Role of heparan sulfate proteoglycans surrounding osteoblast lineage cells

Bone matrix consists of hydroxyapatite and organic components such as type I collagen and non-collagenous proteins. Extracellular matrices of bone tissue generally mean these organic components in bone matrix. However, a few extracellular matrices are present in the intercellular space of osteoblast lineage cells surrounding blood vessels. Our immunohistochemical analysis indicates that heparan sulfate proteoglycans (HSPGs) localize in this space as extracellular matrix and/or membrane-bound forms. Heparan sulfate (HS) chains could bind heparin-binding proteins, including fibronectin, and several growth factors such as bone morphogenetic proteins via their highly negative charges. HSPGs in bone tissue may help to establish the microenvironment for differentiation of osteoblasts, osteoclasts, and hematopoietic cells.