To support the high rates of oxygen consumption associated with being warm-blooded, the hearts of birds and mammals must pump rapidly and frequently. These high heart rates are made possible by a network of conductive tissue that spreads across the heart, controlling its contraction. The evolutionary origin of this conductive tissue has long been a mystery; it has never been identified in reptiles, the shared ancestors of birds and mammals, leading scientists to wonder whether the conductive network evolved independently in the two taxa. Cold-blooded reptiles have lower metabolic rates, and their oxygen requirements can be fulfilled by the spongy tissue that comprises their inner hearts in the absence of a conduction network.
Bjarke Jensen and colleagues from University of Amsterdam (The Netherlands) and Aarhus University (Denmark) have now solved this mystery by showing that the spongy tissue in reptile hearts is the precursor to the conductive tissue in warm-blooded animals. Genetic analysis of adult Anolis lizards, Xenopus frogs and zebrafish compared with embryonic chickens, mice and humans uncovered the common molecular structure (PLoS ONE 7, e44231; 2012). The discovery adds to our knowledge about the complex conduction network of the human heart, which may lead to a better understanding of certain heart conditions.
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