Formation of the Heart Loop
At the beginning of the next phase of development, the heart, as described earlier, is essentially a straight tube with a caudal venosus end and cranial arterial end. It lies within the pericardial cavity and is attached posteriorly only by the dorsal mesocardium.
10-SOMITE AND 14-SOMITE STAGES
A series of chambers forms in
sequence within the tube. At the venous end the sinus venosus develops
left and right horns and receives blood from the common cardinal veins,
vitelline veins, and umbilical veins (two initially, one later). These veins
return blood from the embryo, umbilical vesicle
(yolk sac), and placenta, respectively (see Plate 4-5). From the sinus venosus, blood flows into a primordial atrium,
a primordial ventricle, a chamber called the bulbus cordis, and
then out the arterial end of the heart tube through the truncus arteriosus.
Blood from the truncus arteriosus flows into a dilated aortic sac ventral
to the foregut. The first pair of pharyngeal (branchial) arch arteries (aortic
arches) connect the aortic sac to the dorsal aortae. The
ventricle and bulbus cordis grow faster than the other parts of the heart tube.
Because its two ends are fixed, the heart tube is forced to bend in order to
adapt to the available pericardial space. The bend is between the ventricle and
bulbus cordis, forming a bulboventricular loop that extends ventrally
and to the right. A result of the bend is the more cranial and dorsal position
of the atrium and sinus venosus behind the truncus
arteriosus outflow tract of the heart tube. At the same time, perforations
appear in the dorsal mesocardium, leading to its disappearance as the openings
increase in size. The result is the formation of the transverse pericardial
sinus, the space between the arteries and veins at the top of the adult
heart that connects the left and right aspects of the pericardial cavity.
At the close of this phase of
development, numerous diverticula appear in the ventral aspect of the
early ventricle and in the proximal third of the bulbus cordis.
These diverticula project initially into the cardiac jelly and later into the myocardium, expanding the capacity of the heart sections involved and becoming the mechanism of formation of the trabecular muscle of the ventricular walls. At this point, although bending and local elaborations have changed its appearance, the heart still consists of essentially a single tube. A small, interventricular septum appears marking the start of left and right ventricles and an interventricular canal, but these are still in sequence with regard to blood flow. Although still a tube, the heart’s external appearance already strongly suggests its future four-chambered structure. The elaboration of septa will further define the atria and ventricles. The embryo is now about 3.2 mm long and approximately 25 days old, and it possesses 20 somites (see Plate 4-6).
