| Summary: | The mammalian heart is the first organ formed in the embryo and is composed of four chambers:
right atrium (RA), right ventricle (RV), left atrium (LA), and left ventricle (LV), and three layers:
the endocardium, the myocardium, and the pericardium.
The endocardium is formed by simple squamous epithelium known as endothelium, which lines the
chambers and is joined to myocardium by a thin layer of connective tissue. The pericardium is
comprised of two distinct sublayers: the inner serous pericardium, which is formed by visceral
pericardium or epicardium and parietal pericardium separated by pericardial cavity, and the outer
fibrous pericardium made of dense connective tissue. The myocardium is the muscular layer of the
heart, it is the thickest one and is located between the endocardium and the pericardium.
With regard to cellular composition, the heart is principally formed by cardiomyocytes (CM),
cardiac fibroblasts (cFib) , vascular smooth muscle cells (localized fundamentally within the
myocardium), vascular endothelial cells (located within the myocardium and endocardium) and
mesothelial cells present in the pericardium 1–3
. CM are specialized cells with a complex filament
structure responsible for the control of the rhythmic beating of the heart and present heterogeneity
depending on the location, morphology, and function, including atrial, ventricular, sinoatrial nodal,
atrioventricular nodal, His bundle, and Purkinje fibers 4,5
.
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