The heart works tirelessly to pump blood around the body. But, ironically, heart muscle itself is one of the tissues most reliant on an uninterrupted oxygen supply. Conventionally, there are two routes via which oxygen may reach the heart: a dedicated coronary supply or the residual oxygen in returning venous blood. In mammals such as us, the coronary arteries – regally named because of their resemblance to a crown enveloping the heart – supply the vast majority of oxygen to the heart. They immediately branch off the aorta, carrying fully oxygenated blood, and penetrate throughout the heart muscle. Their importance is revealed by the devastation that ensues if one of these channels becomes blocked, resulting in a heart attack. Yet, most bony fishes do not have a dedicated coronary supply. In those that do, the crown more closely resembles a delicate tiara and only supplies blood to the superficial compact layer of the heart. The majority of the heart is spongy and scavenges the leftover oxygen from venous blood returning after circulating around the body.
Whilst the hearts of bony fishes have been well characterised, much less is known about cartilaginous fishes, such as sharks, which are even more distantly related to mammals. However, gross morphological studies, spanning over a century, suggested that their coronary vessels are more extensive than those of bony fishes and enter the spongy myocardium. To provide the first quantification of coronary vasculature in sharks, Georgina Cox and her colleagues at the University of British Columbia meticulously compared vascularisation of the heart in a shark, the Pacific spiny dogfish, with that of a representative bony fish, the rainbow trout.
Under the microscope, Cox and her colleagues saw what they expected in trout hearts; the atrium and spongy myocardium were devoid of coronary vessels, which only decorated the peripheral compact layer. In the dogfish, however, the coronary arteries uniformly permeated the entire heart, including the atrium and spongy core of the ventricle. Also, the vessels in the trout heart were slightly narrower than those in the dogfish, which is probably because they have smaller red blood cells.
The team further investigated the vessel density in the trout compact layer and found that it was about twice that in the dogfish heart. They believe that this is a sign of the much higher heart rate, and therefore oxygen demand, in trout and suggest that it may be interesting for future studies to compare other sharks and bony fishes with varying heart rates to verify the hypothesis. Nonetheless, it was clear that the coronary arteries in the dogfish heart, whilst less dense, are more far reaching and play a more general role in oxygenating the entirety of the heart.
Because the hearts of bony fishes are so reliant on oxygen in venous blood, but mammals and birds have extensive coronary networks, it has been tempting to speculate that coronary arteries only rose to prominence relatively recently, possibly during the evolution of endothermy. However, by detailing the important crown of vessels in the dogfish heart, Cox and her colleagues suggest that an expansive coronary circulation is likely to have evolved earlier in some of the more ancient vertebrates.
