Dr. Shreetal Rajan Nair

Introduction  Human heart starts to develop during the 3 rd week of embryonic life. Till then the needs of the embryo are met through simple diffusion of blood between the germ layers.  Cardiogenesis in humans is associated with complex morphogenetic events

Area of discussion  Anatomic  Molecular  Clinical aspects

Cardiac development  Early development Formation of the trilaminar embryo Origin of cardiogenic cells Formation of bilateral heart fields Formation of the heart tube Folding of the heart tube Looping of the heart tube Cardiac developmental abnormalities

The Beginnings…………(fetal landmarks)  Day 0 : Fertilisation forming zygote initiating embryogenesis  2 cell stage; 4 cell stage; morula  Week 1 : implantation ( as a blastocyst)  Week 2 : bilaminar stage (epiblast,hypoblast)  Week 3 : gastrulation ;primitive streak,notochord and neural plate begin to form  Week 4: heart begins to form.

Week 1 – beginning of development  Day 1-Fertilisation and formation of zygote  Day 2 – 2 cell blastula  Day 3 – 4 cell blastula  Day 4 – morula ( 32 cell stage)  Day 5 – blastocyst ( inner cell mass of embryoblast and outer cell mass called trophoblast)  Day 6 - implantation

1 st week

BLASTOCYST FORMATION

WEEK 2 – FORMATION OF BILAMINAR EMBRYO

Week 3

SUMMARY- Early development  Rule of 2’s for 2 nd week 2 germ layers (bilaminar disk): epiblast, hypoblast. 2 cavities :amniotic cavity,yolk sac  Rule of 3’s for 3 rd week 3 germ layers ( gastrula) : ectoderm,mesoderm and endoderm

 Cardiac Embryogenesis

Sequence of events  Occurs towards the end of the 3 rd week  Day I8 - cardiac precursor cells seen in the form of blood islands  Day 20 - first intraembryonic blood vessels seen  Day 21- Folding, heart tube formation,looping  Day 22 – heart starts to beat, ebb and flow initially  Day 28 – embryonic circulation established

Cardiac development  Early development Formation of trilaminar embryo Origin of cardiogenic cells Formation of bilateral heart fields Formation of the heart tube Folding of the heart tube Looping of the heart tube Cardiac developmental abnormalities

 The developing blood vessels and heart tube can be seen in an embryo at approximately 18 days.  When looking down at this early embryo you can see multiple blood islands dispersed throughout the embryo.

Origin of cardiac precursor cells  The heart primordium arises predominantly from the mesoderm in the cardiogenic region of the trilaminar embryo.

Cardiac precursor cells  First heart field (FHF)  Second heart field (SHF)  Proepicardium  Cardiac neural crest cells

Cardiac precursor cells  FHF  SHF  CNC  Proepicardium

Cardiac development  Early development Origin of cardiogenic cells Formation of bilateral heart fields Formation of the heart tubes Folding of the heart tube Looping of the heart tube Cardiac development abnormalities

The concept of heart fields  Two distinct mesodermal heart fields that share a common origin appear to contribute cells to the developing heart in a temporally and spatially specific manner.  Using special technologies to mark progenitor cells two heart fields (the primary and secondary) have been characterised.

 The heart tube derived from the primary heart field may predominantly provide a scaffold that enables a second population of cells to migrate and expand into cardiac chambers.  These additional cells arise from an area often referred to as the secondary heart field (SHF), or anterior heart field, based on its location anterior and medial to the crescent- shaped primary heart field

HEART FIELDS  SHF cells cross the pharyngeal mesoderm into the anterior and posterior portions, contributing to the formation of the outflow tract, future right ventricle, and atria

Pathophysiology

Cardiac development  Early development Origin of cardiogenic cells Formation of bilateral heart fields Formation of the heart tube Folding of the heart tube Looping of the heart tube Cardiac developmental abnormalities

 The flat germ disk transforms into a tubular structure during the fourth week of development  This is achieved through a process of differential growth causing the embryo to fold in two different dimensions

Formation of the endocardial tube  The heart initially forms from two tubes located bilaterally (on either side) of the trilaminar embryo in the cranial (head)

 This primitive, bilateral heart tubes each contains an inner layer of endocardium, a middle layer of cardiac jelly, and an outer layer of myocardium region

Folding of the embryo  1. Craniocaudal axis due to the more rapid growth of the neural tube forming the brain at its cephalic end. Growth in this direction will cause the embryo to become convex shaped.  2.Lateral folding, causing the two lateral edges of the germ disk to fold forming a tube-like structure

THE CARDIOGENIC AREA Heart Tube

Formation of the endocardial tube  The primitive heart tubes then fuse in the ventral midline to form the linear or straight heart tube in a cranial to caudal direction

 Simultaneously the heart tube shows a series of dilatations. From cranial to caudal these are: Bulbous cordis Ventricle Atrium Sinus venosus

 The arterial trunk will divide to separate the pulmonary and systemic supply.  The bulbus and the ventricle will later differentiate into the right and left ventricle

Arterial end of the heart  Bulbus cordis represents the arterial end of heart. It consists of  proximal part called the conus  a distal part called truncus arteriosus.  The truncus continues distally with the aortic sac.

VENOUS END OF THE HEART TUBE  The sinus venosus represents the venous end of the heart. One vitelline vein from the yolksac; one umbilical vein from the placenta and one common cardinal vein from the bodywall,joins each horn of the sinus venosus.

 After the formation of the head fold, this tube lies dorsal to the pericardial cavity and ventral to the foregut.  Splanchnopleuric mesoderm lining the dorsal side of the pericardial cavity proliferates to forma thick layer called the myoepicardial mantle.

 When the invagination is complete, the myoepicardial mantle completely surrounds the heart tube  It gives rise to the cardiac muscle (MYOCARDIUM) and also to the visceral layer of the pericardium (EPICARDIUM)

EXTERIOR OF THE HEART  Heart tube is suspended from the dorsal wall of the pericardial cavity by 2 layers of pericardium that constitutes dorsal mesocardium

 A hole forms in the dorsal mesocardium which increases in size.  Gradually Mesocardium disappears and the heart tube lies free within the pericardial cavity  Mesocardium disappears to form the transverse sinus of the pericardium

Cardiac development  Early development Origin of cardiogenic cells Formation of bilateral heart fields Formation of the heart tube Folding of the heart tube Looping of the heart tube Cardiac developmental abnormalities

Cardiac looping  Looping of the heart tube allows the straight heart tube to form a more complex structure reminiscent of the adult heart. Most cardiac looping occurs during the fourth week and completes during the fifth week of development

 The linear heart tube develops differential growth of the heart tube in comparison with the foregut  The direction of cardiac looping is determined by an asymmetric signalling system which affects the position of both thoracic and abdominal contents

 In all vertebrates, there is differential growth within the heart tube itself resulting in posterior, leftward, slower growth and anterior, rightward, faster growth resulting in rightward looping. This positioning results in the future right ventricle taking an anterior and rightward location with reference to the future left ventricle

 Further disproportionate growth of the heart tube in comparison to the foregut results in bending of the heart tube at the inflow as well as within the ventricular segment eventually positioning the inflow and future left ventricular segments posteriorly and to the left, with the future right ventricle and outflow segments anteriorly and to the right

 The straight heart tube begins to elongate with simultaneous growth in the bulbus cordis and primitive ventricle  This forces the heart to bend ventrally and rotate to the right, forming a C-shaped loop with convex side situated on the right.  The ventricular bend moves caudally and the distance between the outflow and inflow tracts diminishes.  The atrial and outflow poles converge and myocardial cells are added, forming the truncus arteriosus

 Hence an S-shape is formed with the first bend of the 'S' being the large ventricular bend while the bend at the junction of the atrium and sinus venosus forms the second 'S' bend

The cardiac tube grows at a greater longitudinal rate then the rest of the embryo, causing it to fold. As it does this it falls to the right. This is known as d-looping. It may fall to the left in an l-loop: this will lead to a malformed heart. Below are chick embryo dissections showing the two types of loop. The cardiac tube grows at a greater longitudinal rate then the rest of the embryo, causing it to fold. As it does this it falls to the right. This is known as d-looping. It may fall to the left in an l-loop: this will lead to a malformed heart. Below are chick embryo dissections showing the two types of loop. normal d-loop l-loop

The fold of the loop is principally at the junction of bulbus cordis and ventricle. Note in panel C that the two end up side by side. The left ventricle will develop from the ventricle, and the right ventricle will develop from the bulbus cordis. (And an l-loop will result in ventricular inversion with the left ventricle on the right. Note also that the arterial trunk is above the developing right ventricle.

Time line of cardiogenesis

MOLECULAR ASPECTS IN CARDIOGENESIS  Transcriptional regulators  Epigenetic regulation by microRNAs (miRNA)

ROLE OF RETINOIC ACID  HENSON’S NODE which contains retinoic acid, serves as an embryonic organizer that confers information required to direct the ultimate fate of mesodermal cells during early embryogenesis.  Exogenous retinoic acid is extremely teratogenic at that stage  Greatest effect is on the arterial pole.

Cardiac development  Early development Origin of cardiogenic cells Formation of bilateral heart fields Formation of the heart tubes Folding of the heart tube Looping of the heart tube Cardiac development abnormalities

Pathophysiology  Abnormal left-right signalling  Looping defects  Defects due to abnormal migration of cells of primary and secondary heart fields and of cardiac neural crest

One disease – several mechanisms – several genes

TRANSCRIPTIONAL REGULATORS

ABNORMALITIES OF DEVELOPMENT  From Fertilization to Primitive Heart Tube Abnormal development at this stage of embryogenesis almost always results in embryonic death because of the critical nature of the early circulation to the further growth and development of the embryo and fetus.

DEFECTIVE EXPRESSION OF TRANSCRIPTIONAL FACTORS  Absence of Hand2 (dHAND) – results in RV HYPOPLASIA or ABSENT RV

Abnormalities due to abnormal left-right signalling and dorso ventral polarity  HETEROTAXY SYNDROMES  DORV  DILV

Pathophysiology

 In humans, mirror-image reversal of left-right asymmetry is often associated with normal organ development ( simple dextrocardia or situs inversus totalis) but discordance of thoracic and visceral asymmetry universally results in defective organogenesis, the most common being heterotaxy syndrome.

HETEROTAXY SYNDROMES

Abnormalities of looping  Ventricular Inversion with Transposition of the Great Arteries

 There is currently considerable research in animate models on the genes known to control left-right development. Similarly, congenitally corrected transposition of the great arteries is thought to result from both an abnormality of looping and of outflow tract development.  Many other complex abnormalities involving both ventricles and outflow tract are thought to have at least some abnormality in the looping process.

Defective migration of cells  HYPOPLASIA OF RVOT/MPA - TOF  ABSENCE OF RVOT/MPA- TA  ABSENCE OF AORTO-PULMONARY SEPTUM – TA  MALALINGMENT OF AORTA AND LV – ABNORMAL WEDGING -TOF  ABNORMAL MYOCARDIAL TRABECULATION- LV/RV NON COMPACTION

DEVELOPMENT ABNORMALITIES

BIBLIOGRAPHY 1. Braunwald’s Heart Disease 9 th edition 2. Hurst’s THE HEART 13 th edition, chapter Moss and Adam’s Heart Disease, 7 th edition 4. Harrison’s Text Book of Internal Medicine,18 th edition 5. Embryology and Congenital Heart Disease,Paolo Angelini, MD 6. Cardiac Chamber Formation:Development, Genes, and Evolution ; ANTOON F. M. MOORMAN AND VINCENT M. CHRISTOFFELS; 83: 1223–1267, 2003; /physrev Regulation of myocardial gene expression during heart development Diego Franco,Jorge Domínguez, María del Pilar de Castro, Amelia Aránega Rev Esp Cardiol. 2002;55: Vol. 55 Núm Molecular embryology for an understanding of congenital heart diseases Hiroyuk i Yamagishi et al. Anat Sci Int (2009) 84:88–94 ; Received: 27 May 2008 / Accepted: 16 June 2008 / Published online: 7 April 2009; Japanese Association of Anatomists Cardiovascular Embryology ; R. Abdulla,G. A. Blew,M.J. Holterman ; Pediatr Cardiol 25:191–200, 2004

Bibliography 10.Wherefore heart thou? Embryonic origins of cardiogenic mesoderm; Katherine E. Yutzey,Margaret L. Kirby, Developmental Dynamics, volume 223 ;issue 3;pages 307–320, March 2002