Embryology of Heart

Topics: Heart, Blood, Left ventricle Pages: 7 (1109 words) Published: November 7, 2012
Development of the heart
October -08 -12

• 15 days after conception
○ Begins as flat sheet of mesodermal cells (cells of the mesoderm layer in an embryo) • Day 18-20
○ The cells form a horse -shoe shape (cardiogenic region)
○ Angiogenic cell clusters coalesce to form right and left endocardial tubes • Day 21
○ Two endocardial tubes are completely fused
• Day 22
○ Heart begins beating
• Day 23
○ Heart tube forms constrictions prior to looping

The first indication of heart development is around
day 16- 19. Clusters of angiogenetic cells form a
"horseshoe- shaped" cluster anterior and lateral to
the brain plate.

Following cardiac mesoderm involution during
gastrulation, the two endocardial tubes fuse along
the embryonic midline. The heart starts to beat at
day 22, but the circulation does not start until days
27 to 29. The single tubular heart develops many
constrictions outlining future structures.

Prior to looping the heart tubes forms constrictions
defining separate components of the heart, these are
(cranial to caudal): Bulbus Cordis , Ventricle and
Atrium. The primitive atrium is still paired and
connects caudally to the paired sinus venosus. The
heart tube will bend ventrally, caudally and slightly
to the right.

At this stage the paired sinus venosus extend
laterally and give rise to the sinus horns. As the
cardiac looping is developing the paired atria form a
common chamber and move into the pericardial sac.

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• Day 21
○ Linear heart tube bends toward right side
• Day 22
○ Chambers begin to form
○ Left ventricle is expands to a large size and is spherical  Formed by most of the heart tube
○ Right ventricle is small
 Anterior to left ventricle
• Day 26
○ As the heart continues to loop, the right and left ventricle arrange themselves so that they are beside one another

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beside one another

○ Atria are arranged side by side on the posterior side
 Begin to expand
• Day 28
○ Ventricles grow downward
 Now they are anterior and inferior to the atria
• Day 34
○ Two septa grow in order to separate the atria
 Septum primum grows from the ventral and posterior walls of the atrium □ A hole is formed in the septum primum, called the foramen (ostium) secundum □ Allows blood to go from right atrium to left atrium

 Septum secundum grows from the dorsal and posterior walls □ Thick and muscular, unlike the septum primum which is thin • Day 50
○ Septum primum fuses with atroventricular/endocardial cushions (cells located at the atroventricular border to separate the atria and the ventricles) ○ The two septums partially overlap
○ Foramen ovale (orifice between the two flaps) is kept open  Allows maternal circulation to the heart
 When the child is born, the lungs inflate, applying pressure on the septum primum against the septum secundum to completely fuse the septa

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the septum secundum to completely fuse the septa

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• Day 38
○ Cells that join the left and right ventricles grow upward • Day 46
○ Cells grow toward endocardial cushions
 Stops growing before the ventricles are completely separated □ Later in development, membranous septum is formed to connect the muscular septum to the endocardial cushions

• Day 35
○ Outflow tract (truncus arteriosus) is a single tube that is connected to both ventricles • Day 40
○ Outflow tract undergoes epithelial-mesenchymal transformation (EMT)  Forms internal outflow tract cushions (truncoconal swellings) that converge towards one another
• Day 45
○ Base of outflow tract rotates clockwise
 Length of tract shortens
○ Truncoconal swellings rotate and fuse at midline
• Day 56
○ Aorta and pulmonary outflow tracts are separated
○ Neural crest cels at base of tract helps in outflow tract septation ○...
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