Heart Wall - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 296

6.2 Wall Structure

The heart wall is composed of 3 layers: the internal thin endocardium ,the
thick muscular myocardium , and the external thin epicardium . Cardiac muscle is
an involuntary striated muscle. The heart is enclosed in a double-wall sac, the
pericardium.
The double-layered pericardium is constituted of the outer parietal pericardium
and the inner visceral pericardium, or epicardium, which is thus attached to
the myocardium. The pericardial cavity separates the 2 pericardium layers. It
contains a lubrificating fluid ( V

25-35 ml). The parietal pericardium consists of
an outer layer of thick, fibrous connective tissue and an inner serous layer with a
mesothelium and connective tissue. The anchoring parietal pericardium is attached
to the diaphragm and fuses with the outer wall of blood vessels entering and leaving
the heart. The visceral pericardium has an external layer of flat mesothelial cells
lying on a support tissue that contains elastic fibers. The coronary arteries reside in
the epicardium and send branches into the myocardium.
Heart cellular population includes cardiomyocytes (at least two-thirds of all
cardiac cells), nodal cells, endothelial and vascular smooth muscle cells, and
fibroblasts. These various cell types are embedded in the collagen-rich extracellular
matrix.
Collagen forms diverse connected structures, such as cables (collagen fibers),
struts (larger crosslinked collagen bundles), nets (porous sheets of collagens), and
gels (filamentous collagens associated with glycosaminoglycans). These structures
bind cardiomyocytes via integrins (Vol. 1 - Chap. 7. Plasma Membrane) that interact
with the cytoskeleton. In fact many components of the extracellular matrix bind to
plasmalemmal integrins (collagen-1 to

∼

α 3 β 1 -, fibronectin to α 3 β 1 -and α 5 β 1 -, and

laminin to α 7 β 1 -integrins).

Integrins are constituents of adhesomes and signalosomes, particularly in
mechanotransduction and electromechanical coupling, as integrins colocalize
with gap junction proteins. Moreover, connections between integrins and the
cytoskeleton regulate the functioning of mechanosensitive ion channels. The
cytoskeleton can regulate the activity of channels implicated in cardiomyocyte
depolarization and subsequent repolarization [ 566 ].
Collagenous connections between adjacent cardiomyocytes register sarcomere
landmarks to ensure synchronous contraction and relaxation. Furthermore, the
collagen network maintains gap junctions between contiguous cardiomyocytes as
well as between cardiomyocytes and nodal cells to favor the electrical connectivity.
The collagen turnover is regulated by: (1) stimulators, such as angiotensin-2, 43
endothelin-1, catecholamines, aldosterone, fibroblast growth factor FGF2, insulin-
like growth factor, among others, as well as (2) inhibitors, such as prostaglandins,
nitric oxide, natriuretic peptides, etc. [ 567 ].

α 1 β 1 -, α 3 β 1 -, and

43 Angiotensin-2 favors the production of collagen-1 and -3.