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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