On the nature of the horny scales of the pangolin

The pangolin scale is a horny derivative of the epidermis. It is complex in structure and is divisible into three distinct regions. The dorsal plate forms approximately one-sixth of the scale thickness. It is composed of flattened solid keratinized cells without basophilic nuclear remnants. This region tends to fray easily. The dorsal plate contains bound phospholipids and sulphydryl groups but is weak in disulphide bonds.
The bulk of the scale is made up of the intermediate plate formed of less flattened cells without basophilic nuclei. This region is rich in disulphide bonds but contains no appreciable bound phospholipids or sulphydryl groups.
The ventral plate is only a few cells thick and is rich in bound phospholipids, which also occur in the underlying scale bed epidermis.
These three regions of the scale are formed from separate epidermal germinal areas which do not develop a granular layer. Keratohyalin granules are, however, formed in the epidermis between the scales.
It is suggested on the basis of histological structure and dishribution of chemical constituents that pangolin scales are probably homologous with primate nails.
Evidence against the views that they are homologous with reptilian scales or are derived from compressed hairs is presented.     

Plantar epidermis of the guinea pig and characteristics of the stratum corneum.

The guinea pig plantar epidermis was examined by light-microscopical histochemical methods and by transmission electron microscopy. Autolysis of cell structure was much less complete in guinea pig plantar horny layer than in the back, and stainable cytoplasm was retained in keratinized cells but organelles were lost except for some degraded ultrastructural remnants. By light microscopy the whole thickness of the horny layer showed bound phospholipid and bound cysteine, and there was a weak cystine reaction at the peripheries of the keratinized cells. In ultrastructure the keratohyalin contained slightly larger subparticles than in the back skin. The horny layer was not divisible into basal, intermediate and superficial regions as in hairy skin. The stratum lucidum of light microscopy was not defined in electron micrographs. Osmium-stained cytoplasmic material was retained in horny cells about to be desquamated, in contrast to the empty appearance of these cells in hairy skin. Epidermal cells in plantar skin have ultrastructural cytoplasmic processes which are longer than they are broad. In the horny layer these interdigitate with those of neighbouring cells and are held together by lateral demonsomal junctions. Probably this gives mechanical strength against shearing forces experienced by the plantar horny layer.     

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.     

Protein and carbohydrate histochemistry in relation to the keratinization in the epidermis of Barbus sophor (Cyprinidae, Pisces)

The distribution of protein and carbohydrate constituents in the epidermis of Barbus sophor is described in order to give a better understanding of its cellular organization and physiology.
Various cytochemical techniques show the keratinized nature of the outer free margins of the polygonal cells in the most-superficial layer. These contain appreciable amounts of cysteine bound sulphydryl groups, basic proteins, protein bound NH₂ groups, ribonucleic acid and calcium and give a strong Papanicolaou's reaction. Absence of cystine bound disulphide groups suggests that the cornified layer in B. sophor is probably mechanically weak as adjacent keratin chains remain unbonded. The polygonal cells showing keratinization at the outer free margins remain metabolically active and are not sloughed off at the surface. This is in contrast to the keratinized epidermis of other teleosts so far reported in which the keratinized cells are dead and are sloughed off at the surface.
In addition to keratinization the polygonal cells undergo mucogenesis synthesizing sulphated acid mucopolysaccharides.
The presence of eosinophilic granular cells in the epidermis is interesting. The possible role of these cells in the protection of the epidermis has been discussed. The epidermis on the inner surface of the scale is very thin so it may not have much protective significance in these areas.     

Distribution and characterization of proteins associated with cornification in the epidermis of gecko lizard

The distribution and molecular weight of epidermal proteins of gecko lizards have been studied by ultrastructural, autoradiographic, and immunological methods. Setae of the climbing digital pads are cross-reactive to antibodies directed against a chick scutate scale beta-keratin but not against feather beta-keratin. Cross-reactivity for mammalian loricrin, sciellin, filaggrin, and transglutaminase are present in alpha-keratogenic layers of gecko epidermis. Alpha-keratins have a molecular weight in the range 40-58 kDa. Loricrin cross-reactive bands have molecular weights of 42, 50, and 58 kDa. Bands for filaggrin-like protein are found at 35 and 42 kDa, bands for sciellin are found at 40-45 and 50-55 kDa, and bands for transglutaminase are seen at 48-50 and 60 kDa. The specific role of these proteins remains to be elucidated. After injection of tritiated histidine, the tracer is incorporated into keratin and in setae. Tritiated proline labels the developing setae of the oberhautchen and beta layers, and proline-labeled proteins (beta-keratins) of 10-14, 16-18, 22-24 and 32-35 kDa are extracted from the epidermis. In whole epidermal extract (that includes the epidermis with corneous layer and the setae of digital pads), beta-keratins of low-molecular weight (10, 14-16, and 18-19 kDa) are prevalent over those at higher molecular weight (34 and 38 kDa). In contrast, in shed epidermis of body scales (made of corneous layer only while setae were not collected), higher molecular weight beta-keratins are present (25-27 and 30-34 kDa). This suggests that a proportion of the small beta-keratins present in the epidermis of geckos derive from the differentiating beta layer of scales and from the setae of digital pads. Neither small nor large beta-keratins of gecko epidermis cross-react with an antibody specifically directed against the feather beta-keratin of 10-12 kDa. This result shows that the 10 and 14-16 kDa beta-keratins of gecko (lepidosaurian) have a different composition than the 10-12 kDa beta-keratin of feather (archosaurian). It is suggested that the smaller beta-keratins in both lineages of sauropsids were selected during evolution in order to build elongated bundles of keratin filaments to make elongated cells. Larger beta-keratins in reptilian scales produce keratin aggregations with no orientation, used for mechanical protection.     

Cornification in reptilian epidermis occurs through the deposition of keratin‐associated beta‐proteins (beta‐keratins) onto a scaffold of intermediate filament keratins

The isolation of genes for alpha‐keratins and keratin‐associated beta‐proteins (formerly beta‐keratins) has allowed the production of epitope‐specific antibodies for localizing these proteins during the process of cornification epidermis of reptilian sauropsids. The antibodies are directed toward proteins in the alpha‐keratin range (40–70 kDa) or beta‐protein range (10–30 kDa) of most reptilian sauropsids. The ultrastructural immunogold study shows the localization of acidic alpha‐proteins in suprabasal and precorneous epidermal layers in lizard, snake, tuatara, crocodile, and turtle while keratin‐associated beta‐proteins are localized in precorneous and corneous layers. This late activation of the synthesis of keratin‐associated beta‐proteins is typical for keratin‐associated and corneous proteins in mammalian epidermis (involucrin, filaggrin, loricrin) or hair (tyrosine‐rich or sulfur‐rich proteins). In turtles and crocodilians epidermis, keratin‐associated beta‐proteins are synthesized in upper spinosus and precorneous layers and accumulate in the corneous layer. The complex stratification of lepidosaurian epidermis derives from the deposition of specific glycine‐rich versus cysteine‐glycine‐rich keratin‐associated beta‐proteins in cells sequentially produced from the basal layer and not from the alternation of beta‐ with alpha‐keratins. The process gives rise to Oberhäutchen, beta‐, mesos‐, and alpha‐layers during the shedding cycle of lizards and snakes. Differently from fish, amphibian, and mammalian keratin‐associated proteins (KAPs) of the epidermis, the keratin‐associated beta‐proteins of sauropsids are capable to form filaments of 3–4 nm which give rise to an X‐ray beta‐pattern as a consequence of the presence of a beta‐pleated central region of high homology, which seems to be absent in KAPs of the other vertebrates. J. Morphol., 2013. © 2012 Wiley Periodicals, Inc.     

Comparative immunolocalization of keratin‐associated beta‐proteins (beta‐keratins) supports a new explanation for the cyclical process of keratinocyte differentiation in lizard epidermis

Lizard epidermis is made of beta‐ and alpha‐layers. Using Western blot tested antibodies, the ultrastructural immunolocalization of specific keratin‐associated beta‐proteins in the epidermis of different lizard species reveals that glycine‐rich beta‐proteins (HgG5) localize in the beta‐layer, while glycine–cysteine‐medium‐rich beta‐proteins (HgGC10) are present in oberhautchen and alpha‐layers. This suggests a new explanation for the formation of different epidermal layers during the shedding cycle in lepidosaurian epidermis instead of an alternance between beta‐keratins and alpha‐keratins. It is proposed that different sets of genes coding for specific beta‐proteins are activated in keratinocytes during the renewal phase of the shedding cycle. Initially, glycine–cysteine‐medium‐rich beta‐proteins with hydrophilic and elastic properties accumulate over alpha‐keratins in the oberhautchen but are replaced in the next cell layer with glycine‐rich hydrophobic beta‐proteins forming a resistant, stiff, and hydrophobic beta‐layer. The synthesis of glycine‐rich proteins terminates in mesos and alpha‐cells where these proteins are replaced with glycine–cysteine‐rich beta‐proteins. The pattern of beta‐protein deposition onto a scaffold of intermediate filament keratins is typical for keratin‐associated proteins and the association between alpha‐keratins and specific keratin‐associated beta‐proteins during the renewal phase of the shedding cycle gives rise to epidermal layers possessing different structural, mechanical, and texture properties.     

Origin of feathers: Feather beta (beta) keratins are expressed in discrete epidermal cell populations of embryonic scutate scales

The feathers of birds develop from embryonic epidermal lineages that differentiate during outgrowth of the feather germ. Independent cell populations also form an embryonic epidermis on scutate scales, which consists of peridermal layers, a subperiderm, and an alpha stratum. Using an antiserum (anti-FbetaK) developed to react specifically with the beta (beta) keratins of feathers, we find that the feather-type beta keratins are expressed in the subperiderm cells of embryonic scutate scales, as well as the barb ridge lineages of the feather. However, unlike the subperiderm of scales, which is lost at hatching, the cells of barb ridges, in conjunction with adjacent cell populations, give rise to the structural elements of the feather. The observation that an embryonic epidermis, consisting of peridermal and subperidermal layers, also characterizes alligator scales (Thompson, 2001. J Anat 198:265-282) suggests that the epidermal populations of the scales and feathers of avian embryos are homologous with those forming the embryonic epidermis of alligators. While the embryonic epidermal populations of archosaurian scales are discarded at hatching, those of the feather germ differentiate into the periderm, sheath, barb ridges, axial plates, barbules, and marginal plates of the embryonic feather filament. We propose that the development of the embryonic feather filament provides a model for the evolution of the first protofeather. Furthermore, we hypothesize that invagination of the epidermal lineages of the feather filament, namely the barb ridges, initiated the formation of the follicle, which then allowed continuous renewal of the feather epidermal lineages, and the evolution of diverse feather forms.     

Avian skin development and the evolutionary origin of feathers

The discovery of several dinosaurs with filamentous integumentary appendages of different morphologies has stimulated models for the evolutionary origin of feathers. In order to understand these models, knowledge of the development of the avian integument must be put into an evolutionary context. Thus, we present a review of avian scale and feather development, which summarizes the morphogenetic events involved, as well as the expression of the beta (beta) keratin multigene family that characterizes the epidermal appendages of reptiles and birds. First we review information on the evolution of the ectodermal epidermis and its beta (beta) keratins. Then we examine the morphogenesis of scutate scales and feathers including studies in which the extraembryonic ectoderm of the chorion is used to examine dermal induction. We also present studies on the scaleless (sc) mutant, and, because of the recent discovery of "four-winged" dinosaurs, we review earlier studies of a chicken strain, Silkie, that expresses ptilopody (pti), "feathered feet." We conclude that the ability of the ectodermal epidermis to generate discrete cell populations capable of forming functional structural elements consisting of specific members of the beta keratin multigene family was a plesiomorphic feature of the archosaurian ancestor of crocodilians and birds. Evidence suggests that the discrete epidermal lineages that make up the embryonic feather filament of extant birds are homologous with similar embryonic lineages of the developing scutate scales of birds and the scales of alligators. We believe that the early expression of conserved signaling modules in the embryonic skin of the avian ancestor led to the early morphogenesis of the embryonic feather filament, with its periderm, sheath, and barb ridge lineages forming the first protofeather. Invagination of the epidermis of the protofeather led to formation of the follicle providing for feather renewal and diversification. The observations that scale formation in birds involves an inhibition of feather formation coupled with observations on the feathered feet of the scaleless (High-line) and Silkie strains support the view that the ancestor of modern birds may have had feathered hind limbs similar to those recently discovered in nonavian dromaeosaurids. And finally, our recent observation on the bristles of the wild turkey beard raises the possibility that similar integumentary appendages may have adorned nonavian dinosaurs, and thus all filamentous integumentary appendages may not be homologous to modern feathers.     

Immunolocalization of keratin‐associated beta‐proteins in developing epidermis of lizard suggests that adhesive setae contain glycine–cysteine‐rich proteins

The localization of specific keratin‐associated beta‐proteins (formerly referred to as beta‐keratins) in the embryonic epidermis of lizards is not known. Two specific keratin‐associated beta‐proteins of the epidermis, one representing the glycine‐rich subfamily (HgG5) and the other the glycine‐cysteine medium‐rich subfamily (HgGC10), have been immunolocalized at the ultrastructural level in the lizard Anolis lineatopus. The periderm and granulated subperiderm are most immunonegative for these proteins. HgG5 is low to absent in theOberhäutchen layer while is present in the forming beta‐layer, and disappears in mesos‐ and alpha‐layers. Instead, HgGC10 is present in the Oberhäutchen, beta‐, and also in the following alpha‐layers, and specifically accumulates in the developing adhesive setae but not in the surrounding cells of the clear layer. Therefore, setae and their terminal spatulae that adhere to surfaces allowing these lizards to walk vertically contain cysteine–glycine rich proteins. The study suggests that, like in adult and regenerating epidermis, the HgGC10 protein is not only accumulated in cells of the beta‐layer but also in those forming the alpha‐layer. This small protein therefore is implicated in resistance, flexibility, and stretching of the epidermal layers. It is also hypothesized that the charges of these proteins may influence adhesion of the setae of pad lamellae. Conversely, glycine‐rich beta‐proteins like HgG5 give rise to the dense, hydrophobic, and chromophobic corneous material of the resistant beta‐layer. This result suggests that the differential accumulation of keratin‐associated beta‐proteins over the alpha‐keratin network determines differences in properties of the stratified layers of the epidermis of lizards. J. Morphol. 2013. © 2012 Wiley Periodicals, Inc.     

Low‐cysteine alpha‐keratins and corneous beta‐proteins are initially formed in the regenerating tail epidermis of lizard

During tail regeneration in lizards, the stratified regenerating epidermis progressively gives rise to neogenic scales that form a new epidermal generation. Initially, a soft, un‐scaled, pliable, and extensible epidermis is formed that is progressively replaced by a resistant but non‐extensible scaled epidermis. This suggests that the initial corneous proteins are later replaced with harder corneous proteins. Using PCR and immunocytochemistry, the present study shows an upregulation in the synthesis of low‐cysteine type I and II alpha‐keratins and of corneous beta‐proteins with a medium cysteine content and a low content in glycine (formerly termed beta‐keratins) produced at the beginning of epidermal regeneration. Quantitative PCR indicates upregulation in the production of alpha‐keratin mRNAs, particularly of type I, between normal and the thicker regenerating epidermis. PCR‐data also indicate a higher upregulation for cysteine‐rich corneous beta‐proteins and a high but less intense upregulation of low glycine corneous protein mRNAs at the beginning of scale regeneration. Immunolabeling confirms the localization of these proteins, and in particular of beta‐proteins with a medium content in cysteine initially formed in the wound epidermis and later in the differentiating corneous layers of regenerating scales. It is concluded that the wound epidermis initially contains alpha‐keratins and corneous beta‐proteins with a lower cysteine content than more specialized beta‐proteins later formed in the mature scales. These initial corneous proteins are likely related to the pliability of the wound epidermis while more specialized alpha‐keratins and beta‐proteins richer in glycine and cysteine are synthesized later in the mature and inflexible scales. J. Morphol. 278:119–130, 2017. ©© 2016 Wiley Periodicals,Inc.     

Adaptation to the land: The skin of reptiles in comparison to that of amphibians and endotherm amniotes

The adaptation to land from amphibians to amniotes was accompanied by drastic changes of the integument, some of which might be reconstructed by studying the formation of the stratum corneum during embryogenesis. As the first amniotes were reptiles, the present review focuses on past and recent information on the evolution of reptilian epidermis and the stratum corneum. We aim to generalize the discussion on the evolution of the skin in amniotes. Corneous cell envelopes were absent in fish, and first appeared in adult amphibian epidermis. Stem reptiles evolved a multilayered stratum corneum based on a programmed cell death, intensified the production of matrix proteins (e.g., HRPs), corneous cell envelope proteins (e.g., loricrine-like, sciellin-like, and transglutaminase), and complex lipids to limit water loss. Other proteins were later produced in association to the soft or hairy epidermis in therapsids (e.g., involucrin, profilaggrin-filaggrin, trichohyalin, trichocytic keratins), or to the hard keratin of hairs, quills, horns, claws (e.g., tyrosine-rich, glycine-rich, sulphur-rich matrix proteins). In sauropsids special proteins associated to hard keratinization in scales (e.g., scale beta-keratins, cytokeratin associated proteins) or feathers (feather beta-keratins and HRPs) were originated. The temporal deposition of beta-keratin in lepidosaurian reptiles originated a vertical stratified epidermis and an intraepidermal shedding layer. The evolutions of the horny layer in Therapsids (mammals) and Saurospids (reptiles and birds) are discussed. The study of the molecules involved in the dermo-epidermal interactions in reptilian skin and the molecular biology of epidermal proteins are among the most urgent future areas of research in the biology of reptilian skin.     

Immunocytochemistry indicates that glycine‐rich beta‐proteins are present in the beta‐layer,while cysteine‐rich beta‐proteins are present in beta‐ and alpha‐layers of snake epidermis

Immunolocalization of glycine‐rich and cysteine–glycine‐medium‐rich beta‐proteins (Beta‐keratins) in snake epidermis indicates a different distribution between beta‐ and alpha‐layers. Acta Zoologica, Stockholm. The epidermis of snakes consists of hard beta‐keratin layers alternated with softer and pliable alpha‐keratin layers. Using Western blot, light and ultrastructural immunolocalization, we have analyzed the distribution of two specific beta‐proteins (formerly beta‐keratins) in the epidermis of snakes. The study indicates that the antibody HgG5, recognizing glycine‐rich beta‐proteins of 12–15 kDa, is poorly or not reactive with the beta‐layer of snake epidermis. This suggests that glycine‐rich proteins similar to those present in lizards are altered during maturation of the beta‐layer. Conversely, a glycine–cysteine‐medium‐rich beta‐protein (HgGC10) of 10–12 kDa is present in beta‐ and alpha‐layers, but it is reduced or disappears in precorneous and suprabasal cells destined to give rise to beta‐ and alpha‐cells. Together with the previous studies on reptilian epidermis, the present results suggest that beta‐proteins rich in glycine mainly accumulate on a scaffold of alpha‐keratin producing a resistant and hydrophobic beta‐layer. Conversely, beta‐proteins lower in glycine but higher in cysteine accumulate on alpha‐keratin filaments present in beta‐ and alpha‐layers producing resistant but more pliable layers.     

Immunolocalization of keratin‐associated beta‐proteins (beta‐keratins) in the regenerating lizard epidermis indicates a new process for the differentiation of the epidermis in lepidosaurians

The process of keratinocyte differentiation was analyzed in the regenerating epidermis of the lizard Anolis carolinensis, where the genes coding for beta‐proteins (beta‐keratins) are known. The regenerating epidermis forms all epidermal layers found in normal scales (Oberhäutchen‐, beta‐, mesos‐, and alpha‐layer). Three specific proteins representing the larger families of beta‐proteins, glycine‐rich (HgG5, 28% glycine, 3.6% cysteine), glycine‐cysteine medium‐rich (HgGC10, 13% glycine, 14.5% cysteine), and glycine‐cysteine rich (HgGC3, 30.4% glycine, 8.7% cysteine) have been immunolocalized at the ultrastructural level. HgG5 is only present in differentiating beta‐cells, a weak or no labeling is observed in Oberhäutchen and is absent in alpha‐cells. The protein is located in the pale corneous material forming the compact beta‐layer but is absent in mature Oberhäutchen cells. HgGC10 is present among beta‐packets in Oberhäutchen and beta‐cells but disappears in more compact and electron‐pale corneous material. The labeling disappears in mesos‐cells and is present with variable intensity in alpha‐cells, whereas lacunar and clear‐cells are low labeled to unlabeled. HgGC3 is sparse or absent in beta‐cells but is lightly present in the darker corneous material of differentiating and mature alpha‐cells, lacunar‐cells, and clear‐cells. The study suggests that while glycine‐rich proteins (electron‐pale) are specifically used for building the resistant and hydrophobic beta‐layer, cysteine–glycine rich proteins (electron‐denser) are used to form the pliable corneous material present in the Oberhäutchen and alpha‐cells. The differential accumulation of beta‐proteins on the alpha‐keratin cytoskeleton scaffold and not the alternance of beta‐ with alpha‐keratins allow the differentiation of different epidermal layers. © 2012 Wiley Periodicals, Inc.     

Ultrastructure of the epidermis of the lizard (Lacerta vivipara) at the resting stage of the sloughing cycle

The ultrastructure of the epidermis of the lizard ( Lacerta vivipara ) one day after sloughing is described. The non-keratinized layers of the epidermis are essentially similar in structure to those of amphibians and mammals. The cells of the basal layer are not however separated from each other by the large spaces described in the amphibian (Farquhar & Palade, 1965). The middle layers of the epidermis at this stage of the sloughing cycle produce neither the characteristic mucous granules found in amphibians nor the keratohyalin granules of mammals. A small number of granules corresponding in size and location to the "Odland bodies" of both mammalian and amphibian epidermis are, however, present. The intermediate layer cells also contain a number of bodies similar in appearance to those described by Farquhar & Palade as lysosomes in amphibian skin. These structures are both osmium iodide and acid phosphatase positive. Unlike the condition in amphibians and mammals, the cytoplasm of cells in the layer immediately beneath the keratinized strata is honeycombed with small vesicles, and contains large irregular vacuoles of uncertain content. Certain nonkeratinizing elements within the epidermis are tentatively interpreted as nerve terminations. Two morphologically distinct keratinized strata can be distinguished, the inner stratum consisting of flattened cells similar to those of the stratum corneum of mammalian epidermis; individual cell outlines cannot be distinguished in the outer stratum, which has a structure similar to that of avian feather keratin. A shallow surface zone of the outer keratinized stratum has been identified as the Oberhautchen. This consists of longitudinally disposed leaflets or laminae which are responsible for the sculptured pattern of the epidermal surface. The observations reported here provide a basis for analysis of changes occurring at other stages of the sloughing cycle.     

Immunolocalization of specific beta‐proteins in pad lamellae of the digits in the lizard Anolis carolinensis suggests that cysteine‐rich beta‐proteins provides flexibility

Knowledge of beta‐protein (beta‐keratin) sequences in Anolis carolinensis facilitates the localization of specific sites in the skin of this lizard. The epidermal distribution of two new beta‐proteins (beta‐keratins), HgGC8 and HgG13, has been analyzed by Western blotting, light and ultrastructural immunocytochemistry. HgGC8 includes 16 kDa members of the glycine‐cysteine medium‐rich subfamily and is mainly expressed in the beta‐layer of adhesive setae but not in the setae. HgGC8 is absent in other epidermal layers of the setae and is weakly expressed in the beta‐layer of other scales. HgG13 comprises members of 17‐kDa glycine‐rich proteins and is absent in the setae, diffusely distributed in the beta layer of digital scales and barely present in the beta‐layer of other scales. It appears that the specialized glycine‐cysteine medium rich beta‐proteins such as HgGC8 in the beta‐layer, and of HgGC10 and HgGC3 in both alpha‐ and beta‐layers, are key proteins in the formation of the flexible epidermal layers involved in the function of these modified scales in adaptation to contact and adhesion on surfaces. J. Morphol. 275:504–513, 2014. © 2013 Wiley Periodicals, Inc.     

Immunocytochemical localization of sulfhydryl oxidase in mammalian epidermis suggests that the enzyme cross‐links keratins in the granular and transitional corneous layers

The epidermis of representative mammalian species including humans has been examined for the presence of sulfhydryl oxidase, an enzyme likely involved in the oxidation of corneous proteins containing sulfhydryl groups in the epidermis. A database search indicates that the enzyme shares common sequences in numerous mammalian species so that an antibody against the human sulfhydryl oxidase 2 has been utilized on other species. The immunofluorescent study on the epidermis of the platypus (monotreme), red kangaroo (marsupials), hamster and human (placentals) reveals a prevalent labelling in the granular, transitional and lowermost part of the stratum corneous layer. The detailed ultrastructural immunogold study of the human epidermis reveals a diffuse and uneven labelling in the paler component of the composite keratohyalin granules or among keratin filaments of the transitional layer while the labelling disappears in the corneous layer. The study supports the hypothesis of the participation of the enzyme in the oxidative process that determines the formation of stable disulphide groups among keratins and other corneous proteins of the stratum corneum. This process gives rise to the resistant cell corneous envelope of keratinocytes in addition to the isopeptide bonds that derive from the catalytic action of epidermal transglutaminase on several corneous proteins.     

How and when the regional competence of chick epidermis is established: feathers vs. scutate and reticulate scales, a problem en route to a solution

Most of the chick body is covered with feathers, while the tarsometatarsus and the dorsal face of the digits form oblong overlapping scales (scuta) and the plantar face rounded nonoverlapping scales (reticula). Feathers and scuta are made of beta-keratins, while the epidermis of reticula and inter-appendage or apteria (nude regions) express a-keratins. These regional characteristics are determined in skin precursors and require an epidermal FGF-like signal to be expressed. Both the initiation of appendages, their outline and pattern depend on signals from the dermis, while their asymmetry and outgrowth depend on epidermal competence. For example, the plantar dermis of the central foot pad induces reticula in a plantar or feathers in an apteric epidermis, in a hexagonal pattern starting from the medial point. By manipulating Shh levels in the epidermis, the regional appendage type can be changed from scuta or reticula to feather, whereas the inhibition of Wnt7a, together with a downregulation of Shh gives rise to reticula and in extreme cases, apteria. During morphogenesis of plantar skin, the epidermal expression of En-1, acting as a repressor both of Wnt7a and Shh, is linked to the formation of reticula. Finally, in birds, the complex formation of feathers, which can be easily triggered, even in the extra-embryonic somatopleure, may result from a basic genetic program, whereas the simple formation of scales appears secondarily derived, as requiring a partial (scuta) or total (reticula) inhibition of epidermal outgrowth and beta-keratin gene expression, an inhibition lost for the scuta in the case of feathered feet breeds.     

The proteins of the keratin component of bird's beaks.

Birds' beaks have an outer shell of hard keratin which consists almost entirely of proteins which are very rich in glycine [about 30 residues per 100 residues (residues %)], contain moderate levels of tyrosine and serine (each about 8 residues %), and which have relatively low contents of cystine (about 2-5 residues %), lysine, histidine, isoleucine and methionine. Major protein fractions in the S-carboxymethyl form isolated from the beaks of six different orders of birds have similar amino acid compositions, isoelectric points (pH 4-2-4-9) and molecular weights (13,000-14,500). Detailed chromatographic electrophoretic and compositional studies of the proteins of kookaburra beak reveal them to be a family of closely related proteins with only limited heterogeneity, in contrast to mammalian keratin systems. The major kookaburra beak fraction is similar in overall composition and molecular weight to fowl epidermal scale, kookaburra claw and turtle scute proteins and shows some resemblance to reptile claw protein. Beaks also contain small amounts of protein which are distinctly different from the major fraction but which resemble feather keratin proteins in composition and size.     

Identification, expression, and localization of β keratin gene products during development of avian scutate scales

Epidermal-dermal interactions influence morphogenesis and expression of the beta keratin gene family during development of scales in the embryonic chick. The underlying mechanisms by which these interactions control beta keratin expression are not understood. However, the present study of beta keratin gene expression during avian epidermal differentiation contributes new information with which to investigate the role of tissue interactions in this process. Using beta keratin-specific synthetic oligonucleotide probe, beta keratin mRNA was hybrid-selected from total poly A+ RNA of scutate scales. Seven beta keratin polypeptides were translated in vitro and could be identified by their positions in two-dimensional gels among the detergent-insoluble extracts of scutate scale epidermis. In vivo phosphorylation studies suggested that an additional three beta keratin polypeptides were present as phosphoproteins. The temporal appearance of beta keratin mRNA and the corresponding polypeptides was followed during scutate scale development. Polyclonal antiserum made against two of the beta keratin polypeptides was used for immunohistochemical and immunogold electron-microscopic analysis of beta keratin tissue distribution. Immunological reactivity was observed specifically along the outer scale surface in epidermal cells above the stratum germinativum. Immunogold beads were localized on 3-nm filament bundles. In situ hybridization with a beta keratin-specific RNA probe demonstrated that mRNA accumulated in the same regional manner as the polypeptides. This selective expression of beta keratin genes in specific regions of the developing scutate scale suggests that epidermal-dermal interactions provide not only for morphological events, but also for control of complex patterns of histogenesis and biochemical differentiation.