Defective histogenesis and morphogenesis in the anterior shank skin of the scaleless mutant

Feather arrays are patterned by interacting signalling and cell density waves

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

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.

Evolutionary origin of the feather epidermis

The formation of scales and feathers in reptiles and birds has fascinated biologists for decades. How might the developmental processes involved in the evolution of the amniote ectoderm be interpreted to shed light on the evolution of integumental appendages? An Evo-Devo approach to this question is proving essential to understand the observation that there is homology between the transient embryonic layers covering the scale epidermis of alligators and birds and the epidermal cell populations of embryonic feather filaments. Whereas the embryonic layers of scutate scales are sloughed off at hatching, that their homologues persist in feathers demonstrates that the predecessors of birds took advantage of the ability of their ectoderm to generate embryonic layers by recruiting them to make the epidermis of the embryonic feather filament. Furthermore, observations on mutant chickens with altered scale and feather development (Abbott and Asmundson [1957] J. Hered. 18:63-70; Abbott [1965] Poult. Sci. 44:1347; Abbott [1967] Methods in developmental biology. New York: Thomas Y. Crowell) suggest that the ectodermal placodes of feathers, which direct the formation of unique dermal condensations and subsequently appendage outgrowth, provided the mechanism by which the developmental processes generating the embryonic layers diverged during evolution to support the morphogenesis of the epidermis of the primitive feather filament with its barb ridges.

Avian skin development and the evolutionary origin of feathers

The discovery of several dinosaurs with filamentous integumentary appendages of different morphologies has stimulated models for the evolutionary origin of feathers. In order to understand these models, knowledge of the development of the avian integument must be put into an evolutionary context. Thus, we present a review of avian scale and feather development, which summarizes the morphogenetic events involved, as well as the expression of the beta (beta) keratin multigene family that characterizes the epidermal appendages of reptiles and birds. First we review information on the evolution of the ectodermal epidermis and its beta (beta) keratins. Then we examine the morphogenesis of scutate scales and feathers including studies in which the extraembryonic ectoderm of the chorion is used to examine dermal induction. We also present studies on the scaleless (sc) mutant, and, because of the recent discovery of "four-winged" dinosaurs, we review earlier studies of a chicken strain, Silkie, that expresses ptilopody (pti), "feathered feet." We conclude that the ability of the ectodermal epidermis to generate discrete cell populations capable of forming functional structural elements consisting of specific members of the beta keratin multigene family was a plesiomorphic feature of the archosaurian ancestor of crocodilians and birds. Evidence suggests that the discrete epidermal lineages that make up the embryonic feather filament of extant birds are homologous with similar embryonic lineages of the developing scutate scales of birds and the scales of alligators. We believe that the early expression of conserved signaling modules in the embryonic skin of the avian ancestor led to the early morphogenesis of the embryonic feather filament, with its periderm, sheath, and barb ridge lineages forming the first protofeather. Invagination of the epidermis of the protofeather led to formation of the follicle providing for feather renewal and diversification. The observations that scale formation in birds involves an inhibition of feather formation coupled with observations on the feathered feet of the scaleless (High-line) and Silkie strains support the view that the ancestor of modern birds may have had feathered hind limbs similar to those recently discovered in nonavian dromaeosaurids. And finally, our recent observation on the bristles of the wild turkey beard raises the possibility that similar integumentary appendages may have adorned nonavian dinosaurs, and thus all filamentous integumentary appendages may not be homologous to modern feathers.

Origin of archosaurian integumentary appendages: the bristles of the wild turkey beard express feather-type beta keratins

The discovery that structurally unique "filamentous integumentary appendages" are associated with several different non-avian dinosaurs continues to stimulate the development of models to explain the evolutionary origin of feathers. Taking the phylogenetic relationships of the non-avian dinosaurs into consideration, some models propose that the "filamentous integumentary appendages" represent intermediate stages in the sequential evolution of feathers. Here we present observations on a unique integumentary structure, the bristle of the wild turkey beard, and suggest that this non-feather appendage provides another explanation for some of the "filamentous integumentary appendages." Unlike feathers, beard bristles grow continuously from finger-like outgrows of the integument lacking follicles. We find that these beard bristles, which show simple branching, are hollow, distally, and express the feather-type beta keratins. The significance of these observations to explanations for the evolution of archosaurian integumentary appendages is discussed.

Origin of feathers: Feather beta (beta) keratins are expressed in discrete epidermal cell populations of embryonic scutate scales

The feathers of birds develop from embryonic epidermal lineages that differentiate during outgrowth of the feather germ. Independent cell populations also form an embryonic epidermis on scutate scales, which consists of peridermal layers, a subperiderm, and an alpha stratum. Using an antiserum (anti-FbetaK) developed to react specifically with the beta (beta) keratins of feathers, we find that the feather-type beta keratins are expressed in the subperiderm cells of embryonic scutate scales, as well as the barb ridge lineages of the feather. However, unlike the subperiderm of scales, which is lost at hatching, the cells of barb ridges, in conjunction with adjacent cell populations, give rise to the structural elements of the feather. The observation that an embryonic epidermis, consisting of peridermal and subperidermal layers, also characterizes alligator scales (Thompson, 2001. J Anat 198:265-282) suggests that the epidermal populations of the scales and feathers of avian embryos are homologous with those forming the embryonic epidermis of alligators. While the embryonic epidermal populations of archosaurian scales are discarded at hatching, those of the feather germ differentiate into the periderm, sheath, barb ridges, axial plates, barbules, and marginal plates of the embryonic feather filament. We propose that the development of the embryonic feather filament provides a model for the evolution of the first protofeather. Furthermore, we hypothesize that invagination of the epidermal lineages of the feather filament, namely the barb ridges, initiated the formation of the follicle, which then allowed continuous renewal of the feather epidermal lineages, and the evolution of diverse feather forms.

Dorsal dermis of the scaleless (sc/sc) embryo directs normal feather pattern formation until day 8 of development

We have examined the ability of the scaleless (sc/sc) backskin dermis (6 to 16 days of incubation) to regulate pattern formation using the presumptive scutate scale epidermis from 11-day normal embryos as the responding tissue. Prior to 8 days of incubation the sc/sc backskin dermis is able to induce hexagonally patterned and uniformly oriented feather germs in normal epidermis. This ability is lost during day 8 and follows a central to lateral gradient. Such gradients are characteristic of normal feather development in the spinal tract. We discuss the change in the inductive ability of the sc/sc dermis in relation to the stabilization of the feather pattern, which occurs all at once throughout the dorsal dermis at 7.5-8 days of development. After day 8 until day 10, the sc/sc backskin dermis only supports the formation of sporadic, unpatterned feather germs; thereafter it will not support feather formation.

Role of epidermal-dermal tissue interactions in regulating tenascin expression during development of the chick scutate scale

During normal chicken development tenascin begins to accumulate in the dermis of anterior metatarsal skin at the time of scutate scale ridge formation, and is localized in a distinct pattern along the outer scale surface. Anterior metatarsal skin from scaleless (sc/sc) embryos, which do not form scutate scales, begins to accumulate tenascin 4 days later than normal skin. This study shows that normal and scaleless anterior metatarsal dermis accumulate the same tenascin isoforms and undergo the same isoform changes in the post-hatch period, but there is less tenascin accumulated in scaleless dermis and there is no pattern to its distribution. In both normal and scaleless anterior metatarsal skin, tenascin mRNA is localized in the dermis and is distributed in the same way as the protein. Thus, scaleless skin is defective in the ability to accumulate appropriate amounts of tenascin and to maintain the tenascin in the patterned manner of normal. Recombinant skin cultures show that epidermal-dermal interactions are required for tenascin accumulation. The dermis specifies the way that tenascin is organized, but interaction with epidermis is required to maintain this organization. The epidermal role appears to be permissive because in heterotypic recombinants, neither scaleless anterior metatarsal epidermis nor normal footpad epidermis changes the way that tenascin appears in the normal anterior metatarsal dermis; and in reciprocal recombinants, normal anterior metatarsal epidermis does not change the way tenascin is accumulated in either scaleless anterior metatarsal dermis or normal footpad dermis.

Pattern formation in chick feather development: distribution of beta 1-integrin in normal and scaleless embryos

We have examined the immunolocalization of beta 1-integrin during feather development in the spino-lumbar tract of the backskin from normal and scaleless chick embryos. beta 1-integrin appears during early feather development in three distinct phases which correspond to important developmental events. The first phase (5-5 1/2 days of incubation; Hamburger and Hamilton [H.H.] stage 27) represents the period prior to the formation of dermis. During this phase, beta 1-integrin antiserum labels mesenchymal cells located in the central region of the spino-lumbar tract where the initiation site for feather development is located. The second phase (5 1/2-7 1/2 days of incubation; H.H. stages 28-32) corresponds to the period during which dermis is formed. The cells that make up the dermis are readily distinguished by their lack of beta 1-integrin immunostaining. The third phase (7 1/2-10 days of incubation; H.H. stages 33-36) begins with the sudden appearance of beta 1-integrin in the central and lateral regions of the dermis. The pattern of beta 1-integrin immunostaining in scaleless backskin becomes different from that of normal backskin during this phase. In normal backskin the dermal condensations of feather germs are not labeled with the beta 1-integrin antiserum. This produces a heterogeneous immunostaining pattern very similar to the pattern seen for Type I collagen (Mauger et al. [1982] Dev. Biol. 94:93-105). In contrast, homogeneous immunostaining is observed in the dermis of scaleless backskin. The initial time of appearance, manner of appearance, and pattern of integrin expression in the third phase suggest that beta 1-integrin may be involved in the stabilization of the feather pattern. We also observed the appearance of beta 1-integrin on the epidermal basal cells during the time of feather follicle formation. The beta 1-integrin antiserum reacts strongly with the baso-lateral surfaces of normal basal cells, yet the basal surfaces of the scaleless basal cells are unstained. This lack of immunostaining along the basal surfaces of the scaleless basal cells may relate to the abnormal adhesion between the epidermis and dermis in scaleless backskin.

Apoptosis in human skin development: morphogenesis, periderm, and stem cells

During human skin development, embryonic- and fetal-specific periderm cells and incompletely keratinized cells are replaced by keratinocytes that differentiate while stratifying to form the fully functional epidermis. Proliferating basal cells of fetal skin also develop into epidermal appendages such as hair follicles and glands. We demonstrate that programmed cell death, not emphasized in conventional epidermal biology, has an important function in establishing the final architecture of the human epidermis and its appendages. Immunohistochemical localization of transglutaminases in fetal periderm, intermediate epidermal cells, and within appendages coincides with DNA fragmentation indicating that apoptosis is involved in deletion of these stage-specific cells and remodeling of appendages. The data also suggest that terminal differentiation of epidermal cells might be a specialized form of apoptosis. The pattern of expression of bcl-2, a gene associated with survival of some cells, is exclusive of the distribution patterns of markers of the cell death pathway. Bcl-2 protein is correlated with specific morphogenetic events in hair follicles and eccrine sweat glands, and its presence in single cells of the hair follicle bulge suggests that Bcl-2 may be a stem cell marker.

Region-specific patterns of beta keratin expression during avian skin development

The transient embryonic layers primarily composed of a periderm and subperiderm cover most regions of the chick embryo and are the first suprabasal cell layers covering the body ectoderm. This study presents evidence for regional variation in the expression of beta keratin in the embryonic layers. Here we show that the embryonic layers covering the anterior metatarsal region of the chicken hindlimb (scutate scale forming region) produce several members of the beta keratin family of polypeptides, designated beta (beta) 1-7. These specific polypeptides are later expressed in this region exclusively in the thick, cornified beta strata of mature scutate scales. In contrast to this sequence of events, the embryonic layers overlying the epidermis of the ventral foot pad (reticulate scale-forming region) and those covering the epidermis in apteric regions of the body produce beta keratin polypeptides beta 1-3 and beta 2,3, respectively, but no subsequent expression of these proteins occurs in the mature epidermises of these regions. Furthermore, we find that the embryonic layers of the skin overlying the anterior metatarsal region of birds homozygous for the mutation "scaleless" (sc/sc), which completely lack scutate scales, produce the same members of the beta keratin family, beta 1-7, as the embryonic layers and beta strata of normal scutate scales. Thus, the accumulation of specific beta keratin polypeptides in the developing anterior metatarsal region appears to occur in two distinct phases; first, an early region-specific expression in cells of the embryonic layers followed by a second phase of expression which occurs in conjunction with appendage morphogenesis.(ABSTRACT TRUNCATED AT 250 WORDS)

Avian scale development. XVII: The epidermis of the scaleless (sc/sc) anterior metatarsal skin is determined, but the dermis lacks permissive cues for the patterned expression of the determined state

Embryos homozygous for the gene scaleless (sc/sc) completely lack scutate scales and the beta strata which characterize terminal differentiation of the scale ridges located on the anterior metatarsal region of the foot. Although the sc/sc epidermis cannot undergo scale morphogenesis, it can respond to the inductive dermal ridges of normal scutate scales by generating beta strata. Recently, we discovered that the anterior metatarsal epidermis of normal embryos becomes committed to the formation of beta strata prior to morphogenesis of definitive scale ridges. Here, we examined the possibility that the sc/sc anterior metatarsal epidermis also becomes determined, i.e., committed to scutate scale-specific terminal differentiation. Experimental tissue recombinants were used to assess the ability of the sc/sc epidermis to generate beta strata. The results show that the germinative cells of the 15-day sc/sc epidermis are committed to generating beta strata, even though they have not undergone scutate scale morphogenesis. Thus, the mechanisms involved in establishing epidermal determination must differ form those regulating scale morphogenesis. In addition, we examined the formation of patterned, permissive cues in the anterior metatarsal and footpad dermises of sc/sc embryos. Analysis of recombinants showed that both the 15- and 20-day dermises from the sc/sc anterior metatarsal region fail to provide cues for beta stratum formation, when associated with the determined 15-day scutate scale epidermis. Likewise, the 15-day sc/sc footpad dermis cannot support beta stratum formation. However, 20-day sc/sc footpad dermis is able to support the generation of a few abnormally patterned beta strata, demonstrating that sc/sc dermis which has experienced even limited morphogenesis is able to provide permissive cues for the terminal differentiation of the scutate scale epidermis.

Avian scale development

Germinative cells of the scutate scale epidermis from 15-day embryos are committed to appendage-specific, beta stratum formation in association with a foreign dermis. Commitment precedes the time (17 days of development) at which beta strata are actually present in their site-specific locations along the outer surface of each scutate scale. This observation suggested the possibility that commitment to beta stratum formation might be occurring as the outer epidermal surface of each scutate scale first becomes established between 12 and 13 days of development. It is at this time that the scale epidermis loses its ability to participate in feather morphogenesis and cell proliferation becomes restricted to a true stratum basale. To examined the ability of the presumptive scutate scale epidermis to generate beta strata in the absence of the inductive scutate scale dermis, scutate scale epidermis from 11-, 12-, and 13-day embryos was recombined with 15-day reticulate scale dermis and grown for 7 or 9 days. The dermis of reticulate scales does not induce beta stratum formation, but it does support differentiation of a beta stratum by the determined 15-day scutate scale epidermis. Using immunohistological and biochemical analyses of beta-keratins, we find that each of these presumptive scutate scale epidermises is competent to generate appendage-specific beta strata in the absence of the scutate scale dermis. This determination is occurring prior to scale ridge morphogenesis and differentiation of the epidermis into the distinct outer and inner epidermal surfaces of the scale ridge. The restricted distribution of beta strata to the apical domes of individual reticulate-like scales demonstrates that the germinative cells of the committed epidermises are responding to patterned cues. This study also suggests that all basal cells of the presumptive scutate scale epidermis are initially endowed with the ability to generate cells that form a beta stratum.

The initial expression and patterned appearance of tenascin in scutate scales is absent from the dermis of the scaleless (sc/sc) chicken

Morphogenesis of the anterior metatarsal skin (scutate scale region), from 9.5 to 12 days of development, results in the formation of orderly patterned scale ridges. It is after the initial formation of the Definitive Scale Ridge that the characteristic outer and inner epidermal surfaces differentiate. The hard, plate-like beta stratum, with its unique beta keratins, characterizes the epidermis of the outer surface, while the epidermis of the inner surface elaborates an alpha stratum. The anterior metatarsal region of the scaleless mutant does not undergo scale morphogenesis. Therefore, scale ridges do not form nor do the outer and inner epidermal surfaces with their characteristic beta and alpha strata. We have found that the extracellular matrix molecule, tenascin, first appears in the scutate scale dermis at 12 days of development when the scale ridge is established. Tenascin is found in the dermis only under the scale ridge and is not associated with the dermal-epidermal junction. Tenascin is not found in scaleless anterior metatarsal dermis at this time. As outgrowth of the Definitive Scale Ridge takes place, tenascin distribution correlates closely with the formation of the outer epidermal surface of each scale ridge. By 16 days of development tenascin is also found in close association with the dermal-epidermal junction. Tenascin does not appear in scaleless anterior metatarsal dermis until 16 days of development and then it is randomly and sparsely distributed at the dermal-epidermal junction. Tenascin's initial appearance and pattern of distribution in the scutate scale dermis and its abnormal expression in the scaleless dermis suggest that morphogenesis plays a significant role in regulation of its expression.

Avian scale development. XIII. Epidermal germinative cells are committed to appendage-specific differentiation and respond to patterned cues in the dermis

The ability of the germinative cell population of scutate scale epidermis to continue to generate cells that undergo their appendage-specific differentiation (beta stratum formation), when associated with foreign dermis, was examined. Tissue recombination experiments were carried out which placed anterior metatarsal epidermis (scutate scale forming region) from normal 15-day chick embryos with either the anterior metatarsal dermis from 15-day scaleless (sc/sc) embryos or the dermis from the metatarsal footpad (reticulate scale forming region) of 15-day normal embryos. Neither of these dermal tissues are able to induce beta stratum formation in the simple ectodermal epithelium of the chorion, however, the footpad dermis develops an appendage-specific pattern during morphogenesis of the reticulate scales, while the sc/sc dermis does not. Morphological and immunohistological criteria were used to assess appendage-specific epidermal differentiation in these recombinants. The results show that the germinative cell population of the 15-day scutate scale epidermis is committed to generating suprabasal cells that follow their appendage-specific pathways of histogenesis and terminal differentiation. Of significance is the observation that the expression of this determined state occurred only when the epidermis differentiated in association with the footpad dermis, not when it was associated with the sc/sc dermis. The consistent positioning of the newly generated beta strata to the apical regions of individual reticulate-like appendages demonstrates that the dermal cues necessary for terminal epidermal differentiation are present in a reticulate scale pattern. The observation that beta stratum formation is completely missing in the determined scutate scale epidermis when associated with the sc/sc dermis adds to our understanding of the sc/sc defect. The present data support the conclusion of earlier studies that the anterior metatarsal dermis from 15-day sc/sc embryos lacks the ability to induce beta stratum formation in a foreign epithelium. In addition, these observations evoke the hypothesis that the sc/sc dermis either lacks the cues (generated during scutate and reticulate scale morphogenesis) necessary for terminal differentiation of the determined scutate scale epidermis or inhibits the generation of a beta stratum.

Expression of the cell adhesion molecules, L-CAM and N-CAM during avian scale development

To examine the involvement of cell adhesion molecules in the inductive epithelial-mesenchymal interactions during avian scale development, a study of the spatiotemporal distribution of L-CAM and N-CAM was undertaken. During scutate scale development, L-CAM and N-CAM are expressed together in cells of the transient embryonic layers destined to be lost at hatching. The ongoing linkage of the cells of these layers by both CAMs sets them apart, early in development, as unique cell populations. L-CAM and N-CAM were also expressed simultaneously at the basal surface of the early germinative cells where signal transduction is presumed to occur. In spite of the differences in cell shape, adhesion, density and proliferative state between populations of epidermal placode and interplacode cells, the expression of L-CAM and N-CAM appeared to be uniform and nondiscriminating for these discrete cell lineages. The same pattern of L-CAM and N-CAM expression was observed during morphogenesis of reticulate scales that develop without placode formation. While L-CAM and N-CAM are present during the early stages of scale development and most likely function in cell adhesion, the data do not support a role for these adhesion molecules in the formation of the morphogenetically critical placode and interplacode cell populations. In both scale types, L-CAM became predominantly epithelial, and N-CAM became predominantly dermal as histogenesis occurred. Initially, N-CAM was concentrated near the basal lamina where it may be involved in the reciprocal epidermal-dermal interactions required for morphogenesis. However, as development of the scales progressed, N-CAM disappeared from the tissues. L-CAM expression continued in the epidermis and was intense on all suprabasal cells undergoing differentiation into either an alpha-stratum or beta-stratum. However, L-CAM was more prevalent on the basal cells of alpha-keratinizing regions than on the basal cells of beta-keratinizing regions.

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.

Identification, expression, and localization of beta keratin gene products during development of avian scutate scales

Epidermal-dermal interactions influence morphogenesis and expression of the beta keratin gene family during development of scales in the embryonic chick. The underlying mechanisms by which these interactions control beta keratin expression are not understood. However, the present study of beta keratin gene expression during avian epidermal differentiation contributes new information with which to investigate the role of tissue interactions in this process. Using beta keratin-specific synthetic oligonucleotide probe, beta keratin mRNA was hybrid-selected from total poly A+ RNA of scutate scales. Seven beta keratin polypeptides were translated in vitro and could be identified by their positions in two-dimensional gels among the detergent-insoluble extracts of scutate scale epidermis. In vivo phosphorylation studies suggested that an additional three beta keratin polypeptides were present as phosphoproteins. The temporal appearance of beta keratin mRNA and the corresponding polypeptides was followed during scutate scale development. Polyclonal antiserum made against two of the beta keratin polypeptides was used for immunohistochemical and immunogold electron-microscopic analysis of beta keratin tissue distribution. Immunological reactivity was observed specifically along the outer scale surface in epidermal cells above the stratum germinativum. Immunogold beads were localized on 3-nm filament bundles. In situ hybridization with a beta keratin-specific RNA probe demonstrated that mRNA accumulated in the same regional manner as the polypeptides. This selective expression of beta keratin genes in specific regions of the developing scutate scale suggests that epidermal-dermal interactions provide not only for morphological events, but also for control of complex patterns of histogenesis and biochemical differentiation.

Development and keratinization of the epidermis in the common lizard, Anolis carolinensis

Epithelial-mesenchymal interactions play important roles in the development of the vertebrate integument with its diverse appendages. As a result of these interactions, specific morphogenetic events occur which result in the formation of distinct epidermal appendages. Following the early morphogenetic events involving cell proliferation and movement, other developmental events such as stratification, histotypic differentiation, and terminal cytodifferentiation occur in the epidermis. Using the common lizard Anolis carolinensis, we are seeking to obtain a better understanding of the relationship between the various developmental events and the expression of alpha and beta keratins, with the aim of eventually understanding the mechanisms by which tissue-specific keratinization patterns are established in the integument. As a first step, we have used immunoblot analyses and indirect immunofluorescence procedures with antisera specific for either alpha or beta keratins to determine the temporal and spatial appearance of these keratins at specific developmental stages. We have found that: 1) There are relatively low molecular weight alpha keratin polypeptides present in the epidermis early in development as morphogenesis is taking place. 2) After morphogenesis occurs and histogenesis is well under way, the alpha keratins which characterize the adult epidermis appear. 3) Only alpha keratins are found in the basal cells of all regions of the epidermis. 4) beta keratins are found only in the suprabasal layers of well-developed scales and show region-specific distribution in overlapping scales.

Expression of beta keratin genes during skin development in normal and sc/sc chick embryos

The expression of RNA sequences specific for scale beta (beta)-keratins has been followed during skin development in normal and scaleless (sc/sc) embryos. Total RNA from skin at various stages (36-46) of development, as well as newly hatched chicks, was immobilized on nitrocellulose paper and hybridized with a [32P]cDNA probe to beta-keratins (pCSK-12). Sequences for beta-keratins showed patterns of expression which were specific for each genotype and scale type examined. During the development of normal scutate scales, which are characterized by the formation of a beta stratum, RNA with beta-keratin sequences first appeared at stage 40, and continued to accumulate through hatching. RNA with beta keratin sequences appeared in scaleless skin between stages 40 and 41, was greatly diminished by stage 44, and was no longer present at stage 46. In normal reticulate scales, which like scaleless skin, do not develop a beta stratum accumulation of RNA with beta-keratin sequences was limited to a brief embryonic period between stages 42 and 44. These patterns of RNA expression correlated well with the appearance of beta-keratin polypeptides, suggesting that beta-keratin synthesis may be controlled at the level of keratin mRNA transcription. Correlations between the patterns of beta-keratin expression and histological events suggest that the brief accumulation of beta-keratin mRNA in scaleless skin and normal reticulate scales is related to the formation of the subperiderm (a protective layer of cells, peculiar to embryonic skin) while the continuous accumulation of beta-keratin mRNA during scutate scale development reflects the formation of a beta stratum.

Different protein synthetic patterns in scale-forming, feather-forming, and apteric embryonic chick dermis

We have examined the protein synthetic profile of embryonic chick dermis from different regions of both wild-type and scaleless mutant embryos by two-dimensional polyacrylamide gel electrophoresis to determine if differences in inductive capability are associated with different patterns of gene expression. We have found proteins preferentially synthesized in dorsal dermis and anterior tarsometatarsal dermis at stages when these tissues are active in inducing feather or scale histogenesis, respectively, in the epidermis. Apteric dermis, which is unable to induce epidermal derivative formation, synthesizes a subset of the proteins specific to each region. Scaleless mutant dermis, which does not participate in feather or scale formation in vivo, synthesizes all of the dorsal dermis-specific or tarsometatarsal dermis-specific proteins appropriate to its regional origin. However, it lacks one protein common to all types of dermis tested, and synthesizes one protein inappropriate for its location. Examination of the protein synthetic profile of dorsal and anterior tarsometatarsal dermis at early stages of development reveals that young dorsal dermis, which can only form feathers, possesses the protein synthetic pattern specific to that region. Young tarsometatarsal dermis, which has the potential to form either feathers or scales, synthesizes the proteins we have identified as specific to dorsal and older tarsometatarsal dermis. These results suggest that different protein synthetic patterns are associated with different inductive potentials. However, combining young tarsometatarsal dermis with dorsal epidermis, which causes the formation of feathers, does not alter the pattern of proteins synthesized by the dermis. While this result may be due to an artifact of the culture system, an alternative explanation is that the protein synthesis pattern is not related to the type of epidermal derivative induced, but to the pattern in which the derivatives are induced. This is supported by the observation that the feathers formed in recombinants of tarsometatarsal dermis and dorsal epidermis are arranged in a scale pattern.

Differences in the histogenesis and keratin expression of avian extraembryonic ectoderm and endoderm recombined with dermis

The responses of the chorionic ectoderm and allantoic endoderm (from 8-day chick embryos) to dermal induction were compared through tissue recombinants grafted onto the chorioallantoic membrane. The chorionic epithelium formed the appropriate epidermis with a fully developed stratum corneum in response to both spur and scutate scale dermises. Analysis of these recombinant epidermal tissues by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) demonstrated that tissue-specific expression of the alpha (alpha) and beta (beta) keratin polypeptides occurred. In addition, indirect immunofluorescence studies with antisera to alpha or beta keratins showed that the beta stratum, which characterizes the epidermis of spurs and scutate scales, was formed, and the alpha keratins were distributed as in the normal epidermal tissues. In contrast, although the allantoic endoderm became stratified in association with either spur or scutate scale dermis, a stratum corneum with a beta stratum did not develop. SDS-PAGE analysis demonstrated that while the characteristic beta keratins of scutate scales and spur were not detected, most of the alpha keratins normally elaborated by these structures were present, suggesting that even without histogenesis of a stratum corneum the expression of alpha keratins of endoderm could be regulated in a tissue-specific manner by dermis. This study also demonstrated that there are differences in the abilities of the chorionic and allantoic epithelia to respond to the same dermal cues, which may reflect earlier restrictions in their developmental potentials.

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.

Avian feather development: relationships between morphogenesis and keratinization

Morphogenesis and expression of the alpha and beta keratin polypeptides are controlled by epidermal-dermal interactions during development of avian skin derivatives. We have examined the relationship between morphogenesis of the embryonic feather and expression of the feather alpha and beta keratins by routine histology, indirect-immunofluorescence, and SDS-PAGE. Initially beta keratins are expressed only in the feather sheath. Following barb ridge morphogenesis beta keratins can be detected in the barb ridge, coincident with the differentiation of barb ridge cells into eight distinct morphological types. Beta keratinization occurs in gradients; from feather apex to base, and from periphery of the barb ridge to the interior. The onset of beta keratinization in the barb ridges is paralleled by an increase in the major feather beta keratin polypeptides, as detected by SDS-PAGE. The alpha keratins are present in both the periderm and feather sheath at early stages of feather development, but become greatly reduced after hatching, when the down feather emerges from the sheath.

The effects of Janus Green B on the temporal and spatial pattern of feather germ morphogenesis

The normal timing and appearance of feather germs was perturbed by injecting the dye Janus Green B into the amniotic fluid of chick embryos at late stage 28, prior to the first appearance of feather germs. This treatment prevented feather germ morphogenesis in some regions while elsewhere it delayed normal morphological development. The Janus Green B effect lasted for approximately 98 hours. Feather regions, which normally form epidermal placodes during the period of treatment, showed the longest delays in subsequent feather germ formation and were the most likely to remain featherless. These results suggest that the epidermal placode stage is critical for feather germ formation. Janus Green B appears to prevent feather germ morphogenesis by interfering with development prior to this critical stage. Since severely affected regions fail to recover their capacity to form feather germs, even after the period of sensitivity to the dye, a limited period of competence is suggested for feather germ formation.

Altered keratin biosynthesis follows inhibition of scale morphogenesis by hydrocortisone

Hydrocortisone, administered onto the chorioallantoic membrane (CAM) of 7- to 10-day-old chick embryos, inhibits scale development, in a dose- and stage-dependent manner. The response is also region specific in that hydrocortisone treatment, at a specific dose and time, will completely block scutellate and interstitial scale development while leaving other scale types unaffected. Using histological, biochemical, and immunofluorescence techniques, we have shown that inhibition of scutellate scale morphogenesis prevents the subsequent formation of a beta stratum and alters expression of the alpha keratins. These data support the hypotheses that each avian scale type has its own distinctive temporal, morphological, and biochemical pattern of development; and in the case of scutellate scale development, hydrocortisone treatment alters keratin biosynthesis by interfering with earlier steps in morphogenesis.

Avian scale development. XI. Initial appearance of the dermal defect in scaleless skin

The chicken mutant, scaleless, is characterized by the total absence of scutate scales. Previous experiments have shown that the scaleless defect is expressed by the epidermal cells while the dermal cells are able to participate in normal scale morphogenesis. However, in association with 14- to 16-day scaleless dermis, normal epidermis or the simple ectoderm of the chorion failed to develop scutate scale epidermis with its characteristic beta stratum. Thus the question arises: since the scaleless dermis starts out functioning normally, when does it become defective? Heterogenetic, heterotopic associations have been performed between 7.5-day to 11.5-day scaleless dermis and a neutral responding tissue, the midventral apteric epidermis, from 10.5-day normal embryos. The results show that up until 9.5 day of incubation the scaleless dermis is able to give instructions for normal scutate scale formation, if combined with normal epidermis. However, after 9.5 days, the scaleless dermis is not able to induce scale formation in normal apteric epidermis. Thus, the functional defect of the scaleless dermis occurs during the time (9 to 10 days of incubation) when epidermal placodes appear in normal embryos. From the present data, at least two explanations are possible. Either the scaleless epidermis is unable to respond to the placode inducing properties being provided by the scaleless dermis and because an epidermal placode does not form the scaleless dermis becomes defective, or the scaleless epidermis does not provide some earlier cue necessary for the scaleless dermis to acquire its placode inducing capabilities.

Avian scale development. X. Dermal induction of tissue-specific keratins in extraembryonic ectoderm

Epidermal-dermal tissue interactions regulate morphogenesis and tissue-specific keratinization of avian skin appendages. The morphogenesis of scutate scales differs from that of reticulate scales, and the keratin polypeptides of their epidermal surfaces are also different. Do the inductive cues which initiate morphogenesis of these scales also establish the tissue-specific keratin patterns of the epidermis, or does the control of tissue-specific keratinization occur at later stages of development? Unlike feathers, scutate and reticulate scales can be easily separated into their epidermal and dermal components late in development when the major events of morphogenesis have been completed and keratinization will begin. Using a common responding tissue (chorionic epithelium) in combination with scutate and reticulate scale dermises, we find that these embryonic dermises, which have completed morphogeneis, can direct tissue-specific stratification and keratinization. In other words, once a scale dermis has acquired its form, through normal morphogenesis, it is no longer able to initiate morphogenesis of that scale, but it can direct tissue-specific stratification and keratinization of a foreign ectodermal epithelium, which itself has not undergone scale morphogenesis.

Epigenesis in developing avian scales. I. Qualitative and quantitative characterization of finite cell populations

Throughout a period from day 8.5 to day 12.5 of incubation of a chick embryo, a finite cell population of scale epidermis was characterized from various view points such as cellular organization, position, shape, area, number of constituent cells, density, and cell proliferation activity. In this study, the preparation of whole mount specimens was found to be quite valuable. On day 8.5, cells in the prospective scale region could be morphologically distinguished in the tarsometatarsus at a certain distance proximally away from the tarsometatarsal-phalangeal joint. --On day 9.25, about 1,100 cells became highly columnar in shape and densely associated, forming a placode structure. In both distally and proximally adjacent regions of this placode, the cells were semiquadrate in shape and loosely associated, leading to the formation of the interplacode structures. Such contrasting difference in cell organization between placode and interplacode was preserved from day 9.25 to day 11. During this period, both the area and number of constituent cells increased greatly in the placode and only slightly in the interplacode. However, cell proliferation activity was completely suppressed in the placode, and quite active in the interplacode. The activity in cell proliferation proved to be inversely correlated with the density of basal cells. Throughout the present study, it has been demonstrated that the early development of scale epidermis is achieved through a coordinated activity of the two discrete cell populations: the placode and interplacode.

Correcting the phenotype of the epidermis from chick embryos homozygous for the gene scaleless (sc/sc)

Scutate scales are completely missing in the scaleless (sc/sc) mutant chicken. Organ cultures consisting of epidermis from sc/sc embryos combined with normal (+/+) scale dermis of the same developmental age produce the scaleless phenotype, but the same scaleless epidermis in combination with normal dermis from more differentiated embryonic scales forms perfectly normal scales.

Morphogenesis of conjunctival papillae from normal and scaleless chick embryos

Morphogenesis of avian conjunctival papillae follows a predictable temporal and spatial pattern and is in some manner directly related to the introduction of the underlying scleral ossicles. We have been able, using Scanning Electron Microscopy (SEM), to correlate all of Murray's ('43) histological stages (1--6) of papillae development, with changes in elevation and morphology of the surface of the conjunctiva. The first indication of morphogenesis is the formation of "papillae primordia." The centers of these primordia exhibit decreased intercellular contact, and become elevated as radially symmetrical humps whose surfaces are composed of rounded cells with numerous microvillar projections. As the papillae become asymmetrical and elongate, cells near the tip of the papillae enlarge and develop microridges. During regression of the papillae, single clusters of cells appear to become lost from the surfaces of the papillae into the surrounding fluid. In contrast to normal chick embryos, those homozygous for papillae and underlying scleral ossicles (Palmoski and Goetinck, '70). SEM of the mutant conjunctival surface indicates that these papillae do not exhibit all of Murray's ('43) histological stages and are morphologically abnormal. Data from the present SEM study of the normal and scaleless conjunctiva are discussed in relation to those data of other investigators, and we suggest that Stage 4 in papillae development is critical to scleral ossicle formation.

Epidermal-dermal recombinations with embryonic naked and normal back skin of Gallus domesticus

Birds exhibiting a varying featherless condition resulting from the recessive sex-linked gene naked (n) were used to investigate whether the gene altered the dermis or the epidermis. By splitting 7-day normal and naked skin into its dermal and epidermal components, and heterotypically recombining and growing it in chambers on the chorio-allantoic membrane (CAM), it was found that the epidermis of the naked birds is the site of mutant gene action. A histological study of developing normal and naked skin was done and the structure of the naked feather is elucidated.

Abnormal scale morphogenesis: relationship of polypeptide patterns to action of the scaleless gene

The polyacrylamide gradient gel electrophoresis (PAGGE) pattern of polypeptides isolated from normal scuttate scale epidermis of 1-week-old chicks was different from that of the anterior shank epidermis from 1-week-old scaleless mutant chicks. The PAGGE patterns of polypeptides isolated from normal and scaleless reticulate scale epidermis (from 1-week-old chicks) differed by only one band, whereas comparison of mutant's scuttate and reticulate patterns showed three band differences. These data are discussed in relation to the action of the scaleless gene on early morphogenesis of the two types of scales.

Avian scale development. Absence of an "epidermal placode" in reticulate scale morphogenesis

Timed-sequence studies have shown that reticulate scales on the ventral footpads of birds do not undergo "epidermal placode" formation during their morphogenesis, but arise as symmetrical evaluations similar to the scales of snakes and lizards. Unlike the scutellate scales on the dorsal surface of the foot, in which the formation of an "epidermal placode" and its subsequent morphogenesis result in disticnt outer and inner epidermal surfaces, the reticulate scales elaborate only one type of epidermal surface.

Epidermal-dermal tissue interactions between mutant and normal embryonic back skin: site of mutant gene activity determining abnormal feathering is in the epidermis

The site of the scaleless gene's activity in the development of abnormal feathers was determined by reciprocally recombining epidermis and dermis between normal and scaleless chick embryos and culturing the recombinants for seven days on the chorioallantoic membrane. When recombined with a common dermal source, feather development is enhanced by scaleless high line as compared to scaleless low line epidermis. Against a common responding tissue, 7-day normal back epidermis, significant differences were not found in feather inducing ability between normal, scaleless high line and scaleless low line dermis. It was concluded that, in relation to abnormal feathering, these tissue interactions reveal that the site of the scaleless gene's activity is the epidermis. A model of tissue interaction in the development of normal and abnormal feathers is presented. According to the model, the focus of the scaleless mutation and the genes accumulated by selection for high or low feather numbers is the epidermis, the effect being that the reactivity of the epidermis to dermal stimuli is altered. Subsequently, the epidermis controls the morphogenetic organization of the dermis. The scaleless dermis is presumed to contain normal positional information for the determination of feather structure and pattern.

Abnormal morphogenesis of feather structures and pattern in the chick embryo integument. II. Histological description

The development of skin and feathers in highly feathered scaleless mutants and normal Single Comb White Leghorn chick embryos was analyzed histologically. In addition, the growth of mutant feathers in chorioallantoic membrane culture is reported as a verification of several inferences made from observations of serially staged fixed specimens. The most striking feature of scaleless high line feather development is the widespread appearance of condensed or nearly condensed dermis. The discrete arrangement of normal placodes with underlying condensed dermis is replaced in the mutant by a heterogeneously shaped group of extremely large islands of columnar ("placodized") epithelium as long as 3,000 micron. The shape and extent of the condensed areas of dermis reflect the shape and extent of the overlying "placodized" epithelium. The polarity of the epidermis in normal feather germs, i.e., thicker epidermis on the posterior surface, is absent in mutant feather germs. This absence of epidermal polarity is reflected in the aberrant outgrowth of the mutant feather primordial. In the mutant, the basal cell layer of the epidermis invades the dermal core of the aberrant feather germs and may form barb vane ridges or feather sheaths. This process had no counterpart in the development of normal down feathers.