Heart Development

Where are the heart fields located in the early embryo?

At the cranial end of the trilaminar germ disc in the splanchnic mesoderm.

When do heart fields begin differentiating?

Around Day 19.

What are heart fields?

Populations of progenitor cells that will develop into different parts of the heart.

What is the primary heart field also called?

The cardiac crescent.

Where do precardiac cells move after leaving the primitive streak?

Cranially, where they form the primary heart field.

Where does the secondary heart field form?

Medial to the primary heart field.

Which heart field initially forms the primary heart tube?

The primary heart field.

When do secondary heart field cells migrate into the heart tube?

After the primary heart tube has formed.

What structures does the secondary heart field contribute to?

The inflow and outflow tracts, most of the atria and right ventricle, and some skeletal muscles in the head and neck.

What could be affected by a defect in the secondary heart field?

The inflow and outflow tracts, atria, right ventricle, and some head and neck skeletal muscles.

What is the early model of secondary heart field development?

It proposed that secondary heart field cells mainly contributed to the inflow and outflow tracts.

How did the newer model change the understanding of the secondary heart field?

It showed that the secondary heart field contributes to more structures, including most of the atria and right ventricle.

How does embryonic folding contribute to heart development?

Lateral and head folding bring the heart fields together at the midline, where they fuse to form the primary heart tube.

When does lateral and head folding relocate the heart to the thoracic cavity?

Between Days 18 and 22.

Where do the heart fields fuse during folding?

At the midline, ventral to the gut.

What does fusion of the heart fields create?

The primary heart tube.

When has formation of the tubular heart occurred?

By the end of the third week.

Where is the tubular heart located?

Inside the pericardial coelom.

What are the three layers of the primary heart tube?

Endocardial lining, cardiac jelly, and myocardium.

What is the endocardium?

The inner lining of the primary heart tube.

What is cardiac jelly?

Extracellular matrix secreted between the myocardium and endocardium.

What is the myocardium?

The muscle layer of the heart tube.

In which direction does blood flow through the primary heart tube?

Blood enters at the caudal end and exits through the cranial end.

What is the first heart structure to form?

The primary heart tube.

When does rhythmic contraction of heart muscle tissue begin?

Around Day 21.

When does active pumping of blood begin?

Around Days 24–25.

When does cardiac looping occur?

Around Day 23.

What happens during cardiac looping?

The primary heart tube folds and loops to establish the future heart chambers in their correct spatial locations.

What shape does the heart tube take during cardiac looping?

An S shape.

Why is cardiac looping important?

It positions the future heart chambers in the correct spatial locations.

What is the first asymmetrical structure to develop in the embryo?

The heart.

Where does the inflow tract become positioned during looping?

Dorsal to the outflow tract (conotruncus).

What happens to the atrium as the heart continues to grow?

It bulges out on either side of the heart, behind the conotruncus.

What is the correct order of early heart development?

Folding, looping, then septation.

What are the three circulatory arcs in the embryonic heart?

Intraembryonic, vitelline, and placental arcs.

What is the pathway of the intraembryonic arc?

Cardinal veins → heart → conotruncus → aortic arches → dorsal aorta.

What is the pathway of the vitelline arc?

Vitelline veins → heart → dorsal aorta → vitelline arteries.

What is the vitelline arc associated with?

The yolk sac.

What happens to the vitelline arc as the yolk sac disappears?

It gradually diminishes.

What is the pathway of the placental arc?

Umbilical vein → heart → dorsal aorta → umbilical artery.

Where do all three embryonic circulatory arcs enter the heart?

At the sinus venosus at the caudal end of the heart.

What happens to oxygenated and deoxygenated blood at the sinus venosus?

They mix.

Which vessel carries oxygenated blood from the placenta to the embryo?

The umbilical vein.

Which vessels carry blood away from the heart toward the body?

The arteries.

Which embryonic circulatory arc remains at birth?

The intraembryonic arc.

How many chambers does the adult heart have?

Four.

What is systemic circulation?

Blood flow from the left side of the heart to the body.

What is pulmonary circulation?

Blood flow from the right side of the heart to the lungs.

What type of blood leaves the right ventricle?

Deoxygenated blood.

Where does blood from the left ventricle travel?

Toward the body through systemic circulation.

What type of blood travels through the pulmonary artery?

Deoxygenated blood.

What is the main difference between fetal and adult circulation?

Fetal circulation uses three shunts to bypass the lungs and liver; adult circulation sends blood through the lungs for oxygenation and through the liver normally.

What is septation?

The process of dividing the primary heart tube into separate chambers and channels.

When does heart septation begin?

Around Day 28.

When is heart septation generally complete?

By around three months.

What are the four types of heart septation?

Atrioventricular, atrial, ventricular, and outflow tract septation.

What does atrioventricular septation separate?

The atria from the ventricles.

What does atrial septation separate?

The left and right atria.

What does ventricular septation separate?

The left and right ventricles.

What does outflow tract septation separate?

The aorta and pulmonary artery.

What begins atrioventricular septation?

The formation of endocardial cushions.

What are endocardial cushions?

Thickenings at the junction of the atrium and ventricle that are rich in cardiac jelly.

What happens when the endocardial cushions grow and meet?

They divide the atrioventricular canal into left and right channels.

What does the right atrioventricular canal develop into?

The tricuspid valve.

What does the left atrioventricular canal develop into?

The mitral (bicuspid) valve.

What does atrial septation accomplish?

It divides the atria into the left and right atrium.

Where does blood continually enter during fetal atrial septation?

The right atrium through the sinus venosus.

Does fetal atrial septation completely close off blood flow between the atria?

No. Blood continues to pass from the right atrium to the left atrium.

What is the septum primum?

The first wall that grows downward between the atrial chambers.

Where does the septum primum grow?

Toward the atrioventricular canal and endocardial cushions.

What is the foramen primum?

The initial opening between the septum primum and the endocardial cushions.

What happens to the foramen primum?

It closes as the septum primum grows toward the endocardial cushions.

What is the foramen secundum?

A second opening that forms within the septum primum before the foramen primum closes.

What is the septum secundum?

A second wall that grows to the right of the septum primum.

What is the foramen ovale?

An oval-shaped opening that develops in the septum secundum.

Why are the foramen ovale and foramen secundum not aligned?

Their non-overlapping positions allow the opening to close quickly at birth.

Which atrial septum is thin and flexible?

The septum primum.

Which atrial septum is thick and rigid?

The septum secundum.

How does fetal blood flow keep the atrial passageway open?

Right-to-left blood flow pushes the flexible septum primum and keeps the passageway open.

Why are there two layers in the fetal atrial septum?

The septum primum and septum secundum create overlapping layers that allow blood to pass before birth but help close the opening after birth.

What creates the interventricular septum?

Ventricular septation.

What are the two parts of the interventricular septum?

The muscular septum and membranous septum.

Where does the muscular interventricular septum grow from?

The apex of the ventricular loop.

In which direction does the muscular interventricular septum grow?

Upward toward the endocardial cushions.

What is the origin of the muscular septum?

It grows from the ventricular wall.

What is the origin of the membranous septum?

Neural crest cells that migrate into the heart.

Why is the membranous septum a common site for defects?

It is derived from neural crest cells that must migrate into the heart and contribute to its formation.

Which part of the interventricular septum is thinner?

The membranous septum.

Which part of the interventricular septum is thicker and muscular?

The muscular septum.

What is the outflow tract called in the primary heart tube?

The conotruncus.

How many channels does the original outflow tract have?

One channel.

What happens during outflow tract septation?

The single outflow tract separates into the aorta and pulmonary artery.