Development of Major Blood Vessels
The early embryonic vascular system is plexiform (intercalating).
Preferential flow related to the development of organ systems, however, leads
to enlargement of certain channels in the plexus. This expansion is brought
about in part by the fusion and confluence of adjacent smaller vessels and by
the enlargement of individual capillaries. Thus a number of vascular systems develop.
As the embryo grows, new organs appear; others are transient and disappear. The
various vascular systems are also continuously modified to satisfy changing
needs.
Initially, the arteries and veins consist simply of endothelial tubes and cannot be distinguished from each other histologically. In later development, typical vessel walls are differentiated from the surrounding mesenchyme. The final pattern of the vascular system is genetically determined and varies with the animal species. Variations are, however, extremely common in both arterial and venous patterns, and local modifications occur in cases of abnormal development of organs.
The major arteries in an early
embryo are represented by a pair of vessels, the dorsal aortae, which run with
the long axis of the embryo and form the continuation of the endocardial heart
tubes. Because of the changing position of the cardiogenic mesoderm containing
the heart tubes, the cranial portion of each dorsal aorta comes to describe an
arc on both sides of the foregut, thus establishing the first pair of aortic
arch arteries, termed aortic arches (see Plate 4-14).
In primitive vertebrates, six pairs
of aortic arches appear in conjunction with the development of the
corresponding pharyngeal (“branchial”) arches, which are transverse swellings
of mesenchyme flanking the foregut ventrally and laterally. The pharyngeal
arches and their blood supply initially evolved in part to form the gills
(branchiae) of aquatic vertebrates, thus their original designation as
“branchial arches.” In humans and other lung-breathing vertebrates, five of the
six pharyngeal arches are present (the fifth pair of arches and arteries
are rudimentary) only in early embryonic life; they become greatly modified in
development as their mesenchyme differentiates into the facial skeleton and
many other structures and tissues of the head and neck. As part of this
process, certain aortic arches (pharyngeal arch arteries) are retained and
modified to form the large arteries of the neck and thorax.
Near the end of the third week (3-mm
embryo), the first pair of arches is large; the second pair is just forming.
The junction of the truncus arteriosus and the first pair of arches is somewhat
dilated and is called the aortic sac. From this aortic sac, subsequent
aortic arches originate, and new arches are added as the heart and aortic sac
undergo relatively caudal displacement. Distally, the dorsal aortae fuse to
form a single artery; this fusion progresses in a cranial direction, in both an
absolute and a relative sense.
Two days later (4-mm embryo), the
first arch has largely disappeared, but part of it persists as a portion of the
maxillary artery. The second arch also regresses; all that remains of it
is the tiny stapedial artery in the middle ear. The third arch is well
developed and large. The fourth and sixth arches form as ventral
and dorsal sprouts from the aortic sac and dorsal aortae, respectively, fuse
with each other. The ventral portion of the sixth arches already have their
major branches, the proximal portions of the left and right primitive pulmonary arteries, even though the arch itself has not yet been
completed (see Plate 4-14).
Soon after the end of the fifth week
(10-mm embryo), the first two aortic arches have disappeared as such; the
third, fourth, and sixth arches are large. The truncus arteriosus and proximal
aortic sac have divided into the ascending aorta and pulmonary trunk. The trunk
is aligned with the left side of the sixth arch, which becomes the ductus
arteriosus, a shunt from the pulmonary trunk into the aorta. The sixth
aortic arch on the right disappears, and the left and right pulmonary arteries
remain connected to the pulmonary trunk. Intersegmental arteries form
between the somites; the seventh cervical pair will play an important role in
the formation of the subclavian arteries, and are located at about the level
where the dorsal aortae join each other.
By now, the aortic arch system has
largely lost its original symmetric pattern (see Plate 4-15). The dorsal aortae between the third and fourth arches have
disappeared on each side, and the third arches begin to elongate as the heart
descends farther. The aortic sac becomes the arch of the aorta and
brachiocephalic trunk, and the third arches extending from these become the
left and right common carotid arteries, respectively. The fourth arch on the
left becomes the short, descending part of the aortic arch between the left
common carotid artery and the slightly more distal connection with the ductus
arteriosus. On the right, the fourth arch becomes the proximal portion of the
right subclavian artery. The rest of this artery derives from a segment of the
right dorsal aorta and right, seventh intersegmental artery. The right dorsal
aorta disappears between the developing right subclavian artery and the site
where the right dorsal aorta joins the left. If this segment persists as an
abnormal origin of the right subclavian artery, it will form a “vascular sling”
behind the foregut.
In summary, it may be helpful to think
of the aortic arch derivatives from a purely topographic perspective. The
pharyngeal (and aortic) arch territories in the embryo extend from the jaws to
the thorax. Aortic arches I and II largely disappear, but arch I contributes to the maxillary arteries in the head; and arch
VI gives rise to the pulmonary arteries and ductus arteriosus in the
mediastinum. The common carotid arteries span the halfway point of the
territory of the arches, so it makes sense that these arteries come from arch
III in the middle of the sequence. Arch V is not functional, leaving arch IV to
contribute to the arteries between the common
carotid and pulmonary vessels: the arch of the aorta and the proximal part of
the right subclavian artery (see Plates 4-14 and 4-15).
MAJOR SYSTEMIC VEINS
The development of the great
systemic veins is a complex process of clinical importance. Few organ systems
in the body are so subject to variations and anomalies in their final, fully
developed state. Although generally of little functional significance to the individual, the many venous variations and
anomalies can cause confusion in diagnostic angiocardiographic studies and
potentially disastrous accidents when surgical correction of cardiac anomalies
is attempted.
In the early embryo the major veins
develop from an initially plexiform network, and a number of channels run
mainly in a longitudinal direction. Three main pairs of veins connect to the
sinus venosus. The vitelline veins carry the blood from the umbilical
vesicle (yolk sac), the first site of the production of embryonic blood cells.
The umbilical veins, initially paired, bring blood from the chorionic villi (developing placenta) into
the embryo, entering the sinus venosus lateral to the vitelline veins. The cardinal
venous system is entirely intra embryonic.
The paired anterior cardinal veins drain the cranial region of the
embryo. The posterior cardinal veins arise somewhat later; they drain
the body of the embryo, including the large mesonephric kidneys, the
first functioning embryonic/fetal kidneys. The anterior and posterior cardinal
veins join to form the short common cardinal veins that enter the right
and left horns of the sinus venosus just lateral to the umbilical veins.
Soon after the posterior cardinal
veins have been established, a new venous system develops as a pair of veins,
the subcardinal veins, appear medially to the posterior cardinal veins
(see Plate
4-16). Their main function is
to drain the urogenital system of the developing embryo: first the mesonephric
kidneys and gonads, then the metanephric kidneys (future adult kidneys),
gonads, and suprarenal glands. Cranially, the subcardinal veins empty into the
posterior cardinal veins.
In the 5-week embryo (8 to 10 mm),
the cardinal venous system is symmetric and equally developed bilaterally, but
this will rapidly change. The vitelline veins in
the region of the septum transversum, the developing liver, and around
the duodenum have broken up into an anastomosing plexus that
will give rise to hepatic veins and the proximal portion of the hepatic portal
system of veins. The original left and right vitelline veins connecting this
plexus with the sinus venosus are now called hepatocardiac channels. The
left vitelline disappears, but the right vein becomes greatly enlarged and
persists as the terminal, posthepatic part of the inferior vena cava (IVC). The
superior vena cava derives from the right common cardinal vein. The right
umbilical vein disappears, and the left umbilical vein connects with the
vitelline venous plexus, after which its proximal portion connecting to the
sinus venosus also disappears. All the umbilical venous blood now enters the
vitelline venous (liver) plexus. A direct route, the ductus venosus, is
created between the left umbilical vein and the right hepatocardiac channel
(future IVC), allowing most of the umbilical venous blood to enter the right
atrium through the IVC.
The subcardinal veins have gained
importance, and numerous anastomoses with the posterior cardinal veins have
been established. The growing mesonephric kidneys have brought the left and
right subcardinal veins closer together, and an anastomosing plexus of
veins has developed between them, the intersubcardinal anastomosis. The right subcardinal vein connects to the right hepatocardiac
channel to form the hepatic segment of the IVC.
A new venous system appears
bilaterally in the caudal region of the embryo, although some view this system
as originating from the posterior cardinal veins. It consists of sacrocardinal
veins that empty into the posterior cardinal veins. Two smaller caudal
veins, the sacrocardinal vein and caudal vein, form the iliac
system of veins (common, internal, external branches) and veins of the pelvis.
As the subcardinal veins enlarge,
the left posterior cardinal vein decreases in size, and the left horn of
the sinus venosus, the future coronary sinus, becomes attenuated. Venous
return is shifting to the right side of the embryo. Most the left and right
posterior cardinal veins soon disappear as the subcardinal system continues to
develop.
The right subcardinal vein and its
anastomosis with the right hepatocardiac channel (from the right vitelline
vein) rapidly become the principal venous channel to the heart. The anastomosis
becomes the intrahepatic component of the IVC, and the
right subcardinal vein becomes an infrahepatic segment of the IVC. The
subcardinal veins lose their cranial connections with the posterior cardinal
veins; remaining branches become the renal and suprarenal veins as well as the
gonadal veins.
Yet another new venous system
appears in the form of two longitudinal channels, the supracardinal veins (see
Plate 4-16). Cranially, the veins empty into the terminal part of the posterior
cardinal veins, and caudally, anastomose with the subcardinal veins. The
supracardinal veins form the azygos system of veins that drain the thoracic
body wall by way of the intercostal veins, taking over this function
from the posterior cardinal veins. The cranial part of the left supracardinal
vein degenerates; the caudal thoracic portion forms the hemiazygos vein that
connects over the midline with the right supracardinal vein (developing azygos
vein). The azygos vein forms an arch that drains the azygos system into the
superior vena cava (SVC). The azygos develops from the terminal portion of the
right supracardinal vein and right posterior cardinal vein. Recall that the
right common cardinal vein forms the SVC. The right supracardinal vein below
its connection to the subcardinal vein enlarges to become the inferior segment
of the IVC. The left supracardinal disappears. If the terminal part of the IVC
fails to develop from the right vitelline vein, blood from the lower part of
the body will enter the right atrium through a greatly enlarged azygos vein.
The upper limbs and head and neck
are drained by veins that empty into the left and right anterior cardinal
veins (see Plate
4-17). An anastomosis between
them forms the left brachiocephalic vein, which brings superior venous return
to the right side of the embryo, the same shift that occurs in venous return
below the heart. The right anterior cardinal vein becomes the right
brachiocephalic vein that continues into the SVC (from right common cardinal
vein). The left horn of the sinus venosus has attenuated further. The left
anterior and common cardinal veins become the ligament of the left SVC (ligament of Marshall), which is continuous
with the coronary sinus (from left horn of sinus venosus). The most common
anomaly of the anterior cardinal veins is a persistent left SVC. An oblique
vein of the left atrium may also persist. The only functioning remnant of the
left anterior cardinal vein is the small, left
superior intercostal vein.
The inferior vena cava derives from
more primordia than any other vessel and thus merits a summary. The
contributions are as follows.
1.
Terminal
part, right vitelline vein : Posthepatic segment
2.
Subcardinohepatic
anastomosis : Hepatic
segment
3.
Part of
right subcardinal vein : Renal segment
4.
Right
supracardinal vein: Prerenal segment
Of the original six connections to
the sinus venosus paired vitelline, umbilical,
and common cardinal veins only two remain: derivatives
of the right common cardinal vein (superior vena cava) and
right vitelline vein (IVC). Terminal part of the right vitelline vein is also
the hepatocardiac channel.
With three vascular systems in the
early embryo and three sequential systems of cardinal veins, it is not
surprising that variations and anomalies are extremely common.
