The Desmoid and Endochondral Ossification Centers of the Skull
The bones of the neurocranium originate primarily through membranous (desmoid) ossification or through a combination of desmoid and endochondral ossification, as is the case with the sphenoid bone, the temporal bone and the occipital bone. Of these bones, the parts that belong to the base of the skull tend to be endochondral and those that are a part of the neurocranium tend to be desmoid. The connective tissue around the primitivebrain functions as the preliminary stage for the desmoid bone that grows in the form of rounded, flat plates of interstitial tissue (fig. 3.4.). The cells for these interstitial plates develop from the ectoderm (neural crest) and not, as is the case in the remaining bone tissue, from the mesoderm. In membranous ossification, the osteoblasts develop directly from mesenchymal cells. They occupy a special position in the skeletal development process, which originates through endochondral ossification nearly everywhere else. The occipital bone and the elements that will form the base of the skull and the jaw develop from mesodermal tissue. For the base of the skull, that is mesoderm from the paraxial somites and the pharyngeal arches (fig. 3.3.). The osteogenesis of these parts of the skull shows a relationship to the osteogenesis that occurs in the extremities