The craniovertebral junction develops from somites that differentiate into sclerotomes around the notochord. The central pivot includes the dens, C2 vertebral body, and basiocciput. It is surrounded by the foramen magnum and atlantal rings. The proatlas, from the 4th occipital sclerotome, is important in development. Ossification of C2 occurs in three phases, with fusion completing by age 16-18. Abnormalities can result in instability or neural compression and include segmentation errors, assimilation, or developmental disorders affecting metabolism or inflammation.

1. Embryology &
Development of the
Craniovertebral Junction
Shashank Gandhi, MD

2. Craniovertebral Junction
Divided into 2 portions:
● Central pivot
o Dens, C2 vertebral body, basiocciput
● 2 ringed structures surrounding central pivot
o Foramen magnum ring
 clivus, exocciput, occipital condyles, opisthion
o Atlantal ring

3. Embryology
● Notochord forms early axial skeleton
● During 4th week gestation, 42 somites
formed
o 4 occipital, 8 cervical, 12 thoracic, 5 lumbar, 5
sacral, 8-10 coccygeal
● Each somite differentiates
o outer dermatome
o inner myotome
o medial sclerotome - form vertebrae around notocord
 4 occipital sclerotomes
● 4th Occipital Sclerotome - Proatlas - important for CVJ
development

5. Formation of CVJ

6. Sclerotomes in Formation of CVJ
**DO NOT RESEGMENT

7. Sclerotome Origins of CVJ

8. Posterior Fossa
● Expansion occurs by endochondral
reabsorption, sutural growth, & boney
accretion until 16-18 years of age
● Growth of basion elongated basiocciput &
lowers frontal margins of foramen magnum
● Downward displacement of cerebellum &
rotation of occipital and temporal lobes
causes resorptive drift down and backward
of opisthion

9. Development of C2
● Apical dens from proatlas
● Dens from 1st cervical sclerotome
● VB of C2 from 2nd cervical sclerotome
● Odotoid separate from VB at birth by
cartilaginous bands - 2 synchondroses
● 3 phases of synchondrosis & ossification
o 1st - 4mo fetus at VB/neural arches
o 2nd - 6mo fetus basal dental segment
 ossification of lower syndrondrosis at birth
o 3rd - 3-5 yrs at apical dental segment
 not completely fused to basal dens until 6-9 yrs
o Fully completed by age 16-18

10. Developmental Phases of C2

11. Developmental Abnormalities
● In general abnormalities result in:
o Central pivot
 instability
 however, basilar impression & retroflexed
odontoid can cause neural compression
o 2 rings (atlas & axis)
 deformity & crowding
 however, hypoplasia/aplasia can cause
weakening/loss of ligamentous structures -
instability

12. Congenital Anomalies & Malformations
● Occipital bone
o Manifestations of occipital vertebra
 clivus segmentations
 remnants around foramen magnum
 atlas variants
 dens segmentations anomalies
o Basilar invagination
o Condylar hypoplasia
o Assimilation of atlas

13. Congenital Anomalies & Malformations
● Atlas
o Assimilation of atlas
o Atlantoaxial fusion
o Aplasia of atlas arches
● Axis
o Irregular atlantoaxial segmentation
o Dens dysplasias
 Ossiculum terminale persistens
 Os odontoideum
 Hypoplasia-aplasia
o Segmentation failure of C2/3

14. Developmental & Acquired Abnormalities
● Foramen Magnum
o Secondar basilar invagination (Paget’s dz,
osteomalacia, rheumatoid cranial settling, Rickets)
o Foraminal stenosis (achondroplasia)
● Atlantoaxial Instability
o Errors in metabolism (Morquio’s syndrome)
o Down syndrome
o Infection (Grisel’s syndrome)
o Inflammatory (rheumatoid arthritis)
o Traumatic OA & AA dislocation, os odontoideum
o Tumors (NF)