Development of the Heart

1
Development of the Heart

• ANHB 2212 2006 Week 5
• Avinash Bharadwaj

2
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.

3
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.

4
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 heartlet us
begin at the beginning.
5
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
  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.

6
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).

7
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.
• Remember that the tube is not partitioned at this
  stage.
• 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.

8
The Tube Bends

• 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 cordiswhich 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.

9
The Chambers

• 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.

10
The Interior

• A portion of the ventral wall of the
  bulbo-ventricular loop is removed to show the
  interior.
• 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.

11
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!

12
Interatrial septum

• Partitioning
• Right to left passage
• Mechanism for closing the passage

13
(No Transcript)
14
Septum Primum

• This is a sagittal section seen from the right.

AVC
V
15
Foramen Primum

• Foramen primum
• Between
• the septum and
• the AV Cushions

16
Passage is a Must!

• Foramen secundum
• Foramen primum about to disappear

17
Septum Secundum

• To the right of primum
• Foramen primum has disappeared

18
Foramen ovale

• F. Ovale
• In septum secundum
• Further

19
The Valve

• Two septa
• Two foramina

20
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

21
Left Atrium

• Four pulmonary veins
• Common opening
• Absorption of veins into atrium
• Rough part - auricle

22
The Ventricular Septum

• Three Parts
• Interventricular septum
• AV Cushions
• Spiral Septum

23
Ventricular Septum
24
Foetal Circulation

• Very little pulmonary flow
• Placental Circulation
• Right to Left Passages

25

• IVC Blood from placenta
• Ductus venosus
• F. ovale
• Ductus arteriosus

26
Changes At Birth

• Closure of interatrial septum
• Closure of ductus arteriosus
• Closure of ductus venosus

27
Congenital Heart Disease

• Septal Defects Atrial and Ventricular
• Endocardial cushion defects
• Aorticopulmonary defects
• PDA
• Others

Last Slide