Immunocytochemical analysis of the process of keratinization of the epidermis of snakes

Papillary and Callous Scales in the Integument of Agamid Lizards (Agamidae, Sauria) as a Phenomenon of Extraordinary Development of the Corneous Layers

Scaled integument of six species of the genus Acanthocercus and Laudakia nupta, family Agamidae was studied using light and scanning electron microscopy. Gross observation revealed the presence of two types of modified scales in the males. The enlarged scales covered with an extremely thick β-corneous layer were detected in the dorsal and ventral surfaces of the tail base and on the palmar and plantar limb surfaces of all species. After detachment of the β-layer, the surface of such scales was covered with high papillae ("papillary scales"). The callous scales were found in the precloacal region of Acanthocercus species and in both precloacal and mid-ventral regions of L. nupta. Modified scales were found in some females and subadut specimens, and absent in juveniles. A prominent papillary layer characterized the dermis of both scale types. It was assumed that well-developed dermal papillae in such scales expended the total surface area of stratum germinativum and created a pool of proliferated cells in the interpapillary loops to increase the production of differentiating keratinocytes. These processes were undoubtedly associated with the formation of a thick and resistant corneous layer that distinguished both types of scales. Functional role, a taxonomic value, and analogies with normal and pathological epidermis in birds and mammals are discussed.

Developmental mechanisms of longitudinal stripes in the Japanese four-lined snake

The developmental mechanisms of color patterns formation and its evolution remain unclear in reptilian sauropsids. We, therefore, studied the pigment cell mechanisms of stripe pattern formation during embryonic development of the snake Elaphe quadrivirgata. We identified 10 post-ovipositional embryonic developmental stages based on external morphological characteristics. Examination for the temporal changes in differentiation, distribution, and density of pigment cells during embryonic development revealed that melanophores first appeared in myotome and body cavity but not in skin surface at Stage 5. Epidermal melanophores were first recognized at Stage 7, and dermal melanophores and iridophores appeared in Stage 9. Stripe pattern first appeared to establish at Stage 8 as a spatial density gradient of epidermal melanophores between the regions of future dark brown longitudinal stripes and light colored background. Our study, thus, provides a comprehensive pigment-cell-based understanding of stripe pattern formation during embryonic development. We briefly discuss the importance of the gene expression studies by considering the biologically relevant theoretical models with standard developmental staging for understanding reptilian color pattern evolution.

Immunolocalization of sulfhydryl oxidase in reptilian epidermis indicates that the enzyme participates mainly to the hardening process of the beta-corneous layer

Reptilian skin is tough and scaled representing an evolutionary adaptation to the terrestrial environment. The presence of sulfhydryl oxidase during the process of hardening of the corneous layer in reptilian epidermis has been analyzed by immunocytochemistry and immunoblotting. Sulfhydryl oxidase-like immunoreactivity of proteins in the 50-65 kDa range of molecular weight is mainly observed in the transitional and pre-corneous layers of crocodilians, chelonian, and in the forming beta-layer of lepidosaurians. The ultrastructural localization of the enzyme by immunogold in lizard epidermis during renewal and resting stages shows that the labeling is mainly distributed in the cytoplasm and along the accumulating beta-packets of differentiating beta-cells while it appears very low to undetectable in differentiating alpha-cells of the lacunar, clear, mesos, and alpha-layers. The labeling however becomes absent or undetectable also in the fully mature beta-layer. The study shows that an oxidative enzyme is likely responsible of the cross-linking of the numerous cysteines present in the main proteins accumulated in corneocytes of reptilian epidermis, known as corneous beta-proteins (beta-keratins). This process of disulphide bond formation is probably largely responsible for the formation of hard beta-corneous layers in reptilian scales, a difference with alpha-corneous layers where substrate proteins of transglutaminase appear predominant.

Cross-immunoreactivity between the LH1 antibody and cytokeratin epitopes in the differentiating epidermis of embryos of the grass snake Natrix natrix L. during the end stages of embryogenesis

The monoclonal anti-cytokeratin 1/10 (LH1) antibody recognizing K1/K10 keratin epitopes that characterizes a keratinized epidermis of mammals cross-reacts with the beta and Oberhäutchen layers covering the scales and gastrosteges of grass snake embryos during the final period of epidermis differentiation. The immunolocalization of the anti-cytokeratin 1/10 (LH1) antibody appears in the beta layer of the epidermis, covering the outer surface of the gastrosteges at the beginning of developmental stage XI, and in the beta layer of the epidermis, covering the outer surface of the scales at the end of developmental stage XI. This antibody cross-reacts with the Oberhäutchen layers in the epidermis covering the outer surface of both scales and gastrosteges at developmental stages XI and XII just before its fusion with the beta layers. After fusion of the Oberhäutchen and beta layers, LH1 immunolabeling is weaker than before. This might suggest that alpha-keratins in these layers of the epidermis are masked by beta-keratins, modified, or degraded. The anti-cytokeratin 1/10 (LH1) antibody stains the Oberhäutchen layer in the epidermis covering the inner surface of the gastrosteges and the hinge regions between gastrosteges at the end of developmental stage XI. However, the Oberhäutchen of the epidermis covering the inner surfaces of the scales and the hinge regions between scales does not show cytokeratin 1/10 (LH1) immunolabeling until hatching. This cross-reactivity suggests that the beta and Oberhäutchen layers probably contain some alpha-keratins that react with the LH1 antibody. It is possible that these alpha-keratins create specific scaffolding for the latest beta-keratin deposition. It is also possible that the LH1 antibody cross-reacts with other epidermal proteins such as filament-associated proteins, i.e., filaggrin-like. The anti-cytokeratin 1/10 (LH1) antibody does not stain the alpha and mesos layers until hatching. We suppose that the differentiation of these layers will begin just after the first postnatal sloughing.

Isolation of a new class of cysteine-glycine-proline-rich beta-proteins (beta-keratins) and their expression in snake epidermis

Scales of snakes contain hard proteins (beta-keratins), now referred to as keratin-associated beta-proteins. In the present study we report the isolation, sequencing, and expression of a new group of these proteins from snake epidermis, designated cysteine-glycine-proline-rich proteins. One deduced protein from expressed mRNAs contains 128 amino acids (12.5 kDa) with a theoretical pI at 7.95, containing 10.2% cysteine and 15.6% glycine. The sequences of two more snake cysteine-proline-rich proteins have been identified from genomic DNA. In situ hybridization shows that the messengers for these proteins are present in the suprabasal and early differentiating beta-cells of the renewing scale epidermis. The present study shows that snake scales, as previously seen in scales of lizards, contain cysteine-rich beta-proteins in addition to glycine-rich beta-proteins. These keratin-associated beta-proteins mix with intermediate filament keratins (alpha-keratins) to produce the resistant corneous layer of snake scales. The specific proportion of these two subfamilies of proteins in different scales can determine various degrees of hardness in scales.

Localization and Characterization of Specific Cornification Proteins in Avian Epidermis

Little is known about proteins involved in the formation of the stratum corneum in the avian apteric epidermis. The present immunocytochemical, autoradiographic and electrophoretic study shows that antibodies against characteristic proteins of mammalian cornification (alpha-keratins, loricrin, sciellin, filaggrin, transglutaminase) recognize avian epidermal proteins. This suggests the presence of avian protein with epitopes common to related mammalian proteins. These proteins may also be involved in the formation of the cornified core and cell envelope of mature avian corneocytes. The immunoblotting study suggests that protein bands, cross-reactive for antibodies against loricrin (45, 52-57 kDa), sciellin (54, 84 kDa), filaggrin (32, 38, 45-48 kDa), and transglutaminase (40, 50, 58 kDa), are present in the avian epidermis. Immunocytochemistry shows that immunoreactivity for the above proteins is localized in the transitional and lowermost corneous layer of apteric epidermis. Their epitopes are rapidly masked/altered in cornifying cells and are no longer detectable in mature corneocytes. In scaled epidermis a thick layer made of beta-keratins of 14-18, 20-22, and 33 kDa is formed. Only in feathered epidermis (not in scale epidermis), an antifeather chicken beta-keratin antibody recognized a protein band at 8-12 kDa. This small beta-keratin is probably suitable for the formation of long, axial filaments in elongated barb, barbule and calamus cells. Conversely, the larger beta-keratins in scales are irregularly deposited forming flat plates. Tritiated histidine coupled to autoradiography show an absence of both keratohyalin and histidine-rich proteins in adult feathered and scaled epidermis. Most of the labeling appears in proteins within the range of beta- and alpha-keratins. These data on apteric epidermis support the hypothesis of an evolution of the apteric and interfollicular epidermis from the expansion of hinge regions of protoavian archosaurians.

Differentiation of snake epidermis, with emphasis on the shedding layer

Little is known about specific proteins involved in keratinization of the epidermis of snakes. The presence of histidine-rich molecules, sulfur, keratins, loricrin, transglutaminase, and isopeptide-bonds have been studied by ultrastructural autoradiography, X-ray microanalysis, and immunohistochemistry in the epidermis of snakes. Shedding takes place along a shedding complex, which is composed of two layers, the clear and the oberhautchen layers. The remaining epidermis comprises different layers, some of which contain beta-keratins and others alpha-keratins. Weak loricrin, transglutaminase, and sometimes also iso-peptide-bond immunoreactivities are seen in some cells, lacunar cells, of the alpha-layer. Tritiated histidine is mainly incorporated in the shedding complex, especially in dense beta-keratin filaments in cells of the oberhautchen layer and to a small amount in cells of the clear layer. This suggests the presence of histidine-rich, matrix proteins among beta-keratin bundles. The latter contain sulfur and are weakly immunolabeled for beta-keratin at the beginning of differentiation of oberhautchen cells. After merging with beta cells, the dense beta-keratin filaments of oberhautchen cells become immunopositive for beta-keratin. The uptake of histidine decreases in beta cells, where little dense matrix material is present, while pale beta-keratin filaments increase. During maturation, little histidine labeling remains in electron-dense areas of the beta layer and in those of oberhautchen spinulae. Some roundish dense granules of oberhautchen cells rich in sulfur are negative to antibodies for alpha-keratin, beta-keratin, and loricrin. The granules eventually merge with beta-keratin, and probably contribute to the formation of the resistant matrix of oberhautchen cells. In conclusion, beta-keratin, histidine-rich, and sulfur-rich proteins contribute to form snake microornamentations.

Immunolocalization and characterization of cornification proteins in snake epidermis

Little is known about specific proteins involved in keratinization of the epidermis of snakes, which is composed of alternating beta- and alpha-keratin layers. Using immunological techniques (immunocytochemistry and immunoblotting), the present study reports the presence in snake epidermis of proteins with epitopes that cross-react with certain mammalian cornification proteins (loricrin, filaggrin, sciellin, transglutaminase) and chick beta-keratin. alpha-keratins were found in all epidermal layers except in the hard beta- and alpha-layers. beta-keratins were exclusively present in the oberhautchen and beta-layer. After extraction and electrophoresis, alpha-keratins of 40-67 kDa in molecular weights were found. Loricrin-like proteins recorded molecular weights of 33, 50, and 58 kDa; sciellin, 55 and 62 kDa; filaggrin-like, 52 and 65 kDa; and transglutaminase, 45, 50, and 56 kDa. These results suggest that alpha-layers of snake epidermis utilize proteins with common epitopes to those present during cornification of mammalian epidermis. The beta-keratin antibody on extracts from whole snake epidermis showed a strong cross-reactive band at 13-16 kDa. No cross-reactivity was seen using an antibody against feather beta-keratin, indicating absence of a common epitope between snake and feather keratins.

Formation of the corneous layer in the epidermis of the tuatara (Sphenodon punctatus, Sphenodontida, Lepidosauria, Reptilia)

The formation of the stratum corneum in the epidermis of the reptile Sphenodon punctatus has been studied by histochemical, immunohistochemical, and ultrastructural methods. Sulfhydryl groups are present in the mesos and pre-alpha-layer but disappear in the keratinized beta-layer and in most of the mature alpha-layer. This suggests a complete cross-linking of keratin filaments. Tyrosine increases in keratinized layers, especially in the beta-layer. Arginine is present in living epidermal layers, in the presumptive alpha-layer, but decreases in keratinized layers. Histidine is present in corneous layers, especially in the intermediate region between the alpha- and a new beta-layer, but disappears in living layers. It is unknown whether histidine-rich proteins are produced in the intermediate region. Small keratohyalin-like granules are incorporated in the intermediate region. The plane of shedding, as confirmed from the study on molts, is located along the basalmost part of the alpha-layer and may involve the degradation of whole cells or cell junctions of the intermediate region. A specific shedding complex, like that of lizards and snakes, is not formed in tuatara epidermis. AE1-, AE2-, or AE3-positive alpha-keratins are present in different epidermal layers with a pattern similar to that previously described in reptiles. The AE1 antibody stains the basal and, less intensely, the first suprabasal layers. Pre-keratinized, alpha- and beta-layers, and the intermediate region remain unlabeled. The AE2 antibody stains suprabasal and forming alpha- and beta-layers, but does not stain the basal and suprabasal layers. In the mature beta-layer the immunostaining disappears. The AE3 antibody stains all epidermal layers but disappears in alpha- and beta-layers. Immunolocalization for chick scale beta-keratins labels the forming and mature beta-layer, but disappears in the mesos and alpha-layer. This suggests the presence of common epitopes in avian and reptilian beta-keratins. Low molecular weight alpha-keratins present in the basal layer are probably replaced by keratins of higher molecular weight in keratinizing layers (AE2-positive). This keratin pattern was probably established since the beginning of land adaptation in amniotes.

Immuno-Cross reactivity of transglutaminase and cornification marker proteins in the epidermis of vertebrates suggests common processes of soft cornification across species

In differentiating mammalian keratinocytes proteins are linked to the plasma membrane by epidermal transglutaminases through N-epsilon-(gamma-glutamyl)-lysine isopeptide bonds to form the cornified cell envelope. The presence of transglutaminases and their protein substrates in the epidermis of nonmammalian vertebrates is not known. The present study analyses the presence and localization of the above proteins in the epidermis using immuno-cross reactivity across different classes of amniotes. After immunoblotting, some protein bands appear labelled for loricrin, sciellin, and transglutaminase in most species. These proteins are scarce to absent in the epidermis of aquatic species (goldfish and newt) where a stratum corneum is absent or very thin. The molecular weight of transglutaminase immunoreactive bands generally varies between 40 to 62 kDa, with the most represented bands at 52-57 kDa in most species. The more intense loricrin- and sciellin-immunoreactive bands are seen at 50-55-62 kDa, but are weak or absent in aquatic vertebrates. Loricrine-like immunoreactivity is present in the epidermis where alpha-(soft)-keratinization occurs. Isopeptide bonds are mainly associated to bands in the range of 50-62 kDa. In vertebrates where hard-keratin is expressed (the beta-keratin corneous layer of sauropsids and in feathers) or in hair cortex of mammals, no loricrin-like, transglutaminase-, and isopeptide-bond-immunoreactivities are seen. Immunoblotting however shows loricrin-, sciellin-, and trasnsglutaminase-positive bands in the corneous layers containing beta-keratin. Histologically, the epidermis of most amniotes shows variable transglutaminase immunoreactivity, but isopeptide-bond and sciellin immunoreactivities are weak or undetactable in most species. The limitations of immunohistochemical methods are discussed and compared with results from immunoblotting. In reptilian epidermis transglutaminase is mainly localized in 0.15-0.3 microm dense granules or diffuse in transitional alpha-keratogenic cells. In beta-keratogenic cells few small dense granules show a weak immunolabeling. Transglutaminase is present in nuclei of terminal differentiating alpha- and beta-keratinocytes, as in those of mature inner and outer root sheath. The present study suggests that keratinization based on loricrin, sciellin and transglutaminase was probably present in the stratum corneoum of basic amniotes in the Carboniferous. These proteins were mainly maintained in alpha-keratogenic layers of amniotes but decreased in beta-keratogenic layers of sauropsids (reptiles and birds). The study suggests that similar proteins for the formation of the cornified cell envelope are present in alpha-keratinocytes across vertebrates but not in beta-keratinocytes.

Characterization of beta-keratins and associated proteins in adult and regenerating epidermis of lizards

Reptilian epidermis contains two types of keratin, soft (alpha) and hard (beta). The biosynthesis and molecular weight of beta-keratin during differentiation of lizard epidermis have been studied by autoradiography, immunocytochemistry and immunoblotting. Tritiated proline is mainly incorporated into differentiating and maturing beta-keratin cells with a pattern similar to that observed after immunostaining with a chicken beta-keratin antibody. While the antibody labels a mature form of beta-keratin incorporated in large filaments, the autoradiographic analysis shows that beta-keratin is produced within the first 30 min in ribosomes, and is later packed into large filaments. Also the dermis incorporates high amount of proline for the synthesis of collagen. The skin was separated into epidermis and dermis, which were analyzed separately by protein extraction and electrophoresis. In the epidermal extract proline-labeled proteic bands at 10, 15, 18-20, 42-45, 52-56, 85-90 and 120 kDa appear at 1, 3 and 5 h post-injection. The comparison with the dermal extract shows only the 85-90 and 120 kDa bands, which correspond to collagen. Probably the glycine-rich sequences of collagen present also in beta-keratins are weakly recognized by the beta-1 antibody. Immunoblotting with the beta-keratin antibody identifies proteic bands according to the isolation method. After-saline or urea-thiol extraction bands at 10-15, 18-20, 40, 55 and 62 kDa appear. After extraction and carboxymethylation, weak bands at 10-15, 18-20 and 30-32 kDa are present in some preparations, while in others also bands at 55 and 62 kDa are present. It appears that the lowermost bands at 10-20 kDa are simple beta-keratins, while those at 42-56 kDa are complex or polymeric forms of beta-keratins. The smallest beta-keratins (10-20 kDa) may be early synthesized proteins that are polymerized into larger beta-keratins which are then packed to form larger filaments. Some proline-labeled bands differ from those produced after injection of tritiated histidine. The latter treatment does not show 10-20 kDa labeled proteins, but tends to show bands at 27, 30-33, 40-42 and 50-62 kDa. Histidine-labeled proteins mainly localize in keratohyalin-like granules and dark keratin bundles of clear-oberhautchen layers of lizard epidermis, and their composition is probably different from that of beta-keratin.

Immunocytochemical and autoradiographic studies on the process of keratinization in avian epidermis suggests absence of keratohyalin

The process of keratinization in apteric avian epidermis and in scutate scales of some avian species has been studied by autoradiography for histidine and immunohistochemistry for keratins and other epidermal proteins. Acidic or basic alpha-keratins are present in basal, spinosus, and transitional layers, but are not seen in the corneous layer. Keratinization-specific alpha-keratins (AE2-positive) are observed in the corneous layer of apteric epidermis but not in that of scutate scales, which contain mainly beta-keratin. Alpha-keratin bundles accumulate along the plasma membrane of transitional cells of apteric epidermis. In contrast to the situation in scutate scales, in the transitional layer and in the lowermost part of the corneous layer of apteric epidermis, filaggrin-like, loricrin-like, and transglutaminase immunoreactivities are present. The lack of isopeptide bond immunoreactivity suggests that undetectable isopeptide bonds are present in avian keratinocytes. Using immunogold ultrastructural immunocytochemistry a low but localized loricrin-like and, less, filaggrin-like labeling is seen over round-oval granules or vesicles among keratin bundles of upper spinosus and transitional keratinocytes of apteric epidermis. Filaggrin-and loricrin-labeling are absent in alpha-keratin bundles localized along the plasma membrane and in the corneous layer, formerly considered keratohyalin. Using ultrastructural autoradiography for tritiated histidine, occasional trace grains are seen among these alpha-keratin bundles. A different mechanism of redistribution of matrix and corneous cell envelope proteins probably operates in avian keratinocytes as compared to that of mammals. Keratin bundles are compacted around the lipid-core of apteric epidermis keratinocytes, which do not form complex chemico/mechanical-resistant corneous cell envelopes as in mammalian keratinocytes. These observations suggest that low amounts of matrix proteins are present among keratin bundles of avian keratinocytes and that keratohyalin granules are absent.

Synthesis of interkeratin matrix in differentiating lizard epidermis: An ultrastructural autoradiographic study after injection of tritiated proline and histidine

During epidermal differentiation in mammals, keratins and keratin-associated matrix proteins rich in histidine are synthesized to produce a corneous layer. Little is known about interkeratin proteins in nonmammalian vertebrates, especially in reptiles. Using ultrastructural autoradiography after injection of tritiated proline or histidine, the cytological process of synthesis of beta-keratin and interkeratin material was studied during differentiation of the epidermis of lizards. Proline is mainly incorporated in newly synthesized beta-keratin in beta-cells, and less in oberhautchen cells. Labeling is mainly seen among ribosomes within 30 min postinjection and appears in beta-keratin packets or long filaments 1-3 h later. Beta-keratin appears as an electron-pale matrix material that completely replaces alpha-keratin filaments in cells of the beta-layer. Tritiated histidine is mainly incorporated into keratohyalin-like granules of the clear layer, in dense keratin bundles of the oberhautchen layer, and also in dense keratin filaments of the alpha and lacunar layer. The detailed ultrastructural study shows that histidine-labeling is localized over a dense amorphous material associated with keratin filaments or in keratohyalin-like granules. Large keratohyalin-like granules take up labeled material at 5-22 h postinjection of tritiated histidine. This suggests that histidine is utilized for the synthesis of keratins and keratin-associated matrix material in alpha-keratinizing cells and in oberhautchen cells. As oberhautchen cells fuse with subjacent beta-cells to form a syncytium, two changes occur : incorporation of tritiated histidine, but uptake of proline increases. The incorporation of tritiated histidine in oberhautchen cells lowers after merging with cells of the beta-layer, whereas instead proline uptake increases. In beta-cells histidine-labeling is lower and randomly distributed over the cytoplasm and beta-keratin filaments. Thus, change in histidine uptake somehow indicates the transition from alpha- to beta-keratogenesis. This study indicates that a functional stratum corneum in the epidermis of amniotes originates only after the association of matrix and corneous cell envelope proteins with the original keratin scaffold of keratinocytes.