Embryonic and early fetal period development and morphogenesis of human craniovertebral junction.

Several studies have focused on the cartilaginous, articular, and ligamentous development of the craniovertebral joint (CVJ), but there are no unifying criteria regarding the origin and morphogenetic timetable of the structures that make up the CVJ. In our study, serial sections of 53 human embryonic (n = 27) and fetal (n = 26) specimens from O'Rahilly stages 17-23 and 9-13 weeks, respectively, have been analyzed. Our results demonstrate that the chondrification of the pars basioccipitalis and exoccipitalis becomes observable at stage 19, and all future bones in the CVJ are in their cartilaginous form except for the future odontoid process. In addition, two chondrification centers appear for the body of the axis. From stage 21, the apical, alar, and transverse atlantal ligaments begin to acquire a ligamentous structure and the odontoid process initiates its chondrogenic phase. Stage 22 witnesses the first signs of the articular cavities of the atlanto-occipital joint, and by stage 23 all joints have cavities except for the transverse-odontoid joint, which will wait until week 9. In week 10, the ossification of the basilar part of the occipital bone begins, followed by the rest of the structures except for the odontoid process, which will start at week 13, thus completing the osteogenesis of all bones in the CVJ. The results of this study could help in establishing the anatomical basis of the normally functioning CVJ and for detecting its related pathologies, abnormalities, and malformations.