Modeling Edar expression reveals the hidden dynamics of tooth signaling center patterning.

When patterns are set during embryogenesis, it is expected that they are straightly established rather than subsequently modified. The patterning of the three mouse molars is, however, far from straight, likely as a result of mouse evolutionary history. The first-formed tooth signaling centers, called MS and R2, disappear before driving tooth formation and are thought to be vestiges of the premolars found in mouse ancestors. Moreover, the mature signaling center of the first molar (M1) is formed from the fusion of two signaling centers (R2 and early M1). Here, we report that broad activation of Edar expression precedes its spatial restriction to tooth signaling centers. This reveals a hidden two-step patterning process for tooth signaling centers, which was modeled with a single activator-inhibitor pair subject to reaction-diffusion (RD). The study of Edar expression also unveiled successive phases of signaling center formation, erasing, recovering, and fusion. Our model, in which R2 signaling center is not intrinsically defective but erased by the broad activation preceding M1 signaling center formation, predicted the surprising rescue of R2 in Edar mutant mice, where activation is reduced. The importance of this R2-M1 interaction was confirmed by ex vivo cultures showing that R2 is capable of forming a tooth. Finally, by introducing chemotaxis as a secondary process to RD, we recapitulated in silico different conditions in which R2 and M1 centers fuse or not. In conclusion, pattern formation in the mouse molar field relies on basic mechanisms whose dynamics produce embryonic patterns that are plastic objects rather than fixed end points.

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.

Fish Scales Dictate the Pattern of Adult Skin Innervation and Vascularization.

As animals mature from embryonic to adult stages, the skin grows and acquires specialized appendages, like hairs, feathers, and scales. How cutaneous blood vessels and sensory axons adapt to these dramatic changes is poorly understood. By characterizing skin maturation in zebrafish, we discovered that sensory axons are delivered to the adult epidermis in organized nerves patterned by features in bony scales. These nerves associate with blood vessels and osteoblasts above scales. Osteoblasts create paths in scales that independently guide nerves and blood vessels during both development and regeneration. By preventing scale regeneration and examining mutants lacking scales, we found that scales recruit, organize, and polarize axons and blood vessels to evenly distribute them in the skin. These studies uncover mechanisms for achieving comprehensive innervation and vascularization of the adult skin and suggest that scales coordinate a metamorphosis-like transformation of the skin with sensory axon and vascular remodeling.

The sequence of lanugo pattern development on the trunk wall in human fetuses.

BACKGROUND: Due to a growing interest in developmental disorders, and in the long-term skin appendage diseases, both in the cosmetic industry and among specialists in dermatology (broadly defined), there is an increasing number of papers on hair development. The publications by the present team of authors are part of this trend. OBJECTIVES: The aim of the study was to describe the topography and typology of skin pilosity patterns in human fetuses. MATERIAL AND METHODS: A total of 278 fetuses (141 male and 137 female) were qualified for the study. The gestational age ranged from 69 to 226 days after conception. All fetuses were taken from a local collection. RESULTS: The study revealed that the first single hairs occur on the posterior wall of the trunk in the 17th week of fetal life, and on the anterior wall between the 18th and 19th week. It was found that in human fetuses lanugo appears statistically significantly later on the skin of the anterior of the trunk than on its posterior. The difference in absolute time is almost 2 weeks of fetal life. No other differences were found in the development cycle of lanugo on the anterior and posterior walls of the trunk. A full pattern was first observed on the posterior wall of the trunk in a fetus in the 19th week, and on the anterior wall in the 21st week. It was found that the process of lanugo development was completed on the posterior wall in the 23rd week, and on the surface of the abdomen in the 26th week. CONCLUSIONS: The lanugo developmental cycle, consisting in the appearance of the first single hairs, then partial hair and subsequently the formation of final patterns, is the same on both walls of the trunk.

Dominant-negative Sox18 function inhibits dermal papilla maturation and differentiation in all murine hair types.

SOX family proteins SOX2 and SOX18 have been reported as being essential in determining hair follicle type; however, the role they play during development remains unclear. Here, we demonstrate that Sox18 regulates the normal differentiation of the dermal papilla of all hair types. In guard (primary) hair dermal condensate (DC) cells, we identified transient Sox18 in addition to SOX2 expression at E14.5, which allowed fate tracing of primary DC cells until birth. Similarly, expression of Sox18 was detected in the DC cells of secondary hairs at E16.5 and in tertiary hair at E18.5. Dominant-negative Sox18 mutation (opposum) did not prevent DC formation in any hair type. However, it affected dermal papilla differentiation, restricting hair formation especially in secondary and tertiary hairs. This Sox18 mutation also prevented neonatal dermal cells or dermal papilla spheres from inducing hair in regeneration assays. Microarray expression studies identified WNT5A and TNC as potential downstream effectors of SOX18 that are important for epidermal WNT signalling. In conclusion, SOX18 acts as a mesenchymal molecular switch necessary for the formation and function of the dermal papilla in all hair types.

Ectopic Atoh1 expression drives Merkel cell production in embryonic, postnatal and adult mouse epidermis.

Merkel cells are mechanosensitive skin cells whose production requires the basic helix-loop-helix transcription factor Atoh1. We induced ectopic Atoh1 expression in the skin of transgenic mice to determine whether Atoh1 was sufficient to create additional Merkel cells. In embryos, ectopic Atoh1 expression drove ectopic expression of the Merkel cell marker keratin 8 (K8) throughout the epidermis. Epidermal Atoh1 induction in adolescent mice similarly drove widespread K8 expression in glabrous skin of the paws, but in the whisker pads and body skin ectopic K8+ cells were confined to hair follicles and absent from interfollicular regions. Ectopic K8+ cells acquired several characteristics of mature Merkel cells in a time frame similar to that seen during postnatal development of normal Merkel cells. Although ectopic K8+ cell numbers decreased over time, small numbers of these cells remained in deep regions of body skin hair follicles at 3 months post-induction. In adult mice, greater numbers of ectopic K8+ cells were created by Atoh1 induction during anagen versus telogen and following disruption of Notch signaling by conditional deletion of Rbpj in the epidermis. Our data demonstrate that Atoh1 expression is sufficient to produce new Merkel cells in the epidermis, that epidermal cell competency to respond to Atoh1 varies by skin location, developmental age and hair cycle stage, and that the Notch pathway plays a key role in limiting epidermal cell competency to respond to Atoh1 expression.

Signaling in tooth, hair, and mammary placodes.

Skin appendages develop from placodes involving reciprocal interactions between the surface ectoderm and the underlying mesenchyme during embryogenesis. Despite their distinct shapes and functions, during early development similar morphological changes are observed among different skin appendages. Previous analyses of genetically modified mice have shown that these skin placodes share many aspects of molecular and cellular programs controlled by a relatively small number of signaling pathways during induction, morphogenesis, and transition to bud stage and beyond. This chapter focuses on the major signaling pathways that are reiteratively utilized to control the early developmental processes of placodes for teeth, hair follicles, and mammary glands. I update knowledge on the roles played by individual pathways and cross talk among them in these placodes and discuss similarities as well as differences among the skin appendages.

Light-emitting hair follicles: studying skin regeneration with in vivo imaging.

Cutting-edge imaging technologies and new luminescent and fluorescent genetic tools now make it possible to study hair regeneration in vivo in real time at the microscopic single-cell level and at the macroscopic level of hair follicle populations. These technologies also allow for noninvasive assessment of the skin's clinically relevant homeostatic parameters, such as oxidative stress levels and pH.

Hair loss in infancy.

Hair diseases represent a significant portion of cases seen by pediatric dermatologists although hair has always been a secondary aspect in pediatricians and dermatologists training, on the erroneous basis that there is not much information extractable from it. Dermatologists are in the enviable situation of being able to study many disorders with simple diagnostic techniques. The hair is easily accessible to examination but, paradoxically, this approach is often disregarded by non-dermatologist. This paper has been written on the purpose of trying to serve in the diagnostic process of daily practice, and trying to help, for example, to distinguish between certain acquired and some genetically determined hair diseases. We will focus on all the data that can be obtained from our patients' hair and try to help on using the messages given by hair for each patient. Quite often it is extremely hard to distinguish between abnormality and normality in neonatal hair aspects. We will specially focus in the most common physiological changes that may mislead to an incorrect diagnosis. Specific treatment for those hair diseases that do have one, and basic general approach to improve the cosmetic appearance of hair, will be also be discussed for those hair disturbances that do not have a specific treatment.

Transgenic flash mice for in vivo quantitative monitoring of canonical Wnt signaling to track hair follicle cycle dynamics.

Hair follicles (HFs) upon development enter a lifelong cycle of growth, regression, and resting. These phases have been extensively studied at the cellular and molecular levels for individual HFs. However, HFs group into domains with coordinated cycling strongly influenced by their environment. These macroscopic hair domains have been difficult to study and can be influenced by physiological or pathological conditions, such as pregnancy or skin wounds. To robustly address this issue, we generated a mouse model for quantitative monitoring of ß-catenin activity reflecting HF cycle dynamics macroscopically by using live bioluminescence imaging. These mice allowed live tracking of HF cycles and development, and highlighted hair regenerative patterns known to occur through macro-environmental cues, including initiation events, propagating anagen and border stability, and allowed refinement of a mechanistic mathematical model that integrates epidermal cell population dynamics into an excitable reaction-diffusion model. HF cycling could be studied in situations of pregnancy, wound healing, hair plucking, as well as in response to cyclosporine or Wnt3a stimulation. In conclusion, we developed a model for analysis of HF cycling at the macroscopic level that will allow refined analysis of hair cycle kinetics as well as its propagation dynamics.

Spatial and temporal control of laminin-332 and -511 expressions during hair morphogenesis.

Hair is one of the smallest organs, but has many important functions to mammals. Hair morphogenesis occurs through the reciprocal exchange of epithelial and mesenchymal signals. There are some reports about the expression of laminin-511 and -332 during hair morphogenesis, but are no reports of the chronological expression and function of laminin-511 and its counter regulator laminin-332 during hair morphogenesis. Our results of immunoblotting revealed that laminin-332 proteins were detected at stage 0 and downregulated during stage 1 to stage 2, and then recovered at stage 3. However, laminin α5 expression was constant throughout stages 0-3. According to the results of semi-quantitative RT-PCR, the mRNA expression of all laminin-332 subunits increased gradually from stage 0 to stage 2, while the mRNA expression of all laminin-511 subunits remained constant from stage 0 to stage 3. Our results suggest that the proper expression of laminin-332 and laminin-511 may regulate appropriate hair morphogenesis.

Keratin 79 identifies a novel population of migratory epithelial cells that initiates hair canal morphogenesis and regeneration.

The formation of epithelial tubes underlies the development of diverse organs. In the skin, hair follicles resemble tube-like structures with lumens that are generated through poorly understood cellular rearrangements. Here, we show that creation of the hair follicle lumen is mediated by early outward movement of keratinocytes from within the cores of developing hair buds. These migratory keratinocytes express keratin 79 (K79) and stream out of the hair germ and into the epidermis prior to lumen formation in the embryo. Remarkably, this process is recapitulated during hair regeneration in the adult mouse, when K79(+) cells migrate out of the reactivated secondary hair germ prior to formation of a new hair canal. During homeostasis, K79(+) cells line the hair follicle infundibulum, a domain we show to be multilayered, biochemically distinct and maintained by Lrig1(+) stem cell-derived progeny. Upward movement of these cells sustains the infundibulum, while perturbation of this domain during acne progression is often accompanied by loss of K79. Our findings uncover previously unappreciated long-distance cell movements throughout the life cycle of the hair follicle, and suggest a novel mechanism by which the follicle generates its hollow core through outward cell migration.

Foetal presentation of cartilage hair hypoplasia with extensive granulomatous inflammation.

Cartilage-hair-hypoplasia is a rare autosomal recessive metaphyseal dysplasia due to RMRP (the RNA component of the RNase MRP ribonuclease mitochondrial RNA processing complex) gene mutations. So far, about 100 mutations have been reported in the promoter and the transcribed regions. Clinical characteristics include short-limbed short stature, sparse hair and defective cell-mediated immunity. We report herein the antenatal presentation of a female foetus, in whom CHH was suspected from 23 weeks' gestation, leading to a medical termination of the pregnancy at 34 weeks gestation, and thereafter confirmed by morphological and molecular studies. Post-mortem examination confirmed short stature and limbs, and revealed thymic hypoplasia associated with severe CD4 T-cell immunodeficiency along with extensive non caseating epithelioid granulomas in almost all organs, which to our knowledge has been described only in five cases. Molecular studies evidenced on one allele the most frequently reported founder mutation NR_003051: g.70A>G, which is present in 92% of Finnish patients with Cartilage Hair Hypoplasia. On the second allele, a novel mutation consisting of a 10 nucleotide insertion at position -18 of the promoter region of the RMRP gene (M29916.1:g.726_727insCTCACTACTC) was detected. The founder mutation was inherited from the father, and the novel mutation from the mother. To our knowledge, this case report represents the first detailed foetal analysis described in the literature.

Expression of Foxi3 is regulated by ectodysplasin in skin appendage placodes.

BACKGROUND: Foxi3 is a member of the large forkhead box family of transcriptional regulators, which have a wide range of biological activities including manifold developmental processes. Heterozygous mutation in Foxi3 was identified in several hairless dog breeds characterized by sparse fur coat and missing teeth. A related phenotype called hypohidrotic ectodermal dysplasia (HED) is caused by mutations in the ectodysplasin (Eda) pathway genes. RESULTS: Expression of Foxi3 was strictly confined to the epithelium in developing ectodermal appendages in mouse embryos, but no expression was detected in the epidermis. Foxi3 was expressed in teeth and hair follicles throughout embryogenesis, but in mammary glands only during the earliest stages of development. Foxi3 expression was decreased and increased in Eda loss- and gain-of-function embryos, respectively, and was highly induced by Eda protein in embryonic skin explants. Also activin A treatment up-regulated Foxi3 mRNA levels in vitro. CONCLUSIONS: Eda and activin A were identified as upstream regulators of Foxi3. Foxi3 is a likely transcriptional target of Eda in ectodermal appendage placodes suggesting that HED phenotype may in part be produced by compromised Foxi3 activity. In addition to hair and teeth, Foxi3 may have a role in nail, eye, and mammary, sweat, and salivary gland development.

Specifying and sustaining pigmentation patterns in domestic and wild cats.

Color markings among felid species display both a remarkable diversity and a common underlying periodicity. A similar range of patterns in domestic cats suggests a conserved mechanism whose appearance can be altered by selection. We identified the gene responsible for tabby pattern variation in domestic cats as Transmembrane aminopeptidase Q (Taqpep), which encodes a membrane-bound metalloprotease. Analyzing 31 other felid species, we identified Taqpep as the cause of the rare king cheetah phenotype, in which spots coalesce into blotches and stripes. Histologic, genomic expression, and transgenic mouse studies indicate that paracrine expression of Endothelin3 (Edn3) coordinates localized color differences. We propose a two-stage model in which Taqpep helps to establish a periodic pre-pattern during skin development that is later implemented by differential expression of Edn3.

Keratinocyte-specific ablation of the NF-κB regulatory protein A20 (TNFAIP3) reveals a role in the control of epidermal homeostasis.

The ubiquitin-editing enzyme A20 (tumor necrosis factor-α-induced protein 3) serves as a critical brake on nuclear factor κB (NF-κB) signaling. In humans, polymorphisms in or near the A20 gene are associated with several inflammatory disorders, including psoriasis. We show here that epidermis-specific A20-knockout mice (A20(EKO)) develop keratinocyte hyperproliferation, but no signs of skin inflammation, such as immune cell infiltration. However, A20(EKO) mice clearly developed ectodermal organ abnormalities, including disheveled hair, longer nails and sebocyte hyperplasia. This phenotype resembles that of mice overexpressing ectodysplasin-A1 (EDA-A1) or the ectodysplasin receptor (EDAR), suggesting that A20 negatively controls EDAR signaling. We found that A20 inhibited EDAR-induced NF-κB signaling independent from its de-ubiquitinating activity. In addition, A20 expression was induced by EDA-A1 in embryonic skin explants, in which its expression was confined to the hair placodes, known to be the site of EDAR expression. In summary, our data indicate that EDAR-induced NF-κB levels are controlled by A20, which functions as a negative feedback regulator, to assure proper skin homeostasis and epidermal appendage development.

Sphere formation restores and confers hair-inducing capacity in cultured mesenchymal cells.

Interactions between epithelial and dermal cells are essential for hair follicle morphogenesis and maintenance. In experimental trials of hair regeneration, isolated dermal cells have been shown to possess hair-inducing capacity. However, dermal cells lose this potential immediately after cultivation. Sphere-forming multipotent cells derived from the dermis possess hair-inducing capacity. These previous findings raise the question of whether hair-inducing capacity depends on the identity as dermal cells or the process of sphere formation. To address this issue, we compared the in vitro and in vivo characteristics of two-dimensionally cultured or thereafter sphere formation-induced dermal and lung mesenchymal cells. We show that sphere-forming mesenchymal cells exhibited higher expression of Wnt signalling genes. Sphere-forming cells but not two-dimensionally cultured cells possessed in vivo hair-inducing capacity. These data suggest that various mesenchymal cells attain hair-inducing capacity through the process of sphere formation.

Two frizzled planar cell polarity signals in the Drosophila wing are differentially organized by the Fat/Dachsous pathway.

The regular array of distally pointing hairs on the mature Drosophila wing is evidence for the fine control of Planar Cell Polarity (PCP) during wing development. Normal wing PCP requires both the Frizzled (Fz) PCP pathway and the Fat/Dachsous (Ft/Ds) pathway, although the functional relationship between these pathways remains under debate. There is strong evidence that the Fz PCP pathway signals twice during wing development, and we have previously presented a Bidirectional-Biphasic Fz PCP signaling model which proposes that the Early and Late Fz PCP signals are in different directions and employ different isoforms of the Prickle protein. The goal of this study was to investigate the role of the Ft/Ds pathway in the context of our Fz PCP signaling model. Our results allow us to draw the following conclusions: (1) The Early Fz PCP signals are in opposing directions in the anterior and posterior wing and converge precisely at the site of the L3 wing vein. (2) Increased or decreased expression of Ft/Ds pathway genes can alter the direction of the Early Fz PCP signal without affecting the Late Fz PCP signal. (3) Lowfat, a Ft/Ds pathway regulator, is required for the normal orientation of the Early Fz PCP signal but not the Late Fz PCP signal. (4) At the time of the Early Fz PCP signal there are symmetric gradients of dachsous (ds) expression centered on the L3 wing vein, suggesting Ds activity gradients may orient the Fz signal. (5) Localized knockdown or over-expression of Ft/Ds pathway genes shows that boundaries/gradients of Ft/Ds pathway gene expression can redirect the Early Fz PCP signal specifically. (6) Altering the timing of ds knockdown during wing development can separate the role of the Ft/Ds pathway in wing morphogenesis from its role in Early Fz PCP signaling.

Cyclic expression of lhx2 regulates hair formation.

Hair is important for thermoregulation, physical protection, sensory activity, seasonal camouflage, and social interactions. Hair is generated in hair follicles (HFs) and, following morphogenesis, HFs undergo cyclic phases of active growth (anagen), regression (catagen), and inactivity (telogen) throughout life. The transcriptional regulation of this process is not well understood. We show that the transcription factor Lhx2 is expressed in cells of the outer root sheath and a subpopulation of matrix cells during both morphogenesis and anagen. As the HFs enter telogen, expression becomes undetectable and reappears prior to initiation of anagen in the secondary hair germ. In contrast to previously published results, we find that Lhx2 is primarily expressed by precursor cells outside of the bulge region where the HF stem cells are located. This developmental, stage- and cell-specific expression suggests that Lhx2 regulates the generation and regeneration of hair. In support of this hypothesis, we show that Lhx2 is required for anagen progression and HF morphogenesis. Moreover, transgenic expression of Lhx2 in postnatal HFs is sufficient to induce anagen. Thus, our results reveal an alternative interpretation of Lhx2 function in HFs compared to previously published results, since Lhx2 is periodically expressed, primarily in precursor cells distinct from those in the bulge region, and is an essential positive regulator of hair formation.

beta-catenin activity in the dermal papilla regulates morphogenesis and regeneration of hair.

The activity of keratinocytes in the hair follicle is regulated by signals from a specialized mesenchymal niche, the dermal papilla (DP). Here, mice expressing cre recombinase in the DP were developed to probe the interaction between follicular keratinocytes and the DP in vivo. Inactivation of the beta-catenin gene within DP of fully developed hair follicles results in dramatically reduced proliferation of the progenitors and their progeny that generate the hair shaft, and, subsequently, premature induction of the destructive phase of the hair cycle. It also prevents regeneration of the cycling follicle from stem cells. Gene expression analysis reveals that beta-catenin activity in the DP regulates signaling pathways, including FGF and IGF, that can mediate the DP's inductive effects. This study reveals a signaling loop that employs Wnt/beta-catenin signaling in both epithelial progenitor cells and their mesenchymal niche to govern and coordinate the interactions between these compartments to guide hair morphogenesis.