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

Cell differentiation in the embryonic periderm and in scaffolding epithelia of skin appendages

Terminal differentiation of epithelial cells is critical for the barrier function of the skin, the growth of skin appendages, such as hair and nails, and the development of the skin of amniotes. Here, we present the hypothesis that the differentiation of cells in the embryonic periderm shares characteristic features with the differentiation of epithelial cells that support the morphogenesis of cornified skin appendages during postnatal life. The periderm prevents aberrant fusion of adjacent epithelial sites during early skin development. It is shed off when keratinocytes of the epidermis form the cornified layer, the stratum corneum. A similar role is played by epithelia that ensheath cornifying skin appendages until they disintegrate to allow the separation of the mature part of the skin appendage from the adjacent tissue. These epithelia, exemplified by the inner root sheath of hair follicles and the epithelia close to the free edge of nails or claws, are referred to as scaffolding epithelia. The periderm and scaffolding epithelia are similar with regard to their transient functions in separating tissues and the conserved expression of trichohyalin and trichohyalin-like genes in mammals and birds. Thus, we propose that parts of the peridermal differentiation program were coopted to a new postnatal function during the evolution of cornified skin appendages in amniotes.

Establishment of a culture model for the prolonged maintenance of chicken feather follicles structure in vitro

Protocols allowing the in vitro culture of human hair follicles in a serum free-medium up to 9 days were developed 30 years ago. By using similar protocols, we achieved the prolonged maintenance in vitro of juvenile feather follicles (FF) microdissected from young chickens. Histology showed a preservation of the FF up to 7 days as well as feather morphology compatible with growth and/or differentiation. The integrity of the FF wall epithelium was confirmed by transmission electron microscopy at Day 5 and 7 of culture. A slight elongation of the feathers was detected up to 5 days for 75% of the examined feathers. By immunochemistry, we demonstrated the maintenance of expression and localization of two structural proteins: scaffoldin and fibronectin. Gene expression (assessed by qRT-PCR) of NCAM, LCAM, Wnt6, Notch1, and BMP4 was not altered. In contrast, Shh and HBS1 expression collapsed, DKK3 increased, and KRT14 transiently increased upon cultivation. This indicates that cultivation modifies the mRNA expression of a few genes, possibly due to reduced growth or cell differentiation in the feather, notably in the barb ridges. In conclusion, we have developed the first method that allows the culture and maintenance of chicken FF in vitro that preserves the structure and biology of the FF close to its in vivo state, despite transcriptional modifications of a few genes involved in feather development. This new culture model may serve to study feather interactions with pathogens or toxics and constitutes a way to reduce animal experimentation.

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).

Review: cornification, morphogenesis and evolution of feathers

Feathers are corneous microramifications of variable complexity derived from the morphogenesis of barb ridges. Histological and ultrastructural analyses on developing and regenerating feathers clarify the three-dimensional organization of cells in barb ridges. Feather cells derive from folds of the embryonic epithelium of feather germs from which barb/barbule cells and supportive cells organize in a branching structure. The following degeneration of supportive cells allows the separation of barbule cells which are made of corneous beta-proteins and of lower amounts of intermediate filament (IF)(alpha) keratins, histidine-rich proteins, and corneous proteins of the epidermal differentiation complex. The specific protein association gives rise to a corneous material with specific biomechanic properties in barbules, rami, rachis, or calamus. During the evolution of different feather types, a large expansion of the genome coding for corneous feather beta-proteins occurred and formed 3-4-nm-thick filaments through a different mechanism from that of 8-10 nm IF keratins. In the chick, over 130 genes mainly localized in chromosomes 27 and 25 encode feather corneous beta-proteins of 10-12 kDa containing 97-105 amino acids. About 35 genes localized in chromosome 25 code for scale proteins (14-16 kDa made of 122-146 amino acids), claws and beak proteins (14-17 kDa proteins of 134-164 amino acids). Feather morphogenesis is periodically re-activated to produce replacement feathers, and multiple feather types can result from the interactions of epidermal and dermal tissues. The review shows schematic models explaining the translation of the morphogenesis of barb ridges present in the follicle into the three-dimensional shape of the main types of branched or un-branched feathers such as plumulaceous, pennaceous, filoplumes, and bristles. The temporal pattern of formation of barb ridges in different feather types and the molecular control from the dermal papilla through signaling molecules are poorly known. The evolution and diversification of the process of morphogenesis of barb ridges and patterns of their formation within feathers follicle allowed the origin and diversification of numerous types of feathers, including the asymmetric planar feathers for flight.

Quantitative characterization and comparative study of feather melanosome internal morphology using surface analysis

A successful feather development implies in a precise orchestration of cells in the follicle, which culminates in one of the most complex epidermal structures in nature. Melanocytes contribute to the final structure by delivering melanosomes to the barb and barbule cells. Disturbance to the tissue during the feather growth can damage the final structure. Here, melanosomes seen in an unusual outgrowth on the barb cortex of a flight feather are reported and compared to commonly observed melanosomes embedded in the cortex. Transmission Electron Microscopy in scanning-transmission mode (STEM) generated images coupled with secondary electron detection. The two classes of melanosomes were registered on images combining transmitted and secondary electron signals. Image processing allowed surface analyses of roughness and texture of the internal morphology of these organelles. Results showed that the two classes of melanosomes are significantly distinct internally, indicating that different physiological processes up to feather maturation could have occurred. Surface analysis methods are not regularly used in cell biology studies, but here it is shown that it has great potential for microscopic image analysis, which could add robust information to studies of cell biology events.

A cis-regulatory mutation of PDSS2 causes silky-feather in chickens

Silky-feather has been selected and fixed in some breeds due to its unique appearance. This phenotype is caused by a single recessive gene (hookless, h). Here we map the silky-feather locus to chromosome 3 by linkage analysis and subsequently fine-map it to an 18.9 kb interval using the identical by descent (IBD) method. Further analysis reveals that a C to G transversion located upstream of the prenyl (decaprenyl) diphosphate synthase, subunit 2 (PDSS2) gene is causing silky-feather. All silky-feather birds are homozygous for the G allele. The silky-feather mutation significantly decreases the expression of PDSS2 during feather development in vivo. Consistent with the regulatory effect, the C to G transversion is shown to remarkably reduce PDSS2 promoter activity in vitro. We report a new example of feather structure variation associated with a spontaneous mutation and provide new insight into the PDSS2 function.

The feather is an excellent model for evolution and development due to its complex structure and vast diversity. Some chickens have silky-feather because of a loss of hooklets in pennaceous feathers, while most chickens have the wild-type normal feather. Hooklets are formed in the last differentiation stage of the life cycle of a pennaceous feather. Chickens with silky-feather are homozygous for a recessive allele (hookless, h). Silkie chicken from China is one of the breeds showing the fascinating silky-feather phenotype and the breed has been known for hundreds of years. In this study, we mapped the silky-feather locus to an 18.9 kb interval and identified a single nucleotide polymorphism (SNP) completely associated with silky-feather. The causative mutation is located 103 base pairs upstream of the coding sequence of prenyl (decaprenyl) diphosphate synthase, subunit 2 (PDSS2). The expression of the PDSS2 gene is decreased in silky-feather skin during feather development in vivo. The silky-feather allele also reduces the PDSS2 promoter activity in vitro. This is the first report of feather structure variation associated with PDSS2 and provides new insight into molecular signaling in the late development stage of feather morphogenesis.

Ultrastructural immunocytochemistry suggests that periderm granules in embryonic chick epidermis contain beta-defensins

The capability to express an innate antimicrobial action through the production of beta-defensins in the skin of chick embryos has been studied by immunocytochemistry. The immunolabeling of periderm granules present in the initial embryonic epidermis of the chick is obtained with a lizard anti-defensin antibody (Ac-BD-15) that recognizes a homologous peptide in the chick beta-defensin 9. Similar granules, termed honeycomb granules, are also stored in some basophilic granulocytes of adult and hatchling chicks, but did not show immunoreactivity to the antibody. It is unclear whether this morphology indicates different organelles, content or lack of available antigens due to organelle packaging. The remarkable ultrastructural similarity between periderm granules and the honeycomb granules present in basophil granulocytes further indicates that embryonic keratinocytes can synthesize antimicrobial peptides together with corneous proteins. These peptides may participate in the formation of an epidermal microbial barrier during ontogenesis or protect the embryo in case of damage to the amniotic-periderm barrier. These transient organelles, apart from the early keratinization of the epidermis that forms the initial barrier before hatching may also contribute to the formation of an efficient anti-microbial barrier against the penetration of microbes during embryonic life until hatching when the adaptive immunity system is still immature.

Cross-immunoreactivity between the LH1 antibody and cytokeratin epitopes in the differentiating epidermis of embryos of the grass snake Natrix natrix L. during the end stages of embryogenesis

The monoclonal anti-cytokeratin 1/10 (LH1) antibody recognizing K1/K10 keratin epitopes that characterizes a keratinized epidermis of mammals cross-reacts with the beta and Oberhäutchen layers covering the scales and gastrosteges of grass snake embryos during the final period of epidermis differentiation. The immunolocalization of the anti-cytokeratin 1/10 (LH1) antibody appears in the beta layer of the epidermis, covering the outer surface of the gastrosteges at the beginning of developmental stage XI, and in the beta layer of the epidermis, covering the outer surface of the scales at the end of developmental stage XI. This antibody cross-reacts with the Oberhäutchen layers in the epidermis covering the outer surface of both scales and gastrosteges at developmental stages XI and XII just before its fusion with the beta layers. After fusion of the Oberhäutchen and beta layers, LH1 immunolabeling is weaker than before. This might suggest that alpha-keratins in these layers of the epidermis are masked by beta-keratins, modified, or degraded. The anti-cytokeratin 1/10 (LH1) antibody stains the Oberhäutchen layer in the epidermis covering the inner surface of the gastrosteges and the hinge regions between gastrosteges at the end of developmental stage XI. However, the Oberhäutchen of the epidermis covering the inner surfaces of the scales and the hinge regions between scales does not show cytokeratin 1/10 (LH1) immunolabeling until hatching. This cross-reactivity suggests that the beta and Oberhäutchen layers probably contain some alpha-keratins that react with the LH1 antibody. It is possible that these alpha-keratins create specific scaffolding for the latest beta-keratin deposition. It is also possible that the LH1 antibody cross-reacts with other epidermal proteins such as filament-associated proteins, i.e., filaggrin-like. The anti-cytokeratin 1/10 (LH1) antibody does not stain the alpha and mesos layers until hatching. We suppose that the differentiation of these layers will begin just after the first postnatal sloughing.

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.

Selective biodegradation of keratin matrix in feather rachis reveals classic bioengineering

Flight necessitates that the feather rachis is extremely tough and light. Yet, the crucial filamentous hierarchy of the rachis is unknown-study hindered by the tight chemical bonding between the filaments and matrix. We used novel microbial biodegradation to delineate the fibres of the rachidial cortex in situ. It revealed the thickest keratin filaments known to date (factor >10), approximately 6 microm thick, extending predominantly axially but with a small outer circumferential component. Near-periodic thickened nodes of the fibres are staggered with those in adjacent fibres in two- and three-dimensional planes, creating a fibre-matrix texture with high attributes for crack stopping and resistance to transverse cutting. Close association of the fibre layer with the underlying 'spongy' medulloid pith indicates the potential for higher buckling loads and greater elastic recoil. Strikingly, the fibres are similar in dimensions and form to the free filaments of the feather vane and plumulaceous and embryonic down, the syncitial barbules, but, identified for the first time in 140+ years of study in a new location-as a major structural component of the rachis. Early in feather evolution, syncitial barbules were consolidated in a robust central rachis, definitively characterizing the avian lineage of keratin.