Foetal Heart Development

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					                                                        Foetal Heart
The premordium of the heart forms in the cardiagenic plate
located at the cranial end of the embryo. Angiogenic cell clusters
which lie in a shoe-shape configuration in the plate coalesce to
form the two endocardial tubes. These tubes are then forced into
the thoracic region due to cephalic and lateral foldings where
they fuse together forming a single endocardial tube. The tube
can be subdevided into primordial heart chambers starting
caudally at the inflow end; the sinus venosus, primitive atria,
ventricle and bulbis cordis to form the right and left atria, right
and left ventricles and two great vessels the pulmonary artery
and the aorta. By the end of the eigth week partitioning is
completed and the fetal has has formed.
                                   Table 1. Week 3

Week   Day      Length                                       Event

 III   16/17   Presomite   Angiogenic cell clusters lie in the cardiogenic plate

       18/20   Presomite   Endocardial tubes are formed and begin to move towards each other.

                           Endocardial tubes begin to fuse to form a single heart tube. The heart
                1,8 mm
       21-22                   tube is forced into the thoracic region. Due to cephalic and lateral
               2 somites
               Table 2 Week 4
Week   Day        Length                         Event

                              Endocardial heart tubes has completely
                                  fused to form a single heart tube. The
                  2,0mm           heart begins to beat.
 IV     22
                 4 somites        Splanchnopleuricmesoderm invests the
                                  heart tube and forms the pericardial

                2,2-2,5mm     The heart tube begins to grow rapidly,
                7/8 somites       forcing it to fold upon itself.

                              The centrally located sinoatrial junction
                                  begins to shift to the right.
        24      14 somites
                                  Perforations appear in the dorsal

                              The bulboventricular loop is formed. The
       25-28     3,2-5mm
                                  septum primum appears.

                              The ventricular septum appears as a small
                                  ridge on the floor of the common
                                  ventricle. The ventricle begin to
        28                        dialate. A single pulmonary artery
                16 somites
                                  grows from the outer dorsal wall of the
                                  left atrium. The endocardial cushions
               Table 3 Week 4 - 6

Week   Day           Length                     Event

 V      29          6-7mm      Truncal swellings appear.

                               Perforations appear in septum

                               The bulboventricular flange begins to
                                   recede. The atrioventricular canal
                                   gains a “dog bone” appearance.
       32-33         9mm
                                   The ostium secundum is formed
                                   by the free edge of the septum

                               The sinoatrial junction has shifted
                                   completely to the right.

                               The septum secundum ceases to
 VI     37           14mm          grow; its free edge forms the
                                   foramen ovale.

                               Superior and inferior endocardial
                                   cushions fuse.
                         Table 4 Weeks 7-8

Week        Day              Length                      Event

                                       The ventricular septum ceases to grow.
VII          46
                                           The coronary sinus is formed.

                                       The interventricular canal is completely
       End 7th week

                                       The outflow tracts (aorta and pulmonary
       Early in 8th wk
                                           trunk) are completely separated.
Angiogenic Cell Clusters

  The origins of the heart tube are clusters of the angiogenic cells
  which are located in the cardiogenic plate. The cardiogenic plate,
  which is derived from the splanchnoplueric mesoderm, is located
  cranial and lateral to the neural plate.
Endocardial Tube
  Endocardial Tube

The angiogenic cell clusters coalesce to form the right and left endocardial tubes.
Each tube is continuous cranially with a dorsal aorta, its outflow tract and caudally
with a vitteloumbilical vein, its inflow tract. The lateral and cranial folding of the
embryo forces the tubes into the thoracic cavity. As a result these tubes comes to
lie closer to each other and begin to fuse in a cranial to caudal direction. At
approximately day 21 they are completely fused, as seen in the scanning electron
Heart Tube   1. The newly formed heart tube bulges into
                the pericardial cavitiy and is attached to
                the dorsal wall by a fold of tissue, the
                dorsal mesoderm. This is a derivative of
                foregut splanchnoplueric mesoderm.
                Eventually this will rupture leaving the
                heart tube suspended in the pericardial
                cavity anchored cranially by the dorsal
                aortae and caudally by the vitelloumbilical
                veins. As it bulges into the cavity it
                becomes invested in a layer of
                myocardium. A layer of acellular matrix,
                the cardiac jelly, seperates the
                myocardium and the endothelial heart
                tube.The newly formed heart tube may be
                devided into regions starting caudally:
                    I. Sinus Venosus - consisting of the
                          right and left horns
                    II. Paired primitive atria - these
                          structures will later fuse
                    III. Atrioventricular sulcus - devides
                          the atria and the primitive ventricle
                    IV. Interventricular sulcus - devides
                          the primitive ventricle and the
                          bulbus cordis
                    V. Bulbus cordis - is devided as
                    VI. bulbus cordis - the proximal
                          portion froms the right ventricle.
                    VII. conus cordis
                    VIII. truncus arteriosus
                    IX. Aortic sac
             2. Formation of the bulboventricular loop can
                be visualized in this scanning micrograph.
Heart Tube
By the time the heart tube has formed the bulboventricular loop, the two primitive
right and left atria have fused to form a common atrium. Note that it lies cranial
to the primitive ventricle and dorsal to the bulbus cordis. The truncus arteriosus
lies on the roof of the common atrium causing a depression and indicates where
septation of the atrium will occur.

     Atrial Partitioning
The partitioning of the atrium begins with the appearance of the septum
primum at about the 28th day. This is a crest of tissue that grows from the
endocardial cushions. The ostium formed by the edge of the septum
primum is the ostium primum.

  Atrial Partitioning
Before the septum primum fuses with the endocardial cushions,
perforations appear in the upper portion of the septum primum. These
perforations will coelasce to form the ostium secundum.

 Atrial Partitioning
Unlike the septum primum, septum secundum does not fuse with the
endocardial cushions. Its free edge forms the foramen ovale. The left
venous valve and the septum spurium, located on the dorsal wall of the
right atrium, fuse with the septum secundum as it grows.

 Atrial Partitioning
At the end of the seventh week the human heart has reached its final stage of development.
Because the fetus does not use its lungs, most of the blood is diverted to the systemic circulation.
This is accomplished by a right to left shunting of blood that occurs between the atria. The
foramen ovale and the septum primum control this right and left communication. The septum
primum acts as a valve over the foramen ovale. At birth the child will use its lungs for the first time
and consequently more blood will flow into the pulmonary circulation. The pressure increase in
the left atrium will force the septum to be pushed up against the septum secundum. Shortly
thereafter the two septa fuse to form a common atrial septum.

    Atrial Partitioning
Unlike the atria, the sinus vinosus remains a paired structure with right and left horns. Each
horn receives venous blood from the three vessels:
1. Vitelline Vein
2. Umbulical Vein
3. Common Cardianal Vein

 Fate of the Sinus Venosus
Communication between the sinus venosus and the primitive atrium, the sinoatrial oriface, is centrally located.
Gradually the sinoatrial oriface shifts to the right due to the shunting of the blood to the right until the sinus
   venosus communicates with only the right atrium. The fate of each structure is as follows:
the right sinus horn becomes enlarged
the right anterior cardianl vein becomes the superior vena cava
the right vetelline vein becomes the inferior vena cava
the right umbulical vein is obliterated

Fate of the Sinus Venosus
Internationally, the sinoatrial oriface is flanked by two valves, the right
and left venous valves. Superiorly these two valves meet to form the
septum spurium. Note that the left horn opens up underneath the
oriface of the right horn (sinoatrial orifice). This is the orifice of the
coronary sinus.

Fate of the Sinus Venosus
Further into the development the right sinus horn is incorporated into the
expanding right atrium. As the atrium expands the smooth tissue of the
sinus venosus displaces the trabeculated tissue of the primitive right
atrium anteriorly and laterally where it becomes the adult right auricle. The
smooth tissue forms part of the atrium called the sinus venarum. Crista
Terminalis, a ridge of tissue located to the right of the sinoatrial oriface,
forms the boundry between the auricle and the sinus venrum.

 Fate of the Sinus Venosus
During the part of the fourth week an outgrowth of the pulmonary vein
appear from the left atrium. This sprout will burificate until there are four
veins. These vessels will then grow towards the lung buds.

  Pulmonary Vein
The left atrium begins to expand gradually intussuscepting the four
branches. As the atrial wall expands, the smooth tissue of the
pulmonary veins is incorporated into the wall of the atrium and
displaces the trabeculated tissue anteriorly and laterally which will
then form the adult auricles. Compare this process to the formation of
the adult auricle.

 Pulmonary Vein
Recall that the proximal bulbus cordis gives rise to the right ventricle. Thus, blood flows from the primitive atrium to the
left ventricle then to the right ventricle. There is no direct communication between the atria and the right ventricle even
after the formation of the bulboventricular loop. The atrioventricular canal must shift to the right in order to achieve
communication to the right ventricle. During this shift the proximal bulbus widens and the bulboventricular flange begins
to recede. Swellings of mesenchymal tissues, the endocardial cushions, appear on the borders of the atrioventricular
canal. There are four cushions: inferior and superior (ventral and dorsal), left and right. The first appear before the latter.
These swellings give the atrioventricular canal a “dogs bone” shape.

       Atrioventricular Canal
Atrioventricular Canal
In the newly formed bulboventricular loop the primitive right and left
ventricles appear as expansions in the heart tube. Externally the
interventricular sulcus seperates the right and left ventricles and internally
they are seperated by the bulboventricular flange. Note, the right ventricles
rise from the proximal bulbus cordis.

Ventricle Formation
During the shifting of the atrioventricular canal the proximal bulbus cordis expands forming the
right ventricle. Both ventricles will continue to expand until the late 7th/early 8th week. The growth
of the ventricles is due to the centrifugal growth of the myocardium and the diverticulation of the
internal walls. (This is what gives the ventricle its trabeculated appearance). The muscular
interventricular septum forms as a result of the expanding ventricles. The walls of the right and left
ventricles grow in opposition to each other to form the muscular septum. Thus, the septum will
cease to grow when the ventricular walls are no longer expanding.

    Ventricle Formation
The final morphological change in the heart is the partioning of the outflow tract - the truncus arteriosus and the conus
cordis - into the aorta and the pulmonary trunk. This is accomplished by the development of a septum that forms in the
outflow tract and the emergence of the two great vessels.The septum forms from two pairs of swellings which grow from
the walls of the outflow tract. These are the truncus swellings and the conus swellings.Truncal swellings: Right which
grows distally and to the left. Left inferior which grows distally and to the right. Both develop at the proximal part of the
truncus and procedd to grow in two directions 1) distally towards the aortic sac and 2) into the lumen of the outflow tract
where the will eventually fuse together.Conus swellings: Right dorsal which is continuous with the right superior. Left
ventral which is continuous with the left inferior. Like the truncal swelling, the conal swellings grow distally and towards
each other, however, they appear after the first pair. These conus swellings eventually fuse with the truncal swellings.

       Outflow Tract Formation
Oxygenated blood from the placenta the umbulical venous blood (P02, 35 mmHg; Saturation 80%) either bypass
the liver through the ductus venosus and enter directly the right atrium (mostly rich O2 blood) or it transverse the
liver and enter the inferior vena cava. (Diagram depicting the Fetal Circulation) Inferior vena cava (IVC) blood
contains umbulical, lower body and portal venous blood. It represents more or less 70% of the total systemic venous
return to the heart. 25% of the IVC blood passes directly into the left atrium through the foramen ovale. Here the
blood mixes with the pulmonary venous blood. Left atrial blood (pO2 - 27 mmHg; O2 Saturation 65%) passes into
the left ventricle and supplies the coronary arteries, the head (the brain), and the upper body. Only 10% of this blood
ejected from the ventricle crosses the aortic isthmus to contribute flow to the descending aorta.
Superior vena cava (SVC) blood contributes only about 25% of the total venous return to the heart and has a
significantly lower pO2 and oxygen saturation (14 mmHg and 40%). Practically all SVC blood flows into the right
ventricle and the pulmonary trunk. 8% is diverted into the highresistance pulmonary vascular circuit. The rest is
ejected through the ductus arteriosus into the descending thoracic aorta, with a pO2 of 21 mmHg and an O2
saturation of 60%. Because of the crossing streams of the IVC and SVC blood in the right atrium, organs distal to
the ductus arteriosus are supplied with blood that has a lower pO2 and O2 saturation than that of the brain and
upper extremities. Thus the placenta receives blood with low O2 saturation, maximizing the effeciency of placental
gas exchange.
In the late gestation, pressures in the foetal left and right ventricles are the same (approximately 70 mmHg systolic).
The pressure is transmitted into the pulmonary arteries and ascending and descending thoracic aorta. The left and
right sides of the foetal circulation function more or less parralell, that is a major distinction from the postnatal (in
series) circulation. Combined ventricular output (CVO), that passes through the placenta is more or less equal to the
volume of right ventricular blood that passes through the lungs after birth.
In the foetus the right ventricle ejects approximately 65% and the left ventricle only about 35% of the CVO. Because
8% flows through the lungs, about 60% of the total CVO crosses the ductus arteriosus into the descending thoracic

      The Fetal Circulation

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