Ossification of bony nerve canals in the human head: a comparison between the endochondral and membranous bones.

Cranial nerves and their branches pass through bony canals in the head. To reveal an ossification manner for closing the nerve canal, we examined histological sections from 32 human fetus heads at 9-18 and 28-40 weeks. Corresponding to types of bones that was penetrated by nerves, the infraorbital nerve (ION) and zygomatic nerve (ZN) were chosen for nerves passing through a membranous bone, while the maxillary nerve (MXN) and hypoglossal nerve (HGN) were chosen for nerves through the endochondral bone. The ION and ZN were sandwiched by membranous bones. The membranous bone trabeculae connected to the secondarily-developed periosteal bone and the latter partly closed the ION canal prenatally, but the closure of the ZN canal was most likely to occur postnatally. Therefore, rather than membranous bone trabeculae, the periosteal bone contributed much to closure of the nerve canal. Because it connected to the periosteum, the orbital muscle might provide a mechanical load facilitating the ossification around the ION. The MXN first passed through the cartilaginous sphenoid and, later, the perichondral bone appeared along the nerve in combination with the additional membranous ossification. Near the HGN canal, a small center of endochondral ossification appeared and the secondarily-developed bone reached the perichondrium of the canal, but the typical periosteal ossification was absent. Consequently, the perichondral or periosteal ossification along the nerve canal seemed to play a major role for a complete closure of the canal postnatally. Modern radiology might show the canal much thicker than the nerve itself in infants.