Serial transverse histological sections of the human craniovertebral
junction (CVJ) of 4 normal human
embryos (aged 45 to 58 d) and of a fetus (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 short eminence of the axis
forming a third occipital condyle with the caudal 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
notochord was constant throughout. The normal course of the vertebral
arteries was already established and
the chondrified vertebral foramina showed progressive closure. The findings
confirm that the odontoid
process is not derived solely from the centrum of C1 and that there is
a
‘natural basilar invagination’ of C2
during normal embryonic development. On the basis of the observed shape
and developmental pattern of
structures of the cartilaginous human CVJ, we suggest that certain
pathologies are likely to originate during
the chondrification phase of development.