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.