Histological Microstructure of the Claws of the African Clawed Frog, Xenopus laevis (Anura: Pipidae): Implications for the Evolution of Claws in Tetrapods HILLARY C. MADDIN Ã1 , SORIN MUSAT-MARCU 2 , AND ROBERT R. REISZ 1 1 Department of Biology, University of Toronto at Mississauga Mississauga, Ontario, Canada L5L 1C6 2 Research Transition Facility, Edmonton, Alberta, Canada T6G 2E1 ABSTRACT Claws are consistent components of amniote anatomy and may thus be implicated in the success of the amniote invasion of land. However, the evolutionary origin of these structures in tetrapods is unclear. Claws are present in certain extant non-amniotes, such as Xenopus laevis, the African clawed frog. The histology of the soft tissue component of the claws of X. laevis is described and compared with the amniote condition in order to gain new information on the question of homology of claws in these two groups based on patterns of keratinization. The X. laevis claw sheath is composed of a localized thickening of the corneous region of the epidermis that envelops the terminal phalanx. Noted differences between the non-cornified layers of the epidermis of the claw and non-claw region are the overall grainier appearance of the cells and an increased abundance of desmosomes in the intermediate spinosus cells. The biochemical identity of the sheath keratin(s) is inferred to be different from that of non-claw region epidermis, based on histological differences and differences in stain affinity between the two regions. The microstructure of the frog claw differs from that of amniotes in several respects, including the lack of a specified zone of growth near the base of the claw. Amphibians and amniotes, therefore, have very different patterns of claw sheath growth. Observations do not support homology of claws on a structural level in these two groups; however, further experimental work may confirm a conserved pattern of cornification in these structures in tetrapods. J. Exp. Zool. (Mol. Dev. Evol.) 308B:1– 10, 2007. r 2007 Wiley-Liss, Inc. How to cite this article: Maddin HC, Musat-Marcu S, Reisz RR. 2007. Histological microstructure of the claws of the african clawed frog, Xenopus laevis (anura: pipidae): implications for the evolution of claws in tetrapods. J. Exp. Zool. (Mol. Dev. Evol.) 308B:[page range]. The term claw is typically applied to the structure located at the terminus of the digit that is composed primarily of a thick, outer keratinous sheath tightly associated with the inner skeleton, such that the entire structure is highly functional and can be manipulated by muscular action. Claws have aided tetrapods in the exploitation of many habitats, yet very little is known about the origin and evolutionary history of these critical structures. Claws are present in virtually all amniotes. Rarely claws also occur in species of extant non-amniote tetrapods. In these lissamphibian taxa, claws are important functional structures, as they are in amniotes. Xenopus laevis uses its claws to shred and tear food before consuming it (Schoonbee et al., ’92) while clawed salamanders use their claws to cling to the substrate in fast-flowing mountain streams (Noble, ’31; Vorobeyva et al., 2000). Despite the apparent functional similarity of tetrapod claws, their homology remains in question. The distribution of claws across Tetrapoda reveals the two extant groups possessing these Published online in Wiley InterScience (www.interscience.wiley. com). DOI: 10.1002/jez.b.21145. Received 13 June 2006; Revised 5 October 2006; Accepted 11 October 2006 Grant sponsor: Natural Sciences and Engineering Council (Canada) Discovery Grant; Grant sponsor: University of Toronto at Mississauga; Grant sponsor: Department of Biology (UTM). Ã Correspondence to: H.C. Maddin, Current address: Biological Sciences, University of Calgary, 2500 University Drive NW, Calgary, Alberta, Canada T2N 1N4. E-mail: hcmaddin@ucalgary.ca r 2007 WILEY-LISS, INC. JOURNAL OF EXPERIMENTAL ZOOLOGY (MOL DEV EVOL) 308B:1–10 (2007)