J Exp Zool B Mol Dev Evol 2007 May 15;3083:259-68. doi: 10.1002/jez.b.21145.

Histological microstructure of the claws of the African clawed frog, Xenopus laevis (Anura: Pipidae): implications for the evolution of claws in tetrapods.

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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.

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