A discovery has been made by a team of international molecular scientists that star ascidians, also known as sea squirts, have pacemaker cells similar to that of the human heart. The studies, published in the Journal of Experimental Zoology, Ecological Genetics and Physiology, may reveal new insights into the early evolution of the heart, as star ascidians are one of the closest related invertebrates to mammals.
Research leader Annette Hellbach from the Max Planck Institute of Biochemistry in Germany and her team anticipated finding clusters of HCN cells, the markers for pacemakers, at either end of the Botryllus schlosseri ascidian heart.
"The Botryllus schlosseri heart beats from one end to the other, stops for a short while and then starts to beat in the other direction. It would make sense to have two pacemakers on both ends from which the heartbeat is initiated, however, we found several HCN positive cells spread along the cardiac tube."
The team interpreted the finding as an evolutionary precursor to the elaborate cardiac conduction system found in mammals, which consists of clusters of pacemaker cells located in defined spots.
In comparison to mammals, the cells in the B. schlosseri heart seemed to be randomly distributed along the heart. In spite of this, as with mammals, the HCN cells played a vital role in generating the heartbeat.
It was also discovered, that as in mice, the cells responded to the blocking chemicals Cilobradine and Zatebradine by reducing the heartbeat, concluding that it increases the possibility that the cells work in a similar molecular function.
"Our study reveals that the presence of HCN channels and their role in generating the heartbeat is shared between B. schlosseri and mammals. This makes colonial ascidians such as B. schlosseri insightful models for studies on the origins and evolution of vertebrate innovations, such as the pacemaker."
Written by Petra Rattue