Development of the Heart

Question 1

Heart Defects - Causes

Answer 1

genetic and teratogenic

• Effects depend on timing of exposure
• Defects will often be multifactorial

Question 2

Development of the heart

Answer 2

• Formation of the 3 germ layers (trilaminar disc): ectoderm; endoderm; mesoderm
• Formation of the primitive heart tube (in the mesoderm)
• Folding and looping of the heart tube
• Septum formation in the common atrium
• Endocardial cushions and the atrioventricular canal
• Septum formation in the ventricles
• Septum formation in the truncus arteriosus
• Growth of atria and ventricles
• Valve formation

Question 3

disc transformation

Answer 3

The bilaminar disc becomes trilaminar (gastrulation) in the early stages of development (only 2 weeks post conception) and the 3 layers are ectoderm; endoderm; and mesoderm that are formed during the 3rd week

Question 4

The Heart Tube

Answer 4

forms in the mesoderm from angiogenetic clusters, as a horseshoe at the cephalic end of the trilaminar disc

Question 5

As the embryo grows diffusion…

Answer 5

is no longer sufficient to provide nutrients and development of both blood and vascular structures becomes necessary

Question 6

beginning of heart development

Answer 6

By day 18

Question 7

Development of the heart begins as

Answer 7

angiogenetic clusters coalesce to form a horseshoe in the mesoderm
As a result of folding of the disc the two limbs of the horseshoe fuse to form a single heart tube that lies in the thorax and initially consists of endothelial cells only

Question 8

The trilaminar disc folds in 2 directions

Answer 8

cephalic-caudal and lateral folding which starts towards the end of week 3 e.g 18 days

Question 9

Folding

Answer 9

the head and tail ends fold and meet to form 2 lateral folds at the umbilicus

the lateral folding swings the 2 limbs of the horseshoe medially so that they fuse as a single heart tube

Question 10

what does cephalo-caudal folding cause

Answer 10

the heart tube to effectively migrate from the head of the embryo, through the neck and in to the thorax where it elongates and develops the three layers

Question 11

what are the three layers involved in heart development

Answer 11

epicardium - visceral pericardium
myocardium - cardiac muscle
endocardium - endothelial lining

Question 12

when does the looping and folding occur

Answer 12

starts early in week 4 at about 22 days

Question 13

what occurs during the looping and folding

Answer 13

The 2 “ends” fold towards each other and to the right, pushing the “apex” of the loop to the left, and rotating slightly so that the right side of the heart tends to be more anterior.

Question 14

Dextrocardia

Answer 14

If the “ends” fold to the left, the developing heart is pushed to the right

Question 15

Heart of a 5mm embryo (28days)

Answer 15

The developing heart pushes in to the pericardial sac; the ventricles begin to trabeculate

Question 16

In Summary

Answer 16

The venous end (inflow) receives blood from the cardinal, umbilical and vitelline veins; it comes to lie behind the arterial end (outflow)
(In the fully developed heart the atria and great veins are behind the ventricles and the roots of the great arteries.)
The atrium bulges out on each side of the bulbus cordis
The proximal part of the bulbus cordis will form the RV The middle part forms the outflow of the ventricles The distal part forms the PT and Aorta
Between the atrium and ventricle is a narrowing - the atrioventricular canal

Question 17

where are endocardial cushions derived from

Answer 17

neural crest cells

Question 18

enlargement of endocardial cushions forms

Answer 18

• interatrial septum
• the membranous part of the interventricular septum
• AV valves (tricuspid and mitral)
• formation of the PT and Aorta from the truncus arteriosus

Question 19

The tissue in the narrowing between the single atrium and the single ventricle swells to form

Answer 19

endocardial cushions

that grow to meet in the middle and divide the atrioventricular canal into right (tricuspid) and left (mitral) channels

Question 20

Formation of the interatrial septum (summary)

Answer 20

• Septum primum grows down towards the endocardial, atrio- ventricular cushions. Blood passes from RA to LA via the foramen (ostium) primum
• Before the foramen primum closes, perforations appear in the upper part of the septum - develop into the foramen (ostium) secundum
• Septum secundum (stiff, muscular and to the right of S. Primum) grows down, but does not fuse with Endocardial Cushion and forms foramen ovale
• Blood passes from RA to LA thro 2 openings: foramen ovale and foramen secundum
• Formed between weeks 5 and 6, embryo grows from 5 mm to17mm
• At post mortem 10 % of people have a probe patency of the fossa or foramen ovale i.e. anatomically patent but functionally sealed

Question 21

Atrial Septal Defect ASD

Answer 21

• more common in females

Question 22

Patent Foramen Ovale (PFO)

Answer 22

may be asymptomatic
Foramen secundum defect or Foramen primum Probably caused by excessive cell death or resorption of
septum primum
A large opening between the RA and LA leads to significant shunting from left to right

Question 23

when is the formation of the inter-ventricular septum completed by

Answer 23

the end of the 7th week

Question 24

what are the four elements involved in the formation of the interventricular septum

Answer 24

• endocardial cushions form the left and right ridges in the conus
• an extension of the inferior trio-ventricular cushion will contribute to the membranous portion of the inter-ventricular septum.
• proliferation of, and ventricular growth around, forms the muscular portion of the IVS
• The ridges (i.e. septum) advance inferiorly towards the muscular IVS, making the temporary interventricular foramen smaller and smaller; ventricular growth makes the muscular septum effectively advance towards the truncal septum too

Question 25

what do the left and right truncates ridges spiral and fuse to form

Answer 25

conotruncal septum

Question 26

The interventricular foramen is closed by

Answer 26

the membranous part of the interventricular septum, derived from the truncal ridges and the inferior atrio-ventricular cushion

Question 27

Ventriculo-Septal Defects (VSD)

Answer 27

Most common congenital cardiac malformation

More common in males

Question 28

Fallot’s tetralogy

Answer 28

Truncal septum deviates right and does not meet the interventricular septum

Question 29

Atrial Growth

Answer 29

The atria grow, but also need to incorporate the adjacent veins (sinus venosus).
In the right atrium, the crista terminalis marks the change and the original atrium shows muscular ridges (musculi pectinati) while the atrium derived from foetal vein (sinus venosus) is smooth

Question 30

Ventricular Growth

Answer 30

The two ventricles “balloon” to grow around and away from the muscular septum dividing them

Question 31

Formation of the Valves

Answer 31

Mitral and Tricuspid, endocardial cushion growth and cavitation to form papillary muscles and chordae tendineae

Question 32

Formation of the Great Vessels

Answer 32

Arterial development is intimately associated with the sequential formation of the pharyngeal or gill arches (weeks 4 and 5) that have their own cranial nerve and arterial supplies
Arches give rise to structures in the head and neck
As each pharyngeal arch develops an artery arises from the aortic sac (the part of the truncus arteriosus that will become the Ao), grows through the gill arch and joins the dorsal aorta

Question 33

more on the development of the great vessels

Answer 33

Development is sequential (earlier arches progressing or resorbing as later ones arise).
The 2nd and 5th arches hardly develop, giving rise to nothing particularly notable
Left 6th arch forms the pulmonary trunk and ductus arteriosus Right 6th arch also contributes to the pulmonary trunk.

Question 34

The right and left pulmonary arteries grow within

Answer 34

the developing lungs and connect to the 6th arches that are forming the pulmonary trunk

Question 35

Coarctation of Aorta

Answer 35

Aortic narrowing after the origin of the left subclavian due to an abnormality in the aortic media and intimal proliferation
May be proximal or distal to the ductus arteriosus
Postductal is more common and blood travels in the subclavian to internal thoracic to intercostal vessels and back to the thoracic aorta; femoral pulses will be weak

Question 36

Recurrent Laryngeal Nerves

Answer 36

The nerves branch from the vagus nerves and hook around the 6th arch, before “recurring” up to the larynx
On the left, the arch persists as the ductus and ligamentum arteriosus(um), so the RLN is carried in to the thorax
On the right, the 5th and 6th arches regress and the RLN hooks around the 4th arch, i.e. the subclavian artery at the root of the neck, NOT in the thorax