Chondrocytes and osteoblasts are two primary cell types in the skeletal system that are differentiated from common mesenchymal progenitors. It is believed that osteoblast differentiation is controlled by distinct mechanisms in intramembranous and endochondral ossification. We have found that ectopic canonical Wnt signaling leads to enhanced ossification and suppression of chondrocyte formation. Conversely, genetic inactivation of beta-catenin, an essential component transducing the canonical Wnt signaling, causes ectopic formation of chondrocytes at the expense of osteoblast differentiation during both intramembranous and endochondral ossification. Moreover, inactivation of beta-catenin in mesenchymal progenitor cells in vitro causes chondrocyte differentiation under conditions allowing only osteoblasts to form. Our results demonstrate that beta-catenin is essential in determining whether mesenchymal progenitors will become osteoblasts or chondrocytes regardless of regional locations or ossification mechanisms. Controlling Wnt/beta-catenin signaling is a common molecular mechanism underlying chondrocyte and osteoblast differentiation and specification of intramembranous and endochondral ossification.

A critical step in skeletal morphogenesis is the formation of synovial joints, which define the relative size of discrete skeletal elements and are required for the mobility of vertebrates. We have found that several Wnt genes, including Wnt4, Wnt14, and Wnt16, were expressed in overlapping and complementary patterns in the developing synovial joints, where ␤-catenin protein levels and transcription activity were up-regulated. Removal of ␤-catenin early in mesenchymal progenitor cells promoted chondrocyte differentiation and blocked the activity of Wnt14 in joint formation. Ectopic expression of an activated form of ␤-catenin or Wnt14 in early differentiating chondrocytes induced ectopic joint formation both morphologically and molecularly. In contrast, genetic removal of ␤-catenin in chondrocytes led to joint fusion. These results demonstrate that the Wnt/␤-catenin signaling pathway is necessary and sufficient to induce early steps of synovial joint formation. Wnt4, Wnt14, and Wnt16 may play redundant roles in synovial joint induction by signaling through the ␤-catenin-mediated canonical Wnt pathway.[Keywords: Wnt; ␤-catenin; joint formation; skeletal development] Supplemental material is available at http://www.genesdev.org. Formation of synovial joints between different skeletal elements is essential for the mobility of vertebrates. The number and position of joints also determine characteristic skeletal patterns in each vertebrate species by defining the size and shape of skeletal elements. As alterations of early patterning signals often lead to changes in the position and number of joints in the developing limb (Dahn and Fallon 2000;Suzuki et al. 2004), understanding the regulation of joint formation in the limb will also provide critical insights into how early-limb patterning is linked to later skeletal morphogenesis at the molecular level.In the developing limb, studies of descriptive embryology have shown that skeletal elements form through temporally and spatially regulated processes that include mesenchymal condensation, elongation, branching, and/ or segmentation (Shubin and Alberch 1986). Most of the synovial joints in the limb form through segmentation of a pre-existing cartilage rod. For instance, in the developing forelimb, the initial de novo mesenchymal condensation forms the cartilage anlagen of the humerus, the growth and branching of which then produce a Y-shaped bifurcation. It is the segmentation of this Y-shaped cartilage primordium that forms the elbow joint that separates the radius and ulna from the humerus (Shubin and Alberch 1986).Synovial joint formation starts from the differentiation of newly differentiated chondrocytes into flattened and densely packed interzone cells (for review, see Archer et al. 2003), which express joint-specific markers such as Gdf5 and lose the expression of chondrocytespecific markers such as ColII (Craig et al. 1987;Nalin et al. 1995;Morrison et al. 1996;Storm and Kingsley 1996). Later in development, the interzone cells differentiate and form three laye...

Zygomorphic flowers, with bilateral (dorsoventral) symmetry, are considered to have evolved several times independently in flowering plants. In Antirrhinum majus, floral dorsoventral symmetry depends on the activity of two TCP-box genes, CYCLOIDEA (CYC) and DICHOTOMA (DICH). To examine whether the same molecular mechanism of floral asymmetry operates in the distantly related Rosid clade of eudicots, in which asymmetric flowers are thought to have evolved independently, we investigated the function of a CYC homologue LjCYC2 in a papilionoid legume, Lotus japonicus. We showed a role for LjCYC2 in establishing dorsal identity by altering its expression in transgenic plants and analyzing its mutant allele squared standard 1 (squ1). Furthermore, we identified a lateralizing factor, Keeled wings in Lotus 1 (Kew1), which plays a key role in the control of lateral petal identity, and found LjCYC2 interacted with Kew1, resulting in a double mutant that bore all petals with ventralized identity to some extents. Thus, we demonstrate that CYC homologues have been independently recruited as determinants of petal identities along the dorsoventral axis in two distant lineages of flowering plants, suggesting a common molecular origin for the mechanisms controlling floral zygomorphy.dorsoventral axis ͉ floral development ͉ keeled wings in Lotus ͉ LjCYC2 ͉ squared standard