| Summary: | The Greenland shark (Somniosus microcephalus) live up to 392 ± 120 years, making it the world’s oldest-living vertebrate [1]. 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 blockface scanning electron microscopy according to the Ellisman protocol [2]. 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. The mitochondrial volume density observed in the Greenland shark may reflect aerobic need relative to its cold environment [3]. This high mitochondrial content could have happened through mitochondrial biogenesis through a molecular pathway contributing to longevity in a variety of species [4]. We observed mitochondrial syncitia which are clues for mitochondrial fusion. Mitochondrial morphology is shaped by mitochondrial dynamics, including mitochondrial fusion which is essential to maintain a normal cardiac function [5]. The shape of the cardiomyocyte nuclei and the heterochromatic structure further support a phenotype resilient to age. We conclude that the subcellular characteristics of the cardiac myocyte in the Greenland shark reflect a healthy and youthful phenotype. In the future, our dataset will be complemented with an increased sampling ...
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