Heart Wall - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 270

Tabl e 6. 1. Main stages of cardiac development. The precardiac mesoderm forms a primitive
tubular heart that starts beating at about 3 weeks of gestation in humans. The heart maintains
its pumping function despite its continuous remodeling into a 4-chamber organ. The heart has an
asy mmetric structure with anterior-posterior, cranial-caudal, and left-right polarity.

Primary heart fields Two mesoderm lateral regions that appear shortly

after gastrulation with precursors of cardiac
lineages (early myocardium)

Cardiac crescent Merging of heart fields at the embryo midline

Linear heart tube Endothelial tube surrounded by a single layer

of myocardial cells

Looped heart Tube elongation and looping with gain of progenitors

from the secondary heart field

Atrioventricular canal Tube remodeling with a segment between atrial

development and ventricular regions

Endocardial cushion formation Outflow tract and atrioventricular cushions, i.e.,

precursors of heart valves; epithelial-to-mesenchymal
transition of endocardial cells

Heart wall trabeculation Myocardium differentiation into 2 layers, an outer

compact zone and an inner trabeculated zone;
coordinated proliferation and differentiation
of myocytes; crosstalk between trabecular myocytes
and endocardial cells

Chamber and outflow Formation of 4 heart chambers and aorta

tract septation and pulmonary artery

Different myocardial progenitors form the various regions of the heart.
The embryonic heart begins to pump blood before the development of valved
chambers. The pumping action results from suction into the heart tube [ 482 ].
The blood stream in the developing heart favors the directed fluid motion in a
stressed gel-like medium. The embryonic heart is transformed from a simple tube
into a 4-chambered pump via multiple transluminal septation.
Two migratory cell populations invade the heart-forming region to regulate
cardiogenesis. The cardiac neural crest originates from the neural tube. After
migration into the cardiac outflow tract, it is involved in the septation of the aortic
sac [ 483 ]. Epicardial cells that originate from the proepicardial serosa epithelium
migrate onto the surface of the heart and create the epicardium and subepicardial
connective tissue, the coronary vascular system (endothelial cells, smooth myocytes,
and perivascular fibroblasts), intermyocardial and subendocardial connective tissue,
as well as a small number of endocardial cells close to the sinus venosus [ 483 ].
The proepicardium gives rise to the myocardial stromal and vascular smooth
muscle cells, but few coronary endothelial cells. Coronary vessels arise from
angiogenic sprouts of the sinus venosus that returns blood to the embryonic
heart [ 484 ]. Sprouting venous endothelial cells dedifferentiate as they invade the
myocardium. These cells differentiate into arterial and capillary cells. Cells on the
surface redifferentiate into venous cells.