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Abstract
The early embryonic heart is a multi-layered tube consisting of (1) an outer myocardial tube; (2) an inner endocardial tube; and (3) an extracellular matrix layer interposed between the myocardium and endocardium, called “cardiac jelly” (CJ). During the past decades, research on CJ has mainly focused on its molecular and cellular biological aspects. This review focuses on the morphological and biomechanical aspects of CJ. Special attention is given to (1) the spatial distribution and fiber architecture of CJ; (2) the morphological dynamics of CJ during the cardiac cycle; and (3) the removal/remodeling of CJ during advanced heart looping stages, which leads to the formation of ventricular trabeculations and endocardial cushions. CJ acts as a hydraulic skeleton, displaying striking structural and functional similarities with the mesoglea of jellyfish. CJ not only represents a filler substance, facilitating end-systolic occlusion of the embryonic heart lumen. Its elastic components antagonize the systolic deformations of the heart wall and thereby power the refilling phase of the ventricular tube. Non-uniform spatial distribution of CJ generates non-circular cross sections of the opened endocardial tube (initially elliptic, later deltoid), which seem to be advantageous for valveless pumping. Endocardial cushions/ridges are cellularized remnants of non-removed CJ.
Highlights
The heart is the first organ to form and function in vertebrate embryos
During the initial phase of its pumping activity, the vertebrate heart has the design of a tubular conduit that generates unidirectional blood flow by a valveless pumping mechanism [1]
“cardiac jelly“ (CJ), for characterization of the cell-free, gelatinous substance filling the myoendocardial space of the embryonic heart tube [13]
Summary
During the initial phase of its pumping activity, the vertebrate heart has the design of a tubular conduit that generates unidirectional blood flow by a valveless pumping mechanism [1]. ITnhtheree“forergeu, lbaetsioidneosfsbelvoeordalfloowth”er[2c9u,3r1re].nCtloymkmnoewntns functions, embryonic jellies act as hydraulic skeletons This short phase of revived interest in the organ-scale biophysics of CJ was followed by a relatively long phase of remarkable progress in our understanding of the molecular and cellular biological aspects of the CJ. On the significance of the CJ for the mechanical pump action of the early embryonic heart tube are missed in contemporary monographs and textbooks on cardiac development [32,33]. We hope that this article will contribute to a better understanding of the morphogenesis and pumping function of the early embryonic heart of vertebrates
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