FORMATION OF THE SEMILUNAR VALVES
Three tubercles form on the inside of both the ascending aorta and pulmonary artery at the junction of the truncus arteriosus and conus cordis (truncoconal junction). As they grow into the lumen, they become cuplike and together form a semilunar valve in each vessel.
CLOSURE OF THE INTERVENTRICULAR FORAMEN
The dorsal (inferior) endocardial cushion expands into the interventricular foramen and fuses with the upper edge of the muscular portion of the interventricular septum. Simultaneously, it also fuses with the lower edge of the conus septum. As a result a thick partition is formed that closes the foramen. The partition later becomes thin and fibrous to form the membranous part of the interventricular septum.
FORMATION OF THE ATRIOVENTRICULAR (A-V) VALVES
The tissue around each atrioventricular canal bulges into the canal and the ventricle below it. The bulges become flaps of tissue that are attached in some areas to the ventricular wall. Three flaps develop around the right canal forming the right atrioventricular (tricuspid) valve. Two flaps develop around the left canal forming the left atrioventricular (mitral) valve. The attachments to the ventricular wall are replaced by dense connective tissue cords called chordae tendineae, which are connected to fingerlike processes of ventricular muscle known as papillary muscles.
SEPTUM FORMATION IN THE RIGHT ATRIUM
The ostium primum disappears as the caudal edge of the septum primum fuses with the endocardial cushions.
Simultaneously the cranial part of the septum primum breaks down forming the ostium secundum.
Most of the septum spurium and the left venous valve become incorporated into the septum secundum obliterating the septovalvular space. This causes the septum secundum to lengthen along the right side of the septum primum partially covering the ostium secundum.
The right venous valve (valve of sinus venosus) grows caudally to join the fused endocardial cushion near the orifices of the inferior vena cava and coronary sinus.
CHANGES IN THE EXTERNAL CONFIGURATION
The heart moves into the upper thoracic region.
The left ventricle becomes conical shaped with its blunted end forming the apex of the heart.
The transverse part of the sinus venosus and the left sinus horn become incorporated into the coronary sinus in the dorsal part of the atrioventricular sulcus.
Venous channels from the right and left lung sacs join near the heart to form the common pulmonary vein, which empties into the left atrium.
ARTERIES
DERIVATIVES OF THE AORTIC SAC, AORTIC ARCHES AND PAIRED DORSAL AORTAS
After the heart moves caudally through the cervical segments, the neck region becomes apparent. The arteries between the heart and the brain necessarily increase in length and narrow in relation to surrounding structures. Many dramatic changes occur in the aortic sac, aortic arches and the paired dorsal aortas. In most instances the changes differ on each side.
Aortic sac—The segment of the left horn of the aortic sac between the origins of the fourth and sixth aortic arches becomes the proximal part of the definitive aortic arch. The corresponding segment of the right horn forms the brachiocephalic artery.
Aortic arches—On both sides, the common carotid artery is derived by an elongation of that segment of the horn of the aortic sac between the origins of the third and fourth aortic arches. The proximal segment of the internal carotid artery is formed by the third aortic arch, its distal segment by the cranial extension of the paired dorsal aorta rostral to the carotid duct. The external carotid artery, a new vessel, joins the internal carotid artery at its point of origin. On the left the distal part of the definitive aortic arch is formed by the fourth aortic arch and the segment of the paired dorsal aorta between the carotid duct and the seventh dorsal intersegmental artery; on the right side these segments form the proximal part (base) of the right subclavian artery. The proximal part of the sixth arch on each side becomes the stem of the right or left pulmonary artery. Its distal part on the right loses its connection with the dorsal aorta and disappears. The connection with the dorsal aorta is retained on the left forming the ductus arteriosus. The pulmonary arteries course to their respective lung through secondary channels located ventral to the primary bronchi.
Paired dorsal aortas—On both sides the segment of the dorsal aorta between the third and fourth arches (carotid duct) disappears. On the right side the segment between the origin of the seventh dorsal intersegmental artery and the junction with the left dorsal aorta also disappears. When this happens, the caudal segment of the left dorsal aorta becomes the stem of the descending aorta, and all of the blood for the lower half of the body flows toward the left.
As the cerebellum becomes established, the stems of the definitive arteries supplying it can be identified. The met- and myelencephalic arteries are replaced by the pontine, superior cerebellar and anterior and posterior inferior cerebellar arteries.
The seventh dorsal intersegmental artery forms all of the left subclavian artery and all but the proximal part of the right subclavian artery. The axial artery (definitive axillary artery) that passes through the brachial plexus to supply the derivatives of the upper limb bud is a lateral branch of its ventral division. The vertebral artery and thyrocervical trunk represent, respectively, post- and precostal anastomoses of its dorsal and ventral divisions with corresponding segments of more cranial intersegmented arteries. The costocervical trunk and internal thoracic (mammary) artery are formed, respectively, by precostal and ventral anastomoses of its ventral division with corresponding segments of more caudally situated dorsal intersegmental arteries.
AORTA
The formation of the ascending aorta is described in Chapter 6, Section VII, A. The development of the aortic arch is described above. The fused portion of the dorsal aortas forms the descending aorta. Additional definitive arteries that can be identified are as follows.
The dorsal intersegmental arteries arising from the thoracic part of the descending aorta become the intercostal and subcostal arteries. Those from the lumbar part become the lumbar arteries.
The lateral segmental or mesonephric arteries supply not only the mesonephros but also the adjacent gonad and suprarenal gland.
Many of the definitive branches of the celiac, superior mesenteric and inferior mesenteric arteries (ventral segmental arteries) can be identified.
VEINS
VITELLINE VEINS
The distal segment of the vitelline veins disappears at varying times. It sometimes remains prominent after the yolk stalk disappears, in which case it courses from the yolk sac through the umbilical coelom to the dorsal mesentery of the midgut loop where it joins the superior mesenteric vein.
When the duodenum changes to the shape of an arc, the superior mesenteric vein then passes ventral to the lower part of the arc, becoming the portal vein between the dorsal and ventral pancreas after it receives the splenic vein. The portal vein courses to the ductus venosus in the liver by passing dorsal to the upper part of the duodenal arc.
UMBILICAL VEIN
The single umbilical vein enlarges and passes through the liver as the ductus venosus, which terminates in the inferior vena cava.
Afferent veins to the hepatic sinusoids arise from the ductus venosus close to the entrance of the portal vein. Efferent veins from the hepatic sinusoids drain into the junction where the ductus venosus empties into the inferior vena cava.
CARDINAL VEINS
Left Common Cardinal Vein
The left common cardinal vein continues to narrow and eventually disappears so that it no longer drains into the coronary sinus.
Precardinal Vein Tributaries
Venous channels peripheral to the brain separate into superficial and deep portions. The superficial channels develop into sinuses within the ectomeninx. The deep channels become the cerebral veins. A narrow channel called the primitive transverse sinus passes ventrolateral to the pons area and connects the anterior and middle cerebral plexuses. A more prominent anastomosis is the primitive sigmoid sinus, which lies ventrolateral to the myelencephalon and connects the middle and posterior cerebral plexuses.
The stem of the anterior cerebral plexus receives the primitive tentorial sinus from the cerebral vesicle and the primitive marginal sinus from the diencephalon.
The rostral segment of the primary head vein between the internal carotid artery and trigeminal ganglion becomes the primitive cavernous sinus, which receives the primitive maxillary and supraorbital veins. The primary head vein becomes the precardinal vein lateral to the parachordal cartilage in the vicinity of cranial nerves IX, X and XI. In the cervical segments the precardinal vein becomes the internal jugular vein.
The internal jugular vein receives a large tributary in the cervicomandibular area that represents the external jugular vein. In the caudal part of the neck it is joined by the subclavian vein. The vessel between this junction and the heart is formed by the caudal segment of the precardinal vein and the common cardinal vein. On the right side the segment becomes the right brachiocephalic vein and superior vena cava. It disappears on the left when an anastomotic channel that will become the left brachiocephalic vein develops ventral to the aortic arch and connects the left internal jugular and subclavian veins with the superior vena cava.
Postcardinal Vein
The postcardinal vein either disappears or is incorporated into other channels. Its cranial segment on the right connects the azygos line vein that develops medial to the sympathetic trunk to the caudal segment of the precardinal vein. Its intermediate segment becomes a plexus dorsal to the kidney before it disappears completely. The interpostcardinal anastomosis between the caudal segments contributes to the formation of the distal part of the inferior vena cava.
Subcardinal Vein
The intersubcardinal anastomosis will become the major portion of the left renal vein. The cranial segment of the subcardinal vein on the right side dilates to form the suprarenal part of the inferior vena cava. On the left this segment becomes the left suprarenal vein. The caudal segment of the subcardinal vein on each side is incorporated into the gonadal vein.
Supracardinal Vein
A third longitudinal channel develops dorsomedial to the postcardinal vein and connects to both the postcardinal and subcardinal veins of the corresponding side. Only part of the caudal segment remains on the right side. It connects the postcardinal anastomosis with the intersubcardinal anastomosis forming the infrarenal part of the inferior vena cava. The sub- and supracardinal anastomosis on the left is incorporated into the renal vein.
Inferior Vena Cava
The inferior vena cava is the largest definitive vein in the body. It begins in the lower lumbar region at the junction of the common iliac veins and ends in the right atrium. It is a composite structure forming only on the right side from the dilated segments of four major venous channels. Beginning caudally these channels are the 1) interpostcardinal anastomosis, 2) right supracardinal vein, 3) right subcardinal vein and 4) right hepatocardiac vein (proximal right vitelline vein).
PULMONARY VEINS
Distinct right and left pulmonary veins from their respective lung sac join near the left atrium to form the common pulmonary vein. The common pulmonary vein disappears as it becomes incorporated into the left atrial wall.
LYMPHATICS
The jugular lymph sac enlarges and extends cranially to the upper cervical segments lateral to the internal jugular vein. Near its termination in the internal jugular vein it receives the axillary lymph sac from the proximal part of the upper limb.
An iliac lymph sac appears around the iliac blood vessels from the lower limb.
The mesenteric lymph sac begins as small spaces dorsal to the inferior vena cava and aorta.
Source: Atlas of Human Embryos.