Cornification in the claw of the amphibianXenopus laevis(Pipidae, Anura) and comparison with claws in amniotes

Abstract

An ultrastructural, immunocytochemical and electrophoretic study on keratinocytes forming the corneous layer of claws in the frog Xenopus laevis has been conducted in order to analyze the process of cornification in amphibians and compare it with cornification in amniote claws. During claw differentiation pre-corneous keratinocytes accumulate dense corneous bundles containing keratin filaments and inter-keratin material. Ribosomes are associated with keratin bundles, suggesting that neo-synthesized matrix molecules are rapidly aggregated onto the keratin filaments, turning the complex into an amorphous, electron-dense, and resistant horny material. Most proteins of the claw are acidic to neutral keratins of 45-66 kDa, but other protein spots outside the molecular weight range of keratins are also present. These proteins of 16-17 kDa with pI at 7.0-7.3 contain little histidine and may represent 'amphibian keratin associated proteins' (aKAPs). The association of keratins and aKAPs determine the formation of the hard corneous material of claws and likely of the other corneous structures present in amphibian skin, such as the horny beak and horny nuptial pads. The cornification in amphibian claws, compared to the process of cornification present in amniote claws, shows common aspects, but is more simplified in amphibians.

Structural diversity of nuptial pads in Phyllomedusinae (Amphibia: Anura: Hylidae)

“…Maddin et al (2007) pointed out that the biochemical identity of the particular keratins of the dark claws of X. laevis is yet unknown. Although Alibardi et al (2010a, b) provided useful insights into the nature of jaw sheath and Xenopus ' claws keratins, they demonstrated that the situation is complex and that further research must be done to fully characterize the identity of keratins and keratin associated proteins. The PAAB reaction in this work indicated an increase in sulfhydryl and disulfide groups in nuptial pads in the outermost part of the cells of the stratum granulosum and the stratum corneum.…”

Immunolabeling indicates that sulfhydryl oxidase is absent in anamniote epidermis but marks the process of cornification in the skin of terrestrial vertebrates

“…The present observations derive from skin samples of different vertebrates previously collected, fixed and embedded in plastic resins (Bioacryl or Lowcryl; see details in Alibardi, 2002, 2005, 2008, 2010a, 2010b; Alibardi & Thompson, 2002; Alibardi & Toni, 2004; Alibardi & Segalla, 2011). The tissue were resectioned in successive areas and collected on microscopic slides to be utilized for the present survey.…”

“…Samples of the skin collected from different regions of the body were immediately fixed for 5–8 hr in modified Carnoy fixative (9 parts of 80% ethanol+1 part of acetic acid), or in 4% paraformaldehyde in 0.1 mol L −1 phosphate buffer, pH 7.4. The tissues were dehydrated and embedded in Bioacryl or Lowcryl hydrophilic resins under UV as previously specified (Alibardi, 2002, 2005, 2008, 2010a, 2010b; Alibardi & Segalla, 2011; Alibardi & Thompson, 2002; Alibardi & Toni, 2004). The tissues were sectioned using an ultramicrotome, and 2 to 4‐μm‐thick sections were collected on chromoalume–gelatin‐coated slides, dried and later utilized for histology or immunofluorescence studies.…”

Perspectives on Hair Evolution Based on Some Comparative Studies on Vertebrate Cornification

“…A‐C). Recent studies have indicated that, further to mucus and intermediate keratin filaments (alpha‐keratin) in fish and amphibian epidermis, also nonkeratin proteins seem to associate to keratins (Alibardi, ,; Alibardi and Segalla, ). We have indicated these nonkeratin proteins as putative fish or amphibian keratin‐associated proteins (fKAPs and aKAPs) although they probably have no sequence homology to mammalian KAPs (Powell and Rogers, ’94; Rogers et al, ).…”

“…Recent studies on amphibian claws have indicated the presence of alpha‐keratins and nonkeratin proteins associate with the keratin filaments that are probably implicated in the increase of claw hardness (Maddin et al, 2007, ; Alibardi, , ). The presence of KAbetaPs associated with hard keratins has been detected in various reptilian claws (Alibardi and Toni, ; Figs.…”