Rj. Tarpley et al., OBSERVATIONS ON THE EXTERNAL MORPHOLOGY AND VASCULATURE OF A FETAL HEART OF THE BOWHEAD WHALE, BALAENA-MYSTICETUS, The Anatomical record, 247(4), 1997, pp. 556-581
Background: Specialized demands within the aquatic environment for ove
r some 60 million years have shaped unique morphological expressions i
n the whales, dolphins, and porpoises (Cetacea). Detailed consideratio
n of these features, particularly in the great whales, has often been
constrained by difficulties in securing adequate specimens for study.
We had the opportunity to examine external heart morphology in a rarel
y obtained and prepared specimen from the bowhead whale, Balaena mysti
cetus, Methods: The external morphology and in situ relations of a for
malin-perfused heart were examined grossly in a near-term bowhead fetu
s. Latex injections assisted visualization of coronary vasculature. Ma
gnetic resonance imaging was used to clarify heart positioning within
the thoracic cavity in two younger (early and mid-gestational) intact
fetuses. Results: The heart was globular in form, with a blunt apex an
d wide base; it was laterally broad relative to height yet considerabl
y compressed between nearly planar atrial (diaphragmatic) and auricula
r (sternocostal) surfaces. The heart constituted 0.01 of body mass in
the near-term fetus. Within the thoracic cavity, the heart tilted forw
ard on its long axis, placing the great basal vessels in the region of
the thoracic inlet. The aorta extended forward from mid-base in paral
lel with the pulmonary trunk, arched sharply to the left, producing in
succession the brachiocephalic trunk, left common carotid artery, and
the left subclavian artery. Bifurcation of the brachiocephalic trunk
yielded the right common carotid and right subclavian arteries. The di
stal portion of the aortic arch was linked to the pulmonary trunk via
the ductus arteriosus. The aorta then swung caudally over the heart ba
se, descending beneath the bodies of the thoracic vertebrae. The ascen
ding aorta featured three bulbous sinuses immediately distal to the th
ree semilunar cusps of the aortic valve. Originating along the distal
boundaries of the left and right sinuses were the left and right coron
ary arteries. The arteries were similar in size and, because each sent
contributions along their respective coronary and intel-ventricular g
rooves, the heart can be described as bilateral relative to arterial s
upply. Anastomoses were common within and between the two arteries. Ve
nous return from the heart was comprised of the great, middle, and rig
ht cardiac veins, all three converging in the coronary sinus. The righ
t cardiac vein also included tributaries that emptied directly into th
e right atrium. Conclusions: External heart morphology in the fetal bo
whead whale examined was distinguished by a laterally broad conformati
on with significant compression between its cranial and caudal surface
s. Aortic bulb configuration in combination with an expandable aortic
arch may support blood service to the heart during diastole. Vascular
service to the heart featured a complex vessel network with extensive
intraarterial and intravenous anastomoses that enable many alternate b
lood perfusion pathways and may be adaptive to water-column-pressure f
luctuations experienced by a large diving mammal. (C) 1997 Wiley-Liss,
Inc.