Pediatr Cardiol 25:191–200, 2004
DOI: 10.1007/s00246-003-0585-1
Cardiovascular Embryology
R. Abdulla,1 G. A. Blew,2 M.J. Holterman3
1
2
3
Pediatric Cardiology, The University of Chicago. MC4051, 5841 S. Maryland Ave., Chicago, IL 60637-1470, USA
School of Biomedical Visualization, University of Illinois at Chicago, 840 S. Wood Street, Chicago, IL 60637, USA
Department of Surgery, University of Illinois at Chicago, 840 S. Wood Street, Chicago, IL 60637, USA
Abstract. During the first 20 days of development,
the human embryo has no cardiovascular structure.
Over the next month, the heart and great vessels
complete their development and look very much like
they will at full gestation. This amazing process
transforms isolated angiogenic cell islets into a complex, four-chambered structure. During this transformation, the single heart tube begins to beat at 23
days of development and by 30 days blood circulates
through the embryo.
Keywords: Heart — Cardiovascular — Embryology
— Primitive heart — Heart looping — Outflow tract
septation
This review of human embryology attempts to document the many different, and sometimes disputing,
theories of the development of the heart and its great
vessels. The goal is to provide a broad spectrum and
detailed information for those interested in the field
of pediatric cardiology. Many details were intentionally left out, such as molecular biology issues,
because it is impossible to include this ever-expanding
topic together with morphogenesis in one article.
Many publications are available for understanding
molecular biology and neural crest involvement in the
development of the cardiovascular system [11, 12, 14,
15, 17, 23, 24, 32–35, 38].
It is difficult to describe or use two-dimensional
(2-D) imagery when describing a three-dimensional
(3-D) object. Despite this fact, we continue to describe in our literature, lectures, and conferences the
heart using 2-D terminology and illustrations, expecting the audience to recreate a mental 3-D figure.
Unfortunately, the inability to conceive what is being
described is frequent, leading to confusion, the need
Correspondence to: R. Abdulla, email: rabdulla@peds.bsd.
uchicago.edu
for repetition and elaboration, or, worse, misunderstanding and error.
Pediatric cardiologists, particularly those in
training, frequently realize when examining a heart
from an autopsy that their understanding of spatial
relationship of cardiac structures of that particular
lesion was wrong. This difficulty becomes even more
immense when dealing with a 3-D object in a state of
continual and complex change, such as that of the
cardiovascular system during its embryological development. Therefore, it becomes increasingly useful
to depict these changes with four-dimensional imagery (i.e., computer animations depicting 3-D
structures changing over time). The task of preparing
these animations is enormous, requiring expertise in
computer medical illustration and mastery over userhostile software. This is possible for only a few of us,
and even then it is time-consuming and costly.
The use of computer-generated 3-D images and
animations in the field of cardiac embryology is becoming more frequent. This technique is implemented
in research as well as to create educational images [1,
13, 19–21, 45].
In the Internet version of this article, movie animations demonstrating cardiovascular development
are presented. Embryonic folding, heart tube looping,
and development of systemic venous drainage are
demonstrated in different movie animations. These
images were created using current information about
the development of these structures. On the other
hand, a different animation shows a process that can
be used to create 3-D objects using histological slices
from human embryos. Stage 14 sliced embryos from
the Carnegie collection of human embryos from the
National Library of Medicine in Washington, DC,
were digitized, the cardiovascular structures were
traced, and the various slices were then stacked up
using special computer software. This animation
demonstrates how actual 3-D structures can be scientifically reassembled for better understanding
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Pediatric Cardiology Vol. 25, No. 3, 2004
ential growth causing the embryo to fold in two different dimensions:
1. Craniocaudal axis due to the more rapid growth of
the neural tube forming the brain at its cephalic
end. Growth in this direction will cause the embryo to become convex shaped.
2. Lateral folding, causing the two lateral edges of
the germ disk to fold forming a tube-like structure.
Fig. 1. The Carnegie collection of embryos includes various stages
of whole and sliced embryos. Digital images of slides of sliced
embryos are made, with various structures traced using specialized
software. Subsequently, 3-D images are electronically reconstructed. This image depicts a slice from a stage 14 embryo with 3-D
reconstruction, demonstrating the dorsal half of the embryo (white)
as well as a 3-D reconstruction of the heart. See animation of this
process in the Web version of this issue.
(Fig. 1). After 3-D cardiac structures from sequentially staged embryos are created, the images can
serve as templates for the animation process. These
can then be studied from various vantage points and
provide embryologically correct teaching tools to
facilitate the comprehension of cardiac development
(Fig. 2).
The first indication of any cardiovascular development occurs on approximately day 18 or 19. Prior to
embryonic folding, angiogenic cell clusters on either
side of the neural crest coalesce to form capillaries in
the mesoderm of the germ disk. These capillaries then
join to form a pair of blood vessels on each side of the
neural crest (total of four blood vessels). These blood
vessels run along the long axis of the germ disk, with
one pair of blood vessels at the lateral edge of the
embryo (one on each edge) and the other pair more
medially on either side of the neural tube. The blood
vessels on either side of the neural tube join at their
cranial end.
As the embryo folds in its lateral dimension, it
causes the lateral edges of the germ disk to approach
each other until they meet, causing the embryo to
acquire a tubular form [16, 25]. The two outer
endocardial tubes will come close to each other in the
median of the embryo, ventral to the primitive gut,
and start fusing cranially to caudally, thus forming a
single median tube—the primitive heart tube [16, 41].
The Primitive Heart
Embryonic Folding
Early in the third week of development, the germ disk
has the appearance of a flat oval disk and is composed of two layers: the epiblast and the hypoplast.
The first faces the amniotic cavity and the latter faces
the yolk sac. A primitive groove, ending caudally
with the primitive pit surrounded by a node, first
appears at approximately 16 days of development
and extends half the length of the embryo. The
primitive groove serves as a conduit for epiblast cells
that detach from the edge of the groove and migrate
inwards toward the hypoblast and replace it to form
the endoderm. After the endoderm is formed, cells
from the epiblast continue to migrate inwards to infiltrate the space between the epiblast and the endoderm to form the intraembryonic mesoderm. After
this process is complete, the epiblast is termed the
ectoderm [16, 25, 37] (Fig. 3).
The flat germ disk transforms into a tubular
structure during the fourth week of development [16,
25, 35]. This is achieved through a process of differ-
The first intraembryonic blood vessels are noted on
day 20, and 1–3 days later the formation of the single
median heart tube is complete. The heart starts to
beat on day 22, but the circulation does not start until
days 27–29 [35].
The single tubular heart develops many constrictions outlining future structures. The cranialmost area is the bulbus cordis, which extends cranially into the truncus arteriosus. This, in turn, is
connected to the aortic sac and through the aortic
arches to the dorsal aorta [35]. The primitive ventricle
is caudal to the bulbus cordis and the primitive atrium is the caudal-most structure of the tubular heart.
The atrium connects to the sinus venosus, which receives the vitelline veins (from the yolk sac) and
common cardinal (from the embryo) and umbilical
(from primitive placenta) veins. The primitive atrium
and sinus venosus lay outside the caudal end of the
pericardial sac, and the truncus arteriosus is outside
the cranial end of the pericardial sac. Some publications have introduced new terminology describing the
segments of the primitive heart. Wenink and Gitten-
R. Abdulla et al.: Cardiovascular Embryology
193
Fig. 2. The sequence of events resulting in the union of
the two lateral endocardial tubes to form the single
endocardial tube. The rest of the embryo is not shown.
The embryo starts as a flat disk(A). The lateral endocardial vessels located on either side of a flat embryo
disk come closer together as the embryo folds along its
long axis to transform a flat structure into a tubular
shape (B). As the edges of the flat embryo meet to form
this tubular structure, the two lateral endocardial vessels unite (C), forming a single heart tube at the ventral
aspect of the embryo (D). This process occurs on approximately day 20 or 21 of development. See animation of this process in the Web version of this issue.
Fig. 4. The single heart tube shows constrictions outlining future
structures.
Fig. 3. Cells from the epiblast detach and migrate through the
primitive groove to form the endoderm and mesoderm layers.
berger-deGroot [44] support the use of inlet, outlet,
and arterial segments as proposed by Anderson and
Becker [3, 4, 10] (Fig. 4).
Looping of the primitive heart occur on approximately day 23 of development [22]. It was initially suggested that this is due to faster growth of the
bulboventricular portion of the heart compared to
the pericardial sac and the rest of the embryo [35].
However, it has been shown that the heart will loop
even when the pericardial sac is removed, as seen
when the heart is cultured in vitro [24, 41]. It seems
that the process of looping is a genetic property of the
myocardium and not related to differential growth
[41].
As the heart tube loops, the cephalic end of the
heart tube bends ventrally, caudally, and slightly to
the right. The bulboventricular sulcus becomes visible
from the outside, and from the inside a primitive
interventricular foramen forms. The internal fold
formed by the bulboventricular sulcus is known as
the bulboventricular fold. The bulboventricular seg-
ment of the heart is now U shaped; the bulbus cordis
forms the right arm of the U-shaped heart tube and
the primitive ventricle forms the left arm. The looping
of the bulboventricular segment of the heart will
cause the atrium and sinus venosus to become dorsal
to the heart loop [41]. At this stage, the paired sinus
venosus extends laterally and gives rise to the sinus
horns.
As the cardiac looping progresses, the paired
atria form a common chamber and move into the
pericardial sac. The atrium now occupies a more
dorsal and cranial position and the common atrioventricular junction becomes the atrioventricular canal, connecting the left side of the common atrium to
the primitive ventricle [35]. At this stage, the heart has
a smooth lining except for the area just proximal and
just distal to the bulboventricular foramen, where
trabeculations form. The primitive ventricle will
eventually develop into the left ventricle and the
proximal portion of the bulbus cordis will form the
right ventricle. The distal part of the bulbus cordis, an
elongated structure, will form the outflow tract of
both ventricles, and the truncus arteriosus will form
the roots of both great vessels. The bulbus cordis
gradually acquires a more medial position due to the
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Pediatric Cardiology Vol. 25, No. 3, 2004
Fig. 5. Looping of the single endocardial
heart tube transforms it into a complex fourchamber structure. Looping starts on day 23
of development, and the four-chambered
heart is evident on day 27.
growth of the right atrium, forcing the bulbus to be in
the sulcus in between the two atria [42] (Fig. 5).
and the right umbilical vein connects to the vitelline
system through the ductus venosus (which is derived
from the vitelline veins) [26] (Fig. 6).
Systemic Venous System
Pulmonary Circulation
On day 21, there is a common atrium as a result of
fusion of the two endocardial tubes. The common
atrium communicates with two sinus horns, a left and
a right horn, representing the unfused ends of the
endocardial tubes [16]. These two horns will form the
sinus venosus.
The sinus venosus is located dorsal to the atria.
The following veins drain into the sinus venosus on
each side: the common cardinal vein, which drains
from the anterior cardinal vein (draining the cranial
part of the embryo); the posterior cardinal vein
(draining the caudal part of the embryo); the umbilical vein (connecting the heart to the primitive placenta); and the vitelline vein (draining the yolk sac,
gastrointestinal system, and the portal circulation).
On week 4, the sinus venosus communicates with
the common atrium. During week 7, the sinoatrial
communication becomes more right sided, connecting
it to the right atrium. At 8 weeks, the distal end of the
left cardinal vein degenerates, and the more proximal
portion of it now connects through the anastomosing
vein (left brachiocephalic vein) to the right anterior
cardinal vein (right brachiocephalic vein), thus forming the superior vena cava. The left posterior cardinal
vein also degenerates, and the left sinus horn receiving
venous blood from the heart becomes the coronary
sinus. The right vitelline vein becomes the inferior
vena cava, and the right posterior cardinal vein becomes the azygos vein. All this is completed in week 8
of development. The left umbilical vein degenerates
Airways, Lung Parenchyma, and Distal Pulmonary
Arteries
On day 21 of development, a groove forms in the
floor of the foregut just dorsal to the heart. This is
termed the pharyngeal groove, which develops to
form the pharynx. On day 23, the laryngotracheal
groove, a median structure in the pharyngeal region,
develops. The edges of the laryngotracheal tube fuse
to form the larynx and trachea cranially and the right
and left main bronchi and right and left lung buds
distally. The growth and branching of the lung buds,
together with the surrounding mesoderm, form the
distal airways, lung parenchyma, and pulmonary
blood vessels. By week 16 of gestation, a full complement of preacinar airways and blood vessels have
formed. The pulmonary arteries in utero are muscular, similar to that of the aorta. The thick, muscular
walls of pulmonary arteries extend much further into
distal arteries than what is seen in adults. Thinning of
distal pulmonary arteries occurs postnatally as the
pulmonary vascular resistance decreases after the
onset of breathing and improved oxygenation [29].
Proximal Pulmonary Arteries
The proximal main pulmonary artery develops from
the truncus arteriosus, whereas the distal main pul-
R. Abdulla et al.: Cardiovascular Embryology
195
Fig. 6. Development of the systemic venous
drainage. These schematics represent dorsal
views of the heart. (a) At week 4 of development,
there is symmetrical systemic venous drainage
into the two sinus venosus horns. (b) At week 7
of development, there is degeneration of some of
the systemic veins. (c) At week 8 of development,
the central systemic venous anatomy as seen in a
term infant. Normal and abnormal development
of systemic venous drainage are shown in movie
clips in the Web version of this issue. IVC, inferior vena cava; SVC, superior vena cava.
monary artery and the proximal right pulmonary
artery develop from the ventral sixth aortic arch artery. The distal right pulmonary artery and the left
pulmonary arteries form from the post branchial arteries, which develop from the lung buds and surrounding mesoderm. The ductus arteriosus develops
from the distal left sixth aortic arch artery.
Pulmonary Venous System
A primitive vein sprouts out of the left atrium, which
bifurcates twice to give four pulmonary veins that
grow toward the developing lungs. The lung buds
develop from the foregut. A plexus of veins is formed
in the mesoderm enveloping the bronchial buds; these
veins will meet with the developing pulmonary veins
out of the left atrium to establish a connection during
week 5 of gestation. As the left atrium develops, it
progressively incorporates the common pulmonary
vein into the left atrial wall until all four pulmonary
veins enter the posterior wall of the left atrium separately. The incorporated pulmonary veins form the
smooth posterior wall of the left atrium, whereas the
trabeculated portion of the left atrium comes to occupy a more ventral aspect [16, 35].
Atrioventricular Canal
The atrioventricular valves form during the fifth to
eighth week of development [26]. Initially, endocardial cushion tissue forms bulges at the atrioventricular junction. These bulges have the appearance
of valves, and although such tissue may play an im-
portant role in the eventual formation of the atrioventricular valves, endocardial cushion tissues are not
the precursors of the mitral and tricuspid valves [16,
43].
The atrioventricular junction is guarded by two
masses of endocardial cushions—a superior and inferior cushion. These two masses will meet in the
middle, thus dividing the common atrioventricular
canal into right and left atrioventricular orifices. The
process through which these two cushions fuse is not
clear [18], and the role of apoptosis in this process is
debatable. The fusion of the two endocardial cushions results in the formation of two atrioventricular
orifices. In addition, the atrioventricular cushion
appears to play a role in the closure of the interatrial
communication at the edge of the primum atrial
septum. This septum grows toward the atrioventricular endocardial cushion and fuses with it [41].
The formation of the atrioventricular valve starts
when the atria and inlet portion of the ventricle enlarge; the atrioventricular junction (or canal) lags
behind. Such a process causes the sulcus tissue to
invaginate into the ventricular cavity, forming a
hanging flap. The endocardial cushion tissue is located at the tip of this flap, which is formed from
three layers—the outer layer from atrial tissue, the
inner layer from ventricular tissue, and the middle
layer from invaginated sulcus tissue. The inlet portion
of the ventricles then becomes undermined, forming
the tethering cords holding the newly formed valve
leaflets. The inner sulcus tissue will eventually come
in contact with the cushion tissue at the tip of valve
leaflets, thus interrupting the muscular continuity
between the atria and ventricles [16] (Fig. 7).
196
Fig. 7. Formation of atrioventricular valves.
The Atria and Atrial Septum
The atria of the mature heart have more than one
origin. The trabeculated portions (appendages) of the
right and left atria are from the primitive atria,
whereas the smooth-walled posterior portions of the
left and right atria originate from the incorporation
of venous blood vessels. The posterior aspect of the
left atrium is formed by the incorporation of the
pulmonary veins, whereas the posterior smooth portion of the right atrium is derived from the sinus
venosus.
The two sinus horns are initially paired structures; later, they fuse to give a transverse sinus
venosus. The entrance of the sinus venosus shifts
rightward to eventually enter into the right atrium
exclusively. The veins draining into the left sinus
venosus (left common cardinal, umbilical, and vitelline veins) eventually degenerate. The left sinus
venosus will become smaller because it will drain only
the venous circulation of the heart, becoming the
coronary sinus.
The sinus venosus orifice of the right atrium is
slit-like and to the right of the undeveloped septum
primum [16]. The sinus venosus now connecting to
the right atrium will assume a more vertical position.
The sinoatrial junction will become guarded by two
valve-like structures, resulting from the invagination
of the atrial wall at the right and left sinoatrial
junction. This orifice enlarges, with the superior and
inferior vena cavae and the coronary sinus opening
separately and directly into the right atrium. The
right and left sinoatrial valves join at the top, forming
the septum spurium. This septum and the two sinoatrial valve-like structures obliterate and are not appreciated in the mature heart [41].
Atrial septation starts when the common atrium
becomes indented externally by the bulbus cordis and
truncus arteriosus. This indentation will correspond
internally with a thin sickle-shaped membrane developing in the common atrium on day 35 [39]. This
membrane divides the atrium into right and left
chambers. It grows from the posterosuperior wall and
extends toward the endocardial cushion of the atrio-
Pediatric Cardiology Vol. 25, No. 3, 2004
ventricular canal. This is the septum primum. The
septum primum initially has a concave-shaped edge
growing toward the atrioventricular canal. This orifice
connecting the two atria is called the ostium primum.
As the superior and inferior endocardial cushions
fuse, thus dividing the atrioventricular canal into a
right and left orifice, the concave lower edge of the
septum primum fuses with it, obliterating the ostium
primum. However, just before this happens fenestrations appear in the posterosuperior part of the septum
forming the ostium secundum, thus maintaining a
communication between the two atria [41]. The ostium
secundum and superior vena cava later acquire a more
anterosuperior position, although they maintain their
relationship with each other; this is achieved through
the growth of the atria [41].
These fenestrations then coalesce and form a
larger fenestration. Meanwhile, another sickle-shaped
membrane develops on the anterosuperior wall of the
right atrium, just right of the septum primum and left
of the sinus venosus valve. It grows and covers the
ostium secundum, which continues to allow blood
passage since the two membranes do not fuse. The
septum secundum grows toward the endocardial
cushion, leaving only an area at the posterosuperior
part of the interatrial septum where the septum primum continues to exist as the foramen ovale membrane. The septum primum disappears from the
posterosuperior portion of interatrial septation and
the edge of the septum secundum forms the rim of the
fossa ovalis [44] on approximately day 42 of development (Figs. 8 and 9).
Ventricular Septation
Ventricular septation is a complex process involving
different septal structures from various origins and
positioned at various planes [2, 27, 28, 31]. These
structures eventually meet to complete the separation
of the right and left ventricles.
Muscular Interventricular Septum
During the fifth week, on approximately day 30, a
muscular fold extending from the anterior wall of the
ventricles to the floor appears at the middle of the
ventricle near the apex and grows toward the atrioventricular valves with a concave ridge. Most of the
initial growth is achieved by growth of the two ventricles on either side of the ventricular septum. In
addition, trabeculations from the inlet region coalesce
to form a septum, which grows into the ventricular
cavity at a slightly different plane than that of the
primary septum; this is the inlet interventricular
septum, which is in the same plane of that of the
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197
Fig. 8. The atrial septum is formed by the septum
primum and septum secundum. A movie clip depicting this process can be viewed in the Web version of this issue. AV, atrioventricular; IVC, inferior
vena cava; SVC, superior vena cava.
Outflow Tract Septum
Fig. 9. 3-D depiction of atrial septum formation. See animation in
Web version of this issue.
atrial septum. The point of contact between these two
septa will cause the edge of the primary septum to
protrude slightly into the right ventricular cavity,
forming the trabecular septomarginalis. The fusion of
these two septa forms the bulk of the muscular
interventricular septum. This septum will then come
into contact with the outflow septum (Fig. 10).
The interventricular foramen, which is bordered
by the concave upper ridge of the muscular interventricular septum, the fused atrioventricular canal
endocardial tissue, and the outflow tract septation
ridges, never actually closes. Instead, communication
between the left ventricle and the right ventricle is
closed at the end of week 7 by growth of three
structures—the right and left bulbar ridges and the
posterior endocardial cushion tissue—that baffle the
left ventricular output through a newly formed left
ventricular outflow tract (LVOT). The LVOT is
posterior to a right ventricular outflow tract, connecting the right ventricle to the pulmonary trunk.
The cardiac outflow tract includes the ventricular
outflow tract and the aortopulmonary septum. There
has been much debate regarding this process. This
section provides a summary of various theories [9, 36,
40].
In 1942, Kramer suggested that there are three
embryological areas: the conus, the truncus, and the
pulmonary arterial segments. Each segment develops
two opposing ridges of endocardial tissue; the opposing pairs of ridges and those from various segments meet to form a septum separating two outflow
tracts and aortopulmonary trunks. The aortopulmonary septum is formed by ridges separating the fourth
(future aortic arch) and the sixth (future pulmonary
arteries) aortic arches. The truncus ridges are formed
in the area where the semilunar valves are destined to
be formed, thus forming the septum between the ascending aorta and the main pulmonary artery. The
conus ridges form just below the semilunar valves and
from the septation between the right and left ventricular outflow tracts.
Van Mierop [41] agreed that there are three pairs
of ridges forming in the aortopulmonary, truncus, and
conus regions. However, he stated that the pairs of
ridges fuse independently and later on fuse with each
other to complete the septation. His theory indicates
that the truncus ridges form first, and as they fuse they
form a truncal septum. This septum then fuses with
the aortopulmonary septum, which is formed by
invagination of the dorsal wall of the aortic sac between the fourth and the sixth aortic arch arteries
(Fig. 11). Asami [7], Pexieder [36, 37], and Orts Llorca
et al. [7], concur with Van Mierop’s theory; however,
Asami believes that these ridges fuse in the opposite
direction of that indicated by Van Mierop (i.e., from
the outflow tract to the aortopulmonary region). On
the other hand, Pexieder and Orts Llorca believe that
198
Fig. 10. Formation of ventricular septum.
there are only two septa—a conotruncal (or bulbar)
and an aortopulmonary septum.
In 1989, Bartlings et al. introduced a new theory.
They stated that the septation process of the ventricular outflow tracts, pulmonary and aortic valves,
and the great vessels is mostly caused by a single
septation complex, which they termed aortopulmonary septum. This septation complex develops at the
junction of the muscular ventricular outflow tract
with the aortopulmonary vessel. This junction has a
saddle shape, allowing the right ventricular outflow
tract to be long with a short main pulmonary artery,
whereas the left ventricular outflow tract becomes
short with a long ascending aorta (Fig. 12). The
ventricular outflow septation is formed by condensed
mesenchyme, embedded in the endocardial cushion
tissue just proximal to the level of the aortopulmonary valves. The condensed mesenchyme will come in
close contact with the outflow tract myocardium,
from the area just above the bulboventricular fold,
and participate in the septation of the outflow tract
by providing an analogue to muscle tissue [6–9].
Myocardium in contact with the mesenchymal arch
grows rapidly and forms the bulk of the outflow
septum, continuous with the primary fold on the
parietal wall of the right ventricle and the myocardium on the right side of the primary septum.
Conduction System
Primary myocardium, found in the early heart tube,
gives rise to the contracting myocardium (of the atria
and ventricles) and the conducting myocardium
(nodal and ventricular conducting tissue). Conducting myocardial tissue is frequently referred to as being highly specialized tissue, implying that it has a
homogenous function. In reality, some portions, such
Pediatric Cardiology Vol. 25, No. 3, 2004
Fig. 11. One theory of formation of the outflow tract and vascular
septation. LV, left ventricular; LVOT, left ventricular outflow
tract; RV, right ventricle; RVOT, right ventricular outflow tract.
Fig. 12. Diagram depicting the theory of ventricular outflow and
great vessels’ septation by Bartlings et al. [9]. Numbers indicate
specific aortic arch arteries.
as nodal tissue, are slow conducting and resemble less
developed primary myocardium, whereas other portions, such as ventricular conduction tissue, are fast
conducting [30].
The embryological origin and formation of the
sinus and atrioventricular nodal tissue is not clear.
The ventricular conduction system formation is better known. The latter starts with the formation of an
encircling ring of conducting myocardial tissue
around the bulboventricular foramen. The dorsal
portion of the ring will become the bundle of His. The
portion of the ring covering the septum will become
the left and right bundle branches. The anterior
portion of the ring is called the septal branch and it
disappears during normal embryological development. Other portions of this specialized tissue that
form and later disappear are the right atrioventricular
ring bundle and the retroartic branch. The right
atrioventricular ring forms due to the rightward shift
of the common atrioventricular valve, which originally connects the common atrium to the primitive
R. Abdulla et al.: Cardiovascular Embryology
Fig. 13. Degenerated aortic arch arteries (AAA) and the final great
vessels anatomy.
(left) ventricle. This results in a shift of the specialized
myocardium rightward in a ring shape around the
right atrioventricular orifice, only later to disappear.
The retroarotic branch is formed as a result of the
leftward shift of the outflow tract, causing some of
the specialized conducting tissue to move and to be
situated behind the aorta.
Development of Pericardial Sac
The right and left intracelomic cavities approach the
midline as the two heart tubes are fusing into a medial tube (day 21). The two cavities approach each
other and surround the heart tube. The ventral mesoderm is immediately absorbed and the two cavities
communicate. The dorsal mesoderm persists until day
25. After the mesoderm is absorbed, the heart becomes suspended from the cranial and caudal ends.
A band of connective tissue grows from the epicardium into the atrioventricular junction when the
heart is four chambered, resulting in separation of
atrial and ventricular myocardium. The bundle of His
remains the only means of electrical conduction from
atria to ventricles. The sinoatrial node, atrioventricular node, and the bundle of His receive sympathetic and parasympathetic nervous supply
throughout the rest of gestation and even after birth
to complete the development of the cardiac conduction system.
Aortic Arches
The first pair of aortic arches is formed by the curving
of the ventral aorta to meet the dorsal aorta; these
199
will eventually contribute to the external carotid arteries (Fig. 13). The second pair of aortic arch arteries
appears in week 4. These regress rapidly and only a
portion remains, which forms the stapedial and hyoid
arteries. The third pair of the aortic arch arteries
appears at approximately the end of the fourth week;
these will give rise to the common carotid arteries and
the proximal portion of the internal carotid arteries.
The distal portion of the internal carotid arteries is
formed by the cranial portions of the dorsal aorta.
The fourth aortic arch arteries develop soon after the
third arch arteries. Their development differs on the
left from that on the right. On the left side, they
persist, connecting the ventral aorta to the dorsal
aorta and forming the aortic arch. On the right, they
form the proximal portion of the right subclavian
artery. The fifth pair of aortic arch arteries is rudimentary and does not develop into any known vessels; this pair of aortic arch arteries is not seen in
many embryo specimens. The sixth aortic arch arteries develop in the middle of the fifth week. The
proximal portions develop into the main and right
pulmonary arteries, whereas the distal portion of the
left aortic arch artery develops into the ductus arteriosus (Fig. 13).
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