Development of the heart Dr Rania Gabr

oBJECTIVES Describe the formation and position of the heart tube. Explain the mechanism of formation of the cardiac loop. Discuss the development of sinus venosus. Explain how cardiac septa are formed. Describe the septum formation in the common atrium. Discuss the septum formation in the Atrioventricular canal. Discuss the septum formation in the Truncus arteriosus and Bulbus Cordis Describe the septum formation in the ventricles.

Angiogenesis The vascular system as well as the blood elements are Mesodermal in origin. The splanchnic mesodermal cells proliferate and form cell clusters called “angiogenic clusters” or “blood islands” which lie in front and on either side of the anterior part of the embryonic disc.

Formation of the Cardiogenic field Clusters of angiogenetic cells form a "horseshoe-shaped" cluster anterior and lateral to the brain plate.

Origin of the heart tube: Mesoderm of the cardiogenic plate at the 3rd week.

Development of the heart tube

- The clusters form endothelial vessels which fuse to form the right and left endocardial heart tubes - As a result of folding of embryo in transverse direction the two endocardial heart tubes come close to each other and fuse to form a single endocardial tube

Development: It starts by the formation of 2 heart tubes each has a cranial end and a caudal end. The newly formed heart tube(1tube) has an arterial end (cranial end) connected to the dorsal aorta and

3-common cardinal veins - A caudal end (venous end) (embedded in the septum transversum) receives 3 pairs of veins, they are: 1-umbilical 2-vitelline and 3-common cardinal veins

Lateral folding Lateral folding results in fusion of the caudal portion of the paired endocardial tubes

Lateral body folding occurs as well as head folding. - The heart tube bulges into the dorsal surface of the pericardial cavity - It is suspended by dorsal& ventral mesocardium which disappear Pericardial cavity Dorsal mesocardium

The heart tube continues to elongate forming the cardiac loop. -The cardiac loop invaginates in the pericardial cavity. -Hence the heart is covered by 2 layers of the pericardium: visceral layer internally and parietal one externally and in between lies the pericardial cavity. hhhhhhhhhhh llkj bbb

2 constrictions appear in this tube dividing the heart tube into 3 chambers externally, those chambers are: Bulbus cordis cranially Ventricle caudal to it, Atrium caudal to the ventricle. - The grooves from above downwards are: 1-Bulbo-ventricular groove. 2- Ventriculo-atrial groove.

The bulbus cordis is elongated and form the truncus arteriosus. - At the venous end another groove appears and separates the atrial part from the sinus venosus. This groove is called 3-sino-atrial groove..

Parts of the Heart Tube Chambers of the heart tube: Three grooves are formed in the tube will form four chambers: Sinus venosus, Primitive atrium, Primitive ventricle and Bulbus cordis. 1st aortic arch

---“S”-shaped heart: The heart tube continues to grow and bend, atrium shifts in the dorso-cranial direction; sinus venousus located at caudal portion of atrium.

Development of the sinus venosus The sinus venosus receives venous blood from the right and the left horns. Again each horn receives 3 veins: umbilical-vitelline and common cardinal veins. The sinus is widely connected to the primitive atrium. through the Sino-atrial orifice. jjjjjj

Venous blood drainage shifts from left to the right side. The communication between the sinus and the atrium is shifted to the right. The right umbilical vein and the left vitelline vein are obliterated during 5th week Left common cardinal vein is obliterated at 10th week. The left sinus horn regresses, and what remains only oblique vein of left atrium and coronary sinus. Right sinus horn is enlarged and incorporated into the right atrium, to form the smooth- walled part of right atrium.

Septum Formation in Common Atrium Formation of septum primum- sickle shaped crest from the roof of common atrium Ostium primum- opening between the lower rim of septum primum and endocardial cushion End of Fourth Week

An extension is growing from the endocardial cushions, closing the ostium primum, but before that, a new opening appears in the upper portion of septum primum, that is ostium secundum. Later, a new septum develops to the right of septum primum, that is septum secundum, which is a crescent shaped , incomplete partition, with an opening inferiorly, that is foramen ovale.

---after birth, the two septa fuse to separate the atrium completely ---before birth, blood can flow from the right atrium towards the left atrium ---after birth, the two septa fuse to separate the atrium completely

In 25% of normal population, the foramen ovale remains ‘probe patent’.

Further Differentiation of the Atria Development of the right atrium: - It is formed of 2 parts: Smooth part, which is formed from absorption of the right horn of sinus venosus to be a part of the right atrium. ( sinus venarum) 2. Rough part, originated from the primitive atrial chamber of the heart tube.

Development of the left atrium: It is formed of 2 parts: 1.Smooth part, which is formed from absorption of the pulmonary vein to be a part of the left atrium. 2.Rough part, originated from the primitive atrial chamber of the heart tube.

After birth foramen ovale is closed by: 1- A decrease in Rt. atrial pressure due to occlusion of placental circulation 2- An increase in Lt. atrial pressure due to increased pulmonary venous return.

Septum primum is pushed and fuses with septum secundum to form interatrial septum Lower margin of septum secundum forms annulus ovalis - Septum primum forms floor of fossa ovalis

Septum in the atrio-ventricular canal Superior and inferior endocardial cushions fuse , to form a partition dividing the canal into right and left atrioventricular canals. Two lateral cushions appear on the right and left.

Each atrioventricular orifice is surrounded by local proliferations of mesenchymal tissue, these form the valves, under which the tissues are vacuolated, but the valves remain attached to the ventricular walls by cords of the papillary muscles.

Division of Atrioventricular canal day 30 embryo day 35 embryo

Septum in the truncus arteriosus and Bulbus cordis During 5th week, two opposing ridges appear in the truncus, which are also called truncus swellings or cushions, one superior and to the right and the other is inferior and to the left. These swellings grow distally in a spiral way, or twisting around each other. Fusion of the truncus swellings results in formation of aorticopulmonary septum, which divides the truncus into aortic and pulmonary channels.

Semilunar valves After formation of truncus septum (aorticopulmonary septum), three small tubercles or swellings appear in each of pulmonary and aortic channels. The tubercles hollow out at their upper surfaces, forming Semilunar valves. Neural crest cells contribute to these tubercles.

Formation of the Semilunar Valves

Formation of the muscular interventricular septum During 4th week muscular part of interventricular septum extends upwards from the floor of the primitive ventricle - Interventricular foramen is a space between the upper part of the interventricular septum and the septum intermedium Septum intermedium

Septum Formation in the Ventricles

Septum Formation in the Ventricles Muscular part of IV septum Formed by apposition and gradual fusion of medial walls of expanding ventricles due to continuous growth of myocardium outside and trabecula formation inside Membranous part of IV septum formed from: Inferior endocardial cushion Right bulbus swelling L bulbus swelling

Development of valves: I. Mitral & tricuspid valves: - Atrioventricular cushions appear at the atrioventricular orifice. These cushions are, ventral& dorsal cushions. - The 2 cushions grow towards each other, till they meet & fuse together in the middle of the orifice. - As a result of this fusion, the single atrioventricular orifice will divide into two (right & left) orifices. - The cushions will form the cusps of mitral & tricuspid orifices.

Pulmonary & aortic valves: 4 bulboventricular cushions appear at bulboventricular orifice. These cushions are 2 (right & left) major cushions& (dorsal & ventral) minor cushions. The major cushions grow to meet each other in the middle of the orifice. Thus the single Bulboventricular orifice will divide into pulmonary & aortic orifices. The cushions excavate from their cranial surface forming semilunar cusps of aortic & pulmonary valves. After rotation the pulmonary orifice will have 2 anterior cusps & one Posterior. While aortic orifice will have 2 posterior cusps and one anterior

Components of the two ventricles Right ventricle develops from: 1- Right part of primitive ventricle forms the rough trabeculated part 2- Right part of bulbus cordis forms the smooth part (infundibulum) Left ventricle develops from: 1- Left part of primitive ventricle forms the rough trabeculated part 2- Left part of bulbus cordis forms the smooth part (aortic vestibule)

Formation of the atrioventricular valves and chordae tendinae Atrioventricular canal is divided by anterior & posterior endocardial cushions Each cushion is thinned by blood stream forming tricuspid & mitral valves but remain attached to wall of ventricle by muscular cord -muscular cord degenerates and replaced by connective tissue forming chordae tendinae

Fate of 5 dilatations of the heart tube Adult Structure Embryonic Dilatation Proximal part of ascending aorta Proximal part of pulmonary trunk Truncus Arteriosus Smooth part of Rt. Ventricle (Infundibulum) Smooth part of Lt. Ventricle (Aortic vestibule) Bulbus Cordis Trabeculated part of right ventricle Trabeculated part of left ventricle Primitive Ventricle Trabeculated part of right atrium Trabeculated part of left atrium Primitive Atrium Smooth part of Rt. Atrium (sinus venarum) Lower part of SVC Coronary sinus Oblique vein of left atrium Sinus Venosus

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