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The heart forms early during embryogenesis (Fig. 232-1), circulating blood, nutrients, and oxygen to the other developing organs while continuing to grow and undergo complex morphogenetic changes. Early cardiac progenitors arise within crescent-shaped fields of lateral splanchnic mesoderm under the influence of multiple signals and migrate to the midline to form the linear heart tube: a single layer of endocardium and a single layer of cardiomyocyte precursors.

FIGURE 232-1

A. Schematic depiction of a transverse section through an early embryo depicts the bilateral regions where early heart tubes form. B. The bilateral heart tubes subsequently migrate to the midline and fuse to form the linear heart tube. C. At the early cardiac crescent stage of embryonic development, cardiac precursors include a primary heart field fated to form the linear heart tube and a second heart field fated to add myocardium to the inflow and outflow poles of the heart. D. Second heart field cells populate the pharyngeal region before subsequently migrating to the maturing heart. E. Large portions of the right ventricle and outflow tract and some cells within the atria derive from the second heart field. F. The aortic arch arteries form as symmetric sets of vessels that then remodel under the influence of the neural crest to form the asymmetric mature vasculature. LA, left atrium; LV, left ventricle; RA, right atrium; RV, right ventricle.

The linear heart tube undergoes asymmetric looping, that coordinates with chamber specification and multilayer growth of different regions of the heart tube to produce the presumptive atria and ventricles. Cells continue to migrate into the heart at both ends from later, or second, heart fields in pharyngeal mesoderm as looping and growth occur. These cells exhibit distinctive gene expression (e.g., Islet-1) and distinctive physiology (e.g., calcium handling), contributing to discrete areas of the adult heart, including the right atrium and the right ventricle. Different embryologic origins of cells within the right and left ventricles help explain why some forms of congenital and adult heart diseases affect regions of the heart to varying degrees.

After looping and chamber formation, a series of morphogenetic events divide the left and right sides of the heart, separate the atria from the ventricles, and form the aorta and pulmonary artery from the truncus arteriosus. Cardiac valves form between the atria and the ventricles and between the ventricles and the outflow vessels. Early in development, the single layer of myocardial cells secretes an extracellular matrix rich in hyaluronic acid, or “cardiac jelly,” which accumulates within the endocardial cushions, precursors of the cardiac valves. Signals from overlying myocardial cells trigger migration, invasion, and phenotypic changes in underlying endocardial cells, which undergo an epithelial-mesenchymal transformation to invade and populate the endocardial cushion matrix with cells. Mesenchymal cells then proliferate and ...

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