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Development of the Heart
ANHB 2212 – 2006 – Week 5
Avinash Bharadwaj
Retrospect…
The development of the heart is the first of the series of topics that deal with
the embryology of organs and systems. This part of embryological study
is often called systemic or ‘special’ embryology or organogenesis, as
opposed to ‘general’ or early embryology.
Throughout systemic embryology, we need to recognise that the starting
point is the formation of the trilaminar embryo, that is, a flat embryo with
three germ layers – ectoderm, mesoderm and endoderm. At the extreme
head end of the embryo the ectoderm and endoderm are in contact
without intervening mesoder. This area is called the prochordal (“in front
of the notochord”) plate.
We have mentioned earlier that the lateral plate mesoderm splits to form the
coelomic cavity. Further, the head and tail ends of the embryo undergo
folding. The embryo also folds on the sides (lateral folds). Folding of the
embryo converts it from a flat plate into a tube.
Cardiovascular System
Cardiovascular system includes the heart and the blood vessels. A
detailed description of regional blood vessels is beyond the scope of
this unit, and we restrict ourselves to the development of the heart.
Embryonic development of any organ involves complex processes. Given
this complexity, it is amazing that a vast majority of human beings are
born without any of the steps going wrong. However, these errors of
development do occur; at the gross, histological and even molecular
level. An in-depth study of these errors is the subject of advanced
study, largely in the medical context. In this unit, we shall mention
some of the inborn defects (“congenital defects”) to illustrate some
principles. This applies to the development of the heart.
On the other hand, from a scientific perspective, the development of the
heart does have an interesting evolutionary story to tell.
From the Level 1 units and the gross anatomical study last week, we need
to recapitulate some basic anatomical facts about the heart.
We know that the heart has two receiving chambers (atria) and two
pumping chambers (ventricles), with partitions or septa (singular –
septum) between right and left chambers. We also understand the
precise distinction between arteries and veins as vessels bringing
blood towards the heart and taking it away from the heart respectively.
Postnatal vs Foetal Circulation
• Postnatal
Body RA RV Lungs LA LV Body
The basic difference between postnatal and foetal circulation is that foetal lungs are
nonfunctional. Effectively, blood from the right side of the heart has nowhere to go
and needs to be ‘shunted’ to the left. Such a shunting passage exists between the
right and the left atria. However, if no blood flows through the right ventricle, that
chamber will fail to develop. Thus some blood does pass to the RV. As it is
pumped into the pulmonary artery, it needs to be shunted again, this time to the
aorta. This illustrated below.
But we are jumping too far ahead! This was mentioned as one of the basic principles
of the development of the heart…let us begin at the beginning.
• Foetal
Body RA RV Lungs LA LV Body
Earliest Development
Cardiovascular system makes its first appearance while the
embryo is still flat. Clusters of mesodermal cells specialise
to form blood cells. Mesodermal cells around these flatten
to form endothelium of blood vessels. These clusters are H
called blood islands of angiogenic (“blood vessel-forming”)
cell clusters.
In the accompanying diagram note that these form a curve
reaching well beyond the neural plate and the notochord.
A mass of mesoderm, called cardiogenic area, near the
head end (H) will give rise to the heart.
The sagittal section below illustrates the three germ layers,
prochordal plate and the cardiogenic area.
Prochordal plate
Cardiogenic area
Head Fold
With the formation of the head fold (shown in the blue
circle), note how the cardiogenic area changes its
position. Also observe that the endoderm (yellow)
is beginning to form the gut tube. At this stage only
the head and tail ends of the digestive tube are
recognisable.
In the lowest picture, the gut tube is better seen and
the heart is in fact in the form of a tube (red).
Heart Tube
The Heart Tube
In the picture on the left the relationships of the
heart, the gut tube and the liver are clearer.
In the magnified picture of the heart tube, the tail
end is the venous end and the cranial end is the
arterial end. The changing shape of the tube
also makes it possible to recognise the primitive
chambers of the tube.
Heart Remember that the tube is not partitioned at this
Live stage.
r Hereafter, for descriptive convenience, we shall
view this tube in the vertical position, with the
caudal (venous) end below and the cranial
(arterial) end at the top as shown below.
The Tube Bends
V B D
A
V SV
This picture shows three successive stages in the growth of the tube. The tube, as it grows,
cannot be accommodated within the pericardial cavity and undergoes bending.
The primitive chambers of the heart are recognisable, and are labelled in the last picture.
SV – sinus venosus (receives veins from the body), A – atrium, V – ventricle. The ventricle
continues into the ‘bulbus cordis’which in turn leads to the arterial end.
Two terms are used somewhat confusingly for the parts at the arterial end. These are
conus arteriosus and truncus arteriosus. In our discussion we shall simply say ‘arterial
end’ of the heart.
The Chambers
A A
A
B-V Loop
V
Left view Front view
Recognise the chambers in these two views. In the view from the left side, the sinus
venosus is partly hidden. Note that with the bending of the tube the atrium is now dorsal
and the loop formed by the ventricle and the bulbus cordis (bulbo-ventricular loop) is
ventral.
In the next slide we shall examine the interior of the unpartitioned heart.
The Interior
A portion of the ventral wall of the bulbo-
ventricular loop is removed to show the
interior.
A-Ar Since there is no partition, there is a single
passage from the atrium to the ventricle.
This passage is the atrioventricular canal.
Note the direction of blood flow through
the bulboventricular loop.
Also note that the single vessel leading out of
the heart has given rise to what are called
aortic arches.
RA LA
AVC
Left – Right Partitioning
• Interatrial septum
• Interventricular septum
• Spiral (aortico-pulomonary) septum
• Endocardial cushions (A-V cushions)
• Functional requirements
• There must always be a right to left passage!
Interatrial septum
• Partitioning
• Right to left passage
• Mechanism for closing the passage
A
V
Septum Primum
• This is a sagittal
section seen from the
right.
AVC
V
Foramen Primum
• Foramen primum :
Between
the septum and
the AV Cushions
Passage is a Must!
• Foramen secundum
• Foramen primum
about to disappear
Septum Secundum
• To the right of primum
• Foramen primum has
disappeared
Foramen ovale
• F. Ovale –
• In septum secundum
• Further…
The ‘Valve’
• Two septa
• Two foramina
Sinus Venosus
• Originally a symmetrical structure
• Venous return more to the right
• Left horn becomes smaller
• Opening shifts to the right
• Later – part of right atrium
Left Atrium
• Four pulmonary veins
• Common opening
• “Absorption” of veins into atrium
• Rough part - auricle
The Ventricular Septum
Three Parts
– Interventricular septum
– AV Cushions
– Spiral Septum
Ventricular Septum
R
Membranous
Muscular Spiral
(Aorticopulmonary)
Foetal Circulation
• Very little pulmonary flow
• Placental Circulation
• Right to Left Passages
• IVC :
Blood from
placenta
– Ductus
venosus
• F. ovale
• Ductus
arteriosus
Changes At Birth
• Closure of interatrial septum
• Closure of ductus arteriosus
• Closure of ductus venosus
Congenital Heart Disease
• Septal Defects – Atrial and Ventricular
• Endocardial cushion defects
• Aorticopulmonary defects
• PDA
• Others
Last Slide
