Development of the heart 1

Objectives:Understand early development of blood vessels.Basic understanding of the early stages of heart development.Describe the formation and position of the heart tube.Discus the development of sinus venosus.Describe partioning by septa and chambers formation.Discus congenital malformations.Gain knowledge of fetal circulation.

THE CARDIOVASCULAR SYSTEMThe cardiovascular system is the first functional system; blood begins to circulate by the end of the third week of development.

Blood vessels:Mesenchymal cells of the splanchnic mesoderm group together to form blood islands (angiogenic clusters).The clusters develop a lumen.The cells on the periphery become endothelium; the cells in the middle become blood cellsThe clusters spread toward one another and fuse to form the vessels.

Heart Formation:The cardiogenic area develops cranial to the oropharyngeal membrane during the third week of development; the cranial folding of the embryo pushes the cardiogenic area caudally. This area (one on each side) develops into the endocardial heart tubes.

As the embryo folds, the endocardial heart tubes approach each other and fuse to form a single endocardial tube.Differential growth in the heart tube causes the heart to bend.

Blood flow thourgh the primitive heart:Common cardinal viens, umbilical veins and vitelline veins drain into the sinus venosus--> atrium -->atrioventricular canal--> venricle--> bulbus cordis--> truncus arteriosus--> aortic sac--> aortic arches--> dorsal aortae.

Looping of heart tube (Days 23-28): The primitive atrium loops up behind and above the primitive ventricle and behind and to the left of the bulbus cordis.

As the heart is bending and enlarging, the internal partitioning of the original single chamber is occurring; this partitioning is occuring simultaneously in the atrium and the ventricle during days 27-37.The atrioventricular partition produces one atrium and one ventricle.

The endocardial cushions (which contain neural crest cells) approach each other and fuse, leaving an opening on each side which is the site where the atrioventricular valves develop (tricuspid and bicuspid).The atrial septum divides the primitive atrium into two atria.

The septum primum:Grows toward the developing endocardial cushions from the superior part of the atrium.The space between the inferior flap of the septum primum and the endocardial cushions is the foramen primum.

The foramen primum is a space that is gradually getting smaller as the septum primum extends downward.Eventually the foramen primum is obliterated BUT by the time it is obliterated, a second foramen has appeared in the upper part of the septum primum .

This second foramen is the foramen secundum; thus, the flow of blood from the right atrium into the left atrium is maintained. This is one of the mechanisms to bypass the non-functional fetal lungs.Both the foramen primum and the foramen secundum are in the septum primum.

The septum secundum:This is a second flap that develops and grows downward from the atrium toward the ventricle and upward from the ventricle to the atrium (there are two segments to this septum); it is located just to the right of the septum primum.The two segments of the septum secundum do NOT fuse together.

The septum secundum overlaps the foramen secundum in the septum primum, forming an incomplete partition, the foramen ovale. Most of the atrial septum is formed by the septum primum.The interventricular septum grows from the bottom of the ventricle and fuses with the downgrowing part of the endocardial cushion.The bottom part = the muscular part of the septum. The top part = membraneous part of septum.

Right atrium: As the heart continues to grow, the right side of the sinus venosus is incorporated into the right side of the primitive atrium. The primitive atrial wall is pushed ventrally, eventually becoming the right auricle. In the adult heart, the right auricle contains pectinate muscle derived from the primitive atrium, whereas the sinus venarum is derived from the sinus venosus and therefore smooth-walled. 

Left atrium: The left side of the primitive atrium sprouts a pulmonary vein which branches and sends two veins toward each of the developing lungs. The trunk of this pulmonary vein is incorporated into the left side of the primitive atrium, forming the smooth wall of the adult left atrium. The left side of the primitive atrium is pushed forward and eventually becomes the trabeculated left auricle.

What embryonic structures are incorporated into the adult right atrium?

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