Development of endocardial, myocardial, epicardial layers and derivatives


As stated before, the cardiac compartments develop as progressively enlarging balloon-shaped distensions at the outer curvature of the atrial and ventricular loops. In the ventricular region this ballooning is accompanied by the formation of myocardial trabeculae, visible from stage 12 onward, which allows the ventricles to increase in size in the absence of a coronary circulation [Van Mierop & Kutsche, 1984]. Moreover, they enhance contractility [Challice & Virágh, 1973] and play an important role in the coordination of intraventricular conduction [De Jong et al., 1992]. Growth of the ballooning ventricles is achieved by tissue proliferation in the outer (compact) myocardium, which initially is only a few cell layers thick [Rumyantsev, 1977; Thompson et al., 1995; Henderson & Copp, 1998]. It is hypothesized that, as a result, the inner myocardial and endocardial layers, that do not contribute to this expansion, “crack open”. The tissue bridges between the thus formed excavations, that become subsequently recoated with endocardium, are to become the primary trabecula. This “craking open” may be facilitated by the peristaltoid contraction patterns [Thompson et al., 2000] as well as the ballooning process itself [Sedmera et al., 2000]. This process repeats itself continuously during development, leading to a radially thickening layer of trabecula, with a proliferating outer layer that retains its compact structure [Steding & Seidl, 1980; Mikawa et al., 1992].


Ventral View Heart Tube Sinus Venosus Coronary Vasculature Capillary Plexus 
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© Springer Science+Business Media, LLC 2007

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