title: Scaling of left ventricle cardiomyocyte ultrastructure across development in the kangaroo Macropus fuliginosus.
abstract: The heart and left ventricle of the marsupial western grey kangaroo Macropus fuliginosus exhibit biphasic allometric growth, whereby a negative shift in the trajectory of cardiac growth occurs at pouch exit. In this study, we used transmission electron microscopy to examine the scaling of left ventricle cardiomyocyte ultrastructure across development in the western grey kangaroo over a 190-fold body mass range (0.355-67.5 kg). The volume-density (%) of myofibrils, mitochondria, sarcoplasmic reticuli and T-tubules increase significantly during in-pouch growth, such that the absolute volume (ml) of these organelles scales with body mass (Mb; kg) with steep hyperallometry: 1.41Mb (1.38), 0.64Mb (1.29), 0.066Mb (1.45) and 0.035Mb (1.87), respectively. Maturation of the left ventricle ultrastructure coincides with pouch vacation, as organelle volume-densities scale independent of body mass across post-pouch development, such that absolute organelle volumes scale in parallel and with relatively shallow hypoallometry: 4.65Mb (0.79), 1.75Mb (0.77), 0.21Mb (0.79) and 0.35Mb (0.79), respectively. The steep hyperallometry of organelle volumes and volume-densities across in-pouch growth is consistent with the improved contractile performance of isolated cardiac muscle during fetal development in placental mammals, and is probably critical in augmenting cardiac output to levels necessary for endothermy and independent locomotion in the young kangaroo as it prepares for pouch exit. The shallow hypoallometry of organelle volumes during post-pouch growth suggests a decrease in relative cardiac requirements as body mass increases in free-roaming kangaroos, which is possibly because the energy required for hopping is independent of speed, and the capacity for energy storage during hopping could increase as the kangaroo grows.
authors.affiliation: Department of Ecology and Environmental Science, School of Biological Sciences, University of Adelaide, Adelaide, SA 5005, Australia edward.snelling@adelaide.edu.au.
authors.forename: Edward P
authors.initials: EP
authors.lastname: Snelling
authors.affiliation: Department of Ecology and Environmental Science, School of Biological Sciences, University of Adelaide, Adelaide, SA 5005, Australia.
authors.forename: David A
authors.initials: DA
authors.lastname: Taggart
authors.affiliation: School of Anatomy, Physiology and Human Biology, University of Western Australia, Crawley, WA 6009, Australia.
authors.forename: Shane K
authors.initials: SK
authors.lastname: Maloney
authors.affiliation: Department of Zoology, University of British Columbia, Vancouver, BC, Canada V6T 1Z4 Faculty of Land and Food Systems, University of British Columbia, Vancouver, BC, Canada V6T 1Z4.
authors.forename: Anthony P
authors.initials: AP
authors.lastname: Farrell
authors.affiliation: School of Medical Sciences, University of Adelaide, Adelaide, SA 5005, Australia.
authors.forename: Christopher M
authors.initials: CM
authors.lastname: Leigh
authors.affiliation: Adelaide Microscopy, University of Adelaide, Adelaide, SA 5005, Australia.
authors.forename: Lyn
authors.initials: L
authors.lastname: Waterhouse
authors.affiliation: Adelaide Microscopy, University of Adelaide, Adelaide, SA 5005, Australia.
authors.forename: Ruth
authors.initials: R
authors.lastname: Williams
authors.affiliation: Department of Ecology and Environmental Science, School of Biological Sciences, University of Adelaide, Adelaide, SA 5005, Australia.
authors.forename: Roger S
authors.initials: RS
authors.lastname: Seymour
mesh_terms: D000818:Animals; D001835:Body Weight; D006352:Heart Ventricles / Q000254:growth & development / Q000648:ultrastructure; D007614:Macropodidae / Q000033:anatomy & histology* / Q000254:growth & development; D008928:Mitochondria / Q000648:ultrastructure; D032383:Myocytes, Cardiac / Q000648:ultrastructure*; D009210:Myofibrils / Q000648:ultrastructure; D012519:Sarcoplasmic Reticulum / Q000648:ultrastructure*; D020257:Ventricular Remodeling
keywords: Heart; Laplace; Marsupial; Ontogeny; Remodelling; Stereology