A shark from Napoleonic wars: 3D segmentations of the organelles from the Greenland shark (Somniosus microcephalus) cardiac myocytes provide insights on extreme longevity

The Greenland shark (Somniosus microcephalus) live up to 392 ± 120 years, making it the world’s oldest-living vertebrate. Because cardiovascular diseases are synonymous with age in humans, we aimed to understand how the heart of this vertebrate can beat since Shakespearian times without failing. Our objective was to elucidate morphological characteristics of organelles associated with natural aging, the mitochondria and the nuclei. Heart tissue samples from the compact region of the Greenland shark ventricle were collected from a ~200 year old female Greenland shark and processed for serial block-face scanning electron microscopy according to the Ellisman protocol. Serial images were collected using Gatan 3View and analysed with IMOD. Heart tissue samples from female Greenland shark (aged 108-220 years-old) preserved in formalin were processed following immunohistochemistry procedures. Image analysis was performed using ImageJ. Approximately 1,200 mitochondria were reconstructed providing a mitochondrial volume density of 69% which is higher than that found in other polar fishes, and similar to that found in highly aerobic muscles such as billfish heater cells, which may reflect aerobic need relative to its cold
nvironment. It can be a consequence of mitochondrial biogenesis which is known to contribute to longevity in a variety of species. Clues for mitochondrial fusion, the shape of the cardiomyocyte nuclei and the heterochromatin arrangement further support a phenotype resilient to age. In the future, our dataset will be complemented with an increased sampling size, comparisons with juvenile Greenland shark cardiac myocytes and molecular assessments investigating mitochondrial dynamics.