Histological Heterogeneity: Tissue-Level Diversity of Lizard Osteoderms
Osteoderms are bone-rich elements that form within the dermis of various vertebrates including many species of lizard. Lizard osteoderms demonstrate variability in size, shape, and body-wide distribution but a detailed comparative assessment of their microstructure is lacking. Here, I characterized the histological diversity of osteoderms from representative members of the lizard groups Gekkota (geckos), Scincomorpha (scincids, cordylids, Gerrhosaurids), and Anguimorpha (anguids, Shinisaurus, helodermatids, varanids). In virtually all lizards, osteoderms are composed primarily of bone, albeit with a heterogeneous and often laminated fibrillary organization. Histologically, most osteoderms are dominated by lamellar, woven-fibred, and/or Sharpey-fibred bone, although details of the organization and relative contribution of each bone matrix differs between genera. In addition, we found that multiple genera from each major group also develop a highly mineralized, collagen and cell-poor capping tissue – a feature previously restricted to only a handful of species. Singularly, the osteoderm-like elements from the gekkotan Geckolepis entirely lack bone and instead are composed of a plate of collagen topped with mineralized capping tissue.