The heart is the first functioning organ to form during development. Similar to other organ primordia, the embryonic heart forms as a simple tube — in this case, a straight muscle-wrapped tube situated on the ventral side of the embryo. During gastrulation, the cardiac progenitors reside in the lateral plate mesoderm but maintain close contact with the underlying endoderm. In amniotes, these bilateral heart fields are initially organized as a pair of flat epithelia that move toward the embryonic midline and fuse above the anterior intestinal portal (AIP) to form the heart tube. This medial motion is typically attributed to active mesodermal migration over the underlying endoderm. In this view, the role of the endoderm is two-fold: to serve as a mechanically passive substrate for the crawling mesoderm and to secrete various growth factors necessary for cardiac specification and differentiation.

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