Cardiac development, by Lisa McCabe.
Hello, my name is Lisa McCabe.
I’m a clinical nurse specialist at Children’s
Hospital Boston in the cardiovascular program.
I will be discussing with you today Cardiac
development.
The information I swill share is consistent
with our practice here at Children’s Hospital
Boston.
You may want to adapt this information to
your own institutional practice.
Fetal Development.
During the first week of fetal life, the fertilized
egg develops into a blastocyte and implants
in the mother’s uterus.
During the second week of fetal life, the
blastocyte implants deeper into the uterine
wall, and a primitive placenta begins to form.
During the third week of fetal life, the primitive
umbilical cord develops.
Also at this time, the blastocyte develops
into a three-layered disk.
The three layers are: the endoderm, mesoderm
and ectoderm.
Specific body systems will develop from each
layer.
The endoderm, or inner layer, gives rise to
the primitive intestinal tube, mucous membranes,
glands, lung buds, urinary tract, and yolk
sac.
The mesoderm, of middle layer, gives rise
to the heart and vascular system, the dermis,
subcutaneous tissue, muscles, skeleton, sex
glands, lymph glands, kidneys, connective
tissue, and blood cells.
And finally the ectoderm, or outer layer,
gives rises to the epidermis, hair, sebaceous
glands, sweat glands, and nervous system.
Cardiogenesis.
Early in the development, the primitive heart
develops two tubes that merge into one tube.
The single tube begins to swell, and develops
into various anatomic features of the heart.
The heart begins o beat by week three.
In normal cardiac development, the cardiac
tube will twist and turns on itself in a rightward
direction.
This is called dextral-looping.
This results in the right ventricle developing
on the right side of the heart and the left
ventricle developing on the left side of the
heart.
Abnormal looping in a leftward direction is
called leval-looping.
This results in the right ventricle developing
on the left side of the heart and the left
ventricle developing on the right side of
the heart.
Atrial Septation.
Entering into the fifth week of fetal life,
the atrial septum and ventricular septum begins
to form.
The atrial septum grows in layers, and includes
the tissues of septum premium, septum secondo,
and endocardial cushion tissue.
The endocardial cushion tissue is located
in the middle of the heart.
From this tissue arises the tricuspid valve,
mitral valve, part of the atrial septum, and
part of the ventricular septum.
Septum prium grows downward between the right
and left atrium, and eventually fuses with
the endocardial cushion tissues.
The septum secudnum grows parallel to septum
primum.
Both septum primum and septum secundum develop
with holes in them to create a passageway
for blood to flow from right atrium to left
atrium through the foramen ovale.
As long as the pressures on the right atrium
are higher than pressures in the left atrium,
the passage will stay open to allow the blood
to flow from right to left.
Errors may occur during atrial septation.
A secundum atrial septal defect is an opening
in the middle of the atrial septum, produce
when the tissue of septum primum does not
reach septum secundum adequately to completely
close the wall or when a tiny hole is left
in the septum primum as the tissues grow.
A primum atrial septal defect is an opening
in the lower part of the atrial septum, near
the tricuspid and mitral valves.
It results from problems in the growth of
the endocardial cushion tissues.
A defect here is often associated with a defect
in the mitral valve, as well as tricuspid
valve.
The most severe form of this is an atrial
ventricular canal defect.
Truncal Septation.
The aorta and pulmonary artery develop from
a single tubular structure: the truncus arteriosis.
Two areas of thickened tissue project into
the lumen of this tube on the right and left
side.
As they continue to grow in size, these ridges
meet and fuse to form the septum, which takes
a spiral course toward the end of this tube.
This septum divides the truncus into two vessels:
the aorta and the pulmonary artery.
The conal tissue involved in the septation
of the truncus arteriosis also directs placement
of the aorta and pulmonary artery over their
related ventricles.
Ventricular Septation.
By the fifth week of fetal life, the atria
completely separate into the right atrium
and left atrium.
Also at this time, the ventricular septum
continues to develop.
The muscular portion of the ventricular septum
develops from the apex of the common ventricle.
The membranous septum develops from the endocardial
cushion tissues.
Conal tissue contributes to the complete closure
of the septum between the aortic and pulmonary
valves.
Errors may occur during ventricular septation.
Muscular defects can occur in any portion
of the muscular septum.
Small defects often close on their own as
the tissues develop.
Membranous defects are located behind the
septal leaflet of the tricuspid valve.
Sub-pulmonary defects are caused by deficiency
of the conal septum.
Atrial ventricular canal type defects are
caused by defects in the endocardial cushion
tissues of the intraventricular septum.
Arch Formation.
The aorta and pulmonary artery and their branching
vessels are formed from the brachial arch
arteries.
Initially there are six pairs of brachial
arch arteries.
The first, second, and fifth pairs disappear,
forming ligaments that hold the heart in place.
The third aortic arch forms the common carotid
artery, external carotid artery, and internal
carotid artery.
The fourth aortic arch forms part of the final
aortic arch, as well as the proximal portion
of the right subclavian artery.
The sixth aortic arch produces the proximal
segment of the branch pulmonary arteries,
the ductus arteriosis, and provides pulmonary
blood flow via the branch that develops lung
buds.
Errors may occur in arch formation that result
in the following defects: coarctation of the
aorta, interrupted aortic arch, aortic atresia,
patent ductus arteriosis, and vascular ring.
In just a few short weeks, the human heart
has undergone major development.
By the third week of fetal life, the heart
is beating.
BY day 44, the fetal heart resembles the postnatal
heart in structure and function.
It will continue to grow and develop over
the next 30 to 32 weeks.
Many of the features that may results in a
heart defect, however, have also begun to
develop.
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