Mesenchymal Condensations: The Architects for Morphogenesis & Growth of Skeletal Elements

The formation of the skull base and the axial and appendicular skeleton occurs by the process of endochondral ossification. During the first step in this process mesenchymal cells condense into cellular templates for the cartilage models of future bones. A combination of ectodermal signals and responses by mesenchymal cells to local hypoxia control the early patterning processes in and around the condensations and result in activation of pathways which regulate cellular polarity and stimulate chondrocyte differentiation. The formation of cartilaginouse models of defined shapes requires cellular interactions that are critically dependent on cellular polarity. During the condensation process, the mesenchymal cells exhibit preferred orientations as they move from the periphery towards the center, such that their primary cilia and Golgi regions are pointing towards the central axes of the condensations. Following chondrocytic differentiation and establishment of primary ossification centers, this orientation is maintained by chondrocytes in the epiphyseal growth plate regions of endochondral bones. Thus, the cellular organization, critical for postnatal growth of endochondral bones, is established during the formation of mesenchymal condensations.