Differentiation of the epidermis in turtle: an immunocytochemical, autoradiographic and electrophoretic analysis

A comprehensive exploration of diverse skin cell types in the limb of the desert tortoise (Testudo graeca) through light, transmission, scanning electron microscopy, and immunofluorescence techniques

The Greek tortoise, inhabiting harsh desert environments, provides a compelling case for investigating skin adaptations to extreme conditions. We have utilized light microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and immunofluorescence analysis to describe the structure of the arid-adapted limb skin in the Greek tortoise. Our aim was to identify the cell types that reflect the skin adaptation of this tortoise to arid conditions. Utilizing seven antibodies, we localized and elucidated the functions of various skin cells, shedding light on how the tortoise adapts to adverse environmental conditions. Our findings unveiled numerous scales on the limbs, varying in size and color, acting as protective armor against abrasions, bites, and other potential threats in their rocky habitats. The epidermis comprises four layers: stratum basalis, stratum spinosum, peri-corneous layer, and stratum corneum. Cytokeratin 14 (CK14) was explicitly detected in the basal layer of the epidermis, suggesting a role in maintaining epidermal integrity and cellular function. Langerhans cells were observed between epidermal cells filled with ribosomes and Birbeck granules. Numerous dendritic-shaped Langerhans cells revealed through E-Cadherin signify strong immunity in tortoises' skin. Melanophores were identified using the Melan-A antibody, labeling the cytoplasm, and the SOX10 antibody, labeling the nucleus, providing comprehensive insights into melanophores morphology and distribution. Two types of melanophores were found: dendritic below the stratum basalis of the epidermis and clustered oval melanophores in the deep dermal layer. Varied melanophores distribution resulted in a spotted skin pattern, potentially offering adaptive camouflage and protection against environmental challenges. Numerous myofibroblasts were discerned through alpha-smooth actin (α-SMA) expression, indicating that the Greek tortoise's skin possesses a robust tissue repair and remodeling capacity. B-cell lymphocytes detected via CD20 immunostaining exhibited sporadic distribution in the dermis, concentrating in lymphoid aggregates and around vessels, implying potential roles in local immune responses and inflammation modulation. Employing Tom20 to identify skin cells with abundant mitochondria revealed a notable presence in melanophores and the basal layer of the epidermis, suggesting high metabolic activity in these cell types and potentially influencing cellular functions. These findings contribute to our comprehension of tortoise skin anatomy and physiology, offering insights into the remarkable adaptations of this species finely tuned to their specific environmental habitats.

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.

Clinical and histological findings of cutaneous wound healing in the red-eared slider turtle (Trachemys scripta elegans) housed in unheated outdoor enclosures

BACKGROUND

Cutaneous wounds are common in chelonians. The clinical and histological features of wound healing in these species are not well described and this prevents evaluation of new therapies.

OBJECTIVES

To describe clinical and histopathological features of cutaneous wound healing in the red-eared slider turtle (Trachemys scripta elegans).

ANIMALS

Twenty four healthy adult females housed in outdoor facilities with free access to water and exposed to daily variations in temperature.

METHODS

Full thickness 6 mm skin biopsy punch wounds were created in the rear limbs. The turtles were assigned to Group 1 (n = 12 for clinical evaluation) and Group 2 (n = 12 for microscopic study). Group 1 was photographed on Day 1 and weekly, until 28 days post wounding. Wound retraction was expressed as the percentage of perimeter reduction. For Group 2, three skin wounds were sampled at 2, 7, 14, 21, 28, 42, 60 and 135 days post wounding for histological study. The avidin-biotin-peroxidase (ABC) staining method was used to evaluate five commercial antibodies.

RESULTS

Wound contraction was limited; crust persisted at least 28 days. Re-epithelialization was complete by Day 14 in many animals; active inflammation persisted until 28 days; connective tissue re-constitution and remodelling was achieved from 42 to 135 days. Antibodies AE1/AE3, Factor VIII, MAC 387, CD3 and NCL-MSA showed cross reactivity with the cell counterpart in turtle tissues.

CONCLUSIONS AND CLINICAL IMPORTANCE

Second intention wound healing progressed slowly and with an indolent behaviour. Microscopically there was marked overlapping of the inflammatory and proliferative phases over a long time period.

Beta-keratins of turtle shell are glycine-proline-tyrosine rich proteins similar to those of crocodilians and birds

This study presents, for the first time, sequences of five beta-keratin cDNAs from turtle epidermis obtained by means of 5'- and 3'-rapid amplification of cDNA ends (RACE) analyses. The deduced amino acid sequences correspond to distinct glycine-proline-serine-tyrosine rich proteins containing 122-174 amino acids. In situ hybridization shows that beta-keratin mRNAs are expressed in cells of the differentiating beta-layers of the shell scutes. Southern blotting analysis reveals that turtle beta-keratins belong to a well-conserved multigene family. This result was confirmed by the amplification and sequencing of 13 genomic fragments corresponding to beta-keratin genes. Like snake, crocodile and avian beta-keratin genes, turtle beta-keratins contain an intron that interrupts the 5'-untranslated region. The length of the intron is variable, ranging from 0.35 to 1.00 kb. One of the sequences obtained from genomic amplifications corresponds to one of the five sequences obtained from cDNA cloning; thus, sequences of a total of 17 turtle beta-keratins were determined in the present study. The predicted molecular weight of the 17 different deduced proteins range from 11.9 to 17.0 kDa with a predicted isoelectric point of 6.8-8.4; therefore, they are neutral to basic proteins. A central region rich in proline and with beta-strand conformation shows high conservation with other reptilian and avian beta-keratins, and it is likely involved in their polymerization. Glycine repeat regions, often containing tyrosine, are localized toward the C-terminus. Phylogenetic analysis shows that turtle beta-keratins are more similar to crocodilian and avian beta-keratins than to those of lizards and snakes.

Immunological characterization of a newly developed antibody for localization of a beta-keratin in turtle epidermis

Turtle scutes are made of hard (beta)-keratins. In order to study size and localization of beta-keratins in turtle shell, we produced a rat polyclonal antiserum against a turtle scute beta-keratin of 13-16 kDa, which allowed the immunolocalization of the protein in the epidermis. In immunoblots the antiserum recognized turtle beta-keratins but showed variable cross-reactivity with lizard, snake, and avian beta-keratins. The turtle antiserum appears less cross-reactive than a chicken scale antiserum (Beta-1). In bidimensional immunoblots, three main protein spots at 15-16 kDa with pI at 7.3, 6.8, 6.4, and an unresolved large spot at 40-45 kDa with pI around 5 were more constantly obtained. The latter may result from the aggregation of the smaller beta-keratin protein. The corneous layer of the carapace and plastron of various species of chelonians appeared immunofluorescent. The ultrastructural immunolocalization showed sparse labeling over beta-keratin filaments of cells of the horny layer of both carapace and plastron. The study for the first time shows that the isolated protein band derived from a component of the beta-keratin filaments of the corneous layer of turtles. This antibody can be used for further studies on beta-keratin expression and sequencing in chelonian shell. No labeling was present over other cell organelles or layers of turtle epidermis and it was absent in non-epidermal cells. The specificity for turtle beta-keratin suggests that the antiserum recognizes some epitope/s specific for chelonians beta-keratins, and that it also variably recognizes other reptilian and avian beta-keratins.

Alpha- and beta-keratins of the snake epidermis

Snake scales contain specialized hard keratins (beta-keratins) and alpha- or cyto-keratins in their epidermis. The number, isoelectric point, and the evolution of these proteins in snakes and their similarity with those of other vertebrates are not known. In the present study, alpha- and beta-keratins of snake molts and of the whole epidermis have been studied by using two-dimensional electrophoresis and immunocytochemistry. Specific keratins in snake epidermis have been identified by using antibodies that recognize acidic and basic cytokeratins and avian or lizard scale beta-keratin. Alpha keratins of 40-70 kDa and isoelectric point (pI) at 4.5-7.0 are present in molts. The study suggests that cytokeratins in snakes are acidic or neutral, in contrast to mammals and birds where basic keratins are also present. Beta keratins of 10-15 kDa and a pI of 6.5-8.5 are found in molts. Some beta-keratins appear as basic proteins (pI 8.2) comparable to those present in the epidermis of other reptiles. Some basic "beta-keratins" associate with cytokeratins as matrix proteins and replace cytokeratins forming the corneous material of the mature beta-layer of snake scales, as in other reptiles. The study also suggests that more forms of beta-keratins (more than three different types) are present in the epidermis of snakes.

Characterization of beta-keratins in lizard epidermis: electrophoresis, immunocytochemical and in situ-hybridization study

Lizard scales are composed of alpha-(cyto-) keratins and beta-keratins. The characterization of the molecular weight and isoelectric point (pI) of alpha- and beta-keratins of lizard epidermis (Podarcis sicula) has been done by using two-dimensional electrophoresis, immunoblotting, and immunocytochemistry. Antibodies against cytokeratins, against a chicken scale beta-keratin or against lizard beta-keratin bands of 15-16kDa, have been used to recognize alpha- and beta-keratins. Acid and basic cytokeratins of 42-67kDa show a pI from 5.0 to 8.9. This indicates the presence of specific keratins for the formation of the stratum corneum. Main protein spots of beta-keratin at 15-17kDa, and pI at 8.5, 8.2, and 6.7, and one spot at 10kDa and pI at 7.3 were recognized. Therefore, beta-keratins are mainly basic proteins, and are used for the formation of the hard corneous layer of the epidermis. Ultrastructural immunocytochemistry confirms that beta-keratin is packed into large and dense bundles of beta-keratin cells of lizard epidermis. The use of a probe against a lizard beta-keratin in situ-hybridization studies confirms that the mRNA for beta-keratins is present in beta-cells and is localized around or even associated with beta-keratin filaments.

Distribution and Characterization of Keratins in the Epidermis of the Tuatara (Sphenodon punctatus; Lepidosauria, Reptilia)

Reptilian scales are mainly composed of alpha-and beta-keratins. Epidermis and molts from adult individuals of an ancient reptilian species, the tuatara (Sphenodon punctatus), were analysed by immunocytochemistry, mono- and bi-dimensional electrophoresis, and western blotting for alpha- and beta-keratins. The epidermis of this reptilian species with primitive anatomical traits should represent one of the more ancient amniotic epidermises available. Soft keratins (AE1- and AE3-positive) of 40-63 kDa and with isoelectric points (pI) at 4.0-6.8 were found in molts. The AE3 antibody was diffusely localised over the tonofilaments of keratinocytes. The lack of basic cytokeratins may be due to keratin alteration in molts, following corneification or enzymatic degradation of keratins. Hard (beta-) keratins of 16-18 kDa and pI at 6.8, 8.0, and 9.2 were identified using a beta-1 antibody produced against chick scale beta-keratin. The antibody also labeled filaments of beta-cells and of the mature, compact beta-layer. We have shown that beta-keratins in the tuatara resemble those of lizards and snakes, and that they are mainly basic proteins. These proteins replace cytokeratins in the pre-corneoum beta-layers, from which a hard, mechanically resistant corneoum layer is formed over scales. Beta-keratins may have both a fibrous and a matrix role in forming the hard texture of corneoum scales in this ancient species, as well as in more recently evolved reptiles.

Skin structure and cornification proteins in the soft-shelled turtle Trionyx spiniferus

In contrast to most chelonians, the fully aquatic soft-shelled turtles have a smooth, unscaled, and pliable shell. The skin of the shell, tail, limbs, and neck of juveniles of Trionyx spiniferus has been studied by ultrastructural, immunocytochemical, and immunoblotting methods. The epidermis of the carapace and plastron has a thick corneous layer composed of alpha-corneocytes surrounded by a cornified cell envelope. The softer epidermis is similar to that of the shell but the epidermis and corneous layer are much thinner. Pre-corneous cells in both soft and shell epidermis are rich in vesicles produced in the Golgi apparatus and smooth endoplasmic vesicles, and contain numerous dense-core mucus-like and vesicular (lamellar) bodies. Secreted material is present among corneocytes where it probably forms an extensive intercellular lipid-mucus waterproof barrier. The dermis is very thick and composed of several layers of collagen bundles that form a plywood-patterned dermis. This dermis constitutes a strong mechanical barrier that compensates for the low content in beta-keratin, and lack of cornified scutes and dermal bones. The growth of the shell mainly occurs along the lateral margins. Immunocytochemistry reveals the presence of some beta-keratin in soft and shell epidermis, and this is confirmed by immunoblotting where bands at 18 and 32-35 kDa are present. Other proteins of the cornified cell envelope (loricrin and sciellin) or associated to lipid trafficking (caveolin-1) are also detected by immunoblotting. Loricrin positive bands at 24 and 57 kDa are present while bands cross-reactive for sciellin are seen at 45 and 53 kDa. Caveolin-1 positive bands are seen at 21-22 kDa. The presence of these proteins indicates that the epidermis is both coriaceous and waterproof. These results suggest that the shell of Trionyx is tough enough to be as mechanically efficient as the hard shell of the other turtles. At the same time, a soft shell is lighter, its shape is more easily controlled by muscles, and it allows a more controlled closure and retraction of limbs and neck inside the shell. Thus, the shell makes the animal more streamlined for swimming.

Scale keratin in lizard epidermis reveals amino acid regions homologous with avian and mammalian epidermal proteins

Small proteins termed beta-keratins constitute the hard corneous material of reptilian scales. In order to study the cell site of synthesis of beta-keratin, an antiserum against a lizard beta-keratin of 15-16 kDa has been produced. The antiserum recognizes beta-cells of lizard epidermis and labels beta-keratin filaments using immunocytochemistry and immunoblotting. In situ hybridization using a cDNA-probe for a lizard beta-keratin mRNA labels beta-cells of the regenerating and embryonic epidermis of lizard. The mRNA is localized free in the cytoplasm or is associated with keratin filaments of beta-cells. The immunolabeling and in situ labeling suggest that synthesis and accumulation of beta-keratin are closely associated. Nuclear localization of the cDNA probe suggests that the primary transcript is similar to the cytoplasmic mRNA coding for the protein. The latter comprises a glycine-proline-rich protein of 15.5 kDa that contains 163 amino acids, in which the central amino acid region is similar to that of chick claw/feather while the head and tail regions resemble glycine-tyrosine-rich proteins of mammalian hairs. This is also confirmed by phylogenetic analysis comparing reptilian glycine-rich proteins with cytokeratins, hair keratin-associated proteins, and claw/feather keratins. It is suggested that different small glycine-rich proteins evolved from progenitor proteins present in basic (reptilian) amniotes. The evolution of these proteins originated glycine-rich proteins in scales, claws, beak of reptiles and birds, and in feathers. Some evidence suggests that at least some proteins contained within beta-keratin filaments are rich in glycine and resemble some keratin-associated proteins present in mammalian corneous derivatives. It is suggested that glycine-rich proteins with the chemical composition, immunological characteristics, and molecular weight of beta-keratins may represent the reptilian counterpart of keratin-associated proteins present in hairs, nails, hooves, and horns of mammals. These small proteins produce a hard type of corneous material due to their dense packing among cytokeratin filaments.

Ultrastructural and immunohistochemical observations on the process of horny growth in chelonian shells

The process of growth of horny scutes of the carapace and plastron in chelonians is poorly understood. In order to address this problem, the shell of the terrestrial tortoise Testudo hermanni, the freshwater turtle Chrysemys picta, and the soft shelled turtle Trionix spiniferus were studied. The study was carried out using immunohistochemistry, electron microscopy and autoradiography following injection of tritiated histidine. The species used in the present study illustrate three different types of shell growth that occur in chelonians. In scutes of Testudo and Chrysemys, growth mainly occurs in the hinge regions by the production of cells that accumulate beta-keratin and incorporate tritiated histidine. Newly produced bundles of alpha- and beta-keratin incorporate most of the histidine. No keratohyalin is observed in the epidermis of any of the species studied here. In Testudo, newly generated corneocytes containing beta-keratin form a corneous layer to form the growing rings of scutes. In Chrysemys, newly generated corneocytes containing beta-keratin form the new, expanded corneous layer. In the latter species, at the end of the growing season (autumn/fall), thin corneocytes containing little beta-keratin are produced underneath the corneous layer, and gradually form a scission layer. In the following growing season (spring-summer) the shedding layer matures and determines the loss of the outer corneous layer. In this way, scutes expand their surface at any new molt. In Trionix, no distinct scutes and hinge regions are present and during the growing season, new corneocytes are mainly produced along the perimeter of the shell. Corneocytes of Trionix contain little beta-keratin and form a thick corneous layer in which cells resemble the alpha-layer of the softer epidermis of the limbs, tail and neck. Neither keratohyalin nor specific histidine incorporation was observed in these cells. Corneocytes are gradually lost from the epidermal surface. Dermal scutes are absent in Trionix, but the dermis is organized in 6-10 layers of plywood-patterned collagen bundles. The stratified layers gradually disappear toward the growing border of the shell. The mode of growth of horny scutes in these different species of chelonians is discussed.

Cytochemical, biochemical and molecular aspects of the process of keratinization in the epidermis of reptilian scales

The characteristics of scaled skin of reptiles is one of their main features that distinguish them from the other amniotes, birds and mammals. The different scale patterns observed in extant reptiles result from a long evolutive history that allowed each species to adapt to its specific environment. The present review deals with comparative aspects of epidermal keratinization in reptiles, chelonians (turtles and tortoises), lepidosaurian (lizards, snakes, sphenodontids), archosaurians (crocodilians). Initially the morphology and cytology of reptilian scales is outlined to show the diversity in the epidermis among different groups. The structural proteins (alpha-keratins and associated proteins), and enzymes utilized to form the corneous layer of the epidermis are presented. Aside cytokeratins (alpha-keratins), used for making the cytoskeleton, reptilian alpha-keratinocytes produce interkeratin (matrix) and corneous cell envelope proteins. Keratin bundles and degraded cell organelles constitute most of the corneous material of alpha-keratinocytes. Matrix, histidine-rich and sulfur-rich proteins are produced in the soft epidermis and accumulated in the cornified cell envelope. Main emphasis is given to the composition and to the evolution of the hard keratins (beta-keratins). Beta-keratins constitute the hard corneous material of scales. These small proteins are synthesized in beta-keratinocytes and are accumulated into small packets that rapidly merge into a compact corneous material and form densely cornified layers. Beta-keratins are smaller proteins (8-20 kDa) in comparison to alpha-keratins (40-70 kDa), and this size may determine their dense packing in corneocytes. Both glycine-sulfur-rich and glycine-proline-rich proteins have been so far sequenced in the corneous material of scales in few reptilian species. The latter keratins possess C- and N-amino terminal amino acid regions with sequence homology with those of mammalian hard keratins. Also, reptilian beta-keratins possess a central core with homology with avian scale/feather keratins. Multiple genes code for these proteins and their discovery and sequentiation is presently an active field of research. These initial findings however suggest that ancient reptiles already possessed some common genes that have later diversified to produce the specific keratin-associated proteins in their descendants: extant reptiles, birds and mammals. The evolution of these small proteins in lepidosaurians, chelonians and archosaurians represent the next step to understand the evolution of cornification in reptiles and derived amniotes (birds and mammals).

Immunolocalization and characterization of beta-keratins in growing epidermis of chelonians

Beta-keratins constitute most of the corneous material of carapace and plastron of turtles. The production of beta-keratin in the epidermis of a turtle and tortoise (criptodirians) and of a species of pleurodiran turtle was studied after injection of tritiated proline during the growth of carapace, plastron and claws. Growth mainly occurs near hinge regions along the margins of scutes and along most of the claws (growing regions). Proline incorporation occurs mainly in the growing centers, and is more specifically associated with beta-keratin synthesis. Proline-labeled bands of protein at 12-14 kDa and 25-27 kDa, and 37 kDa, in the molecular weight range of beta-keratins, were isolated from the soft epidermis of turtles 3 h after injection of the labeled amino acid. After extraction of epidermal proteins, an antibody directed against a chicken beta-keratin was used for immunoblotting. Bands of beta-keratin at 15-17 kDa, 22-24 kDa, and 36-38 kDa appear in all species. Beta-keratin is present in the growing and compact stratum corneum of the hard (shell) and soft (limbs, neck and tail) epidermis. This was confirmed using a specific antibody against a turtle beta-keratin band of 15-16 kDa. The latter antibody recognized epidermal protein bands in the range of 15-16 kDa and 29-33 kDa, and labels beta-keratin filaments. This result indicates that different forms of beta-keratins are produced from low molecular weight precursors or that larger aggregate form during protein preparation. The present study shows that beta-keratin is abundant in the scaled epidermis of tortoise but also in the soft epidermis of pleurodiran and cryptodiran turtles, indicating that this form of hard keratin is constitutively expressed in the epidermis of chelonians.

Immunological characterization and fine localization of a lizard beta-keratin

Scales of lizards contain beta-keratin of poorly known composition. In the present study, a rat polyclonal serum against a lizard beta-keratin of 14-15 kDa has been produced and the relative protein has been immunolocalized in the epidermis. The observations for the first time show that the isolated protein band derives from the extraction of a protein component of the beta-keratin filaments of lizard epidermis. In immunoblots and immunocytochemistry, the antiserum recognizes most lizard beta-keratins, but produces a variable cross-reactivity with snake beta-keratins, and weak or no reactivity with beta-keratins isolated from tuatara, turtles, alligator and birds. In bidimensional immunoblots of lizard epidermis, three main spots at 15-16 kDa with isoelectric point at 7.0, 7.6 and 8.0, and an unresolved large spot at 29-30 kDa and with pI at 7.5-8.0, are obtained, may be derived from the aggregation of smaller beta-keratin proteins. The ultrastructural immunolocalization with the antibody against lizard beta-keratin shows that only small and large beta-keratin filaments of beta-cells of lizard epidermis are labeled. Keratin bundles in oberhautchen cells are less immunolabeled. Beta-keratin is rapidly polymerized into beta-packets that merge into larger beta-keratin filaments. No labeling is present over other cell organelles or cell layers of lizard epidermis, and is absent in non-epidermal cells. The antiserum recognizes epitope(s) characteristics for lizard beta-keratins, partially recognized in snakes and absent in non-lepidosaurian species. This result indicates that beta-keratins among different reptilian groups posses different immunoreactive regions.

Structural and Immunocytochemical Characterization of Keratinization in Vertebrate Epidermis and Epidermal Derivatives

This review presents comparative aspects of epidermal keratinization in vertebrates, with emphasis on the evolution of the stratum corneum in land vertebrates. The epidermis of fish does not contain proteins connected with interkeratin matrix and corneous cell envelope formation. Mucus-like material glues loose keratin filaments. In amphibians a cell corneous envelope forms but matrix proteins, aside from mucus/glycoproteins, are scarce or absent. In reptiles, birds, and mammals specific proteins associated with keratin become relevant for the production of a resistant corneous layer. In reptiles some matrix, histidine-rich and sulfur-rich corneous cell envelope proteins are produced in the soft epidermis. In avian soft epidermis low levels of matrix and cornified proteins are present while lipids become abundant. In mammalian keratinocytes, interkeratin proteins, cornified cell envelope proteins, and transglutaminase are present. Topographically localized areas of dermal-epidermal interactions in amniote skin determine the formation of skin derivatives such as scales, feathers, and hairs. New types of keratin and associated proteins are produced in these derivatives. In reptiles and birds beta-keratins form the hard corneous material of scales, claws, beaks, and feathers. In mammals, small sulfur-rich and glycine-tyrosine-rich proteins form the corneous material of hairs, horns, hooves, and claws. Molecular studies on reptilian beta-keratins show they are glycine-rich proteins. They have C- and N-terminal amino acid regions homologous to those of mammalian proteins and a central core with homology to avian scale/feather keratins. These findings suggest that ancient reptiles already possessed some common genes that later diversified to produce some keratin-associated protein in extant reptiles and birds, and others in mammals. The evolution of these small proteins represents the more recent variation of the process of cornification in vertebrates.