The Dorsal Integument of the Southern Long-Nosed Armadillo Dasypus hybridus (Cingulata, Xenarthra), and a Possible Neural Crest Origin of the Osteoderms. Discussing Evolutive Consequences for Amniota

Osteoderms are present in most tetrapod lineages with considerable lineage-specific variation. It has been hypothesized that osteoderms are a case of “deep homology” in craniates, but the embryonic origin of osteoderms -and other related postcranial exoskeletal elements- is still under debate. Most authors support its mesodermal origin, while others suggest that osteoderms are derived from neural crest scleroblastic cells in sauropsids. The armadillos (Xenarthra, Cingulata) are the only living mammals and the only extant synapsids with osteoderms. Here, we aim to identify skeletogenic neural crest cells in the dorsal skin of armadillos in order to assess if osteoderms have a neuroectodermal origin in mammals, similar to what is observed in sauropsids. For this purpose, skin samples from fetuses and newborn specimens of Dasypus hybridus were processed and the embryological development of osteoderms was characterized using different immunohistochemical markers (HNK-1, PDGFR α, S-100, and C5). For the first time, we report cell populations that were reactive to skeletogenic neural crest markers, indicating an ectomesenchymal origin of the mammalian osteoderms. Our results demonstrate similar molecular expression for mammals as in sauropsids and, therefore, this strongly suggests that osteoderms in both groups would have a homologous embryonic origin.