Development of the Heart Flashcards
Heart Defects - Causes
genetic and teratogenic
- Effects depend on timing of exposure
- Defects will often be multifactorial
Development of the heart
- 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
disc transformation
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
The Heart Tube
forms in the mesoderm from angiogenetic clusters, as a horseshoe at the cephalic end of the trilaminar disc
As the embryo grows diffusion…
is no longer sufficient to provide nutrients and development of both blood and vascular structures becomes necessary
beginning of heart development
By day 18
Development of the heart begins as
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
The trilaminar disc folds in 2 directions
cephalic-caudal and lateral folding which starts towards the end of week 3 e.g 18 days
Folding
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
what does cephalo-caudal folding cause
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
what are the three layers involved in heart development
epicardium - visceral pericardium
myocardium - cardiac muscle
endocardium - endothelial lining
when does the looping and folding occur
starts early in week 4 at about 22 days
what occurs during the looping and folding
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.
Dextrocardia
If the “ends” fold to the left, the developing heart is pushed to the right
Heart of a 5mm embryo (28days)
The developing heart pushes in to the pericardial sac; the ventricles begin to trabeculate
In Summary
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
where are endocardial cushions derived from
neural crest cells
enlargement of endocardial cushions forms
- interatrial septum
- the membranous part of the interventricular septum
- AV valves (tricuspid and mitral)
- formation of the PT and Aorta from the truncus arteriosus
The tissue in the narrowing between the single atrium and the single ventricle swells to form
endocardial cushions
that grow to meet in the middle and divide the atrioventricular canal into right (tricuspid) and left (mitral) channels
Formation of the interatrial septum (summary)
- 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
Atrial Septal Defect ASD
- more common in females
Patent Foramen Ovale (PFO)
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
when is the formation of the inter-ventricular septum completed by
the end of the 7th week
what are the four elements involved in the formation of the interventricular septum
- 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
what do the left and right truncates ridges spiral and fuse to form
conotruncal septum
The interventricular foramen is closed by
the membranous part of the interventricular septum, derived from the truncal ridges and the inferior atrio-ventricular cushion
Ventriculo-Septal Defects (VSD)
Most common congenital cardiac malformation
More common in males
Fallot’s tetralogy
Truncal septum deviates right and does not meet the interventricular septum
Atrial Growth
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
Ventricular Growth
The two ventricles “balloon” to grow around and away from the muscular septum dividing them
Formation of the Valves
Mitral and Tricuspid, endocardial cushion growth and cavitation to form papillary muscles and chordae tendineae
Formation of the Great Vessels
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
more on the development of the great vessels
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.
The right and left pulmonary arteries grow within
the developing lungs and connect to the 6th arches that are forming the pulmonary trunk
Coarctation of Aorta
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
Recurrent Laryngeal Nerves
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
