Heart Wall - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 269

Diverse cell populations are structural determinants of heart function. Cardiomy-
ocytes represent the main cell population ( 75%) in the myocardium. Cardiac

fibroblast is the main producer of the extracellular matrix with its collagen and
elastin bundles and interconnected basement membranes.
Vascular cells are close to cardiomyocytes to efficiently supply nutrients.
Parasympathetic and sympathetic nerve fibers extend within the myocardium. The
right and left vagus nerves primarily innervate the sinoatrial and atrioventricular
nodes responsible for the genesis and proper propagation of the electrochemical
wave, respectively. Sympathetic nerves are located inside the atria and ventricles.

∼

6.1 Cardiogenesis and Cardiac Remodeling

The heart is the first organ to form during embryogenesis. This first functional organ
has a mesodermal origin. Growth and remodeling of the embryonic myocardium
adapt dynamically to fit the changing needs of a developing body. Its contraction
supports heart shaping via a feedback between the hemodynamic stress field and
cardiac cells.
Soon after gastrulation, the heart starts to develop from cells of the mesoderm
(Chap. 11 ). The main stages of cardiogenesis are briefly described in Table 6.1 .
In vertebrates, the heart is initially built from the fusion of the 2 endocardial tubes
that arise in the splanchnic mesoderm. The resulting primitive heart tube at the
ventral midline undergoes a series of movements and remodelings to form the
chambered organ. Disturbances of cardiogenesis cause cardiac malformations.
Markers of initial cardiac differentiation include Heart and neural crest deriva-
tives expressed HAND1 and HAND2 transcription factors, or class-A basic helix-
loop-helix proteins bHLHa27 and bHLHa26, T-box transcription factor TBx5, as
well as myosin regulatory light chain MLC2. 1
Among the first features of anteroposterior patterning, atrial (aMHC1) and
ventricular (vMHC1) myosin heavy chains are restricted to the anterior and posterior
poles of the heart tube, respectively.
Two myosin heavy chain genes are expressed in the mouse heart: β MHC, or

MyH7, in embryonic cardiomyocytes, and α MHC, or MyH6, postnatally. Efforts

cause hypertrophy and a shift to fetal β MHC with negative repercussion on the

cardiac function.
Organ morphogenesis involves the orchestration of cell differentiation,
proliferation, and migration (Fig. 6.1 ). During early embryogenesis, the heart is
a single, relatively straight tube that bends and twists. The cardiac tube is composed
of an outer layer of myocardium and an inner lining of endocardial cells, separated
by an extensive extracellular matrix, the so-called cardiac jelly. Local protuberances
of the cardiac jelly and associated mesenchymal cells form cardiac cushions.

1 A.k.a. regulatory myosin light chain MyL9.