FGF-2 signaling is sufficient to induce dermal condensations during feather development

Embryonic feather bud development - A keystone model for vertebrate organogenesis

The development of feathers in the embryonic skin has been used as a model for biological self-organisation for many decades. The availability, size and ease of manipulation of the skin has enabled it to serve as a model revealing concepts of epithelial-mesenchymal interaction, origins of periodic patterns in the anatomy, and the effects of growth factors and structural and mechanical properties on tissue development. These efforts provide a rich history of observation, informing continued development of new concepts in this system. Here we review the process of early feather bud development, the understanding gained from decades of experimentation, and current debate and future directions for progress.

Chromosome-level genome sequencing and multi-omics of the Hungarian White Goose (Anser anser domesticus) reveals novel miRNA-mRNA regulation mechanism of waterfowl feather follicle development

The Hungarian White Goose (Anser anser domesticus) is an excellent European goose breed, with high feather and meat production. Despite its importance in the poultry industry, no available genome assembly information has been published. This study aimed to present Chromosome-level and functional genome sequencing of the Hungarian White Goose. The results showed that the genome assembly has a total length of 1115.82 Mb, 39 pairs of chromosomes, 92.98% of the BUSCO index, and contig N50 and scaffold N50 were up to 2.32 Mb and 60.69 Mb, respectively. Annotation of the genome assembly revealed 19550 genes, 286 miRNAs, etc. We identified 235 expanded and 1,167 contracted gene families in this breed compared with the other 16 species. We performed a positive selection analysis between this breed and four species of Anatidae to uncover the genetic information underlying feather follicle development. Further, we detected the function of miR-199-x, miR-143-y, and miR-23-z on goose embryonic skin fibroblast. In summary, we have successfully generated a highly complete genome sequence of the Hungarian white goose, which will provide a great resource to improve our understanding of gene functions and enhance the studies on feather follicle development at the genomic level.

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.

Periodic pattern formation during embryonic development

During embryonic development many organs and structures require the formation of series of repeating elements known as periodic patterns. Ranging from the digits of the limb to the feathers of the avian skin, the correct formation of these embryonic patterns is essential for the future form and function of these tissues. However, the mechanisms that produce these patterns are not fully understood due to the existence of several modes of pattern generation which often differ between organs and species. Here, we review the current state of the field and provide a perspective on future approaches to studying this fundamental process of embryonic development.

Identification of genes involved in regulating the development of feathered feet in chicken embryo

The genetic and developmental factors driving the diverse distribution and morphogenesis of feathers and scales on bird feet are yet unclear. Within a single species, Guangxi domestic chickens exhibit dramatic variety in feathered feet, making them an accessible model for research into the molecular basis of variations in skin appendages. In this study, we used H&E staining to observe the morphogenesis of feathered feet, scaled feet and wings skin at different embryonic stages in Longsheng-Feng chickens and Guangxi Partridge chickens. We selected 4 periods (E6, E7, E8, and E12) that play an important role in feather development and performed transcriptome sequencing to screen for candidate genes associated with feathered feet. Through comparison and analysis of transcriptome data, we identified a set of differently expressed genes (DGEs), which were enriched in appendage organ development, hindlimb morphogenesis, activation of transcription factor binding, and binding of sequence-specific DNA in the cis-regulatory region. In addition, we identified some feathered feet-related genes by analyzing the classical signaling pathways that regulate feather development. Finally, we identified candidate genes that regulate feathered feet formation, which include TBX5, PITX1, ZIC1, FGF20, WNT11, WNT7A, WNT16, and SHH. Interestingly, we found that TBX5 was significantly overexpressed in the skin of the feathered feet and had the highest expression at E7 (P < 0.01), whereas PITX1 expression was significantly reduced at E7(P < 0.01). It is hypothesized that TBX5 and PITX1 regulate the development of hair follicles through the Wnt/β-catenin signaling pathway at E7. Our results provide a theoretical basis for investigating the molecular regulatory mechanisms underlying the formation of chicken feathered feet.

Evo Devo of the Vertebrates Integument

All living jawed vertebrates possess teeth or did so ancestrally. Integumental surface also includes the cornea. Conversely, no other anatomical feature differentiates the clades so readily as skin appendages do, multicellular glands in amphibians, hair follicle/gland complexes in mammals, feathers in birds, and the different types of scales. Tooth-like scales are characteristic of chondrichthyans, while mineralized dermal scales are characteristic of bony fishes. Corneous epidermal scales might have appeared twice, in squamates, and on feet in avian lineages, but posteriorly to feathers. In contrast to the other skin appendages, the origin of multicellular glands of amphibians has never been addressed. In the seventies, pioneering dermal-epidermal recombination between chick, mouse and lizard embryos showed that: (1) the clade type of the appendage is determined by the epidermis; (2) their morphogenesis requires two groups of dermal messages, first for primordia formation, second for appendage final architecture; (3) the early messages were conserved during amniotes evolution. Molecular biology studies that have identified the involved pathways, extending those data to teeth and dermal scales, suggest that the different vertebrate skin appendages evolved in parallel from a shared placode/dermal cells unit, present in a common toothed ancestor, c.a. 420 mya.

Systems for intricate patterning of the vertebrate anatomy

Periodic patterns form intricate arrays in the vertebrate anatomy, notably the hair and feather follicles of the skin, but also internally the villi of the gut and the many branches of the lung, kidney, mammary and salivary glands. These tissues are composite structures, being composed of adjoined epithelium and mesenchyme, and the patterns that arise within them require interaction between these two tissue layers. In embryonic development, cells change both their distribution and state in a periodic manner, defining the size and relative positions of these specialized structures. Their placement is determined by simple spacing mechanisms, with substantial evidence pointing to a variety of local enhancement/lateral inhibition systems underlying the breaking of symmetry. The nature of the cellular processes involved, however, has been less clear. While much attention has focused on intercellular soluble signals, such as protein growth factors, experimental evidence has grown for contributions of cell movement or mechanical forces to symmetry breaking. In the mesenchyme, unlike the epithelium, cells may move freely and can self-organize into aggregates by chemotaxis, or through generation and response to mechanical strain on their surrounding matrix. Different modes of self-organization may coexist, either coordinated into a single system or with hierarchical relationships. Consideration of a broad range of distinct biological processes is required to advance understanding of biological pattern formation. This article is part of the theme issue 'Recent progress and open frontiers in Turing's theory of morphogenesis'.

Molecular Signaling and Nutritional Regulation in the Context of Poultry Feather Growth and Regeneration

The normal growth and regeneration of feathers is important for improving the welfare and economic value of poultry. Feather follicle stem cells are the basis for driving feather development and are regulated by various molecular signaling pathways in the feather follicle microenvironment. To date, the roles of the Wnt, Bone Morphogenetic Protein (BMP), Notch, and Sonic Hedgehog (SHH) signaling pathways in the regulation of feather growth and regeneration are among the best understood. While these pathways regulate feather morphogenesis in different stages, their dysregulation results in a low feather growth rate, poor quality of plumage, and depilation. Additionally, exogenous nutrient intervention can affect the feather follicle cycle, promote the formation of the feather shaft and feather branches, preventing plumage abnormalities. This review focuses on our understanding of the signaling pathways involved in the transcriptional control of feather morphogenesis and explores the impact of nutritional factors on feather growth and regeneration in poultry. This work may help to develop novel mechanisms by which follicle stem cells can be manipulated to produce superior plumage that enhances poultry carcass quality.

Exploration of key regulators driving primary feather follicle induction in goose skin

The primary feather follicles are universal skin appendages widely distributed in the skin of feathered birds. The morphogenesis and development of the primary feather follicles in goose skin remain largely unknown. Here, the induction of primary feather follicles in goose embryonic skin (pre-induction vs induction) was investigated by de novo transcriptome analyses to reveal 409 differentially expressed genes (DEGs). The DEGs were characterized to potentially regulate the de novo formation of feather follicle primordia consisting of placode (4 genes) and dermal condensate (12 genes), and the thickening of epidermis (5 genes) and dermal fibroblasts (17 genes), respectively. Further analyses enriched DEGs into GO terms represented as cell adhesion and KEGG pathways including Wnt and Hedgehog signaling pathways that are highly correlated with cell communication and molecular regulation. Six selected Wnt pathway genes were detected by qPCR with up-regulation in goose skin during the induction of primary feather follicles. The localization of WNT16, SFRP1 and FRZB by in situ hybridization showed weak expression in the primary feather primordia, whereas FZD1, LEF1 and DKK1 were expressed initially in the inter-follicular skin and feather follicle primordia, then mainly restricted in the feather primordia. The spatial-temporal expression patterns indicate that Wnt pathway genes DKK1, FZD1 and LEF1 are the important regulators functioned in the induction of primary feather follicle in goose skin. The dynamic molecular changes and specific gene expression patterns revealed in this report provide the general knowledge of primary feather follicle and skin development in waterfowl, and contribute to further understand the diversity of hair and feather development beyond the mouse and chicken models.

Expression patterns of three JAK-STAT pathway genes in feather follicle development during chicken embryogenesis

The Janus kinase (JAK)-signal transducer and activator of transcription (STAT) (JAK-STAT) pathway is shown to restrain the hair follicles in catagen and telogen and prevent anagen reentry in murine hair follicle cycling. The early roles of JAK-STAT pathway genes in skin development remain uncharacterized in mouse and chicken models. Here, we revealed the expression patterns of three JAK-STAT pathway genes (JAK1, JAK2, and TYK2) in chicken embryonic skin at E6-E10 stages which are key to feather follicle morphogenesis. Multiple sequence alignment of the three genes from chicken and other species all showed a closely related homology with birds like quail and goose. Whole mount in situ hybridization (WISH) revealed weak expression of JAK1, JAK2, and TYK2 in chicken skin at E6 and E7, and followed with the focally restricted signals in the feather follicles of neck and body skin located dorsally at E8 for JAK1, E9 for TYK2 and E10 for JAK2 gene. All three genes displayed stronger expression in feather follicles of neck skin than that of body skin. The expression levels of JAK1 and TYK2 were much stronger than those of JAK2. Quantitative real-time PCR (qRT-PCR) analysis revealed the increased expression tendency for JAK2 both in the neck and body skin from E6 to E10, and the much stronger expression in neck and body skin at later stages (E8-E10) than earlier stages (E6 and E7) for JAK1 and TYK2. Overall, these findings suggest that JAK1 and TYK2, not JAK2 are important to specify the feather follicle primordia, and to arrange the proximal-distal axis of feather follicles, respectively, during the morphogenesis of feather follicles in embryonic chicken skin.

Feather Evolution from Precocial to Altricial Birds

Birds are the most abundant terrestrial vertebrates and their diversity is greatly shaped by the feathers. How avian evolution is linked to feather evolution has long been a fascinating question. Numerous excellent studies have shed light on this complex relationship by investigating feather diversity and its underlying molecular mechanisms. However, most have focused on adult domestic birds, and the contribution of feather diversity to environmental adaptation has not been well-studied. In this review, we described bird diversity using the traditional concept of the altricial-precocial spectrum in bird hatchlings. We combined the spectrum with a recently published avian phylogeny to profile the spectrum evolution. We then focused on the discrete diagnostic character of the spectrum, the natal down, and propose a hypothesis for the precocial-to-altricial evolution. For the underlying molecular mechanisms in feather diversity and bird evolution, we reviewed the literature and constructed the known mechanisms for feather tract definition and natal down development. Finally, we suggested some future directions for research on altricial-precocial divergence, which may expand our understanding of the relationship between natal down diversity and bird evolution.

Feather arrays are patterned by interacting signalling and cell density waves

Feathers are arranged in a precise pattern in avian skin. They first arise during development in a row along the dorsal midline, with rows of new feather buds added sequentially in a spreading wave. We show that the patterning of feathers relies on coupled fibroblast growth factor (FGF) and bone morphogenetic protein (BMP) signalling together with mesenchymal cell movement, acting in a coordinated reaction-diffusion-taxis system. This periodic patterning system is partly mechanochemical, with mechanical-chemical integration occurring through a positive feedback loop centred on FGF20, which induces cell aggregation, mechanically compressing the epidermis to rapidly intensify FGF20 expression. The travelling wave of feather formation is imposed by expanding expression of Ectodysplasin A (EDA), which initiates the expression of FGF20. The EDA wave spreads across a mesenchymal cell density gradient, triggering pattern formation by lowering the threshold of mesenchymal cells required to begin to form a feather bud. These waves, and the precise arrangement of feather primordia, are lost in the flightless emu and ostrich, though via different developmental routes. The ostrich retains the tract arrangement characteristic of birds in general but lays down feather primordia without a wave, akin to the process of hair follicle formation in mammalian embryos. The embryonic emu skin lacks sufficient cells to enact feather formation, causing failure of tract formation, and instead the entire skin gains feather primordia through a later process. This work shows that a reaction-diffusion-taxis system, integrated with mechanical processes, generates the feather array. In flighted birds, the key role of the EDA/Ectodysplasin A receptor (EDAR) pathway in vertebrate skin patterning has been recast to activate this process in a quasi-1-dimensional manner, imposing highly ordered pattern formation.

Involvement of selective epithelial cell death in the formation of feather buds on a bioengineered skin

Selective cell death by apoptosis plays important roles in organogenesis. Apoptotic cells are observed in the developmental and homeostatic processes of several ectodermal organs, such as hairs, feathers, and mammary glands. In chick feather development, apoptotic events have been observed during feather morphogenesis, but have not been investigated during early feather bud formation. Previously, we have reported a method for generating feather buds on a bioengineered skin from dissociated skin epithelial and mesenchymal cells in three-dimensional culture. During the development of the bioengineered skin, epithelial cavity formation by apoptosis was observed in the epithelial tissue. In this study, we examined the selective epithelial cell death during the bioengineered skin development. Histological analyses suggest that the selective epithelial cell death in the bioengineered skin was induced by caspase-3-related apoptosis. The formation of feather buds of the bioengineered skin was disturbed by the treatment with a pan-caspase inhibitor. The pan-caspase inhibitor treatment suppressed the rearrangement of the epithelial layer and the formation of dermal condensation, which are thought to be essential step to form feather buds. The suppression of the formation of feather buds on the pan-caspase inhibitor-treated skin was partially compensated by the addition of a GSK-3β inhibitor, which activates Wnt/β-catenin signaling. These results suggest that the epithelial cell death is involved in the formation of feather buds of the bioengineered skin. These observations also suggest that caspase activities and Wnt/β-catenin signaling may contribute to the formation of epithelial and mesenchymal components in the bioengineered skin.

Calcium oscillations coordinate feather mesenchymal cell movement by SHH dependent modulation of gap junction networks

Collective cell migration mediates multiple tissue morphogenesis processes. Yet how multi-dimensional mesenchymal cell movements are coordinated remains mostly unknown. Here we report that coordinated mesenchymal cell migration during chicken feather elongation is accompanied by dynamic changes of bioelectric currents. Transcriptome profiling and functional assays implicate contributions from functional voltage-gated Ca²⁺ channels (VGCCs), Connexin-43 based gap junctions, and Ca²⁺ release activated Ca²⁺ (CRAC) channels. 4-Dimensional Ca²⁺ imaging reveals that the Sonic hedgehog-responsive mesenchymal cells display synchronized Ca²⁺ oscillations, which expand progressively in area during feather elongation. Inhibiting VGCCs, gap junctions, or Sonic hedgehog signaling alters the mesenchymal Ca²⁺ landscape, cell movement patterns and feather bud elongation. Ca²⁺ oscillations induced by cyclic activation of opto-cCRAC channels enhance feather bud elongation. Functional disruption experiments and promoter analysis implicate synergistic Hedgehog and WNT/β-Catenin signaling in activating Connexin-43 expression, establishing gap junction networks synchronizing the Ca²⁺ profile among cells, thereby coordinating cell movement patterns.

The molecular mechanisms regulating mesenchymal cell movements are unclear. Here, the authors show in chicken feather elongation that SHH/WNT signalling establishes gap-junctions, enabling synchronized Ca^2 + oscillations to emerge for and in turn coordinating directed cell migration.

Hair follicle dermal condensation forms via Fgf20 primed cell cycle exit, cell motility, and aggregation

Mesenchymal condensation is a critical step in organogenesis, yet the underlying molecular and cellular mechanisms remain poorly understood. The hair follicle dermal condensate is the precursor to the permanent mesenchymal unit of the hair follicle, the dermal papilla, which regulates hair cycling throughout life and bears hair inductive potential. Dermal condensate morphogenesis depends on epithelial Fibroblast Growth Factor 20 (Fgf20). Here, we combine mouse models with 3D and 4D microscopy to demonstrate that dermal condensates form de novo and via directional migration. We identify cell cycle exit and cell shape changes as early hallmarks of dermal condensate morphogenesis and find that Fgf20 primes these cellular behaviors and enhances cell motility and condensation. RNAseq profiling of immediate Fgf20 targets revealed induction of a subset of dermal condensate marker genes. Collectively, these data indicate that dermal condensation occurs via directed cell movement and that Fgf20 orchestrates the early cellular and molecular events.

All mammal hair springs from hair follicles under the skin. These follicles sit in the dermis, beneath the outermost skin layer, the epidermis. In the embryo, hair follicles develop from unspecialized cells in two tissues, the epithelium and the mesenchyme, which will later develop into the dermis and epidermis, respectively. As development progresses, the cells of these tissues begin to cluster, and signals passing back and forth between the epithelium and mesenchyme instruct the cells what to do. In the mesenchyme, cells called fibroblasts squeeze up against their neighbors, forming patches called dermal condensates. These mature into so-called dermal papillae, which supply specific molecules called growth factors that regulate hair formation throughout lifetime.

Fibroblasts in the developing skin respond to a signal from the epithelium called fibroblast growth factor 20 (Fgf20), but we do not yet understand its effects. It is possible that Fgf20 tells the cells to divide, forming clusters of daughter cells around their current location. Or, it could be that Fgf20 tells the cells to move, encouraging them to travel towards one another to form groups.

To address this question, Biggs, Mäkelä et al. examined developing mouse skin grown in the laboratory. They traced cells marked with fluorescent tags to analyze their behavior as the condensates formed. This revealed that the Fgf20 signal acts as a rallying call, triggering fibroblast movement. The cells changed shape and moved towards one another, rather than dividing to create their own clusters. In fact, they switched off their own cell cycle as the condensates formed, halting their ability to divide. A technique called RNA sequencing revealed that Fgf20 also promotes the use of genes known to be active in dermal condensates.

Dermal papillae control hair growth, and transplanting them under the skin can form new hair follicles. However, these cells lose this ability when grown in the laboratory. Understanding how they develop could be beneficial for future hair growth therapy. Further work could also address fundamental questions in embryology. Condensates of cells from the mesenchyme also precede the formation of limbs, bones, muscles and organs. Extending this work could help us to understand this critical developmental step.

Neural crest and the origin of species-specific pattern

For well over half of the 150 years since the discovery of the neural crest, the special ability of these cells to function as a source of species-specific pattern has been clearly recognized. Initially, this observation arose in association with chimeric transplant experiments among differentially pigmented amphibians, where the neural crest origin for melanocytes had been duly noted. Shortly thereafter, the role of cranial neural crest cells in transmitting species-specific information on size and shape to the pharyngeal arch skeleton as well as in regulating the timing of its differentiation became readily apparent. Since then, what has emerged is a deeper understanding of how the neural crest accomplishes such a presumably difficult mission, and this includes a more complete picture of the molecular and cellular programs whereby neural crest shapes the face of each species. This review covers studies on a broad range of vertebrates and describes neural-crest-mediated mechanisms that endow the craniofacial complex with species-specific pattern. A major focus is on experiments in quail and duck embryos that reveal a hierarchy of cell-autonomous and non-autonomous signaling interactions through which neural crest generates species-specific pattern in the craniofacial integument, skeleton, and musculature. By controlling size and shape throughout the development of these systems, the neural crest underlies the structural and functional integration of the craniofacial complex during evolution.

A novel sex-linked mutant affecting tail formation in Hongshan chicken

The Hongshan chicken is a Chinese indigenous breed that has two distinctly different tail types. Some chickens have stunted tails as compared to the normal phenotype, and they are termed rumpless. Rumplessness in other chicken breeds was caused by a reduction in the number of coccygeal vertebrae. However, X-ray examination showed that rumpless Hongshan chickens possess the normal number of coccygeal vertebrae. Our analyses of the main tail feathers and tissue sections led us to speculate that their stunted tail appearance may be the result of abnormal feather development. To investigate the genetic mechanism underlying rumplessness in Hongshan chickens, we analyzed the results of various crosses. The results indicated that rumplessness is a Z-linked dominant character. In addition, we chose some normal and rumpless individuals for pool-sequencing. Nucleotide diversity and Fst were calculated, and a selective sweep was detected on the Z chromosome. These analyses allowed us to reduce the search area to 71.8–72 Mb on the Z chromosome (galGal5.0). A pseudogene LOC431648 located in this region appeared a strong candidate involving in Wnt/β-catenin signaling pathway to regulate feather development in chickens.

Hierarchical patterning modes orchestrate hair follicle morphogenesis

Two theories address the origin of repeating patterns, such as hair follicles, limb digits, and intestinal villi, during development. The Turing reaction-diffusion system posits that interacting diffusible signals produced by static cells first define a prepattern that then induces cell rearrangements to produce an anatomical structure. The second theory, that of mesenchymal self-organisation, proposes that mobile cells can form periodic patterns of cell aggregates directly, without reference to any prepattern. Early hair follicle development is characterised by the rapid appearance of periodic arrangements of altered gene expression in the epidermis and prominent clustering of the adjacent dermal mesenchymal cells. We assess the contributions and interplay between reaction-diffusion and mesenchymal self-organisation processes in hair follicle patterning, identifying a network of fibroblast growth factor (FGF), wingless-related integration site (WNT), and bone morphogenetic protein (BMP) signalling interactions capable of spontaneously producing a periodic pattern. Using time-lapse imaging, we find that mesenchymal cell condensation at hair follicles is locally directed by an epidermal prepattern. However, imposing this prepattern's condition of high FGF and low BMP activity across the entire skin reveals a latent dermal capacity to undergo spatially patterned self-organisation in the absence of epithelial direction. This mesenchymal self-organisation relies on restricted transforming growth factor (TGF) β signalling, which serves to drive chemotactic mesenchymal patterning when reaction-diffusion patterning is suppressed, but, in normal conditions, facilitates cell movement to locally prepatterned sources of FGF. This work illustrates a hierarchy of periodic patterning modes operating in organogenesis.

The anatomical placode in reptile scale morphogenesis indicates shared ancestry among skin appendages in amniotes

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.

Regulatory Differences in Natal Down Development between Altricial Zebra Finch and Precocial Chicken

Birds can be classified into altricial and precocial. The hatchlings of altricial birds are almost naked, whereas those of precocial birds are covered with natal down. This regulatory divergence is thought to reflect environmental adaptation, but the molecular basis of the divergence is unclear. To address this issue, we chose the altricial zebra finch and the precocial chicken as the model animals. We noted that zebra finch hatchlings show natal down growth suppressed anterior dorsal (AD) skin but partially down-covered posterior dorsal (PD) skin. Comparing the transcriptomes of AD and PD skins, we found that the feather growth promoter SHH (sonic hedgehog) was expressed higher in PD skin than in AD skin. Moreover, the data suggested that the FGF (fibroblast growth factor)/Mitogen-activated protein kinase (MAPK) signaling pathway is involved in natal down growth suppression and that FGF16 is a candidate upstream signaling suppressor. Ectopic expression of FGF16 on chicken leg skin showed downregulation of SHH, upregulation of the feather growth suppressor FGF10, and suppression of feather bud elongation, similar to the phenotype found in zebra finch embryonic AD skin. Therefore, we propose that FGF16-related signals suppress natal down elongation and cause the naked AD skin in zebra finch. Our study provides insights into the regulatory divergence in natal down formation between precocial and altricial birds.

Generation of bioengineered feather buds on a reconstructed chick skin from dissociated epithelial and mesenchymal cells

Various kinds of in vitro culture systems of tissues and organs have been developed, and applied to understand multicellular systems during embryonic organogenesis. In the research field of feather bud development, tissue recombination assays using an intact epithelial tissue and mesenchymal tissue/cells have contributed to our understanding the mechanisms of feather bud formation and development. However, there are few methods to generate a skin and its appendages from single cells of both epithelium and mesenchyme. In this study, we have developed a bioengineering method to reconstruct an embryonic dorsal skin after completely dissociating single epithelial and mesenchymal cells from chick skin. Multiple feather buds can form on the reconstructed skin in a single row in vitro. The bioengineered feather buds develop into long feather buds by transplantation onto a chorioallantoic membrane. The bioengineered bud sizes were similar to those of native embryo. The number of bioengineered buds was increased linearly with the initial contact length of epithelial and mesenchymal cell layers where the epithelial-mesenchymal interactions occur. In addition, the bioengineered bud formation was also disturbed by the inhibition of major signaling pathways including FGF (fibroblast growth factor), Wnt/β-catenin, Notch and BMP (bone morphogenetic protein). We expect that our bioengineering technique will motivate further extensive research on multicellular developmental systems, such as the formation and sizing of cutaneous appendages, and their regulatory mechanisms.

Development, regeneration, and evolution of feathers

The feather is a complex ectodermal organ with hierarchical branching patterns. It provides functions in endothermy, communication, and flight. Studies of feather growth, cycling, and health are of fundamental importance to avian biology and poultry science. In addition, feathers are an excellent model for morphogenesis studies because of their accessibility, and their distinct patterns can be used to assay the roles of specific molecular pathways. Here we review the progress in aspects of development, regeneration, and evolution during the past three decades. We cover the development of feather buds in chicken embryos, regenerative cycling of feather follicle stem cells, formation of barb branching patterns, emergence of intrafeather pigmentation patterns, interplay of hormones and feather growth, and the genetic identification of several feather variants. The discovery of feathered dinosaurs redefines the relationship between feathers and birds. Inspiration from biomaterials and flight research further fuels biomimetic potential of feathers as a multidisciplinary research focal point.

The origin and loss of periodic patterning in the turtle shell

The origin of the turtle shell over 200 million years ago greatly modified the amniote body plan, and the morphological plasticity of the shell has promoted the adaptive radiation of turtles. The shell, comprising a dorsal carapace and a ventral plastron, is a layered structure formed by basal endochondral axial skeletal elements (ribs, vertebrae) and plates of bone, which are overlain by keratinous ectodermal scutes. Studies of turtle development have mostly focused on the bones of the shell; however, the genetic regulation of the epidermal scutes has not been investigated. Here, we show that scutes develop from an array of patterned placodes and that these placodes are absent from a soft-shelled turtle in which scutes were lost secondarily. Experimentally inhibiting Shh, Bmp or Fgf signaling results in the disruption of the placodal pattern. Finally, a computational model is used to show how two coupled reaction-diffusion systems reproduce both natural and abnormal variation in turtle scutes. Taken together, these placodal signaling centers are likely to represent developmental modules that are responsible for the evolution of scutes in turtles, and the regulation of these centers has allowed for the diversification of the turtle shell.

The dermal papilla: an instructive niche for epithelial stem and progenitor cells in development and regeneration of the hair follicle

The dermal papilla (DP) of the hair follicle is both a chemical and physical niche for epithelial progenitor cells that regenerate the cycling portion of the hair follicle and generate the hair shaft. Here, we review experiments that revealed the importance of the DP in regulating the characteristics of the hair shaft and frequency of hair follicle regeneration. More recent work showed that the size of this niche is dynamic and actively regulated and reduction in DP cell number per follicle is sufficient to cause hair thinning and loss. The formation of the DP during follicle neogenesis provides a context to contemplate the mechanisms that maintain DP size and the potential to exploit these processes for hair preservation or restoration.

Fgf20 governs formation of primary and secondary dermal condensations in developing hair follicles

In hair follicle development, a placode-derived signal is believed to induce formation of the dermal condensation, an essential component of ectodermal organs. However, the identity of this signal is unknown. Furthermore, although induction and patterning of hair follicles are intimately linked, it is not known whether the mesenchymal condensation is necessary for inducing the initial epithelial pattern. Here, we show that fibroblast growth factor 20 (Fgf20) is expressed in hair placodes and is induced by and functions downstream from epithelial ectodysplasin (Eda)/Edar and Wnt/β-Catenin signaling to initiate formation of the underlying dermal condensation. Fgf20 governs formation of primary and secondary dermal condensations in developing hair follicles and subsequent formation of guard, awl, and auchene hairs. Although primary dermal condensations are absent in Fgf20 mutant mice, a regular array of hair placodes is formed, demonstrating that the epithelial patterning process is independent of known histological and molecular markers of underlying mesenchymal patterns during the initial stages of hair follicle development.

Module-based complexity formation: periodic patterning in feathers and hairs

Patterns describe order which emerges from homogeneity. Complex patterns on the integument are striking because of their visibility throughout an organism’s lifespan. Periodic patterning is an effective design because the ensemble of hair or feather follicles (modules) allows the generation of complexity, including regional variations and cyclic regeneration, giving the skin appendages a new lease on life. Spatial patterns include the arrangements of feathers and hairs in specific number, size, and spacing.We explorehowa field of equivalent progenitor cells can generate periodically arranged modules based on genetic information, physical–chemical rules and developmental timing. Reconstitution experiments suggest a competitive equilibrium regulated by activators/inhibitors involving Turing reaction-diffusion. Temporal patterns result from oscillating stem cell activities within each module (microenvironment regulation), reflected as growth (anagen) and resting (telogen) phases during the cycling of feather and hair follicles. Stimulating modules with activators initiates the spread of regenerative hair waves, while global inhibitors outside each module (macroenvironment) prevent this. Different wave patterns can be simulated by cellular automata principles. Hormonal status and seasonal changes can modulate appendage phenotypes, leading to ‘organ metamorphosis’, with multiple ectodermal organ phenotypes generated from the same precursors. We discuss potential novel evolutionary steps using this module-based complexity in several amniote integument organs, exemplified by the spectacular peacock feather pattern. We thus explore the application of the acquired knowledge of patterning in tissue engineering. New hair follicles can be generated after wounding. Hairs and feathers can be reconstituted through self-organization of dissociated progenitor cells.

Sprouty/FGF signaling regulates the proximal-distal feather morphology and the size of dermal papillae

In a feather, there are distinct morphologies along the proximal-distal axis. The proximal part is a cylindrical stalk (calamus), whereas the distal part has barb and barbule branches. Here we focus on what molecular signaling activity can modulate feather stem cells to generate these distinct morphologies. We demonstrate the drastic tissue remodeling during feather cycling which includes initiation, growth and resting phases. In the growth phase, epithelial components undergo progressive changes from the collar growth zone to the ramogenic zone, to maturing barb branches along the proximal-distal axis. Mesenchymal components also undergo progressive changes from the dermal papilla, to the collar mesenchyme, to the pulp along the proximal-distal axis. Over-expression of Spry4, a negative regulator of receptor tyrosine kinases, promotes barb branch formation at the expense of the epidermal collar. It even induces barb branches from the follicle sheath (equivalent to the outer root sheath in hair follicles). The results are feathers with expanded feather vane regions and small or missing proximal feather shafts (the calamus). Spry4 also expands the pulp region while reducing the size of dermal papillae, leading to a failure to regenerate. In contrast, over-expressing Fgf10 increases the size of the dermal papillae, expands collar epithelia and mesenchyme, but also prevents feather branch formation and feather keratin differentiation. These results suggest that coordinated Sprouty/FGF pathway activity at different stages is important to modulate feather epidermal stem cells to form distinct feather morphologies along the proximal-distal feather axis.

Chicken sine oculis binding protein homolog (sobp), a novel gene that may regulate feather development

Feathers are appendages of skin in avians and provide a model for analyzing skin appendage morphogenesis. The feathers of Chinese Silky (CS) and White Leghorn (WL) chickens have distinct phenotypes. Based on preliminary genetic results, we cloned the chicken gene sobp (encoding sine oculis binding protein homolog, which is expressed at a higher level in the dorsal skin and in the feather follicle in the WL compared with the CS. The reverse-transcription PCR and quantitative real-time PCR indicated that sobp was expressed in many tissues and was continuously expressed during embryonic development in both strains. Northern and Western blotting indicated that the mRNA of sobp was approximately 5 kb and the SOBP protein was approximately 96 kDa. The expressions of signaling molecules that affect feather development were similar. This study is the first report of sobp expression in chickens. Our results suggest that sobp might regulate distinct feather type.

Cryptic patterning of avian skin confers a developmental facility for loss of neck feathering

Vertebrate skin is characterized by its patterned array of appendages, whether feathers, hairs, or scales. In avian skin the distribution of feathers occurs on two distinct spatial levels. Grouping of feathers within discrete tracts, with bare skin lying between the tracts, is termed the macropattern, while the smaller scale periodic spacing between individual feathers is referred to as the micropattern. The degree of integration between the patterning mechanisms that operate on these two scales during development and the mechanisms underlying the remarkable evolvability of skin macropatterns are unknown. A striking example of macropattern variation is the convergent loss of neck feathering in multiple species, a trait associated with heat tolerance in both wild and domestic birds. In chicken, a mutation called Naked neck is characterized by a reduction of body feathering and completely bare neck. Here we perform genetic fine mapping of the causative region and identify a large insertion associated with the Naked neck trait. A strong candidate gene in the critical interval, BMP12/GDF7, displays markedly elevated expression in Naked neck embryonic skin due to a cis-regulatory effect of the causative mutation. BMP family members inhibit embryonic feather formation by acting in a reaction-diffusion mechanism, and we find that selective production of retinoic acid by neck skin potentiates BMP signaling, making neck skin more sensitive than body skin to suppression of feather development. This selective production of retinoic acid by neck skin constitutes a cryptic pattern as its effects on feathering are not revealed until gross BMP levels are altered. This developmental modularity of neck and body skin allows simple quantitative changes in BMP levels to produce a sparsely feathered or bare neck while maintaining robust feather patterning on the body.

Spots and stripes: pleomorphic patterning of stem cells via p-ERK-dependent cell chemotaxis shown by feather morphogenesis and mathematical simulation

A key issue in stem cell biology is the differentiation of homogeneous stem cells towards different fates which are also organized into desired configurations. Little is known about the mechanisms underlying the process of periodic patterning. Feather explants offer a fundamental and testable model in which multi-potential cells are organized into hexagonally arranged primordia and the spacing between primordia. Previous work explored roles of a Turing reaction-diffusion mechanism in establishing chemical patterns. Here we show that a continuum of feather patterns, ranging from stripes to spots, can be obtained when the level of p-ERK activity is adjusted with chemical inhibitors. The patterns are dose-dependent, tissue stage-dependent, and irreversible. Analyses show that ERK activity-dependent mesenchymal cell chemotaxis is essential for converting micro-signaling centers into stable feather primordia. A mathematical model based on short-range activation, long-range inhibition, and cell chemotaxis is developed and shown to simulate observed experimental results. This generic cell behavior model can be applied to model stem cell patterning behavior at large.

BMP2 and BMP7 play antagonistic roles in feather induction

Feathers, like hairs, first appear as primordia consisting of an epidermal placode associated with a dermal condensation that is necessary for the continuation of their differentiation. Previously, the BMPs have been proposed to inhibit skin appendage formation. We show that the function of specific BMPs during feather development is more complex. BMP2 and BMP7, which are expressed in both the epidermis and the dermis, are involved in an antagonistic fashion in regulating the formation of dermal condensations, and thus are both necessary for subsequent feather morphogenesis. BMP7 is expressed earlier and functions as a chemoattractant that recruits cells into the condensation, whereas BMP2 is expressed later, and leads to an arrest of cell migration, likely via its modulation of the EIIIA fibronectin domain and alpha4 integrin expression. Based on the observed cell proliferation, chemotaxis and the timing of BMP2 and BMP7 expression, we propose a mathematical model, a reaction-diffusion system, which not only simulates feather patterning, but which also can account for the negative effects of excess BMP2 or BMP7 on feather formation.

Abortive placode formation in the feather tract of the scaleless chicken embryo

The featherless phenotype of the scaleless mutant provides a model for delineating the process of feather follicle formation. Initial studies established that the mutation affects the epidermis and suggested that epidermis is unable to respond to signals from underlying dermis, or propagate a reciprocal signal. The work presented here demonstrates that scaleless epidermis does indeed respond to the initial inductive signals from dermis, as indicated by the localization of nuclear beta-catenin and transient focal expression of genes expressed in the placode of wild-type feather rudiments. In the sporadic "escaper" feathers that form in scaleless, expression of many genes associated with the progression of feather development is comparable to that in wild-type embryos. An exception is the ectodysplasin receptor gene Edar, which is expressed at lower levels in mutant feather buds. These observations suggest that the scaleless mutation impairs the locally augmented expression of Edar required to stabilize the placodal fate and sustain feather development.

Phospholipase D1 activation through Src and Ras is involved in basic fibroblast growth factor-induced neurite outgrowth of H19-7 cells

Phospholipase D (PLD) is implicated in a variety of physiological processes that reveal it to be a member of the signal transducing phospholipases. We found that PLD1 is activated when basic fibroblast growth factor (bFGF) stimulates neurite outgrowth of an immortalized hippocampal cell line (H19-7). Overexpression of PLD1 in H19-7 cells dramatically elongated bFGF-induced neurite outgrowth and increased PLD activity. Transfection of DN-rPLD1 blocked bFGF-induced PLD activation and completely inhibited neurite outgrowth induced by bFGF, suggesting that PLD1 activation is important in bFGF-induced neurite outgrowth of H19-7 cells. PLD activation and neurite outgrowth induced by bFGF was dependent on phospholipase C gamma (PLC-gamma) and Ca2+, but not protein kinase C (PKC). Furthermore, inhibition of Src and Ras partially blocked bFGF-induced PLD activation and neurite outgrowth, respectively. Coinhibition of Src and Ras completely blocked bFGF-induced PLD activation, suggesting that Src and Ras independently regulate PLD1 activation. Interestingly, bFGF-induced PLD activation and neurite outgrowth did not require ERK1/2 activated by Ras. Taken together, this study demonstrates that bFGF activates PLD1 through PLC-gamma activation, which leads to neurite outgrowth in H19-7 cells. Furthermore, our results show that PLD1 activation by bFGF is regulated by Src and Ras independently.

Molecular signaling in feather morphogenesis

The development and regeneration of feathers have gained much attention recently because of progress in the following areas. First, pattern formation. The exquisite spatial arrangement provides a simple model for decoding the rules of morphogenesis. Second, stem cell biology. In every molting, a few stem cells have to rebuild the entire epithelial organ, providing much to learn on how to regenerate an organ physiologically. Third, evolution and development ('Evo-Devo'). The discovery of feathered dinosaur fossils in China prompted enthusiastic inquiries about the origin and evolution of feathers. Progress has been made in elucidating feather morphogenesis in five successive phases: macro-patterning, micro-patterning, intra-bud morphogenesis, follicle morphogenesis and regenerative cycling.

Syndecan-4 dependent FGF stimulation of mouse vibrissae growth

The development, maintenance and regeneration of epithelial appendages such as hairs or vibrissae depend on reciprocal interactions between the epidermal and the dermal components of the integument. Growth factors are among a number of signaling molecules that have been identified during these developmental events. Growth factors such as fibroblast growth factors (FGFs) bind cell surface heparan sulfate proteoglycans (HSPGs) on their heparan sulfate side chains and as such these proteoglycans act as co-receptors for FGF receptors (FGFRs) by forming a ternary signaling complex of HSPG, FGFR and FGF. The syndecans make up a family (syndecan-1-4) of transmembrane HSPGs. In the present study we examined the growth response of mouse vibrissae to HSPG-binding growth factors as a function of the presence or absence of syndecan-4 in an organ culture system. Syndecan-4 is expressed on keratinocytes that make up the inner root sheath of the vibrissa. Vibrissae from wild-type mice, but not from syndecan-4 null mice, displayed a statistically significant and dose-dependent growth response to FGF-1, FGF-2 and FGF-7. In contrast, a statistically significant growth response is seen in vibrissae from both wild-type and syndecan-4 null mice when the culture medium is supplemented with either hepatocyte growth factor (HGF) that binds to HSPG, insulin that does not bind to HSPG or 5% fetal bovine serum. The syndecan-4 dependent effect of FGF-1, -2 and -7 on the transcriptional activity of IRS expressed genes and of genes involved in cell proliferation reveals a number of different response patterns. In vivo, the vibrissae of syndecan-4 null mice are shorter and have a smaller diameter than those of wild-type mice and this phenotype may result from a suboptimal response to growth factors. Syndecan-1, which is expressed in the outer root sheath of the vibrissae shaft, does not influence the response of the vibrissae to FGF-1, -2 and -7 and the length and diameter of vibrissae of syndecan-1 null mice do not differ from those of wild-type mice.

Syndecans: new kids on the signaling block

Cell-associated proteoglycans provide highly complex and sophisticated systems to control interactions of extracellular cell matrix components and soluble ligands with the cell surface. Syndecans, a conserved family of heparan- and chondroitin-sulfate carrying transmembrane proteins, are emerging as central players in these interactions. Recent studies have demonstrated the essential role of syndecans in modulating cellular signaling in embryonic development, tumorigenesis, and angiogenesis. In this review, we focus on new advances in our understanding of syndecan-mediated cell signaling.

Quail-duck chimeras reveal spatiotemporal plasticity in molecular and histogenic programs of cranial feather development

The avian feather complex represents a vivid example of how a developmental module composed of highly integrated molecular and histogenic programs can become rapidly elaborated during the course of evolution. Mechanisms that facilitate this evolutionary diversification may involve the maintenance of plasticity in developmental processes that underlie feather morphogenesis. Feathers arise as discrete buds of mesenchyme and epithelium, which are two embryonic tissues that respectively form dermis and epidermis of the integument. Epithelial-mesenchymal signaling interactions generate feather buds that are neatly arrayed in space and time. The dermis provides spatiotemporal patterning information to the epidermis but precise cellular and molecular mechanisms for generating species-specific differences in feather pattern remain obscure. In the present study, we exploit the quail-duck chimeric system to test the extent to which the dermis regulates the expression of genes required for feather development. Quail and duck have distinct feather patterns and divergent growth rates, and we exchange pre-migratory neural crest cells destined to form the craniofacial dermis between them. We find that donor dermis induces host epidermis to form feather buds according to the spatial pattern and timetable of the donor species by altering the expression of members and targets of the Bone Morphogenetic Protein, Sonic Hedgehog and Delta/Notch pathways. Overall, we demonstrate that there is a great deal of spatiotemporal plasticity inherent in the molecular and histogenic programs of feather development, a property that may have played a generative and regulatory role throughout the evolution of birds.