109 - Initiating Factors of Osteoderm Development in the American Alligator (Alligator Mississippiensis) and Comparisons to Heterotopic Ossification

Poster Board Number: 109
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Abstract Body : Osteoderms are bones that form in the dermis of several reptilian species and have been a source of scientific curiosity since at least the 1800s. Though there are several hypotheses for their function, a consensus has not yet emerged. Understanding cellular and molecular events that initiate and drive osteoderm development may clarify their functional significance and provide insight into several pathologies similarly characterized by the formation of bone within soft tissues (heterotopic ossification). It is thought that the origin and growth of some heterotopic ossification lesions are mediated through mechanotransduction, or the conversion of mechanical forces on the extracellular matrix to biochemical signals that initiate bone formation. Dubansky & Dubansky (2018) reported the first detailed histological description of the development of the osteoderm in the American alligator, and identified discrete developmental stages based on the contributing cell mediators, organization of collagen fibers, and degree of matrix mineralization. However, questions remained regarding the type of collagen and the identity of the cell precursors that are involved at different stages and the factors that trigger these developmental processes. To address some of these unanswered questions, differential staining techniques characterized changes in the fibers of the developing extracellular matrix; and gross dissection demonstrated extrinsic collagen attachments to the developing bone. Staining showed early stage osteoderms exhibit an extensive type III collagen fiber scaffold, which is later replaced by organized, thick type I collagen fibers within the mineralizing matrix; and organized, thin type I collagen at the proliferating edge of the matrix. We further demonstrated that these collagen fibers are under tension in specific and organized planes relative to the body axis, and that these fibers correlate with collagenous attachments of the deep fascia and the tendons of some muscles. Based on these data, we hypothesize that the development of osteoderms in alligators is triggered in part by mechanical forces acting through the dorsal scales during normal movements of the head, neck, trunk, and tail during post-embryonic and juvenile growth phases, and that this developmental mechanism is comparable to the development of heterotopic ossification lesions via mechanotransduction.