Heart Development

Question 1

What is the critical period of cardiac development?

Answer 1

3-6 embryonic weeks (5-8 gestational weeks).

Question 2

What does the heart develop from? what happens to this? what is the primary myocardium?

Answer 2

The heart develops out of mesodermal tissue that forms two roughly parallel endocardial heart tubes that ultimately fuse together into a single endocardial tube. While these tubes are fusing myocardial tissue (also from mesoderm) develops and surrounds the endocardial heart tube.

These two heart tubes fuse starting at their cranial ends and progress caudally resulting in a single thin‐walled endocardial heart tube

While the endocardial heart tubes form and fuse, other lateral visceral mesoderm cells begin to form a muscular layer called the primordial myocardium surrounding the heart tube

Question 3

When does the heart begin to beat? what is its size at this point?

Answer 3

The heart begins to contract by the end of the 4th week and is only 2‐3mm long

Question 4

what happens to the heart tube? neural tube cells do what?

Answer 4

As the single heart tube grows in diameter and length it also folds and undergoes a counterclockwise rotation and develops the several embryonic dilations.

As these dilations form, cranial neural crest cells migrate from the neural tube, through the branchial arches to the outflow region of the heart tube. Here they will contribute to the formation of the aorticopulmonary septum

Question 5

What is the sinus venosus? fuses with?

Answer 5

A. sinus venosus

1. is a common venous dilation that receives blood from cardinal venous system a. the cardinal venous system is the main venous drainage of the embryo proper and is destined to form the SVC, IVC, and azygous system
2. fuses with the primordial atrium and is destined to become the smooth surfaced portion of the right atrium

Question 6

what is the primordial/primitive atrium? develops?

Answer 6

B. primordial /primitive atrium

1. starts as a single chamber of the heart tube’s caudal segment
2. begins to develop R and L atrial chambers due to the formation of an overlapping partition comprised of the septum primum(develops 1st, on left) and the septum secundum(develops 2nd, on right)

Question 7

What is the pimordial ventricle? what develops from it?

Answer 7

primordial ventricle

1. starts off as a single chamber of the heart tube’s cranial segment; begins to develop R & L chambers due to the formation of a primordial muscular interventricular septum (IVS) that originates caudally (near future heart apex) and grows cranially.
2. a primordial membranous IVS also begins to develop from the cranial region of the ventricle

Question 8

what is the bulbus cordis?

Answer 8

bulbus cordis

1. a transient dilation of the embryonic heart part of which is destined to contribute to the formation of the conus arteriosus and aortic (cardiac) outflow tract as well as the trabeculae carnaeof the right ventricle

Question 9

What is the cardiac outflow tract? eventually its the site of the? explain that structure?

Answer 9

Cardiac Outflow tract

The cardiac outflow tract (OFT) is a transient structure at the arterial pole of the heart connecting the embryonic ventricles with the aortic sac.
Eventually this will be the site of the truncus arteriosus

1. a common arterial trunk that extends cranially from the bulbuscordis
2. destined to become the aorta and pulmonary trunk
3. origin of truncus arteriosus gives rise to aortic valve and pulmonary valve

Question 10

What are the cardiac neural crest cells?

Answer 10

Cardiac Neural Crest Cells form the Outflow Tract cardiac neural crest cells *form the cardiac tract outflow tract so a disturbance in this grouping of neural crest can result in cardiac septation defects

ex: DiGeorge’ssyndrome is characterized by atypical neural crest cell migration. This may result in reduced function of the thymus, thyroid, and parathyroid glands as well as cardiovascular defects, such as persistent truncus arteriosus and abnormalities of the aortic arch arteries.

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*cardiac neural crest cells are a type of neural crest cells that migrate to create the aortic arches (more later) and the cardiac outflow tract; proper migration of the cardiac neural crest cells is necessary for normal cardiovascular development

Question 11

what is dextrocardia?

Answer 11

• heart tube folds and twists in an abnormal clockwise rotation when viewed down the cranial‐caudal longitudinal axis of the embryonic heart
• results in a heart that is a mirror image of a “normal” heart with the apex directed to the right

Question 12

explain the radiology of dextrocardia?

Answer 12

Question 13

what does the truncus arteriosus become? Bulbus cordis? Primitive ventricle? Primitive artium? Sinus venosus?

Answer 13

Truncus arteriosus: Aorta Pulmonary trunk

Bulbus cordis: Smooth part of right ventricle (infundibulum) Smooth part of left ventricle (aortic vestibule)

Primitive ventricle: Trabeculated part of right ventricle Trabeculated part of left ventricle

Primitive atrium: Pectinate wall of right atrium (right auricle) Pectinate wall of left atrium (left auricle)

Sinus venosus: Smooth part of right atrium (sinus venarum) Coronary sinus

Question 14

what are endocardial cushions? a defect associated with the endocardial cushions? what is the defect associated with?

Answer 14

Endocardial cushions
Endocardial cushions fuse and close the atrioventricular canals
A defect associated with the endocardial cushions: • Tricuspid atresia leading to hypoplasticright heart • Associated with maternal use of lithium

Question 15

What is artial partitioning?

Answer 15

A. The single chamber becomes separated by two septa (primum& secundum) that grow from opposite ends of the primordial atrium. A defect (foramen ovale) forms in the septum primumthat is overlapped (on the right) by the septum secundum. The septum primumacts as a one way valve for the foramen ovale. This foramen and the valve allow for one way flow from the right atrium to the left atrium during fetal life

B. Failure of complete partitioning results in a patent foramen ovale

Question 16

explain ventricular partitioning?

Answer 16

A. The interventricularseptum forms from a muscular portion growing cranially from the apex of the ventricle and is met (late 7th week) by a caudally growing membranous portion of the septum (derived from the aorticopulmonaryseptum). Prior to the end of the 7th week there is an interventricularforamen at the superior edge of the muscular septum and some blood mixes between the left and right ventricles.

B. Impaired development in this process results in ventricular septal defects

Question 17

Explain partitioning of the truncus arteriosus and bulbus cordis? Abnormal neural crest cells lead to? Abnormal myocardium? abnormal endocardium?

Answer 17

The outflow tract is one of the most common sites of cardiac abnormalities, as it requires normal development and proliferation of multiple cell types. Abnormalities commonly occur via defects in the following areas:

• Neural crest cells ‐abnormal migration/proliferation leads to a septationdefect
• Myocardium ‐abnormal rotation/leads to an alignment defect
• Endocardium ‐proliferation leads to a cushion defect

Question 18

The cardiac outflow tract includes what? what is the aorticopulmonary septum? explain the steps to divide the bulbus cordis and the truncus arteriosus?

Answer 18

the cardiac outflow tract includes the infundibulum, pulmonary trunk, aortic vestibule, and ascending aorta
theaorticopulmonary septum(akaspiral septum) develops to partition the bulbuscordisand truncus arteriosus, eventually fusing with the interventricular septum

i. bulbuscordisdivides into the infundibulum and the aortic vestibule ii. truncus arteriosus is separated into the pulmonary trunk and ascending aorta
iii. Septationof the outflow tract into the aorta and pulmonary trunk begins with two truncal swellings (orbulbarridges) on the left and right sides of the bulbous. These swellings grow towards each other, and in opposing winding/spiraling directions. The winding nature of this growth results in the normal adult relationship of the aorta and pulmonary trunks.
iv. At the same time these septa grow inferiorly into the ventricle, thus forming the membranous portion of the interventricular septum

Question 19

Explain the derivatives of the aortic arch?

Answer 19

1 Largely disappears (part of maxillary artery in head)

2L a r g e l y disappears

3 Common and internal carotid arteries

4( R i g h t ) subclavian artery and aortic arch (on left side)

5 Disappears

6 Infundibulum and L & R pulmonary arteries, Ductus arteriosus

Question 20

Which cells are the first to become electrically active? When does the SA node develop what happens to it?

Answer 20

Cardiomyocytes in the caudal heart tube are the first to become electrically active and become the “pacemaker”.
The SA node, which develops during the fifth week, initially develops in the sinus venosusand then is incorporated into the RA. The AV node arises slightly superior to the endocardial cushions. Fibresforming the bundle of His develop from fast‐conducting ventricular myocardium while the SA and AV nodes are formed from the slow‐conducting myocardium of the inflow tract and AV canal. Connective tissue grows in from the epicardium, forming the cardiac skeleton that separates conduction in the atria and ventricles.
SA node initially develops in the sinus venosusand then is incorporated into the RA. AV node arises slightly superior to the endocardial cushions.

Question 21

blood flowing to the heart?

Answer 21

1. blood returning from the placenta is oxygenated and carries nutrients back to the heart through the single umbilical vein
2. blood also returns to the heart through the following veins: a. vitelline veins:drain blood from future gastrointestinal tract, will become portal system
   b. cardinal veins:form SVC and IVC tributaries to become thecavalsystemof venous return.

Question 22

Blood leaving the embryonic heart?

Answer 22

1. blood from the primitive embryonic heart pumps intoaortic arches, which are formed by neural crest cells
2. the right and left dorsal aortas fuse to form the single midline aorta, coursing along the length of the embryo.
3. some of the blood enters the vitelline arteries to supply the future gastrointestinal tract, and some passes to the placenta via a pair of umbilical arteries, where gases, nutrients, and metabolic wastes are exchanged.

Question 23

What is the ductus arteriosis?

Answer 23

Direct connection between the pulmonary trunk (or L pulmonary artery) to the arch of the aorta which allows for blood (75% of CO) to bypass the fetal pulmonary circulation and enter the systemic circulation

So blood returning from the systemic circulation (including the placenta) enters the systemic circulation without passing through the non‐functional lungs

• connects pulmonary trunk to aortic arch during fetal development
• over 75% of the fetal output from the right ventricle bypasses the pulmonary circuit through the ductus arteriosis
• Classified as a right to left arterial‐arterial shunt

Question 24

Post natal changes for the ductus arteriosus?

Answer 24

The high plasma prostaglandins and low O2 levels present during fetal life decrease and increase, respectively, to cause a stenosis(narrowing) of the DA and a resultant decrease of flow through it; usually complete by 1 month [recall post‐natally, the fibrous remnant of the ductus arteriosus is called the ligamentum arteriosum]

Question 25

Foramen ovale is what?

Answer 25

a. clamping of the umbilical cord causes an abrupt drop in the umbilical venous pressure and hence right atrium pressure; furthermore the drop in pulmonary vascular resistance and increase in pulmonary blood pressure increases the left atrium pressure
b. these above two factors cause the valve of the foramen ovaleto seal against the left interatrialseptum thereby closing off the foramen ovale
c. in most cases, a gradual permanent seal forms between the valve and the interatrial septum within a few weeks or years d.postnatally, the slight oval depression found in the interatrialseptum seen from the right atrium side is called the fossa ovalis

Question 26

Explain the umbilical arteries?

Answer 26

A. approximately 50% of the combined cardiac output of the RV plus LV supplies the fetal tissues

B. the remaining 50% passes through the bilateral (i.e. R&L) umbilical aa. which exit the fetus as branches of the R&L internal iliac aa. through the umbilicus (navel)

C. next, the two umbilical arteries enter the umbilical cord twisting and perfuse the placenta.

D. The placenta (Latin: round cake) receives blood from the R&L umbilical arteries. 1. fetal blood and maternal blood never mix, but are separated by a thin placental membrane to allow fetal waste products to diffuse into the maternal circulation and maternal nutrients and oxygen to diffuse into the fetal

Question 27

What are the changes that occur to the umbilical arteries?

Answer 27

A. clamping the umbilical cord causes the extra‐abdominal and distal intra‐abdominal segments of the R&L umbilical arteries to become obliterated

B. the proximal segments of the R&L umbilical arteries maintain patency and supply blood to some of the pelvic viscera such as the bladder

Question 28

Summarize the umbilical vein?

Answer 28

A. High O2, nutrient‐rich fetal blood leaves the placenta and is returned to the fetal circulation through the single umbilical vein.

B. about 50% of the blood in the umbilical vein enters the hepatic portal vein to percolate through the fetal liver. This blood then exits the liver in the hepatic veins which drain into the IVC.

C. the other 50% of the blood in the umbilical vein bypasses the liver circuit through the ductus venosusto reach the IVC directly

Question 29

What is the ductus venosus?

Answer 29

A. usually within about 3 hours of birth (but sometimes taking days), the smooth muscle in the walls of the ductus venosuscontracts by an unknown mechanism

B. as a result of this contraction, blood is prevented from entering the IVC directly and must pass entirely through the hepatic circulation

C. post‐natally, the obliterated fibrous remnant of the vestigial ductus venosusis called the ligamentum venosum

Question 30

summarize the the fetal and post natal blood flow?

Answer 30

in the fetus, there is significant shunting of blood such that: i. the fetal pulmonary circuit receives relatively little blood ii. both the right ventricle and left ventricle output, together, pump a significant fraction of blood (≈ 50%) to the fetal systemic circuit

postnatally, the right ventricle & left ventricle outputs are independent but equal in the volume of blood that each ventricle pumps : right ventricle pumps to pulmonary circuit; left ventricle pumps to systemic circuit

Question 31

Conversion from fetal to post natal circulation pattern post birth is initiated because?

Answer 31

a. neonate taking first breath i. air enters and expands lung → causes drop in pulmonary vascular resistance to blood flow → increases pulmonary blood flow → drop in blood pressure in right atrium, right ventricle, and pulmonary trunk
b. neonate separating from placenta by cutting umbilical cord i. loss of low vascular resistance placenta → increases neonate’s overall systemic vascular resistance → increases blood pressure in aorta, le ventricle, and le atrium

Question 32

what are the two contributing structures of the interventricular septum?

Answer 32

msucular and membranous portions (neural crest cells)