Cells in focus
Osteoblasts: novel roles in orchestration of skeletal architecture

https://doi.org/10.1016/S1357-2725(03)00107-9Get rights and content

Abstract

Osteoblasts are located on bone surfaces and are the cells responsible for bone formation through secretion of the organic components of bone matrix. Osteoblasts are derived from mesenchymal osteoprogenitor cells found in bone marrow and periosteum. Following a period of secretory activity, osteoblasts undergo either apoptosis or terminal differentiation to form osteocytes surrounded by bone matrix. Osteoblasts secrete a characteristic mixture of extracellular matrix proteins including type I collagen as the major component as well as proteoglycans, glycoproteins and γ-carboxylated proteins. Cells of the osteoblast lineage also provide factors essential for differentiation of osteoclasts (bone-resorbing cells). By regulating osteoclast differentiation and activity in response to systemic influences, osteoblasts not only play a central role in regulation of skeletal architecture, but also in calcium homeostasis. Inadequate osteoblastic bone formation in relation to osteoclastic resorption results in osteoporosis, a disease characterised by enhanced skeletal fragility.

Cellfacts

  • Osteoblasts are the cells responsible for bone formation.

  • Osteoblasts indirectly control levels of bone resorption.

  • Osteoblasts play a key role in the pathophysiology of osteoporosis and the resulting fractures, which constitute a major public health burden in developed countries.

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.

Section snippets

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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