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Abstract
Cranial sutures are sites of bone deposition in the growing craniofacial skeleton. Through the addition of bone at osteogenic fronts, the cranial sutures eventually fuse (Opperman 2000). However, sutures also assist in the active maintenance of suture patency during development and act as shock absorbers during birth (Moss 1959, Warren 2003). Thus, it is likely that, in response to mechanical strain, the normal cell biology of sutures and the underlying dura represents a balance of both osteoinductive and counter-regulatory signals. The purpose of these experiments is to investigate changes in dural cell biology and intact calvarial sutures caused by tensile strain. More specifically, we propose to investigate: 1) the effect of intermittent tensile strain on the biology of dural cell cultures derived from tissue underlying both fusing and patent calvarial sutures, and 2) changes in the cell biology of normally patent calvarial sutures in response to tensile strain exerted across their structure. In all of these experiments special attention will be paid to both the osteoinductive (increased rates of osteoblast differentiation and increased expression of osteoinductive genes) and counter-regulatory effects (the expression of factors known to restrict osteogenesis) that accompany the exertion of exogenous tensile strain. We hypothesize that the exertion of both intermittent tensile strain on dural cell cultures, and static tensile strain on calvarial sutures in organ culture, results in increased rates of osteoblast differentiation, increased expression of osteoinductive genes, and decreased expression of counter-regulatory signals required to maintain suture patency. A growing body of evidence has implicated various forms of biomechanical force in altered dural cell biology, sutural morphogenesis, and the pathogenesis of craniosynostosis. Clinical reports and experimental evidence suggest that intrauterine constraint (likely resulting in increased intermittent compressive force) may play a causal role in the pathogenesis of craniosynostosis (Kirschner 2002, Hunenk02001). Recent laboratory reports also suggest that static tensile strain produces increased rates of dural cell proliferation, growth factor production (TGFbeta-l, FGF-2), and increased rates of osteoblast differentiation (Fong 2003). Furthermore, a growing body of research suggests that tensile strain exerted upon calvarial sutures in organ culture results in increased rates of osteoblast differentiation, and increased expression of osteoinductive proteins (Ikegame 2001, Yu 2001). Despite these advances, the effect of intermittent tensile strain on dural cell cultures has not been investigated. Furthermore, the role of counter-regulatory factors (Noggin, TGFbeta-3) in maintaining suture patency in response to mechanical perturbation has not been examined.