GENERAL
The heart is located primarily in the cervical segments. The mesocardium is separated into cranial and caudal parts by a gap that will produce the transverse sinus in the pericardial cavity.
As the epimyocardial space disappears, three layers become apparent: a) the endocardium or thin lining of the heart chambers, b) the myocardium or thick muscular layer and c) the epicardium or thin surface layer.
SEPTUM FORMATION IN THE TRUNCUS ARTERIOSUS AND AORTIC SAC
A pair of opposing ridges called right (inferior) and left (superior) truncus swellings develops in the form of a spiral in the wall of the truncus arteriosus.
These two swellings grow toward one another, meet and fuse, thus forming the truncus septum that divides the lumen of the truncus arteriosus into two vessels, the ascending aorta and the pulmonary trunk. Because of the spiral arrangement of the swellings, the two vessels twist upon one another.
The area where the first four pairs of aortic arches arise from the aortic sac moves to the right. The point where the sixth pair of aortic arches arises from the sac moves to the left and becomes separated from the other arches by the thick aorticopulmonary septum. This septum is continuous with the spiral truncus septum thus causing the pulmonary trunk to course to the sixth aortic arches and the ascending aorta to course to the remaining arches.
SEPTUM FORMATION IN THE CONUS CORDIS
Larger but slower developing swellings appear in the wall of the conus cordis. The right conus swelling forms along the right dorsal wall and becomes continuous with the left truncus swelling. The left conus swelling forms along the left ventral wall and becomes continuous with the right truncus swelling.
As the conus swellings grow toward one another, they fuse to form the conus septum that divides the conus into two portions: a) a ventrolateral portion that joins with the primitive right ventricle thereby forming the outflow tract of the definitive right ventricle and b) a dorsomedial portion that joins the primitive left ventricle thus forming the outflow tract of the definitive left ventricle.
SEPTUM FORMATION IN THE ATRIOVENTRICULAR (A-V) CANAL
As the bulboventricular flange recedes, the atrioventricular canal enlarges and moves to the right. Atrial blood then passes directly into both ventricles.
Two mesenchymal proliferations called endocardial cushions develop in the wall of the atrioventricular canal. The ventral (superior) cushion projects from the ventrocranial wall of the canal; the dorsal (inferior) cushion projects from the dorso-caudal wall. As the two cushions meet and fuse, they divide the previously single canal into two canals, one on the right and one on the left. Two other smaller proliferations called lateral cushions project from the right and left walls of the canal.
The right conus swelling becomes continuous with both the right lateral and dorsal cushions on the right side of the atrioventricular canal.
SEPTUM FORMATION IN THE VENTRICLES
The walls of the two ventricles expand in a caudal direction as the myocardium grows at a rapid rate. Muscular bands called trabeculae carneae cordis are produced in the inner part of the myocardium.
As the ventricles expand, their walls become adjacent caudal to the secondary interventricular foramen where they form the muscular portion of the interventricular septum.
Proliferation of the dorsal atrioventricular cushion reduces the size of the secondary intraventricular foramen.
SEPTUM FORMATION IN THE ATRIUM AND INCORPORATION OF THE RIGHT SINUS HORN AND COMMON CARDINAL VEIN
As both portions of the atrial chamber continue to expand, the septum primum lengthens and approaches the endocardial cushions. This gradually narrows the ostium primum and better defines the boundary between the right and left atria.
A common pulmonary vein from the region of the branching lung buds courses through the caudal part of the mesocardium to join the left atrial wall.
The right horn of the sinus venosus and the right common cardinal vein continue to enlarge and become incorporated into the right atrium. This pulls the narrow transverse portion of the sinus venosus along with the left sinus horn and common cardinal vein to a position dorsal to the heart in the atrioventricular sulcus.
With the formation of the left venous valve, the sinoatrial orifice then has a valve on each side. Cranial to the orifice the venous valves join together to produce a ridge called the septum spurium. The bay area between the septum spurium and septum primum is called the septovalvular space.
The joining of the right sinus horn to the right atrium and the shifting of the venous return to the right side cause the right atrium to dilate and bulge to the right. This produces an infolding in the roof of the septovalvular space giving rise to the septum secundum.
ARTERIES
AORTIC ARCHES
As the sixth pair of aortic arches develops, the first and second pairs disappear. The distal segment of the right sixth arch and the segment of the right dorsal aorta between the seventh intersegmental artery and the point of fusion of the two dorsal aortas begin to narrow and eventually disappear. According to some accounts a fifth pair of aortic arches develops but is very transient.
The origins of the arches from the aortic sac realign themselves in such a way that the third and fourth pairs arise from horns of the ventral part of the sac and become continuations of the ascending aorta. The sixth pair arises from the dorsal part of the sac and becomes a continuation of the pulmonary trunk.
The proximal part of the internal carotid artery is a remnant of the third arch. Its distal part is derived from the cranial segment of the dorsal aorta. The external carotid artery arises as a new branch from the ventral portion of the third arch. The segment of the dorsal aorta between the third and fourth arches begins to narrow and is called the carotid duct. It eventually disappears. The sixth pair of arches gives rise to a pulmonary artery on each side near the pulmonary trunk. Each artery courses to its respective lung bud.
Near the brain the internal carotid artery gives branches to the eye and diencephalon and then terminates as anterior and middle cerebral arteries to the telencephalon. The anterior cerebral artery will join its counterpart on the other side to form the anterior communicating artery across the midline.
The dorsal intersegmental arteries in the cervical segments anastomose into a longitudinal channel called the vertebral artery. Ventral to the myelencephalon the vertebral artery on each side joins its counterpart in the midline to form the single basilar artery. The basilar artery supplies the myel- and metencephalon before it bifurcates near the mesencephalon into right and left posterior cerebral arteries that supply the di- and mesencephalon and later on the telencephalon. An anastomosis called the posterior communicating artery develops on each side connecting the posterior cerebral and internal carotid arteries. A circular vascular channel called the Circle of Willis is thereby completed at the base of the brain.
Just cranial to the fusion point each dorsal aorta gives rise to an axial artery to the upper limb bud. The axial artery begins as a bud from the seventh cervical intersegmental artery.
COMMON DORSAL AORTA (AORTA)
The point of fusion of the paired dorsal aortas into a single aorta moves from the midcervical to the lower cervical segments.
The celiac, superior mesenteric and inferior mesenteric arteries become distinct, single vessels arising from the ventral aspect of the dorsal aorta. With the disappearance of the yolk stalk the distal part of the original vitelline artery eventually disappears. The proximal segment becomes the superior mesenteric artery that supplies the derivatives of the midgut.
Numerous lateral segmental branches of the dorsal aorta course to the mesonephros as mesonephric arteries.
The dorsal aorta bifurcates in the L-4 segment into right and left common iliac arteries each of which, after giving rise to the axial artery of the lower limb bud, continues into the umbilical cord as an umbilical artery. The axial artery develops as a bud of the fifth lumbar intersegmental artery.
VEINS
VITELLINE VEINS
The anastomotic network formed between the paired vitelline veins around the duodenum becomes a single channel that courses first to the left and then dorsal to the duodenum. This channel lies between the dorsal and ventral pancreas and represents the portal vein.
With the disappearance of the yolk stalk the distal tributaries of the vitelline veins eventually disappear. The proximal tributaries unite to form the superior mesenteric vein, which drains the midgut derivatives. The superior mesenteric vein courses through the dorsal mesentery to the pancreas area where it receives the splenic vein. It then continues to the liver as the portal vein.
As the left hepatocardiac vein disappears, blood from the left side of the liver is diverted to the right side. This causes dilation of the right hepatocardiac vein, which empties into the right horn of the sinus venosus.
UMBILICAL VEINS
An anastomosis forms between the proximal segment of each umbilical vein and the hepatic sinusoids. This segment of each umbilical vein is soon surrounded by the rapidly expanding liver cords and becomes incorporated into the liver, thus causing all of the umbilical blood to course through the liver sinusoids.
The entire right umbilical vein disappears forcing all of the chorionic blood to flow to the liver through a single channel, the former left umbilical vein.
As blood flow through the liver increases, a large channel called the ductus venosus develops as a direct communication between the umbilical vein and the right hepatocardiac vein thus allowing umbilical blood to bypass the hepatic sinusoids in its course to the heart. Blood in the portal vein also empties into the ductus venosus.
Some blood continues to enter the hepatic sinusoids by way of afferent veins, which arise from the ductus venosus. Sinusoidal blood drains by way of efferent veins into the right hepatocardiac vein.
CARDINAL VEINS
Common Cardinal Veins
The left common cardinal vein narrows and moves to the right in the dorsal atrioventricular sulcus as the left sinus horn is pulled to the right.
The right sinus horn becomes incorporated into the wall of the right atrium. The right common cardinal vein will become the terminal segment of the superior vena cava.
Precardinal Vein
The precardinal vein is a continuation of the large primary head vein that receives the primitive maxillary vein and the anterior, middle and posterior cerebral plexuses.
It narrows as it passes through the cervical region where it receives the marginal vein from the upper limb bud.
Postcardinal Vein
The cranial segment of the postcardinal vein begins to narrow as the subcardinal vein increases in size.
The caudal segment of the postcardinal vein receives the marginal vein from the lower limb bud and anastomoses in the midline with its counterpart on the other side.
Subcardinal Vein
The subcardinal vein on each side enlarges and courses longitudinally along the medial aspect of the mesonephros and gonadal ridge.
The right subcardinal vein establishes a connection with the right hepatocardiac vein.
The right subcardinal vein anastomoses across the midline with its counterpart on the other side.
COMMON PULMONARY VEIN
As the septum primum divides the common atrium into two chambers, veins from the region of the branching lung buds join together and enter the left atrium as a common pulmonary vein.
LYMPHATICS
Lymphatic channels begin as spaces or clefts in the mesenchymal tissue. A particularly large space develops in the cervical region called the jugular lymph sac which joins the precardinal vein.
Source: Atlas of Human Embryos.