The heart tube is located primarily in the upper cervical segments and remains suspended dorsally by the mesocardium. Continued growth of the tube moves the bulboventricular part in a ventrocaudal direction. The initially paired atria, which were outside the pericardial cavity, fuse into a common chamber and move in a dorsocranial direction into the pericardial cavity. As a result of both movements the atrioventricular junction comes to lie in a more central position. The heart remains a single tube, but the location of its four definitive chambers becomes apparent.
The wall of the primitive ventricle becomes trabeculated to form the primitive left ventricle.
The bulbus cordis lies on the ventral side to the right with the deep bulboventricular sulcus at its left margin. The bulbus cordis can be subdivided into three parts:
The proximal part next to the primitive left ventricle is large with trabeculated walls and forms the primitive right ventricle. The primitive right and left ventricles are separated on the outside by a shallow interventricular groove. On the inside they communicate through the large primary interventricular foramen that is bounded cranially by a ridge of myocardium called the bulboventricular flange.
The middle part is cone shaped and called the conus cordis. It has smooth walls and courses obliquely toward the midline to form the outflow channels of both ventricles.
The distal part is the narrow, smooth-walled truncus arteriosus. It is located in the midline and joins the aortic sac.
The common atrial chamber communicates with the primitive left ventricle through the single atrioventricular (A-V) canal near the middle of the heart.
The common atrial chamber is divided into right and left portions by a ventral depression caused by the conus cordis and truncus arteriosus. The portions communicate with each other through a constricted area called the ostium primum. As each portion of the atrial chamber bulges cranially, a ridge proliferation between them produces the septum primum. The atrial walls remain temporarily smooth. Muscular trabeculae begin to form in the walls and produce bulges on the inner surface. The bulges will develop into muscular ridges called musculi pectinati.
Initially, the paired primordia of the sinus venosus, like the atria, are located entirely outside the pericardial cavity. They fuse into a common chamber and move in a dorsocranial direction. The chamber is not yet entirely within the pericardial cavity, coming to lie between the cavity and the hepatic trabeculae. It develops a large right and a small left horn. The right horn dilates and joins with the right atrium through the large sinoatrial orifice. A redundant fold of the wall to the right of the orifice forms the right venous valve. Each sinus horn receives three veins: a) the common cardinal vein, b) the umbilical vein and c) the vitelline vein by way of the hepatocardiac vein.
The walls of the heart tube remain double layered with a thin inner layer called the endocardium and a thick outer layer called the epimyocardium. A trabeculum of muscle tissue forms in the epimyocardium in the region of the ventricles. The endomyocardial space continues to separate the two layers except in the sinoatrial region.
ARTERIES
Four pairs of aortic arches become apparent and connect the aortic sac with the paired dorsal aortas above the primitive pharynx. The first pair loses its connection with the dorsal aorta and begins to disappear in the mandibular process of the first branchial arch. The internal carotid artery is the rostral continuation of the dorsal aorta and lies in the roof of the primitive pharynx on each side of the hypophyseal pouch. The caudal three pairs of aortic arches course through the central portion of their respective branchial arch.
The large, paired dorsal aortas course caudally on either side of the midline between the notochord and the primitive gut. They gradually approach each other and fuse in the midline in the midcervical region. In the thoracic and upper lumbar regions the dorsal aorta is a large, single vessel. It bifurcates into right and left common iliac arteries in the lower lumbar region. The median sacral artery continues in the midline into the tail bud. After each common iliac artery gives rise to a small axial artery to the lower limb bud, it continues into the umbilical cord as an umbilical artery carrying blood to the chorionic villi. The umbilical arteries initially fuse in the proximal part of the cord but later separate completely.
Branches of the single dorsal aorta are divided into three groups:
Dorsal intersegmental arteries, which arise at every level between somites to supply the dorsal body wall and the spinal cord. In the region of each limb bud, one artery gives off a lateral branch that enlarges to form the axial artery.
Lateral segmental arteries, which supply the derivatives of the intermediate mesoderm (pro- , meso- , and metanephroi).
Ventral segmental arteries, which course through the dorsal mesentery to supply the gut and its derivatives. The artery to the fore- and midgut junction area is the celiac artery. One of the arteries to the midgut proper will become the superior mesenteric artery and one to the hindgut will become the inferior mesenteric artery. Several of the arteries to the midgut join together ventral to the gut to form the large vitelline artery. The vitelline artery courses in the yolk stalk bringing blood to the vitelline plexus in the yolk sac wall.
VEINS
Three distinct pairs of venous channels bring blood to the sinus venosus: 1) vitelline veins from the yolk sac, 2) umbilical veins from the chorionic villi and 3) cardinal veins from the embryo proper.
Vitelline veins—After draining the yolk sac the right and left vitelline veins form dorsal and ventral anastomoses around the primitive duodenum. Then they break up into hepatic sinusoids as a result of invasion by the hepatic trabeculae. The sinusoids drain into the horns of the sinus venosus by short right and left hepatocardiac veins.
Umbilical veins—Blood in the chorionic villi moves to the embryo through the common umbilical vein in the umbilical cord. As the vein enters the embryo it divides into right and left umbilical veins, which break up into an extensive plexus in the lateral body wall. Each vein reforms and courses in the lateral body wall to join its respective horn of the sinus venosus. The proximal part of the left umbilical vein begins to disappear and in the region of the hepatic trabeculae anastomoses with the hepatic sinusoids.
Cardinal veins—Blood from the cranial part of the embryo drains to the heart through right and left precardinal veins. Blood from the caudal part of the embryo drains to the heart through right and left postcardinal veins. The pre- and postcardinal veins join on each side of the heart to form a short common cardinal vein, which enters its respective horn of the sinus venosus.
Precardinal vein—The cranial segment of the precardinal vein is called the primary head vein, which receives the anterior, middle and posterior cerebral plexuses. It passes medial to the trigeminal ganglion but ventrolateral to the otocyst. The precardinal veins course to the heart lateral to the dorsal aortas.
Postcardinal vein—The postcardinal vein begins in the tail bud and passes in a longitudinal manner toward the heart in the lateral part of the dorsal body wall between the somites and mesonephros. It receives intersegmental veins at every level. A new longitudinal channel called the subcardinal vein begins to develop along the medial side of the mesonephros. It gradually enlarges and eventually takes over the drainage of the expanding mesonephros. The subcardinal vein drains into the postcardinal vein by way of transverse anastomoses.
Source: Atlas of Human Embryos.