Philadelphia – Can a bird have a heart attack?  A recent paper published by researchers at the Perelman School of Medicine at the University of Pennsylvania suggests that cardiovascular disease may be an unfortunate consequence of mammalian evolution.  The study, published in a recent issue of the journal Blood demonstrates that the same features of blood platelets that may have provided an evolutionary advantage to early mammals now predispose humans to cardiovascular disease.

"The biology of platelets has been studied in great detail in the context of human disease, but almost nothing is known about why mammals have platelets, whereas no other species do," said lead study author Alec A. Schmaier, PhD, an MD/PhD student in the lab of Mark Kahn, MD, professor of Medicine at Penn. "This new line of research suggests that platelets could have allowed mammals to better survive traumatic injury by being able to form cellular clots in arterial blood vessels.  The price for this evolutionary change may be modern cardiovascular diseases."

Platelets are small circulating cells that have no nucleus and form clots at sites of vessel injury.  Platelets are required to prevent excessive bleeding following traumatic injury, but they also form clots at sites of atherosclerotic plaques in the blood vessels that lead to stroke and heart attack.  Drugs that inhibit the function of platelets, including aspirin and clopidogrel, are the main weapons for treating heart attack and stroke.

Despite being a vital element of the blood clotting system, platelets are only found in mammals, whereas all non-mammalian vertebrates, including birds, have thrombocytes.  About twice the diameter of platelets, thrombocytes contain a nucleus. Studies performed in the 1970s suggested they have a clotting function similar to platelets, but extensive studies of thrombocytes using modern experimental techniques have not been performed. 

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