Basic - Primitive Heart Tube
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The heart is the first organ to function within an embryo. It starts to function at the beginning of the fourth week when the nutritional and oxygen requirements of the growing embryo can no longer be met by diffusion from the placenta. The heart initially forms from two tubes located bilaterally (on either side) of the trilaminar embryo in the cranial (head) region. The image on the right shows these primitive tubes developing in an embryo approximately 18 days after conception.
When looking down at this early embryo, you can see multiple blood islands dispersed throughout the embryo. These will form the early blood vessels. At the most cranial end of the embryonic disc, these blood islands are actually the primitive heart tube. From the side you can see one of the heart tubes and heart cavity developing in this position.
Embryonic Folding
The disc-like embryo then undergoes a process of folding, in which both the cranial and lateral parts of the embryo fold ventrally (forwards). This brings the heart-forming region to a ventral (frontal) position. The following animation shows the development of the heart tubes and how embryonic folding brings them to fuse in the midline.
<Flowplayer height="540" width="720" autoplay="true">Heart folding 002.flv</Flowplayer>
From here we can see the primitive heart from a ventral view as it consists of the two tubes. These tubes fuse together (as seen in the diagram on the right) to form a single, primordial heart tube, situated in the midline of the embryo, ventral to the pharynx.
Segments of the Heart Tube
At this stage, the tube already has minor constrictions within it indicating sections of the heart tube that will form parts of the adult heart. The most caudal (tail end) segment of the heart tube is the sinus venosus which will later become the ends of the major veins carrying blood to the heart as well as parts of the atria. The next segments are the primitive atrium and primitive ventricle which will become the atria and ventricles of the adult heart. Cranial to these segments are the bulbus cordis, most of which will become the right ventricle, and the truncus arteriosus which forms the pulmonary and aortic trunks carrying blood away from the heart.
Heart Tube Looping
This tubular heart undergoes a process of looping during week four of development to form a shape that resembles that of the adult heart. It initially forms a C-shape (with the convex portion of the C situated on the right side of the embryo) and then an S-shape. Eventually the atria are brought backwards and upwards, so that they lie cranially and behind the ventricles.
