Mechanical stress may initiate osteoderm development in the American alligator (<i>Alligator mississippiensis</i>)

Osteoderms are bones that form in the dermis of several species, but their development and function are not well understood. Comparing cellular and molecular events that initiate and drive pathologies similarly characterized by ossification of soft tissues (heterotopic ossification) may provide insight into how osteoderms develop. It is thought that the origin of some heterotopic ossification lesions is mediated through mechanotransduction, or the conversion of mechanical forces on the extracellular matrix (ECM) to biochemical signals that initiate bone formation. Discrete osteoderm developmental stages in alligators were previously established based on morphology; however, questions remain regarding the type of collagen, the identity of cell precursors, and factors that initiate their development. In this study, we investigated changes in the ECM in different stages of osteoderm development. We found that in dorsal cervical scales, an organized reticulin fiber scaffold precedes bone formation, and that before and during ossification, collagen fibers are under tension in specific planes of the body. These tensed collagen fibers correspond to attachments of the superficial and deep cervical fasciae and the tendons of some muscles of the jaws, neck, and shoulders. Osteoderms are therefore exoskeletal elements that likely distribute mechanical forces to the skin during normal body movements, and we hypothesize that these mechanical forces play a role in stimulating ossification at these sites via mechanotransduction. This developmental mechanism may be comparable to the development of some trauma-induced heterotopic ossification lesions that are similarly initiated by mechanotransduction.