Mode of Ossification and Extracellular Fiber Characterization of Osteoderm Matrix in the American Alligator (Alligator mississippiensis) and Comparisons to Inherited and Acquired Heterotopic Ossification Disorders

Abstract

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. The function of osteoderms is further obscured, since there is little histomorphological information on these structures. Indeed, understanding the cellular and molecular events that initiate and drive osteoderm development may clarify the functional significance of these structures. Dubansky & Dubansky (2018) conducted the first histological description of the development of the osteoderm in the American alligator, and identified discrete developmental stages based on the organization of collagen fibers and contributing cell mediators, as well as the degree of mineralization of the matrix. However, questions remain regarding the type of collagen and the identity of the cell precursors that are involved at different stages, as well as the existence of a hyaline cartilage model. In this study, differential staining techniques characterized changes in the fibrous component of the developing matrix of the osteoderm, and investigated whether ossification proceeds via an endochondral mechanism. Early stage osteoderms exhibit an extensive reticulin (type III collagen) fiber scaffold, which is replaced by disorganized, thick type I collagen fibers in mineralizing maturing matrix, and highly organized, thin type I collagen at the proliferating edge of the matrix. Despite morphological similarities to cartilage in some areas, staining showed that hyaline cartilage was absent and, therefore, the mode of ossification more resembled intramembranous ossification, than endochondral ossification. Osteoderm development resembles Heterotopic Ossification (HO) disorders in humans and is being used as a model to study several forms of this pathology. Based on this study, osteoderm histomorphology most closely resembles the inherited disorder Progressive Osseous Heteroplasia, in which pathological bone formation is restricted to the dermis and superficial fascia and proceeds via intramembranous ossification. Documenting similarities and differences among the various types of HO lesions and osteoderms may provide insights into the disease progression and future therapeutic strategies.Support or Funding InformationThis research was supported by a Faculty‐Student Research and Creative Activity Internal Grant from Tarleton State University, Office of Research and Innovation.