A review of ectochondral bone and the role of membranes in shaping endochondral bones of the skull.

Bones of the skull are traditionally categorized as derived from either endochondral or intramembranous bone. In our previous work, we have observed the interaction of different tissue types in growth of the skull. We find the dichotomy of intramembranous and endochondral bone to be too restrictive, limiting our interpretation of sources of biological variation. Here, we advocate for the use of the term ectochondral bone to describe bone that originates from an endochondral model but is directed in its subsequent growth by membranes and other fascial attachments. Growth of the alisphenoid and orbitosphenoid are described as two examples of ectochondral bone, influenced in their shape primarily by the surrounding soft tissues. Ectochondral bone may be an ideal mechanism for rapidly evolving new phenotypes. Instead of evolving novelties by altering morphology of the cartilage template, novel features may be formed by ectochondral ossification, a more direct and rapid mode of osteogenesis than that of the cartilage template.