EMBRYOLOGY - DEVELOPMENT OF

CONOTRUNCAL REGION

Dr Julian Johny Thottian

Introduction

The Conus- also known as Infundibulum (Keith 1909 ) The Conotruncus comprises collectively two myocardial sub segments, the Bulbus cordis and the Truncus arteriosus

BULBUS CORDIS –refers to the ventricular outflow tract TRUNCUS ARTERIOSUS- embryologic precursor of great

arteries.

D A Goor etal

EARLY HEART

Definitions

Conotruncus -The conotruncus is the outflow region of the developing heart. It consists of: Conus cordis and Truncus Arteriosis. Conus- Inferior to the aortic and pulmonary valves . Truncus- Superior to the valves that is continuous with the ventral aorta (aortic sac).

Bulbus cordis- also known as the conotruncus lies ventral to primitive ventricle.

Together with primitive ventricle it forms the ventricle of the formed heart. Keith (1909)

The developing two main truncal cushions and the underlying two conal cushions are perfectly aligned and no line of demarcation between the two is identifiable in mammals

(Van Mierop and Patterson, 1980)

The net result is….

Proper alignment of the outlet septum with ventricular trabecular septum with membraneous septum in between.

Proper posterior alignment of left outflow tract with left ventricle and aorto mitral continuity.

Mechanism – differential growth and apoptosis.

Secondary heart field

Area of the ventral pharyngeal mesoderm (Kelly et al, 2001; Mjaadvedt et al, 2001; Waldo et al, 2001) - Pre cardiac Splanchnic Mesodermic -region providing myocardial precursor cells, which migrate to the Out flow Tract area of the developing cardiac tube, where they build up the Conotruncal myocardium as well as smooth muscle cells joining the caudal portion of the aortic sac (Waldo et al, 2005).

The topography of the SHF.

Rochais F et al. Circulation Research 2009;104:933-942

Copyright © American Heart Association

Molecular aspect

SHF expressed NKx2.5 and Gata4 transcription

factors.NKx2.5- and Gata4 SHF-committed cells join

and incorporate themselves into the outflow tract of the primary heart tube, these cells undergo terminal myocardial differentiation under the induction of the local primary myocardial Bmp2 factor (Waldo et al 2001)

Wnt, fibroblast growth factor, bone morphogenetic protein, Hedgehog, and retinoic acid are all involved in signalling.

SHF contributes to the outflow tract (OFT), right ventricle, and inflow region

Illustration showing the core features of the Wnt, Fgf, Bmp, Hh, and Notch signaling pathways.

Rochais F et al. Circulation Research 2009;104:933-942

Copyright © American Heart Association

NEURAL CREST CELLS

Crest develops from - Dorsal neural tube. It overlaps the vagal neural crest and migrates to populate the pharyngeal arches 3, 4 and 6 (producing structures in the head) and to the heart, forming connective tissue that separates the great vessels of the heart.

Other Migration Locations: Pharyngeal arches and Truncus arteriosus , aorticopulmonary septum and the smooth muscle of great arteries.

Anterior of the aorta to become the four pre-aortic ganglia (celiac ganglion, superior mesenteric ganglion, inferior mesenteric ganglion and aortical renal ganglia)

Role of neural crest cells

Neural crest cells modulate the SHF cells. It plays a role in elongation of the OFT.

Ablation of these cells cause failure of migration of SHF cells to conotruncus. (Kelly et al 2002)

They provide the cells for entire conotruncal septum

Cardiac neural crest cells

Sequential events related to conotruncal septum

Embryo – Aged 18-22 days

Explanation

The heart tube is convoluted to forms five straight segments (limbs), and in-between them, four curves. The proximal segment of the heart tube (starting at the venous end) is the A-V

canal. It is oriented posteroanteriorly. First curve or proximal bend- the heart tube makes a 90 degree turn toward the right to become the proximal transverse limb, or the interventricular foramen.

Curve 2 the heart tube makes a 90° turn cephalically to become the ascending limb .

Curve 3 the heart tube turns in 90degree medially to form the distal transverse limb.

Curve 4 the heart tube turns in 90degree toward the back of the embryo to form the terminal limb, which is cephalad and parallel to the A-V canal.

Each curve has a Greater and a Lesser curvature. The Lesser curvature of curve 2 is the Conoventricular Flange

Conotruncus and ostium bulbi

Border between meta ampulla and conus is the Ostium bulbi, or the conoventricular

junction. On the right the Ostium bulbi is the

transition from the trabeculated ventricular

endocardium to the smooth conal endocardium. On the left the Ostium bulbi is lower edge of conoventricular flange.

Shift or rotation of ostium bulbi

Ostium bulbi shifts toward the left to cephalically and override the IVF. This critical process provides the conus with an access to the left ventricle.

Truncal and conal cushions

Conotruncal ridges

The conotruncal ridges are arranged in a spiral course, like riflings of a gun barrel.

Two main opposing dextrosuperior and sinistroinferior truncal endocardial cushions appear.

Occupying respectively a dorsal and a ventral oblique position, these cushions extend from the junction between the aortic sac and the truncus arteriosus down to the beginning of the conus, where they align with the dextrodorsal and sinistroventral conal cushions, respectively

(Van Mierop and Patterson, 1980).

Aortic truncus

Pulmonary truncus

Truncal rotation The truncus rotates about 90-110° in a

counterclockwise direction. This counterclockwise rotation (torsion) of the truncus, which follows the earlier counterclockwise rotation of the ostium bulbi, unwinds the coiled course of the conotruncal ridges . As a result, the aortic truncus is transferred to the same side as the aortic conus (left side) and the aortic and pulmonary trunks become coiled, this situation is seen in the definitive heart.

Absorption of conus

Marked shortening of the conus and the equivalent lengthening of the aorta and pulmonary arteries.

The aortic conotruncus is reduced in length from 700 to 400 microns

The length of the pulmonary conotruncus is reduced from 880 microns to 600 microns

D A Goor et al

Absorption of the bilateral proximal conuses brought the distal conus septum toward the ventricular septum, and absorption of the distal aortic conus accounts for the fibrous continuity between the aortic and mitral valves.

The truncus is continuous distally with the aortic sac (ventral aorta) which is devoid of endocardial cushions.

At the same time, the septum aortopulmonale grows from the dorsal wall of the aortic sac toward the truncal septum to fuse with it. As a result of the fusion of these two septa the aortic sac is divided into the ascending aorta and the pulmonary artery .

Muscular elements arising from the right ventricle invade the conus septum. Once the conus septum is muscularized it

receives the anatomic appearance of the crista supraventricularis.

Crista supraventricularis

Summarize

Effect of conus absorption- 1. “Migration“ of the distal conus septum

toward the heart where it assumes its definitive position in the interventricular septum

2. Additional absorption of the distal aortic conus accounts for the fibrous continuity seen in the mature heart between the aortic and mitral valves

Inversion of conotruncus- 2 stagesStage1 – Inversion of ostium bulbi at same time of

loopingStage 2- Rotation of truncus which occurs after the

formation of septum aortopulmonale. Ostium bulbar rotation causes the anatomic

concordance between the left ventricle and the proximal aortic conus

Truncal torsion in similar manner and bring the semilunar valves to the same sides as their proximal conuses and unwinding the spiral course of the conotruncal ridges.

Basis for conotruncal defects

Ward et al. (2005) and Ward and Kirby (2006) emphasize that a short outflow tract,

through SHF ablation and through experimental NC ablation and with consequent low SHF cellular output to the conotruncal region, does not allow a normal conotruncal rotation.

PTA, tetralogy of Fallot (TF), pulmonary atresia with ventricular septal defect (VSD), and double-outlet right ventricle (DORV) as a consequence of the primary short conotruncal morphology.

Contd…

Myocardialisation of the ridges gives a zippering effect resulting in fusion.

Fusion occurs in a distal to proximal direction during the sixth week, allowing for cleavage of the aorta and pulmonary trunk. The spiralling nature of the ridges causes the pulmonary trunk to twist around the aorta.

Formation of the Aorta and Pulmonary Artery

Truncal septation and development of arterial valves

The external wall of the truncal myocardium creates some sort of rim known as myocardial cuff, which appears to cover the root of the aortic sac (Thompson and Fitzharris, 1979)

Dextrosuperior and sinistroinferior truncal endocardial cushions extend from the junction between the aortic sac and the truncus arteriosus down to the beginning of the conus, where they align with the dextrodorsal and sinistroventral conal cushions, respectively (Van Mierop and Patterson, 1980).

FORMATION OF SEMILUNAR VALVES

The mesenchymal truncal septum undergoes a complex differentiation process leading to the formation of the right and left pulmonary valve cusps and of the right coronary and left coronary aortic valve cusps.

Two additional intercalated truncal endocardial swellings appear to occupy a parietal position on the right and on the left side of the truncus arteriosus. After the normal counterclockwise conotruncal rotation, the right intercalated cushion becomes the posterior noncoronary aortic valve cusp and the left intercalated cushion becomes the anterior pulmonary valve cusp.

Semilunar Valve Formation

Development of sinus of valsalva

Caudal elongation of the aortic sac, concomitantly with the downward retraction

of the truncus arteriosus, is the one that allows the development of the intrapericardial portions of the great arteries and of the arterial walls of the sinuses of Valsalva

MOLECULAR ASPECT

Sox4 and NF-Atc transcription factors -mainly involved in this developmental phase

Sox4 regulates the normal development and fusion of the truncal endocardial cushions. [Schilham et al. (1996) and Ya et al. (1998)]

Targeted disruption of the NF-Atc gene produced absence of both arterial valves. Ranger et al. (1998)

Conotruncal anomalies

TOF DORV TGA PTA

TOF Conal septum deviates anteriorly- faulty partition

of conotruncal septum. Abnormal conal rotation takes place Mal rotation of trunco-bulbar ridges causes

misalignment of septum and straddling of aorta over VSD

Another mechanism-hypoplasia and under development of the pulmonary infundibulum causes infundibular hypoplasia.

Van Praagh et al {Amj Card 1970;26:25-33}

Truncus arteriosus

Due to incomplete or failed septation of the embryonic truncus arteriosus. Aortopulmonary and interventricular defects are believed to represent an abnormality of conotruncal septation.

Van Praagh {Amj Card 1965 ;16;406-425}

TGA

Arrest of both proximal & distal conal rotation lead to the transposition group of diseases, in which the aorta is dextroposed on the right side of the pulmonary artery & has no continuity with left ventricle

D A Goor et al

Contd..

Faulty absorption of the conus Absent leftward shift of the

conoventricular junction account for the variability of

transposition

DORV

Impaired morphogenesis of either the outflow portion (conotruncus) or the conoventricular flange

Abnormal connection between the muscular ventricular septum and the conus septum

And hence sub aortic flow path from right ventricle

Originally both vessels arise from RV and if no conoventricular shifting occurs then DORV

(Manner et al , Thorac cardiovasc surg 1995 )

THANK YOU

Book references

Moss & Adams` Heart Disease in infants, children and adolescents. Vol 2 Development of conotruncus 891-892, 906,911,1040,1102

Avery Diseases of newborn. William Tauesch. Development of conotruncus. Pg 972

Langman`s essential embryology. Thomas W Sadler Conotruncal inversions Pg 49

Clinical recognition of congenital heart disease by Joseph Perloff. Diagnosis of conotruncal anomalies

Article references

1. DA Goor and Walton Lillihei Circulation 1972;46:375-384

2. Francesca Rochais et al Circulation 2009;104 :933-942

3. Angelo Restivo et al THE ANATOMICAL RECORD PART A 288A:936–943 (2006)

4. Keith A. 1909. The Hunterian lectures on malformations of the heart. Lancet 12:359–364.

5. Mjaadvedt CH et al 2001. The outflow tract of the heart is recruited from a novel heart forming field. Dev Biol 238:97–109.

6. Waldo K et al 2001. Conotruncal myocardium arises from a secondary heart field. Development 128:3179–3188.

7. Ward C, Kirby ML. 2006. The secondary heart field: understanding conotruncal defects from a developmental perspective. Curr Cardiol Rev 2:5–20.

8. Van Praagh {Amj Card 1965 ;16;406-425} 9. Van Mierop LHS et al 1978. Pathogenesis of

persistent truncus arteriosus in light of observations made in a dog embryo with the anomaly. Am J Cardiol 41:755–762.

QUIZ

QUESTION 1

SHF CELLS ARE DERIVED FROM1. Pharyngeal ectoderm2. Pharyngeal mesoderm3. Pharyngeal endoderm4. Dorsal pericardial wall

QUESTION 2

NO: OF TRUNCAL AND CONAL CUSHIONS1. 2 & 42. 4 & 23. 3 & 44. 4 & 3

QUESTION 3

MAIN SIGNALING PATHWAYS IN OUTFLOW TRACT DEVELOPMENT INCLUDE ALL EXCEPT.

1. WNT 2. FGF 3. SHH4. NOTCH5. TBX5

QUESTION 4

NUMBER OF ROTATIONS THAT TAKE PLACE IN THE CONOTRUNCAL REGION

A . 3B . 4C . 2D . 1

QUESTION 5

SHF CONTRIBUTES TO ALL EXCEPT1. RV2. LV3. OUTFLOW TRACT4. INFLOW REGION

QUESTION 6

WHICH GENE IS INVOLVED IN TRUNCAL VALVE FORMATION

1. Sox 42. Tbx53. Fgf 104. Shh

QUESTION 7

SEPTUM AORTOPULMONALE IS DERIVED FROM

1. Shf & Ncc2. Fhf3. Septum transversus4. Dorsal pericardial wall

QUESTION 8

CATCH 22 INCLUDES ALL EXCEPT1. Cardiac anomalies2. Hyperparathyroidism3. Thymic hypolasia4. Cleft palate 5. Abnormal facies

QUESTION 9

DEVELOPMENT OF THE CONOTRUNCUS OCCURS FROM

1. 2-4 weeks 2. 5-7 weeks3. 7-9 weeks4. 12-14 weeks

QUESTION 10

WHICH IS NOT A NORMAL DEVELOPMENTAL PROCESS

1. AV shift to right2. Ostio bulbar shift to left3. Clockwise rotation of truncus4. Counter clockwise rotation of ostium

bulbi