Download The heart develops from mesoderm

Dr. Frank CT Voon
The Development of the Heart
12th October 2010
The Heart develops from Mesoderm
The entire cardiovascular system develops from mesoderm.
It begins development in the middle of the third week.
At this time, diffusion of nutrients into the cells of the early embryo is inadequate.
Mesoderm gives rise to hemangioblasts, cardiac myoblasts and mesenchyme.
Hemangioblasts give rise to hemopoietic stem cells and angioblasts.
Angioblasts develop into the endothelial tubes which become the endothelium of the
heart or endocardium.
The cardiac myoblasts which surround the endocardium will become the myocardium.
The rest of the mesoderm or mesenchyme will form the epicardium.
The Endothelial Tube
Mesenchymal cells located anterior and lateral to the neural plate, cluster together as
groups of hemangioblasts.
The cells on the periphery form the endothelium (the inner lining of the walls of blood
vessels), while those in the center become blood cells.
These vessels join up to form a U-shaped endothelial tube.
With cephalocaudal folding of the embryo, the cranial end of the endocardial tube is
brought forward and downwards to the front of the thorax.
The Endocardial Tube
Together with cephalocaudal folding, the embryo also undergoes lateral folding which
brings the two lateral ends of the endothelial tube to meet and fuse to from an endocardial
tube.
The endothelium of this endocardial tube is called the endocardium.
The endocardium is surrounded by mesenchyme.
Mesenchyme is the general name for mesodermal cells.
These mesodermal cells surrounding the endocardium become cardiac myoblasts.
This occurs as the result of a process known as induction.
Pharyngeal endoderm induces the adjacent splanchnic layer of the lateral plate mesoderm
to form myoblasts.
The cardiac myoblasts surrounding the endocardium thus forms the myocardium.
Other mesenchyme surrounding the myoblasts will form the epicardium and pericardium
lining a pericardial cavity.
The primitive heart tube
This tube that now consists of endocardium, myocardium and epicardium is known as the
primitive heart tube.
The primitive heart tube lies within the pericardial cavity.
As the heart tube enlarges, it forms 4 primordia: the sinus venosus, common atrium,
embryonic ventricle and bulbus cordis.
These primordia are parts of the heart tube that have enlarged more than the other parts in
between them so that they appear as heart vesicles.
The sinus venosus, common atrium, embryonic ventricle and bulbus cordis are primordia
and so will develop into the following parts in the adult heart.
Development of the chambers of the heart
The common atrium will be divided by an interatrial septum to form the right atrium and
the left atrium.
The sinus venosus will be form the smooth posterior part of the right atrium.
The embryonic ventricle develops into the adult or definitive left ventricle.
The right ventricle in the adult develops from the proximal one-third of the bulbus cordis.
The middle one-third of the bulbus cordis is called the conus cordis.
The infundibulum in the right ventricle and the aortic vestibule in the left ventricle
develop from the conus cordis.
The distal one-third of the bulbus cordis is called the truncus arteriosus.
The pulmonary trunk and the aorta develop from the truncus arteriosus.
The truncus arteriosus will be divided by a vertical spiral septum to form the pulmonary
trunk and aorta.
The cardiac loop
As the heart tube elongates faster than the pericardial cavity, it bends forwards and
towards the left to form a cardiac loop.
The cardiac loop is initially in the form of a C-loop and then with further elongation and
bending, an S-loop.
This results in the sinus venosus being most posterior, with the common atrium just in
front of it.
The embryonic ventricle and bulbus cordis are in front of them both.
The embryonic ventricle is usually more towards the left side, and the bulbus cordis is
more on the right side.
If the heart tube elongates and bends forwards and towards the right, it will result in the
condition of dextrocardia.
Here, the position of the heart has been reversed so that the embryonic ventricle is on the
right and the bulbus cordis is on the left side.
The Atria
Blood flows anteriorly from the sinus venosus into the common atrium, which then flows
through an atrioventricular canal into both the embryonic ventricle and bulbus cordis.
The common atrium will further be divided into 2 to become the right atrium and left
atrium of the adult heart.
The embryonic atrium becomes the right and left auricles (with musculi pectinati).The
sinus venosus becomes absorbed or incorporated into the right atrium, giving rise to its
smooth-walled part. The pulmonary veins become incorporated into the left atrium giving
rise to its smooth part.
The Ventricles
The embryonic ventricle develops into the adult (definitive) left ventricle.
The proximal one-third of the bulbus cordis develops into the adult right ventricle.
The growth of the myocardium increases the size of the embryonic ventricles.
The size of the cavity that can hold blood within each chamber or ventricle is increased
through selective cell death which also forms the trabeculae (trabeculae carneae and
chordae tendinae) and results in the muscular part of the interventricular septum.
The smooth part of the interventricular septum is formed from mesenchymal swellings
from the truncoconal ridges (truncal swellings and conal swellings) and from the inferior
atrioventricular cushion.
The Great Vessels
The bulbus cordis has 3 parts:
a proximal one-third which will give rise to the right ventricle;
a middle one-third (called the conus cordis) which will form the infundibulum of the right
ventricle and the aortic vestibule of the left ventricle; and,
a distal one-third called the truncus arteriosus, which will be divided by a septum to form
the great vessels (pulmonary trunk and aorta).
Both truncus arteriosus and conus cordis are separated by a septum which develops from
the left and right truncoconal ridges.
The truncal and conal swellings whilst growing towards each other, also twist around
each other, giving rise to the spiral course of the septum.
This will result in the aorta and pulmonary trunk twisting around each other.
Stages in Interatrial Septum Formation
– Atrioventricular endocardial cushions (inferior & superior)
– Septum Primum
– Ostium Primum
– Septum Secundum
– Ostium Secundum
– Foramen Ovale
Congenital abnormalities
Coarctation of the Aorta
Atrial Septal Defect
Ventricular Septal Defect
Patent Ductus Arteriosus
Rubella (German measles)