Formation of the Heart Tube
ONE-SOMITE
AND TWO-SOMITE STAGES
As the primitive, bilaterally symmetric cardiovascular system appears, shaping of the embryo during the fourth week profoundly influences the relative position of the cardiac portion of this system. The trilaminar embryonic disc folds into a cylinder, and the amnion tucks around the embryo on each side. The amnion also envelops the head end of the embryo as the ectodermal tube of the forebrain rapidly increases in size in a cranial and ventral direction. The result is a 180-degree sagittal plane rotation of the cardiogenic mesoderm and oropharyngeal membrane, which were originally cranial to the neural plate and the developing neural tube. The heart is now caudal to the oropharyngeal membrane rather than cranial, and the heart locates dorsal to the developing pericardial cavity (see Plate 4-3).
The folding of the gastrula also
results in (1) formation of an endodermal gut tube consisting of a foregut,
midgut, and hindgut and (2) each dorsal aorta leaving the cranial end of the
endocardial tubes and curving dorsally
around the foregut. Thus the first pair of aortic arches, or pharyngeal
(branchial) arch arteries, appear.
With the folding of the trilaminar
embryonic disc, the endoderm is shaped into a tube within the embryo. The midgut
is continuous with the umbilical vesicle (yolk sac) that extends ventrally
from the embryo. The foregut is an extension into the head region of the
embryo, and the hindgut is a caudal extension. The umbilical vesicle is
pressed against the connecting stalk, and the intraembryonic part becomes a
narrow duct connected to the midgut. With the rotation of
the cardiogenic mesoderm and oropharyngeal membrane from growth of the
forebrain, the developing heart swings to a position ventral to the foregut, a
relationship retained in the adult. The dorsal aortae are dorsal to the
foregut, and pairs of pharyngeal (branchial) arch arteries flank the foregut to
connect the heart tubes with the aortae.
As a result of all these changes,
the endothelial heart (endocardial) tubes lie closer and parallel to each
other.
They quickly fuse into a single tube
in a craniocaudal direction.
FOUR-SOMITE AND SEVEN-SOMITE STAGES
As indicated earlier, the heart tube
is dorsal to the developing pericardial cavity. As the tube enlarges and bends,
it bugles into the underlying coelom. As the heart tube comes to rest entirely
within the pericardial cavity, it is suspended by the two opposing epithelial
layers of the pericardial sac, the dorsal mesocardium. A ventral
mesocardium never develops.
The mesodermal tissue surrounding
the endothelial heart (endocardial) tube, meanwhile, has
differentiated into three layers. The inner layer immediately around the
endothelium is initially thick, gelatinous connective tissue called the cardiac
jelly. The next layer is the cellular primitive myocardium. The
third (outer) layer consists of flat mesothelial cells that also line the
remaining pericardial cavity. The cardiac jelly
disappears. The endocardial tubes persist as the inner lining of the heart
chambers, the endocardium. The primitive myocardium elaborates and
matures to become the muscular wall of the heart, the myocardium. The simple squamous
epi cardium, although
continuous with the rest of the pericardial sac, derives from cells overlying
the sinus venosus of the heart tube that migrate over the heart (see Plate 4-4).
The embryo now has seven somites, is
about 2.2 mm long, and is approximately 23 days old. About 3 days have elapsed
between the appearance of intraembryonic vasculogenesis and the formation of
the endocardial tube. About this time, or slightly earlier, the heart begins to
beat. No known cardiac anomaly can be attributed to the developmental phases
described thus far, except the rare cases of acardia seen infrequently in twins with a common placental circulation.
