Embryology Flashcards
ORIGIN OF THE HEART
The vascular system appears in the middle of the 3rd week of development.
Reason: Diffusion only is no longer enough to satisfy the nutritional requirements of the growing
embryo.
DEVELOPMENT OF THE PRIMITIVE HEART TUBE
The primitive heart tube is initially a straight tube, consisting of the truncus arteriosus, bulbus
cordis, primitive ventricle, primitive atrium and sinus venosus.
The atrium and sinus venosus initially lie outside the pericardium.
Blood flows from the venous side to the arterial side of the heart tube.
As the heart tube elongates, the cephalic portion bends in a ventral and caudal direction and to
the right, whereas the caudal portion bends in a dorso-cranial direction and to the left. This
bending results in the formation of the cardiac loop.
At approximately 28 days contractions are thought to begin in the ventriculobulbar portion of the
heart, and the heart beat is initiated.
PARTITIONING OF THE PRIMITIVE HEART:
The endocardial cushions develop in the atrioventricular and trunco-conal regions of the heart
and play a central role in the formation of the heart septa, atrioventricular, aortic and pulmonary
canals.
These cushions therefore play a role in several cardiac malformations.
They develop from the neural crest and therefore, cardiac abnormalities are often associated with
craniofacial defects.
a) Interatrial septum
A sickle-shaped crest, called the septum primum grows from the roof of the common atrium at
the end of the 4th week.
The opening between the lower rim of this septum and the endocardial cushions is called the
ostium primum.
Growths from the endocardial cushions in a superior direction eventually closes the opstium
primum.
Before the closure is completed, perforations in the ostium primum occur which coalesce to form
the ostium secundum, which ensures blood flow from the right to the left atrium.
As the lumen of the right atrium expands, a second crescent-shaped septum, the septum
secundum, grows from the roof of the RA in an inferior direction, but never forms a complete
partition between the two atria, although it overlaps the ostium secundum to some extent.
The opening left by the septum secundum is called the oval foramen.
The upper part of the septum primum disappears gradually and the remaining lower part forms
the valve of the oval foramen.
The passage between the two atria is thus an obliquely elongated cleft.
After birth, the valve is pressed against the oval foramen due to higher pressure in the left atrium,
and thus the foramen is closed.
b) Atrioventricular canals
The superior and inferior endocardial cushions fuse to separate the left and right atrioventricular
canals from each other.
c) Interventricular septum
The medial walls of the expanding ventricles eventually become fused, forming the muscular part
of the interventricular septum.
Initially there is a communication between the two ventricles through an opening superior to the
upper edge of this septum (the interventricular foramen).
This foramen is eventually closed by outgrowths from the endocardial cushions, forming the
membranous part of the interventricular septum (end of 7th week).
d) Bulbus cordis & truncus arteriosus
During the 5th week, swellings or cushions develop in the truncus arteriosus.
These swellings are located on the right superior wall and the left inferior wall of the truncus.
They grow towards each other to divide the truncus into two, eventually forming a spiral septum
in the truncus, called the aortico-pulmonary septum.
Similar swellings develop in the conus cordis, which forms the outflow tracts of the ventricles and
eventually fuses with the truncus septum.
The septum thus consists of an anterolateral portion (outflow tract of right ventricle and pulmonary
trunk), and a posteromedial portion (outflow tract of the left ventricle and aorta).
FURTHER FORMATION OF THE ATRIA & VENTRICLES
Initially, a single pulmonary vein develops as an outgrowth from the posterior wall of the LA.
This vein develops connections with the developing lung buds and eventually form several
branches.
Some of these branches get incorporated into the left atrium, forming the smooth part of the LA.
Eventually, four pulmonary veins enter the left atrium
FETAL CIRCULATION
Reasons for differences in blood circulation before and after birth
The fetus does not breath – not a prominent pulmonary circulation (i.e., lungs are bypassed to a
great extent)
The fetus does not eat – not a prominent portal circulation between the gut and the liver (i.e., liver
is bypassed to a great extent).
A sphincter mechanism regulates flow of the umbilical blood through the liver sinusoids, and
prevents sudden overfilling of the heart during uterine contraction.
Overloading of the lungs is prevented by the shunt of blood from the right to the left atrium and
also through the ductus arteriosus to the aorta instead of the pulmonary arteries (due to high
resistance in the pulmonary vessels).
Changes at birth as well as the derivatives of fetal circulation which occur
Changes are caused by cessation of placental blood flow and the beginning of respiration (i.e.,
increased blood flow through lungs and pressure in the left heart).
The following changes take place at birth:
a) Closure of the umbilical arteries
b) Closure of the umbilical vein
c) Closure of the ductus venosus
d) Closure of the ductus arteriosus
e) Closure of the oval foramen
Classification of congenital heart disease.
Congenital heart diseases can be either classified as Acyanotic with left to right shunt/ with no shunt or Cyanotic
Acyanotic Congenital Heart Diseases
With left to right shunt: V.A.P
Ventricular septal defect
Atrial septal defect
Patent ductus arteriosus
P and V can lead to eisenmengers syndrome
With no shunt:
B.E.D.C.P.C
Bicuspid aortic valve Congenital aortic stenosis Coarctation of aorta Dextrocardia Pulmonary stenosis, tricuspidstenosis Ebstein's anomaly
Mention the Acyanotic Congenital Heart Diseases
Ventricular Septal defect
Atrial Septal Defect
Patent ductus arteriousus
Coarctation of the Aorta
Ventricular septal defect:
In this condition one or more holes are present in the membranous or muscular
ventricular septum.
Atrial septal defect:
There are two main types: ostium secundum 90% where there is a defect in the part of the
septum which does not involve the atrioventricular valves and ostium primum where the defect
does involve the atrioventricular valves.
Patent ductus arteriosus:
This is a persistent embryonic vessel which connects the pulmonary artery and the aorta. The
shunt is from the aorta to the pulmonary artery unless pulmonary hypertension has supervened.
Coarctation of the aorta
Coarctation of the aorta is a relatively common defect that accounts for 5-8% of all congenital
heart defects. Coarctation of the aorta may occur as an isolated defect or in association with
various other lesions, most commonly bicuspid aortic valve and ventricular septal defect (VSD).
It is more common in males
Coarctation of the aorta
Coarctation of the aorta is a relatively common defect that accounts for 5-8% of all congenital
heart defects. Coarctation of the aorta may occur as an isolated defect or in association with
various other lesions, most commonly bicuspid aortic valve and ventricular septal defect (VSD).
It is more common in mal
Mention the Cyanotic Congenital Heart Disease
Eisenmenger’s syndrome
Tetralogy of Fallot
Ebstein’s anomaly
Truncus arteriosus
Transposition of the great vessels
Tricuspid atresia
Total anomalous pulmonary venous drainage
Eisenmenger’s syndrome
Ebstein’s anomaly
This is a very rare lesion. The abnormality is a downward displacement of the tricuspid valve
apparatus into the right ventricle so that the right atrium becomes very large and consists partly
of ventricular muscle, while the right ventricle becomes small. An atrial septal defect is
commonly associated.
Tetralogy of Fallott
There are four features which are due to a single development abnormailty;
(i) ventricular septal defect VSD; (ii) right ventricular outflow obstruction, which determines
the severity of the condition, and can be at the pulmonary valve or infundibular level;
(iii) an aorta which overrides the VSD and is res onsible for the cyanosis and
(iv) right ventricular hypertrophy secondary to outflow obstruction.
Truncus arteriosus
Truncus arteriosus is a rare type of heart disease in which a single blood vessel (truncus
arteriosus) comes out of the right and left ventricles, instead of the normal 2 vessels (pulmonary
artery and aorta
Transposition of the great vessels
Transposition of the great arteries is a serious but rare heart defect present at birth (congenital),
in which the two main arteries leaving the heart (aorta and pulmonary trunk) are reversed
(transposed). The condition is also called dextro-transposition of the great arteries.
Tricuspid atresia
Tricuspid atresia is a type of heart disease that is present at birth (congenital heart disease), in
which the tricuspid heart valve is missing or abnormally developed. The defect blocks blood flow
from the right atrium to the right ventricle.
Total anomalous pulmonary venous drainage
Total anomalous pulmonary venous connection (TAPVC) consists of an abnormality of blood flow
in which all 4 pulmonary veins drain into systemic veins or the right atrium with or without
pulmonary venous obstruction. Systemic and pulmonary venous blood mix in the right atrium.
The important point to determine is whether or not signs of pulmonary hypertension are present.
Congenital heart disease in which a shunt from the left to the right side of the circulation occurs
leads to an increase in pulmonary blood flow. This can cause reactive pulmonary hypertension
so that the pulmonary pressures exeed the systemic pressures. When this happens the systemic
to pulmonary (left to right) shunt will reverse. This right to left shunt leads to deoxygenated blood
being mixed in the systemic circulation, results in cyanosis.
Ebstein’s anomaly
This is a very rare lesion. The abnormality is a downward displacement of the tricuspid valve
apparatus into the right ventricle so that the right atrium becomes very large and consists partly
of ventricular muscle, while the right ventricle becomes small. An atrial septal defect is
commonly associated.
Tetralogy of Fallott
There are four features which are due to a single development abnormailty;
(i) ventricular septal defect VSD; (ii) right ventricular outflow obstruction, which determines
the severity of the condition, and can be at the pulmonary valve or infundibular level;
(iii) an aorta which overrides the VSD and is res onsible for the cyanosis and
(iv) right ventricular hypertrophy secondary to outflow obstruction.
Truncus arteriosus
Truncus arteriosus is a rare type of heart disease in which a single blood vessel (truncus
arteriosus) comes out of the right and left ventricles, instead of the normal 2 vessels (pulmonary
artery and aorta
Transposition of the great vessels
Transposition of the great arteries is a serious but rare heart defect present at birth (congenital),
in which the two main arteries leaving the heart (aorta and pulmonary trunk) are reversed
(transposed). The condition is also called dextro-transposition of the great arteries.
Tricuspid atresia
Tricuspid atresia is a type of heart disease that is present at birth (congenital heart disease), in
which the tricuspid heart valve is missing or abnormally developed. The defect blocks blood flow
from the right atrium to the right ventricle.
Total anomalous pulmonary venous drainage
Total anomalous pulmonary venous connection (TAPVC) consists of an abnormality of blood flow
in which all 4 pulmonary veins drain into systemic veins or the right atrium with or without
pulmonary venous obstruction. Systemic and pulmonary venous blood mix in the right atrium.
