Formation of Cardiac Septa
At the close of the preceding phase of development, the heart completely occupies the pericardial cavity. Blood flows in a single path through the sinus venosus and atrium, through an atrioventricular canal into the left ventricle, though an interventricular canal above the free edge of the primordial interventricular septum into the right ventricle, then out the bulbus cordis and truncus arteriosus. The stage is now set for the septation of the heart, which lasts about 10 days. No major changes occur in the external appearance of the heart. The formation of the various cardiac septa occurs more or less simultaneously; for descriptive purposes, however, it is necessary to consider their development separately.
Blood flow through the heart is
first separated into left and right sides by dorsal and ventral endocardial cushions
that appear about day
24. Endocardial cushions are swellings of mesenchymal tissue that grow toward each
other, then fuse to divide the atrioventricular (A-V) canal into left
and right A-V canals (see Plate 4-7). The primordial atrium
begins to shape into a left atrium and a right atrium, although at this early stage
the atria are in wide communication with each other. With the appearance of the
endocardial cushions and primordial interventricular septum, the four chambers can
be identified, but blood from the sinus venosus still enters the heart in one location, the right atrium, and it exits
the heart in one location, the right ventricle. Half the blood in the right atrium
passes through the right A-V canal into the right ventricle, then out through the
bulbus cordis and truncus arteriosus. The other half of the blood in the right atrium
passes into the left atrium, through the left A-V canal into the left ventricle,
then to the right ventricle and the same exit path through bulbus cordis and truncus
arteriosus.
ATRIA, ATRIAL SEPTUM, AND
PULMONARY VEINS
The atria are divided by two adjacent
septa that function as a valve permitting blood to flow from primitive right
atrium to left atrium, but not the other direction. This plays a crucial role in
having blood bypass the nonfunctioning prenatal lungs, and in converting the
prenatal circulatory pattern to the postnatal configuration soon after the
first breath of the newborn.
The truncus arteriosus forms a depression
on the external surface of the common atrium that corresponds on the inside to a
crescent-shaped ridge, the septum primum (see Plate 4-7). It grows inferiorly
toward the fused endocardial cushions. The transient interatrial passage inferior
to it is the foramen primum, which disappears as the septum primum fuses
with the endocardial cushions. Before this happens, holes appear high up on the
septum primum and coalesce to form a foramen secundum in the septum primum.
On the right atrium side of the septum primum, a thick, muscular, septum secundum
grows inferiorly toward the endocardial cushions. It also has a crescent shape
that, with continued growth, will circumscribe an oval foramen in the septum secundum,
the foramen ovale. The foramen secundum is at a higher level than the foramen
ovale. Oxygen-rich blood in the fetal inferior vena cava is directed at the foramen
ovale. It pushes the septum primum away from the septum secundum to allow blood
to pass through the foramen secundum into the left atrium. With increased blood
pressure in the left atrium at birth from increased pulmonary blood flow, however,
the septum primum is pressed against the relatively stiff septum secundum, effectively
closing the foramen ovale. After fusion of septum primum with septum secundum, the
foramen ovale becomes the fossa ovalis of the right atrium. Growth of the
septum secundum occurs during the fifth and sixth weeks.
A single embryonic pulmonary vein,
present in a 5- to 6-mm embryo, develops as an outgrowth of the posterior left atrial
wall. It connects with the splanchnic plexus of veins in the region of the developing
lung buds. Later in development, the vein itself and parts of its first four branches
(two from the left lung and two from the right) expand tremendously and become incorporated
into the left atrium to form the larger, smooth, posterior part of the adult atrium.
In the fully developed heart, the original embryonic left atrium is represented by little more than the trabeculated
atrial appendage (auricle). The intrapulmonary part of the splanchnic venous plexus
ultimately loses its connections with the systemic veins and drains exclusively
by way of the pulmonary veins.
On the right side, the right sinus horn
is similarly incorporated into the right atrium; it enlarges mainly in its vertical
diameter, and the relative distance between the common cardinal vein (proximal superior
vena cava) and the inferior vena cava (from the right vitelline vein) increases. The original embryonic
right atrium becomes the right atrial appendage (auricle), containing the
earliest-appearing pectinate muscles. A lateral wall with pectinate muscle will
grow to become the largest component of the right atrial wall. Thus the primitive
right atrium becomes the right atrial appendage with its pectinate muscle; the right
horn of the sinus venosus becomes the smooth back wall of the right atrium; and
new pectinate muscle develops into the lateral wall.
The primordial right and left ventricles
are little more than sequential local widening of the original cardiac tube, and
they are connected to each other by a smooth walled,
relatively narrow channel, the primary interventricular foramen. As the heart
tube folds, the interventricular foramen is bounded inferiorly by the developing
interventricular septum (see Plate 4-8). Completion of the ventricular separation
is intimately related to division of the outflow tract of the primitive heart tube:
the bulbus cordis and truncus arteriosus.
In an embryo of about 4 to 5 mm, the
A-V canal still leads into the primitive left ventricle, and blood can reach the
primitive right ventricle only by way of the primary interventricular foramen. After
division of the A-V canal by the endocardial cushions into left and right A-V canals,
blood still must pass from the left ventricle to
the right ventricle before exiting the heart. If the interventricular septum
simply grew to fuse with the endocardial cushions, there would be no exit of blood
from the left ventricle.
Enlargement of the ventricles is accomplished
by centrifugal growth of the myocardium, always closely followed by increasing
diverticulation and formation of trabeculae internally; this prevents the compact
outer layer of the myocardium from becoming too thick and solid. Typically, the ventricles of the embryonic
heart consist of a tremendous mass of trabeculae enclosed by a rather thin outer
layer of compact myocardium. Most of the trabeculae eventually disappear. Of the
remaining trabeculae, some coalesce to form larger structures such as papillary
muscles and the moderator band; others are reduced to thin, fibrous strands
(e.g., chordae tendineae) that connect the papillary muscles to the
atrioventricular valve cusps (see Plate 4-7).
The primary interventricular septum is
thick and gives rise to the inferior, muscular part of the adult septum. Again,
it does not continue to grow to fuse with the endocardial cushions. Instead, it
will fuse with a septum that divides the bulbus cordis and truncus arteriosus. The
primary interventricular foramen never closes, but actually enlarges
and, in the fully developed heart, gives access to the aortic vestibule,
the smooth upper part of the left ventricle that leads to the aortic valve.
TRUNCUS ARTERIOSUS AND BULBUS CORDIS
Distal to the ventricles in the outflow
part of the heart tube is a dilatation, the bulbus cordis, followed by a
tapering truncus arteriosus (see Plate 4-8). Potentially confusing terms
have been used to describe these structures; some consider these two outflow chambers
to be a single structure, sometimes referred to as the bulbus cordis, sometimes
as the truncus arteriosus. Another term used for the interface between the two is
the “conus cordis.” Word combinations are also used, such as “truncoconal” to describe
septal swellings (see Plate 4-9). This section uses more recent and common terms,
the bulbus cordis leading to the truncus arteriosus.
The bulbus cordis and truncus arteriosus
are divided lengthwise by a spiral septum, also called the aorticopulmonary
septum, named after the ascending aorta and pulmonary
trunk that are derived from the truncus arteriosus (“arterial trunk”). Two streams
of blood spiral through this part of the heart tube, and longitudinal septa form
in the path of least resistance between the two streams (see Plate 4-10).
The process begins in the 6-mm embryo
at the end of the fourth week and is completed near the end of the sixth week (14-
to 15-mm embryo). It proceeds in a distal to proximal direction; the truncus arteriosus
is divided first, followed by the bulbus cordis. The
two opposing ridges dividing the bulbus cordis are called left and right bulbar
ridges, which are the proximal parts of the developing spiral septum. The ridges
are continuous with the attached edges of the muscular interventricular septum.
The fusion of these ridges with each other, with the interventricular septum, and
with the endocardial cushions completes division of the ventricles and creates an
outflow path for each chamber. The thin, upper,
membranous interventricular septum derives from an extension of tissue on
the right side of the endocardial cushions. It fuses with the muscular interventricular
septum and bulbar ridges of the spiral septum. Membranous septal defects are the
most common heart defect (25% of all congenital heart defects), partly because three
basic primordia (interventricular septum, spiral septum, endocardial cushions) are
required to fuse in an area of very dynamic blood flow. There is considerable opportunity
for a failure of fusion of these elements at the location of the membranous interventricular
septum.
The spiral septum is a suitable synonym
for the aorticopulmonary septum because of its orientation in the outflow part of
the embryonic heart tube and the resulting relationship of the adult arterial derivatives
of the chambers it divides. The ascending aorta arises posterior to the pulmonary
trunk in the adult heart, spirals up to the right of
the pulmonary trunk, and continues anteriorly to form the aortic arch that passes
over the bifurcation of the trunk into left and right pulmonary arteries.
The bulbus cordis is incorporated into
the ventricles, forming the upper, smooth-walled, outflow part of each ventricle:
the conus arteriosus in the right ventricle, just below the pulmonic
semilunar valve, and the aortic vestibule of the left ventricle, leading
to the aortic semilunar valve (see Plate 4-11). The inferior,
trabeculated part of each ventricle derives from the primitive
ventricle. Because of the oblique orientation of the bulbar (spiral septum) ridges,
part of the primitive right ventricle is captured by the left ventricle when the
ridges fuse to the interventricular septum. As a result, the embryonic interventricular
foramen above the primary interventricular septum is retained as the interface
between aortic vestibule and trabecular part of left ventricle.
 |
| HEART TUBE DERIVATIVES |
SINUS VENOSUS
The cardiovascular system in the early
embryo is paired and symmetric. At about day 23, the four-somite stage, the paired
endothelial heart tubes fuse, beginning in the bulboventricular region and progressing
toward the venous pole of the heart. The sinus venosus maintains its paired
condition. Early in the fourth week, a central unpaired part of the sinus venosus
opens into the primitive atrium and right and left sinus horns.
At this stage the sinus venosus receives
three pairs of veins. Most medially, at the junction of the sinus horns and the
central portion, the vitelline veins enter the floor of the sinus. Lateral
to the vitelline veins, the umbilical veins enter the sinus horns from below,
with the common cardinal veins coming from above. The proximal parts of the
umbilical veins soon disappear (and the distal segment of the left umbilical vein
connects with the developing inferior vena cava). Because anastomotic channels develop
between the right and left systemic veins (e.g., future left brachiocephalic vein),
and blood flow is preferential to the right side of the embryo, the right horn of
the sinus venosus and the right proximal cardinal and vitelline veins become more
important, whereas their left counterparts are greatly reduced in size. Thus the
right sinus horn becomes larger, more vertical, and incorporated into the part of
the primitive atrium that will become the right atrium. The right horn will form
the smooth posterior wall of the right atrium, the sinus venarum, named after
its sinus venosus origin. The communication between the sinus venosus and the developing
right atrium is now limited to the right sinus horn. The left horn of the sinus
venosus becomes the coronary sinus. The left common cardinal vein usually
disappears.
 |
| PARTITIONING OF THE HEART TUBE – ATRIAL SEPTATION |
The sinoatrial orifice is
tall and narrow, and the folds on either side constitute the valve of the sinus
venosus, with the right fold larger than the left fold. The vertical dimension increases until a constriction in the middle
creates separate openings for the developing superior and inferior venae cavae.
The left fold of the valve fuses with the septum secundum to become part of the
interatrial septum. The cranial part of the right valve fold becomes a thick, vertical
ridge of muscle, the crista terminalis, that marks the boundary between the
two primordia (sinus venosus and primitive atrium) that contribute to the right
atrial wall. Posterior to the crista terminalis is
the smooth-walled sinus venarum; anterior to the crista terminalis is the wall of
the right atrium lined with pectinate muscle, including the right atrial
appendage (auricle). The inferior part of the right fold of the valve of the sinus
venosus becomes the valve of the inferior vena cava and the smaller valve of the
coronary sinus.
Plate 4-11 summarizes the primitive heart
tube chambers and their adult derivatives.
ATRIOVENTRICULAR AND SEMILUNAR VALVES
The atrioventricular valve cusps
form from extensions of mesenchyme and ventricular muscle surrounding the A-V canals
(see Plate 4-7). The cusps are initially thick and fleshly, becoming thin and fibrous
later. The left, bicuspid mitral valve initially has four cusps of equal
size. The left and right cusps diminish in size and are usually identifiable in
the adult valve as very small left and right commissural cusps. The remaining
two cusps are the anterior (aortic) cusp and posterior cusp. The right
tricuspid valve develops similarly, except three cusps develop instead of
the original four (becoming two) in the mitral valve. The lateral cusp and part
of the anterior cusp develop first. The medial cusp overlying the membranous interventricular
septum develops later. The papillary muscles and their chordae tendineae develop
from trabecular muscle. Initially thick and fleshy, the chordae tendineae become
thin and fibrous as their muscular component disappears. Development of the basic
structure of the mitral valve is completed by the end of the sixth week; the tricuspid
valve is completed soon after (see Plates 4-12 and 4-13).
The primordia of the aortic and pulmonary
semilunar valves appear near the end of partitioning of the truncus arteriosus by
the truncal component of the spiral
septum. Four swellings of mesenchyme surround the lumen of the truncus arteriosus
(see Plate 4-9). Left and right swellings are divided by the aorticopulmonary
septum to form left and right valve cusps in both the ascending aorta and pulmonary
trunk. The anterior swelling forms the anterior cusp in the pulmonary valve,
and the posterior swelling in the truncus arteriosus forms the posterior cusp of
the aortic valve.
Excavation of the superior surfaces of the swellings and later thinning result in the semilunar shape of each cusp. Dilatation of the proximal origins of the ascending aorta and pulmonary trunk gives rise to the pulmonary and aortic sinuses, the expanded space between each cusp and the walls of the arteries. The left and right coronary arteries arise from the left and right aortic sinuses (of Valsalva), respectively.
