Formation of the heart tube
- Cardiac mesoderm is specified early during gastrulation and is located in the anterolateral portion of the embryo, forming the primary heart fields.
- Endothelial precursor cells form tubular structures in the bilateral primary heart fields, and during the 4th week, lateral folding brings these tubes together to fuse in the midline resulting in formation of the endothelial and myocardial components of the linear heart tube.
- Then anterior/posterior folding of the embryo brings the heart tube to lie ventral to the foregut as the embryo takes on a convex shape.
- Next, the primary heart tube sinks into what will become the pericardial cavity: fixed at cranial and caudal ends by the dorsal mesocardium.
- By day 22, the primary heart tube wall will consist of three components:
- The endothelium, which forms an inner lining and is continuous with developing blood vessels;
- The myocardium, which is formed when the surrounding splanchnic mesoderm and the cardiac progenitor cells it contains invests the endothelium of the primary heart tube.
- And the cardiac jelly, a largely acellular extracellular matrix, which lies between layers of endothelium and myocardium.
- Constrictions within the primary heart tube forms five primary segments: these outline critical future structures (from caudal to cranial):
- Sinus venosus - receives venous inflow from right and left sinus horns
- Primitive atrium - forms definitive right and left atria
- Primitive ventricle - forms definitive left ventricle
- Bulbus cordis - forms majority of definitive right ventricle
- Conotruncal segment - outflow tract which is subdivided into two sections
- Conus arteriosus - incorporated into right and left ventricles
- Truncus arteriosus - divides to form ascending aorta and pulmonary trunk
- These segments are additionally separated by two primary sulci
- Atrioventricular sulcus - separates the primitive atrium from primitive ventricle
- Bulboventricular sulcus - separates the primitive ventricle from bulbus cordis
- The aortic sac is located at the most cranial portion of the primitive heart tube and later gives rise to six pairs of aortic arches.
- Blood flow is initially caudal to cranial. The sinus venosus receives blood from the bilateral vitelline veins (from yolk sac), the common cardinal veins (from embryo), and the umbilical veins (from primitive placenta) while the truncus arteriosus serves as the outflow tract of the developing heart.
- The heart is the human body's first functioning organ. The heart tube begins to beat around day 22 -23 with peristaltic waves and circulation can be recorded via Doppler by day 27-29.
Looping of the heart tube
- The primary heart tube begins the process of looping on approximately the 23rd day, following activation of gene cascade that determines right-left symmetry (Ashworth).
- Cells migrate from the splanchnic mesoderm (secondary heart field) at the venous (caudal) and arterial (cranial) poles to then contribute to growth of the heart tube.
- Lengthening of the fixed heart tube and dissolution of dorsal mesocardium allow rightward bending and torsion to occur as the heart takes on a C-shaped formation.
- Looping reverses the positions of the primitive atrium and ventricle, modifying the initially C-shaped heart into more of an S-shape. This is achieved by bending of the cranial outflow tract toward the right and ventrally, and movement of the primitive ventricle toward the left, both dorsally and superiorly.
- Elongation of the primary heart tube is commonly complete by day 28. Additional remodeling brings the primordial chambers into the final spatial arrangement present through the remainder of gestation and adulthood.
- Migration of neural crest cells combined with synthesis and expression of specific molecules by myocardium into cardiac jelly induces formation, migration, and proliferation of mesenchymal cells from endocardium forming endocardial cushion tissue (ECT).
- The ECT is crucial for proper formation of the fibrous portions of atrial and ventricular septa, bulbar ridges dividing the outflow tract into right and left sides, and forming the cardiac skeleton of the heart.
Formation of cardiac chambers
- Both atria and ventricles form by ballooning growth from primary heart tube
- Expansions from the primitive atrium balloon to either side of the outflow tract occurs to form the primordia of both right and left atrial appendages.
- The expansions that balloon outward from the ventricular loop, forms the apical components of both the right and left ventricles; initially supporting the circumference of the outflow tract and atrioventricular canal, respectively.