Comparative Anatomy and Histology Reveal the American Alligator (Alligator mississippiensis) Exhibits Regenerative Capacity of the Tail

Abstract

Reptiles are the only amniotes that maintain the capacity to lose and regrow entire appendages. Among reptiles, tail regeneration has been extensively studied in Lepidosauria, the subclass that includes lizards, but this study presents the first anatomical and histological evidence of tail regeneration in an archosaur, the American alligator (Alligator mississippiensis). All animals were wild caught juveniles or sub‐adults with an average total body length of 133.4±29.2cm (n=3). Although the duration of tail regrowth is unknown, the average length of the regenerated tail was 15.7±7.3cm. The regenerated tail was externally distinct from the original tail. The scales were compacted and lacked organization as well as dorsal scutes. Gross dissection, radiographs, and magnetic resonance imaging revealed that the caudal vertebrae were replaced by a ventrally‐positioned, unsegmented endoskeleton. This contrasts with lepidosaurs, where the regenerated tail is radially organized around a central endoskeleton, but shares commonality with urodele amphibians that regrow a tail with a clear dorsal‐ventral axis. Immunohistochemistry confirmed that this alligator structure was composed of cartilage, which formed a hollow rod with foramina distributed along the proximal‐distal axis. Furthermore, in contrast to the original tail, the regenerated tail lacked skeletal muscle and instead featured adipose and fibrous connective tissue supplied by a dense network of newly established axons and blood vessels. Despite substantial regrowth of the tail, the deposition of extensive connective tissue is reminiscent of mammalian wound healing or fibrosis. Using Herovici’s stain, it was shown that the regenerated tail is composed of both type I and type III collagen fibers. The lack of muscle contrasts with the lepidosaurs, which regrow functional skeletal muscle groups, but shares similarities with regenerated limbs in Xenopus adult frogs, which have a cartilaginous endoskeleton surrounded by connective tissue, but lacking skeletal muscle. Overall, this study of regeneration in the alligator identifies a distinct pattern of regrowth with features in common with lepidosaurs, amphibia, and wound repair in mammals. Further analysis will help to shed light on the conservation of regeneration in amniote vertebrates. Support or Funding Information This work was funded by the College of Liberal Arts and Sciences at Arizona State University to KK.