Transient agonism of the sonic hedgehog pathway triggers a permanent transition of skin appendage fate in the chicken embryo

Organizational principles of integumentary organs: Maximizing variations for effective adaptation

The integument serves as the interface between an organism and its environment. It primarily comprises ectoderm-derived epithelium and mesenchyme derived from various embryonic sources. These integumentary organs serve as a barrier defining the physiological boundary between the internal and exterior environments and fulfill diverse functions. How does the integument generate such a large diversity? Here, we attempt to decipher the organizational principles. We focus on amniotes and use appendage follicles as the primary examples. The integument begins as a simple planar sheet of coupled epithelial and mesenchymal cells, then becomes more complex through the following patterning processes. 1) De novo Turing periodic patterning process: This process converts the integument into multiple skin appendage units. 2) Adaptive patterning process: Dermal muscle, blood vessels, adipose tissue, and other components are assembled and organized around appendage follicles when present. 3) Cyclic renewal: Skin appendage follicles contain stem cells and their niches, enabling physiological molting and regeneration in the adult animal. 4) Spatial variations: Multiple appendage units allow modulation of shape, size, keratin types, and color patterns of feathers and hairs across the animal's surface. 5) Temporal phenotypic plasticity: Cyclic renewal permits temporal transition of appendage phenotypes, i.e. regulatory patterning or integumentary metamorphosis, throughout an animal's lifetime. The diversities in (4) and (5) can be generated epigenetically within the same animal. Over the evolutionary timescale, different species can modulate the number, size, and distributions of existing ectodermal organs in the context of micro-evolution, allowing effective adaptation to new climates as seen in the variation of hair length among mammals. Novel ectodermal organs can also emerge in the context of macro-evolution, enabling animals to explore new ecological niches, as seen in the emergence of feathers on dinosaurs. These principles demonstrate how multi-scale organ adaption in the amniotes can maximize diverse and flexible integumentary organ phenotypes, producing a vast repertoire for natural selection and thereby providing effective adaptation and evolutionary advantages.

Exacerbated sonic hedgehog signalling promotes a transition from chemical pre-patterning of chicken reticulate scales to mechanical skin folding

Many examples of self-organized embryonic patterning can be attributed to chemically mediated systems comprising interacting morphogens. However, mechanical patterning also contributes to the emergence of biological forms. For example, various studies have demonstrated that diverse patterns arise from elastic instabilities associated with the constrained growth of soft materials, which generate wrinkles, creases and folds. Here, we show that between days 12 and 13 of in ovo development, transient experimentally increased activity of the sonic hedgehog pathway in the chicken embryo, through a single intravenous injection of smoothened agonist (SAG), abolishes the Turing-like chemical patterning of reticulate scales on the ventral footpad and promotes a transition to mechanical labyrinthine skin folding. Using in situ hybridization, nanoindentation and labelling of proliferating cells, we confirm that skin surface folding is associated with the loss of signalling placode pre-patterning as well as increased epidermal growth and stiffness. Using additional in ovo hydrocortisone treatments, we also demonstrate that experimentally induced hyper-keratinization of the skin mechanically restricts SAG-induced folding. Finally, we verify our experimental findings with mechanical growth simulations built from volumetric light sheet fluorescence microscopy data. Overall, we reveal that pharmacological perturbation of the underlying gene regulatory network can abolish chemical skin appendage patterning and replace it with self-organized mechanical folding.

In vivo sonic hedgehog pathway antagonism temporarily results in ancestral proto-feather-like structures in the chicken

The morphological intricacies of avian feathers make them an ideal model for investigating embryonic patterning and morphogenesis. In particular, the sonic hedgehog (Shh) pathway is an important mediator of feather outgrowth and branching. However, functional in vivo evidence regarding its role during feather development remains limited. Here, we demonstrate that an intravenous injection of sonidegib, a potent Shh pathway inhibitor, at embryonic day 9 (E9) temporarily produces striped domains (instead of spots) of Shh expression in the skin, arrests morphogenesis, and results in unbranched and non-invaginated feather buds-akin to proto-feathers-in embryos until E14. Although feather morphogenesis partially recovers, hatched treated chickens exhibit naked skin regions with perturbed follicles. Remarkably, these follicles are subsequently reactivated by seven weeks post-hatching. Our RNA-sequencing data and rescue experiment using Shh-agonism confirm that sonidegib specifically down-regulates Shh pathway activity. Overall, we provide functional evidence for the role of the Shh pathway in mediating feather morphogenesis and confirm its role in the evolutionary emergence and diversification of feathers.

Self-organized patterning of crocodile head scales by compressive folding

Amniote integumentary appendages constitute a diverse group of micro-organs, including feathers, hair and scales. These structures typically develop as genetically controlled units1, the spatial patterning of which emerges from a self-organized chemical Turing system2,3 with integrated mechanical feedback4,5. The seemingly purely mechanical patterning of polygonal crocodile head scales provides an exception to this paradigm6. However, the nature and origin of the mechanical stress field driving this patterning remain unclear. Here, using precise in ovo intravenous injections of epidermal growth factor protein, we generate Nile crocodile embryos with substantially convoluted head skin, as well as hatchlings with smaller polygonal head scales resembling those of caimans. We then use light-sheet fluorescence microscopy to quantify embryonic tissue-layer geometry, collagen architecture and the spatial distribution of proliferating cells. Using these data, we build a phenomenological three-dimensional mechanical growth model that recapitulates both normal and experimentally modified patterning of crocodile head scales. Our experiments and numerical simulations demonstrate that crocodile head scales self-organize through compressive folding, originating from near-homogeneous skin growth with differential stiffness of the dermis versus the epidermis. Our experiments and theoretical morphospace analyses indicate that variation in embryonic growth and material properties of skin layers provides a simple evolutionary mechanism that produces a diversity of head-scale patterns among crocodilian species.

Evolution, development, and regeneration of tooth-like epithelial appendages in sharks

Sharks and their relatives are typically covered in highly specialized epithelial appendages embedded in the skin called dermal denticles; ancient tooth-like units (odontodes) composed of dentine and enamel-like tissues. These 'skin teeth' are remarkably similar to oral teeth of vertebrates and share comparable morphological and genetic signatures. Here we review the histological and morphological data from embryonic sharks to uncover characters that unite all tooth-like elements (odontodes), including teeth and skin denticles in sharks. In addition, we review the differences between the skin and oral odontodes that reflect their varied capacity for renewal. Our observations have begun to decipher the developmental and genetic shifts that separate these seemingly similar dental units, including elements of the regenerative nature in both oral teeth and the emerging skin denticles from the small-spotted catshark (Scyliorhinus canicula) and other chondrichthyan models. Ultimately, we ask what defines a tooth at both the molecular and morphological level. These insights aim to help us understand how nature makes, replaces and evolves a vast array of odontodes.

A chromosome-level genome assembly of the Asian house martin implies potential genes associated with the feathered-foot trait

The presence of feathers is a vital characteristic among birds, yet most modern birds had no feather on their feet. The discoveries of feathers on the hind limbs of basal birds and dinosaurs have sparked an interest in the evolutionary origin and genetic mechanism of feathered feet. However, the majority of studies investigating the genes associated with this trait focused on domestic populations. Understanding the genetic mechanism underpinned feathered-foot development in wild birds is still in its infancy. Here, we assembled a chromosome-level genome of the Asian house martin (Delichon dasypus) using the long-read High Fidelity sequencing approach to initiate the search for genes associated with its feathered feet. We employed the whole-genome alignment of D. dasypus with other swallow species to identify high-SNP regions and chromosomal inversions in the D. dasypus genome. After filtering out variations unrelated to D. dasypus evolution, we found six genes related to feather development near the high-SNP regions. We also detected three feather development genes in chromosomal inversions between the Asian house martin and the barn swallow genomes. We discussed their association with the wingless/integrated (WNT), bone morphogenetic protein, and fibroblast growth factor pathways and their potential roles in feathered-foot development. Future studies are encouraged to utilize the D. dasypus genome to explore the evolutionary process of the feathered-foot trait in avian species. This endeavor will shed light on the evolutionary path of feathers in birds.

The avian ectodermal default competence to make feathers

Feathers originate as protofeathers before birds, in pterosaurs and basal dinosaurs. What characterizes a feather is not only its outgrowth, but its barb cells differentiation and a set of beta-corneous proteins. Reticula appear concomitantly with feathers, as small bumps on plantar skin, made only of keratins. Avian scales, with their own set of beta-corneous proteins, appear more recently than feathers on the shank, and only in some species. In the chick embryo, when feather placodes form, all the non-feather areas of the integument are already specified. Among them, midventral apterium, cornea, reticula, and scale morphogenesis appear to be driven by negative regulatory mechanisms, which modulate the inherited capacity of the avian ectoderm to form feathers. Successive dermal/epidermal interactions, initiated by the Wnt/β-catenin pathway, and involving principally Eda/Edar, BMP, FGF20 and Shh signaling, are responsible for the formation not only of feather, but also of scale placodes and reticula, with notable differences in the level of Shh, and probably FGF20 expressions. This sequence is a dynamic and labile process, the turning point being the FGF20 expression by the placode. This epidermal signal endows its associated dermis with the memory to aggregate and to stimulate the morphogenesis that follows, involving even a re-initiation of the placode.

Evolutionary origin of Hoxc13-dependent skin appendages in amphibians

Cornified skin appendages, such as hair and nails, are major evolutionary innovations of terrestrial vertebrates. Human hair and nails consist largely of special intermediate filament proteins, known as hair keratins, which are expressed under the control of the transcription factor Hoxc13. Here, we show that the cornified claws of Xenopus frogs contain homologs of hair keratins and the genes encoding these keratins are flanked by promoters in which binding sites of Hoxc13 are conserved. Furthermore, these keratins and Hoxc13 are co-expressed in the claw-forming epithelium of frog toe tips. Upon deletion of hoxc13, the expression of hair keratin homologs is abolished and the development of cornified claws is abrogated in X. tropicalis. These results indicate that Hoxc13-dependent expression of hair keratin homologs evolved already in stem tetrapods, presumably as a mechanism for protecting toe tips, and that this ancestral genetic program was coopted to the growth of hair in mammals.

Genetic Basis and Evolution of Structural Color Polymorphism in an Australian Songbird

Island organisms often evolve phenotypes divergent from their mainland counterparts, providing a useful system for studying adaptation under differential selection. In the white-winged fairywren (Malurus leucopterus), subspecies on two islands have a black nuptial plumage whereas the subspecies on the Australian mainland has a blue nuptial plumage. The black subspecies have a feather nanostructure that could in principle produce a blue structural color, suggesting a blue ancestor. An earlier study proposed independent evolution of melanism on the islands based on the history of subspecies divergence. However, the genetic basis of melanism and the origin of color differentiation in this group are still unknown. Here, we used whole-genome resequencing to investigate the genetic basis of melanism by comparing the blue and black M. leucopterus subspecies to identify highly divergent genomic regions. We identified a well-known pigmentation gene ASIP and four candidate genes that may contribute to feather nanostructure development. Contrary to the prediction of convergent evolution of island melanism, we detected signatures of a selective sweep in genomic regions containing ASIP and SCUBE2 not in the black subspecies but in the blue subspecies, which possesses many derived SNPs in these regions, suggesting that the mainland subspecies has re-evolved a blue plumage from a black ancestor. This proposed re-evolution was likely driven by a preexisting female preference. Our findings provide new insight into the evolution of plumage coloration in island versus continental populations, and, importantly, we identify candidate genes that likely play roles in the development and evolution of feather structural coloration.

Molecular and Cellular Characterization of Avian Reticulate Scales Implies the Evo-Devo Novelty of Skin Appendages in Foot Sole

Among amniotic skin appendages, avian feathers and mammalian hairs protect their stem cells in specialized niches, located in the collar bulge and hair bulge, respectively. In chickens and alligators, label retaining cells (LRCs), which are putative stem cells, are distributed in the hinge regions of both avian scutate scales and reptilian overlapping scales. These LRCs take part in scale regeneration. However, it is unknown whether other types of scales, for example, symmetrically shaped reticulate scales, have a similar way of preserving their stem cells. In particular, the foot sole represents a special interface between animal feet and external environments, with heavy mechanical loading. This is different from scutate-scale-covered metatarsal feet that function as protection. Avian reticulate scales on foot soles display specialized characteristics in development. They do not have a placode stage and lack β-keratin expression. Here, we explore the molecular and cellular characteristics of avian reticulate scales. RNAscope analysis reveals different molecular profiles during surface and hinge determination compared with scutate scales. Furthermore, reticulate scales express Keratin 15 (K15) sporadically in both surface- and hinge-region basal layer cells, and LRCs are not localized. Upon wounding, the reticulate scale region undergoes repair but does not regenerate. Our results suggest that successful skin appendage regeneration requires localized stem cell niches to guide regeneration.

Protocol for the rapid intravenous in ovo injection of developing amniote embryos

We present a technique for precise drug delivery into the vascular system of developing amniote embryos via injection into chorioallantoic veins underlying the eggshell membrane. We describe steps for incubating and candling eggs, removing the shell to expose underlying veins, and precise intravenous injection. In addition to chicken embryos, this protocol is applicable to other amniote species that lay hard-shell eggs, including crocodiles and tortoises. This technique is rapid, is reproducible, is of low cost, and will provide an important resource for developmental biologists. For complete details on the use and execution of this protocol, please refer to Cooper & Milinkovitch.¹.