Revealing the 3D sub-micron organization of melanosomes across mammalia with nanoCT

Abstract Mammalian hair has long been used and studied as an exceptional biological material of interest due to its unique thermal and mechanical properties. However, the sub-micron internal structural assemblies within hair that drive these macroscale characteristics remain poorly understood. In this study, we investigate the internal composition of hair using samples from musk ox ( Ovibos maschatus ) and that of other therian species utilizing nanoCT and SEM. Imaging revealed a unique, highly conserved “fiber-like-motif” where melanin, in the form of melanosomes, are aligned in a stringed architecture throughout the hair cortex. Our findings show that this motif is conserved across not only extant placental mammal species, including black sheep ( Ovis aries ), yak ( Bos grunniens ), and chinchilla ( Chinchilla chinchilla ), but also marsupial opossum ( Didelphis virginiana ) hair and extinct mammoth ( Mammuthus primigenius ). Further, the packing density and aspect ratios of these melanosomes were investigated which can provide insight into the functional differences of the hairs between species. As a whole this work provides unique insight into the spatial organization and morphology of melanosomes in mammalian hair which could be vital for our evolutionary and functional understanding of hair in mammalian species.