col]agen. Osteoblasts are responsible for the produetion ofcollagen and non-coltagenous proteins including osteocalcin, bone sialoprotein, osteopontin, and osteonectin3・ "). They also