Observations on the histochemistry and ultrastructure of regenerating caudal epidermis of the tuatara Sphenodon punctatus (Sphenodontida, Lepidosauria, Reptilia)

Regeneration in Reptiles Generally and the New Zealand Tuatara in Particular as a Model to Analyse Organ Regrowth in Amniotes: A Review

The ability to repair injuries among reptiles, i.e., ectothermic amniotes, is similar to that of mammals with some noteworthy exceptions. While large wounds in turtles and crocodilians are repaired through scarring, the reparative capacity involving the tail derives from a combined process of wound healing and somatic growth, the latter being continuous in reptiles. When the tail is injured in juvenile crocodilians, turtles and tortoises as well as the tuatara (Rhynchocephalia: Sphenodon punctatus, Gray 1842), the wound is repaired in these reptiles and some muscle and connective tissue and large amounts of cartilage are regenerated during normal growth. This process, here indicated as “regengrow”, can take years to produce tails with similar lengths of the originals and results in only apparently regenerated replacements. These new tails contain a cartilaginous axis and very small (turtle and crocodilians) to substantial (e.g., in tuatara) muscle mass, while most of the tail is formed by an irregular dense connective tissue containing numerous fat cells and sparse nerves. Tail regengrow in the tuatara is a long process that initially resembles that of lizards (the latter being part of the sister group Squamata within the Lepidosauria) with the formation of an axial ependymal tube isolated within a cartilaginous cylinder and surrounded by an irregular fat-rich connective tissue, some muscle bundles, and neogenic scales. Cell proliferation is active in the apical regenerative blastema, but much reduced cell proliferation continues in older regenerated tails, where it occurs mostly in the axial cartilage and scale epidermis of the new tail, but less commonly in the regenerated spinal cord, muscles, and connective tissues. The higher tissue regeneration of Sphenodon and other lepidosaurians provides useful information for attempts to improve organ regeneration in endothermic amniotes.

Autotomy and Regeneration in Squamate Reptiles (Squamata, Reptilia): Defensive Behavior Strategies and Morphological Characteristics (Using Computer Microtomography Methods)

Abstract

It has been noted that caudal autotomy as a way of defending against predators in recent reptiles is characteristic solely of lepidosaurs and is absent in crocodiles and turtles. It was found that, in the order Rhynchocephalia and in representatives of the majority of families of lizards, intravertebral (IntraVB) autotomy is a widespread phenomenon, whereas agamid lizards and some snakes do not have a break plane, and their tails break between adjacent vertebrae (intervertebral (InterVB) autotomy). The frequencies of occurrence of InterVB autotomy and regeneration in six species of agamas of the genus Paralaudakia were analyzed. Six types of regenerate’s characteristic of the studied group and the anatomical structure of the knob -shaped jagged regenerate are described on the basis of the results of computed microtomography (micro-CT). Phenomena of autotomy and regeneration are discussed in the phylogenetic context.

Tail regeneration in Lepidosauria as an exception to the generalized lack of organ regeneration in amniotes

The present review hypothesizes that during the transition from water to land, amniotes lost part of the genetic program for metamorphosis utilized in larvae of their amphibian ancestors, a program that in extant fish and amphibians allows organ regeneration. The direct development of amniotes, with their growth from embryos to adults, occurred with the elimination of larval stages, increases the efficiency of immune responses and the complexity of nervous circuits. In amniotes, T-cells and macrophages likely eliminate embryonic-larval antigens that are replaced with the definitive antigens of adult organs. Among lepidosaurians numerous lizard families during the Permian and Triassic evolved the process of tail autotomy to escape predation, followed by tail regeneration. Autotomy limits inflammation allowing the formation of a regenerative blastema rich in the immunosuppressant and hygroscopic hyaluronic acid. Expression loss of developmental genes for metamorphosis and segmentation in addition to an effective immune system, determined an imperfect regeneration of the tail. Genes involved in somitogenesis were likely lost or are inactivated and the axial skeleton and muscles of the original tail are replaced with a nonsegmented cartilaginous tube and segmental myotomes. Lack of neural genes, negative influence of immune system, and isolation of the regenerating spinal cord within the cartilaginous tube impede the production of nerve and glial cells, and a stratified spinal cord with ganglia. Tissue and organ regeneration in other body regions of lizards and other reptiles is relatively limited, like in the other amniotes, although the cartilage shows a higher regenerative capability than in mammals.

Gecko Adhesion in Space and Time: A Phylogenetic Perspective on the Scansorial Success Story

An evolutionary perspective on gecko adhesion was previously hampered by a lack of an explicit phylogeny for the group and of robust comparative methods to study trait evolution, an underappreciation for the taxonomic and structural diversity of geckos, and a dearth of fossil evidence bearing directly on the origin of the scansorial apparatus. With a multigene dataset as the basis for a comprehensive gekkotan phylogeny, model-based methods have recently been employed to estimate the number of unique derivations of the adhesive system and its role in lineage diversification. Evidence points to a single basal origin of the spinulate oberhautchen layer of the epidermis, which is a necessary precursor for the subsequent elaboration of a functional adhesive mechanism in geckos. However, multiple gains and losses are implicated for the elaborated setae that are necessary for adhesion via van der Waals forces. The well-supported phylogeny of gekkotans has demonstrated that convergence and parallelism in digital design are even more prevalent than previously believed. It also permits the reexamination of previously collected morphological data in an explicitly evolutionary context. Both time-calibrated trees and recently discovered amber fossils that preserve gecko toepads suggest that a fully-functional adhesive apparatus was not only present, but also represented by diverse architectures, by the mid-Cretaceous. Further characterization and phylogenetically-informed analyses of the other components of the adhesive system (muscles, tendons, blood sinuses, etc.) will permit a more comprehensive reconstruction of the evolutionary pathway(s) by which geckos have achieved their structural and taxonomic diversity. A phylogenetic perspective can meaningfully inform functional and performance studies of gecko adhesion and locomotion and can contribute to advances in bioinspired materials.

Scar-free cutaneous wound healing in the leopard gecko, Eublepharis macularius

Cutaneous wounds heal with two possible outcomes: scarification or near-perfect integumentary restoration. Whereas scar formation has been intensively investigated, less is known about the tissue-level events characterising wounds that spontaneously heal scar-free, particularly in non-foetal amniotes. Here, a spatiotemporal investigation of scar-free cutaneous wound healing following full-thickness excisional biopsies to the tail and body of leopard geckos (Eublepharis macularius) is provided. All injuries healed without scarring. Cutaneous repair involves the development of a cell-rich aggregate within the wound bed, similar to scarring wounds. Unlike scar formation, scar-free healing involves a more rapid closure of the wound epithelium, and a delay in blood vessel development and collagen deposition within the wound bed. It was found that, while granulation tissue of scarring wounds is hypervascular, scar-free wound healing conspicuously does not involve a period of exuberant blood vessel formation. In addition, during scar-free wound healing the newly formed blood vessels are typically perivascular cell-supported. Immunohistochemistry revealed widespread expression of both the pro-angiogenic factor vascular endothelial growth factor A and the anti-angiogenic factor thrombospondin-1 within the healing wound. It was found that scar-free wound healing is an intrinsic property of leopard gecko integument, and involves a modulation of the cutaneous scar repair program. This proportional revascularisation is an important factor in scar-free wound healing.

Repeated origin and loss of adhesive toepads in geckos

Geckos are well known for their extraordinary clinging abilities and many species easily scale vertical or even inverted surfaces. This ability is enabled by a complex digital adhesive mechanism (adhesive toepads) that employs van der Waals based adhesion, augmented by frictional forces. Numerous morphological traits and behaviors have evolved to facilitate deployment of the adhesive mechanism, maximize adhesive force and enable release from the substrate. The complex digital morphologies that result allow geckos to interact with their environment in a novel fashion quite differently from most other lizards. Details of toepad morphology suggest multiple gains and losses of the adhesive mechanism, but lack of a comprehensive phylogeny has hindered efforts to determine how frequently adhesive toepads have been gained and lost. Here we present a multigene phylogeny of geckos, including 107 of 118 recognized genera, and determine that adhesive toepads have been gained and lost multiple times, and remarkably, with approximately equal frequency. The most likely hypothesis suggests that adhesive toepads evolved 11 times and were lost nine times. The overall external morphology of the toepad is strikingly similar in many lineages in which it is independently derived, but lineage-specific differences are evident, particularly regarding internal anatomy, with unique morphological patterns defining each independent derivation.

First evidence of unicellular glands in the general epidermis of terrestrial reptiles

The general integument of reptiles is traditionally defined as being dry, but we report here the discovery of unicellular mucoid glands (UCMG) in the dorsal skin of lizards of the genus Phelsuma (Gekkonidae). To this end, the skin of these lizards and of some others for comparison was studied by scanning electron microscopy and transmission electron microscopy. These photographs showed that the development and function of the UCMGs are related to the skin's sloughing cycle. The UCMGs differentiate at scattered locations from Oberhäutchen cells of the inner (new) epidermal generation, above the differentiating beta-keratin layer. While the inner generation matures, the UCMG increases in size; unlike the surrounding Oberhäutchen cells, it does not develop the spinules that characterize gecko skin. When, upon sloughing, the inner generation becomes the new outer generation, and the Oberhäutchen forms the skin surface, the UCMGs, several per scale, dot the surface as mucus-inflated "blebs" projecting from the surrounding spinulate Oberhäutchen, each nesting in a shallow pit of the underlying beta-keratin. On the surface, the UCMGs rupture and the mucus appears to dissipate in cords, flowing over the tips of the spinules, and incorporating minute foreign bodies. It is concluded that, due to the low wettability of the spinulate surface (derived from the spacing of the spinules), the cords brush off easily, with the mucus functioning as a cleaning agent.

Epidermal differentiation in embryos of the tuatara Sphenodon punctatus (Reptilia, Sphenodontidae) in comparison with the epidermis of other reptiles

Studying the epidermis in primitive reptiles can provide clues regarding evolution of the epidermis during land adaptation in vertebrates. With this aim, the development of the skin of the relatively primitive reptile Sphenodon punctatus in representative embryonic stages was studied by light and electron microscopy and compared with that of other reptiles previously studied. The dermis organizes into a superficial and deep portion when the epidermis starts to form the first layers. At embryonic stages comparable with those of lizards, only one layer of the inner periderm is formed beneath the outer periderm. This also occurs in lizards and snakes so far studied. The outer and inner periderm form the embryonic epidermis and accumulate thick, coarse filaments (25-30 nm thick) and sparse alpha-keratin filaments as in other reptiles. Beneath the embryonic epidermis an oberhautchen and beta-cells form small horny tips that represent overlapping borders along the margin of beta-cells that overlap other beta-cells (in a tile-like arrangement). The tips resemble those of agamine lizards but at a small scale, forming a lamellate-spinulated pattern as previously described in adult epidermis. The embryonic epidermis matures by the dispersion of coarse filaments among keratin at the end of embryonic development and is shed around hatching. The presence of these matrix organelles in the embryonic epidermis of this primitive reptile further indicates that amniote epidermis acquired interkeratin matrix proteins early for land adaptation. Unlike the condition in lizards and snakes, a shedding complex is not formed in the epidermis of embryonic S. punctatus that is like that of the adult. Therefore, as in chelonians and crocodilians, the epidermis of S. punctatus also represents an initial stage that preceded the evolution of the shedding complex for moulting.

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).

Water relations of tetrapod integument

The vertebrate integument represents an evolutionary compromise between the needs for mechanical protection and those of sensing the environment and regulating the exchange of materials and energy. Fibrous keratins evolved as a means of strengthening the integument while simultaneously providing a structural support for lipids, which comprise the principal barrier to cutaneous water efflux in terrestrial taxa. Whereas lipids are of fundamental importance to water barriers, the efficacy of these barriers depends in many cases on structural features that enhance or maintain the integrity of function. Amphibians are exceptional among tetrapods in having very little keratin and a thin stratum corneum. Thus, effective lipid barriers that are present in some specialized anurans living in xeric habitats are external to the epidermis, whereas lipid barriers of amniotes exist as a lipid-keratin complex within the stratum corneum. Amphibians prevent desiccation of the epidermis and underlying tissues either by evaporating water from a superficial aqueous film, which must be replenished, or by shielding the stratum corneum with superficial lipids. Water barrier function in vertebrates generally appears to be relatively fixed, although various species have`plasticity' to adjust the barrier effectiveness facultatively. While it is clear that both phenotypic plasticity and genetic adaptation can account for covariation between environment and skin resistance to water efflux, studies of the relative importance of these two phenomena are few. Fundamental mechanisms for adjusting the skin water barrier include changes in barrier thickness, composition and physicochemical properties of cutaneous lipids,and/or geometry of the barrier within the epidermis. While cutaneous lipids have been studied extensively in the contexts of disease and cosmetics,relatively little is known about the processes of permeability barrier ontogenesis related to adaptation and environment. Advances in such knowledge have didactic significance for understanding vertebrate evolution as well as practical application to clinical dermatology.

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.

Evolutionary origin of the feather epidermis

The formation of scales and feathers in reptiles and birds has fascinated biologists for decades. How might the developmental processes involved in the evolution of the amniote ectoderm be interpreted to shed light on the evolution of integumental appendages? An Evo-Devo approach to this question is proving essential to understand the observation that there is homology between the transient embryonic layers covering the scale epidermis of alligators and birds and the epidermal cell populations of embryonic feather filaments. Whereas the embryonic layers of scutate scales are sloughed off at hatching, that their homologues persist in feathers demonstrates that the predecessors of birds took advantage of the ability of their ectoderm to generate embryonic layers by recruiting them to make the epidermis of the embryonic feather filament. Furthermore, observations on mutant chickens with altered scale and feather development (Abbott and Asmundson [1957] J. Hered. 18:63-70; Abbott [1965] Poult. Sci. 44:1347; Abbott [1967] Methods in developmental biology. New York: Thomas Y. Crowell) suggest that the ectodermal placodes of feathers, which direct the formation of unique dermal condensations and subsequently appendage outgrowth, provided the mechanism by which the developmental processes generating the embryonic layers diverged during evolution to support the morphogenesis of the epidermis of the primitive feather filament with its barb ridges.

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.

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.

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.

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.

Ultrastructural autoradiographic and immunocytochemical analysis of setae formation and keratinization in the digital pads of the gecko Hemidactylus turcicus (Gekkonidae, Reptilia)

The modified subdigital scales of some lizards allow them to climb vertical surfaces. This is due to the action of millions of tiny setae present in the digital pads. Setae are mainly composed of beta-keratin which may have some modality of aggregation similar to that of barbs and barbules of feathers. Keratins and associated proteins are involved in the organization of setae. The formation of setae in the climbing pad lamellae of the gecko Hemidactylus turcicus has been analyzed under the electron microscope after injection of tritiated histidine and immunocytochemistry for a chick scale beta-keratin. Setae are made up of dense and pale filaments, both oriented along the longer axis of setae. Beta-keratin is present in the oberhautchen layer and in the growing setae which are highly modified oberhautchen cells. Most of the immunolabeling concentrated in the central part of setae. This cross-reactivity suggests that some epitopes in chick beta-keratin are also present in gecko setae. Four hours after injection of tritiated histidine, the labeling is localized over setae, in particular in the dense filaments and less in the pale filaments. Some labeling is also seen in the keratinaceous material present in the cytoplasm of clear cells, which are believed to mold setae. The present observations suggest that both beta-keratin and denser matrix proteins, possibly incorporating histidine, are packed into growing setae. These proteins may be mixed to form pale and dense filaments oriented along the longer axis of setae, a pattern resembling that of barb and barbule cells of feathers. The role of matrix material in the orientation of the deposited beta-keratin during setal outgrowth is discussed with the problem of barb and barbule differentiation in avian feathers.

Ultrastructural localization of histidine-labelled interkeratin matrix during keratinization of amphibian epidermis

Interkeratin histidine-rich proteins (filaggrins) play a functional role in aggregation of keratin filaments into dense bundles during terminal differentiation of mammalian keratinocytes when forming the dense matrix of the stratum corneum. The origin of the stratum corneum during adaptation to land in amphibious vertebrate progenitors was probably linked with the synthesis of matrix proteins. However, whether similar proteins are present in living amphibians is unknown. The possible involvement of interkeratin matrix molecules rich in basic amino acids such as histidine during keratinization of amphibian epidermis has been evaluated in the present study by ultrastructural autoradiography after administration of tritiated histidine. At 4 and 8 h post-injection, labelling was mainly localized over electron-dense amorphous material or irregular granules in between keratin filaments in cells of the upper intermedium, replacement, and immature corneous layers. Nuclear material incorporating tritiated histidine was also present in the maturing corneous layer. Small mucous-like granules did not take up tritiated histidine and X-ray microanalysis indicated that the latter granules contained sulphur. The present study suggests that small amounts of histidine-rich molecules which were not sufficient to form microscopically-visible keratohyalin granules were present in ancestral amphibian epidermis. However, this material was sufficient to promote aggregation of keratin filaments in the cytoplasm of amphibian differentiating keratinocytes, especially near the external corneous cell envelope. Electron-dense material associated with the corneous cell envelope also contained sulphur as indicated by X-ray microanalysis. It is unknown whether sulphur is derived from either sulphated mucins, or disulphide bonds in aggregated keratins, or specific sulphur-rich proteins.