129
the origin of macrophages. The traditional model states that blood monocytes give
rise to all tissue macrophages. The second hypothesis assumes a prenatal coloniza-
tion of tissues by resident macrophages derived from embryonic yolk sac progeni-
tors. These macrophages persist throughout adulthood and self-renew without input
from circulating monocytes [ 93 – 95 ].
Macrophages secrete a variety of cytokines, pro-inflammatory molecules, andtrophic mediators. Some of these have been suggested to inhibit apoptosis in
hypoxic cardiomyocytes or promote neonatal heart regeneration [ 96 ]. In various
cardiac diseases, such as ischemic heart disease and idiopathic dilated cardiomy-
opathy, the expansion of macrophage populations occurs [ 97 ]. Cardiac macrophages
may contribute to tissue remodeling during chronic pressure-overload heart failure
or heart fibrosis through the activation of myofibroblasts [ 98 , 99 ]. The role of mac-
rophages in inflammation after a myocardial infarct indicates that these cells are
absolutely necessary for adequate wound healing and scar formation [ 100 ].
There is only one scientific article that describes the link between cardiac macro-phages and physical exercise. In a study by Botta et al. [ 101 ], the infiltration of the
hearts of diabetic mice by F4/80+ macrophages was attenuated by exercise, which
consisted of animals running on a motorized exercise wheel system.
7 Cardiac Endothelial Cells and Pericytes
Endothelial cells resemble simple squamous epithelial cells. They have their own
basal lamina and line blood vessels. The internationally accepted Terminologia
Histologica describes them as an epithelial tissue [ 38 ]. On the other hand, they are
of mesodermal origin and can produce collagen type IV. Therefore, endothelial cells
are also considered to be connective tissue cells. Inside the heart, there are two dif-
ferent populations of cardiac endothelial cells:
- vascular endothelium (lining the luminal surface of the coronary vessels; Fig. 8.8)
- endocardial endothelium (a continuous monolayer of cells that line the cavities
of the heart; Fig. 8.9).The differences between these types of endothelial cells are apparent only on theultrastructural level. For example, endocardial endothelial cells have Weibel-Palade
bodies in their cytoplasm, which contain von Willebrand factor [ 102 ]. Additionally,
endocardial endothelium has a different cell shape, cytoskeletal organization, and
permeability than vascular endothelium [ 103 ]. From an embryological point of
view, vascular endothelium originates from the epicardium, and endocardial endo-
thelium originates from the cardiogenic plate [ 104 ].
Both vascular and endocardial endothelial cells play a role in controlling thecontractility of cardiomyocytes by releasing various biologically active autocrine
and paracrine agents. Cardiac endothelial cells produce nitric oxide, endothelin-1,
prostaglandin I (2), angiotensin II, and other factors [ 104 , 105 ]. All of these sub-
8 The Non-cardiomyocyte Cells of the Heart. Their Possible Roles...