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Bony Lesions in Early Tetrapods and the Evolution of Mineralized Tissue Repair

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Bony Lesions in Early Tetrapods and the Evolution of Mineralized Tissue Repair Paleobiology, 45(4), 2019, pp. 676–697 DOI: 10.1017/pab.2019.31 Article Bony lesions in early tetrapods and the evolution of mineralized tissue repair Eva C. Herbst , Michael Doube , Timothy R. Smithson, Jennifer A. Clack, and John R. Hutchinson Abstract.—Bone healing is an important survival mechanism, allowing vertebrates to recover from injury and disease. Here we describe newly recognized paleopathologies in the hindlimbs of the early tetrapods Crassi- gyrinus scoticus and Eoherpeton watsoni from the early Carboniferous of Cowdenbeath, Scotland. These path- ologies are among the oldest known instances of bone healing in tetrapod limb bones in the fossil record (about 325 Ma). X-ray microtomographic imaging of the internal bone structure of these lesions shows that they are characterized by a mass of trabecular bone separated from the shaft’s trabeculae by a layer of cortical bone. We frame these paleopathologies in an evolutionary context, including additional data on bone healing and its pathways across extinct and extant sarcopterygians. These data allowed us to synthesize information on cell-mediated repair of bone and other mineralized tissues in all vertebrates, to reconstruct the evolutionary history of skeletal tissue repair mechanisms. We conclude that bone healing is ancestral for sar- copterygians. Furthermore, other mineralized tissues (aspidin and dentine) were also capable of healing and remodeling early in vertebrate evolution, suggesting that these repair mechanisms are synapomorphies of vertebrate mineralized tissues. The evidence for remodeling and healing in all of these tissues appears con- currently, so in addition to healing, these early vertebrates had the capacity to restore structure and strength by remodeling their skeletons. Healing appears to be an inherent property of these mineralized tissues, and its linkage to their remodeling capacity has previously been underappreciated. Eva C. Herbst* and John R. Hutchinson. Structure and Motion Laboratory, Royal Veterinary College, Hawkshead Lane, Hatfield, Hertfordshire AL9 7TA, United Kingdom. E-mail: [email protected], [email protected] Michael Doube. Department of Infectious Diseases and Public Health, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong. E-mail: [email protected] Timothy R. Smithson and Jennifer A Clack. University Museum of Zoology, Cambridge, Downing Street, Cambridge, CB2 3EJ, United Kingdom. E-mail: [email protected], [email protected] Accepted: 13 August 2019 First published online: 27 September 2019 *Correspondence to: Eva C. Herbst, [email protected]. Data available from the Dryad Digital Repository: https://doi.org/10.5061/dryad.30kp1qg; and Figshare Digital Repository: https://doi.org/10.6084/m9.figshare.9211643 Introduction are to regain competitive function. Despite its Bones have a remarkable ability to heal after broad distribution in extant taxa and clear traumatic injury and in the face of infectious, role in enhancing evolutionary fitness within neoplastic, and other insults. Up to half of and across species, an evolutionary context juvenile humans experience and survive bone for bone healing has not yet been elucidated. fracture (Jenkins et al. 2018). Wild animals The broad category of bone repair may be such as hawks, bottle-nosed dolphins, urban subdivided into several different categories: foxes, and serrasalmid fish (the group includ- regeneration, nonregenerative large-scale heal- ing piranhas) have high rates of fracture inci- ing, remodeling, and biophysical annealing dence and survival (de Smet 1977; Harris (Fig. 1). In regeneration (Fig. 1A), whole limb, 1978; Roth et al. 2002; Kolmann et al. 2018). In tail, and fin segments regrow via the formation human and veterinary medicine, fracture repair of a blastema, which comprises dedifferen- may be enhanced by supporting bones with tiated cells (Egawa et al. 2014; see Akimenko splints, casts, slings, surgical implants, and et al. [2003] and Akimenko and Smith [2007] medication; wild animals are not afforded for fin regeneration). We divide nonregenera- such care and must heal by themselves if they tive large-scale healing into three subcategories: © 2019 The Paleontological Society. All rights reserved. This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted re-use, distribution, and reproduction in any medium, provided the original work is properly cited. 0094-8373/19 Downloaded from http://pubs.geoscienceworld.org/paleobiol/article-pdf/45/4/676/4955869/s0094837319000319a.pdf by guest on 29 September 2021 EVOLUTION OF SKELETAL REPAIR 677 FIGURE 1. Generalized schematic of bone repair mechanisms: A, regeneration; B, bone healing; C, remodeling; and D, biophysical annealing. *For healing of dermal bone in various taxa (via cartilaginous or fibrous callus formation), see Moss (1962), Irwin and Ferguson (1986), and Geurtzen et al. (2014). For endochondral healing mechanisms with surgical fixation, see Shapiro (2008). fracture repair, bony collar formation (sub- collar of subperiosteal woven bone (Uthgen- periosteal reactive bone and exostoses), and nant et al. 2007; Shapiro 2008; Marsell and modeling and remodeling around space-filled Einhorn 2011). We do not include healing lesions (such as cysts, abscesses, and neo- of stress fractures or microfractures in the plasms), which for brevity we refer to collect- category of fracture repair, because the healing ively as bone healing (Fig. 1B). Bony collar process of stress and microfractures omits fibro- formation is modeling “outside” the bone cartilaginous callus formation. organ, by the formation of subperiosteal react- Bone healing usually incorporates remodel- ive bone and exostoses. An example of bony ing. During fracture repair, woven bone in the collar formation is in equine “bucked shins,” callus is removed and replaced by mature where bone forms in response to high-strain lamellar bone, restoring the strength of the cyclic injury (Nunamaker 2002). Modeling bone organ (Shapiro 2008). In our terminology and remodeling around space-filling lesions (following Frost 1991), remodeling refers to the help heal bone in response to disease and infec- process of bone resorption by osteoclasts and tion. Here we use the term fracture repair only to deposition by osteoblasts without an overall refer to the healing processes that occur after change in organ shape (i.e., formation follows gross organ breakage into separate fragments. resorption on the same surface), and modeling This “secondary” or “indirect” fracture repair refers to unbalanced formation and resorption, process is characterized by rapid fibrous/cartil- leading to a net change in organ shape (e.g., cal- aginous callus formation that closes the fracture lus formation) or size (e.g., during growth) gap and stabilizes interfragmentary movement (Fig. 1B,C). Modeling and remodeling function (Shapiro 2008; Marsell and Einhorn 2011). In during normal bone growth; modeling contrast, incomplete and fatigue (“stress”)frac- increases bone size during development, and tures that are stable and lack a fracture gap heal modeling and remodeling enable growth to by secondary osteonal remodeling that fixes the occur while keeping dynamic strains consistent intracortical cracks and/or by deposition of a (Biewener et al. 1986;Frost1991). Furthermore, Downloaded from http://pubs.geoscienceworld.org/paleobiol/article-pdf/45/4/676/4955869/s0094837319000319a.pdf by guest on 29 September 2021 678 EVA C. HERBST ET AL. bone remodeling can function in mineral 2018). Paleopathologies can provide insight homeostasis (Doherty et al. 2015). Remodeling into the evolution of bone repair, because repairs microfractures in cortical bone (Burr paleopathologies are direct evidence of bone’s 1993; Mori and Burr 1993; Bentolila et al. response to insult. 1998). Bone’s healing response can consist of Although it is often difficult to determine the tissue formation only (i.e., modeling), for exact etiology of a paleopathology (Moodie example, subperiosteal bone deposition form- 1917), bone healing, regeneration, and remod- ing a bony collar. Remodeling is not always eling can be distinguished from one another necessary to reconnect two fractured segments in the fossil record. For example, the presence or to infill a lesion, but is advantageous in the of a fracture callus in a fossil indicates that the healing pathway, because it re-establishes full animal survived the fracture and started the strength in the healed structure (Shapiro fracture repair process. Bone healing may also 2008). Due to its tissue maintenance function manifest as an unusual bone formation in and ability to repair microfracture and stress response to nonfracture insult or disease; some- fracture independently, we place remodeling in times it is not possible to specify the type of its own category within bone repair, while rec- bone healing (response to fracture vs. other dis- ognizing its contribution to the later stages of ease). Remodeling can be identified by the pres- fracture repair. Modeling around space-filling ence of secondary osteons. Microcracks (and lesions, and bony collar formation, can also be hence, their repair) are difficult to identify in followed by remodeling. However, the subcat- the fossil record; however, there is evidence of egories of bone healing are united by the char- remodeling as part of growth (Giles et al. acteristic that they are modeling processes, with 2013) and as
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