Development of The Human Heart
July 10th, 2005 by dr-jabbar-best1)The Formation of Cardiogenic Field
Cardiogenic Field consists of horseshoe-shaped endothelial
tube, surrounded by myoblast and the pericardial cavity over
it. The cardiac progenitor cells lie in the epiblast immediately
lateral to the primitive streak that will migrate toward cranium
and ventral to buccopharyngeal membrane and neural fold.
Then it will reside in splanchnic layer of lateral plate
mesoderm. The underlying pharyngeal endoderm will induced
the progenitor cells to form cardiac myoblasts. At the same
time, blood islands also appear in the splanchnic layer of
lateral plate mesoderm that will give rise to the 2 heart tube
which in turn will fuse to form horshoe-shaped endothelial-lined
tube. The intraembryonic coelom related to this cardiogenic
field will form pericardial cavity.
2)Formation and Position of Heart Tube
The fusion of the 2 heart tube is due to the lateral folding of
the embryo except at its caudalmost ends. At the same time,
the brain growth and cephalic folding will pull the
buccopharyngeal membrane forward and move the heart at
first toward cervical region and finally into the thorax. The
developing heart tube bulges more and more into pericardial
cavity but attached to the dorsal side of the pericardial cavity
by dorsal mesocardium which then dissappear to form
transverse pericardial sinus(connection between both sides of
pericardial cavity). During above events, myocardium secretes
a thick extracellular matrix rich in hyaluronic acid to separate
with the endothelium and mesothelial cells from sinus
venosus region migrate over the heart to form epicardium
3)Formation of Cardiac Loop
The cephalic portion will bends ventrocaudally and to the right
The caudal portion will bends dorsocranially and to the left.
Then, the conotruncal portion will move more medial position
from its initial position in the right side that will results in
transvers dilatation of the atrium bulging on each side of the
bulbus cordis
During the above events,
a) there will be dilatation appear on this tube along its
craniocaudal axis that will separate the heart tube into several
regions, from cranial ro caudal end, bulbus cordis, ventricle,
atrium, and sinus venosus.
b) there will also be the formation of common atrium from
initial paired structure and incorporation of it into the
pericardial cavity
c) the atrioventricular junction remains narrow to form
atriventricular canal
d) bulbus cordis can be divided into 3 parts
i) proximal third portion which is broadest called
trabeculated part of the right ventricle
ii) the narrow middle portion called conus cordis that will
form outflow tracts for both ventricles
iii) the distal part called truncus arteriosus that will form
roots and proximal part of aortic and pulmonary channel
e) the junction between ventricle and bulbus cordis externally
will remains narrow called bulboventricular sulcus and also
called primary interventricular foramen
f) smooth walled of heart tube will form primitive trabeculae
just proximal and distal to primary interventricular foramen.
Development of Sinus Venosus
The sinus venosus receive venous blood from right and left
horn which in turn receive three vein which is i)vitelline vein
ii)umbilical vein iii)common cardinal vein
The communication of sinus venosus and primitive atrium
(sinuatrial orifice) is wide but soon it will be narrowed and
shifted to the right because of the left-to-right shunts of blood.
The sinuatrial orifice is guarded by right and left venous valve
which fused to form septum spurium dorsocranially
Then obliteration of right umbilical vein, left vitelline vein and
left common cardinal vein will occur afterwards.
Development of Atria
>The Right Atrium
The right atrium is formed from the right half of the primitive
atrium(musculi pectinati) and the incorporated right horn of
sinus venosus(sinus venarum).
After the shift of the sinuatrial orifice to the right, the right horn
of sinus venosus will be incorporated into the atrium by the
enlargement of the sinuatrial orifice.
The right venous valve will expand and divide into crista
terminalis, valve of inferior vena cava and valve of coronary
sinus.
The left venous valve with the septum spurium will fuse with
the developing atrial septum
The right common cardinal vein will become the superior vena
cava, the right vitelline vein will form inferior vena cava and
the left umbilical vein will remains small to form coronary
sinus.
>The Left Atrium
The left atrium of the heart is form from the left half of the
primitive atrium and the incorporation of pulmonary veins.
Initially, just to the left of septum primum, there will be only
one pulmonary vein develops as an outgrowth of posterior left
atrial wall, but soon the pulmonary veins and its total 4
branches will be incorporated to form the large smooth wall of
left atrium where finally the the left atrium will receive
oxygenated bloods from 2 pulmonary vein from each side after
birth.
Development of Interatrial Septum
The septum primum will grow from the roof of common atrium
towards the endocardial cushion and will stop to form the
ostium primum that connect right atrium with the left atrium.
the ostium primum will be closed by the growth of superior and
inferior endocardial cushion along the edge of septum primum.
At the same time the ostium primum is closed, the death of
the cells on the upper portion of septum primum will perforates
the septum primum, where the coalescence of these
perforations will create an opening called ostium secundum.
The incorporation of right sinus venosus into the right atrium
will form a fold called septum secundum. Septum secundum
will grow towards endocardial cushions but stops to form and
opening on its lower portion called foramen ovale.
The remaining part of septum primum will form the valve of
foramen ovale. The oblique elongated cleft is vital for the
embryo as the pulmonary circulation is not yet developed and
thus it creates the need for the flow of blood from right atirum
directly to the left atrium.
After birth, the left atrium pressure will increase that will
pressed the valve of foramen ovale on the septum secundum.
Formation of Cardiac Septa
The actively growing masses of tissue that will approach each
other and dividing the lumen into two separate canals or only
the growth of masses of tissue on one side that will grow and
will approach the other side of the lumen.
This masses of tissue is also known as endocardial cushion
that will occur in conotruncal region and in atriovenricular
region.
This endocardial cushions will assists in formation of
interatrial septum and interventricular septum, AV and
semilunar valves.
Formation of interventricular septum
The medial walls of expanding ventrical will become apposed
and gradually merged to form muscular part of interventricular
septum and sometimes these two walls do not merge
completely and this deep apical cleft is called interventricular
foramen which will shrink in size by the completion of conus
septum.
The completion of the interventricular septum is by
outgrowth of the inferior endocardial cushion along the top
muscular interventricular septum
Formation of Septum in Truncus Arteriosus and Conus Cordis
In Truncus Arteriosus
The right superior wall of the truncus arteriosus will grow
distally and to the left while the left inferior wall will grow
distally and to the right. They will fuse and twist while growing
towards the aortic sac, foreshadowing the spiral course of
conus septum.
In Conus Cordis
The right dorsal swellings and the left ventral swelling will grow
towards each other and distal toward truncus arteriosus septum.
When the conus swellings have fused, it will twist and divide the conus into anterolateral portion and posteromedial portion which is outflow part of right ventricle and left ventricle respectively.
The Formation of Valves
>The Atrioventricular valve
After fusion of AV endocardial orifice, it will be surrounded by local proliferation of messenchymal tissue. The venrticular surface of this tissue will be hollows and thins by the bloodstream and finally will form the valves that remains attached to the ventricular wall
Then Muscular tissue in the cord degenerates and replaced by dense connective tissue.
the valve consists of connective tissue covered by endocardium and connected to papillart muscles by means of chordae tendinae.
>The Semilunar Valve
When partitionin of truncus almost complete, primordia of semilunar valve become visible as small tubercles on main truncus swelling where one of each pair is assigned for aortic and pulmonary channel.
then a third tubercle appears opposite the fusion of the fused truncus swelling
Gradually the tubercle will hollows out at the upper surface to form semilunar valve.
(Written by jabbar, sources from Langman’s Medical Embryology)
