Keratin synthesis during development of the embryonic chick feather

Immunolocalization of epidermal differentiation complex proteins reveals distinct molecular compositions of cells that control structure and mechanical properties of avian skin appendages

The epidermal differentiation complex (EDC) is a cluster of genes that encode structural proteins of skin derivatives with variable mechanical performances, from the scales of reptiles and birds to the hard claws and beaks, and to the flexible but resistant corneous material of feathers. Corneous proteins with or without extended beta-regions are produced from avian genomes, and include the largely prevalent corneous beta proteins (CβPs, formerly indicated as beta-keratins), and minor contribution from histidine-rich proteins, trichohyalin-like proteins (scaffoldin), loricrin, and other proteins rich in cysteine or other types of amino acids. The light-microscopic and ultrastructural immunolocalization of major and minor EDC-proteins in avian skin (feather CβPs, EDKM, EDWM, EDMTFH, EDDM, and scaffoldin) suggests that each specific appendage consists of a particular mix of these proteins in addition to the main proteins containing a peculiar beta-region of 34 amino acids, indicated as feather/scale/claw/beak CβPs (fCβPs, sCβPs, cCβPs, bCβPs). This indicates that numerous proteins of the EDC are added to the variable meshwork of intermediate filament keratins to produce avian epidermis with different mechanical and functional properties. Although the specific roles for these proteins are not known they likely make an important contribution to the final material properties of the different skin appendages of birds. The highest number of sauropsid CβPs is found in birds, suggesting a relation to the evolution of feathers, and additional epidermal differentiation proteins have contributed to the evolutionary adaptations of avian skin.

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.

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.

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.

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

The present ultrastructural and immunocytochemical study on the embryonic feathers of the zebrafinch, an altricial passerine bird, describes cellular differentiation of developing downfeathers. Barb ridges are folds of the original epidermis of the embryonic feather germ in which the basal-apical polarity of epidermal cells is upset. The result is the loss of most germinal activity of basal cells of the barb ridges so that only the embryonic epidermal layers remain. The more external layer is the primary periderm, followed by 4-6 layers of inner-periderm cells that mature into feather sheath and barb vane ridge cells. The following layer, the subperiderm, produces a small type of beta-keratin typical of feathers. In barb ridges, the subperiderm layer is displaced to form barbule plates and barb cells. The formation of branching barbules occurs by the presence of barb vane ridge cells that function as spacers between barbule cells. The fourth layer is homologous to the germinal layer of the epidermis, but in barb ridges it rapidly loses the germinal capability and becomes the cyclindrical layer of marginal plates. The study indicates that a necrotic process determines the carving out of the final feather shape, although apoptosis may also play a role. In fact, after barb and barbule cells have formed a keratinized syncitium, retraction of the vascular bed determines anoxia with the resultant necrosis of all feather cells. Only those of the keratinized syncitium remain to form the feather while supportive cells disappear. The sheath covering the barb and barbule syncitium is lost by the formation of a sloughing layer following degeneration of external barb ridge vane cells and loss of the sheath. It is proposed that the evolution of the morphogenetic process of barb ridge formation was peculiar to tubular outgrowths of the integument of archosaurian reptiles that evolved into birds. Once established in the embryonic programmes of skin morphogenesis of ancient birds, variations in the process of barb ridge morphogenesis allowed the fusion of ridges into large or branched ridges that originated the rachis. This process produced pennaceous feathers, among which were those later used for flight. The present study stresses that the morphogenetic process of barb ridge formation determines the concomitant appearance of barbs and barbules. As a consequence, intermediate forms of evolving feathers with only barbs but not barbules are unlikely or are derived from alteration of the above basic morphogenetic mechanism.

Isolation of a mRNA encoding a glycine-proline-rich beta-keratin expressed in the regenerating epidermis of lizard

During scale regeneration in lizard tail, an active differentiation of beta-keratin synthesizing cells occurs. The cDNA and amino acid sequence of a lizard beta-keratin has been obtained from mRNA isolated from regenerating epidermis. Degenerate oligonucleotides, selected from the translated amino acid sequence of a lizard claw protein, were used to amplify a specific lizard keratin cDNA fragment from the mRNA after reverse transcription with poly dT primer and subsequent polymerase chain reaction (3'-rapid amplification of cDNA ends analysis, 3'-RACE). The new sequence was used to design specific primers to obtain the complete cDNA sequence by 5'-RACE. The 835-nucleotide cDNA sequence encodes a glycine-proline-rich protein containing 163 amino acids with a molecular mass of 15.5 kDa; 4.3% of its amino acids is represented by cysteine, 4.9% by tyrosine, 8.0% by proline, and 29.4% by glycine. Tyrosine is linked to glycine, and proline is present mainly in the central region of the protein. Repeated glycine-glycine-X and glycine-X amino acid sequences are localized near the N-amino and C-terminal regions. The protein has the central amino acid region similar to that of claw-feather, whereas the head and tail regions are similar to glycine-tyrosine-rich proteins of mammalian hairs. In situ hybridization analysis at light and electron microscope reveals that the corresponding mRNA is expressed in cells of the differentiating beta-layers of the regenerating scales. The synthesis of beta-keratin from its mRNA occurs among ribosomes or is associated with the surface of beta-keratin filaments.

Scale keratin in lizard epidermis reveals amino acid regions homologous with avian and mammalian epidermal proteins

Small proteins termed beta-keratins constitute the hard corneous material of reptilian scales. In order to study the cell site of synthesis of beta-keratin, an antiserum against a lizard beta-keratin of 15-16 kDa has been produced. The antiserum recognizes beta-cells of lizard epidermis and labels beta-keratin filaments using immunocytochemistry and immunoblotting. In situ hybridization using a cDNA-probe for a lizard beta-keratin mRNA labels beta-cells of the regenerating and embryonic epidermis of lizard. The mRNA is localized free in the cytoplasm or is associated with keratin filaments of beta-cells. The immunolabeling and in situ labeling suggest that synthesis and accumulation of beta-keratin are closely associated. Nuclear localization of the cDNA probe suggests that the primary transcript is similar to the cytoplasmic mRNA coding for the protein. The latter comprises a glycine-proline-rich protein of 15.5 kDa that contains 163 amino acids, in which the central amino acid region is similar to that of chick claw/feather while the head and tail regions resemble glycine-tyrosine-rich proteins of mammalian hairs. This is also confirmed by phylogenetic analysis comparing reptilian glycine-rich proteins with cytokeratins, hair keratin-associated proteins, and claw/feather keratins. It is suggested that different small glycine-rich proteins evolved from progenitor proteins present in basic (reptilian) amniotes. The evolution of these proteins originated glycine-rich proteins in scales, claws, beak of reptiles and birds, and in feathers. Some evidence suggests that at least some proteins contained within beta-keratin filaments are rich in glycine and resemble some keratin-associated proteins present in mammalian corneous derivatives. It is suggested that glycine-rich proteins with the chemical composition, immunological characteristics, and molecular weight of beta-keratins may represent the reptilian counterpart of keratin-associated proteins present in hairs, nails, hooves, and horns of mammals. These small proteins produce a hard type of corneous material due to their dense packing among cytokeratin filaments.

Ultrastructural Localization of Tritiated Histidine in Down Feathers of the Chick

During the differentiation of cells in developing down feathers of the chick embryo, keratin and associated proteins are synthesized. Previous studies indicated that a histidine-rich protein with a different amino acid composition but similar molecular weight and localization of feather keratin is produced in forming feathers. The precise localization of the histidine-rich protein, either in feather barb and barbule cells and/or in supportive cells (sheath, barb vane ridge and cylindrical cells) is not known. The present ultrastructural autoradiographic study on developing feathers in the chick embryo shows the subcellular localization of histidine-labeled molecules, presumably representing histidine-rich proteins. Two hours after injection of tritiated histidine in chick embryos, the labeling is mainly present in the cytoplasm or is associated with forming keratin filaments of barb and barbule cells. Neither keratin filaments nor dense granules of barbule cells are specifically or prevalently labeled with tritiated histidine. No labeling is seen in periderm granules or in keratinaceous dark granules of sheath and barb vane ridge cells localized among barbule cells. The present study indicates that histidine-rich material rapidly associates with newly synthesized filaments of keratin. This observation suggests that histidine-labeled material contributes to the formation of long keratin filaments with axial orientation that are utilized for the elongation of barb and barbule cells.

Immunological characterization and fine localization of a lizard beta-keratin

Scales of lizards contain beta-keratin of poorly known composition. In the present study, a rat polyclonal serum against a lizard beta-keratin of 14-15 kDa has been produced and the relative protein has been immunolocalized in the epidermis. The observations for the first time show that the isolated protein band derives from the extraction of a protein component of the beta-keratin filaments of lizard epidermis. In immunoblots and immunocytochemistry, the antiserum recognizes most lizard beta-keratins, but produces a variable cross-reactivity with snake beta-keratins, and weak or no reactivity with beta-keratins isolated from tuatara, turtles, alligator and birds. In bidimensional immunoblots of lizard epidermis, three main spots at 15-16 kDa with isoelectric point at 7.0, 7.6 and 8.0, and an unresolved large spot at 29-30 kDa and with pI at 7.5-8.0, are obtained, may be derived from the aggregation of smaller beta-keratin proteins. The ultrastructural immunolocalization with the antibody against lizard beta-keratin shows that only small and large beta-keratin filaments of beta-cells of lizard epidermis are labeled. Keratin bundles in oberhautchen cells are less immunolabeled. Beta-keratin is rapidly polymerized into beta-packets that merge into larger beta-keratin filaments. No labeling is present over other cell organelles or cell layers of lizard epidermis, and is absent in non-epidermal cells. The antiserum recognizes epitope(s) characteristics for lizard beta-keratins, partially recognized in snakes and absent in non-lepidosaurian species. This result indicates that beta-keratins among different reptilian groups posses different immunoreactive regions.

Synthesis of interkeratin matrix in differentiating lizard epidermis: An ultrastructural autoradiographic study after injection of tritiated proline and histidine

During epidermal differentiation in mammals, keratins and keratin-associated matrix proteins rich in histidine are synthesized to produce a corneous layer. Little is known about interkeratin proteins in nonmammalian vertebrates, especially in reptiles. Using ultrastructural autoradiography after injection of tritiated proline or histidine, the cytological process of synthesis of beta-keratin and interkeratin material was studied during differentiation of the epidermis of lizards. Proline is mainly incorporated in newly synthesized beta-keratin in beta-cells, and less in oberhautchen cells. Labeling is mainly seen among ribosomes within 30 min postinjection and appears in beta-keratin packets or long filaments 1-3 h later. Beta-keratin appears as an electron-pale matrix material that completely replaces alpha-keratin filaments in cells of the beta-layer. Tritiated histidine is mainly incorporated into keratohyalin-like granules of the clear layer, in dense keratin bundles of the oberhautchen layer, and also in dense keratin filaments of the alpha and lacunar layer. The detailed ultrastructural study shows that histidine-labeling is localized over a dense amorphous material associated with keratin filaments or in keratohyalin-like granules. Large keratohyalin-like granules take up labeled material at 5-22 h postinjection of tritiated histidine. This suggests that histidine is utilized for the synthesis of keratins and keratin-associated matrix material in alpha-keratinizing cells and in oberhautchen cells. As oberhautchen cells fuse with subjacent beta-cells to form a syncytium, two changes occur : incorporation of tritiated histidine, but uptake of proline increases. The incorporation of tritiated histidine in oberhautchen cells lowers after merging with cells of the beta-layer, whereas instead proline uptake increases. In beta-cells histidine-labeling is lower and randomly distributed over the cytoplasm and beta-keratin filaments. Thus, change in histidine uptake somehow indicates the transition from alpha- to beta-keratogenesis. This study indicates that a functional stratum corneum in the epidermis of amniotes originates only after the association of matrix and corneous cell envelope proteins with the original keratin scaffold of keratinocytes.

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.

Fine structure of the developing epidermis in the embryo of the American alligator (Alligator mississippiensis, Crocodilia, Reptilia)

The morphological transition from the simple epidermis that contacts the amniotic fluid of embryonic crocodilians to the adult epidermis required in a terrestrial environment has never been described. We used light and electron microscopy to study the development, differentiation and keratinisation of the epidermis of the American alligator, Alligator mississippiensis, between early and late stages of embryonic skin formation. In early embryonic development, the epidermis consists of a flat bilayer. As it develops, the bilayered epidermis comes to lie beneath the peridermis. Glycogen is almost absent from the bilayered epidermis but increases in basal and suprabasal cells when scales form. Glycogen disappears from suprabasal cells that accumulate keratin. The peridermis and 1 or 2 subperidermal layers form an embryonic epidermis that is partially or totally lost before hatching. These cells accumulate coarse filaments and form reticulate bodies. Mucous and lamellate granules are produced in the Golgi apparatus and are partly secreted extracellularly. The embryonic cells darken with the formation of larger reticulate bodies that aggregate with intermediate filaments and other cell organelles, as their nuclear chromatin condenses. Thin beta-cells resembling those of scutate scales of birds develop beneath the embryonic epidermis and form a stratified beta-layer that varies in thickness in different body regions. The epidermis differentiates first in the back, tail and belly. At the beginning of beta-cell differentiation, the cytoplasm contains sparse bundles of alpha-keratin filaments, glycogen and lipid droplets or vacuoles apparently derived from the endoplasmic reticulum and Golgi apparatus. These organelles disappear rapidly as irregular bundles of electron-dense beta-keratin filaments accumulate and form larger bundles. The larger bundles consist of 3 nm thick electron-pale keratin microfibrils and are derived from the assemblage of beta-keratin molecules produced by ribosomes. While in mammals the epidermal barrier is formed by alpha-keratinocytes, in the alligator the barrier is formed by beta-keratin cells. The beta-layer is reduced or absent from the small hinge region between scales. In the latter areas the barrier is made of alpha or a mixture of alpha/beta keratinocytes. Thus alligators resemble birds where the beta-keratin molecules are deposited directly over an alpha-keratin scaffold, rather than an initial production of beta-keratin packets which then merge with alpha-keratin, as occurs in the 'Chelonia and Lepidosauria. The pigmentation of the epidermis of embryos is mostly derived from epidermal melanocytes.

Histidine-rich protein B of embryonic feathers is present in the transient embryonic layers of scutate scales

Based on its amino acid composition and N-terminal sequence, a polypeptide (HRP-B) has been identified as a member of the avian histidine-rich protein (HRP) family. An antiserum against HRP-B has been used to localize this polypeptide in developing feathers and scales of chick embryos. HRP-B was first detectable in the barb ridge cells of feathers at 13 days of incubation and progressively appeared in the distal/proximal and peripheral/central gradients observed previously for the feather-type beta keratins in developing feathers. The HRP-B polypeptide was detected only in the embryonic layers of scutate scales. It first appeared at 16 days of incubation and was not found in the differentiated beta strata of these scales. At no time during the development of reticulate scales or apteric skin regions did the epidermal cells or cells of the embryonic layers express HRP-B. The transient expression of HRP-B by the embryonic layers of the scutate scale epidermis is discussed in light of the feather-forming potential of the presumptive epidermis of the scutate scale-forming region.

Retinoic acid induction of featherlike structures from reticulate scales

Retinoic acid-induced transformation of reticulate scales to feather-like structures (Dhouailly and Hardy, '78) provides a useful model to study biochemical differentiation in avian skin. In this study, immunofluorescent analysis of reticulate scale-feathers (RSFs) indicates that they contain beta keratin in feather barbs and, thus, are true feathers, biochemically. Epidermal cells that would otherwise produce only alpha keratin in reticulate scales are induced to reorganize and differentiate into barb ridge cells that accumulate feather beta keratins. The mechanism for these dramatic morphological and biosynthetic responses to retinoic acid is unknown.

Genes for hair and avian keratins

This paper briefly reviews the present level of understanding of the genes that code for hair keratins and the keratins of avian feather and scale. The emphasis to date has been on the structure of the genes, the derivation of amino acid sequences for several of the proteins from the coding sequences, and the organization of the different gene families within genomic DNA. Genomic sequences for the proteins of the three main gene families for wool keratin have been isolated from sheep genomic libraries and their detailed analysis by DNA sequencing is proceeding. Already, representative sequences for the genes of the microfibril (alpha-filament or IF) proteins and for the cysteine-rich and HGT matrix proteins are available. With these sequences as sources of probes, it is now possible to move from organizational and structural studies and undertake the study of the control of the expression of these genes either in wool (hair) follicles in vivo or after transfection with specific keratin genes of suitable epidermal cells in culture. Parallel with these studies is the investigation of the fate of trichohyalin droplets in the inner root sheath during its maturation, which is coordinated with keratinization in the hair itself. These unusual proteins and the changes they undergo suggest a highly specific function. The discovery of that function might come from the use of molecular cloning techniques especially for the synthesis of quantities of protein adequate for study. The avian keratins of feather and scale are coded for by a large family of evolutionarily related genes as shown in recent investigations of the DNA sequences. Again, derived protein sequences have provided a view of the feather and scale structures previously unavailable. Information on the fine structure of the genes is enabling the examination of their expression in vivo during embryonic development and the possible significance in this development of the histidine-rich ("fast") protein. The lack of any detectable transcripts from characterized feather keratin genes injected into Xenopus oocytes is presenting the possibility of testing for tissue-specific protein factors that might be responsible for the specific activation of keratin genes.

Differentiation of embryonic chick feather-forming and scale-forming tissues in transfilter cultures

The dermal-epidermal tissue interaction in the chick embryo, leading to the formation of feathers and scales, provides a good experimental system to study the transfer between tissues of signals which specify cell type. At certain times in development, the dermis controls whether the epidermis forms feathers or scales, each of which are characterized by the synthesis of specific beta-keratins. In our culture system, a dermal effect on epidermal differentiation can still be observed, even when the tissues are separated by a Nuclepore filter, although development is abnormal. Epidermal morphological and histological differentiation in transfilter cultures are distinct and recognizable, more closely resembling feather or scale development, depending on the regional origin of the dermis. Differentiation is more advanced when epidermis is cultured transfilter from scale dermis than from feather dermis, as assessed by morphology and histology, as well as the expression of the tissue-specific gene products, the beta-keratins. Two-dimensional polyacrylamide gel analysis of the beta-keratins reveals that scale dermis cultured transfilter from either presumptive scale or feather epidermis induces the production of 7 of the 9 scale-specific beta-keratins that we have identified. Feather dermis, although less effective in activating the feather gene program when cultured transfilter from either presumptive feather or scale epidermis, is able to turn on the synthesis of 3 to 6 of the 18 feather-specific beta-keratins that we have identified. However, scale epidermis in transfilter recombinants with feather dermis also continues to synthesize many of the scale-specific beta-keratins. Using transmission and scanning electron microscopy, we detect no cell contact between tissues separated by a 0.2-micron pore diameter Nuclepore filter, while 0.4-micron filters readily permit cell processes to traverse the filter. We find that epidermal differentiation is the same with either pore size filter. Furthermore, we do not detect a basement membrane in transfilter cultures, implying that neither direct cell contact between dermis and epidermis, nor a basement membrane between the tissues is required for the extent of epidermal differentiation that we observe.

Analysis of morphogenesis and keratinization in transfilter recombinants of feather-forming skin

The relationships between feather morphogenesis, histogenesis, and biochemical differentiation were examined by recombining backskin epidermis and dermis, from chick embryos (Hamburger-Hamilton stages 27-31), with an intervening Nucleopore filter (pore size of 0.4 micron). The filter inhibited normal feather morphogenesis and histogenesis of barb ridges, yet feather-like filaments, which were free of dermal cells, formed from the epidermal cells. Using indirect immunofluorescence, with antiserum against alpha- and beta-keratins, the biochemical differentiation of the feather-like filaments was compared to normal feathers. In the feather-like filaments resulting from tissues of stages 27-29, cells containing beta keratins were occasionally seen at the periphery of the filaments, yet cells containing alpha-keratins were inappropriately located throughout the filaments. In a few feather-like filaments on recombinants resulting from tissues of stages 29.5-31, cells positive for beta-keratins were found in the center of the filament, but again alpha-keratins were also found. Surrounding these cells there were several layers of cells, arranged circumferentially, resembling sheath cells. Some sheath-like cells contained beta-keratins. We conclude that although feather epidermal cells, which are separated from their dermis by a Nuclepore filter, can undergo limited morphogenesis and the production of alpha- and beta-keratins, normal feather morphogenesis, histogenesis, and biochemical differentiation require the intimate associations of epidermis and dermis.

Isolation of messenger RNA coding for the "fast" protein of embryonic chick feathers

The messenger RNA coding for the "Fast" protein of embryonic chick feathers has been purified from the overwhelming relative amounts of keratin mRNA which are present in the developing feathers. The "Fast" protein mRNA represents about 4-8% of the total mRNA population of the feather. Despite differences between the size of the "Fast" proteins and the keratins the two mRNA species are very similar in molecular weight as judged by electrophoresis under denaturing conditions. However, by electrophoresis in 8 M urea gels at 55 degrees C, the "Fast" protein mRNA could be separated from keratin mRNA, presumably reflecting differences in messenger RNA secondary structure.

The origin of citrulline-containing proteins in the hair follicle and the chemical nature of trichohyalin, an intracellular precursor

The present studies have demonstrated that the medulla and inner root sheath cells develop within their cytoplasm a protein that is unique in composition and is present in the trichohyalin granules. The protein is rich in arginine residues, some of which undergo a side-chain conversion in situ into citrulline residues. An unusual Ca2+-dependent enzyme activity distinguishable from cross-linking transamidase has been detected in the hair follicle and will act in vitro on trichohyalin protein as the natural substrate. The conversion in vivo must occur during the time that the medullary and inner root sheath cells move up the follicle and their cytoplasm fills with cross-linked protein containing citrulline. The function of citrulline in these proteins is not understood but its formation is a major process during hair growth.

[RNA metabolism in chick embryo skin]

Explants from 7, 8, 9, 11, 13-day chick embryonic skin incorporating (3H) Uridine for different periods 1 hr, 3 or 4 hr and a chase with actinomycin) are studied with respect to free (F) or membrane bound (B) cytoplasmic polysomes and to RNA extracted from them. Polysome specific activity decreases at older stages but the amount of polysomes increases due to increased protein synthesis. At each stage B polysomes are less abundant but more radioactive than F polysomes. RNA extracted from each kind is analysed on sucrose gradients: one half of each fraction is precipitated by TCA to estimate total radioactivity, the other is retained on millipore at high salt concentration to estimate radioactivity in messenger-like RNAs due to their poly-A sequences. The pattern of the labelling of the different fractions of RNA changes with the length of incorporation, the stages of explants and the kind of polysomes (F or B); at 11-13 days the incorporation is slow, radioactivity is low and distributed among several peaks of poly-A RNA; at 7-8 days the incorportion is rapid, dispersed throughout the gradient; at 9 days, a midway stage, incorporation is particularly high into 12S and 24S fractions from B RNA. In the 5 studied stages the labelling of this 12S occurs early, remains for a longer time and cannot be chased. These observations suggest stability of the 12S RNA. Since, in 14-day chick embryos, feather keratin m RNA has been shown to sediment at 12S and although our experiments have been done with total skin because this differentiating tissue is the site of extensive interactions between dermis and epidermis, they suggest that this 12S RNA is the actual keratin m RNA and might be synthesised some days before the onset of keratin synthesis. Its template ability will be investigated at earlier stages.

The identification of fibrous proteins in fetal rat epidermis by electrophoretic and immunologic techniques

Two proteins have been identified in extracts of fetal rat skin which are related to the two major fibrous proteins of newborn rat stratum corneum. The relative amount of these proteins increases daily from the 16th to the 20th day (d) of gestation when judged by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis and immunoelectrophoresis using antibody to the purified fibrous protein. Two-dimensional analysis by SDS-polyacrylamide gel electrophoresis and immunoelectrophoresis demonstrates that these two proteins are the only cross-reactive species in the fetal skin from 16d to 19d development. Some additional lower-molecular-weight components can be detected at 20d and 21d. In double-diffusion analysis, cross-reactive proteins in 19d fetal extracts show partial identity but have fewer antigenic sites than proteins in 20d extracts. The 20d protein shows a reaction of identity with purified newborn fibrous protein. Immunofluorescence studies on fetal skin support the prescence of cross-reacting components at 16d development related to the newborn fibrous protein. Intensity of fluorescence increases at 18d and 20d in the spinous and granular cell cytoplasm and in the keratohyaline granules. The stratum corneum, first seen at 20d, is intensely fluorescent. The cellular localization and time of appearance of the cross-reactive proteins suggest that they may be associated with tonofilaments.

Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of proteins of newbonr rat skin. II. Keratohyalin and stratum corneum proteins

Keratohyalin extracts from newborn rat epidermis were prepared by potassium phosphate and citric acid-detergent extraction procedures. These preparations were compared by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis and amino acid analysis. The major band of the potassium phosphate extract has a molecular weight of 48,000. The major bands of the citric acid-detergent preparation have molecular weights of 64,000, 61,500, 57,000 and 54,000. Electrophoresis of S-carboxylmethylated (SCM)-fibrous protein results in two major bands of approximately 57,000 and 64,000. SDS gels of the two preparations of keratohyalin and the SCM-fibrous protein were compared with gels of the insoluble proteins of granular and eluted cornified cells. All of the major bands in the preparations of keratohyalin can be seen in gels of the granular preparation. The two SCM-fibrous protein bands correspond with two prominent bands in gels of the cornified cell preparation. Two bands of the citric acid-extracted keratohyalin sample also have the same mobility. The major band of the potassium phosphate-extracted preparation of keratohyalin corresponds with a third prominent band of the cornified cell preparation. These results suggest that biochemical components of the preparations of keratohyalin are present in both the granular and the cornified layers of newborn rat epidermis.

Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of proteins of newborn rat skin. I. Cell strata and nuclear proteins

The proteins obtained from separated cells of neonatal rat dermis, four cell populations of epidermis, and an epidermal nuclear preparation were analyzed by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis. Comparison of the results of the insoluble proteins of the dermis and epidermis show no similarity of the major protein bands, indicating the effective separation of the dermis and epidermis and absence of cross-contamination. The gels of the soluble proteins of the basal, spinous, and granular layers of the epidermid are very similar. Only the pattern of bands of the cornified cells differs in that some of these bands are absent and at least three new bands are present. The insoluble proteins have specific differences in the protein content related to the cell structure. An example is the nuclear protein bands which correspond with the most prominent bands in the gels of basal and spinous layer proteins, but are absent, with the possible exception of one band, from gels of cornified cell proteins.

Unique and repetitive sequences in multiple genes for feather keratin

Embryonic chick feather keratins are a family of homologous polypeptide chains. The mRNA coding for these has been obtained in a pure state and transcribed into complementary DNA (cDNA) using the reverse transcriptase from avian myeloblastosis virus. Studies on the kinetics of hybridisation and reannealing of cDNA indicate that there are 25-35 different keratin mRNA species in the embryonic chick feather, and a total of 100-240 keratin genes in the chick genome. Each keratin gene contains both a unique and a repetitive sequence. It is proposed that the repetitive sequences are the keratin coding sequences and that the unique sequences correspond to untranslated regions.