Epidermal-dermal tissue interactions between mutant foot skin and normal back skin: a comparison of the inductive capacities of scaleless low line and normal anterior foot dermis

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.

Bmp7 mediates early signaling events during induction of chick epidermal organs

The induction and specification of a large number of vertebrate organs require reciprocal signaling between an epithelium and subjacent mesenchyme. In the formation of integumentary organs, the initial inductive signaling events leading to the formation of the organ primordia stem from the mesenchyme. However, the epithelium must have the capacity to respond to these signals. We demonstrate that bone morphogenetic protein 7 (Bmp7) is an early molecular marker for epidermal organ development during development of feathers and scales of the chick. Bmp7 is expressed broadly in the preplacode epidermis and subsequently becomes localized to the forming placodes of feathers and scales. An examination of Bmp7 expression in the scaleless mutant chicken integument indicates that Bmp7 expression in the epidermis is associated with the ability to form epidermal organs. We show that BMP7 function is necessary for the formation of epidermal placodes in both feather and scale forming epidermis. In addition, precocious expression of Bmp7 in the metatarsal epidermis of the Silkie mutant or treatment of the metatarsus with ectopic BMP7 protein results in feather development from scale forming integument. From these data, we propose that Bmp7 is necessary and sufficient, in a developmental context, to mediate the competence of an epithelium to respond to inductive signals from the underlying mesenchyme to form epidermal organs in the chick. We propose that regulation of Bmp7 in localized areas of the embryonic epidermis facilitates the development of regional formation of integumentary organs.

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.

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 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.

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.

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.

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.

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.