L14 RDA Heart development

Overview

This lecture traces heart development from its tissue origin in the lateral plate mesoderm, through formation and fusion of bilateral endocardial tubes into a single primitive heart tube, folding of that tube into the mature four-chambered shape, generation of left-right asymmetry, and finally septation of the atria, ventricles and outflow tracts. Malformations are covered at each stage: early defects of tube formation/folding, and later septation defects.

Origin: lateral plate mesoderm and the coelom

• The heart, circulatory system and body cavities all derive from lateral plate mesoderm.
• Mesoderm fate is set by position relative to the midline, moving outward: notochord, paraxial mesoderm, intermediate mesoderm, then lateral plate mesoderm (most lateral).
• Lateral plate mesoderm splits into two layers with a space, the coelom, between them: somatic mesoderm (outer, adjacent to ectoderm) and splanchnic mesoderm (inner, adjacent to endoderm).
• Embryonic folding curls this flat trilaminar sheet into a tube. In the folded tube: dorsally lie the neural tube, notochord and intermediate mesoderm; laterally lie the ectoderm (epidermis) and endoderm (gut), with splanchnic mesoderm, the coelom, and somatic mesoderm between them.

Formation of the primitive heart tube

Timeline of early events:

• Day 19: cardiac mesoderm specified; bilateral endocardial tubes form.
• Days 20-21: embryonic folding brings the endocardial tubes together at the midline.
• Day 22: heart begins to beat.
• Day 23: heart begins to fold.
• Day 28: folding complete, septation begins.

Steps:

1. Cardiogenic mesoderm is a horseshoe-shaped “heart-forming area” of cells that gives rise to the heart and great vessels.
2. Interaction between anterior endoderm and splanchnic mesoderm produces bilateral endocardial primordia (tubes), which delaminate from the cardiogenic splanchnic mesoderm.
3. Embryonic folding brings the two cardiac primordia together at the midline, where they fuse into a single tube.
4. The splanchnic mesoderm surrounding the endocardial tube differentiates into myocardium.

Result: a single endocardial tube surrounded by myocardium.

Anatomy of the early heart tube and its folding

• By week 4 the heart beats and blood flows through a paired vascular system:
  • Outflow tracts: a pair of dorsal aortae; four paired aortic arches.
  • Inflow tracts (paired): umbilical vein, vitelline vein, common cardinal vein.
• At 23 days the primitive heart is a straight tube. In order from inflow to outflow: sinus venosus, primitive atrium, ventricle, bulbus cordis (leading to the conotruncus).
• Folding (days 24-28) converts this straight tube into the mature shape:
  • Day 24: the conotruncus and ventricle begin to loop.
  • Day 25: the ventricular loop progresses further into a C-shape (truncus arteriosus and conus cordis now distinguishable).
  • Day 26: folding is largely complete, with the future left and right ventricles lying side by side.
  • Day 28: the mature S-shaped, looped external form is achieved.

Generation of left-right asymmetry

• Embryos are morphologically symmetrical until the heart-folding stages, but molecular evidence of asymmetry exists as early as gastrulation.
• This is shown by asymmetric, left-restricted gene expression (in-situ hybridisation): Lefty-2 (staining at the midline plus one side), Nodal (left-sided staining), and Pitx2 (left-sided staining).
• Mechanism: node cells bear motile cilia. Beating of these cilia establishes a leftward flow of amniotic fluid across the node. This flow is “sensed” by cells and initiates asymmetric gene expression on one side, establishing sidedness.

Early malformations of heart formation

• Failure of specification of cardiac mesoderm -> acardia.
• Failure of fusion of the bilateral endocardial primordia -> acardia.
• Defects of sidedness:
  • Heart tube folding to the right instead of the left -> dextrocardia.
  • Situs inversus.

Septation timeline (weeks 5-10)

• Day 28: septum primum begins to form; muscular ventricular septum begins to form (folding now complete).
• Day 33: atrioventricular valves begin to form (complete by 3 months).
• Day 35: right and left truncoconal ridges begin to form.
• Week 6: definitive atria and auricles are present.
• Day 42: ostium secundum and foramen ovale form as the septum primum meets the septum intermedium.
• Day 46: the muscular ventricular septum ceases to grow.
• Day 56: the coronary sinus is formed.
• Day 63: the semilunar valves are complete.

Atrial septation

1. Septum primum: a thin membrane originating from the superior midline aspect of the common atrium; grows ventro-inferiorly and fuses with the inferior endocardial cushion.
2. Ostium primum: the foramen formed beneath the arching leading edge of the septum primum; it eventually diminishes and is replaced by the ostium secundum.
3. Ostium secundum: perforations that form within the septum primum itself, so the single ostium primum opening is replaced by many small holes that retain cross-atrial flow.
4. Septum secundum: forms alongside the septum primum in a similar way, but as thicker tissue that becomes the definitive interatrial septum.
5. Foramen ovale: the opening within the septum secundum, analogous to the ostium primum but sited in the septum secundum.
6. At birth, the septum primum and septum secundum overlap so that the ostium secundum and foramen ovale together form a one-way flap valve, allowing right-to-left atrial shunting in utero. Final closure of atrial septation only occurs at birth.
7. Atrial septation also brings the superior and inferior endocardial cushions into close proximity, contributing to separation of the atria from the ventricles (see atrioventricular septation).

Atrioventricular septation

• Atrial septation brings the superior and inferior cardiac cushions into close proximity.
• The cushions fuse to form the septum intermedium, which divides the common atrioventricular canal into separate left and right atrioventricular canals.
• Failure of cushion fusion -> persistent (unified) atrioventricular canal.
• Cushion abnormalities are common in Down syndrome (genetic causes overlapping with those of atrial septal defects) and are also associated with maternal diabetes, implicating a role for cardiac neural crest (Kumar et al., 2007, Cardiovasc Diabetol).

Ventricular septation

• The interventricular septum begins to form early, from the floor of the common ventricle; its growth closes the interventricular foramen.
• Its continued growth is coordinated with formation of the outflow tract.

Outflow tract septation

• Begins in week 5, tightly coordinated with completion of ventricular septation; this coordination underlies a large proportion of congenital heart defects.
• The outflow tract is divided by truncoconal septae: swellings that grow from opposite sides of the outflow tract at the junction of the truncus arteriosus and conus cordis.
• These septal cells are of cardiac neural crest origin and migrate ventrally from the hindbrain region.
• The two swellings zip together superiorly and inferiorly to achieve total septation, progressively narrowing the shared lumen into two separate channels.
• By week 9, inferior growth of the truncoconal septum contacts the inferior cardiac cushion/septum intermedium and the ventricular septum, producing (a) separation of the ventricular chambers and (b) separation of the pulmonary and systemic outflow tracts.
• Result: the outflow tracts spiral around one another (an internal H-shaped septum divides the lumen), aligning the future aorta with the left ventricle and the future pulmonary trunk with the right ventricle. Superiorly, septation is complete externally, with the aorta and pulmonary trunk visibly separate and spiralled.

Septation defects and malformations

Atrial septal defects

Two broad categories:

1. Persistent shunting of blood between the atria after birth.
   • Incidence 6 per 10,000; female:male ratio 2:1.
   • Associated with chromosomal abnormalities, including trisomy 21.
   • Common causes: incomplete septum secundum; incomplete closure of ostium primum (a cardiac cushion defect); patent foramen ovale; Holt-Oram (heart-hand) syndrome (TBX5 mutations).
   • Genes implicated: TBX5, GATA4, NKX2.5. TBX5 mutations prevent septum primum progenitor cells from forming.
2. Failure of shunting of blood between the atria before birth.
   • Causes: premature closure of the foramen ovale; ostium secundum defect (failure of the cell death that normally creates the “holes”).
   • Consequence: prevents further development of the left side of the heart.
   • 2nd trimester onset -> hypoplastic heart (leads to death).
   • 3rd trimester onset -> fetal heart failure and arrhythmia (leads to death).

Atrioventricular cushion defects and atrial septal defects share overlapping genetic causes (TBX5, GATA4, NKX2.5) and both cluster with trisomy 21 and maternal diabetes.

Ventricular and outflow (conotruncal) septal defects

1. Ventricular septal defect (generic): involves the membranous (not muscular) portion of the septum; the most common cardiac malformation (12 per 10,000); often associated with other conotruncal malformations.
2. Persistent truncus arteriosus: the conotruncal ridges fail to fuse and descend toward the ventricle, so the outflow tracts are not separated and ventricular septation is incomplete.
3. Transposition of the great vessels: the truncoconal septae fail to spiral as they form, so the right ventricle connects to the aorta and the left ventricle connects to the pulmonary artery (the vessels arise in parallel rather than crossing).
4. Tetralogy of Fallot, four co-occurring features:
   1. Pulmonary stenosis: anterior displacement of the conotruncal septum narrows the right ventricular outflow tract.
   2. Ventricular septal defect.
   3. Overriding aorta.
   4. Right ventricular hypertrophy.

Genes implicated in conotruncal septation malformations include GATA6, TBX1, TBX5 and NKX6.5, though the precise mechanism is unclear.

Self-test

1. Define the coelom and name the two layers of lateral plate mesoderm that border it.
2. Describe the steps, in order, by which bilateral cardiac primordia become a single primitive heart tube surrounded by myocardium.
3. List the chambers of the straight primitive heart tube in order from inflow to outflow.
4. Describe how the straight heart tube folds into its mature external shape between days 24 and 28.
5. Explain the mechanism by which nodal cilia generate left-right asymmetry.
6. Distinguish dextrocardia from situs inversus as outcomes of sidedness defects.
7. Describe the steps of atrial septation from formation of the septum primum to the one-way flap valve present at birth.
8. Distinguish the septum primum from the septum secundum, and the ostium primum from the ostium secundum.
9. Describe how atrioventricular septation occurs and what results if the cushions fail to fuse.
10. What is the incidence and sex ratio of persistent atrial septal shunting after birth, and what are its common causes?
11. Predict the consequence for the fetal heart if the foramen ovale closes prematurely in the 2nd trimester versus the 3rd trimester.
12. Describe how outflow tract (truncoconal) septation occurs, including the embryological origin of the septal cells.
13. Distinguish persistent truncus arteriosus from transposition of the great vessels in terms of the underlying defect.
14. List the four features of Tetralogy of Fallot.
15. A neonate presents with a ventricular septal defect, an overriding aorta, right ventricular hypertrophy and a narrowed right ventricular outflow tract. What diagnosis fits, and what is the primary embryological defect?
16. Explain how a single failure of cardiac neural crest cell migration to the outflow tract could produce both a ventricular septal defect and a great-vessel malformation.

Answers

Reveal answers

1. The coelom is the space between the somatic and splanchnic layers of lateral plate mesoderm; somatic mesoderm lies outer (adjacent to ectoderm), splanchnic mesoderm lies inner (adjacent to endoderm).
2. Cardiogenic (splanchnic) mesoderm interacts with anterior endoderm to form bilateral endocardial primordia, which delaminate; embryonic folding brings these together at the midline where they fuse into a single tube; the surrounding splanchnic mesoderm then differentiates into myocardium, giving a single endocardial tube surrounded by myocardium.
3. Sinus venosus, primitive atrium, ventricle, bulbus cordis (leading to the conotruncus).
4. At day 24 the conotruncus and ventricle begin to loop; by day 25 the loop has progressed into a C-shape; by day 26 folding is largely complete with the future ventricles side by side; by day 28 the mature S-shaped external form is achieved.
5. Node cells bear motile cilia whose beating generates a leftward flow of amniotic fluid across the node; this flow is sensed by cells and triggers asymmetric gene expression on one side, establishing left-right sidedness.
6. Dextrocardia is the heart tube folding to the right instead of the left; situs inversus is a mirror-image reversal of sidedness. Both are defects in generation of sidedness (the transcript does not elaborate further on their distinction beyond listing them as separate sidedness defects).
7. The septum primum grows ventro-inferiorly from the roof of the common atrium and fuses with the inferior endocardial cushion, leaving the ostium primum as a gap beneath it; the ostium primum is replaced by many small perforations (ostium secundum) in the septum primum; the thicker septum secundum then grows alongside it, with its own opening (foramen ovale); at birth the two septa overlap so the ostium secundum and foramen ovale together act as a one-way flap valve.
8. The septum primum is the first, thin membrane to form; the septum secundum is a second, thicker membrane that forms similarly and becomes the definitive interatrial septum. The ostium primum is the transient gap beneath the septum primum’s leading edge; the ostium secundum is the set of small perforations that later form within the septum primum itself.
9. Atrial septation brings the superior and inferior cardiac cushions together; they fuse to form the septum intermedium, dividing the atrioventricular canal into left and right canals. Failure of fusion causes a persistent, unified atrioventricular canal.
10. Incidence is 6 per 10,000, with a female:male ratio of 2:1. Common causes: incomplete septum secundum, incomplete closure of the ostium primum (cardiac cushion defect), patent foramen ovale, and Holt-Oram syndrome (TBX5 mutations); it is also associated with chromosomal abnormalities including trisomy 21.
11. Premature closure of the foramen ovale prevents further development of the left side of the heart. If this occurs in the 2nd trimester it leads to a hypoplastic heart and death; if it occurs in the 3rd trimester it leads to fetal heart failure and arrhythmia, and death.
12. Truncoconal septae, swellings arising from opposite sides of the outflow tract at the truncus arteriosus/conus cordis junction, are formed by cardiac neural crest cells that migrate ventrally from the hindbrain. The swellings zip together superiorly and inferiorly, narrowing the shared lumen into two separate, spiralled channels that align each outflow tract with its correct ventricle.
13. In persistent truncus arteriosus, the conotruncal ridges fail to fuse and descend, so the outflow tracts remain unseparated. In transposition of the great vessels, the truncoconal septae form but fail to spiral, so the great vessels arise in parallel with the right ventricle connected to the aorta and the left ventricle to the pulmonary artery.
14. Pulmonary stenosis, ventricular septal defect, overriding aorta, and right ventricular hypertrophy.
15. Tetralogy of Fallot; the primary defect is anterior displacement of the conotruncal septum, which narrows the right ventricular outflow tract and produces the other three features as consequences.
16. Since the truncoconal septae are formed from migrating cardiac neural crest cells, a failure of this migration would leave the outflow tract septum incomplete or unspiralled; the same defect could therefore both fail to close the ventricular septum (ventricular septal defect) and fail to correctly spiral/position the great vessels (e.g. persistent truncus arteriosus or transposition), since both processes depend on the same neural-crest-derived septal tissue.