ANA 206 Development of the Heart

Question 1

What is gastrulation?

Answer 1

It is the formation of three germ layers —endoderm, ectoderm and mesoderm from the epiblast.

•Happens during the 3rd Week.

Question 2

When the embryoblast differentiates into two layers, what are the layers and what is it called?

Answer 2

•Superficial layer consisting of flat cells —epiblast
•Deep layer consisting of columnar cells —hypoblast.

It is called the bilaminar embryonic disc

Question 3

Describe the formation of the 3 germ layers

Answer 3

the epiblast forms a linear thickening in the midline of embryonic disc called primitive streak.
Epiblast cells in the region of the primitive streak become flask-shaped, detach themselves from the epiblast, and slip beneath it

The flask-shaped cells of epiblast move inward (invaginate) towards the hypoblast, some of which replace the hypoblast cells to form the endoderm

•Others come to lie between the epiblast and the newly formed endoderm to form the mesoderm.

•While the remaining cells of the epiblast will form the ectoderm.

•This stage is referred to as trilaminar embryonic disc.

Question 4

What does the ectoderm form?

Answer 4

Epidermis of the skin and it’s appendages
Epithelial lining of:
Lower part of anal canal
Distal Part of male urethra
Lower part of vagina
External auditory meatus
Oral cavity
Nasal cavity

Lens of the eye
Enamel of teeth
Adenohypophysis of the pituitary gland
Adrenal medulla
Nervous tissue and sense organs (CNS & PNS)

Question 5

What does the endoderm form?

Answer 5

Epithelial lining of the GIT
Respiratory tract
Urinary tract
Biliary tract
Auditory tube and middle ear cavity
Uterus and upper part of vagina

Liver(hepatocytes)
Pancreas (acinar and islet cells)
Thyroid (follicular cells)
Parathyroid ( principle and oxyphil cells)

Question 6

When does the CVS develop in the developing embryo?

Answer 6

At 3 weeks

Question 7

What does the mesoderm form?

Answer 7

Muscles: smooth, cardiac, and
skeletal
• Bones and cartilages
• Connective tissue
Heart, blood vessels, and lymph vessels
• Epithelial lining of blood vessels, lymph
vessels, body cavities, and joint cavities
• Spleen
• Kidney and ureters
• Adrenal cortex
• Testes and ovaries

Question 8

The 3 layers of the heart

Answer 8

endocardium, myocardium & epicardium

Question 9

From their cranial to caudal ends, what are the heart tube dilatations?

Answer 9

1.Truncus arteriosus
2.Bulbus cordis
3.Primitive ventricle
4.Primitive atrium
5.Sinus venosus.

Question 10

Describe the arterial end of the primitive heart tube

Answer 10

The truncus arteriosus represents the arterial end of the heart (vide supra).

•Cranially it is continuous with aortic sac having right and left horns.

•From each horn of aortic sac, the first pharyngeal arch artery arises and passes backwards on the lateral side of the foregut to become continuous with the respective dorsal aorta.

Question 11

Describe the venous end of the primitive heart tube

Answer 11

The sinus venosus represents the venous end of the heart (vide supra).

•Each horn of sinus venosus receives 3 primitive veins:
•Vitelline vein from the yolk sac,
•Umbilical vein from the placenta, and
•Common cardinal vein from the body wall

Question 12

Truncus arteriosus develops into_______

Answer 12

Ascending aorta
Pulmonary trunk

Question 13

Bulbus cordis develops into_______

Answer 13

Smooth upper part of the right ventricle
(conus arteriosus)
Smooth upper part of the left ventricle
(aortic vestibule)

Question 14

Primitive ventricle develops into_______

Answer 14

Trabeculated part of the right ventricle
Trabeculated part of the left ventricle

Question 15

Primitive atrium develops into_______

Answer 15

Trabeculated part of the right atrium
Trabeculated part of the left atrium

Question 16

Sinus venosus develops into_______

Answer 16

Smooth part of the right atrium (sinus
venarum)
Coronary sinus
Oblique vein of the left atrium

Question 17

Role of vitelline veins

Answer 17

Return poorly oxygenated blood from the yolk sac/umbilical vesicle, and enter the sinus venosus

Question 18

Role of umbilical veins

Answer 18

carry well-oxygenated blood from the chorionic sac to the primordial placenta

Question 19

Role of common cardinal veins

Answer 19

Returns poorly oxygenated blood from the body of the embryo, to the heart

Question 20

Describe the rotation of the heart tube

Answer 20

During formation of head fold, the pericardial cavity and heart tube undergoes 180° rotation.

Thus, the heart tube comes to lie on the roof of the pericardial cavity

Question 21

describe the formation of the walls of the heart

Answer 21

Cells of the splanchnopleuric mesoderm proliferate and become thickened to form myoepicardial mantle (the primordial myocardium).

The developing heart tube is separated from the myoepicardial mantle by the cardiac jelly, ground substance secreted by the developing myocardium.

Myoepicardial mantle eventually becomes myocardium.

The endothelial heart tube forms the endocardium of the heart.

The myoepicardial mantle forms the myocardium and the epicardium/visceral pericardium.

Question 22

Describe cardiac looping

Answer 22

The heart tube continues to elongate as cells are added from the SHF to its cranial end.

This lengthening process is essential for normal formation of other parts of the heart and for the looping process.

Inhibition of this lengthening can lead outflow tract defects.

As the outflow tract lengthens, the heart tube begins to bend on day 23.

Cardiac looping involves the cephalic portion (bulbus cordis) of the tube bending ventrally, caudally, and to the right; and the atrial (caudal) portion shifting dorsocranially and to the left.

It is complete by day 28.

Also called bulboventricular loop.
The cardiac loop is suspended by the dorsal mesocardium (DM)

The central part of the DM soon degenerates to form a communication between the right and left sides of the pericardial cavity—the transverse pericardial sinus.

The primitive atrium and sinus venosus now enter the pericardial cavity and occupy position posterosuperior to the ventricle.

Thus, the heart tube now becomes S shaped.
Bulbus cordis and primitive ventricle together form a common chamber.

The atrial chamber that lies behind the above truncus arteriosus expands on either side of truncus arteriosus to produce the auricles.

Question 23

Describe dextrocardia

Answer 23

The most common positional anomaly of the heart
Here, all the chambers of the heart and associated blood vessels are reversed as a mirror image.
It occurs if heart tube bends to the left instead of the right, the heart is displaced to the right, and there is transposition. The dextrocardia may be associated with situs inversus (transposition of abdominal viscera).

Question 24

Describe Ectopia cordis

Answer 24

It is a rare condition in which the heart lies exposed on the surface of the thorax. It occurs due to nonunion of two sternal plates of developing sternum.

Death occurs in most of the cases during first few days of
the birth.

Question 25

What are the stages of partition of the primordial heart?

Answer 25

Partition of AV canal primordial atrium primordial ventricle

Question 26

Describe the partition of the AV canal

Answer 26

Partitioning of the atrioventricular canal (AV canal) Towards the end of the 4th week, endocardiac cushions are formed on the dorsal and ventral wall of the atrioventricular canal These endocardial cushions develop from the cardiac jelly/ extracellular matrix the AV endocardial cushions approach each other and fuse, dividing the AV canal into right and left AV canals These canals partially separate the primordial atrium from the primordial ventricle, and the endocardial cushions function as AV valves

Question 27

What is tetralogy of fallot?

Answer 27

Consists of: Pulmonary stenosis VSD Overriding aorta Right ventricular hypertrophy

Question 28

What are the changes involving blood circulation at birth? (5)

Answer 28

1. Closure of the umbilical arteries to give rise to: superior vesicle artery and umbilical ligament 2. Closure of umbilical vein which will give rise to ligament teres 3. Closure of ductus venosus which will give rise to ligament venosus 4. Closure of ductus arteriosus which will give rise to ligament arteriosus 5. Closure of foramen ovale will give rise to Fossa ovalis of the heart

Question 29

List the fetal structures and adult remanant

Answer 29

Foramen ovale -Fossa ovalis of the heart Ductus arteriosus - Ligamentum arteriosum Left umbilical vein - Extra-hepatic portion Ligamentum teres hepatis - Intra-hepatic portion (ductus venosus) Ligamentum venosum - Left and right umbilical arteries Proximal portions - Umbilical branches of internal iliac arteries Distal portions - Medial umbilical ligaments

Question 30

Discuss the formation of the heart tubes

Answer 30

1. The mesenchymal cells in the cardiogenic area, condense to form two angioblastic cords called cardiogenic cords. These cords get canalized to form two endothelial heart tubes 2. These tubes fuse in a craniocaudal direction to form a single primitive heart tube, However, the caudal ends of two heart tubes fail to fuse. As a result, the caudal end of the heart tube remains bifurcated. 3.

Question 31

Adult derivative of Truncus arteriosus

Answer 31

Ascending aorta Pulmonary trunk

Question 32

Adult derivative of Bulbus cordis

Answer 32

Smooth upper part of the right ventricle (conus arteriosus) Smooth upper part of the left ventricle (aortic vestibule)

Question 32

Adult derivative of Primitive ventricle

Answer 32

Trabeculated part of the right ventricle Trabeculated part of the left ventricle

Question 33

Adult derivative of Primitive atrium

Answer 33

Trabeculated part of the right atrium Trabeculated part of the left atrium

Question 34

Adult derivative of sinus venosus

Answer 34

Smooth part of the right atrium (sinus venarum) Coronary sinus Oblique vein of the left atrium

Question 35

Each horn of sinus venosus receives three primitive veins

Answer 35

(a) vitelline vein from the yolk sac, (b) umbilical vein from the placenta (c) common cardinal vein from the body wall

Question 35

Describe the Formation of Atrioventricular Septum

Answer 35

At the atrioventricular sulcus between the primitive atrium and primitive ventricle. The atrioventricular (AV) septum divides the AV canal into right and left AV canals (partitioning of the AV canal). Neural crest cells will migrate and form two thickenings on the dorsal and ventral walls of the AV canal. These are called atrioventricular, endocardial or AV cushions. They grow towards each other and fuse to form the AV septum (also called septum intermedium) that divides the AV canal into right and left AV canals.

Question 36

Describe the Formation of the Interatrial Septum

Answer 36

Septum primum starts developing from the roof of the primitive atrium. It grows downward towards the septum intermedium (AV septum). The gap between the septum primum and septum intermedium is called foramen primum (ostium primum). As septum primum fuses with AV septum (septum intermedium), a gap forms in the upper part of septum primum called the foramen secundum (ostium secundum). A second crescent-shaped septum now arises from the roof of primitive atrial chamber immediately to the right of septum primum. It is called the septum secundum. The right and left atria now communicate with each other through an oblique valvular passage between the upper margin of the septum primum and the lower margin of the septum secundum. This passage is called foramen ovale.

Question 36

Describe the absorption of sinus venosus into the right atrium

Answer 36

After formation of interatrial septum the right horn and body of sinus venosus are absorbed into the right atrium so that superior vena cava (SVC), inferior vena cava (IVC), and coronary sinus (regressed left horn) now open in the right atrium. With the absorption of sinus venosus in the right atrium, right and left venous valves separate from each other.

Question 37

Describe the formation of the Interventricular Septum

Answer 37

(a) muscular part, (b) bulbar part, and (c) membranous part Muscular part develops from the floor of the ventricle: A median muscular ridge grows upward from the floor of primitive ventricle (near its apex) and reaches almost up to the AV cushions. It forms the muscular part of the IV septum Bulbar part develops from right and left bulbar ridges: From the conical part of common ventricular chamber the two ridges develop in the distal part of bulbus cordis. They grow and approach each other to fuse together to form the bulbar part of the IV septum. The gap between the upper margin of the muscular septum and lower margin of bulbar septum is called IV foramen Membranous part develops as under: The gap between the muscular part and bulbar part of IV septum is filled by proliferation of tissue from the right side of the AV cushions and from the right and left bulbar ridges. Thus, the membranous part of IV septum is derived from three sources: right bulbar ridge, left bulbar ridge, and AV cushions. Formation of membranous part closes the IV foramen.

Question 38

Discuss tetralogy of fallot

Answer 38

This condition occurs due to combinations of four cardiac anomalies 1. pulmonary stenosis 2. overriding aorta (i.e. displacement of aortic orifice to the right to override the IV septum) 3. VSD 4. hypertrophy of the right ventricle. The resultant right-to-left shunting of blood leads to cyanosis. Tetralogy of Fallot is the most common congenital cyanotic heart disease

Question 39

What is a ventricular septal defect (VSD)

Answer 39

It is the most common congenital anomaly of the heart. This defect is most common in the membranous part of the IV septum. It is caused by the failure of the fusion of right and left bulbar ridges with the AV cushions, which form the membranous part of the IV septum. Thus, there occurs a defect in the IV septum that results in communication between the right and left ventricles. Blood flows from the left to the right ventricle. As a result of left-to-right shunting of blood, the output from the left ventricle is reduced. Consequently, the patient complains of excessive fatigue on exertion. Rarely, the defect may be present in the muscular part of the IV septum

Question 40

Discuss Tricuspid atresia

Answer 40

In this condition, there is complete agenesis of tricuspid valves. It is caused by an insufficient amount of tissue in the AV cushion for the formation of the tricuspid valve. The tricuspid atresias are always associated with (a) patent foramen ovale, (b) VSD, (c) underdeveloped right ventricle, and (d) hypertrophy of the right ventricle