Epidermal Placodes Research Articles

Foxi3GFP and Foxi3CreER mice allow identification and lineage labeling of pharyngeal arch ectoderm and endoderm, and tooth and hair placodes.

FOXI3 is a forkhead family transcription factor that is expressed in the progenitors of craniofacial placodes, epidermal placodes, and the ectoderm and endoderm of the pharyngeal arch region. Loss of Foxi3 in mice and pathogenic Foxi3 variants in dogs and humans cause a variety of craniofacial defects including absence of the inner ear, severe truncations of the jaw, loss or reduction in external and middle ear structures, and defects in teeth and hair. To allow for the identification, isolation, and lineage tracing of Foxi3-expressing cells in developing mice, we targeted the Foxi3 locus to create Foxi3GFP and Foxi3CreER mice. We show that Foxi3GFP mice faithfully recapitulate the expression pattern of Foxi3 mRNA at all ages examined, and Foxi3CreER mice can trace the derivatives of pharyngeal arch ectoderm and endoderm, the pharyngeal pouches and clefts that separate each arch, and the derivatives of hair and tooth placodes. Foxi3GFP and Foxi3CreER mice are new tools that will be of use in identifying and manipulating pharyngeal arch ectoderm and endoderm and hair and tooth placodes.

Breed-specific expression mode of the Wnt signalling pathway is involved in feather follicle morphogenesis between Anser cygnoide and Anser anser

ABSTRACT Feathers are important economic products. Anser cygnoide (AC) is superior in reproduction, growth and feather quality, while Anser anser (AA) has an advantage in feather yield. Understanding the development of feather follicles facilitates deliberate crossbreeding. Here, we compared feather follicles morphogenesis between AC and AA. Feathers and dorsal skin were sampled at multiple embryonic days to investigate feather phenotypes, feather follicle morphology and the expression of hub molecules (FZD4, β-catenin, LEF1, TCF4 and c-Myc) in the Wnt signalling pathway. We found a marked difference in plumage colour between the two breeds. Feather colour in AA has undergone a ‘black dorsum’ stage and cavity formation in AC was earlier. Furthermore, different trends in molecular hub expression helped explain the phenotypical and morphological differences: the upregulated Wnt signals in AC at E12 and/or E15, as well as in AA at E18, were associated with early epidermal placode formation and cavity formation in AC, also, related to the development of the secondary feather follicles and ‘black dorsum’ in AA. Summarily, breed-specific expression patterns of Wnt signalling pathway between AC and AA might participate in the shaping of breed-specific feather follicles morphogenesis and feather phenotypes. This work provides references for feather follicles development in geese.

Development and Maintenance of Epidermal Stem Cells in Skin Adnexa.

The skin surface is modified by numerous appendages. These structures arise from epithelial stem cells (SCs) through the induction of epidermal placodes as a result of local signalling interplay with mesenchymal cells based on the Wnt–(Dkk4)–Eda–Shh cascade. Slight modifications of the cascade, with the participation of antagonistic signalling, decide whether multipotent epidermal SCs develop in interfollicular epidermis, scales, hair/feather follicles, nails or skin glands. This review describes the roles of epidermal SCs in the development of skin adnexa and interfollicular epidermis, as well as their maintenance. Each skin structure arises from distinct pools of epidermal SCs that are harboured in specific but different niches that control SC behaviour. Such relationships explain differences in marker and gene expression patterns between particular SC subsets. The activity of well-compartmentalized epidermal SCs is orchestrated with that of other skin cells not only along the hair cycle but also in the course of skin regeneration following injury. This review highlights several membrane markers, cytoplasmic proteins and transcription factors associated with epidermal SCs.

Supernumerary scutes verify a segment-dependent model of the horny shell development in turtles.

Turtle horny shell has a scute pattern, which is conservative through evolution and across species. The discovery of epidermal placodes as the scute primordia and their strict topographical association to the somites of the turtle embryo suggested a new interpretation of the developmental mechanism of the scute pattern. Here, we tested the hypothesis that horny scutes develop from a mosaic of placodes corresponding exactly to the paths of myoseptae, with vertebral and pleural scutes developing staggered in adjacent segments, and marginal scutes developing in every segment. This scheme predicts little variation in marginals and suggests intercalary supernumerary scutes as potential variations for the vertebral and pleural rows. We examined spatial and numerical variations of the horny shell in 655 newly hatched olive ridley sea turtle, Lepidochelys olivacea, which is known to have a highly variable horny shell. In total, 120 patterns of carapacial scutes and 10 patterns of scutes on plastron, differing in the number and position of scutes were found. The number of vertebral scutes varied from 4 to 10. Variations with five, six and seven vertebrals occurred with the greatest and nearly equally frequency (31.5% on average). Pleural scutes were from 5 to 10 at one or both sides, and the typical symmetric pattern for sea turtles with five pairs of pleurals was only seen in ca. 12% of specimens. In contrast, the majority of the specimens (92.7%) had just 13 pairs of marginals, showing a stable normal pattern. Similarly, on plastron the horny scutes were conservative, too; about 85% of specimens standardly had six pairs of plastral scutes and all specimens had four pairs of inframarginals. Despite a high level of variation of vertebral and pleural scutes in olive ridley turtle, all patterns fall into the theoretical spectrum of possible variants predicted by the segment-dependent model of development of the turtle horny shell. Therefore, the results of our analysis support the existence of direct morphogenetic correlation between the number and distribution of normal and supernumerary scutes and metamere organization of the turtle embryo.

1387 Identifying the key niche signals for hair follicle formation

Specialized mesenchymal cells in dermal condensates (DC) play a crucial role in regulating hair follicle (HF) progenitors in epidermal placodes (Pc) to orchestrate HF morphogenesis. Nevertheless, to date the inductive signals during DC specification and how DC cells interplay with Pc progenitors to facilitate HF development remain unknown. Here we identify precursor cells of the DC (pre-DC) before it appears as specialized cell cluster. With fluorescence-activated cell sorting we co-isolate pre-DC and the DC as it matures, together with the Pc at stage 0 HFs, and other lineage-related populations. We then define the gene expression patterns in each population with next-generation RNA sequencing. Through cross-comparisons we define a molecular time-lapse of dynamically changing gene signatures in consecutive developmental stages during the three earliest HF formation stages. Within the stage-specific pre-DC and DC molecular signatures we uncover several putative key hair inductive signals. With genetic manipulation in live embryonic skin cultures we aim to effectively test the role of newly identified signals in their interplay between the niche and placode progenitors that govern hair follicle formation.

Patterns of scute development in turtle shell: Symmetry and asymmetry

The scute mosaic (pholidosis) in the turtle shell is characterized by two phenomena: phylogenetic stability of the general body plan and wide individual variability. The study of turtle morphogenesis allowed this phenomenon to be explained. It is shown that scute rudiments are formed in embryogeny as local epidermal thickenings, placodes; the positions of placodes is strictly confined to certain parts of the body and their foundation is realized in strict sequence. In particular, in the carapace, placodes are formed exclusively opposite the transverse trunk myosepta, in septal depressions of the embryonic integument; at the same time, marginal placodes are formed in each septal depression (i.e., segment by segment); placodes of pleural and central series are formed in alternate position in the even and odd depressions, respectively. The stability of the scute pattern is connected with the basic mechanisms of morphogenesis, providing the maintenance of bilateral symmetry and determination of scute positions by the primary embryonic segmentation. The main sourse for pholidosis variability is the presence in turtle embryos of vacant septal depressions, in which extraordinary placodes could have developed in the case of minor disturbances of embryogenesis. These aberrations are rather frequent and usually result in asymmetry of the scutation. The breaking of turtle pholidosis symmetry is connected with deviations in ontogenesis, such as asymmetrical formation of epidermal placodes, with a shift to one body side for a segment or more, the formation of additional placodes in the vacant septal depressions, atypical fusion of neighboring epidermal placodes, the absence of regular epidermal placodes in typical body segments.

Neural crest αN‐catenin function in cranial ganglia assembly (344.1)

Neural crest cells are crucial for proper vertebrate development, building the craniofacial skeleton and, together with epidermal placode cells, the cranial ganglia of the peripheral nervous system. Defective ganglia function can lead to conditions such as neuralgia and peripheral neuropathies and contribute to other diseases. Migratory neural crest cells are generated from immotile precursors possessing adherens junctions composed of cadherins and αN‐catenin, a protein that also controls formation of central nervous system structures. Although αN‐catenin down‐regulation is critical for initial neural crest cell emigration from the neural tube, a later role for αN‐catenin in the peripheral nervous system is not known. To this end, we evaluated αN‐catenin spatio‐temporal distribution and perturbed αN‐catenin function to elucidate effects on cranial trigeminal ganglia formation in the chick. We find that migratory neural crest cells forming the trigeminal ganglia re‐express αN‐catenin and, as in the trunk, express Cadherin‐7. αN‐catenin perturbation alters trigeminal ganglia assembly through changes in neural crest and placode cell contribution to the ganglia as well as Cadherin‐7 distribution and levels. Importantly, αN‐catenin function is likely conserved in other cranial ganglia. Together, our results highlight a novel role for neural crest cell αN‐catenin in cranial gangliogenesis.Grant Funding Source: Supported by NSF IOS‐0948525

Dermal β-catenin activity in response to epidermal Wnt ligands is required for fibroblast proliferation and hair follicle initiation

Dermal fibroblasts are required for structural integrity of the skin and for hair follicle development. Uniform Wnt signaling activity is present in dermal fibroblast precursors preceding hair follicle initiation, but the functional requirement of dermal Wnt signaling at early stages of skin differentiation and patterning remains largely uncharacterized. We show in mice that epidermal Wnt ligands are required for uniform dermal Wnt signaling/β-catenin activity and regulate fibroblast cell proliferation and initiation of hair follicle placodes. In the absence of dermal Wnt signaling/β-catenin activity, patterned upregulation of epidermal β-catenin activity and Edar expression are absent. Conversely, forced activation of β-catenin signaling leads to the formation of thickened dermis, enlarged epidermal placodes and dermal condensates that result in prematurely differentiated enlarged hair follicles. These data reveal functional roles for dermal Wnt signaling/β-catenin in fibroblast proliferation and in the epidermal hair follicle initiation program.

Expression of nucleosomal protein HMGN1 in the cycling mouse hair follicle

Here we examine the expression pattern of HMGN1, a nucleosome binding protein that affects chromatin structure and activity, in the hair follicle and test whether loss of HMGN1 affects the development or cycling of the follicle. We find that at the onset of hair follicle development, HMGN1 protein is expressed in the epidermal placode and in aggregated dermal fibroblasts. In the adult hair follicle, HMGN1 is specifically expressed in the basal layer of epidermis, in the outer root sheath, in the hair bulb, but not in the inner root sheath and hair shaft. The expression pattern of HMGN1 is very similar to p63, suggesting a role for HMGN1 in the transiently amplifying cells. We also find HMGN1 expression in some, but not all hair follicle stem cells as detected by its colocalization with Nestin and with BrdU label-retaining cells. The appearance of the skin and hair follicle of Hmgn1 −/− mice was indistinguishable from that of their Hmgn1 +/+ littermates. We found that in the hair follicle the expression of HMGN2 is very similar to HMGN1 suggesting functional redundancy between these closely related HMGN variants.

An Ordovician vertebrate from western New South Wales, with comments on Cambro-Ordovician vertebrate distribution patterns

Young, G.C., March, 2009. An Ordovician vertebrate from western New South Wales, with comments on Cambro-Ordovician vertebrate distribution patterns. Alcheringa 33, 79–89. ISSN 0311-5518. A fish plate impression from the Ordovician Rowena Formation in Mutawintji National Park is referred to an indeterminate species of the genus Arandaspis Ritchie & Gilbert-Tomlinson, 1977, genotype of the family Arandaspididae, previously known only from the Middle Ordovician Stairway Sandstone in the Amadeus Basin of central Australia. The Rowena Formation has previously been correlated with units underlying the Stairway Sandstone (Pacoota Sandstone, Horn Valley Siltstone) in the Amadeus Basin, from which a second arandaspid Porophoraspis represents the earliest occurrence in the fossil record that demonstrates two defining developmental characters of the vertebrates (neural crest, epidermal placodes). The new occurrence of Arandaspis is consistent with the Middle Ordovician (Darriwilian) age for the upper part of the Rowena Formation indicated by recent studies of trilobites. The order Arandaspidiformes, also known from the Upper Ordovician of South America (Sacabambaspis), defines a ‘Gondwana Endemic Assemblage’, which may have been established already by the late Cambrian. Although the first phase of vertebrate biomineralization is not yet documented from the Cambro-Ordovician of Asia, the oldest vertebrates in the early Cambrian Chengjiang fauna and diverse agnathans and gnathostomes by Early Silurian time in South China implicate this as a significant area in the evolution of the earliest vertebrates.

Zebrafish eda and edar Mutants Reveal Conserved and Ancestral Roles of Ectodysplasin Signaling in Vertebrates

The genetic basis of the development and variation of adult form of vertebrates is not well understood. To address this problem, we performed a mutant screen to identify genes essential for the formation of adult skeletal structures of the zebrafish. Here, we describe the phenotypic and molecular characterization of a set of mutants showing loss of adult structures of the dermal skeleton, such as the rays of the fins and the scales, as well as the pharyngeal teeth. The mutations represent adult-viable, loss of function alleles in the ectodysplasin (eda) and ectodysplasin receptor (edar) genes. These genes are frequently mutated in the human hereditary disease hypohidrotic ectodermal dysplasia (HED; OMIM 224900, 305100) that affects the development of integumentary appendages such as hair and teeth. We find mutations in zebrafish edar that affect similar residues as mutated in human cases of HED and show similar phenotypic consequences. eda and edar are not required for early zebrafish development, but are rather specific for the development of adult skeletal and dental structures. We find that the defects of the fins and scales are due to the role of Eda signaling in organizing epidermal cells into discrete signaling centers of the scale epidermal placode and fin fold. Our genetic analysis demonstrates dose-sensitive and organ-specific response to alteration in levels of Eda signaling. In addition, we show substantial buffering of the effect of loss of edar function in different genetic backgrounds, suggesting canalization of this developmental system. We uncover a previously unknown role of Eda signaling in teleosts and show conservation of the developmental mechanisms involved in the formation and variation of both integumentary appendages and limbs. Lastly, our findings point to the utility of adult genetic screens in the zebrafish in identifying essential developmental processes involved in human disease and in morphological evolution.

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.

Role of the vitamin D receptor in hair follicle biology

The vitamin D receptor (VDR) is expressed in numerous cells and tissues, including the skin. The critical requirement for cutaneous expression of the VDR has been proven by investigations in mice and humans lacking functional receptors. These studies demonstrate that absence of the VDR leads to the development of alopecia. The hair follicle is formed by reciprocal interactions between an epidermal placode, which gives rise to the hair follicle keratinocytes and the underlying mesoderm which gives rise to the dermal papilla. Hair follicle morphogenesis ends the second week of life in mice. Studies in VDR null mice have failed to demonstrate a cutaneous abnormality during this period of hair follicle morphogenesis. However, VDR null mice are unable to initiate a new hair cycle after the period of morphogenesis is complete, therefore, do not grow new hair. Investigations in transgenic mice have demonstrated that restricted expression of the VDR to keratinocytes is capable of preventing alopecia in the VDR null mice, thus demonstrating that the epidermal component of the hair follicle requires VDR expression to maintain normal hair follicle homeostasis. Studies were then performed to determine which regions of the VDR were required for these actions. Investigations in mice lacking the first zinc finger of the VDR have demonstrated that they express a truncated receptor containing an intact ligand binding and AF2 domain. These mice are a phenocopy of mice lacking the VDR, thus demonstrate the critical requirement of the DNA binding domain for hair follicle homeostasis. Transgenic mice expressing VDRs with mutations in either the ligand-binding domain or the AF2 domain were generated. These investigations demonstrated that mutant VDRs incapable of ligand-dependent transactivation were able to prevent alopecia. Investigations are currently underway to define the mechanism by which the unliganded VDR maintains hair follicle homeostasis.

Epidermal transient down‐regulation of retinol‐binding protein 4 and mirror expression of apolipoprotein Eb and estrogen receptor 2a during zebrafish fin and scale development

Very little is known about the molecular control of skin patterning and scale morphogenesis in teleost fish. We have found radially symmetrical epidermal placodes with down-regulation of retinol-binding protein 4 (rbp4) expression during the initial paired fin and scale morphogenesis in zebrafish (Danio rerio). This finding may be related to changes in keratinocyte cytodifferentiation and/or the integument retinoid metabolism. rbp4 transcripts are expressed afterward in the central epidermis of the scale papilla and gradually extend to the epidermis, covering the growing scale, whereas no transcripts were detected in posterior margin epidermis. In contrast, induction of apolipoprotein Eb (apoeb) and up-regulation of estrogen receptor 2a (esr2a) transcripts were observed in the epidermis at initiator sites of zebrafish ectodermal/dermal appendage morphogenesis. This expression was maintained in the posterior margin epidermis of the formed scales. esr2a was also strongly expressed in neuromasts, whereas no rbp4 and apoeb transcripts were detected in these mechanosensory structures. The observed epidermal molecular events suggest that epidermis patterning is due to an activator-inhibitor mechanism operational at epidermal-dermal interaction sites. rbp4 transcript expression was also strongly down-regulated by 1-phenyl-2-thio-urea (PTU). As this inhibitor is commonly used to block obscuring pigmentation during in situ hybridization studies, this finding suggests that PTU should be used with caution, particularly in studying skin development.

Distinct Stem Cell Populations Regenerate the Follicle and Interfollicular Epidermis

The regeneration of the skin and its appendages is thought to occur by the regulated activation of a dedicated stem cell population. A population of cells in the bulge region of the hair follicle has been identified as the putative stem cell of both the follicle and the interfollicular epidermis. While this assertion is supported by a variety of surrogate assays, there has been no direct confirmation of the normal contribution of these cells to the regeneration of structures other than the cycling portion of the hair follicle. Here, we report lineage analysis revealing that the follicular epithelium is derived from cells in the epidermal placode that express Sonic hedgehog. This analysis also demonstrates that the stem cells resident in the follicular bulge that regenerate the follicle are neither the stem cells of the epidermis nor the source of the stem cells of the epidermis in the absence of trauma.

CDermo-1 misexpression induces dense dermis, feathers, and scales

Reciprocal epithelio-mesenchymal interactions between the prospective epidermis and the underlying dermis are the major driving forces in the development of skin appendages. Feather development is initiated by a still unknown signal from the dermis in feather-forming skin. The morphological response of the ectoderm to this signal is the formation of an epidermal placode, which signals back to the mesenchyme to induce dermal condensations. Together, epidermal and dermal components constitute the outgrowing feather bud. The bHLH transcription factor cDermo-1 is expressed in developing dermis and is the earliest known marker of prospective feather tracts. To test its function during feather development, we forced cDermo-1 expression in embryonic chicken dermis using a retroviral expression vector. In featherless (apteric) regions, cDermo-1 misexpression induced dense, thickened dermis normally observed in feathered skin (pterylae), and leads to the development of regularly spaced and normally shaped ectopic feather buds. In pterylae, cDermo-1 misexpression enhanced feather growth. In hindlimb skin, according to the local skin identity, misexpression of cDermo-1 induced ectopic scale formation. Thus, we show that forced cDermo-1 expression in developing dermis is sufficient to launch the developmental program leading to skin appendage formation. We propose a role of cDermo-1 at the initial stages of feather induction upstream of FGF10.

FGF‐2 signaling is sufficient to induce dermal condensations during feather development

In a previous report, we showed that fibroblast growth factor-2 (FGF-2) is a signal produced by epidermal placode cells during feather development and that this growth factor can induce feathers in scaleless mutant skins that fail to form feathers due to a defective epidermis (Song et al., [1996] Proc Natl Acad Sci USA 93:10246-10249). Here, we test whether FGF-2 is sufficient to induce dermal condensations, structures that normally form under the control of signals from the epidermal placode and are identified on the basis of aggregation of cells, the expression of FGF receptor-1 and bone morphogenetic protein-2 transcripts and the cessation of proliferation of the condensed cells. By using denuded 8-day scaleless dermis as a test system, we have established that FGF-2 is sufficient to induce dermal condensation. We suggest that the primary effect of FGF-2 is an increase in cellular density mediated through cell migration, followed by the expression of dermal condensation-specific genes and cessation of cell proliferation. The FGF-2 effect can be abolished by heparin, suggesting the involvement of heparan sulfate proteoglycans (HSPGs) in growth factor signaling. The spatiotemporal expression of syndecan-3 during feather development suggests that this cell-surface HSPG may be involved in the response of competent embryonic skin dermis to FGF-2.

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.

A new allele of the mouse hairless gene interferes with Hox/LacZ transgene regulation in hair follicle primordia

A new autosomal recessive mouse mutation, causing loss of hair in homozygous mice 2–3 weeks after birth, arose spontaneously in a colony at the National Institute for Medical Research (NIMR), Mill Hill, London in early 1998. Complementation analysis confirmed that this mutation was an allele of the hairless gene ( hr). The gene symbol hr rhbm (hairless-rhino-bald Mill Hill) was assigned to reflect the source of the colony. Here we show the molecular defect in these mutants, which is a substantial deletion at the 3′-end of the hairless gene. Morphological and immunological analysis of the new hairless mutation was performed at early postnatal stages. In an effort to address the molecular and cellular mechanisms of the hairless phenotype, we analysed developmental stages before the establishment of alopecia. Using a HoxLacZ reporter line of transgenic mice, epidermal placode formation was followed in embryos. Homozygous mutant embryos ( hr rhbmh /hr rhbmh ), containing the LacZ reporter under the control of a Hoxb4 gene enhancer, display sharp loss of LacZ staining in epidermal cells invaginating to form the embryonic hair follicle placode. In the light of targeted mutagenesis data involving a Hox gene in the hair development [Genes Dev. 12 (1) (1998) 11], we discuss the potential implication of the hr rhbmh locus in cascades of Hox gene regulation during embryogenesis.