The third through seventh vertebrae of the cervical spine are similar in their development, but development is different for the first and second vertebrae. All the cervical vertebrae develop from primitive sclerotomes [1], but the upper cervical spine, because of its functional complexity evolves from both the occipital and upper cervical sclerotomes (Fig. 1.1). These sclerotomes undergo significant embryologic alterations so as to eventually be able to accommodate free movement of the head on the first two (atlas and axis) cervical vertebrae. In this regard, the skull first must sit firmly on the atlas. This is accomplished by way of the occipital condyles articulating with the lateral masses of C1. This union is ensured by the presence of strong ligaments between these two structures. Thereafter, the skull and C1 must be able to rotate freely on C2. This is facilitated by the dens, which serves as a pivot for this function. The resultant rotatory movement occurs primarily at the C1–C2 level, but it should be underscored that while C1 rotates, within limits, around the dens, on flexion and extension, the skull, C1, and C2 move as a unit.

In terms of the derivation of the various portions of the upper cervical spine from the primitive sclerotomes, the atlas (C1) is derived from the first cervical sclerotome (Fig. 1.1). The axis (C2) is derived from the second cervical sclerotome. This sclerotome gives rise to the body, lateral masses, and neural arch of C2, but not the dens. The dens actually is the body of C1 and as such is derived from the first cervical sclerotome. The ossicle at the tip of the dens (the os terminale) is derived from the fourth occipital sclerotome (specifically known as the proatlas). The dens itself arises from two primordial centers, which eventually fuse with the os terminale to form the mature dens (Fig. 1.2). If they remain separate the dens often is referred to as bifid, but still normal (see Fig. 4.19)

In terms of the development of the third through seventh cervical vertebra, and actually the body and neural arch of C2, there are six chondrofication centers [2]. These are diagrammatically depicted in Fig. 1.3, and while in most cases all these centers unite, if they fail to unite or develop, predictable anomalous configurations result. For example, if the two vertebral body centers fail to completely unite, a sagittal cleft vertebra results (Fig. 1.3b). If the union of the two vertebral body chondrofication centers is more advanced, but yet incomplete, a so-called butterfly vertebra develops (see Fig. 3.1), and when one or other, or both, of the vertebral body chondrofication centers fail to develop, a hemivertebra, or absence of a vertebra, results (Fig. 1.3c, d). Failure of development of the midchondrofication center leads to the absence of the pedicle (Fig. 1.3e), and if failure of the chondrofication center of the neural arch occurs, partial or complete absence of the neural arch results (Fig. 1.3f, g). Finally, if the two neural arches fail to unite posteriorly, a spina bifida results (Fig. 1.3h). However, development of the vertebral bodies at this stage is not complete for later, two vertebral body ossification centers develop. One is ventral and the other dorsal (Fig. 1.4a). If the ossification centers develop but fail to fuse, a coronal cleft vertebra results (Fig. 1.4b). If one or other of these ossification centers fails to develop, a coronal hemivertebra results (Fig. 1.4c, d). If both ossification centers fail to develop, the vertebral body will be hypoplastic or absent. Failure of the ossification centers of the neural arches to develop results in anomalies similar to those seen when the corresponding chondrofication centers fail to develop.