The avian ectodermal default competence to make feathers.

Feathers originate as protofeathers before birds, in pterosaurs and basal dinosaurs. What characterizes a feather is not only its outgrowth, but its barb cells differentiation and a set of beta-corneous proteins. Reticula appear concomitantly with feathers, as small bumps on plantar skin, made only of keratins. Avian scales, with their own set of beta-corneous proteins, appear more recently than feathers on the shank, and only in some species. In the chick embryo, when feather placodes form, all the non-feather areas of the integument are already specified. Among them, midventral apterium, cornea, reticula, and scale morphogenesis appear to be driven by negative regulatory mechanisms, which modulate the inherited capacity of the avian ectoderm to form feathers. Successive dermal/epidermal interactions, initiated by the Wnt/ß-catenin pathway, and involving principally Eda/Edar, BMP, FGF20 and Shh signaling, are responsible for the formation not only of feather, but also of scale placodes and reticula, with notable differences in the level of Shh, and probably FGF20 expressions. This sequence is a dynamic and labile process, the turning point being the FGF20 expression by the placode. This epidermal signal endows its associated dermis with the memory to aggregate and to stimulate the morphogenesis that follows, involving even a re-initiation of the placode.

Evo Devo of the Vertebrates Integument.

All living jawed vertebrates possess teeth or did so ancestrally. Integumental surface also includes the cornea. Conversely, no other anatomical feature differentiates the clades so readily as skin appendages do, multicellular glands in amphibians, hair follicle/gland complexes in mammals, feathers in birds, and the different types of scales. Tooth-like scales are characteristic of chondrichthyans, while mineralized dermal scales are characteristic of bony fishes. Corneous epidermal scales might have appeared twice, in squamates, and on feet in avian lineages, but posteriorly to feathers. In contrast to the other skin appendages, the origin of multicellular glands of amphibians has never been addressed. In the seventies, pioneering dermal-epidermal recombination between chick, mouse and lizard embryos showed that: (1) the clade type of the appendage is determined by the epidermis; (2) their morphogenesis requires two groups of dermal messages, first for primordia formation, second for appendage final architecture; (3) the early messages were conserved during amniotes evolution. Molecular biology studies that have identified the involved pathways, extending those data to teeth and dermal scales, suggest that the different vertebrate skin appendages evolved in parallel from a shared placode/dermal cells unit, present in a common toothed ancestor, c.a. 420 mya.

The Early Origin of Feathers.

Feathers have long been regarded as the innovation that drove the success of birds. However, feathers have been reported from close dinosaurian relatives of birds, and now from ornithischian dinosaurs and pterosaurs, the cousins of dinosaurs. Incomplete preservation makes these reports controversial. If true, these findings shift the origin of feathers back 80 million years before the origin of birds. Gene regulatory networks show the deep homology of scales, feathers, and hairs. Hair and feathers likely evolved in the Early Triassic ancestors of mammals and birds, at a time when synapsids and archosaurs show independent evidence of higher metabolic rates (erect gait and endothermy), as part of a major resetting of terrestrial ecosystems following the devastating end-Permian mass extinction.

Getting to the root of scales, feather and hair: As deep as odontodes?

While every jawed vertebrate, or its recent ancestor, possesses teeth, skin appendages are characteristic of the living clades: skin denticles (odontodes) in chondrichthyans, dermal scales in teleosts, ducted multicellular glands in amphibians, epidermal scales in squamates, feathers in birds and hair-gland complexes in mammals, all of them showing a dense periodic patterning. While the odontode origin of teleost scales is generally accepted, the origin of both feather and hair is still debated. They appear long before mammals and birds, at least in the Jurassic in mammaliaforms and in ornithodires (pterosaurs and dinosaurs), and are contemporary to scales of early squamates. Epidermal scales might have appeared several times in evolution, and basal amniotes could not have developed a scaled dry integument, as the function of hair follicle requires its association with glands. In areas such as amnion, cornea or plantar pads, the formation of feather and hair is prevented early in embryogenesis, but can be easily reverted by playing with the Wnt/BMP/Shh pathways, which both imply the plasticity and the default competence of ectoderm. Conserved ectodermal/mesenchymal signalling pathways lead to placode formation, while later the crosstalk differs, as well as the final performing tissue(s): both epidermis and dermis for teeth and odontodes, mostly dermis for teleosts scales and only epidermis for squamate scale, feather and hair. We therefore suggest that tooth, dermal scale, epidermal scale, feather and hair evolved in parallel from a shared placode/dermal cell unit, which was present in a common ancestor, an early vertebrate gnathostome with odontodes, ca. 420 million years ago.

Response to Comment on "A Jurassic ornithischian dinosaur from Siberia with both feathers and scales".

Lingham-Soliar questions our interpretation of integumentary structures in the Middle-Late Jurassic ornithischian dinosaur Kulindadromeus as feather-like appendages and alternatively proposes that the compound structures observed around the humerus and femur of Kulindadromeus are support fibers associated with badly degraded scales. We consider this hypothesis highly unlikely because of the taphonomy and morphology of the preserved structures.

The vertebrate corneal epithelium: from early specification to constant renewal.

BACKGROUND: The cornea is an ectodermal/neural crest derivative formed through a cascade of molecular mechanisms to give rise to the specific optical features necessary for its refractory function. Moreover, during cornea formation and maturation, epithelial stem cells are sequestered to ensure a constant source for renewal in the adult. RESULTS: Recent progress in the molecular and stem cell biology of corneal morphogenesis and renewal shows that it can serves as a paradigm for epithelial /mesenchymal organ biology. This review will synthesize historical knowledge together with recent data to present a consistent overview of cornea specification, formation, maturation, and maintenance. CONCLUSIONS: This should be of interest not only to developmental biologists but also ophthalmologists, as several human vision problems are known to be rooted in defects in corneal development.

Dinosaur evolution. A Jurassic ornithischian dinosaur from Siberia with both feathers and scales.

Middle Jurassic to Early Cretaceous deposits from northeastern China have yielded varied theropod dinosaurs bearing feathers. Filamentous integumentary structures have also been described in ornithischian dinosaurs, but whether these filaments can be regarded as part of the evolutionary lineage toward feathers remains controversial. Here we describe a new basal neornithischian dinosaur from the Jurassic of Siberia with small scales around the distal hindlimb, larger imbricated scales around the tail, monofilaments around the head and the thorax, and more complex featherlike structures around the humerus, the femur, and the tibia. The discovery of these branched integumentary structures outside theropods suggests that featherlike structures coexisted with scales and were potentially widespread among the entire dinosaur clade; feathers may thus have been present in the earliest dinosaurs.

Evo-devo of the mammary gland.

We propose a new scenario for mammary evolution based on comparative review of early mammary development among mammals. Mammary development proceeds through homologous phases across taxa, but evolutionary modifications in early development produce different final morphologies. In monotremes, the mammary placode spreads out to form a plate-like mammary bulb from which more than 100 primary sprouts descend into mesenchyme. At their distal ends, secondary sprouts develop, including pilosebaceous anlagen, resulting in a mature structure in which mammary lobules and sebaceous glands empty into the infundibula of hair follicles; these structural triads (mammolobular-pilo-sebaceous units or MPSUs) represent an ancestral condition. In marsupials a flask-like mammary bulb elongates as a sprout, but then hollows out; its secondary sprouts include hair and sebaceous anlagen (MPSUs), but the hairs are shed during nipple formation. In some eutherians (cat, horse, human) MPSUs form at the distal ends of primary sprouts; pilosebaceous components either regress or develop into mature structures. We propose that a preexisting structural triad (the apocrine-pilo-sebaceous unit) was incorporated into the evolving mammary structure, and coupled to additional developmental processes that form the mammary line, placode, bulb and primary sprout. In this scenario only mammary ductal trees and secretory tissue derive from ancestral apocrine-like glands. The mammary gland appears to have coopted signaling pathways and genes for secretory products from even earlier integumentary structures, such as odontode (tooth-like) or odontode-derived structures. We speculate that modifications in signal use (such as PTHrP and BMP4) may contribute to taxonomic differences in MPSU development.

The corneal epithelium and lens develop independently from a common pool of precursors.

BACKGROUND: The corneal epithelium (CE) overlays a stroma, which is derived from neural crest cells, and appears to be committed during chick development, but appears still labile in adult rabbit. Its specification was hitherto regarded as resolved and dependent upon the lens, although without experimental support. Here, we challenged CE fate by changing its environment at different stages. RESULTS: Recombination with a dermis showed that CE commitment is linked to stroma formation, which results in Pax6 stabilization in both species. Surgical ablation shows that CE specification has already taken place when the lens placode invaginates, while removal of the early lens placode led to lens renewal. To block lens formation, bone morphogenetic protein (BMP) signaling, one of its last inducing factors, was inhibited by over-expression of Gremlin in the ocular ectoderm. This resulted in lens-less embryos which formed a corneal epithelium if they survived 2 weeks. CONCLUSION: The corneal epithelium and lens share a common pool of precursors. The adoption of the CE fate might be dependent on the loss of a lens placode favoring environment. The corneal fate is definitively stabilized by the migration of Gremlin-expressing neural crest cells in the lens peripheral ectoderm.

Skin, cornea and stem cells - an interview with Danielle Dhouailly. Interviewed by Chuong, Cheng-Ming.

Danielle Dhouailly received her Bachelor of Science degree (Biology) from Paris University. She then worked on a Ph.D. with Philippe Sengel at Grenoble University. After that, she went to Canada and the USA to work with Drs. M. Hardy, R. Sawyer and H. Sun before going back to Grenoble and starting her own laboratory. In the 1970s, she began a series of creative epithelial-mesenchymal recombination experiments among chicken feathers, mouse hairs and lizard scales, and later between rabbit cornea / mouse hairs. Through these original experiments, she elegantly demonstrated that the dermis initiates the formation of cutaneous appendages, while their type is specified by the class and regional origin of the epidermis. Subsequently she showed that the induction of an ectodermal organ, even in an adult epithelium, provokes the appearance of the related tissue stem cells. These works pioneered the concepts which are used in stem cell biology today. Her laboratory now works on the molecular mechanisms underlying these processes. Her papers are typically characterized by an initial insightful observation, followed by rigorous experiments and thoughtful discussions. They are rich with different shades of perspectives, almost like a piece of impressionist art. She loves gardening and her pets. She considers herself a good observer and hard worker driven by curiosity. Her best moments occur when she suddenly becomes enlightened as to an explanation of a basic concept when looking at experimental results or discussing ideas with colleagues. She believes that good results last forever, although interpretations can change. Her advice to young scientists is to be rigorous at the bench, to think hard, and not to be shy to speak up. The following is the story of how this young, female naturalist grew into a well-respected developmental biologist.

A new scenario for the evolutionary origin of hair, feather, and avian scales.

In zoology it is well known that birds are characterized by the presence of feathers, and mammals by hairs. Another common point of view is that avian scales are directly related to reptilian scales. As a skin embryologist, I have been fascinated by the problem of regionalization of skin appendages in amniotes throughout my scientific life. Here I have collected the arguments that result from classical experimental embryology, from the modern molecular biology era, and from the recent discovery of new fossils. These arguments shape my view that avian ectoderm is primarily programmed toward forming feathers, and mammalian ectoderm toward forming hairs. The other ectoderm derivatives - scales in birds, glands in mammals, or cornea in both classes - can become feathers or hairs through metaplastic process, and appear to have a negative regulatory mechanism over this basic program. How this program is altered remains, in most part, to be determined. However, it is clear that the regulation of the Wnt/beta-catenin pathway is a critical hub. The level of beta-catenin is crucial for feather and hair formation, as its down-regulation appears to be linked with the formation of avian scales in chick, and cutaneous glands in mice. Furthermore, its inhibition leads to the formation of nude skin and is required for that of corneal epithelium. Here I propose a new theory, to be further considered and tested when we have new information from genomic studies. With this theory, I suggest that the alpha-keratinized hairs from living synapsids may have evolved from the hypothetical glandular integument of the first amniotes, which may have presented similarities with common day terrestrial amphibians. Concerning feathers, they may have evolved independently of squamate scales, each originating from the hypothetical roughened beta-keratinized integument of the first sauropsids. The avian overlapping scales, which cover the feet in some bird species, may have developed later in evolution, being secondarily derived from feathers.

BMP2 and BMP7 play antagonistic roles in feather induction.

Feathers, like hairs, first appear as primordia consisting of an epidermal placode associated with a dermal condensation that is necessary for the continuation of their differentiation. Previously, the BMPs have been proposed to inhibit skin appendage formation. We show that the function of specific BMPs during feather development is more complex. BMP2 and BMP7, which are expressed in both the epidermis and the dermis, are involved in an antagonistic fashion in regulating the formation of dermal condensations, and thus are both necessary for subsequent feather morphogenesis. BMP7 is expressed earlier and functions as a chemoattractant that recruits cells into the condensation, whereas BMP2 is expressed later, and leads to an arrest of cell migration, likely via its modulation of the EIIIA fibronectin domain and alpha4 integrin expression. Based on the observed cell proliferation, chemotaxis and the timing of BMP2 and BMP7 expression, we propose a mathematical model, a reaction-diffusion system, which not only simulates feather patterning, but which also can account for the negative effects of excess BMP2 or BMP7 on feather formation.

Dermal condensation formation in the chick embryo: requirement for integrin engagement and subsequent stabilization by a possible notch/integrin interaction.

During embryonic development, feathers appear first as primordia consisting of an epidermal placode associated with a dermal condensation. When 7-day chick embryo dorsal skin fragments showing three rows of feather primordia are cultured, they undergo a complete reorganization, which involves the down-regulation of morphogenetic genes and dispersal of dermal fibroblasts, leading to the disappearance of primordia. This loss of organisation is followed by de novo differentiation events. We have used this model to study potential factors involved in the formation of dermal condensations. Activation of Integrins by extracellular Manganese or intracellular Calcium prevents the initial disappearance of the dermal condensations. New primordia formation occurs even after inhibition of the Notch pathway albeit with some fusion between primordia. In conclusion, dermal fibroblast migration requires beta1-Integrin whereas the stability of dermal condensations could depend on Notch/Integrin interaction.

Transdifferentiation of corneal epithelium into epidermis occurs by means of a multistep process triggered by dermal developmental signals.

Differentiated cells of the corneal epithelium are converted to hair, along with their associated stem cells, then interfollicular epidermis, by means of a multistep process triggered by dermal developmental signals. The committed basal cells of the adult corneal epithelium dedifferentiate under the control of signals from an associated embryonic hair-forming dermis, likely Wnts, and revert to a limbal basal cell phenotype. This initial process involves the down-regulation of Pax6 and the loss of expression of corneal-specific keratins and the induction of basal keratinocyte markers. These dedifferentiated cells are able to reinduce dermal condensations, which in turn induce the formation of hair follicles from cells that have lost Pax6 expression, by means of a Noggin-dependent mechanism. An epidermis is subsequently formed by cells derived from the newly segregated hair stem cells.

Transformation of amnion epithelium into skin and hair follicles.

There is increasing interest into the extent to which epithelial differentiation can be altered by mesenchymal influence, and the molecular basis for these changes. In this study, we investigated whether amnion epithelium could be transformed into skin and hair follicles by associating E12.5 to E14.5 mouse amnion from the ROSA 26 strain, with mouse embryonic hair-forming dermis from a wild-type strain. These associations were able to produce fully formed hair follicles with associated sebaceous glands, and skin epidermis. Using beta-galactosidase staining we were able to demonstrate that the follicular epithelium and skin epidermis, but not the associated dermal cells, originated from the amnion. As Noggin and Sonic hedgehog (Shh) were recently shown to be required for early chick ventral skin formation, and able to trigger skin and feather formation from chick amnion, we associated cells engineered to produce those two factors with mouse amnion. In a few cases, we obtained hair buds connected to a pluristratified epithelium; however, the transformation of the amnion was impeded by uncontrolled fibroblastic proliferation. In contrast to an earlier report, none of our control amnion specimens autonomously transformed into skin and hair follicles, indicating that specific influences are necessary to elicit follicle formation from the mouse amnion. The ability to turn amnion into skin and its appendages has practical potential for the tissue engineering of replacement skin, and related biotechnological approaches.

Skin field formation: morphogenetic events.

This chapter is mostly a review of the pioneering work of the Philippe Sengel school in Grenoble carried out in the late sixties and the seventies. The questions raised concerning the morphogenesis of feather tracts were approached by means of microsurgery on chick embryos. P. Sengel and his wife M. Kieny had the feeling that proteins synthesized by the neural tube were required for the formation of feather fields. It was my pleasure to carry on the story from the beginning. Although some clarifications concerning this morphogenesis have been contributed by my group and by a few other laboratories interested in this subject, the most important contributions to recent research have been the elucidation of the nature of the required messages, which will be explored further in other papers in this Issue.

Molecular mechanisms controlling dorsal dermis generation from the somitic dermomyotome.

The initiation of the development of skin appendages (hair/feathers/scales) requires a signal from the competent dense dermis to the epidermis (Dhouailly, 1977). It is therefore essential to understand how to make a competent dermis. In recent years, a few studies have focused on the development of the dorsal dermis from the somitic dermomyotome. Our first aim in this review is to attempt to reconcile the available data on the origin of the dorsal dermis and summarize the present knowledge on the molecular mechanisms implicated in dermal lineage induction. Secondly, we open the discussion on the formation of a loose pre-dermal mesenchyme and more importantly of a dense dermis capable of participating in appendage development. To go further we draw a comparison between the chick and mouse systems to gain a new insight into how to initiate appendage morphogenesis and regulate the extent of hair/feather fields.

Ventral vs. dorsal chick dermal progenitor specification.

The dorsal and the ventral trunk integuments of the chick differ in their dermal cell lineage (originating from the somatic and somatopleural mesoderm respectively) and in the distribution of their feather fields. The dorsal macropattern has a large spinal pteryla surrounded by semi-apteria, whereas the ventral skin has a true medial apterium surrounded by the ventral pterylae. Comparison of the results of heterotopic transplantations of distal somatopleure in place of somatic mesoderm (Mauger 1972) or in place of proximal somatopleure (our data), leads to two conclusions. These are that the fate of the midventral apterium is not committed at day 2 of incubation and that the signals from the environment which specify the ventral and dorsal featherforming dermal progenitors are different. Effectively, Shh, but not Wnt -1 signalling can induce the formation of feather forming dermis from the embryonic somatopleure. Shh is not able, however, to trigger the formation of a feather forming dermis from the extra embryonic somatopleure. This brief report constitutes the first attempt, by comparing old and new preliminary results, to understand whether dermal progenitors at different sites are specified by different signalling pathways.

The different steps of skin formation in vertebrates.

Skin morphogenesis occurs following a continuous series of cell-cell interactions which can be subdivided into three main stages: 1- the formation of a dense dermis and its overlying epidermis in the future appendage fields (macropattern); 2- the organization of these primary homogeneous fields into heterogeneous ones by the appearance of cutaneous appendage primordia (micropattern) and 3- cutaneous appendage organogenesis itself. In this review, we will first show, by synthesizing novel and previously published data from our laboratory, how heterogenetic and heterospecific dermal/epidermal recombinations have allowed us to distinguish between the respective roles of the dermis and the epidermis. We will then summarize what is known from the work of many different research groups about the molecular signaling which mediates these interactions in order to introduce the following articles of this Special Issue and to highlight what remains to done.