The Craniocervical Region: A Developmental Restless Region

The aim of this overview is to demonstrate and discuss several anatomical variations of the craniocervical region in the light of developmental processes. The anterior end of the notochord as well as the axial sclerotomes of the first four somites are included into the skull base and are forming the basioccipital part. If too much material is included, the assimilation of the atlas will occur. If the material of the proatlas is not incorporated or if the hypochordal blastema of the proatlas persists completely or partly, manifestations of the occipital vertebra can be seen. Typical autopsy specimens of these entities are presented and explained. If the lateral parts of the hypochordal blastema of the proatlas persist, the basilar processes can be seen at the lateral anterior border of the occipital foramen. If the medial part of the hypochordal blastema persists, the third condyle (condylus tertius) will occur in the midline plane at the anterior border of the occipital foramen. If the whole hypochordal blastema persists, the prebasioccipital arch (arcus praebasioccipitalis) can be seen. This arch can appear as a horseshoe-like fixed osseous mass or as an isolated horseshoe-like ossicle sitting at the odontoid process like a Napoleon hat. The material of the transverse process of the proatlas can form the paracondyloid process and the related entities. If the atlas is fixed to the skull base by this material, this should not be termed as an assimilation of the atlas. This should be classified better as an atlantooccipital synostosis or fusion. A characteristic feature of the real assimilation of the atlas is that the atlas is "absorbed" by the skull base, so that a new osseous structure occurs.