2 Heart development (Exam 3)

• The formation of the primitive heart and vascular system starts during week 3.
• The heart begins beating by the start of week 4.
• The heart acts as a one-way pump, transferring oxygen and nutrient-rich blood from the placenta to the embryo.
• The umbilical vein carries blood, then oxygen-poor blood is returned to the placenta via the umbilical arteries.

Primary Heart Field

• The primary heart field is located at the rostral end of the embryonic body.
• Mesodermal cells aggregate there, forming longitudinal cellular strands called angioblastic cords.

Heart Development

• Longitudinal folding positions the heart in the thorax region.
• Embryonic folding brings two endocardial tubes into the thorax where they meet, fuse along the midline, and form a single tube.

Heart Tube Differentiation

• The sinus venosus, ventricle, bulbis cordis, atrium, and truncus arteriosus are differentiated.
• The blood enters the sinus venosus on the caudal end of the developing heart.
• The common cardinal veins bring blood from body to sinus venosus.
• The placenta brings blood to the sinus venosus via the umbilical veins.
• The yolk sac also brings blood via the vitelline veins.
• Blood proceeds from the the atrium and then to primitive ventricle.
• The bulbis cordis recieves blood pumped from the ventricle.
• The truncus arteriosus is drained by the bulbis cordus .
• The aortic arches comes from the expanded aortic sac, which is continuous cranially with the truncus.
• Blood moves from the aortic arches into the dorsal aortae to reach the embryonic body, placenta, and yolk sac.
• Folding of the primitive heart tube positions the four developing chambers of the future adult heart into their correct spatial relationships.
• The primitive ventricle shifts ventrally, caudally, and to the right.
• The primitive atria shifts dorsally, cranially, and to the left.

Partitioning the Primitive Heart

• The partitioning of the heart allows the heart to bend and enlarge, which is used to separate the systemic and pulmonary circulations.
• Four partition sets are formed at same time during weeks 4-5 in atria and ventricle.
• These partitions will divide:
  • The atria from the ventricles
  • Both the right and left atria
  • Both the left and right ventricles
  • The pulmonary trunk and ascending aorta

Atrial Septation

• The division of the primitive atrium occurs in two phases.
• An incomplete partition forms before birth for blood flow between atria.
• The partition gets functionally completed at birth.
• The septum primum develops from the roof of the common atrium, extending toward the endocardial cushions, which fuse to make the AV septum.
• The ostium (foramen) primum is between the septum primum and the endocardial cushions.
• Septum secundum gets formed immediately at the right of the septum primum growing down from the roof of the atrium.
• The Septum secundum gradually overlaps septum primum.
• Foramen ovale shunts most blood entering the right atrium to the left atrium before birth.
• Septum primum closes against septum secundum, preventing blood passage in the opposite direction because of their relative rigidness.
• After birth the foramen ovale closes, due to right atrial pressure decreasing (from placental circulation occlusion) and left atrial pressure increasing (from increased pulmonary venous return).
• The anatomical closure happens during the 1st postnatal year
• Septum primum adheres against septum secundum and starts to form fossa ovalis.
• Once foramen ovale closes it becomes named the fossa ovalis, and its an embryological remnant of development.

Partitioning Bulbis Cordis and Truncus Arteriosus

• Without it, thee would only be one outflow from fused ventricles.
• Aorticopulmonary septum develops outflow paths, creating aorta and pulmonary trunk.
• Formation of the aorticopulmonary septum the development must coincide with the interventricular septum to create two separate ventricles.
• The aorticopulmonary septum develops during week 5 from swellings in walls of of the truncus arteriosus and upper region of bulbis cordis.
• Neural crest cells populate the conotruncal ridges within the aorticopulmonary septum
• NC cells complete a septum when they grow toward eacher and divide the bulbis and truncus into two arterial channels, creating both the aorta and the pulmonary trunk.
• Formation begins at the inferior end of the truncus, and then proceeds superiorly and inferiorly.
• Aorticopulmonary septum will form a spiral, which free edges unite in the center of the truncus to form a spiraling wall, which then separates the ascending aorta and pulmonary trunk.

Ventricular Septation

• Is composed of a membranous and muscular portion.
• Closure of the interventricular septum needs fusion of both endocardial cushions with the downward spiraling aorticopulmonary septum.
• Septum is located where the valves attach

Embryonic Structures and Adult Derivatives

• Primitive atria become Auricles of the right and left atria.
• Right horn of sinus venosus becomes the smooth part of the right atrium (sinus venarum).
• Left horn of sinus venosus becomes the Coronary sinus.
• Primitive pulmonary veins becomes the Smooth part of the left atrium.
• Conus cordis (upper bulbis cordis) becomes the Outflow tract for both ventricles: conus arteriosus (infundibulum) for the right ventricle, it turns into the aortic vestibule for the left ventricle, sitting just below the aortic valve.
• Bulbis cordis becomes the Trabeculated right ventricle.
• Primitive ventricle becomes the Trabeculated left ventricle.
• Truncus arteriosus becomes the Ascending aorta and pulmonary trunk.