Cell structure of developing downfeathers in the zebrafinch with emphasis on barb ridge morphogenesis

Identification of pennaceous barbule cell factor (PBCF), a novel gene with spatiotemporal expression in barbule cells during feather development

Bird contour feathers exhibit a complex hierarchical structure composed of a rachis, barbs, and barbules, with barbules playing a crucial role in maintaining feather structure and function. Understanding the molecular mechanisms underlying barbule formation is essential for advancing our knowledge of avian biology and evolution. In this study, we identified a novel gene, pennaceous barbule cell factor (PBCF), using microarray analysis, RT-PCR, and in situ hybridization. PBCF is expressed in barbule cells adjacent to the ramus during pennaceous barbule formation, where these cells fuse with the ramus to establish the feather's branching structure. PBCF expression occurs transiently after melanin pigmentation of the barbule plates but before the expression of barbule-specific keratin 1 (BlSK1). Orthologues of PBCF, predicted to be secreted proteins, are conserved across avian species, with potential homologues detected in reptiles, suggesting an evolutionary lineage-specific adaptation. Additionally, PBCF is expressed in non-vacuolated notochord cells and the extra-embryonic ectoderm of the yolk sac, hinting at its broader developmental significance. The PBCF gene produces two mRNA isoforms via alternative splicing, encoding a secreted protein and a glycophosphatidylinositol (GPI)-anchored membrane-bound protein, indicating functional versatility. These findings suggest that PBCF may be involved as an avian-specific extracellular matrix component in cell adhesion and/or communication, potentially contributing to both feather development and embryogenesis. Further investigation of PBCF's role in feather evolution and its potential functions in other vertebrates could provide new insights into the interplay between development and evolution.

Transcriptome profiling of morphogenetic differences between contour and flight feathers in duck

1. This study examined the transcriptomic profiles of contour and flight feather follicles from two duck breeds to determine the molecular network and the candidate genes associated with contour and flight feather morphogenesis.2. High-throughput RNA sequencing was performed to compare differences in feather follicles between contour and flight feathers in two duck breeds (Heiwu and Nonghua duck).3. Comparing the contour feather follicles with flight feather follicles, 4,757 and 4,820 differentially expressed genes (DEGs) were identified in Heiwu and Nonghua duck respectively. Weighted gene co-expression network analysis (WGCNA) was used to construct a gene co-expression network of all DEGs and identify the key modules and hub genes associated with feather morphogenesis.4. Two key modules were enriched in many pathways involved in feather morphogenesis, such as the Wnt signalling pathway, anatomical structure morphogenesis, and focal adhesion. The CCNA2, TTK, NUF2, ECT2 and INCENP (in one module), and PRSS23, LAMC1, IGFBP3, SHISA5, and APLP2 (in another module) may be essential candidate genes for influencing feather morphology. Moreover, seven transcription factors (TFs) (UBP1, MBD2, ZNF512B, SMAD1, CAPN15, JDP2, KLF10, and MEF2A) were predicted to regulate the essential genes that contribute to feather morphogenesis.5. This work demonstrated gene expression changes of contour and flight feather follicles and is beneficial for further understanding of the complex structure of feathers.

Detection of endogenous lipids in chicken feathers distinct from preen gland constituents

Bird feather lipids are usually attributed to the oily secretion product of the uropygial (preen) gland. We have observed, however, that feathers exhibit a strong reaction with osmium tetroxide (OsO4), even after treatment with detergents. This leads us to postulate the existence of endogenous feather lipids distinct from preen gland lipids. In order to substantiate our hypothesis, we investigated down feathers from a 1-day-old chicken as their uropgygial gland is not functionally active. The results confirmed the osmiophilic reaction, which was concentrated in the center of barbs and strongly reduced after lipid extraction. In these lipid extracts, we identified using thin layer chromatography, cholesterol, various ceramides, glycolipids, phospholipids, and fatty acids, which closely resembled the lipid composition of the water barrier in the chicken-cornified epidermal envelope. This composition is clearly distinct from chicken uropygeal gland secretion (UGS) known to consist of fatty alcohols as part of aliphatic monoester waxes and of free, predominantly saturated, fatty acids. A filter assay showed a strong reactivity between OsO4 and the fatty acids C18:1 and C18:2 and with feather lipid extracts, but not with UGS. These observations were confirmed by gas chromatography detecting unsaturated fatty acids including C18:1 and C18:2 as well as cholesterol exclusively in chicken feathers. Our results indicate that (1) endogenous lipids are detectable in chicken feathers and distinct from UGS and (2) in analogy to the morphogenesis of the cornified envelope of chicken feather lipids that may have derived from cellular feather-precursors, apparently enduring the specific cell death during developmental feather cornification.

Investigation of feather follicle morphogenesis and the expression of the Wnt/β-catenin signaling pathway in yellow-feathered broiler chick embryos

1. This study investigated the pattern of feather follicle morphogenesis and the expression of the Wnt/β-catenin signalling pathway in the skin of yellow-feathered broiler chick embryos during feather development, using haematoxylin and eosin (H&E) staining and Western blot assays, respectively. 2. The results showed that the skin displayed protrusions during embryonic days E7-E9, feather buds elongated during E10-E11 with anterior-posterior and proximal-distal asymmetries, and the epidermis invaginated to form the primary feather follicles (Pfs) at E12. At E13, the formation of the feather follicle and the epidermis at the base of the feather bud further invaginated into the dermis. By E15, Pf formation was essentially complete, and secondary feather follicles (Sfs) appeared. It was speculated that Pfs and Sfs developed independently and that Pfs occurred earlier than Sfs. 3. Quantitative measurements of Pf density reached a maximum at E15 and then decreased gradually. Sf density started to increase from E15. 4. Protein expression levels of β-catenin, TCF4, cyclin D1, and c-Myc were significantly increased during E8-E12 (P < 0.05) and then decreased from E13 to the day of hatching (DOH) (P < 0.05). The result of the β-catenin immunolocalisation signal intensity assay was consistent with the result of the Western blot assay. 5. Collectively, the results indicated that the Wnt/β-catenin signalling pathway is essential for promoting the development of feather follicles, especially during E7-E15.

Transmission electron microscopic and immunohistochemical observations of resting follicles of feathers in chicken show massive cell degeneration

The molting cycle of feathers includes an anagen (growth) stage, a likely catagen stage where the feather follicles degenerate, and a resting stage where fully grown feathers remain in their follicles and are functional before molting. However, the cytological changes involved in the resting and molting stages are poorly known, so the results of an ultrastructural analysis of these processes in adult chick feathers are presented here. The study showed that the dermal papilla shrinks, and numerous cells present increased heterochromatin and free collagen fibrils in the extracellular matrix. Degeneration of the germinal epithelium of the follicle-the papillary collar-occurs with an initial substantial contraction of cells followed by an increase in heterochromatin, vesicle and lipid accumulation, and membrane and organelle degeneration. Desmosomes are still present between degenerating epithelial cells, but ribosomes and tonofilaments disappear. This suggests that cell necrosis initially proceeds as a major contraction resembling apoptosis-a process termed necroptosis, which was previously also shown to occur during the formation of barbs and barbules in mature down and pennaceous feathers. This study suggests that, aside from apoptosis, the collar epithelium degenerates due to external factors, in particular the retraction of blood vessels supplying the dermal papilla. In contrast, revascularization of the dermal papilla triggers a new phase of feather growth (anagen).

Convergent evolution of cysteine-rich proteins in feathers and hair

Background

Feathers and hair consist of cornified epidermal keratinocytes in which proteins are crosslinked via disulfide bonds between cysteine residues of structural proteins to establish mechanical resilience. Cysteine-rich keratin-associated proteins (KRTAPs) are important components of hair whereas the molecular components of feathers have remained incompletely known. Recently, we have identified a chicken gene, named epidermal differentiation cysteine-rich protein (EDCRP), that encodes a protein with a cysteine content of 36%. Here we have investigated the putative role of EDCRP in the molecular architecture and evolution of feathers.

Results

Comparative genomics showed that the presence of an EDCRP gene and the high cysteine content of the encoded proteins are conserved among birds. Avian EDCRPs contain a species-specific number of sequence repeats with the consensus sequence CCDPCQ(K/Q)(S/P)V, thus resembling mammalian cysteine-rich KRTAPs which also contain sequence repeats of similar sequence. However, differences in gene loci and exon-intron structures suggest that EDCRP and KRTAPs have not evolved from a common gene ancestor but represent the products of convergent sequence evolution. mRNA in situ hybridization demonstrated that chicken EDCRP is expressed in the subperiderm layer of the embryonic epidermis and in the barbule cells of growing feathers. This expression pattern supports the hypothesis that feathers are evolutionarily derived from the subperiderm.

Conclusions

The results of this study suggest that convergent sequence evolution of avian EDCRP and mammalian KRTAPs has contributed to independent evolution of feathers and hair, respectively.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-015-0360-y) contains supplementary material, which is available to authorized users.

Dynamic evolution of the alpha (α) and beta (β) keratins has accompanied integument diversification and the adaptation of birds into novel lifestyles

BACKGROUND

Vertebrate skin appendages are constructed of keratins produced by multigene families. Alpha (α) keratins are found in all vertebrates, while beta (β) keratins are found exclusively in reptiles and birds. We have studied the molecular evolution of these gene families in the genomes of 48 phylogenetically diverse birds and their expression in the scales and feathers of the chicken.

RESULTS

We found that the total number of α-keratins is lower in birds than mammals and non-avian reptiles, yet two α-keratin genes (KRT42 and KRT75) have expanded in birds. The β-keratins, however, demonstrate a dynamic evolution associated with avian lifestyle. The avian specific feather β-keratins comprise a large majority of the total number of β-keratins, but independently derived lineages of aquatic and predatory birds have smaller proportions of feather β-keratin genes and larger proportions of keratinocyte β-keratin genes. Additionally, birds of prey have a larger proportion of claw β-keratins. Analysis of α- and β-keratin expression during development of chicken scales and feathers demonstrates that while α-keratins are expressed in these tissues, the number and magnitude of expressed β-keratin genes far exceeds that of α-keratins.

CONCLUSIONS

These results support the view that the number of α- and β-keratin genes expressed, the proportion of the β-keratin subfamily genes expressed and the diversification of the β-keratin genes have been important for the evolution of the feather and the adaptation of birds into multiple ecological niches.

Investigation of characteristics of feather follicle stem cells and their regeneration potential

Feather follicles have the extraordinary ability to regenerate and undergo molting cycles. Being tissue-specific stem cells, feather follicle stem cells (FFSCs) have a strong capacity for proliferation and are presumed to be progenitor cells for various epidermal organs. In order to characterize FFSCs and to understand how the feather epidermis and FFSCs produce such a reliable differentiation program resulting in the formation of complex feathers, We developed a culture scheme to select and expand FFSCs from chick feather follicles. FFSCs were examined with cell profiles, mutilpotential differentiation and immunocytochemical staining. FFSCs from a single clone were capable of self-renewal and proliferation. These cells expressed integrin β1, CD49c, cytokeratin 15 (K15), cytokeratin 19 (K19) and a neural-genic cell marker, nestin, but not a teminal differentiation-related keratinocyte marker, cytokeratin 10 (K10). FFSCs could trans-differentiate into adipocytes, neurocytes and keratinocytes. The formation of micro-feather like structures ex-vivo also revealed the potential of regeneration. These results demonstrate that FFSCs possess the properties of stem/progenitor cells and may therefore serve as a useful model for studying mechanisms of stem cell differentiation and their involvement in organ regeneration.

Interpopulation variation in contour feather structure is environmentally determined in great tits

Background

The plumage of birds is important for flying, insulation and social communication. Contour feathers cover most of the avian body and among other functions they provide a critical insulation layer against heat loss. Feather structure and composition are known to vary among individuals, which in turn determines variation in the insulation properties of the feather. However, the extent and the proximate mechanisms underlying this variation remain unexplored.

Methodology/Principal Findings

We analyzed contour feather structure from two different great tit populations adapted to different winter regimes, one northern population in Oulu (Finland) and one southern population in Lund (Sweden). Great tits from the two populations differed significantly in feather structure. Birds from the northern population had a denser plumage but consisting of shorter feathers with a smaller proportion containing plumulaceous barbs, compared with conspecifics from the southern population. However, differences disappeared when birds originating from the two populations were raised and moulted in identical conditions in a common-garden experiment located in Oulu, under ad libitum nutritional conditions. All birds raised in the aviaries, including adult foster parents moulting in the same captive conditions, developed a similar feather structure. These feathers were different from that of wild birds in Oulu but similar to wild birds in Lund, the latter moulting in more benign conditions than those of Oulu.

Conclusions/Significance

Wild populations exposed to different conditions develop contour feather differences either due to plastic responses or constraints. Environmental conditions, such as nutrient availability during feather growth play a crucial role in determining such differences in plumage structure among populations.

Gap and tight junctions in the formation of feather branches: A descriptive ultrastructural study

The present study has focused on the distribution and ultrastructure of gap and tight junctions responsible for the formation of the barb/barbule branching in developing feathers using immunocytochemical detection. Apart from desmosomes, both tight and gap junctions are present between differentiating barb/barbule cells and during keratinization. While gap junctions are rare along the perimeter of these cells, tight junctions tend to remain localized in nodes joining barbule cells and between barb cells of the ramus. Occludin and connexin-26 but not connexin-43 have been detected between barb medullary, barb cortical and barbule cells during formation of barbs. Gap junctions are present in supportive cells located in the vicinity of barbule cells and destined to degenerate, but no close junctions are present between supportive and barb/barbule cells. Close junctions mature into penta-laminar junctions that are present between mature barb/barbule cells. Immunolabeling for occludin and Cx26 is rare along these cornified junctions. The junctions allow barb/barbule cells to remain connected until feather-keratin form the mature corneous syncytium that constitutes the barbs. A discussion of the role of gap and tight junctions during feather morphogenesis is presented.

Genomic organization and molecular phylogenies of the beta (beta) keratin multigene family in the chicken (Gallus gallus) and zebra finch (Taeniopygia guttata): implications for feather evolution

BACKGROUND

The epidermal appendages of reptiles and birds are constructed of beta (beta) keratins. The molecular phylogeny of these keratins is important to understanding the evolutionary origin of these appendages, especially feathers. Knowing that the crocodilian beta-keratin genes are closely related to those of birds, the published genomes of the chicken and zebra finch provide an opportunity not only to compare the genomic organization of their beta-keratins, but to study their molecular evolution in archosaurians.

RESULTS

The subfamilies (claw, feather, feather-like, and scale) of beta-keratin genes are clustered in the same 5' to 3' order on microchromosome 25 in chicken and zebra finch, although the number of claw and feather genes differs between the species. Molecular phylogenies show that the monophyletic scale genes are the basal group within birds and that the monophyletic avian claw genes form the basal group to all feather and feather-like genes. Both species have a number of feather clades on microchromosome 27 that form monophyletic groups. An additional monophyletic cluster of feather genes exist on macrochromosome 2 for each species. Expression sequence tag analysis for the chicken demonstrates that all feather beta-keratin clades are expressed.

CONCLUSIONS

Similarity in the overall genomic organization of beta-keratins in Galliformes and Passeriformes suggests similar organization in all Neognathae birds, and perhaps in the ancestral lineages leading to modern birds, such as the paravian Anchiornis huxleyi. Phylogenetic analyses demonstrate that evolution of archosaurian epidermal appendages in the lineage leading to birds was accompanied by duplication and divergence of an ancestral beta-keratin gene cluster. As morphological diversification of epidermal appendages occurred and the beta-keratin multigene family expanded, novel beta-keratin genes were selected for novel functions within appendages such as feathers.

Hard cornification in reptilian epidermis in comparison to cornification in mammalian epidermis

The structure of reptilian hard (beta)-keratins, their nucleotide and amino acid sequence, and the organization of their genes are presented. These 13-19 kDa proteins are basic, rich in glycine, proline and serine, and different from cytokeratins. Their mRNAs are expressed in beta-cells. The central part of beta-keratins (this region has been previously termed 'core-box' and is peculiar of all sauropsid proteins) is composed of two beta-folded regions and shows a high identity with avian beta-keratins. This central part present in all beta-keratins, including feather keratins, is the site of polymerization to build the framework of beta-keratin filaments. Beta-keratins appear cytokeratin-associated proteins. Their central region might have originated in an ancestral glycine-rich protein present in stem reptiles from which beta-keratins evolved and diversified into reptiles and birds. Stem reptiles of the Carboniferous period might have possessed glycine-rich proteins derived from exons/domains corresponding to the variable, glycine-rich region of cytokeratins. Beta-keratins might have derived from a gene coding for small glycine-rich keratin-associated proteins. The glycine-rich regions evolved differently in the lineage leading to modern reptiles and birds versus that leading to mammals. In the reptilian lineage some amino acid regions produced by point mutations and amino acid changes might have given rise to originate the central beta-pleated region. The latter allowed the formation of filamentous proteins (beta-keratins) associated with intermediate filament keratins and replaced them in beta-keratin cells. In the mammalian lineage no beta-pleated region was generated in their matrix proteins, the glycine-rich keratin-associated proteins. The latter evolved as glycine-tyrosine-rich, sulphur-rich, and ultra-sulphur-rich proteins that are used for building hairs, horns and nails.

Wedge cells during regeneration of juvenile and adult feathers and their role in carving out the branching pattern of barbs

The present ultrastructural study on regenerating feathers emphasizes the role of supportive cells in determining the branching pattern of barbs. Supportive cells are localized among developing barb and barbule cells, in marginal plates, and underneath the feather sheath, and their differentiative fate, in general, is a form of lipid degeneration. The Latter process determines the carving out of barb branching in both downfeathers and pennaceous feathers. In the latter feathers, some supportive cells (barb vane cells and cylindrical cells of marginal plates) degenerate within each barb ridge leaving separate barbules. Other supportive cells, here termed wedge cells, form columns of cornified material that merge into elongated corneous scaffolds localized among barbs and the rachis. This previously undescribed form of cornification of supportive cells derives from the aggregation of periderm and dense granules present in wedge cells. The latter cells give origin to a corneous material different from feather keratin that may initially sustain the early and soft barbules. After barbules are cornified the supportive cells scaffolds are eventually sloughed as the sheath breaks allowing the new feather to open up and form a planar vane. The corneous material of wedge cells may also contribute to molding of the overlapped nodes of barbule cells that form lateral spines or hooklets in mature barbules. Eventually, the disappearance of wedge cell scaffolding determines the regular spacing of barbs attached to the rachis in order to form a close vane.

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.