Meeting Abstract
The developmental and functional advantages of vertebrate metaplastic hard tissue is still relatively unclear. Metaplastic tissue forms from direct transformation of one cell type to another. Specifically, mineralization of preformed dense connective tissue results in intratendinous metaplastic tissue, and has been described in a variety of taxa and elements including dinosaur cranial elements, ossified tendons, and tendinous entheses. Histological analysis of the extent of metaplastic hard tissues, both across Mammalia and within individual mammalian bones, is lacking. Here we serially sectioned both tibiae and humeri of a nine-banded armadillo, Dasypus novemcinctus [Xenarthra: Cingulata], to identify the existence and patterns of metaplastic tissue. We found metaplastic tissue extensively throughout the diaphysis of both elements, primarily at sites of muscle attachments. The presence of metaplastic tissue in the tibiae and humeri seems consistent with tendinous insertions through fibrous entheses along the length of the two elements. We also found metaplastic tissue interstitially deep in the cortex of both elements. The entrapment of metaplastic tissue within the cortex is likely due to appositional bone growth at the enthesis. However, we also found metaplastic tissue interstitially in regions unassociated with tendinous entheses. Previously, unassociated metaplastic tissue was found extensively in osteoderms and dinosaur cranial ornamentation such as the parietal frill of Triceratops. Study of metaplastic tissue in modern bone, independent of tendinous insertions, can improve our understanding of its potential biomechanical advantages and thereby possibly provide clues to its use in ornamentation.
