Cell differentiation starts with the formation of three germ cell layers (endoderm, mesoderm and ectoderm) during the very early embryonic phase called gastrulation. The neural plate appears as a thickening in the ectoderm layer. It undergoes a folding process termed neurulation. The neural tube is formed by this process, which is controlled by growth factors from the notochord. At the margins of the neural tube, cells of the neural crest are formed. The spinal cord and brain develop from the neural tube. Closure of the neural tube occurs at about 3 weeks. At which stage the cells of the neural crest are at its lateral margins and the notochord ventrally.

The notochord is formed from endoderm. It precedes the formation of the neural tube and modulates its development. It extends the length of the neural tube and acts as a scaffold for the spinal support structures, which develop from surrounding (paraxial) mesoderm and then regresses. The cells of the neural crest contribute to the formation of bone, meninges and the dorsal root ganglia of the spinal cord. The structures of the spine form segmentally from blocks of mesoderm on either side of the notochord arranged as repeating somites. This pattern of repeated segments is known as metamerism and the individual blocks metameres. The post-embryonic remnant of notochord is represented by the nucleus pulposus (Fig. 4.1).

During the 3–6 week stage, up to 44 somites form but then regression leaves 31 somites, each receiving a pair of primitive arteries from the dorsal aorta. These segmental arteries thus supply tissues derived from the neural tube, neural crest and somite that together constitute the metamere, i.e. the precursors of the spinal cord, spinal nerves and paraspinal muscle, skin and bone.

The primitive segmental artery, for each metamere, makes a primary division into dorsomedial and dorsolateral branches. The dorsomedial division supplies the neural tube, neural crest and dorsal epimere (i.e. that part of the somite which contributes to the vertebral column and paraxial muscles). The dorsolateral division supplies all the other structures of the metamere. At its cranial extent, the dorsal aorta contributes to the carotid arteries and at its caudal extent becomes the median sacral artery.

The blood supply to the neural tube develops from a primitive vascular plexus on its surface called the vasa corona supported by dorsomedial segmental arteries. On the ventral surface of the neural tube, longitudinal channels form on either side of the midline from longitudinal connections within the vasa corona. In Tutorial 1, when they developed anterior to the hindbrain, we called these the longitudinal neural system (LNS), though in the spine they are often called ventral longitudinal arteries. They run on either side of the developing median sulcus. As the cord develops, they give branches that enter the sulcus and branches to the vasa corona (Fig. 4.2).

The dorsomedial branches of the segmental artery to the developing spinal cord supply the anterior spinal roots and the ventral longitudinal arteries, i.e. future anterior spinal artery. They also give branches to the dorsal neural tube to supply the dorsal vasa corona and the posterior roots. Posterior longitudinal channels on the dorsolateral surface of the neural tube form later from the plexus of vessels forming the vasa corona and subsequently develop into the posterior spinal arteries. The delay is because the ventral longitudinal arteries supply most of the grey matter within the cord, which precedes the formation of white matter tracts [1].

The formation of longitudinal arteries supplying multiple metameres changes the segmental pattern of blood supply and leads to the development of the adult system of spinal arteries. On the neural tube, craniocaudal midline fusion of the ventral longitudinal arteries occurs after 6 weeks and creates the anterior spinal artery (ASA). Failures of fusion are more often evident in the cervical spinal cord and represent a failure of maturation of this system. They are evident as apparent duplications of the anterior spinal artery in the adult pattern.

At the same time (6–12 weeks), desegmentation occurs. This is a process in which most of the primitive segmental arteries supplying the neural tube regress. Completed, this process leaves only 4–8 ventral spinal arteries supplying the ASA and 10–20 dorsal spinal arteries supplying the vasa corona. The rest of the dorsomedial segmental artery supply is to the other metameric tissues of the spine, i.e. the nerve root, dura and bone. Each spinal artery is named after the nerve it accompanies into the neural foramen.

Concurrently, longitudinal anastomoses between the segmental arteries of each metamere develop around the developing spine. These longitudinal arteries are identified by their position relative to the transverse processes of vertebra, i.e. pre-transverse, transverse, post-transverse. The vertebral artery (VA) in its canal is the most developed transverse longitudinal artery connecting segments.

At this point, we need to consider the development of the arterial supply of the cervical spine and craniocervical junction together to understand the numerical labels used for the cervical vertebrae. There are eight radiculospinal arteries (like the eight cervical spinal nerves) supplying the seven cervical vertebrae. In Fig. 4.3, the somite and vertebral level of the craniocervical region and upper spine are presented in diagram form. The longitudinal multimetameric arteries in the neck are shown (i.e. VA, ascending cervical artery and deep cervical artery) and the proatlantal artery.

Eight cervical segmental arteries develop for the eight cervical somites, and the first cervical somitic artery is the proatlantal artery, which lies above the C1 vertebra. This is because the vertebral bodies develop between somite levels with the intervertebral disc as the centre of the metamere. Thus, the embryonic vertebral body is supplied by two adjacent pairs of somitic (segmental) arteries, and its numbered radiculospinal artery actually arises between vertebrae. The adult pattern of seven cervical vertebral bodies arising from eight cervical somites creates the need for the proatlantal artery nomenclature, as was explained in Tutorial 1.

The intracranial VA gives the C1 spinal artery. The continuation of the cranial vertebral artery represents an ascending branch of its ventral radiculomedullary branch. This takes a medial course to the ventral surface of the future medulla oblongata. A descending ventral radiculomedullary branch runs to the midline surface of the spinal cord to form the cranial origin of the ASA. The other branches of intracranial vertebral artery are derived from the posterior radiculomedullary branch of the first cervical spinal artery, i.e. posterior inferior cerebellar artery (PICA) and the posterolateral spinal artery [2].

A standard adult pattern of spinal arteries is based on this embryonic development with variations at both cranial (cervical) and caudal (lumbosacral) ends. Regional variations will be described later, but first it is worth considering a standard vertebra of the thoracic and lumbar spine with its arterial blood supply arising from the descending aorta. Figure 4.4 shows posterior intercostal arteries running posteriorly on either side of a vertebral body. They give short osseous branches to the vertebral body before dividing into ventral and dorsal branches. An anterior longitudinal anastomosis between segments is shown before the division. The ventral branch becomes an intercostal or lumbar artery. These are numbered by the rib or transverse process under which they run. The dorsal branch connects to a pre-transverse longitudinal anastomosis before giving a radiculospinal branch to the intervertebral foramen. It then passes under the transverse process of it numbered vertebra to supply the posterior muscles and bone of the lamina and spinous process and connect to a posterior-transverse longitudinal anastomosis.

The radiculospinal artery enters the spinal foramen and gives an anterior epidural branch to take part in the retrocorporeal anastomosis. This supplies the bone and dura and anastomoses with its counterpart at the midline. These midline anastomoses occur at disc level and are easily recognised by a hexagon pattern (created by longitudinal and horizontal arteries) on angiography. It reflects the developmental origin of the vertebral body and its blood supply from two pairs of intersegmental arteries. The spinal artery also gives a posterior epidural branch to the prelaminar anastomosis and supplies the dura and bone of the lamina. The prelaminar anastomosis is usually smaller than the retrocorporeal anastomoses with fewer longitudinal anastomoses, but any longitudinal component lies close to the midline and should not be mistaken for the anterior spinal artery. The radiculospinal artery usually then terminates in radicular spinal branches.