Immunocytochemistry suggests that the prevalence of a sub-type of beta-proteins determines the hardness in the epidermis of the hard-shelled turtle

The Story of the Finest Armor: Developmental Aspects of Reptile Skin

The reptile skin is a barrier against water loss and pathogens and an armor for mechanical damages. The integument of reptiles consists of two main layers: the epidermis and the dermis. The epidermis, the hard cover of the body which has an armor-like role, varies among extant reptiles in terms of structural aspects such as thickness, hardness or the kinds of appendages it constitutes. The reptile epithelial cells of the epidermis (keratinocytes) are composed of two main proteins: intermediate filament keratins (IFKs) and corneous beta proteins (CBPs). The outer horny layer of the epidermis, stratum corneum, is constituted of keratinocytes by means of terminal differentiation or cornification which is a result of the protein interactions where CBPs associate with and coat the initial scaffold of IFKs. Reptiles were able to colonize the terrestrial environment due to the changes in these epidermal structures, which led to various cornified epidermal appendages such as scales and scutes, a beak, claws or setae. Developmental and structural aspects of the epidermal CBPs as well as their shared chromosomal locus (EDC) indicate an ancestral origin that gave rise to the finest armor of reptilians.

Development, structure, and protein composition of the corneous beak in turtles

The beak or rhamphotheca in turtles is a horny lamina that replaces the teeth. Its origin, development, structure, and protein composition are here presented. At mid-development stages, the epidermis of the maxilla and mandible gives rise to placodes that enlarge and merge into laminae through an intense cell proliferation. In these expanding laminae, the epidermis gives rise to 5-8 layers of embryonic epidermis where coarse filaments accumulate for the initial keratinization of cells destined to be sloughed before hatching. Underneath the embryonic epidermis of the beak numerous layers of spindle-shaped beta-cells are produced while they are absent in other skin regions. Beta-cells contain hard corneous material and give rise to the corneous layer of the beak whose external layers desquamate due to wearing and mechanical abrasion. Beta-catenin is present in nuclei of proliferating keratinocytes of the germinal layer likely responding to a wnt signal, but also is part of the adhesive junctions located among beak keratinocytes. The thick corneous layer is made of mature corneocytes connected one to another along their irregular perimeter by an unknown cementing material and junctional remnants. Immunolabeling shows that the main components of the horny beak are Corneous Beta Proteins (CBPs) of 10-15 kDa which genes are located in the Epidermal Differentiation Complex (EDC) of the turtle genome. Specific CBPs, in addition to a lower amount of Intermediate Filament Keratins, accumulate in the horny beak. Compaction of the main proteins with other unknown, minor proteins give rise to the hard corneous material of the beak.

Comparative study of collagen distribution in the dermis of the embryonic carapace of soft- and hard-shelled cryptodiran turtles

Turtles are characterized by their typical carapace, which is primarily composed of corneous beta proteins in the horny part and collagen in the dermal part. The formation of the extracellular matrix in the dermis of the carapace in a hard-shelled and a soft-shelled turtle has been compared. The study examines carapace development, with an emphasis on collagen accumulation, in the soft-shelled turtle Pelodiscus sinensis and hard-shelled turtle Trachemys scripta elegans, using comparative morphological and embryological analyses. The histological results showed that collagen deposition in the turtle carapace increased as the embryos developed. However, significant differences were observed between the two turtle species at the developmental stages examined. The microstructure of the dermis of the carapace of P. sinensis showed light and dark banding of collagen bundles, with a higher overall collagen content, whereas the carapacial matrix of T. scripta was characterized by loosely packed and thinner collagenous fiber bundles with a lower percentage of type I collagen. Overall, the formation and distribution of collagen fibrils at specific developmental stages are different between the soft-and hard-shelled turtles. These results indicate that the pliable epidermis of the soft-shelled turtle is supported by a strong dermis that is regularly distributed with collagen and that it allows improved maneuvering, whereas a strong but inflexible epidermis as observed in case of hard-shelled turtles limits movement.

Development, structure, and protein composition of reptilian claws and hypotheses of their evolution

Here, we review the development, morphology, genes, and proteins of claws in reptiles. Claws likely form owing to the inductive influence of phalangeal mesenchyme on the apical epidermis of developing digits, resulting in hyperproliferation and intense protein synthesis in the dorsal epidermis, which forms the unguis. The tip of claws results from prevalent cell proliferation and distal movement along most of the ungueal epidermis in comparison to the ventral surface forming the subunguis. Asymmetrical growth between the unguis and subunguis forces beta-cells from the unguis to rotate into the apical part of the subunguis, sharpening the claw tip. Further sharpening occurs by scratching and mechanical wearing. Ungueal keratinocytes elongate, form an intricate perimeter and cementing junctions, and remain united impeding desquamation. In contrast, thin keratinocytes in the subunguis form a smooth perimeter, accumulate less corneous beta proteins (CBPs) and cysteine-poor intermediate filament (IF)-keratins, and desquamate. In addition to prevalent glycine-cysteine-tyrosine rich CBPs, special cysteine-rich IF-keratins are also synthesized in the claw, generating numerous SS bonds that harden the thick and compact corneous material. Desquamation and mechanical wear at the tip ensure that the unguis curvature remains approximately stable over time. Reptilian claws are likely very ancient in evolution, although the unguis differentiated like the outer scale surface of scales, while the subunguis might have derived from the inner scale surface. The few hair-like IF-keratins synthesized in reptilian claws indicate that ancestors of sauropsids and mammals shared cysteine-rich IF-keratins. However, the number of these keratins remained low in reptiles, while new types of CBPs function to strengthen claws.

Cell proliferation, adhesion, and differentiation of keratinocytes in the developing beak and egg-tooth of the turtle Emydura macquarii

The development of the beak in turtles is poorly known. Beak development has been analyzed by immunofluorescent methods for studying cell proliferation and localization of specific proteins. The flat two-layered epidermis covering the turtle embryo at mid stage of development becomes columnar in the oral region and is associated with an increase of mesenchymal density as in placodes. Using 5BrdU, an intense cell proliferation is observed in the oral and epidermal cells covering the maxilla and mandibular bones, probably stimulated by the underlying mesenchyme in continuation with maxillary and mandibular bones. Expansion and fusion of these placodes give rise to the corneous beak. Beta catenin, mainly junctional but also sparsely detected in nuclei of the germinal layer of the beak epithelium, maintains united the differentiating keratinocytes that form a stratified corneous epithelium. This is initially composed of some layers of large cells that accumulate intermediate filament keratins (IFKs) and give rise to a keratinized embryonic epidermis destined to slough around hatching. The following corneocytes accumulate IFKs but mainly type I/II corneous beta proteins (CBPs) and form a corneous beak. These CBPs appear present with lower intensity in the beak than in the shell, but the higher intensity obtained with a general antibody against CBPs indicates that other CBPs contribute to the composition and stiffness of beak corneous material. The egg-tooth in continuation with the stratum corneum of the maxillary beak develops from a localized proliferation and comprises a thick embryonic epidermis accumulating IFKs under which large beta-cells connected by adhesion proteins accumulate CBPs contributing to hardening of this temporary organ.

Corneous beta proteins of the epidermal differentiation complex (EDC) form large part of the corneous material of claws and rhamphothecae in turtles

The presence of specific protein types in claws and beaks of turtles is poorly known. The present immunological study describes the localization of some of the main corneous beta proteins (CBPs) coded in the epidermal differentiation complex of turtles. Three antibodies here utilized revealed that glycine-, cysteine-, tyrosine-, and valine-rich CBPs are present in differentiating keratinocytes of the beak and of the dorsal (unguis) and ventral (sub-unguis) sides of the claw in different species, semi-aquatic and terrestrial. These proteins provide mechanical resilience to the horny material of claws and beaks through the formation of numerous -S-S- bonds and also hydrophobicity that contributes to preserve wearing of the horny material. The thicker corneous layer of the unguis is made of elongated and partially merged corneocytes, and no or few cells desquamate superficially. Unknown junctional proteins may contribute to maintain corneocytes connected one to another. In contrast, corneocytes of the sub-unguis show an elongated but lenticular shape and form a looser corneous layer whose cells remain separate and desquamate superficially. This suggests that other specific corneous proteins are present in the unguis in comparison with the sub-unguis to determine this different compaction. The wearing process present in the sub-unguis creates a loss of tissue that may favor the slow by continuous apical migration of corneocytes from the unguis into the initial part of the sub-unguis. Beak corneocytes form a compact corneous layer like the unguis but numerous superficial cells desquamate on both outer (epidermal) and inner (oral) sides.

Cystatin immunoreactivity in cornifying layers of the epidermis suggests a role in the formation of the epidermal barrier in amniotes

The presence and localization of cystatin, a cysteine protease inhibitor involved in barrier formation in human and mice epidermis, has been studied in the epidermis of piscine and terrestrial vertebrates using a mouse monoclonal antibody. Cystatin has been localized by Immunostaining in the pre-corneous and corneous layers of monotreme, marsupial and placental mammals, and sparsely in the thin corneous layer of birds. Cystatin-immunolabeling is present in the pre-corneous and corneous layer of crocodilian and turtle epidermis, in the alpha-corneous layer and likely also in the beta-corneous layer of the epidermis in lizards, snakes and the tuatara. In keratinocytes of the pre-corneous (transitional) layers the protein initially shows a peripheral distribution that becomes compacted in mature corneocytes. The protein is not detected using the antibody in the epidermis of cyclostome, teleosts, sarcopterigian fish, and in amphibians. The study concludes that while in fish and amphibians cystatin is absent or however uncertainly localized in the epidermis, the protein instead appears present in the more external pre-corneous and corneous layers of amniotes. This special regionalization suggests a specific role of cystatin in the formation of the corneous epidermal barrier and regulation of desquamation originally evolved in the terrestrial environment.

Comparative Genomics Identifies Epidermal Proteins Associated with the Evolution of the Turtle Shell

The evolution of reptiles, birds, and mammals was associated with the origin of unique integumentary structures. Studies on lizards, chicken, and humans have suggested that the evolution of major structural proteins of the outermost, cornified layers of the epidermis was driven by the diversification of a gene cluster called Epidermal Differentiation Complex (EDC). Turtles have evolved unique defense mechanisms that depend on mechanically resilient modifications of the epidermis. To investigate whether the evolution of the integument in these reptiles was associated with specific adaptations of the sequences and expression patterns of EDC-related genes, we utilized newly available genome sequences to determine the epidermal differentiation gene complement of turtles. The EDC of the western painted turtle (Chrysemys picta bellii) comprises more than 100 genes, including at least 48 genes that encode proteins referred to as beta-keratins or corneous beta-proteins. Several EDC proteins have evolved cysteine/proline contents beyond 50% of total amino acid residues. Comparative genomics suggests that distinct subfamilies of EDC genes have been expanded and partly translocated to loci outside of the EDC in turtles. Gene expression analysis in the European pond turtle (Emys orbicularis) showed that EDC genes are differentially expressed in the skin of the various body sites and that a subset of beta-keratin genes within the EDC as well as those located outside of the EDC are expressed predominantly in the shell. Our findings give strong support to the hypothesis that the evolutionary innovation of the turtle shell involved specific molecular adaptations of epidermal differentiation.