The making of a feather: homeoproteins, retinoids and adhesion molecules

Morphogenesis of fungiform papillae in developing miniature pigs

Objective

Fungiform papillae contain taste buds and play a critical role in mastication and the gustatory system. In this study, we report a series of sequential observations of organogenesis of fungiform papillae in miniature pigs, as well as changes in the expression of BMP2, BMP4, Wnt5a, Sox2, and Notch1 signaling pathway components.

Design

In this study, we investigated the spatiotemporal expression patterns of BMP, Wnt, Sox2 and Notch in the fungiform papillae of miniature pigs at the bud stage (E40), cap stage (E50) and bell stage (E60). Pregnant miniature pigs were obtained, and the samples were processed for histological staining. Immunohistochemistry and real-time PCR were used to detect the mRNA and protein expression levels of BMP2, BMP4, Wnt5a, Sox2, and Notch1.

Results

At E40, fungiform papillae were present on the anterior two-thirds of the tongue in a specific array and pattern. The fungiform papillae were enlarged and basically developed at E50 and were largest at the earlier stage (E60). Most of the BMP2 was concentrated in the epithelial layer and the connective tissue core of the fungal papilloma and gradually accumulated from E40-E60. BMP-4 was weakly expressed in the fungiform papillae epithelia, but BMP-4-positive cells were also observed in the developing tongue muscle at E50 and E60. Wnt5a-positive cells were observed in the fungiform papillae epithelia and developing tongue muscle at all three time points. Sox2-positive cells were observed only in fungiform papillae epithelial cells, and Notch1-positive cells could not be detected.

Conclusions

This study provides primary data regarding the morphogenesis and expression of developmental signals in the fungiform papillae of miniature pigs, establishing a foundation for further research in both this model and humans.

Interference with the retinoic acid signalling pathway inhibits the initiation of teeth and caudal primary scales in the small-spotted catshark Scyliorhinus canicula

The retinoic acid (RA) pathway was shown to be important for tooth development in mammals, and suspected to play a key role in tooth evolution in teleosts. The general modalities of development of tooth and "tooth-like" structures (collectively named odontodes) seem to be conserved among all jawed vertebrates, both with regard to histogenesis and genetic regulation. We investigated the putative function of RA signalling in tooth and scale initiation in a cartilaginous fish, the small-spotted catshark Scyliorhinus canicula. To address this issue, we identified the expression pattern of genes from the RA pathway during both tooth and scale development and performed functional experiments by exposing small-spotted catshark embryos to exogenous RA or an inhibitor of RA synthesis. Our results showed that inhibiting RA synthesis affects tooth but not caudal primary scale development while exposure to exogenous RA inhibited both. We also showed that the reduced number of teeth observed with RA exposure is probably due to a specific inhibition of tooth bud initiation while the observed effects of the RA synthesis inhibitor is related to a general delay in embryonic development that interacts with tooth development. This study provides data complementary to previous studies of bony vertebrates and support an involvement of the RA signalling pathway toolkit in odontode initiation in all jawed vertebrates. However, the modalities of RA signalling may vary depending on the target location along the body, and depending on the species lineage.

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.

Regional specific differentiation of integumentary organs: SATB2 is involved in α- and β-keratin gene cluster switching in the chicken

BACKGROUND

Animals develop skin regional specificities to best adapt to their environments. Birds are excellent models in which to study the epigenetic mechanisms that facilitate these adaptions. Patients suffering from SATB2 mutations exhibit multiple defects including ectodermal dysplasia-like changes. The preferential expression of SATB2, a chromatin regulator, in feather-forming compared to scale-forming regions, suggests it functions in regional specification of chicken skin appendages by acting on either differentiation or morphogenesis.

RESULTS

Retrovirus mediated SATB2 misexpression in developing feathers, beaks, and claws causes epidermal differentiation abnormalities (e.g. knobs, plaques) with few organ morphology alterations. Chicken β-keratins are encoded in 5 sub-clusters (Claw, Feather, Feather-like, Scale, and Keratinocyte) on Chromosome 25 and a large Feather keratin cluster on Chromosome 27. Type I and II α-keratin clusters are located on Chromosomes 27 and 33, respectively. Transcriptome analyses showed these keratins (1) are often tuned up or down collectively as a sub-cluster, and (2) these changes occur in a temporo-spatial specific manner.

CONCLUSIONS

These results suggest an organizing role of SATB2 in cluster-level gene co-regulation during skin regional specification.

Genome-Wide Association Study and Transcriptome Differential Expression Analysis of the Feather Rate in Shouguang Chickens

The feather rate phenotype in chicks, including early-feathering and late-feathering phenotypes, are widely used as a sexing system in the poultry industry. The objective of this study was to obtain candidate genes associated with the feather rate in Shouguang chickens. In the present study, we collected 56 blood samples and 12 hair follicle samples of flight feathers from female Shouguang chickens. Then we identified the chromosome region associated with the feather rate by genome-wide association analysis (GWAS). We also performed RNA sequencing and analyzed differentially expressed genes between the early-feathering and late-feathering phenotypes using HISAT2, StringTie, and DESeq2. We identified a genomic region of 10.0–13.0 Mb of chromosome Z, which is statistically associated with the feather rate of Shouguang chickens at one-day old. After RNA sequencing analysis, 342 differentially expressed known genes between the early-feathering (EF) and late-feathering (LF) phenotypes were screened out, which were involved in epithelial cell differentiation, intermediate filament organization, protein serine kinase activity, peptidyl-serine phosphorylation, retinoic acid binding, and so on. The sperm flagellar 2 gene (SPEF2) and prolactin receptor (PRLR) gene were the only two overlapping genes between the results of GWAS and differential expression analysis, which implies that SPEF2 and PRLR are possible candidate genes for the formation of the chicken feathering phenotype in the present study. Our findings help to elucidate the molecular mechanism of the feather rate in chicks.

Turning induced plasticity into refined adaptations during range expansion

Robustness against environmental fluctuations within an adaptive state should preclude exploration of new adaptive states when the environment changes. Here, we study transitions between adaptive associations of feather structure and carotenoid uptake to understand how robustness and evolvability can be reconciled. We show that feather modifications induced by unfamiliar carotenoids during a range expansion are repeatedly converted into precise coadaptations of feather development and carotenoid accommodation as populations persist in a region. We find that this conversion is underlain by a uniform and coordinated increase in the sensitivity of feather development to local carotenoid uptake, indicative of cooption and modification of the homeostatic mechanism that buffers feather growth in the evolution of new adaptations. Stress-buffering mechanisms are well placed to alternate between robustness and evolvability and we suggest that this is particularly evident in adaptations that require close integration between widely fluctuating external inputs and intricate internal structures.

Immunolocalization and phylogenetic profiling of the feather protein with the highest cysteine content

Feathers are the most complex skin appendages of vertebrates. Mature feathers consist of interconnected dead keratinocytes that are filled with heavily cross-linked proteins. Although the molecular architecture determines essential functions of feathers, only few feather proteins have been characterized with regard to their amino acid sequences and evolution. Here, we identify Epidermal Differentiation protein containing DPCC Motifs (EDDM) as a cysteine-rich protein that has co-evolved with other feather proteins. The EDDM gene is located within the avian epidermal differentiation complex (EDC), a cluster of genes that has originated and diversified in amniotes. EDDM shares the exon-intron organization with EDC genes of other amniotes, including humans, and a gene encoding an EDDM-like protein is present in crocodilians, suggesting that avian EDDM arose by sequence modification of an epidermal differentiation gene present in a common ancestor of archosaurs. The EDDM protein contains multiple sequence repeats and a higher number of cysteine residues than any other protein encoded in the EDC. Immunohistochemical analysis of chicken skin and skin appendages showed expression of EDDM in barb and barbules of feathers as well as in the subperiderm on embryonic scutate scales. These results suggest that the diversification and differential expression of EDDM, besides other EDC genes, was instrumental in facilitating the evolution of the most complex molecular architecture of feathers.

Transcription and microRNA Profiling of Cultured Human Tympanic Membrane Epidermal Keratinocytes

The human tympanic membrane (TM) has a thin outer epidermal layer which plays an important role in TM homeostasis and ear health. The specialised cells of the TM epidermis have a different physiology compared to normal skin epidermal keratinocytes, displaying a dynamic and constitutive migration that maintains a clear TM surface and assists in regeneration. Here, we characterise and compare molecular phenotypes in keratinocyte cultures from TM and normal skin. TM keratinocytes were isolated by enzymatic digestion and cultured in vitro. We compared global mRNA and microRNA expression of the cultured cells with that of human epidermal keratinocyte cultures. Genes with either relatively higher or lower expression were analysed further using the biostatistical tools g:Profiler and Ingenuity Pathway Analysis. Approximately 500 genes were found differentially expressed. Gene ontology enrichment and Ingenuity analyses identified cellular migration and closely related biological processes to be the most significant functions of the genes highly expressed in the TM keratinocytes. The genes of low expression showed a marked difference in homeobox (HOX) genes of clusters A and C, giving the TM keratinocytes a strikingly low HOX gene expression profile. An in vitro scratch wound assay showed a more individualised cell movement in cells from the tympanic membrane than normal epidermal keratinocytes. We identified 10 microRNAs with differential expression, several of which can also be linked to regulation of cell migration and expression of HOX genes. Our data provides clues to understanding the specific physiological properties of TM keratinocytes, including candidate genes for constitutive migration, and may thus help focus further research.

Most Colorful Example of Genetic Assimilation? Exploring the Evolutionary Destiny of Recurrent Phenotypic Accommodation

Evolution of adaptation requires both generation of novel phenotypic variation and retention of a locally beneficial subset of this variation. Such retention can be facilitated by genetic assimilation, the accumulation of genetic and molecular mechanisms that stabilize induced phenotypes and assume progressively greater control over their reliable production. A particularly strong inference into genetic assimilation as an evolutionary process requires a system where it is possible to directly evaluate the extent to which an induced phenotype is progressively incorporated into preexisting developmental pathways. Evolution of diet-dependent pigmentation in birds-where external carotenoids are coopted into internal metabolism to a variable degree before being integrated with a feather's developmental processes-provides such an opportunity. Here we combine a metabolic network view of carotenoid evolution with detailed empirical study of feather modifications to show that the effect of physical properties of carotenoids on feather structure depends on their metabolic modification, their environmental recurrence, and biochemical redundancy, as predicted by the genetic assimilation hypothesis. Metabolized carotenoids caused less stochastic variation in feather structure and were more closely integrated with feather growth than were dietary carotenoids of the same molecular weight. These patterns were driven by the recurrence of organism-carotenoid associations: commonly used dietary carotenoids and biochemically redundant derived carotenoids caused less stochastic variation in feather structure than did rarely used or biochemically unique compounds. We discuss implications of genetic assimilation processes for the evolutionary diversification of diet-dependent animal coloration.

A Complex Structural Variation on Chromosome 27 Leads to the Ectopic Expression of HOXB8 and the Muffs and Beard Phenotype in Chickens

Muffs and beard (Mb) is a phenotype in chickens where groups of elongated feathers gather from both sides of the face (muffs) and below the beak (beard). It is an autosomal, incomplete dominant phenotype encoded by the Muffs and beard (Mb) locus. Here we use genome-wide association (GWA) analysis, linkage analysis, Identity-by-Descent (IBD) mapping, array-CGH, genome re-sequencing and expression analysis to show that the Mb allele causing the Mb phenotype is a derived allele where a complex structural variation (SV) on GGA27 leads to an altered expression of the gene HOXB8. This Mb allele was shown to be completely associated with the Mb phenotype in nine other independent Mb chicken breeds. The Mb allele differs from the wild-type mb allele by three duplications, one in tandem and two that are translocated to that of the tandem repeat around 1.70 Mb on GGA27. The duplications contain total seven annotated genes and their expression was tested during distinct stages of Mb morphogenesis. A continuous high ectopic expression of HOXB8 was found in the facial skin of Mb chickens, strongly suggesting that HOXB8 directs this regional feather-development. In conclusion, our results provide an interesting example of how genomic structural rearrangements alter the regulation of genes leading to novel phenotypes. Further, it again illustrates the value of utilizing derived phenotypes in domestic animals to dissect the genetic basis of developmental traits, herein providing novel insights into the likely role of HOXB8 in feather development and differentiation.

Genetic variation is a key part for the study of evolution, development and differentiation. In domestic animals, many breeds display striking phenotypes that differentiate them from their wild ancestors. Several of these have been related to structural variations, including Fibromelanosis and Rose-comb in chickens, Double-muscled and Osteopetrosis in cattle, Cone degeneration in dogs, and White coat color in pigs. The feather is a type of skin appendages that exists in multiple variants on different body parts, and the derived feathering phenotypes in domestic birds are perfect resources to decipher the mechanisms regulating feather development and differentiation. Here we study the genetics of the Muffs and beard trait, a variant that alters the feather development in the facial area of chickens. We show that this phenotype is associated with a genomic structural variant that leads to an ectopic expression of HOXB8 in the facial skin during feather development. This is thus another example of how structural variants in the genome lead to novel, derived phenotypic changes in domestic animals and suggests an important role for HOXB8 in feather development.

Hoxc8 initiates an ectopic mammary program by regulating Fgf10 and Tbx3 expression and Wnt/β-catenin signaling

The role of Hox genes in the formation of cutaneous accessory organs such as hair follicles and mammary glands has proved elusive, a likely consequence of overlapping function and expression among various homeobox factors. Lineage and immunohistochemical analysis of Hoxc8 in mice revealed that this midthoracic Hox gene has transient but strong regional expression in ventrolateral surface ectoderm at E10.5, much earlier than previously reported. Targeted mice were generated to conditionally misexpress Hoxc8 from the Rosa locus using select Cre drivers, which significantly expanded the domain of thoracic identity in mutant embryos. Accompanying this expansion was the induction of paired zones of ectopic mammary development in the cervical region, which generated between three and five pairs of mammary placodes anterior to the first wild-type mammary rudiment. These rudiments expressed the mammary placode markers Wnt10b and Tbx3 and were labeled by antibodies to the mammary mesenchyme markers ERα and androgen receptor. Somitic Fgf10 expression, which is required for normal mammary line formation, was upregulated in mutant cervical somites, and conditional ablation of ectodermal Tbx3 expression eliminated all normally positioned and ectopic mammary placodes. We present evidence that Hoxc8 participates in regulating the initiation stages of mammary placode morphogenesis, and suggest that this and other Hox genes are likely to have important roles during regional specification and initiation of these and other cutaneous accessory organs.

Regionalisation of the skin

The skin displays marked anatomical variation in thickness, colour and in the appendages that it carries. These regional distinctions arise in the embryo, likely founded on a combinatorial positional code of transcription factor expression. Throughout adult life, the skin's distinct anatomy is maintained through both cell autonomous epigenetic processes and by mesenchymal-epithelial induction. Despite the readily apparent anatomical differences in skin characteristics across the body, several fundamental questions regarding how such regional differences first arise and then persist are unresolved. However, it is clear that the skin's positional code is at the molecular level far more detailed than that discernible at the phenotypic level. This provides a latent reservoir of anatomical complexity ready to surface if perturbed by mutation, hormonal changes, ageing or experiment.

Developmental integration of feather growth and pigmentation and its implications for the evolution of diet-derived coloration

Variation in avian coloration is produced by coordinated pigmentation of thousands of growing feathers that vary in shape and size. Although the functional consequences of avian coloration are frequently studied, little is known about its developmental basis, and, specifically, the rules that link feather growth to pigment uptake and synthesis. Here, we combine biochemical, modeling, and morphometric techniques to examine the developmental basis of feather pigmentation in house finches (Carpodacus mexicanus)--a species with extensive variation in both growth dynamics of ornamental feathers and their carotenoid pigmentation. We found that the rate of carotenoid uptake was constant across a wide range of feather sizes and shapes, and the relative pigmented area of feathers was independent of the total amount of deposited carotenoids. Analysis of the developmental linkage of feather growth and pigment uptake showed that the mechanisms behind partitioning the feather into pigmented and nonpigmented parts and the mechanisms regulating carotenoid uptake into growing feathers are partially independent. Carotenoid uptake strongly covaried with early elements of feather differentiation (the barb addition rate and diameter), whereas the pigmented area was most closely associated with the rate of feather growth. We suggest that strong effects of carotenoid uptake on genetically integrated mechanisms of feather growth and differentiation provide a likely route for genetic assimilation of diet-dependent coloration.

In search of the Golden Fleece: unraveling principles of morphogenesis by studying the integrative biology of skin appendages

The mythological story of the Golden Fleece symbolizes the magical regenerative power of skin appendages. Similar to the adventurous pursuit of the Golden Fleece by the multi-talented Argonauts, today we also need an integrated multi-disciplined approach to understand the cellular and molecular processes during development, regeneration and evolution of skin appendages. To this end, we have explored several aspects of skin appendage biology that contribute to the Turing activator/inhibitor model in feather pattern formation, the topo-biological arrangement of stem cells in organ shape determination, the macro-environmental regulation of stem cells in regenerative hair waves, and potential novel molecular pathways in the morphological evolution of feathers. Here we show our current integrative biology efforts to unravel the complex cellular behavior in patterning stem cells and the control of regional specificity in skin appendages. We use feather/scale tissue recombination to demonstrate the timing control of competence and inducibility. Feathers from different body regions are used to study skin regional specificity. Bioinformatic analyses of transcriptome microarrays show the potential involvement of candidate molecular pathways. We further show Hox genes exhibit some region specific expression patterns. To visualize real time events, we applied time-lapse movies, confocal microscopy and multiphoton microscopy to analyze the morphogenesis of cultured embryonic chicken skin explants. These modern imaging technologies reveal unexpectedly complex cellular flow and organization of extracellular matrix molecules in three dimensions. While these approaches are in preliminary stages, this perspective highlights the challenges we face and new integrative tools we will use. Future work will follow these leads to develop a systems biology view and understanding in the morphogenetic principles that govern the development and regeneration of ectodermal organs.

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.

Evolution of hard proteins in the sauropsid integument in relation to the cornification of skin derivatives in amniotes

Hard skin appendages in amniotes comprise scales, feathers and hairs. The cell organization of these appendages probably derived from the localization of specialized areas of dermal-epidermal interaction in the integument. The horny scales and the other derivatives were formed from large areas of dermal-epidermal interaction. The evolution of these skin appendages was characterized by the production of specific coiled-coil keratins and associated proteins in the inter-filament matrix. Unlike mammalian keratin-associated proteins, those of sauropsids contain a double beta-folded sequence of about 20 amino acids, known as the core-box. The core-box shows 60%-95% sequence identity with known reptilian and avian proteins. The core-box determines the polymerization of these proteins into filaments indicated as beta-keratin filaments. The nucleotide and derived amino acid sequences for these sauropsid keratin-associated proteins are presented in conjunction with a hypothesis about their evolution in reptiles-birds compared to mammalian keratin-associated proteins. It is suggested that genes coding for ancestral glycine-serine-rich sequences of alpha-keratins produced a new class of small matrix proteins. In sauropsids, matrix proteins may have originated after mutation and enrichment in proline, probably in a central region of the ancestral protein. This mutation gave rise to the core-box, and other regions of the original protein evolved differently in the various reptilians orders. In lepidosaurians, two main groups, the high glycine proline and the high cysteine proline proteins, were formed. In archosaurians and chelonians two main groups later diversified into the high glycine proline tyrosine, non-feather proteins, and into the glycine-tyrosine-poor group of feather proteins, which evolved in birds. The latter proteins were particularly suited for making the elongated barb/barbule cells of feathers. In therapsids-mammals, mutations of the ancestral proteins formed the high glycine-tyrosine or the high cysteine proteins but no core-box was produced in the matrix proteins of the hard corneous material of mammalian derivatives.

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.

A dermal HOX transcriptional program regulates site-specific epidermal fate

Reciprocal epithelial-mesenchymal interactions shape site-specific development of skin. Here we show that site-specific HOX expression in fibroblasts is cell-autonomous and epigenetically maintained. The distal-specific gene HOXA13 is continually required to maintain the distal-specific transcriptional program in adult fibroblasts, including expression of WNT5A, a morphogen required for distal development. The ability of distal fibroblasts to induce epidermal keratin 9, a distal-specific gene, is abrogated by depletion of HOXA13, but rescued by addition of WNT5A. Thus, maintenance of appropriate HOX transcriptional program in adult fibroblasts may serve as a source of positional memory to differentially pattern the epithelia during homeostasis and regeneration.

Investigation of Feather Follicle Development in Embryonic Geese

In the present study, the process of feather follicle formation in the Zi goose, a Chinese indigenous breed, was investigated during various stages of embryonic development by using a modified histological processing method. The results showed that the feather placodes evolved initially at embryonic day (E) 12 on the spinal feather tract, emerging as symmetrical structures. Sequentially, the buds elongated from E14 to E16 with anterior-posterior and proximal-distal asymmetries, and invaginated to form the primary feather follicles, which were identified to develop the contour feathers or remiges. The remarkable observation at this stage was the formation of the feather follicle wall, which was understood to be the result of the epidermis surrounding the base and further invaginating into the dermis. With the differentiation of the barbule plates, the various types of feathers were determined. We proved that the secondary feather follicles simply had radially symmetrical barb ridges, with much smaller diameters than the primary follicles, and that they developed only downy feathers. The primary and secondary follicles evolved independently of each other and formed ranks in a linear fashion. Moreover, quantitative measurements of the densities of both follicles confirmed that the density of the primary follicles sharply reached the maximum at E18, and then decreased gradually. Coincidentally, the secondary follicles started to increase from the age of E18, and up to E26 the density of the secondary follicles exceeded that of the primary follicles. Each of the primary feather follicles was richly encircled with muscles, which pointed to a quadrangularly arranged network in the dermis. The present work lays the foundation for further study of the cellular and molecular mechanisms of feather follicle morphogenesis in geese.

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.

Distinct mechanisms underlie pattern formation in the skin and skin appendages

Patterns form with the break of homogeneity and lead to the emergence of new structure or arrangement. There are different physiological and pathological mechanisms that lead to the formation of patterns. Here, we first introduce the basics of pattern formation and their possible biological basis. We then discuss different categories of skin patterns and their potential underlying molecular mechanisms. Some patterns, such as the lines of Blaschko and Naevus, are based on cell lineage and genetic mosaicism. Other patterns, such as regionally specific skin appendages, can be set by distinct combinatorial molecular codes, which in turn may be set by morphogenetic gradients. There are also some patterns, such as the arrangement of hair follicles (hair whorls) and fingerprints, which involve genetics as well as stochastic epigenetic events based on physiochemical principles. Many appendage primordia are laid out in developmental waves. In the adult, some patterns, such as those involving cycling hair follicles, may appear as traveling waves in mice. Since skin appendages can renew themselves in regeneration, their size and shape can still change in the adult via regulation by hormones and the environment. Some lesion patterns are based on pathological changes involving the above processes and can be used as diagnostic criteria in medicine. Understanding the different mechanisms that lead to patterns in the skin will help us appreciate their full significance in morphogenesis and medical research. Much remains to be learned about complex pattern formation, if we are to bridge the gap between molecular biology and organism phenotypes.

Evidence that the satin hair mutant gene Foxq1 is among multiple and functionally diverse regulatory targets for Hoxc13 during hair follicle differentiation

It is increasingly evident that the molecular mechanisms underlying hair follicle differentiation and cycling recapitulate principles of embryonic patterning and organ regeneration. Here we used Hoxc13-overexpressing transgenic mice (also known as GC13 mice), known to develop severe hair growth defects and alopecia, as a tool for defining pathways of hair follicle differentiation. Gene array analysis performed with RNA from postnatal skin revealed differential expression of distinct subsets of genes specific for cells of the three major hair shaft compartments (cuticle, cortex, and medulla) and their precursors. This finding correlates well with the structural defects observed in each of these compartments and implicates Hoxc13 in diverse pathways of hair follicle differentiation. The group of medulla-specific genes was particularly intriguing because this included the developmentally regulated transcription factor-encoding gene Foxq1 that is altered in the medulladefective satin mouse hair mutant. We provide evidence that Foxq1 is a downstream target for Hoxc13 based on DNA binding studies as well as co-transfection and chromatin immunoprecipitation assays. Expression of additional medulla-specific genes down-regulated upon overexpression of Hoxc13 requires functional Foxq1 as their expression is ablated in hair follicles of satin mice. Combined, these results demonstrate that Hoxc13 and Foxq1 control medulla differentiation through a common regulatory pathway. The apparent regulatory interactions between members of the mammalian Hox and Fox gene families shown here may establish a paradigm for "cross-talk" between these two conserved regulatory gene families in different developmental contexts including embryonic patterning as well as organ development and renewal.

Vitamin A nutrition of growing cockatiel chicks (Nymphicus hollandicus)

The experiments examined the physiological response of growing cockatiel chicks to varying levels of dietary vitamin A (VA) or beta-carotene and the rate of liver VA uptake. Adult cockatiels breeding pairs (n=10 pairs) were fed a VA-deficient diet for approximately 90 days prior to onset of egg laying. Breeding pairs were then allowed to feed their chicks diets containing either 0 IU VA/kg, 4000 IU VA/kg, or 2.4 mg beta-carotene/kg. After 5 weeks, chicks fed 0 IU VA developed poor feathering, facial dermatitis and reduced body weight (p<0.05). Liver VA was higher in chicks fed 4,000 IU VA or 2.4 mg beta-carotene vs. those fed 0 IU VA (p<0.05). Duodenal beta-actin and 15,15'-dioxygenase mRNA expression was similar to that of growing chickens, and greatest for cockatiel chicks fed 0 IU VA (p<0.01). Chicks fed 0 IU VA had keratinization of the bursa and oral mucosa, and reduced bursa development and lymphocyte density (p<0.05). Finally, when chicks fed 0 IU VA were orally gavaged with 20 IU VA/g body weight, maximal liver retinol uptake occurred between 0 and 24 h and reached a plateau at 36 h. These data demonstrate that VA deficiency can be prevented with 4,000 IU VA/kg diet or 2.4 mg beta-carotene/kg diet, although beta-carotene conversion to VA may be lower in cockatiels than chickens.

Localization and Characterization of Specific Cornification Proteins in Avian Epidermis

Little is known about proteins involved in the formation of the stratum corneum in the avian apteric epidermis. The present immunocytochemical, autoradiographic and electrophoretic study shows that antibodies against characteristic proteins of mammalian cornification (alpha-keratins, loricrin, sciellin, filaggrin, transglutaminase) recognize avian epidermal proteins. This suggests the presence of avian protein with epitopes common to related mammalian proteins. These proteins may also be involved in the formation of the cornified core and cell envelope of mature avian corneocytes. The immunoblotting study suggests that protein bands, cross-reactive for antibodies against loricrin (45, 52-57 kDa), sciellin (54, 84 kDa), filaggrin (32, 38, 45-48 kDa), and transglutaminase (40, 50, 58 kDa), are present in the avian epidermis. Immunocytochemistry shows that immunoreactivity for the above proteins is localized in the transitional and lowermost corneous layer of apteric epidermis. Their epitopes are rapidly masked/altered in cornifying cells and are no longer detectable in mature corneocytes. In scaled epidermis a thick layer made of beta-keratins of 14-18, 20-22, and 33 kDa is formed. Only in feathered epidermis (not in scale epidermis), an antifeather chicken beta-keratin antibody recognized a protein band at 8-12 kDa. This small beta-keratin is probably suitable for the formation of long, axial filaments in elongated barb, barbule and calamus cells. Conversely, the larger beta-keratins in scales are irregularly deposited forming flat plates. Tritiated histidine coupled to autoradiography show an absence of both keratohyalin and histidine-rich proteins in adult feathered and scaled epidermis. Most of the labeling appears in proteins within the range of beta- and alpha-keratins. These data on apteric epidermis support the hypothesis of an evolution of the apteric and interfollicular epidermis from the expansion of hinge regions of protoavian archosaurians.

Evo-Devo of amniote integuments and appendages

Integuments form the boundary between an organism and the environment. The evolution of novel developmental mechanisms in integuments and appendages allows animals to live in diverse ecological environments. Here we focus on amniotes. The major achievement for reptile skin is an adaptation to the land with the formation of a successful barrier. The stratum corneum enables this barrier to prevent water loss from the skin and allowed amphibian / reptile ancestors to go onto the land. Overlapping scales and production of beta-keratins provide strong protection. Epidermal invagination led to the formation of avian feather and mammalian hair follicles in the dermis. Both adopted a proximal - distal growth mode which maintains endothermy. Feathers form hierarchical branches which produce the vane that makes flight possible. Recent discoveries of feathered dinosaurs in China inspire new thinking on the origin of feathers. In the laboratory, epithelial - mesenchymal recombinations and molecular mis-expressions were carried out to test the plasticity of epithelial organ formation. We review the work on the transformation of scales into feathers, conversion between barbs and rachis and the production of "chicken teeth". In mammals, tilting the balance of the BMP pathway in K14 noggin transgenic mice alters the number, size and phenotypes of different ectodermal organs, making investigators rethink the distinction between morpho-regulation and pathological changes. Models on the evolution of feathers and hairs from reptile integuments are discussed. A hypothetical Evo-Devo space where diverse integument appendages can be placed according to complex phenotypes and novel developmental mechanisms is presented.

Distinct Wnt members regulate the hierarchical morphogenesis of skin regions (spinal tract) and individual feathers

Skin morphogenesis occurs in successive stages. First, the skin forms distinct regions (macropatterning). Then skin appendages with particular shapes and sizes form within each region (micropatterning). Ectopic DKK expression inhibited dermis formation in feather tracts and individual buds, implying the importance of Wnts, and prompted the assessment of individual Wnt functions at different morphogenetic levels using the feather model. Wnt 1, 3a, 5a and 11 initially were expressed moderately throughout the feather tract then were up-regulated in restricted regions following two modes: Wnt 1 and 3a became restricted to the placodal epithelium, then to the elongated distal bud epidermis; Wnt 5a and 11 intensified in the inter-tract region and interprimordia epidermis or dermis, respectively, then appeared in the elongated distal bud dermis. Their role in feather tract formation was determined using RCAS mediated misexpression in ovo at E2/E3. Their function in periodic feather patterning was examined by misexpression in vitro using reconstituted E7 skin explant cultures. Wnt 1 reduced spinal tract size, but enhanced feather primordia size. Wnt 3a increased dermal thickness, expanded the spinal tract size, reduced interbud domain spacing, and produced non-tapering "giant buds". Wnt 11 and dominant negative Wnt 1 enhanced interbud spacing, and generated thinner buds. In cultured dermal fibroblasts, Wnt 1 and 3a stimulated cell proliferation and activated the canonical beta-catenin pathway. Wnt 11 inhibited proliferation but stimulated migration. Wnt 5a and 11 triggered the JNK pathway. Thus distinctive Wnts have positive and negative roles in forming the dermis, tracts, interbud spacing and the growth and shaping of individual buds.

Sonic Hedgehog functions by localizing the region of proliferation in early developing feather buds

Feathers are formed following a series of reciprocal signals between the epithelium and the mesenchyme. Initially, the formation of a dense dermis leads to the induction of a placode in the overlying ectoderm. The ectoderm subsequently signals back to the dermis to promote cell division. Sonic Hedgehog (Shh) is a secreted protein expressed in the ectoderm that has previously been implicated in mitogenic and morphogenetic processes throughout feather bud development. We therefore interfered with Shh signaling during early feather bud development and observed a dramatic change in feather form and prominence. Surprisingly, outgrowth did occur and was manifest as irregular, fused, and ectopic feather domains at both molecular and morphological levels. Experiments with Di-I and BrdU indicated that this effect was at least in part caused by the dispersal of previously aggregated proliferating dermal cells. We propose that Shh maintains bud development by localizing the dermal feather progenitors.

Sculpting skin appendages out of epidermal layers via temporally and spatially regulated apoptotic events

Complex skin appendages are built from the epidermal cells through various cell events. Here we used TUNEL and caspase-3 immuno-localization to examine apoptosis in feather morphogenesis. We deduced three modes. In Mode 1A, apoptosis occurs within the localized growth zone (LoGZ) to regulate growth (feather buds). In Mode 1B, morphogen secreting cells are present adjacent to LoGZ and apoptosis may work to remove such signaling centers (barb ridges). In Mode 2, keratinocytes apoptosed before terminal differentiation and left spaces between branches (marginal plate). In Mode 3A, keratinocytes cornified and flaked off to free skin appendages (feather sheath, pulp epithelium). In Mode 3B, keratinized apoptosed epithelial cells became permanent structures (rachis, ramus, barbules). Thus, different apoptotic modes can have different impacts on morphogenesis. We further tested effects of imbalanced Shh on apoptosis. Shh suppression reduced marginal plate apoptosis and caused abnormal differentiation of barbule plates. Shh over-expression enhanced proliferation in barb ridges. Expression of Patched in the barbule plate epithelia implies a paracrine mechanism. The current work complements our recent work on LoGZ to show how adding and removing cell masses in temporally and spatially specific ways are coordinated to sculpt skin appendages from epidermal layers.

EphA4, RhoB and the molecular development of feather buds are maintained by the integrity of the actin cytoskeleton

The development of feather buds is a highly ordered process involving epithelial-mesenchymal signalling. Cellular morphology is determined by the actin cytoskeleton, which is controlled by networks of regulators such as the GTPases. EphA4 belongs to a receptor tyrosine kinase family that has been consistently shown to regulate the cytoskeleton via Rho family GTPases in neural development and is expressed in early stages of feather bud development though its role has not been defined. We therefore used an in vitro skin culture system to interfere with EphA4 levels in feather buds using anti-sense oligonucleotides, demonstrating a severe effect on both their number and morphological form. Analysis of the Rho family of GTPases revealed that this effect was mediated by the GTPase RhoB, the expression of which was altered in response to altered levels of EphA4. In addition, the inhibition of RhoB mimicked the effects of reduced EphA4 levels on feather development. Significantly, manipulation of cytoskeletal dynamics revealed that those cells undergoing morphogenetic change regulate the patterning signals responsible for initiating feather development. We propose that this molecular maintenance mechanism between EphA4-RhoB and the actin cytoskeleton converges or coordinates with other morphogenic signalling systems to control feather bud development.

Dlx genes integrate positive and negative signals during feather bud development

In the embryonic chicken skin, feather buds and the intervening interbud tissue form in a reiterated and sequential pattern that is dependent on interactions between the epidermis and dermis. Feather promoting and inhibiting signals such as fibroblast growth factors (FGF) and bone morphogenetic proteins (BMP), respectively, direct the formation of this periodic pattern. However, the transcription factors that mediate the response to these signals and transmit this information to downstream effector genes are largely unknown. Here we have explored the DLX transcription factors as candidate transcriptional mediators downstream of the described feather patterning signals. We show that several Dlx members are expressed in the dermis and epidermis of the developing feather buds and their expression is induced in embryonic chick skin by the ectopic activation of BMP and FGF signaling. Misexpression of Dlx in the chick skin leads to both feather loss and feather bud fusions, suggesting that DLX proteins play a negative as well as a positive role in feather development. Moreover, DLX regulates the expression of NCAM and tenascin, molecules that are important for feather bud initiation as well as bud outgrowth and morphogenesis. Our results suggest that DLX transcription factors serve to integrate and transduce feather patterning signals to downstream effector molecules.

Morphological evidence of the importance of epithelial tissue during mouse tongue development

The morphogenesis of fungiform papillae occurs in a stereotyped pattern on the dorsal surface of the tongue in mice from embryonic day 12 (E12) to E17. The histological results and ultrastructural observations showed the development of specific structures in the epithelium into fungiform papillae. Prior to the morphological changes, the Bmp-4 and Shh transcripts are expressed in a restricted area on the dorsal surface. These results suggest that the development of fungiform papillae requires an epithelium and mesenchyme interaction during morphogenesis. In order to obtain direct evidence of the epithelium and mesenchyme interaction during tongue papillae morphogenesis, the formation of fungiform papillae was examined after a recombination assay. In order to confirm the epithelium and mesenchyme interactions during the early development of the mouse tongue, a recombination assay was conducted after the recombination assay at E12.5 and E13.5 for 2 days using an in vitro organ culture. From the recombination assay results, the E13.5 epithelial portion of the fungiform papillae could determine the position of the newly formed fungiform papillae with the epithelial signaling molecules. E13.5 was a critical stage for fungiform papillae morphogenesis. Fungiform papillae can be considered to be small epithelial appendages, which are formed via the epithelium and mesenchyme interactions.

Variations in the morphology, distribution, and arrangement of feathers in domesticated birds

Domesticated birds exhibit a greater diversity in the morphology of their integument and its appendages than their wild ancestors. Many of these variations affect the appearance of a bird significantly and have been bred selectively by poultry and pigeon fanciers and aviculturists for the sake of visual appeal. Variations in feather distribution (e.g., feathering of legs and feet, featherless areas in normally feather-bearing skin) are widespread in chickens and pigeons. Variations in the number of feathers (e.g., increased number of tail feathers, lack of tail feathers) occur in certain pigeon and poultry breeds. Variations in feather length can affect certain body regions or the entire plumage. Variations in feather structure (e.g., silkiness, frilled feathering) can be found in exhibition poultry as well as in pet birds. Variations in feather arrangement (e.g., feather crests and vortices) occur in many domesticated bird species as a results of mutation and intense selective breeding. The causes of variations in the structure, distribution, length and arrangement of feathers is often unknown and opens a wide field for scientific research under various points of view (e.g., morphogenesis, pathogenesis, ethology, etc.). To that extent, variations in the morphology, distribution and arrangement of feathers in domesticated birds require also a concern for animal welfare because certain alleles responsible for integumentary variations in domesticated birds have pleiotropic effects, which often affect normal behaviour and viability.

Molecular biology of feather morphogenesis: a testable model for evo-devo research

Darwin's theory describes the principles that are responsible for evolutionary change of organisms and their attributes. The actual mechanisms, however, need to be studied for each species and each organ separately. Here we have investigated the mechanisms underlying these principles in the avian feather. Feathers comprise one of the most complex and diverse epidermal organs as demonstrated by their shape, size, patterned arrangement and pigmentation. Variations can occur at several steps along each level of organization, leading to highly diverse forms and functions. Feathers develop gradually during ontogeny through a series of steps that may correspond to the evolutionary steps that were taken during the phylogeny from a reptilian ancestor to birds. These developmental steps include 1) the formation of feather tract fields on the skin surfaces; 2) periodic patterning of the individual feather primordia within the feather tract fields; 3) feather bud morphogenesis establishing anterio-posterior (along the cranio-caudal axis) and proximo-distal axes; 4) branching morphogenesis to create the rachis, barbs and barbules within a feather bud; and 5) gradual modulations of these basic morphological parameters within a single feather or across a feather tract. Thus, possibilities for variation in form and function of feathers occur at every developmental step. In this paper, principles guiding feather tract formation, distributions of individual feathers within the tracts and variations in feather forms are discussed at a cellular and molecular level.

Development and evolution of the amniote integument: current landscape and future horizon

This special issue on the development and evolution of the amniote integument begins with a discussion of the adaptations to terrestrial conditions, the acquisition of water-impermeability of the reptilian integument, and the initial formation of filamentous integumentary appendages that prepare the way towards avian flight. Recent feather fossils are reviewed, and a definition of feathers is developed. Hierarchical models are proposed for the formation of complex structures, such as feathers. Molecular signals that alter the phenotype of integumentary appendages at different levels of the hierarchy are presented. Tissue interactions and the roles of keratins in evolution are discussed and linked to their bio-mechanical properties. The role of mechanical forces on patterning is explored. Elaborate extant feather variants are introduced. The regeneration/gene mis-expression protocol for the chicken feather is established as a testable model for the study of biological structures. The adaptations of the mammalian distal limb end organs to terrestrial, arboreal and aquatic conditions are discussed. The development and cycling of hair are reviewed from a molecular perspective. These contributions reveal that the structure and function of diverse integumentary appendages are variations that are superimposed on a common theme, and that their formation is modular, hierarchical and cyclical. They further reveal that these mechanisms can be understood at the molecular level, and that an integrative and organismal approach to studying integumentary appendages is called for. We propose that future research should foster interdisciplinary approaches, pursue understanding at the cellular and molecular level, analyze interactions between the environment and genome, and recognize the contributions of variation in morphogenesis and evolution.

Hox in hair growth and development

The evolutionarily conserved Hox gene family of transcriptional regulators has originally been known for specifying positional identities along the longitudinal body axis of bilateral metazoans, including mouse and man. It is believed that subsequent to this archaic role, subsets of Hox genes have been co-opted for patterning functions in phylogenetically more recent structures, such as limbs and epithelial appendages. Among these, the hair follicle is of particular interest, as it is the only organ undergoing cyclical phases of regression and regeneration during the entire life span of an organism. Furthermore, the hair follicle is increasingly capturing the attention of developmental geneticists, as this abundantly available miniature organ mimics key aspects of embryonic patterning and, in addition, presents a model for studying organ renewal. The first Hox gene shown to play a universal role in hair follicle development is Hoxc13, as both Hoxc13-deficient and overexpressing mice exhibit severe hair growth and patterning defects. Differential gene expression analyses in the skin of these mutants, as well as in vitro DNA binding studies performed with potential targets for HOXC13 transcriptional regulation in human hair, identified genes encoding hair-specific keratins and keratin-associated proteins (KAPs) as major groups of presumptive Hoxc13 downstream effectors in the control of hair growth. The Hoxc13 mutant might thus serve as a paradigm for studying hair-specific roles of Hoxc13 and other members of this gene family, whose distinct spatio-temporally restricted expression patterns during hair development and cycling suggest discrete functions in follicular patterning and hair cycle control. The main conclusion from a discussion of these potential roles vis-à-vis current expression data in mouse and man, and from the perspective of the results obtained with the Hoxc13 transgenic models, is that members of the Hox family are likely to fulfill essential roles of great functional diversity in hair that require complex transcriptional control mechanisms to ensure proper spatio-temporal patterns of Hox gene expression at homeostatic levels.

Disruption of sonic hedgehog signaling alters growth and patterning of lingual taste papillae

Taste buds on the anterior part of the tongue develop in conjunction with epithelial-mesenchymal specializations in the form of gustatory (taste) papillae. Sonic hedgehog (Shh) and Bone Morphogenetic Protein 4 (BMP4) are expressed in developing taste papillae, but the roles of these signaling molecules in specification of taste bud progenitors and in papillary morphogenesis are unclear. We show here that BMP4 is not expressed in the early tongue, but is precisely coexpressed with Shh in papillary placodes, which serve as a signaling center for both gustatory and papillary development. To elucidate the role of Shh, we used an in vitro model of mouse fungiform papillary development to determine the effects of two functional inhibitors of Shh signaling: anti-Shh (5E1) antibody and cyclopamine. Cultured E11.5 tongue explants express Shh and BMP4(LacZ) in a pattern similar to that of intact embryos, localizing to developing papillary placodes after 2 days in culture. Tongues cultured with 5E1 antibody continue to express these genes in papillary patterns but develop more papillae that are larger and closer together than in controls. Tongues cultured with cyclopamine have a dose-dependent expansion of Shh and BMP4(LacZ) expression domains. Both antibody-treated and cyclopamine-treated tongue explants also are smaller than controls. Taken together, these results suggest that, although Shh is not involved in the initial specification of papillary placodes, Shh does play two key roles during pmcry development: (1) as a morphogen that directs cells toward a nonpapillary fate, and (2) as a mitogen, causing expansion of the interplacodal epithelium and underlying mesenchyme.

Shift of localized growth zones contributes to skin appendage morphogenesis: role of the Wnt/beta-catenin pathway

Skin appendage formation represents a process of regulated new growth. Bromodeoxyuridine labeling of developing chicken skin demonstrated the presence of localized growth zones, which first promote appendage formation and then move within each appendage to produce specific shapes. Initially, cells proliferate all over the presumptive skin. During the placode stage they are organized to form periodic rings. At the short feather bud stage, the localized growth zones shifted to the posterior and then the distal bud. During the long bud stage, the localized growth zones descended through the flank region toward the feather collar (equivalent to the hair matrix). During feather branch formation, the localized growth zones were positioned periodically in the basilar layer to enhance branching of barb ridges. Wnts were expressed in a dynamic fashion during feather morphogenesis that coincided with the shifting localized growth zones positions. The expression pattern of Wnt 6 was examined and compared with other members of the Wnt pathway. Early in feather development Wnt 6 expression overlapped with the location of the localized growth zones. Its function was tested through misexpression studies. Ectopic Wnt 6 expression produced abnormal localized outgrowths from the skin appendages at either the base, the shaft, or the tip of the developing feathers. Later in feather filament morphogenesis, several Wnt markers were expressed in regions undergoing rearrangements and differentiation of barb ridge keratinocytes. These data suggest that skin appendages are built to specific shapes by adding new cells from well-positioned and controlled localized growth zones and that Wnt activity is involved in regulating such localized growth zone activity.

The morphogenesis of feathers

Feathers are highly ordered, hierarchical branched structures that confer birds with the ability of flight. Discoveries of fossilized dinosaurs in China bearing 'feather-like' structures have prompted interest in the origin and evolution of feathers. However, there is uncertainty about whether the irregularly branched integumentary fibres on dinosaurs such as Sinornithosaurus are truly feathers, and whether an integumentary appendage with a major central shaft and notched edges is a non-avian feather or a proto-feather. Here, we use a developmental approach to analyse molecular mechanisms in feather-branching morphogenesis. We have used the replication-competent avian sarcoma retrovirus to deliver exogenous genes to regenerating flight feather follicles of chickens. We show that the antagonistic balance between noggin and bone morphogenetic protein 4 (BMP4) has a critical role in feather branching, with BMP4 promoting rachis formation and barb fusion, and noggin enhancing rachis and barb branching. Furthermore, we show that sonic hedgehog (Shh) is essential for inducing apoptosis of the marginal plate epithelia, which results in spaces between barbs. Our analyses identify the molecular pathways underlying the topological transformation of feathers from cylindrical epithelia to the hierarchical branched structures, and provide insights on the possible developmental mechanisms in the evolution of feather forms.

The evolutionary origin and diversification of feathers

Progress on the evolutionary origin and diversification of feathers has been hampered by conceptual problems and by the lack of plesiomorphic feather fossils. Recently, both of these limitations have been overcome by the proposal of the developmental theory of the origin of feathers, and the discovery of primitive feather fossils on nonavian theropod dinosaurs. The conceptual problems of previous theories of the origin of feathers are reviewed, and the alternative developmental theory is presented and discussed. The developmental theory proposes that feathers evolved through a series of evolutionary novelties in developmental mechanisms of the follicle and feather germ. The discovery of primitive and derived fossil feathers on a diversity of coelurosaurian theropod dinosaurs documents that feathers evolved and diversified in nonavian theropods before the origin of birds and before the origin of flight. The morphologies of these primitive feathers are congruent with the predictions of the developmental theory. Alternatives to the theropod origin of feathers are critique and rejected. Hypotheses for the initial function of feathers are reviewed. The aerodynamic theory of feather origins is falsified, but many other functions remain developmentally and phylogenetically plausible. Whatever their function, feathers evolved by selection for a follicle that would grow an emergent tubular appendage. Feathers are inherently tubular structures. The homology of feathers and scales is weakly supported. Feathers are composed of a suite of evolutionary novelties that evolved by the duplication, hierarchical organization, interaction, dissociation, and differentiation of morphological modules. The unique capacity for modular subdivision of the tubular feather follicle and germ has fostered the evolution of numerous innovations that characterize feathers. The evolution of feather keratin and the molecular basis of feather development are also discussed.

Keratinization and lipogenesis in epidermal derivatives of the zebrafinch, Taeniopygia guttata castanotis (Aves, Passeriformes, Ploecidae) during embryonic development

Little is known of the lipid content of beta-keratin-producing cells such as those of feathers, scutate scales, and beak. The sequence of epidermal layers in some apteria and in interfollicular epidermis in the zebrafinch embryo (Taeniopygia guttata castanotis) was studied. Also, the production of beta-keratin in natal down feathers and beak was ultrastructurally analyzed in embryos from 3-4 to 17-18 days postdeposition, before hatching. Two layers of periderm initially cover the embryo, but there are eventually 6-8 over the epidermis of the beak. In the beak and sheath cells of feathers, peridermal granules are numerous at 12-14 days postdeposition but they are less frequent in apteria. These granules swell and disappear during sheath or peridermal degeneration at 15-17 days postdeposition. A thin beta-keratin layer forms under the periderm among feather germs of pterylous areas but is discontinuous or disappears in apteria. In differentiating cells of barbs, barbules, and calamus cells of natal down, electron-dense beta-keratin filaments form bundles oriented along the main axis of these cells. Cells of the pulp epidermis and collar, at the base of the follicle, contain lipids and bundles of alpha-keratin filaments. Degenerating pulp cells show vacuolization and nuclear pycnosis. During beta-keratin packing, keratin bundles turn electron-pale, perhaps due to the addition of lipids to produce the final, homogenous beta-keratin matrix. In contrast to the situation in feathers, in the cells of beak beta-keratin packets are irregularly oriented. In both feather and beak epidermal cells the Golgi apparatus and smooth endoplasmic reticulum produce vesicles containing lipid-like material which is also found among forming beta-keratin. The contribution of lipids or lipoprotein to the initial aggregation of beta-keratin molecules is discussed.