The Distribution of Bone Sialoprotein in Bone Mineralization

Abstract

Bone consists of an organic matrix and hydroxyapatite (HA) crystals. The mechanism by which bone mineralizes the organic matrix is not clearly understood. Collagen provides the framework of bone and may even play a role in the nucleation of the HA crystals through the hole zones within collagen fibers. The hole zones are created by the alternating arrangement of the individual collagen fibrils. Phosphoproteins that are only found in bone are thought to play an important role in the nucleation and organization of the crystals. Bone sialoprotein (BSP) is a bone matrix protein that possesses an Arg-Gly-Asp (RGD) cell attachment region and three polyglutamic acid regions. These two regions of the protein are thought to have an active role in bone mineralization. The RGD region has been shown to bind integrins on the surface of osteoblasts. The negative charge of the glutamic acid regions can bind to HA crystals and also may attract calcium ions that are incorporated into the HA crystals. BSP is heavily phosphorylated through associations between BSP’s serine and threonine content and inorganic phosphate ions. These phosphate ions may be available for use in the mineralization process. It is thought that BSP is aiding in the nucleation of the crystals. In the literature, there exist little histological characterizations of this protein in mature bone. The organization of BSP within already ossified bone may help to ascertain its function. Fish, chicken and pork bone were chosen for their differences in their organization of mineralized bone and growth patterns. Immunohistological sections were made of each bone type to try to identify the organization of the BSP in each. BSP was found to be a component of the organic matrix. Moreover, BSP was more concentrated in areas where there was active mineralization.