The impact of irritant challenge on the skin barrier and myeloid-resident immune cells in women who are postmenopausal is modulated by hormone replacement therapy.

BACKGROUND: Sex hormone changes during menopausal transition contribute to declining skin health. However, how menopause and its treatment by hormone replacement therapy (HRT) impact the skin barrier and immune system is unclear. OBJECTIVES: To examine how menopause and HRT affect the skin barrier and immune cell composition in postmenopausal women following irritant challenge. METHODS: Two cohorts of postmenopausal women were recruited to the study. The first cohort consisted of 10 untreated women [HRT-; mean (SEM) age 56.5 (1.6) years (range 48-63)] and the second was composed of 8 women receiving HRT [HRT+; mean (SEM) age 54.0 (2.1) years (range 48-63)]. Skin irritation was induced by applying topical sodium lauryl sulfate (SLS) 1.25% to occluded buttock skin for 48 h. Clinical assessment was conducted after 24 h, followed by biopsy of both SLS-challenged and unchallenged skin for analysis of skin barrier proteins and immune cell distribution using immunofluorescence. RESULTS: Clinically, there were no significant differences in skin irritant responses between those taking or not taking HRT (including increased skin redness and blood flow). In response to SLS challenge a significant increase in transepidermal water loss (P < 0.05), filaggrin deposition and cytokeratin 10 (K10)+ cell layers (P < 0.01) was observed in individuals receiving HRT compared with the HRT- group. Following SLS challenge in individuals taking HRT, a significant (P < 0.01) reduction in CD207+ cells in the epidermis was observed, accompanied by an increase of CD207+ cells in the dermis, indicative of migrating Langerhans cells (LCs). Significantly fewer migrating LCs were found in those who were not receiving HRT (P < 0.01). Furthermore, the numbers of dermal dendritic cells (DCs), macrophages, and CD11c+CD206- and CD68+CD206- subsets were found to be significantly (P < 0.05) higher in those taking HRT following SLS challenge. CONCLUSIONS: Individuals receiving HRT displayed enhanced skin barrier response to SLS challenge with thicker filaggrin and increased K10+ epidermal cell layers. Following challenge, HRT users exhibited elevated LC, inflammatory DC and macrophage counts in the dermis. These may render skin both more prone to inflammation and more capable of resolving it, while also promoting skin repair.

Changes to a person's sex hormones during the menopause can affect the skin. The effects of the menopause on the immune system and the skin are still unclear. The effects of a treatment called 'hormone replacement therapy' ('HRT') are also still unclear. We investigated the effects of HRT on immune cells and skin barrier function in women who had been through the menopause. To do this, we compared the skin of two groups of women: those who were taking HRT and those who were not. Looking at skin redness and blood flow, we found that the two groups of women had a similar response to their skin being irritated by a chemical called 'SLS'. Yet, the women taking HRT had increased water loss from their skin after SLS was applied. We also found that after having SLS applied, women on HRT had a thicker layer of cells in the top section of their skin that produced more of a protein that helps protect the skin. Women taking HRT also had more inflammatory cells in the deeper layers of their skin after SLS was applied. Overall, our findings suggest that HRT may improve the skin's immune response to irritating substances. HRT could have an effect on the skin's ability to repair itself and on general skin health.

Cosmetic retinoid use in photoaged skin: A review of the compounds, their use and mechanisms of action.

The inevitable attrition of skin due to ultraviolet radiation, termed photoaging, can be partially restored by treatment with retinoid compounds. Photoaged skin in lightly pigmented individuals, clinically presents with the appearance of wrinkles, increased laxity, and hyper- and hypopigmentation. Underlying these visible signs of ageing are histological features such as epidermal thinning, dermal-epidermal junction flattening, solar elastosis and loss of the dermal fibrillin microfibrillar network, fibrillar collagen and glycosaminoglycans. Retinoid compounds are comprised of three main generations with the first generation (all-trans retinoic acid, retinol, retinaldehyde and retinyl esters) primarily used for the clinical and cosmetic treatment of photoaging, with varying degrees of efficacy, tolerance and stability. All-trans retinoic acid is considered the 'gold standard' for skin rejuvenation; however, it is a prescription-only product largely confined to clinical use. Therefore, retinoid derivatives are readily incorporated into cosmeceutical formulations. The literature reported in this review suggests that retinol, retinyl esters and retinaldehyde that are used in many cosmeceutical products, are efficacious, safe and well-tolerated. Once in the skin, retinoids utilize a complex signalling pathway that promotes remodelling of photoaged epidermis and dermis and leads to the improvement of the cutaneous signs of photoaging.

L'altération inévitable de la peau due aux rayons ultraviolets, appelée photovieillissement, peut être partiellement restaurée par un traitement à base de composés rétinoïdes. Chez les personnes à la pigmentation claire, le photovieillissement de la peau se manifeste au plan clinique par l'apparition de rides, un relâchement accru et une hyperpigmentation ou hypopigmentation. Ces signes visibles du vieillissement sont sous­tendus par des caractéristiques histologiques telles que l'amincissement de l'épiderme, l'aplatissement de la jonction dermo­épidermique, l'élastose solaire et la perte du réseau microfibrillaire de fibrilline dermique, du collagène fibrillaire et des glycosaminoglycanes. Les composés rétinoïdes sont constitués de trois générations principales, la première génération (acide tout­trans rétinoïque, rétinol, rétinaldéhyde et esters de rétinyle) étant principalement utilisée pour le traitement clinique et cosmétique du photovieillissement, avec des degrés variables d'efficacité, de tolérance et de stabilité. L'acide tout­trans rétinoïque est considéré comme la référence en matière de rajeunissement de la peau; il s'agit toutefois d'un produit délivré uniquement sur ordonnance, dont l'utilisation est largement limitée au domaine clinique. Les dérivés rétinoïdes sont donc volontiers incorporés ds formulations cosméceutiques. La littérature citée dans cette synthèse bibliographique laisse penser que le rétinol, les esters de rétinyle et le rétinaldéhyde, utilisés dans de nombreux produits cosmétiques, sont efficaces, sûrs et bien tolérés. Une fois dans la peau, les rétinoïdes utilisent une voie de signalisation complexe qui favorise le remodelage de l'épiderme et du derme photovieillis, et conduit à l'amélioration des signes cutanés du photovieillissement.

Skin ageing and topical rejuvenation strategies.

Skin ageing is a complex process involving the additive effects of skin's interaction with its external environment, predominantly chronic sun exposure, upon a background of time-dependent intrinsic ageing. Skin health and beauty is considered one of the principal factors perceived to represent overall 'health and wellbeing'; thus, the demand for skin rejuvenation strategies has rapidly increased, with a worldwide annual expenditure expected to grow from $US24.6 billion to around $US44.5 billion by 2030 (https://www.databridgemarketresearch.com/reports/global-facial-rejuvenation-market). Skin rejuvenation can be achieved in several ways, ranging from laser and device-based treatments to chemical peels and injectables; however, topical skin care regimes are a mainstay treatment for ageing skin and all patients seeking skin rejuvenation can benefit from this relatively low-risk intervention. While the most efficacious topical rejuvenation treatment is application of tretinoin (all-trans retinoic acid) - a prescription-only medicine considered to be the clinical 'gold standard' - a hybrid category of 'cosmeceutical' products at the midpoint of the spectrum of cosmetics and pharmaceutical has emerged. This article reviews the clinical manifestations of skin ageing and the available topical treatments for skin rejuvenation, including retinoids, peptides and antioxidants.

The systemic influence of chronic smoking on skin structure and mechanical function.

One of the major functions of human skin is to provide protection from the environment. Although we cannot entirely avoid, for example, sun exposure, it is likely that exposure to other environmental factors could affect cutaneous function. A number of studies have identified smoking as one such factor that leads to both facial wrinkle formation and a decline in skin function. In addition to the direct physical effects of tobacco smoke on skin, its inhalation has additional profound systemic effects for the smoker. The adverse effects on the respiratory and cardiovascular systems from smoking are well known. Central to the pathological changes associated with smoking is the elastic fibre, a key component of the extracellular matrices of lungs. In this study we examined the systemic effect of chronic smoking (>40 cigarettes/day; >5 years) on the histology of the cutaneous elastic fibre system, the nanostructure and mechanics of one of its key components, the fibrillin-rich microfibril, and the micromechanical stiffness of the dermis and epidermis. We show that photoprotected skin of chronic smokers exhibits significant remodelling of the elastic fibre network (both elastin and fibrillin-rich microfibrils) as compared to the skin of age- and sex-matched non-smokers. This remodelling is not associated with increased gelatinase activity (as identified by in situ zymography). Histological remodelling is accompanied by significant ultrastructural changes to extracted fibrillin-rich microfibrils. Finally, using scanning acoustic microscopy, we demonstrated that chronic smoking significantly increases the stiffness of both the dermis and the epidermis. Taken together, these data suggest an unappreciated systemic effect of chronic inhalation of tobacco smoke on the cutaneous elastic fibre network. Such changes may in part underlie the skin wrinkling and loss of skin elasticity associated with smoking. © 2020 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of Pathological Society of Great Britain and Ireland.

Heterogeneity of fibrillin-rich microfibrils extracted from human skin of diverse ethnicity.

The dermal elastic fibre network is the primary effector of skin elasticity, enabling it to extend and recoil many times over the lifetime of the individual. Fibrillin-rich microfibrils (FRMs) constitute integral components of the elastic fibre network, with their distribution showing differential deposition in the papillary dermis across individuals of diverse skin ethnicity. Despite these differential findings in histological presentation, it is not known if skin ethnicity influences FRM ultrastructure. FRMs are evolutionarily highly conserved from jellyfish to man and, regardless of tissue type or species, isolated FRMs have a characteristic 'beads-on-a-string' ultrastructural appearance, with an average inter-bead distance (or periodicity) of 56 nm. Here, skin biopsies were obtained from the photoprotected buttock of healthy volunteers (18-27 years; African: n = 5; European: n = 5), and FRMs were isolated from the superficial papillary dermis and deeper reticular dermis and imaged by atomic force microscopy. In the reticular dermis, there was no significant difference in FRM ultrastructure between European and African participants. In contrast, in the more superficial papillary dermis, inter-bead periodicity was significantly larger for FRMs extracted from European participants than from African participants by 2.20 nm (p < .001). We next assessed whether these differences in FRM ultrastructure were present during early postnatal development by characterizing FRMs from full-thickness neonatal foreskin. Analysis of FRM periodicity identified no significant difference between neonatal cohorts (p = .865). These data suggest that at birth, FRMs are developmentally invariant. However, in adults of diverse skin ethnicity, there is a deviation in ultrastructure for the papillary dermal FRMs that may be acquired during the passage of time from child to adulthood. Understanding the mechanism by which this difference in papillary dermal FRMs arises warrants further study.

Remodelling of fibrillin-rich microfibrils by solar-simulated radiation: impact of skin ethnicity.

Fibrillin-rich microfibrils (FRMs) constitute integral components of the dermal elastic fibre network with a distinctive ultrastructural 'beads-on-a-string' appearance that can be visualised using atomic force microscopy and characterised by measurement of their length and inter-bead periodicity. Their deposition within the dermis in photoprotected skin appears to be contingent on skin ethnicity, and influences the ultrastructure of papillary - but not reticular - dermal FRMs. Truncation and depletion of FRMs at the dermal-epidermal junction of skin occurs early in photoageing in people with lightly pigmented skin; a process of accelerated skin ageing that arises due to chronic sun exposure. Accumulation of ultraviolet radiation (UVR)-induced damage, either by the action of enzymes, oxidation or direct photon absorption, results in FRM remodelling and changes to ultrastructure. In the current study, the direct effect of UVR exposure on FRM ultrastructure was assayed by isolating FRMs from the papillary and reticular dermis of photoprotected buttock skin of individuals of either black African or white Northern European ancestry and exposing them to solar-simulated radiation (SSR). Exposure to SSR resulted in significant reduction in inter-bead periodicity for reticular dermis-derived FRMs across both cohorts. In contrast, papillary dermal FRMs exhibited significantly increased inter-bead periodicity, with the magnitude of damage greater for African FRMs, as compared to Northern European FRMs. Our data suggest that FRMs of the dermis should be considered as two distinct populations that differentially accrue damage in response to SSR. Furthermore, papillary dermal FRMs derived from black African subjects show greater change following UVR challenge, when extracted from skin. Future studies should focus on understanding the consequences of UVR exposure in vivo, regardless of skin ethnicity, on the molecular composition of FRMs and how this UVR-induced remodelling may affect the role FRMs play in skin homeostasis.

Ageing significantly impacts the biomechanical function and structural composition of skin.

Skin ageing is a complex process involving the additive effects of skin's interaction with its external environment, predominantly chronic sun exposure, upon a background of time-dependent intrinsic ageing. Here, using non-invasive cutometry and ballistometry, we explore the consequences of ageing on the biomechanical function of skin in otherwise healthy White Northern European volunteers. Intrinsic skin ageing caused biomechanical decline; skin loses both resilience (P < 0.01) and elasticity (P < 0.001), which is characterised histologically by modest effacement of rete ridges (P < 0.05) and disorganisation of papillary dermal elastic fibres. At photoexposed sites, biomechanical testing identified significant loss of biomechanical function-particularly in the aged cohort. Photoaged forearm displayed severe loss of resilience (P < 0.001) and elasticity (P < 0.001); furthermore with repetitive testing, fatigue (P < 0.001), hysteresis (P < 0.001) and viscous "creep" (P < 0.001) were exacerbated. Histologically, both young and aged forearm displayed flattening of rete ridges and disruption to the arrangement of elastic fibres. We conclude that maintenance of skin architecture is inherently associated with optimal biomechanical properties. Modest perturbations to skin architecture-as exemplified by intrinsic ageing-result in moderate functional decline. Chronic sun exposure causes fundamental changes to the clinical and histological appearance of skin, and these are reflected by an extreme alteration in biomechanical function.

Cross-linking of structural proteins in ageing skin: an in situ assay for the detection of amine oxidase activity.

With increasing age, dynamic tissues such as lungs, blood vessels and skin lose their ability to both deform and recoil, culminating in tissue stiffening. This loss of tissue elasticity, which profoundly impacts tissue function and thus morbidity, may be due not only to changes in the relative abundance of key extracellular matrix proteins within tissues but also to their accumulation of post-translational modifications. Whilst to date attention has focussed primarily on the age-related non-enzymatic formation of advanced glycation end products, the accumulation of pathological enzyme-mediated cross-links may also lead to age-related tissue stiffening. The lysyl oxidase (LOX) family of enzymes are constitutively expressed in adult tissues and are known to drive the catalysis of cross-links in both fibrillar collagens and elastin. Although immunochemical approaches are commonly used to localise the inactive pro-enzyme of LOX, and biochemical methods are employed to quantify activity in homogenised tissue, they do not allow for the in situ localisation of the enzyme. Thus, we have developed a novel assay to both detect and localise LOX enzyme activity in situ. LOX family members are amine oxidases and this assay uses the principle that an amine substrate in the presence of this class of enzyme will be oxidised to an aldehyde and hydrogen peroxide (H2O2). In turn, H2O2, when combined with luminol and horseradish peroxidase, will produce a light-emitting reaction that can be detected by film autoradiography. The development of a technique to localise specific amine oxidase activity in tissue sections may provide crucial additional information on the exact role played by this class of enzymes in mediating age-related tissue stiffening.

Behaviour and sun exposure in holidaymakers alters skin microbiota composition and diversity.

Introduction: The skin microbiota plays a crucial role in maintaining epidermal homeostasis. Ultraviolet radiation (UVR) and other environmental challenges can impact the skin microbiota through direct and indirect mechanisms. This study aimed to investigate the effects of sun exposure on the skin microbiota and its relationship with individual skin phototypes. Methods: Healthy volunteers (n = 21 [4M, 17 F], mean age 33.2 years) holidayed in a sunny destination for a minimum of 7 days with swabs taken pre-holiday and up to 84 days post-holiday. Participant group was categorised by individual typology angle (ITA) classification and the composition of the skin microbiota was examined using 16S rRNA gene sequencing. Results: In the entire cohort and at all time points, the major bacterial phyla were Actinobacteria, Proteobacteria and Firmicutes. There was a significant change in microbial beta diversity at day 28 post-holiday, compared to baseline, for all participants. However, when participants were segregated into three cohorts dependent on the degree of skin tanning response between baseline (pre-holiday) and immediately one-day post-holiday, there was a reduction in Proteobacteria in the sun-seeking participants 1 day after the holiday, which recovered over time. Discussion: These findings suggest that sun exposure can affect the diversity and composition of the skin microbiota, which may have downstream effects on skin health.

Differential expression of elastic fibre components in intrinsically aged skin.

Intrinsic ageing of the skin is a subtle process resulting in some degree of skin laxity. The dermal elastic fibre network imbues skin with the capacity to recoil and loss of this property contributes to an aged, wrinkled appearance. Whilst elastic fibres have a complex, composite structure which allows them to fulfil multiple roles, the effects of intrinsic ageing on their discrete molecular components has not previously been explored. In this study, we have used a microarray-based approach to perform a novel survey of the changes in gene expression that occur in components of cutaneous elastic fibres as a result of intrinsic ageing. Age-related changes in gene expression were validated at the mRNA and protein levels using quantitative real-time polymerase chain reaction (qPCR) and immunostaining, respectively. The microarray revealed that the majority of elastic fibre network components were unchanged with age. However, three differentially expressed genes were identified: latent TGFß-binding protein (LTBP)-2 which was up-regulated with age (fold change +1.58, P = 0.041); LTBP3 (fold change -1.67, P = 0.025) and the lysyl oxidase-like enzyme (LOXL1, fold change -1.47, P = 0.008) which were both down-regulated with age. Although the changes in gene expression for LTBP3 were not confirmed by either qPCR or immunostaining, the expression and tissue deposition of both LTBP2 and LOXL1 were significantly enhanced in intrinsically aged skin. Whilst the functional implications of these altered expression profiles remains to be elucidated, LTBP2 and LOXL1 are thought to play important roles in controlling and maintaining elastic fibre deposition, assembly and structure via binding to fibulin-5. Consequently, any age-related perturbations in the expression of these components may have important consequences on remodelling of the extracellular matrix and hence on the mechanical properties of intrinsically aged skin.

Fractional Sunburn Threshold UVR Doses Generate Equivalent Vitamin D and DNA Damage in Skin Types I-VI but with Epidermal DNA Damage Gradient Correlated to Skin Darkness.

Public health guidance recommends limiting sun exposure to sub-sunburn levels, but it is unknown whether these can gain vitamin D (for musculoskeletal health) while avoiding epidermal DNA damage (initiates skin cancer). Well-characterized healthy humans of all skin types (I-VI, lightest to darkest skin) were exposed to a low-dose series of solar simulated UVR of 20%-80% their individual sunburn threshold dose (minimal erythema dose). Significant UVR dose responses were seen for serum 25-hydroxyvitamin D and whole epidermal cyclobutane pyrimidine dimers (CPDs), with as little as 0.2 minimal erythema dose concurrently producing 25-hydroxyvitamin D and CPD. Fractional MEDs generated equivalent levels of whole epidermal CPD and 25-hydroxyvitamin D across all skin types. Crucially, we showed an epidermal gradient of CPD formation strongly correlated with skin darkness (r = 0.74, P < 0.0001), which reflected melanin content and showed increasing protection across the skin types, ranging from darkest skin, where high CPD levels occurred superficially, with none in the germinative basal layer, to lightest skin, where CPD levels were induced evenly across the epidermal depth. People with darker skin can be encouraged to use sub-sunburn UVR-exposure to enhance their vitamin D. In people with lighter skin, basal cell damage occurs concurrent with vitamin D synthesis at exquisitely low UVR levels, providing an explanation for their high skin cancer incidence; greater caution is required.

The impact of intrinsic ageing on the protein composition of the dermal-epidermal junction.

The dermal-epidermal junction of human skin exhibits age-related remodelling, resulting in a flattened appearance and reduced surface area. Despite this, a paucity of information is available regarding which protein components change with advancing age. Here we report a significant reduction in the protein distribution of collagen IV (P<0.0001), collagen VII (P<0.001), collagen XVII (P<0.01), integrin ß4 (P<0.001) and laminin-332 (P<0.0001) in intrinsically aged skin. The functional implication of this altered protein composition appears to be loss of structural integrity and may, in part, explain the increased fragility of aged skin.

Thyroid Hormones Enhance Mitochondrial Function in Human Epidermis.

Since it is unknown whether thyroid hormones (THs) regulate mitochondrial function in human epidermis, we treated organ-cultured human skin, or isolated cultured human epidermal keratinocytes, with triiodothyronine (100 pmol/L) or thyroxine (100 nmol/L). Both THs significantly increased protein expression of the mitochondrially encoded cytochrome C oxidase I (MTCO1), complex I activity, and the number of perinuclear mitochondria. Triiodothyronine also increased mitochondrial transcription factor A (TFAM) protein expression, and thyroxine stimulated complex II/IV activity. Increased mitochondrial function can correlate with increased reactive oxygen species production, DNA damage, and accelerated tissue aging. However, THs neither raised reactive oxygen species production or matrix metalloproteinase-1, -2 and -9 activity nor decreased sirtuin1 (Sirt1) immunoreactivity. Instead, triiodothyronine increased sirtuin-1, fibrillin-1, proliferator-activated receptor-gamma 1-alpha (PGC1α), collagen I and III transcription, and thyroxine decreased cyclin-dependent kinase inhibitor 2A (p16(ink4)) expression in organ-cultured human skin. Moreover, TH treatment increased intracutaneous fibrillin-rich microfibril and collagen III deposition and decreased mammalian target of rapamycin (mTORC1/2) expression ex vivo. This identifies THs as potent endocrine stimulators of mitochondrial function in human epidermis, which down-regulates rather than enhance the expression of skin aging-related biomarkers ex vivo. Therefore, topically applied THs deserve further exploration as candidate agents for treating skin conditions characterized by reduced mitochondrial function.

Chemical consequences of cutaneous photoageing.

Human skin, in common with other organs, ages as a consequence of the passage of time, but in areas exposed to solar ultraviolet radiation, the effects of this intrinsic ageing process are exacerbated. In particular, both the severity and speed of onset of age-related changes, such as wrinkle formation and loss of elasticity, are enhanced in photoaged (also termed extrinsically aged) as compared with aged, photoprotected, skin. The anatomy of skin is characterised by two major layers: an outer, avascular, yet highly cellular and dynamic epidermis and an underlying vascularised, comparatively static and cell-poor, dermis. The structural consequences of photoageing are mainly evident in the extracellular matrix-rich but cell-poor dermis where key extracellular matrix proteins are particularly susceptible to photodamage. Most investigations to date have concentrated on the cell as both a target for and mediator of, ultraviolet radiation-induced photoageing. As the main effectors of dermal remodelling produced by cells (extracellular proteases) generally have low substrate specificity, we recently suggested that the differential susceptibility of key extracellular matrix proteins to the processes of photoageing may be due to direct, as opposed to cell-mediated, photodamage.In this review, we discuss the experimental evidence for ultraviolet radiation (and related reactive oxygen species)-mediated differential degradation of normally long lived dermal proteins including the fibrillar collagens, elastic fibre components, glycoproteins and proteoglycans. Whilst these components exhibit highly diverse primary and hence macro- and supra-molecular structures, we present evidence that amino acid composition alone may be a useful predictor of age-related protein degradation in both photoexposed and, as a consequence of differential oxidation sensitivity, photoprotected, tissues.

Hair follicles are required for optimal growth during lateral skin expansion.

The hair follicles (HFs) and the interfollicular epidermis (IFE) of intact mature skin are maintained by distinct stem cell populations. Upon wounding, however, emigration of HF keratinocytes to the IFE plays a role in acute stages of healing. In addition to this repair function, rapidly cycling cells of the upper HF have been observed transiting to the IFE in neonatal skin. Here we report that an absence of HF development leads to shortening and kinking of the mouse tail. These skeletal defects are reduced by stimulating keratinocyte proliferation, suggesting that they arise from impaired epidermal expansion. We confirm that rapidly cycling cells of the HF emigrate to the IFE of the neonatal tail. These results suggest that an absence of HFs results in impaired skin growth that is unable to keep pace with the rapidly elongating axial skeleton of the tail. Thus, in addition to their role in wound repair, HFs can make a significant contribution to lateral expansion of the IFE in the absence of trauma.

An extended epidermal response heals cutaneous wounds in the absence of a hair follicle stem cell contribution.

Hair follicles have been observed to provide a major cellular contribution to epidermal healing, with emigration of stem-derived cells from the follicles aiding in wound reepithelialization. However, the functional requirements for this hair follicle input are unknown. Here we have characterized the keratinocyte stem cell status of mutant mice that lack all hair follicle development on their tail, and analyzed the consequent alterations in epidermal wound healing rate and mechanisms. In analyzing stem cell behavior in embryonic skin we found that clonogenic keratinocytes are relatively frequent in the ectoderm prior to hair follicle formation. However, their frequency in the interfollicular epidermis drops sharply by birth, at which time the majority of stem cells are present within the hair follicles. We find that in the absence of hair follicles cutaneous wounds heal with an acute delay in reepithelialization. This delay is followed by expansion of the region of activated epidermis, beyond that seen in normal haired skin, followed by appropriate wound closure. JID Journal Club article: for questions, answers, and open discussion about this article please go to http://network.nature.com/group/jidclub.