CARTILAGINOUS DEVELOPMENT OF THE HUMAN CRANIOVERTEBRAL JUNCTION AS VISUALIZED BY A NEW 3-DIMENSIONAL COMPUTER RECONSTRUCTION TECHNIQUE

Serial transverse histological sections of the human craniovertebral j unction (CVJ) of 4 normal human embryos (aged 45 to 58 d) and of a fet us (77 d) were used to create 3-dimensional computer models of the CVJ . The main components modelled included the chondrified basioccipital, atlas and axis, notochord, the vertebrobasilar complex and the spinal cord. Chondrification of the component parts of CVJ had already begun at 45 d (Stage 18). The odontoid process appeared to develop from a s hort eminence of the axis forming a third occipital condyIe with the c audal end of the basioccipital. The cartilaginous anterior arch of C1 appeared at 50-53 d (Stages 20-21). Neural arches of C1 and C2 showed gradual closure, but there was still a wide posterior spina bifida in the oldest reconstructed specimen (77 d fetus). The position of the no tochord was constant throughout. The normal course of the vertebral ar teries was already established and the chondrified vertebral foramina showed progressive closure. The findings confirm that the odontoid pro cess is not derived solely from the centrum of C1 and that there is a 'natural basilar invagination' of C2 during normal embryonic developme nt. On the basis of the observed shape and developmental pattern of st ructures of the cartilaginous human CVJ, we suggest that certain patho logies are likely to originate during the chondrification phase of dev elopment.