Mammalian Osteoderm Ultrastructure in the Armored Acomys Spiny Mouse Tail

ABSTRACT The skin of mammals acts as a barrier against pathogens, ultraviolet radiation, and mechanical trauma. Embedded within the dermal tail skin of spiny mice (genus Acomys ) lie osteoderms—finely structured, symmetrically patterned, bone‐like armor plates akin to those in armadillos and non‐avian reptiles. Using 2D and 3D microscopic characterization techniques, we provide details on Acomys osteoderm patterning, cell size and distribution, and an ultrastructural overview of collagen‐mineral relationships in the extracellular matrix. Osteoderms are organized into a periodic array with A‐B‐A‐B‐A‐B patterning, with an angular offset to the central tail axis of roughly 26°, an aspect ratio of ∼3.8, and a cellular mineral lacunar porosity of ∼4.5%. Mature osteoderms in young mice preferentially have vascular channels in their distal‐third portion, and marginally higher cellularity in their proximal portion. Patterned nucleation of nascent osteoderms occurs relative to surrounding guard and awl hairs, and develops through the growth and amalgamation of nanoscale mineral foci. On the epidermal‐facing side of the osteoderm, bundled dermal collagen fibrils have prominent Sharpey's fiber insertions and a hypomineralized band along the osteoderm surface, whereas the dermis on the hypodermal side has parallel‐oriented dermal collagen. This complex architecture may serve to define planes on which autotomy occurs.