Skeletal development--Wnts are in control.

Approximately 200 individual skeletal elements, which differ in shape and size, are the building blocks of the vertebrate skeleton. Various features of the individual skeletal elements, such as their location, shape, growth and differentiation rate, are being determined during embryonic development. A few skeletal elements, such as the lateral halves of the clavicle and parts of the skull are formed by a process called intramembranous ossification, whereby mesenchymal cells differentiate directly into osteoblasts, while the majority of skeletal elements are formed via endochondral ossification. The latter process starts with the formation of a cartilaginous template, which eventually is being replaced by bone. This requires co-regulation of differentiation of the cell-types specific for cartilage and bone, chondrocytes and osteoblasts, respectively. In recent years it has been demonstrated that Wnt family members and their respective intracellular pathways, such as non-canonical and the canonical Wnt/beta-catenin pathway, play important and diverse roles during different steps of vertebrate skeletal development. Based on the recent discoveries modulation of the canonical Wnt-signaling pathway could be an interesting approach to direct stem cells into certain skeletal lineages.