An overview of bone cells and their regulating factors of differentiation

Abstract

Bone is a specialised connective tissue and together with cartilage forms the strong and rigid endoskeleton. These tissues serve three main functions: scaffold for muscle attachment for locomotion, protection for vital organs and soft tissues and reservoir of ions for the entire organism especially calcium and phosphate. One of the most unique and important properties of bone is its ability to constantly undergo remodelling even after growth and modelling of the skeleton have been completed. Remodelling processes enable the bone to respond and adapt to changing functional situations. Bone is composed of various types of cells and collagenous extracellular organic matrix, which is predominantly type I collagen (85-95%) called osteoid that becomes mineralised by the deposition of calcium hydroxyapatite. The non-collagenous constituents are composed of proteins and proteoglycans, which are specific to bone and the dental hard connective tissues. Maintenance of appropriate bone mass depends upon the precise balance of bone formation and bone resorption which is facilitated by the ability of osteoblastic cells to regulate the rate of both differentiation and activity of osteoclasts as well as to form new bone. An overview of genetics and molecular mechanisms that involved in the differentiation of osteoblast and osteoclast is discussed.

Keywords: Bone cells; osteoblasts; osteoclasts; regulations.

Figure 1.

Relationship of OPG/RANK/RANKL ; The control of osteoclastogenesis that emerged in the relationship of OPG/RANK/RANKL. RANKL, expressed on the surface of preosteoblastic/stromal cells. M-CSF, which binds to its receptor, c-fms, on preosteoclastic cells, appears to be necessary for osteoclast development because it is the primary determinant of the pool of these precursor cells. RANKL, however is critical for the differentiation, fusion into multinucleated cells, activation and survival of osteoclastic cells. OPG put a break on the entire system by blocking the effects of RANKL. Khosla, 2001 (55).

Figure 2.

Bone Remodelling Process ; Remodelling process is accomplished by cycles of resorption of old bone by osteoclasts and the subsequent formation of bone by osteoblasts. Modified from Manolagas and Jilka, 1995 (57).