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Summarizing schematic illustration of hypothetic cell dynamic present in developing scales in different sauropsids, which likely remains imprinted after hatching in growing scales. —A. During avian scutate scale morphogenesis (and possibly in crocodile scutate scales?), the placode (in black) has no cell proliferation and presents a dermal condensation (circles in A1). The placode is surrounded by interplacodes where cell division (asterisks) is instead active. The outer scale surface elongates due to cell proliferation and determines the shifting of the placode towards the tip of scales (from 1 to 3 and in dorsal view from 4 to 5). —B. In turtle and crocodilians, the interbump non-proliferating placodes (in black) may be moved into the outer scale surface by the surrounding cell proliferation in interplacodes (see B1–3, and the dorsal view from 4 to 5). An intense cell proliferation in the epidermis near the forming hinge regions gives rise to the interscale epidermis. —C. In lepidosaurians, poorly differentiated placodes but, however, determined scale primordia are initially formed (C1). It is unknown whether cell proliferation is absent in these primordial. The proliferation mainly in the forming outer scale surface determines the initial asymmetry (C2) and later the overlap of adjacent scales (C3). It is unknown whether and where the primordia are moved during scale elongation (hypothetical dorsal view representation in C4–5).
Source publication
Cell proliferation in forming shield scutes has been studied by immunofluorescence in embryos of turtle, alligator and snake after injection of 5-bromo-deoxy-uridine. Hinge regions of scutes in alligator and turtle carapace derive from an initial waving and invagination of the epidermis that contains 5-bromo-deoxy-uridine-labelled cells. This sugge...
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The skin of limbless squamates has an increased contact with the substrate compared with limbed counterparts. Comparatively, the contact with the substrate is intensified in fossorial species, where the whole circumference of the body interacts with the soil during underground locomotion. Although fossoriality in the Squamata, specifically lizards...
Citations
... After this phase, the epidermis becomes waved and forms dome-like symmetric scales that turns into scales of shield or flat type (turtles and crocodilians; Cherepanov, 1985Cherepanov, , 1992Alibardi and Thompson, 1999a,b) or variably overlapped (lizards and snakes) (Fig. 5A-F). During the formation of the dorsal (outer) surface of scales, proliferating cells are localized in the elongating epidermis destined to produce a hard corneous layer (beta-layer) in turtle, crocodilians and lepidosaurian scales (Wu et al., 2004;Alibardi and Minelli, 2016;Di-Poi and Milinkovitch, 2016, Fig. 5 E). In the developing carapace of turtles, where shield scutes are formed, an intense cell proliferation is also present in the folding epidermis of the interscale regions (Fig. 5 C). ...
General cellular aspects of skin development in vertebrates are presented with emphasis on the epidermis of sauropsids. Anamniote skin develops into a multilayered mucogenic and soft keratinized epidermis made of Intermediate Filament Keratins (IFKs) that is reinforced in most fish and few anurans by dermal bony and fibrous scales. In amniotes, the developing epidermis in contact with the amniotic fluid initially transits through a mucogenic phase recalling that of their anamniotes progenitors. A new gene cluster termed EDC (Epidermal Differentiation Complex) evolved in amniotes contributing to the origin of the stratum corneum. The EDC contains numerous genes coding for over 100 types of corneous proteins (CPs). In sauropsids 2-8 layers of embryonic epidermis accumulate soft keratins (IFKs) but do not form a compact corneous layer. The embryonic epidermis of reptiles and birds produces small amount of other, poorly known proteins in addition to IFKs and mucins. In the following development, a resistant corneous layer is formed underneath the embryonic epidermis that is shed before hatching. The definitive corneous epidermis of sauropsids is mainly composed of CBPs (Corneous beta proteins, formerly indicated as beta-keratins) derived from the EDC. CBPs belong to a gene sub-family of CPs unique for sauropsids, contain an inner amino acid region formed by beta-sheets, are rich in cysteine and glycine, and make most of the protein composition of scales, claws, beaks and feathers. In mammalian epidermis CPs missing the beta-sheet region are instead produced, and include loricrin, involucrin, filaggrin and various cornulins. Small amount of CPs accumulate in the 2-3 layers of mammalian embryonic epidermis and their appendages, that is replaced with the definitive corneous layers before birth. Differently from sauropsids, mammals utilize KAPs (keratin associated proteins) rich in cysteine and glycine for making the hard corneous material of hairs, claws, hooves, horns, and occasionally also scales.
... The scutes formed by the interaction of ectoderm and mesenchyme and by changing and expanding the surfaces of the placodes follow the ridge. Scales and tubercles develop last [50,51]. ...
The reptile skin is a barrier against water loss and pathogens and an armor for mechanical damages. The integument of reptiles consists of two main layers: the epidermis and the dermis. The epidermis, the hard cover of the body which has an armor-like role, varies among extant reptiles in terms of structural aspects such as thickness, hardness or the kinds of appendages it constitutes. The reptile epithelial cells of the epidermis (keratinocytes) are composed of two main proteins: intermediate filament keratins (IFKs) and corneous beta proteins (CBPs). The outer horny layer of the epidermis, stratum corneum, is constituted of keratinocytes by means of terminal differentiation or cornification which is a result of the protein interactions where CBPs associate with and coat the initial scaffold of IFKs. Reptiles were able to colonize the terrestrial environment due to the changes in these epidermal structures, which led to various cornified epidermal appendages such as scales and scutes, a beak, claws or setae. Developmental and structural aspects of the epidermal CBPs as well as their shared chromosomal locus (EDC) indicate an ancestral origin that gave rise to the finest armor of reptilians.
... The beta-keratin of turtle shells and the sites of cell proliferation are like those of crocodylians and different from lepidosaurs. (Dalla Valle et al., 2009;Alibardi and Minelli, 2016). The scutes and bony components of the shell are secondarily reduced in some clades of turtles, most notably the freshwater softshell turtles (Trionychidae and Carretochelyidae) and the marine leatherback turtle (Dermochelys coriacea). ...
Most of the more than 11,000 extant species of nonavian reptiles are squamates (lizards and snakes); there are about 360 extant species of turtles, 26 crocodylians, and one rhynchocephalian. Although the diversity of reptiles is greatest in the tropics, many species occur in temperate regions and a few have geographic ranges that extend north of the Arctic Circle. Antarctica is the only continent with no extant reptiles. Oviparity is the ancestral mode of reproduction, but viviparity has evolved repeatedly among squamates. Both genetic sex determination (XX/XY and ZW/ZZ) and environmental sex determination are represented, and genetic, environmental, and non-genetic maternal factors interact in some species. Environmental sex-determination is universal in crocodylians, widespread among turtles, and present in some clades of squamates. Parental care is universal among crocodylians and is present in some species of squamates and turtles. Ectothermy, an ancestral character, is central to the biology of reptiles, and is responsible for their low metabolic rates and their high efficiency of secondary production. Lizards typically eat daily and consume many small prey items, whereas snakes eat less frequently and consume larger prey items relative to their body size. Low metabolic rates make small body sizes energetically feasible for ectotherms, and more than half of the extant species of lizards are smaller than nearly all mammals and birds. Among squamates, the mode of predation – from sit-and-wait to widely foraging – has a strong phylogenetic component and correlates with many elements of ecology, morphology, physiology, and behavior. Many species of snakes and a few lizards are venomous, and some snakes are poisonous because they sequester toxins from their prey. Although most species of reptiles have little economic value, they are important components of energy and nutrient flow in terrestrial ecosystems. Habitat loss, pollution, invasive species, disease, and global climate change affect many species. The life histories of most large species of turtles, lizards, snakes, and crocodylians depend on prolonged adult survival and reproduction, and these species are vulnerable to commercial exploitation.
... Aside from the timing of production of the hard corneous material of scales, mainly composed of corneous beta proteins (CBPs, formerly known as beta-keratins), and knowledge on the sites of cell proliferation that shape snake scales (Alibardi and Minelli 2016), no other information is available on the expression and localization of other proteins in the epidermis and dermis of snake embryos, including cell junctional proteins. The latter are essential for establishing the epidermal stratification that during the shedding cycle determines the formation of the 5-6 epidermal layers that characterize snake epidermis: Oberhautchen, beta-, mesos-, lacunar-alpha and clear layer (Maderson 1965;Roth and Jones 1970;Landmann 1979). ...
... The formation of symmetric and then slanted scales from embryonic stage 32 onward that progressively become very overlapped has been shown to depend on increased cell proliferation in the caudal side of the initial symmetric scale bump (Alibardi and Minelli 2016), like in lizards (Alibardi 1998(Alibardi , 2003Wu et al. 2014). Previous morphological analyses indicated that the dermal cells localized underneath the expanding outer scale surface, the dorsal longer area of the forming scale (Figs. ...
The development of scales and the sequence of epidermal layers during snake embryogenesis has been studied by immunofluorescence for the localization of cell adhesion, adherens, and communicating cell junctional proteins. At about 2nd/3rd of embryonic development in snakes the epidermis forms symmetric bumps at the beginning of scale formation, and they rapidly become asymmetric and elongate forming outer and inner surfaces of the very overlapped scales seen at hatching. The dermis separates a superficial loose from a deeper dense part; the latter is joined to segmental muscles and nerves, likely acting on scale orientation during snake movements. N-cam is present in the differentiating epidermis and mesenchyme of forming scales while L-cam is only/mainly detected in the periderm and epidermis. Mesenchymal N-cam is associated with the epidermis of the elongating dorsal scale surface and with the beta-differentiation that occurs in the overlapping outer surface of scales. Beta-catenin and Connexin-43 show a similar distribution, and they are mainly present in the periderm and differentiating suprabasal keratinocytes likely forming an intense connectivity during epidermal differentiation. Beta-catenin also shows nuclear localization in differentiating cells of the shedding and beta-layers at late stages of scale morphogenesis, before hatching. The study suggests that intensification of adhesion and gap junctions allows synchronization of the differentiation of suprabasal cells to produce the ordered sequence of epidermal layers of snake scales, starting from the shedding complex and the beta-layer.
... Studies of the scute formation during the embryonic development of the Alligatoridae show that scutes grow continuously from a uniform layer of keratinocytes on the epidermis of osteoderms, supported by a network of collagen fibres. [39][40][41][42] Scutes can be divided into a collagen and keratinized section by splitting the hard outer keratin layer from the supporting tissue along the epidermis. Keratin is a chemically inert biological material that is formed when the cytoplasmic content of epidermal cells is replaced by filamentous proteins. ...
Rationale:
The diet of wild Nile crocodiles (Crocodylus niloticus) is difficult to assess because they are cryptic, nocturnal predators that are extremely sensitive to disturbance by observers, and stomach content analysis is challenging, especially in large specimens. Stable light isotope analysis provides a means of assessing their diet but diet-to-tissue discrimination factors have yet to be established for the species.
Methods:
Isotope ratio (15 N/14 N and 13 C/12 C expressed as δ15 N and δ13 C) analyses of scutes, claws and blood of farmed crocodile of a range of sizes were compared with the isotope values of their lifelong diet, which comprises chickens from a single supplier.
Results:
Systematic size dependence in the diet-to-tissue discrimination factors for scute collagen, scute keratin, and claw keratin, is described in regression relationships against the snout to vent length. Fixed values are presented for erythrocytes and blood plasma because blood was not sampled from juveniles.
Conclusions:
The diet-to-tissue discrimination factors allow an assessment of the diet of wild crocodiles. The diet of crocodiles from Lake Flag Boshielo shows a clear ontogenic shift, as has been seen in other studies, and the results strongly indicate a dependence on the terrestrial food web rather than a fish diet. That this population may exploit a terrestrial diet highlights potential conflicts for conserving Nile crocodiles outside of protected areas.
... A similar high proliferative activity is only present in active area of the embryonic turtle epidermis that gives rise to the lager scutes of the carapace and plastron (Alibardi and Minelli 2015) and claws (Alibardi 2009), areas of the epidermis where a strong accumulation of beta-cells occurs. Like in scutes and claws at ES 19 and 21, also in the epidermis of beak keratinocytes move to external epidermal layers in 1-2 days from the injection of 5BrdU, before starting their differentiation into beta-corneocytes. ...
The development of the beak in turtles is poorly known. Beak development has been analyzed by immunofluorescent methods for studying cell proliferation and localization of specific proteins. The flat two-layered epidermis covering the turtle embryo at mid stage of development becomes columnar in the oral region and is associated with an increase of mesenchymal density as in placodes. Using 5BrdU, an intense cell proliferation is observed in the oral and epidermal cells covering the maxilla and mandibular bones, probably stimulated by the underlying mesenchyme in continuation with maxillary and mandibular bones. Expansion and fusion of these placodes give rise to the corneous beak. Beta catenin, mainly junctional but also sparsely detected in nuclei of the germinal layer of the beak epithelium, maintains united the differentiating keratinocytes that form a stratified corneous epithelium. This is initially composed of some layers of large cells that accumulate intermediate filament keratins (IFKs) and give rise to a keratinized embryonic epidermis destined to slough around hatching. The following corneocytes accumulate IFKs but mainly type I/II corneous beta proteins (CBPs) and form a corneous beak. These CBPs appear present with lower intensity in the beak than in the shell, but the higher intensity obtained with a general antibody against CBPs indicates that other CBPs contribute to the composition and stiffness of beak corneous material. The egg-tooth in continuation with the stratum corneum of the maxillary beak develops from a localized proliferation and comprises a thick embryonic epidermis accumulating IFKs under which large beta-cells connected by adhesion proteins accumulate CBPs contributing to hardening of this temporary organ.
... The molecular mechanism that induces the augmented proliferation in the dorsal side forming the unguis with respect to the ventral side, the sub-unguis, remains unknown. This may be similar to the expansion of the outer scale surface of lizard and snake scales, that appears associated with the localization of a special mesenchyme, absent underneath of the inner scale surface (Alibardi, 2015). The signalling molecules associated with the mesenchyme that may induce the formation of a placode (DiPoi & Milinkovitch, 2016), and later of a corneous beta-layer in the unguis and sub-unguis (BMP, Shh, Wnt, see Hamrich, 2001Hamrich, , 2003 remain to be discovered in reptiles. ...
Differential cell proliferation and differentiation in developing and growing claws of turtles and alligator determine their shape. Acta Zoologica (Stockolm). Morphogenesis and cell proliferation in claws of turtle and alligator have been analysed by immunolabelling and thymidine autoradiography. Proliferating cells are randomly distributed in the epidermis and mesenchyme at the tip of forming digits. Developing claws elongate with a dorsal curvature forming the unguis. Numerous proliferating keratinocytes are present in the unguis at 4 hr–1 day after injection of 5BrdU or by PCNA detection. Their size increases before being incorporated into the ungueal corneous layer. Immunolabelling at 3–6 days post‐injection of 5BrdU indicates that proliferating keratinocytes move towards the tip of the claw giving rise to the claw pad. The ventral side of the claw termed sub‐unguis shows a lower proliferation that expands the surface and produces thin corneocytes that are desquamated. Autoradiography and immunolabelling indicates that proliferating cells of growing turtle claws are present along most of the germinal epidermis 7 hr–1 day after injection of tritiated thymidine or 5BrdU. Low labelled corneocytes incorporated into the corneous layer are detected at 6–12 days post‐injection, generally localized towards the tip of the claw, indicating they have migrated distally. The study confirms that claws of reptiles have an extended proliferative zone instead of a localized proximal germinal matrix as in mammalian claws.
... Other labs have also explored properties of scales and found that crocodilian and turtle scales share similarities 41 . This paper highlights that proliferation of these scales differs from those found in leidosaurians which show higher proliferation rates in the outer scale surface. ...
Abstract Amniote skin appendages such as feathers, hairs and scales, provide thermoregulation, physical protection and display different color patterns to attract a mate or frighten an adversary. A long-standing question is whether “reptile scale” and “avian leg scales” are of the same origin. Understanding the relation between avian feathers, avian scales and reptilian scales will enhance our understanding of skin appendage evolution. We compared the molecular and cellular profiles in chicken feather, chicken scales and alligator scales and found that chicken scutate scales are similar to chicken feathers in morphogenesis at the early placode stage. When we compared the expression of the recently identified feather-specific genes and scale-specific genes in these skin appendages, we found that at the molecular level alligator scales are significantly different from both chicken feathers and chicken scales. Furthermore, we identified a similarly diffuse putative stem cell niche in morphologically similar chicken and alligator scales. These putative stem cells participate in alligator scale regeneration. In contrast, avian feathers have a more condensed stem cell niche, which may be responsible for cycling. Thus, our results suggest that chicken and alligator scales formed independently through convergent evolution.
... Hairs, feathers, and scales exhibit substantial developmental specificities, blurring evolutionary relationships among the processes involved. One primary example of developmental divergence among skin appendage types is that hairs, feathers, and avian and turtle scutate scales develop from a characteristic local thickening of the epidermis [the anatomical placode (10)(11)(12)(13)(14)], whereas all authors agree that scales in squamates (snakes and lizards) form from regular dermoepidermal elevations without exhibiting placodes (3,9,(14)(15)(16)(17). Later developmental stages are even more divergent as hair and feather placodes are associated with a dermal condensation and further develop into follicular organs characterized by substantial downward growth (hair follicle) or outgrowth (feather follicle) of the epidermis, whereas mature scales typically develop by asymmetrization of the initial dermoepidermal elevations without showing any apparent sign of dermal condensation. ...
... Hairs, feathers, and scales exhibit substantial developmental specificities, blurring evolutionary relationships among the processes involved. One primary example of developmental divergence among skin appendage types is that hairs, feathers, and avian and turtle scutate scales develop from a characteristic local thickening of the epidermis [the anatomical placode (10)(11)(12)(13)(14)], whereas all authors agree that scales in squamates (snakes and lizards) form from regular dermoepidermal elevations without exhibiting placodes (3,9,(14)(15)(16)(17). Later developmental stages are even more divergent as hair and feather placodes are associated with a dermal condensation and further develop into follicular organs characterized by substantial downward growth (hair follicle) or outgrowth (feather follicle) of the epidermis, whereas mature scales typically develop by asymmetrization of the initial dermoepidermal elevations without showing any apparent sign of dermal condensation. ...
Most mammals, birds, and reptiles are readily recognized by their hairs, feathers, and scales, respectively. However, the lack of fossil intermediate forms between scales and hairs and substantial differences in their morphogenesis and protein composition have fueled the controversy pertaining to their potential common ancestry for decades. Central to this debate is the apparent lack of an “anatomical placode” (that is, a local epidermal thickening characteristic of feathers’ and hairs’ early morphogenesis) in reptile scale development. Hence, scenarios have been proposed for the independent development of the anatomical placode in birds and mammals and parallel co-option of similar signaling pathways for their morphogenesis. Using histological and molecular techniques on developmental series of crocodiles and snakes, as well as of unique wild-type and EDA (ectodysplasin A)–deficient scaleless mutant lizards, we show for the first time that reptiles, including crocodiles and squamates, develop all the characteristics of an anatomical placode: columnar cells with reduced proliferation rate, as well as canonical spatial expression of placode and underlying dermal molecular markers. These results reveal a new evolutionary scenario where hairs, feathers, and scales of extant species are homologous structures inherited, with modification, from their shared reptilian ancestor’s skin appendages already characterized by an anatomical placode and associated signaling molecules.
... The beta-keratin of turtle shells and the sites of cell proliferation are like those of crocodylians and different from lepidosaurs. (Dalla Valle et al., 2009;Alibardi and Minelli, 2016). The scutes and bony components of the shell are secondarily reduced in some clades of turtles, most notably the freshwater softshell turtles (Trionychidae and Carretochelyidae) and the marine leatherback turtle (Dermochelys coriacea). ...
Most of the 9300 extant species of non-avian reptiles are squamates (lizards and snakes); there are only 315 extant species of turtles, 23 crocodilians, and one rhynchocephalian. Although the diversity of reptiles is greatest in the tropics, many species occur in the temperate regions and a few have geographic ranges that extend north of the Arctic Circle. Antarctica is the only continent with no extant reptiles. Ectothermy, an ancestral character, is central to the biology of reptiles, and is responsible for their low metabolic rates and their high efficiency of secondary production. Temperature-dependent sex-determination is universal in crocodilians, widespread among turtles, and present in some lineages of squamates. Among lizards, the mode of predation – sit-and-wait, cruising forager, or widely foraging – has a strong phylogenetic component and correlates with many elements of their ecology, morphology, physiology, and behavior. Lizards typically eat daily and consume many small prey items, whereas snakes eat less frequently and consume larger prey items relative to their body size. Most species of reptiles are small, inconspicuous, and have little obvious economic value, and as a consequence we lack information about the viability of their populations. Climate change, habitat loss, and pollution (including the feminizing effects of estrogen analogs) affect many species, and nearly three million lizards, snakes, turtles, and crocodilians are consumed by trade in hides and pets annually. The life histories of most large species of turtles, lizards, snakes, and crocodilians depend on prolonged adult survival and reproduction, and it is unlikely that these species will long withstand the current rate of commercial exploitation.
