Week 7, the branches of the dorsal aorta are evident in the thoracic region. (1) Visceral ventral segmental branch, (2) visceral lateral segmental branch, (3) intersegmental dorso-lateral branch, (4) dorsal branch of the intersegmental artery, (5) ventral branch of the intersegmental artery (intersegmental arteries from the aortic arch in the cervical region and intercostal and lumbar arteries in the thoracolumbar region)
Embryological modifications occur at the different regions of the spine . The blood supply to each vertebra is similar at all spinal levels. It is divided to supply the vertebral body, pedicle, transverse process and spinous process.
In the cervical region , extraspinal anastomoses from between segments form the vertebral artery, the ascending cervical artery and the deep cervical artery. These vessels are located within ventral and dorsal to the transverse processes of the cervical vertebrae. Additional contributions from the ascending pharyngeal artery or occipital artery may be present at the craniocervical junction.
In the thoracolumbar region , the constant origin of radicular arteries from intercostal and lumbar arteries reflects the persistence of the segmental embryological arrangement. The vertebrae receive blood supply from lumbar arteries.
In the sacral region , the regression of the dorsal aorta forms the median sacral artery. This region is also supplied by the internal iliac vessels.
The primitive arterial vascularization of the spinal cord relies on the dorsal aorta which gives off intersegmental arteries supplying the somites. Each intersegmental artery reaches the ventrolateral aspect of the developing spinal cord and establishes longitudinal anastomoses at the capillary level with intersegmental arteries from the levels above and below.
During development, each radicular artery initially contributes to both the ventral and dorsal spinal arteries. They also supply the spinal roots, dura mater, bony structures, and paraspinal musculature at each level. Then most of the segmental branches regress.
Finally, 6–8 ventral branches of the radicular arteries continue to supply the entire axis of the ventral spinal artery; 10–20 dorsal branches of the radicular arteries provide the blood supply to the dorsal spinal arteries. Of the 62 radicular branches, at the most 7 or 8 truly participate on the vascularization of the spinal cord.
At the end of the third week, one can thus distinguish a fine longitudinal capillary network along each ventrolateral aspect of the spinal cord. The ventral aspect of the cord situated between the capillary network and the dorsal part of the spinal cord are still avascular at this stage.
Radicular arteries form by the division of the primitive intersegmental arteries into a ventral and a dorsal branch. The dorsal branches reach the dorsal aspect of the cord and, like their ventral homologues, form longitudinal anastomotic networks along the surface of spinal cord.
The dorsal radicular arteries give rise to two dorsal longitudinal axes on each side. The larger one courses dorsal to the dorsal root. The other one, which is finer, forms laterally, between the dorsal and ventral nerve roots, in proximity to the dentate ligament. With time, the capillary bed over the ventrolateral surface of the spinal cord progressively extends to its ventromedial aspect.
Between the sixth and tenth weeks of gestation, a single ventral spinal artery forms. Whether this occurs by fusion of the two parallel ventral axes or by a process of partial obliteration and remodelling of the primitive plexiform ventral axis is not clear. Incomplete fusion could explain why the ventral spinal axis is often fenestrated and sometimes duplicated, whereas obliteration and remodelling might account for the commonly tortuous course of the ventral spinal artery.
By the tenth week, the vasculature of the spinal cord essentially corresponds to its postnatal configuration. After this period, only a few significant changes occur. The dorsal anastomotic chains are remodelled into two distinct dorsal spinal arteries. There is a significant reduction in the number of radiculomedullary arteries supplying the spinal cord, with a transition from a regular, symmetric and ubiquitous distribution to an irregular and asymmetric distribution, a transformation that is particularly marked at the level of the inferior third of the spinal cord.
The course of radiculomedullary arteries is modified: during embryological development, the unequal growth of the spinal cord and the spinal column results in an apparent ascension of the cord relative to the column, a phenomenon that continues into postnatal life until about the age of 12 months. Consequently, the radiculomedullary arteries assume an ascending course, which becomes more and more marked as one progresses in a caudal direction.
The number of ventral and dorsal spinal contributors detectable by macroscopic anatomy and angiography decreases with age, until the spinal vascularization seems to be derived from a few branches only, including the artery of the lumbar enlargement or artery of Adamkiewicz. It has been clearly shown, however, that at the macroscopic and microscopic levels, the ventral and dorsal spinal contributors are present at every single level throughout life. The apparent vascular distribution observed in the adult human results from the combination of functional adaptation (largest branches at the level of the largest neuronal masses) and senescence. These observations are clinically important, as they imply that each radicular artery has the potential to become a feeding branch for a vascular malformation.
The definitive adult vascular pattern forms in a cranial-to-caudal direction: fusion and medial displacement of the paired ventrolateral channels form the midline ventral spinal artery; dorsally, two channels coalescence from a pial network on the lateral and dorsal surfaces to form the dorsal spinal arteries.
So, from early embryonic stages, these two intrinsic arterial supplies to the spinal cord remain separated anatomically and functionally.
Blood Supply of the Spinal Structures
The branches of the thoracic aorta are divided into visceral (pericardial, bronchial and oesophageal arteries) and parietal vessels (intercostal, superior phrenic and mediastinal arteries) .
The intercostal vessels are located in the centre of the vertebral bodies. Both right and left branches of the aorta supply each thoracic vertebra. There are 11 intercostal arteries and one subcostal artery arising from the aorta. But usually there are 10 pairs of intercostal arteries. The upper two interspaces are supplied by branches of the subclavian artery. The lowest intercostal artery is known as the subcostal artery and accompanies the twelfth rib.
The intercostal arteries differ on the two sides of the aorta . The arteries on the right side, especially the upper vessels, are longer than those on the left because the aorta lies to the left side of the vertebral column.
On the right side, the intercostal arteries course over the right side of the vertebral bodies, crossed by the thoracic duct, esophagus and major vena azygos, and covered by the pleura and lung.
On the left side, the upper two intercostal vessels are crossed by the left superior intercostal vein, and the next two are crossed by the accessory hemiazygos vein. The lower left intercostal arteries are crossed by the hemiazygos vein (minor). The left pleura and lung cover the left-sided arteries.
The ventral branches initially cross the vertebrae somewhat obliquely because of the downward direction of the ribs. They continue towards the angle of the rib. At the costovertebral articulation, they are crossed by the sympathetic chain. The ventral branches are accompanied by a nerve and a vein to their termination between two intercostal muscles.
The dorsal branches arise from the intercostal arteries opposite the space bounded by the transverse process of the vertebra above, the costal process below and the body of the vertebra medially. The dorsal branches travel towards the intervertebral foramen where they divide into a muscular and a spinal branch. The spinal branch enters the intervertebral foramen where it supplies ligaments, laminae, nerve roots, dura and the spinal cord .
Abdominal Aorta (Fig. 4)
The abdominal aortic branches are divided into three types: parietal (right and left phrenic arteries, and the four pairs of right and left lumbar arteries), visceral (celiac, mesenteric, renal …arteries) and the terminal branches: the right and left common iliac arteries and the median sacral artery. However, this vascular bifurcation is variable and can occur anywhere from the upper half of L3 to the lower border of L5.
The eight lumbar segmental arteries (four on each side) arise from the dorsal aspect of the abdominal aorta. Because the aorta is situated somewhat to the left of the midline, the right lumbar arteries are longer than are those on the left side. These vessels pass through the middle and around the four upper lumbar vertebrae.
As the arteries curve around the bodies of the vertebrae, they pass beneath the sympathetic trunk. The upper two lumbar arteries course under the crura of the diaphragm. The right lumbar arteries pass beneath the vena cava, and the upper two on the right side are situated under the cisterna chyli. Both the right and left lumbar arteries are under the tendinous arch of the psoas muscles situated along the sides of the vertebral bodies.
The arteries continue under the psoas until they arrive at the interval between the transverse process of the vertebrae and the medial edge of the quadratus lumborum muscle . While the lumbar arteries are coursing under the psoas, they are accompanied by rami of the sympathetic chain and the lumbar veins. Ventral to the transverse process, the lumbar arteries are crossed by branches of the lumbar plexus. A fifth pair of lumbar arteries is often given off from the middle sacral artery opposite the fifth lumbar vertebrae.
Like the intercostal arteries, the lumbar arteries course to the foramen and divide into a dorsal vertebral branch and a ventral muscular branch. The ventral branch travels forward between the abdominal muscles and terminates by anastomosing with other abdominal wall arteries.
The origin of the common iliac arteries usually occurs opposite the left side of the middle of the fourth lumbar vertebra. They terminate opposite the lumbosacral articulation by dividing into the external and internal iliac arteries. The external iliac artery continues to the brim of the pelvis to the lower limb; the internal iliac artery descends medially. The right and left arteries differ in their relations to nearby structures.
Because of the bifurcation of the aorta a little to the left of the midline, the right common iliac artery is approximately 5 cm long and the left one is 4 cm. The right common iliac artery is crossed in front by the ureter and the ovarian artery in the female, sympathetic nerve branches descending to the hypogastric plexus, the inferior mesenteric artery termination, the sigmoid colon and the sigmoid mesocolon. From behind, the right iliac artery lies on the right common iliac vein, the end of the left common iliac vein and the beginning of the vena cava. These venous structures separate it from the fourth and the fifth vertebra and the L4L5 interspace. To the right of the right iliac artery are located the inferior vena cava, the termination of the right iliac vein and the psoas. Along the left border of the right common iliac artery are seated the right common iliac vein, the end of the left common iliac vein, and the superior hypogastric plexus .
The shorter left common iliac artery is crossed in front by the ureter, the ovarian artery in the female, branches of the sympathetic nerve and termination of the inferior mesenteric artery, the sigmoid colon and the sigmoid mesocolon. Located behind the left common iliac artery are the lower border of the fifth lumbar vertebra, the L4L5 disc space, the body of L5 and the L5S1 disc interspace. To the left of the left common iliac artery is the psoas muscle. On the right side of the artery are the left common iliac vein, the hypogastric plexus and the middle sacral artery.
At the thoracic and lumbar level, the blood supply is derived from the intercostal and lumbar arteries.
a prevertebral group formed by the artery of the vertebral body and the periosteal branches.
an intracanalicular group formed by the ventral spinal canal branch.
The Ventral Group
It is formed of multiple small periosteal branches coming from the trunk of the intercostal artery which vascularize the peripheral portion of the ventral and lateral sides of the vertebral body.
The aorta lies laterally on the left side and its particular position accounts for the differences observed according to side and level. These are periosteal branches numbering between two and four per intercostal artery on the right side, and often less numerous on the left side where they arise lateral to the insertion of the ventral longitudinal ligament.
ascending vessels arising from the superior surface of the intercostal artery and passing superiorly. They divide into several periosteal branches which supply the superior half of the vertebral body, may give an anastomotic branch to the descending artery of the vertebral body which arises from the suprajacent intercostal artery.
descending vessels which originate from the inferior surface of the intercostal artery pass downwards and give off periosteal branches to supply the inferior half of the vertebral body.
recurrent arteries usually arising from the superior surface of the intercostal artery, and which pass superiorly then transversely and medially, and may anastomose with the corresponding artery on the opposite side.
While all three arrangements may be met at the thoracic and lumbar level, such is not the case in the superior thoracic spine where the branches to the vertebral bodies arise from the ascending segment of the trunk of the intercostal artery, and ascend vertically before penetrating the corresponding vertebrae at the level of the ventral intercostal artery without giving off any collaterals.
The Dorsal Group
The dorsal group provides the majority of the vascularization of the vertebral body and comes from the ventral branch of the retrocorporeal artery via two perforating branches which penetrate the vertebral body through its vascular hilum whose branches vascularize the dorsal side of the vertebral body and most of the central corporeal region.
This group is formed by the anastomotic network behind the vertebral body. The ventral spinal canal branch arises from the dorsospinal artery close to its origin, either in isolation or more frequently via a common trunk with the radicular artery. In certain cases, they may originate directly from the trunk of the intercostal artery in front of the origin of the dorsal branch. This anatomical arrangement is a relatively frequent finding at the lumbar and superior dorsal levels.
The ascending branch passes obliquely upwards and medially, on the dorsal surface of the vertebral body, passes beneath the dorsal longitudinal ligament and then divides at the level of the central part of the vertebral column into several branches: an anastomotic branch to the descending branch of the subjacent intercostal artery, another one to the retrovertebral artery on the other side and several intra-osseous branches which penetrate the body of the vertebrae via the basivertebral foramen (of Hann).
Among the several variations which can exist, it is worth pointing out the isolated origin of the descending branch directly from the trunk of the dorsospinal artery or even one of its terminal branches. In this case, this branch passes behind the spinal nerve, crossing its inferior surface to reach the ventral or inferior part of the intervertebral foramen.
The Costovertebral Joint
The costovertebral vascularization is fed by branches coming from the ventral branch of the intercostal artery and the two branches coming from the dorsospinal artery during its passage through the paravertebral space.
a transverse branch from the ventral intercostal artery of very slender calibre, it arises from the superior surface of the ventral intercostal artery near to its origin, sometimes from the first perforating branch in its proximal segment. It ascends vertically and then it is just near the inferior border of the rib as far as the medial extremity of the transverse process.
branches arising from the supra and subjacent dorsal arteries during their course in the paravertebral gutter, providing a blood supply to the posterior surface of the transverse process.
The Dorsal Arch
the intracanalicular portion supplied by an arterial network situated in the epidural space and formed by branches of the dorsal spinal canal branch;
the dorsal portion supplied by branches of the medial muscular branch of the dorsospinal artery.
It is formed by the dorsal spinal canal artery and its branches. It arises from the trunk of the dorsospinal artery after the radiculomedullary artery, or sometimes via a common trunk with it, but rarely from a branch of division of the dorsospinal artery. It penetrates the spinal canal by passing through the intervertebral foramen, behind the spinal nerve and then runs a variable course in the epidural space and terminates on the median line with respect to the origin of the spinal process in anastomosing with its counterpart from the opposite side.
a vertical anastomotic branch with its adjacent neighbour situated above and just medial to the articular process. Its situation is more lateral than that of the retrovertebral anastomoses.
an osseous branch which is constant and which supplies the lamina.
a branch penetrating the root of the spinal process in the median line.