Induction and patterning of intramembranous bone.

The primary focus of this article is to review intramembranous bone development, that is, ossification that takes place directly. Comparisons with endochondral ossification (ossification with a cartilage precursor) will be made in order to illustrate the differences between these two modes of ossification and to highlight the comparatively sparse information that is available about intramembranous ossification. Despite decades of research into understanding skeletal development, there is still much to learn. Most of the research in this area has focused on the development of the calvariae (or skull bones) as typical intramembranous bones and the development of the limb bones as a typical endochondral bones. Few studies investigate other skeletal elements or compare these processes in a systematic manner. In this review, I focus primarily on condensation formation and skeletal patterning with specific examples from different organisms.