Appropriate biochemical regulation of intramembranous bone growth from sutures is necessary to achieve correct craniofacial morphology. Failure to form sutures (agenesis) or to maintain sutures in their unossified state (craniosynostosis) can result in severe facial dysmorphology. Several factors such as Twist, Msx2, fibroblast growth factors (Fgfs), bone morphogenetic proteins (Bmps) and transforming growth factors-beta (Tgf-betas) regulate suture patency, likely by interacting with one another. Tgf-beta2 and Tgf-beta3 use the same cell surface receptors, yet have opposite effects on suture patency, cellular proliferation and apoptosis within the suture. One possible mechanism by which Tgf-beta3 rescues sutures from obliteration is by regulating the ability of suture cells to respond to Tgf-beta2. As Tgf-beta3 does not regulate protein levels of Tgf-beta2 in sutures, Tgf-beta3 could regulate tissue responsiveness to Tgf-beta2 by regulating Tgf-beta2 access to receptors. Tgf-beta3 is a more potent competitor than Tgf-beta2 for cell surface receptors, so it is proposed that Tgf-beta3 binds to and down-regulates Tgf-beta receptor type I (Tbetar-I) expression by suture cells. This down-regulation would limit the ability of cells to respond to all Tgf-betas, including Tgf-beta2. To test this hypothesis, an in vitro culture model was used in which fetal rat sutures either remain patent or are induced to fuse when cultured in the presence or absence of dura mater, respectively. Tgf-beta3 was added to cultured calvaria and changes in the number of receptor positive cells within the suture were established. Data were compared with that seen in control sutures and in normal sutures in vivo. It was found that the numbers of cells expressing Tbetar-I within the suture matrix increased over time in sutures remaining patent. Osteoblastic cells lining the bone fronts on either side of sutures were Tbetar-I positive during early morphogenesis, but these numbers declined as sutures fused, both in vivo and in vitro. Addition of Tgf-beta3 to calvaria in culture decreased the number of Tbetar-I expressing cells in both fusing and non-fusing sutures, with dramatic decreases in the numbers of osteoblasts expressing Tbetar-I.