Heart Wall - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 297

The myocardium contains mainly cardiomyocytes. In both ventricles, papillary
muscles and chordae tendinae tether the valve. The extracellular matrix couples the
different cardiac cell types and conveys chemical, mechanical, and physical signals
that regulate cell functioning.
The endocardium is composed of 3 layers. The outer layer is made of collagen
fibers, which merge with collagen surrounding adjacent cardiomyocytes. This layer
contains the Purkinje fibers (cardiac Purkinje cells) of the nodal tissue. The thickest
middle endocardial layer contains more regularly arranged collagen fibers and
elastic fibers arranged in parallel. The inner layer is constituted of endothelial cells
(EC), which are continuous with the vessel endothelium. The endocardium is thicker
in the atria than in the ventricles.
The heart has a fibrous skeleton with its central fibrous body, which prevents
early propagation of action potential. The central fibrous body provides extensions:
(1) the valve rings, to which are inserted the cardiac valves and (2) the membra-
nous interventricular septum, 44

that can bulge into the right ventricle to form an

aneurysm.

6.2.1 Cardiofibroblasts

Activated fibroblast, a mature cell, synthesizes constituents of the extracellular ma-
trix, such as collagen, elastic, and reticular fibers, as well as glycosaminoglycans and
glycoproteins. Fibrocyte, a less active cell type, is involved in matrix maintenance.
Cardiac fibroblasts reside in the cardiac wall, where they align in the direction
of myofibers. In the adult human heart, fibroblasts represent about 60% of total cell
population, whereas myocytes occupy about 75% of the myocardium volume [ 568 ].
They not only synthesize extracellular matrix constituents, but also respond to
mechanical and electrical stimuli, and secrete hormones and growth factors involved
in cardiac homeostasis.

6.2.1.1 Heterocellular Connections

Heterocellular gap junctions may couple cardiomyocytes and nodal myocytes
(especially sinoatrial nodal myocytes) to fibroblasts [ 568 ]. Connexin-43 and -45
are produced by both cardiac myocytes and fibroblasts.
In addition, thin, large conductance membrane nanotubes (caliber 50-200 nm),
which are ephemeral connections, enable long-distance intercellular interactions
between cardiac myocytes and fibroblasts [ 569 ]. Membrane nanotubes contain actin

44 The pars membranacea septi correspond to the upper part of the interventricular septum.