The fetal skeleton arises from neural crest and from mesoderm. Here, we provide evidence that each lineage contributes a unique stem cell population to the regeneration of injured adult bones. Using Wnt1Cre::Z/EG mice we found that the neural crest-derived mandible heals with neural crest-derived skeletal stem cells, whereas the mesoderm-derived tibia heals with mesoderm-derived stem cells. We tested whether skeletal stem cells from each lineage were functionally interchangeable by grafting mesoderm-derived cells into mandibular defects, and vice versa. All of the grafting scenarios,except one, healed through the direct differentiation of skeletal stem cells into osteoblasts; when mesoderm-derived cells were transplanted into tibial defects they differentiated into osteoblasts but when transplanted into mandibular defects they differentiated into chondrocytes. A mismatch between the Hox gene expression status of the host and donor cells might be responsible for this aberration in bone repair. We found that initially,mandibular skeletal progenitor cells are Hox-negative but that they adopt a Hoxa11-positive profile when transplanted into a tibial defect. Conversely, tibial skeletal progenitor cells are Hox-positive and maintain this Hox status even when transplanted into a Hox-negative mandibular defect. Skeletal progenitor cells from the two lineages also show differences in osteogenic potential and proliferation, which translate into more robust in vivo bone regeneration by neural crest-derived cells. Thus, embryonic origin and Hox gene expression status distinguish neural crest-derived from mesoderm-derived skeletal progenitor cells, and both characteristics influence the process of adult bone regeneration.

The Wnt signaling pathway plays a central role in bone development and homeostasis. In most cases, Wnt ligands promote bone growth, which has led to speculation that Wnt factors could be used to stimulate bone healing. We gained insights into the mechanism by which Wnt signaling regulates adult bone repair through the use of the mouse strain Axin2^LacZ/LacZ in which the cellular response to Wnt is increased. We found that bone healing after injury is accelerated in Axin2^LacZ/LacZ mice, a consequence of more robust proliferation and earlier differentiation of skeletal stem and progenitor cells. In parallel, we devised a biochemical strategy to increase the duration and strength of Wnt signaling at the sites of skeletal injury. Purified Wnt3a was packaged in liposomal vesicles and delivered to skeletal defects, where it stimulated the proliferation of skeletal progenitor cells and accelerated their differentiation into osteoblasts, cells responsible for bone growth. The end result was faster bone regeneration. Because Wnt signaling is conserved in mammalian tissue repair, this protein-based approach may have widespread applications in regenerative medicine.

Wnt signaling is activated by wounding and participates in every subsequent stage of the healing process from the control of inflammation and programmed cell death, to the mobilization of stem cell reservoirs within the wound site. In this review we summarize recent data elucidating the roles that the Wnt pathway plays in the injury repair process. These data provide a foundation for potential Wnt-based therapeutic strategies aimed at stimulating tissue regeneration.

Tissue regeneration is increasingly viewed as reactivation of a developmental process that, when misappropriated, can lead to malignant growth. Therefore, understanding the molecular and cellular pathways that govern tissue regeneration provides a glimpse into normal development as well as insights into pathological conditions such as cancer. Herein, we studied the role of Wnt signaling in skeletal tissue regeneration. Introduction: Some adult tissues have the ability to regenerate, and among these, bone is one of the most remarkable. Bone exhibits a persistent, lifelong capacity to reform after injury, and continual bone regeneration is a prerequisite to maintaining bone mass and density. Even slight perturbations in bone regeneration can have profound consequences, as exemplified by conditions such as osteoporosis and delayed skeletal repair. Here, our goal was to determine the role of Wnts in adult bone regeneration. Materials and Methods: Using TOPgal reporter mice, we found that damage to the skeleton instigated Wnt reporter activity, specifically at the site of injury. We used a skeletal injury model to show that Wnt inhibition, achieved through adenoviral expression of Dkk1 in the adult skeleton, prevented the differentiation of osteoprogenitor cells. Results: As a result, injury-induced bone regeneration was reduced by 84% compared with controls. Constitutive activation of the Wnt pathway resulting from a mutation in the Lrp5 Wnt co-receptor results in high bone mass, but our experiments showed that this same point mutation caused a delay in bone regeneration. In these transgenic mice, osteoprogenitor cells in the injury site were maintained in a proliferative state and differentiation into osteoblasts was delayed. Conclusions: When considered together, these data provide a framework for understanding the roles of Wnt signaling in adult bone regeneration and suggest a feasible approach to treating clinical conditions where enhanced bone formation is desired.

Human faces exhibit enormous variation. When pathological conditions are superimposed on normal variation, a nearly unbroken series of facial morphologies is produced. When viewed in full, this spectrum ranges from cyclopia and hypotelorism to hypertelorism and facial duplications. Decreased Hedgehog pathway activity causes holoprosencephaly and hypotelorism. Here, we show that excessive Hedgehog activity, caused by truncating the primary cilia on cranial neural crest cells, causes hypertelorism and frontonasal dysplasia (FND). Elimination of the intraflagellar transport protein Kif3a leads to excessive Hedgehog responsiveness in facial mesenchyme, which is accompanied by broader expression domains of Gli1, Ptc and Shh, and reduced expression domains of Gli3. Furthermore, broader domains of Gli1 expression correspond to areas of enhanced neural crest cell proliferation in the facial prominences of Kif3a conditional knockouts. Avian Talpid embryos that lack primary cilia exhibit similar molecular changes and similar facial phenotypes. Collectively, these data support our hypothesis that a severe narrowing of the facial midline and excessive expansion of the facial midline are both attributable to disruptions in Hedgehog pathway activity. These data also raise the possibility that genes encoding ciliary proteins are candidates for human conditions of hypertelorism and FNDs.

No region of our anatomy more powerfully conveys our emotions nor elicits more profound reactions when disease or genetic disorders disfigure it than the face. Recent progress has been made towards defining the tissue interactions and molecular mechanisms that control craniofacial morphogenesis. Some insights have come from genetic manipulations and others from tissue recombinations and biochemical approaches, which have revealed the molecular underpinnings of facial morphogenesis. Changes in craniofacial architecture also lie at the heart of evolutionary adaptation, as new studies in fish and fowl attest. Together, these findings reveal much about molecular and tissue interactions behind craniofacial development.