Observations on the Ultrastructural Morphology of the Bowhead Whale (Balaena mysticetus) Heart

Recent anatomic studies of the bowhead whale (Balaena mysticetus), including cardiovascular investigations, have been undertaken to help elucidate cetacean adaptations to diving. Electron microscopy data on the heart of this species are presented, illustrating unique features such as high glycogen content of the myocardium and a spongiosum subjacent to the endocardium. It is postulated that the high glycogen content may facilitate cardiac metabolism during prolonged submersion, and the spongiosum may dampen pressure effects on the Purkinje fibers during systole. Other findings in the bowhead whale heart, such as the enlarged and displaced Purkinje fibers and dense thickened endocardium, were similar to these features reported in smaller whales. A transverse tubular system, closely associated with the Z line, was present but less developed than that found in some terrestrial mammals.