Transforming growth factor-beta 2 and TGF-beta 3 regulate fetal rat cranial suture morphogenesis by regulating rates of cell proliferation and apoptosis

Cranial vault sutures are the major intramembranous bone growth sites durin g rapid expansion of the neurocranium. To function as bone growth sites, su tures need to remain patent, while allowing rapid bone formation at the edg es of the bone fronts. Premature osseous obliteration of sutures (craniosyn ostosis) by fusion of bone fronts across the suture site prevents further b one formation at this site, often leading to severe facial dysmorphology. A lthough several growth factor receptor and transcription factor mutations h ave been implicated in craniosynostosis, the underlying mechanisms leading to sutural obliteration remain unclear, Previous studies have shown that du ra secreted soluble factors responsible for maintaining suture patency and that suture fusion observed in the absence of dura was preceded by elevated levels of DNA synthesis and collagen production in the suture region. The use of neutralizing antibodies in a fetal calvarial culture model further d emonstrated that removal of transforming growth factor (TGF) -beta 3 activi ty induced premature sutural obliteration, whereas removal of TGF-beta 2 ac tivity prevented sutural obliteration. Data presented here demonstrate that suture obliteration induced by removal of TGF-beta 3 activity was preceded by elevated levels of DNA synthesis, similar to that seen upon removal of the dura. Addition of exogenous TGF-beta 3 to calvaria cultured without dur a both prevented suture obliteration and reduced DNA synthesis to levels co mparable to those seen with intact dura, Addition of exogenous TGF-beta 2 t o calvarial cultures induced sutural fusion accompanied by elevated levels of cell proliferation, However, sutures rescued from obliteration by remova l of TGF-beta 2 activity did not have decreased levels of cell proliferatio n, but rather appeared to be due to inhibited differentiation. In all calva ria in which sutures remained patent in culture, numbers of apoptotic cells were high within the suture, whereas in sutures destined to fuse, numbers of apoptotic cells were low, Results indicate that one of the critical regu lators of suture patency is cell number. Alterations in cell number can tri gger premature differentiation of cells, resulting in sutural obliteration. Furthermore, a complex interplay between closely related molecules is requ ired to maintain cranial vault sutures in an unossified state, while allowi ng new bone to be formed at the edges of the bone fronts. (C) 2000 Wiley-Li ss, Inc.