Introduction

Bone growth, whether longitudinal or appositional, requires a source of differentiated osteoblasts capable of secreting the proteins of bone matrix. Thus, factors influencing both the proliferation and differentiation of osteoblast precursors are important for bone growth. Cells of the osteoblast lineage respond to many environmental influences including soluble factors (hormones, growth factors) and mechanical loading. The proteins of the extracellular matrix, as well as having important structural roles in bone, may also be important local regulators of bone cell function, and may act as long-term insoluble mediators of the actions of soluble factors or load.

In recent years, it has been demonstrated that extracellular matrix proteins, acting through cell-surface receptors such as integrins, can have potent specific effects on cell behaviour, being able to influence cell proliferation, migration and differentiation. The extracellular matrix protein tenascin-C is of particular interest in bone development and growth because of its selective association with the developing skeleton. Tenascin-C (hereafter referred to simply as ‘tenascin’) is a member of the tenascin gene family, composed of at least four members (Chiquet-Ehrismann, Hagios & Matsumoto, 1994). Tenascin is a large hexameric glycoprotein with disulphide-linked subunits, each consisting of several structural domains, including epidermal growth factor-like repeats, fibronectin type III repeats and a fibrinogen-like terminal domain (Fig. 1; Spring, Beck & Chiquet-Ehrismann, 1989). Alternative splicing of the tenascin gene results in the existence of differently sized subunits, the number of possible subunits varying between species. The larger splice variants differ from the smaller variants by the presence of additional fibronectin type III repeats.