Embryology of CVS Abbas A. A. Shawka

Medical student 1st stage

Subjects • Cardiogenic plate • Cardiac looping • Pericardium development

Introduction • In the young embryo epiblast cells wander via the primitive

streak between epi- and hypoblast and form the mesoderm layer out of which various structures arise. Cells, which are destined for cardiogenesis, take up a position that is cranial to the forming neural tube.

• Cardiogenesis takes place via a complex series of steps: 1. Determination of mesoderm- and neural crest cells for

heart formation 2. Growth and differentiation processes to become

cardiomyocytes 3. Migration and transformation processes in order to form

the hear

Cardiogenic plate : The cardiogenic plate is formed by a collection of mesoderm cells in the most anterior part of the embryo.

Transverse section of the embryo showing angioblastic cords in the cardiogenic mesoderm and their relationship to the pericardial coelom.

Longitudinal section through the embryo illustrating the relationship the angioblastic cords to the oropharyngeal membrane, pericardial coelom, and septum transversum.

1 • The embryo is diskshaped. During

the transition from the amniotic cavity to the umbilical vesicle a horseshoe-shaped gap forms in the cranial region, the intraembryonic coelom, probably due to unequal growth. The cranial part of this gap, the pericardial cavity is located above the cardiogenenetic plate in front of the embryo. At this point, this coelom has no connection to the extraembryonic coelom (chorionic cavity).

1 Cardiac anlage (cardiogenetic plate) 2 pericardial cavity (part of the intraembryonic coelom) 3 Embryo 4 Edge of the umbilical vesicle section 5 Edge of the amniotic cavity section 6 Coelom opening

2 • With the extension of the

amniotic cavity and the cranial flexion of the embryo the whole cranial primordium goes through a rotation of 180 degrees with the pericardial cavity. The cardiac primordium lies dorsally to the pericardial cavity (with relation to the embryo).

1 Cardiac anlage (cardiogenetic plate) 2 pericardial cavity (part of the intraembryonic coelom) 3 Embryo 4 Edge of the umbilical vesicle section 5 Edge of the amniotic cavity section 6 Coelom opening

3 • The pericardial cavity is

bent over forwards and has enclosed the cardiac primordium (myoepicardium, cardiac jelly and the endocardiac tube) in the front.

• Dorsally, the cardiac primordium forms the two layers of the pericardial bilaminar structure of a folded envelope, the mesocardium.

1 Cardiac anlage (cardiogenetic plate) 2 pericardial cavity (part of the intraembryonic coelom) 3 Embryo 4 Edge of the umbilical vesicle section 5 Edge of the amniotic cavity section 6 Coelom opening

• Up to the early (at roughly 25 – 27 days) this cardiac anlage is still located in the visceral part of the splanchnic mesoderm (splanchnopleura) above the umbilical vesicle. Out of this mainly develops the myocardium, which is responsible for the very early contractile ability of the embryonic heart. Through the rotation following the cranial folding of the embryo the pericardial cavity comes to lie ventrally from the cardiac anlage and, as things progress, will surround it.

• The blood from the supplying vessels, the umbilical and omphalomesenteric veins, flows caudally over the inflow tract into the cardiac anlage and leaves it via the outflow tract and the aortic arches at the cranial end

• The cardiac tube itself consists of three layers: epicardium, myocardium and endocardium.

• The outermost layer and boundary of the pericardial cavity is the epicardium. The myocardial mantle follows as the next inner layer. Together they form the myoepicardium. The considerable distance from the myocardial mantle to the endocardial tube is filled with cardiac jelly and the cardiac lumen is coated with endocardial cells.

• The heart consists of the myoepicardial mantle, the cardiac jelly and the endocardium.

• In this transversal section the heart finds itself at the level of the rhomboencephalon, i.e., still very cranial. Note the position of the pericardium in relation to the heart, the many layers of the cardiac wall and the provisional existence of a dorsal suspension (mesocardium).

The formation of the venticular loop - the U-shaped heart

• The loop formation in a side view should mainly illustrate how the inflow tract (pink) slips behind the embryonic ventricle (violet) and the outflow tract (yellow). Further, the beginning formation of the sulcus atrio-ventricularis between the atrium and ventricle can be seen well.

1 Saccus aorticus 2 Conus 3 Atrial anlage 4 Ventricular anlage 5 Left horn of the sinus venosus 6 Left umbilical vein 7 Left omphalomesenteric vein 8 Pericardium 9 Sulcus interventricularis

• By the formation of the loop the inflow tract migrates towards the back and upwards. Thereby, more and more from the sinus venosus or the material for the atria is incorporated into the pericardial sac (pericardium). The ventricle part forms a loop towards the front and to the right = Dloop (not easily seen in this picture).

1 Saccus aorticus 2 Conus 3 Atrial anlage 4 Ventricular anlage 5 Left horn of the sinus venosus 6 Left umbilical vein 7 Left omphalomesenteric vein 8 Pericardium 9 Sulcus interventricularis

• The inflow tract is now at the level of the ventricle. Through the forming of the ventricular loop a small and a large curvature comes into being. The sulcus primus corresponds to the small one. The large one is not visible in this view because it empties to the right.

1 Saccus aorticus 2 Conus 3 Atrial anlage 4 Ventricular anlage 5 Left horn of the sinus venosus 6 Left umbilical vein 7 Left omphalomesenteric vein 8 Pericardium 9 Sulcus interventricularis

• The atria now enclose the outflow tract on the left and right. The ventricles grow downwards and in the middle they form an indentation, the sulcus interventricularis (not visible here). This divides it into left and right ventricles. 1 Saccus aorticus

2 Conus 3 Atrial anlage 4 Ventricular anlage 5 Left horn of the sinus venosus 6 Left umbilical vein 7 Left omphalomesenteric vein 8 Pericardium 9 Sulcus interventricularis

Frontal view of heart looping • In the frontal view mainly the formation of

the D-loop of the embryonic ventricle (violet) is easily seen. The inflow tract moves upwards and will be covered by a part of the ventricle. The development of the sulcus primus can be easily recognized in this view. Via the movement of the inflow tract to the right a part (later: right atrium) becomes visible besides the outflow tract

• By the merging of vesicles (plexiform phase of heart development) the tubular heart forms medially, surrounded by a myoepicardiac mantle. The omphalomesenteric veins and, somewhat later, the umbilical veins connect with it via the sinus venosus at the caudal pole of the tubular heart. The pericardial cavity surrounds the entire heart. The septum transversum forms the boundary to the midgut.

• Through an extension of the righthand volume of the heart towards the right and, at the same time, an extension of the whole cardiac loop, a cardiac tube, bent over forwards and to the right, forms, whereby the inflow and outflow tracts remain in the middle.

• With the forming of the loop (Dshaped loop) a large curvature towards the right forms (on the left in the picture) and thereby a small curvature arises on the inside forming a crease that gets increasingly deeper, the sulcus primus.

• These bends become tighter very rapidly so that soon three sections lying opposite to one another can be recognized. The proximal section, consisting of the supply veins, that form the sinus venosus, extends itself backwards and upwards in a cranio-dorsal, left convex bend. The middle section (ventricle part) runs from cranial, dorsal left to caudal, ventral right and becomes a caudal-ventral right convex bend. At the transition to the distal section, the outflow tract, which represents a cranial-ventral left convex bend, a deep crease, the sulcus primus, forms that corresponds to the interior of the crista prima. It has a decisive influence on the septation of the outflow tract.

Key! 1 Pharyngeal pouch I 2 Aortic arch I 3 Anterior intestine 4 Myoepicardiac mantle 5 Pericardial cavity 6 Amniotic cavity 7 Endocardiac tube 8 Midgut 9 Septum transversum 10 Omphalomesenteric vein 11 Umbilical vein 12 Sulcus primus 13 Sulcus atrio-ventricularis 14 later: right atrium

Development of the pericardium 1 Pericardial cavity 2 Cranial end of the embryo A Arterial part (outflow tract) V Venous part (inflow tract)

• The pericardial cavity as well as the cardiogenic plate lie cranially to the embryonic anlage. The pericardial cavity is found dorsal to the cardiogenic tissue. Further, one can distinguish the venous (inflow tract) from the arterial (outflow tract) parts of the cardiac anlage. The venous part of the later cardiac anlage is found more cranially than the arterial.

• After the 180 degree rotation the pericardial cavity is found ventral to the cardiac anlage. Through the rotation, the venous part (inflow tract) of the cardiac anlage now lies caudal of the arterial pole (outflow tract).

• The pericardium covers the cardiac loop from the front and forms a double layered structure with a meso, the mesocardium, along the entire length of the cardiac loop. The visceral layer of the pericardium lies on the heart and the parietal layer is the outer one of the pericardial bilaminar structure.

• The heart begins to bend towards the right and forms a D-loop and the inflow tract is moved upwards next to the outflow tract. Now the mesocardium dissolves in the central part and the heart is only attached on the inand outflow tracts.

• The atrium moves from behind on the right upwards towards the outflow tract. The mesocardium has degenerated in the middle part. Between the atrium and ventricle a sulcus has formed, the sulcus atrioventricularis.

• At the inflow tract, where slowly the sinus venosus is integrated into the right atrium, the pulmonary veins form on the left. The left sinus horn, with its influxes from the left common cardinal, the umbilical and the omphalomesenteric vein; slowly shrinks. The right sinus horn turns so that the right common cardinal vein shows itself upwards. Later, a part of the superior vena cava will be formed from it.

Key! • 1 Aortic sac • 2 Pericardiac meso (Mesocardium) • 3 Sinus venosus • 4 Pericardium parietale • 5 Atrial anlage • 6 Pericardiac cavity • 7 Outflow tract • 8 Right umbilical vein • 9 Right omphalomesenteric vein • 10 Right common cardinal vein (superior vena cava) • 11 Pulmonary vein • 12 Inferior vena cava • 13 Left horn of the sinus venosus (Sinus coronarius)

Descent of the heart

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