Development Of Heart Part3 | Heart embryology | Cardiovascular system | First Aid Lectures | Lec3
Channel: Dr. Jamil Lectures
Date: 03/05/2023

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In this lecture we will give the overview of whole heart embryology from first aid book and we will discuss about important questions for USMLE,PLAB,NLE,AMC and all other medical liscensing exams.


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Development of Heart
● Heart is the first functional organ in vertebrates that develops in
the fetus → Heart begins to develop as early as the 3rd week of
gestation and starts to beat spontaneously by week 4.
● Inner cells mass of blastula differentiates into embryoblast that
forms the bilaminar disk (epiblast and hypoblast)→Bilaminar
disk transforms into trilaminar disk (ectoderm,mesoderm and
endoderm).
Cross-section
● Mesoderm has 3 parts
1) Paraxial mesoderm 2) Intermediate mesoderm 3) Lateral mesoderm
● Lateral mesoderm has further 2 layers
1) Somatic layer 2) Splanchnic layer
● Heart tube develops from the splanchnic layer of lateral mesoderm.
● Vascular Endothelial Growth Factor (VEGF) →Endoderm
secrets VEGF which stimulates the mesoderm to differentiate
into 2 structures
1. outer angioblasts that will form the heart tube and blood vessels
2. Inner hemocytoblasts that will form the blood cells
→ VEGF also stimulates the lateral mesoderm to grow
and form the 2 cavities.
1. Heart tubes
2. Pericardial cavities
● Lateral Folding → During the lateral folding lateral ends of
all layers fuse together to forms a tube
➢ Lateral ends of ectoderm layers fuse together
➢ Lateral ends of mesoderm layers fuse together
➢ Heart tubes from both lateral mesoderm also fuse together
and make 1 heart tube
➢ Pericardial cavities from both lateral mesoderm also fuse
together and make 1 pericardial cavity
➢ Dorsal mesocardium connects the heart tube with the pericardial cavity.
● Heart Tube Layers → Heart tube has 2 layers
➢ Inner layer which forms endocardium
➢ Outer layer which forms myocardium (cardiomyocytes)
➢ Cardiomyocytes secrete jelly-like connective tissue between
endocardium and myocardium called cardiac Jelly.

Heart Tube
★ Divisions
● Heart tube has following divisions and these divisions develop into
following structures.
➢ Aortic sac which leaves out via dorsal aortae
➢ Truncus arteriosus (T.A.) which gives rise
to ascending aorta and pulmonary trunk.
➢ Bulbus cordis (B.C.) which gives rise to the
right ventricle & outflow tract of left and right
ventricles.
➢ Primitive ventricle (P.V.) which gives rise to
trabeculated part of left and right ventricles.
➢ Primitive atria (P.A) which gives rise to
Trabeculated part of left and right atria
➢ Sinus venosus (S.V.) has 2 horns
1) Right horn 2) Left horn
➔ Right horn gives rise to the smooth part of the right atrium.
➔ Left horn gives rise to coronary sinus
➔ Common cardinal veins (outside) ,Umbilical veins (middle) and
Vitelline veins (inside) enter into right and left horns of S.V → These
veins are called inflow tract of S.V.
● Blood enters the heart tube from inflow tract to S.V,through P.A,through P.V,through
B.C,through T.A and leaves out through outflow tract (dorsal aortae) of Aortic sac


Septation of the Heart chambers
Atria
● Septum primum forms from top and grows down towards endocardial cushion,
narrowing foramen/ostium primum (foramen primum is an anterior opening between
septum primum and endocardial cushion)
● Septum primum doesn’t reach the endocardial cushion and a hole/gap develops in
septum primum towards top→This hole is called foramen/ostium secundum that
maintains right-to-left shunt of blood in fetus heart.
● Septum primum continues to grow until it reaches
endocardial cushion and foramen primum
regresses/closes.
● Septum secundum develops on the right side of septum
primum and expands to cover the most of foramen
secundum →It blocks the ostium/foramen secundum
→The remaining passage/ foramen is called the
foramen ovale.
Patent foramen ovale →failure of septum primum and septum secundum to fuse after
birth that results in opened foramen → venous thromboemboli (venous blood clots) can
enter the LA from RA via patent foramen ovale → LA → LV → enter systemic circulation as
“paradoxical embolism” → can cause stroke.
Ventricles



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