Cranial sutures as intramembranous bone growth sites

Intramembranous bone growth is achieved through bone formation within a per iosteum or by bone formation at sutures. Sutures are formed during embryoni c development at the sites of approximation of the membranous bones of the craniofacial skeleton. They serve as the major sites of bone expansion duri ng postnatal craniofacial growth. For sutures to function as intramembranou s bone growth sites, they need to remain in an unossified state, yet allow new bone to be formed at the edges of the overlapping bone fronts. This pro cess relies on the production of sufficient new bone cells to be recruited into the bone fronts, while ensuring that the cells within the suture remai n undifferentiated. Unlike endochondral growth plates, which expand through chondrocyte hypertrophy, sutures do not have intrinsic growth potential. R ather, they produce new bone at the sutural edges of the bone fronts in res ponse to external stimuli, such as signals arising from the expanding neuro cranium. This process allows growth of the cranial vault to be coordinated with growth of the neurocranium. Too little or delayed bone growth will res ult in wide-open fontanels and suture agenesis, whereas too much or acceler ated bone growth will result in osseous obliteration of the sutures or cran iosynostosis. Craniosynostosis in humans, suture fusion in animals, and ind uced suture obliteration in vitro has been associated with mutations or alt erations in expression of several transcription factors, growth factors, an d their receptors. Much of the data concerning signaling within sutures has been garnered from research on cranial sutures; hence, only the cranial su tures will be discussed in detail in this review. This review synthesizes c lassic descriptions of suture growth and pathology with modern molecular an alysis of genetics and cell function in normal and abnormal suture morphoge nesis and growth in a unifying hypothesis. At the same time, the reader is reminded of the importance of the suture as an intramembranous bone growth site. (C) 2000 Wiley-Liss, Inc.