The International Journal of Biochemistry & Cell Biology
Cells in focusOsteoblasts: novel roles in orchestration of skeletal architecture
Introduction
Osteoblasts are the cells found on bone surfaces that are responsible for bone formation. They are cuboidal cells with a round nucleus usually found in a single layer adherent to periosteal or endosteal surfaces of bone (Marks & Odgren, 2002) (Fig. 1). Since the main function of osteoblasts is the secretion of a complex mixture of bone matrix proteins (known as osteoid), active osteoblasts have a prominent Golgi complex and abundant rough endoplasmic reticulum. As polarised cells that secrete osteoid unidirectionally towards the bone surface on which they lie, osteoblasts form tight junctions with adjacent osteoblasts, and possess specialised regions of their plasma membrane modified for vesicular trafficking and secretion (Prêle, Horton, Caterina, & Stenbeck, 2003). Osteoblasts are normally separated from the mineralised bone matrix by a thin layer of unmineralised matrix, the osteoid seam.
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Cell origin and plasticity
Osteoblasts are derived from primitive mesenchymal cells (Aubin & Triffitt, 2002). During embryonic skeletogenesis, they participate in two distinct modes of bone development, intramembranous and endochondral ossification (Marks & Odgren, 2002). The former gives rise to the flat bones of the skull and involves the condensation of mesenchymal cells, followed by their differentiation into osteoblasts and secretion of osteoid to form bone spicules; osteoblasts fill in the spaces between spicules
Functions
Osteoblasts play a central role in creating and maintaining skeletal architecture, and they do this in two ways. They are responsible for deposition of bone matrix and they regulate the differentiation and activity of the bone-resorbing osteoclasts (Fig. 2). In addition, as a result of their ability to regulate osteoclastic activity, osteoblasts indirectly play an important role in calcium homeostasis.
As osteoblasts differentiate from their precursors they begin to secrete bone matrix proteins.
Associated pathologies
There are a number of diseases associated with altered osteoblast function, including osteoporosis, Paget’s disease of bone and osteoarthritis; only osteoporosis will be discussed here. As the cells responsible for bone formation, osteoblasts play a central role in the pathophysiology of osteoporosis. In this disease, reduced bone mass and a deterioration in bone microarchitecture lead to enhanced skeletal fragility; people suffering from osteoporosis are more susceptible to the occurrence of
Acknowledgements
The author would like to thank Dr. R.M.D. Zebaze for helpful advice on the manuscript.
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