Distinct fibroblast lineages determine dermal architecture in skin development and repair. Nature. 2013;504:277-281. [PMID: 24336287 PMCID: PMC3868929 DOI: 10.1038/nature12783]

Morphological Retrospective Study of Peritoneal Biopsies from Patients with Encapsulating Peritoneal Sclerosis: Underestimated Role of Adipocytes as New Fibroblasts Lineage?

Background. Encapsulating peritoneal sclerosis (EPS) is a rare but serious complication of peritoneal dialysis (PD). Besides the endothelial-to-mesenchymal transition (EMT), recently peritoneal adipocytes emerged as a potential source of fibrosis. We performed immunohistochemistry to approach EMT and to localize peritoneal adipocytes in peritoneal biopsies from PD-related EPS patients. Material and Methods. We investigated tissue expression of podoplanin, cytokeratin AE1/AE3 (mesothelium), calretinin (adipocytes), alpha-smooth muscle actin [α-SMA] (mesenchymal cells), interstitial mononuclear cell inflammation, and neoangiogenesis (CD3, CD4, CD8, CD20, CD68, and CD31 immunostainings, resp.). Results. Three patients (1 man/2 women; 17, 64, and 39 years old, resp.) developed EPS after 21, 90, and 164 months of PD therapy. In patients with EPS, we observed (1) loss of AE1/AE3 cytokeratin+ mesothelial cells without any evidence of migration into the interstitium, (2) disappearance of adipose tissue, (3) diffuse infiltration of calretinin+ cells in the areas of submesothelial fibrosis with a huge number of α-SMA and calretinin+ fusiform cells, and (4) increased vascular density. Conclusion. We report that the involvement of EMT in peritoneal fibrosis is difficult to demonstrate and that the calretinin+ adipocytes might be an underestimated component and a new source of myofibroblasts in peritoneal remodeling during PD-related EPS.

Fate of Prominin-1 Expressing Dermal Papilla Cells during Homeostasis, Wound Healing and Wnt Activation

Prominin-1/CD133 (Prom1) is expressed by fibroblasts in the dermal papilla (DP) of the hair follicle (HF). By examining endogenous Prom1 expression and expression of LacZ in the skin of Prom1CreERLacZ (Prom1C-L) mice, in which a CreERT2-IRES-nuclear LacZ cassette is knocked into the first ATG codon of Prom1, we confirmed that Prom1 is expressed in the DP of all developing HFs and also by postnatal anagen follicles. To analyze the fate of Prom1+ DP cells, we crossed Prom1C-L mice with Rosa26-CAG flox/stop/flox tdTomato reporter mice and applied 4-hydroxytamoxifen (4OHT) to back skin at postnatal day (P) 1 and P2. We detected tdTomato+ cells in ~50% of DPs. The proportion of labeled cells per DP increased between P5 and P63, while the total number of cells per DP declined. Following full thickness wounding, there was no migration of tdTomato-labeled cells out of the DP. When β-catenin was activated in Prom1+ DP cells there was an increase in the size of anagen and telogen DP, but the proportion of tdTomato-labeled cells did not increase. We conclude that Prom1+ DP cells do not contribute to dermal repair but are nevertheless capable of regulating DP size via β-catenin-mediated intercellular communication.

Fibroblast heterogeneity and its implications for engineering organotypic skin models in vitro

Advances in cell culture methods, multidisciplinary research, clinical need to replace lost skin tissues and regulatory need to replace animal models with alternative test methods has led to development of three dimensional models of human skin. In general, these in vitro models of skin consist of keratinocytes cultured over fibroblast-populated dermal matrices. Accumulating evidences indicate that mesenchyme-derived signals are essential for epidermal morphogenesis, homeostasis and differentiation. Various studies show that fibroblasts isolated from different tissues in the body are dynamic in nature and are morphologically and functionally heterogeneous subpopulations. Further, these differences seem to be dictated by the local biological and physical microenvironment the fibroblasts reside resulting in "positional identity or memory". Furthermore, the heterogeneity among the fibroblasts play a critical role in scarless wound healing and complete restoration of native tissue architecture in fetus and oral mucosa; and excessive scar formation in diseased states like keloids and hypertrophic scars. In this review, we summarize current concepts about the heterogeneity among fibroblasts and their role in various wound healing environments. Further, we contemplate how the insights on fibroblast heterogeneity could be applied for the development of next generation organotypic skin models.

TAT-Mediated Acidic Fibroblast Growth Factor Delivery to the Dermis Improves Wound Healing of Deep Skin Tissue in Rat

BACKGROUND

The definition of deep tissue injury was derived from multiple clinical cases as "A purple or maroon localized area of discolored intact skin or blood-filled blister due to damage of underlying soft tissue from pressure and/or shear". Acidic fibroblast growth factor (aFGF) significantly improves wound healing under diabetic conditions. However, to date, the therapeutic application of aFGF has been limited, due to its low delivery efficiency and short half-life.

METHODOLOGY/PRINCIPAL FINDINGS

Using an animal model of magnet-induced pressure ulcers, transactivator of transcription protein (TAT)-aFGF was evaluated for transdermal delivery and wound healing. Immunohistochemistry and Western blotting were also performed to determine the expression of transforming growth factor (TGF)-β1, α-smooth muscle actin (α-SMA), CD68, proliferating cell nuclear antigen (PCNA) and TGF-β-receptor II (TGF- βRII) in cultured human dermal fibroblasts. We found that that mice treated with TAT-aFGF had higher accumulation of aFGF in both dermis and subcutaneous tissues compared with mice treated with aFGF alone. In the remodeling phase, TAT-aFGF treatment decreased the expression of α-SMA to normal levels, thereby facilitating normal wound healing processes and abrogating hypertrophic scarring. In human dermal fibroblasts, TAT-aFGF reversed the suppressive effect of TNF-α on α-SMA expression and restored TGF-βRII and TGF-β1 expression.

CONCLUSIONS/SIGNIFICANCE

Our results demonstrate that TAT-aFGF has a favorable therapeutic effect on the healing of subcutaneous deep tissue injury.

Concise Review: Human Dermis as an Autologous Source of Stem Cells for Tissue Engineering and Regenerative Medicine

The exciting potential for regenerating organs from autologous stem cells is on the near horizon, and adult dermis stem cells (DSCs) are particularly appealing because of the ease and relative minimal invasiveness of skin collection. A substantial number of reports have described DSCs and their potential for regenerating tissues from mesenchymal, ectodermal, and endodermal lineages; however, the exact niches of these stem cells in various skin types and their antigenic surface makeup are not yet clearly defined. The multilineage potential of DSCs appears to be similar, despite great variability in isolation and in vitro propagation methods. Despite this great potential, only limited amounts of tissues and clinical applications for organ regeneration have been developed from DSCs. This review summarizes the literature on DSCs regarding their niches and the specific markers they express. The concept of the niches and the differentiation capacity of cells residing in them along particular lineages is discussed. Furthermore, the advantages and disadvantages of widely used methods to demonstrate lineage differentiation are considered. In addition, safety considerations and the most recent advancements in the field of tissue engineering and regeneration using DSCs are discussed. This review concludes with thoughts on how to prospectively approach engineering of tissues and organ regeneration using DSCs. Our expectation is that implementation of the major points highlighted in this review will lead to major advancements in the fields of regenerative medicine and tissue engineering.

SIGNIFICANCE

Autologous dermis-derived stem cells are generating great excitement and efforts in the field of regenerative medicine and tissue engineering. The substantial impact of this review lies in its critical coverage of the available literature and in providing insight regarding niches, characteristics, and isolation methods of stem cells derived from the human dermis. Furthermore, it provides analysis of the current state-of-the-art regenerative approaches using human-derived dermal stem cells, with consideration of current guidelines, to assist translation toward therapeutic use.

An Integrated Transcriptome Atlas of Embryonic Hair Follicle Progenitors, Their Niche, and the Developing Skin

Defining the unique molecular features of progenitors and their niche requires a genome-wide, whole-tissue approach with cellular resolution. Here, we co-isolate embryonic hair follicle (HF) placode and dermal condensate cells, precursors of adult HF stem cells and the dermal papilla/sheath niche, along with lineage-related keratinocytes and fibroblasts, Schwann cells, melanocytes, and a population inclusive of all remaining skin cells. With next-generation RNA sequencing, we define gene expression patterns in the context of the entire embryonic skin, and through transcriptome cross-comparisons, we uncover hundreds of enriched genes in cell-type-specific signatures. Axon guidance signaling and many other pathway genes are enriched in multiple signatures, implicating these factors in driving the large-scale cellular rearrangements necessary for HF formation. Finally, we share all data in an interactive, searchable companion website. Our study provides an overarching view of signaling within the entire embryonic skin and captures a molecular snapshot of HF progenitors and their niche.

Gene Signature of Human Oral Mucosa Fibroblasts: Comparison with Dermal Fibroblasts and Induced Pluripotent Stem Cells

Oral mucosa is a useful material for regeneration therapy with the advantages of its accessibility and versatility regardless of age and gender. However, little is known about the molecular characteristics of oral mucosa. Here we report the first comparative profiles of the gene signatures of human oral mucosa fibroblasts (hOFs), human dermal fibroblasts (hDFs), and hOF-derived induced pluripotent stem cells (hOF-iPSCs), linking these with biological roles by functional annotation and pathway analyses. As a common feature of fibroblasts, both hOFs and hDFs expressed glycolipid metabolism-related genes at higher levels compared with hOF-iPSCs. Distinct characteristics of hOFs compared with hDFs included a high expression of glycoprotein genes, involved in signaling, extracellular matrix, membrane, and receptor proteins, besides a low expression of HOX genes, the hDFs-markers. The results of the pathway analyses indicated that tissue-reconstructive, proliferative, and signaling pathways are active, whereas senescence-related genes in p53 pathway are inactive in hOFs. Furthermore, more than half of hOF-specific genes were similarly expressed to those of hOF-iPSC genes and might be controlled by WNT signaling. Our findings demonstrated that hOFs have unique cellular characteristics in specificity and plasticity. These data may provide useful insight into application of oral fibroblasts for direct reprograming.

REDD1 functions at the crossroads between the therapeutic and adverse effects of topical glucocorticoids

Cutaneous atrophy is the major adverse effect of topical glucocorticoids; however, its molecular mechanisms are poorly understood. Here, we identify stress-inducible mTOR inhibitor REDD1 (regulated in development and DNA damage response 1) as a major molecular target of glucocorticoids, which mediates cutaneous atrophy. In REDD1 knockout (KO) mice, all skin compartments (epidermis, dermis, subcutaneous fat), epidermal stem, and progenitor cells were protected from atrophic effects of glucocorticoids. Moreover, REDD1 knockdown resulted in similar consequences in organotypic raft cultures of primary human keratinocytes. Expression profiling revealed that gene activation by glucocorticoids was strongly altered in REDD1 KO epidermis. In contrast, the down-regulation of genes involved in anti-inflammatory glucocorticoid response was strikingly similar in wild-type and REDD1 KO mice. Integrative bioinformatics analysis of our and published gene array data revealed similar changes of gene expression in epidermis and in muscle undergoing glucocorticoid-dependent and glucocorticoid-independent atrophy. Importantly, the lack of REDD1 did not diminish the anti-inflammatory effects of glucocorticoids in preclinical model. Our findings suggest that combining steroids with REDD1 inhibitors may yield a novel, safer glucocorticoid-based therapies.

Myofibroblasts

Myofibroblasts are activated in response to tissue injury with the primary task to repair lost or damaged extracellular matrix. Enhanced collagen secretion and subsequent contraction - scarring - are part of the normal wound healing response and crucial to restore tissue integrity. Due to myofibroblasts ability to repair but not regenerate, accumulation of scar tissue is always associated with reduced organ performance. This is a fair price to pay by the body for not falling apart. Whereas myofibroblasts typically vanish after successful repair, dysregulation of the normal repair process can lead to persistent myofibroblast activation, for instance by chronic inflammation or mechanical stress in the tissue. Excessive repair leads to the accumulation of stiff collagenous ECM contractures - fibrosis - with dramatic consequences for organ function. The clinical need to terminate detrimental myofibroblast activities has stimulated researchers to answer a number of essential questions: where do myofibroblasts come from, what are the factors leading to their activation, how do we discriminate myofibroblasts from other cells, what is the molecular basis for their contractile activity, and how can we stop or at least control them? This article reviews the current state of the myofibroblast literature by emphasizing their role in ocular repair and fibrosis. It appears that although the eye is quite an extraordinary organ, ocular myofibroblasts behave or misbehave just like their siblings in other organs.

Platelet-Derived Growth Factor Receptor Alpha as a Marker of Mesenchymal Stem Cells in Development and Stem Cell Biology

Three decades on, the mesenchymal stem cells (MSCs) have been intensively researched on the bench top and used clinically. However, ambiguity still exists in regard to their anatomical locations, identities, functions, and extent of their differentiative abilities. One of the major impediments in the quest of the MSC research has been lack of appropriate in vivo markers. In recent years, this obstacle has been resolved to some degree as PDGFRα emerges as an important mesenchymal stem cell marker. Accumulating lines of evidence are showing that the PDGFRα (+) cells reside in the perivascular locations of many adult interstitium and fulfil the classic concepts of MSCs in vitro and in vivo. PDGFRα has long been recognised for its roles in the mesoderm formation and connective tissue development during the embryogenesis. Current review describes the lines of evidence regarding the role of PDGFRα in morphogenesis and differentiation and its implications for MSC biology.

Cellular and molecular mechanisms of repair in acute and chronic wound healing

Summary

A considerable understanding of the fundamental cellular and molecular mechanisms underpinning healthy acute wound healing has been gleaned from studying various animal models, and we are now unravelling the mechanisms that lead to chronic wounds and pathological healing including fibrosis. A small cut will normally heal in days through tight orchestration of cell migration and appropriate levels of inflammation, innervation and angiogenesis. Major surgeries may take several weeks to heal and leave behind a noticeable scar. At the extreme end, chronic wounds – defined as a barrier defect that has not healed in 3 months – have become a major therapeutic challenge throughout the Western world and will only increase as our populations advance in age, and with the increasing incidence of diabetes, obesity and vascular disorders. Here we describe the clinical problems and how, through better dialogue between basic researchers and clinicians, we may extend our current knowledge to enable the development of novel potential therapeutic treatments.

What's already known about this topic?

What does this study add?

Time- and compartment-resolved proteome profiling of the extracellular niche in lung injury and repair

The extracellular matrix (ECM) is a key regulator of tissue morphogenesis and repair. However, its composition and architecture are not well characterized. Here, we monitor remodeling of the extracellular niche in tissue repair in the bleomycin-induced lung injury mouse model. Mass spectrometry quantified 8,366 proteins from total tissue and bronchoalveolar lavage fluid (BALF) over the course of 8 weeks, surveying tissue composition from the onset of inflammation and fibrosis to its full recovery. Combined analysis of proteome, secretome, and transcriptome highlighted post-transcriptional events during tissue fibrogenesis and defined the composition of airway epithelial lining fluid. To comprehensively characterize the ECM, we developed a quantitative detergent solubility profiling (QDSP) method, which identified Emilin-2 and collagen-XXVIII as novel constituents of the provisional repair matrix. QDSP revealed which secreted proteins interact with the ECM, and showed drastically altered association of morphogens to the insoluble matrix upon injury. Thus, our proteomic systems biology study assigns proteins to tissue compartments and uncovers their dynamic regulation upon lung injury and repair, potentially contributing to the development of anti-fibrotic strategies.

Pigment epithelium-derived factor as a multifunctional regulator of wound healing

During dermal wound repair, hypoxia-driven proliferation results in dense but highly permeable, disorganized microvascular networks, similar to those in solid tumors. Concurrently, activated dermal fibroblasts generate an angiopermissive, provisional extracellular matrix (ECM). Unlike cancers, wounds naturally resolve via blood vessel regression and ECM maturation, which are essential for reestablishing tissue homeostasis. Mechanisms guiding wound resolution are poorly understood; one candidate regulator is pigment epithelium-derived factor (PEDF), a secreted glycoprotein. PEDF is a potent antiangiogenic in models of pathological angiogenesis and a promising cancer and cardiovascular disease therapeutic, but little is known about its physiological function. To examine the roles of PEDF in physiological wound repair, we used a reproducible model of excisional skin wound healing in BALB/c mice. We show that PEDF is abundant in unwounded and healing skin, is produced primarily by dermal fibroblasts, binds to resident microvascular endothelial cells, and accumulates in dermal ECM and epidermis. PEDF transcript and protein levels were low during the inflammatory and proliferative phases of healing but increased in quantity and colocalization with microvasculature during wound resolution. Local antibody inhibition of endogenous PEDF delayed vessel regression and collagen maturation during the remodeling phase. Treatment of wounds with intradermal injections of exogenous, recombinant PEDF inhibited nascent angiogenesis by repressing endothelial proliferation, promoted vascular integrity and function, and increased collagen maturity. These results demonstrate that PEDF contributes to the resolution of healing wounds by causing regression of immature blood vessels and stimulating maturation of the vascular microenvironment, thus promoting a return to tissue homeostasis after injury.

Medical cell technologies for treatment of patients suffering from recessive dystrophic epidermolysis bullosa. Method of intracutaneous administration of fibroblasts

Recessive dystrophic epidermolysis bullosa (RDEB) is a severe inherited disease developing due to genetic abnormalities in the synthesis of Type VII collagen by fibroblasts. A low production rate of Type VII collagen and abnormalities related to the formation of anchoring fibrils weaken the epidermis and derma adhesion strength, which results in the formation of blisters or erosions in case of any mechanical injury. Fibroblasts and keratinocytes belong to the key sources of Type VII collagen in the skin. Application of allogeneic fibroblasts is a promising cell technique for treating RDEB patients. The therapeutic effect of fibroblasts intradermal administration is stipulated by high stability of newly synthesized Type VII collagen and its ability to form anchoring fibrils in the area of the dermoepidermal junction. According to experimental and clinical studies, it is possible to boost the content of Type VII collagen in the dermoepidermal junction area and heal long-term skin defects in RDEB patients by means of intradermal administration of allogeneic fibroblasts.

Inflammation, Innate Immunity, and the Intestinal Stromal Cell Niche: Opportunities and Challenges

Stromal cells of multiple tissues contribute to immune-mediated protective responses and, conversely, the pathological tissue changes associated with chronic inflammatory disease. However, unlike hematopoietic immune cells, tissue stromal cell populations remain poorly characterized with respect to specific surface marker expression, their ontogeny, self-renewal, and proliferative capacity within tissues and the extent to which they undergo phenotypic immunological changes during the course of an infectious or inflammatory insult. Extending our knowledge of the immunological features of stromal cells provides an exciting opportunity to further dissect the underlying biology of many important immune-mediated diseases, although several challenges remain in bringing the emerging field of stromal immunology to equivalence with the study of the hematopoietic immune cell compartment. This review highlights recent studies that have begun unraveling the complexity of tissue stromal cell function in immune responses, with a focus on the intestine, and proposes strategies for the development of the field to uncover the great potential for stromal immunology to contribute to our understanding of the fundamental pathophysiology of disease, and the opening of new therapeutic avenues in multiple chronic inflammatory conditions.

Scarless wound healing: chasing the holy grail

Over 100 million patients acquire scars in the industrialized world each year, primarily as a result of elective operations. Although undefined, the global incidence of scarring is even larger, extending to significant numbers of burn and other trauma-related wounds. Scars have the potential to exert a profound psychological and physical impact on the individual. Beyond aesthetic considerations and potential disfigurement, scarring can result in restriction of movement and reduced quality of life. The formation of a scar following skin injury is a consequence of wound healing occurring through reparative rather than regenerative mechanisms. In this article, the authors review the basic stages of wound healing; differences between adult and fetal wound healing; various mechanical, genetic, and pharmacologic strategies to reduce scarring; and the biology of skin stem/progenitor cells that may hold the key to scarless regeneration.

Principles and mechanisms of regeneration in the mouse model for wound-induced hair follicle neogenesis

Wound‐induced hair follicle neogenesis (WIHN) describes a regenerative phenomenon in adult mammalian skin wherein fully functional hair follicles regenerate de novo in the center of large excisional wounds. Originally described in rats, rabbits, sheep, and humans in 1940−1960, the WIHN phenomenon was reinvestigated in mice only recently. The process of de novo hair regeneration largely duplicates the morphological and signaling features of normal embryonic hair development. Similar to hair development, WIHN critically depends on the activation of canonical WNT signaling. However, unlike hair development, WNT activation in WIHN is dependent on fibroblast growth factor 9 signaling generated by the immune system's γδ T cells. The cellular bases of WIHN remain to be fully characterized; however, the available evidence leaves open the possibility for a blastema‐like mechanism wherein epidermal and/or dermal wound cells undergo epigenetic reprogramming toward a more plastic, embryonic‐like state. De novo hair follicles do not regenerate from preexisting hair‐fated bulge stem cells. This suggests that hair neogenesis is not driven by preexisting lineage‐restricted progenitors, as is the case for amputation‐induced mouse digit tip regeneration, but rather may require a blastema‐like mechanism. The WIHN model is characterized by several intriguing features, which await further explanation. These include (1) the minimum wound size requirement for activating neogenesis, (2) the restriction of hair neogenesis to the wound's center, and (3) imperfect patterning outcomes, both in terms of neogenic hair positioning within the wound and in terms of their orientation. Future enquiries into the WIHN process, made possible by a wide array of available skin‐specific genetic tools, will undoubtedly expand our understanding of the regeneration mechanisms in adult mammals.

PDGFRα signaling drives adipose tissue fibrosis by targeting progenitor cell plasticity

Adipose tissue fibrosis occurs during obesity and is associated with metabolic dysfunction. Iwayama et al. identify perivascular cells as fibro/adipogenic progenitors in white adipose tissue and show that PDGFRα targets progenitor cell plasticity as a profibrotic mechanism.

Fibrosis is a common disease process in which profibrotic cells disturb organ function by secreting disorganized extracellular matrix (ECM). Adipose tissue fibrosis occurs during obesity and is associated with metabolic dysfunction, but how profibrotic cells originate is still being elucidated. Here, we use a developmental model to investigate perivascular cells in white adipose tissue (WAT) and their potential to cause organ fibrosis. We show that a Nestin-Cre transgene targets perivascular cells (adventitial cells and pericyte-like cells) in WAT, and Nestin-GFP specifically labels pericyte-like cells. Activation of PDGFRα signaling in perivascular cells causes them to transition into ECM-synthesizing profibrotic cells. Before this transition occurs, PDGFRα signaling up-regulates mTOR signaling and ribosome biogenesis pathways and perturbs the expression of a network of epigenetically imprinted genes that have been implicated in cell growth and tissue homeostasis. Isolated Nestin-GFP⁺ cells differentiate into adipocytes ex vivo and form WAT when transplanted into recipient mice. However, PDGFRα signaling opposes adipogenesis and generates profibrotic cells instead, which leads to fibrotic WAT in transplant experiments. These results identify perivascular cells as fibro/adipogenic progenitors in WAT and show that PDGFRα targets progenitor cell plasticity as a profibrotic mechanism.

Three-Dimensional Histological Structures of the Human Dermis

Spatial information has been shown to be critical for cell differentiation and function. Therefore, a better understanding of skin microstructures is very important for biomimetic and bioengineered scaffolds of engineering skin. The purpose of the study was to generate collagen/elastin-based three-dimensional (3D) images of human dermis to further understand the microstructures of the skin, which is believed to be helpful in the fabrication of bionic engineered skin. Skin samples were fixed, embedded, serially sectioned, stained with aldehyde-fuchsin, and photographed as serial panoramas. Dermal subregions were divided according to dermal depth and distance to hair follicle. The porosity, pore diameters, and wall thickness of human acellular dermal matrix (ADM) were measured by microcomputed tomography (micro-CT). Three-dimensional reconstructed images of collagen and elastic fibers were generated. Our results showed that there were fewer elastic fibers in the subregions close to hair follicles than in the subregions far away from hair follicles (p<0.001), but the collagen fibers were evenly distributed. Both collagen and elastic fibers were found in fewer numbers in the layers either close to the epidermis or close to the hypodermis. The mean proportions of collagen fibers and elastic fibers in the whole dermis were 28.96%±14.63% and 8.06%±3.75%, respectively. The porosity of ADM calculated by micro-CT was 68.3%±5.8%. The mean pore diameter of ADM was 131.2±96.8 μm, and the wall thickness of pores was 207.2±251.7 μm. This study represents for the first time that 3D histological cutaneous structures have been presented, which may be helpful for the next generation of skin engineering.

Skin fibrosis. Identification and isolation of a dermal lineage with intrinsic fibrogenic potential

Dermal fibroblasts represent a heterogeneous population of cells with diverse features that remain largely undefined. We reveal the presence of at least two fibroblast lineages in murine dorsal skin. Lineage tracing and transplantation assays demonstrate that a single fibroblast lineage is responsible for the bulk of connective tissue deposition during embryonic development, cutaneous wound healing, radiation fibrosis, and cancer stroma formation. Lineage-specific cell ablation leads to diminished connective tissue deposition in wounds and reduces melanoma growth. Using flow cytometry, we identify CD26/DPP4 as a surface marker that allows isolation of this lineage. Small molecule-based inhibition of CD26/DPP4 enzymatic activity during wound healing results in diminished cutaneous scarring. Identification and isolation of these lineages hold promise for translational medicine aimed at in vivo modulation of fibrogenic behavior.

Generation and characterization of PDGFRα-GFPCreERT2 knock-In mouse line

Platelet-derived growth factor (PDGF) and its receptor play an important role in embryogenesis. PDGF receptor α (PDGFRα) is expressed specifically in the embryonic day 7.5 (E7.5) mesoderm and in the E9.5 neural crest among other tissues. PDGFRα-expressing cells and their descendants are involved in the formation of various tissues. To trace PDGFRα-expressing cells in vivo, we generated a knock-in mouse line that expressed a fusion protein of green fluorescent protein (GFP), Cre recombinase (Cre), and mutated estrogen receptor ligand-binding domain (ERT2) under the control of the PDGFRα promoter. In these mice, Cre activity in PDGFRα-expressing cells could be induced by tamoxifen treatment. Taken together, our results suggest that the knock-in mouse line generated here could be useful for studying PDGFRα-expressing cells and their descendants in vivo at various stages of development.

A Role of Myocardin Related Transcription Factor-A (MRTF-A) in Scleroderma Related Fibrosis

In scleroderma (systemic sclerosis, SSc), persistent activation of myofibroblast leads to severe skin and organ fibrosis resistant to therapy. Increased mechanical stiffness in the involved fibrotic tissues is a hallmark clinical feature and a cause of disabling symptoms. Myocardin Related Transcription Factor-A (MRTF-A) is a transcriptional co-activator that is sequestered in the cytoplasm and translocates to the nucleus under mechanical stress or growth factor stimulation. Our objective was to determine if MRTF-A is activated in the disease microenvironment to produce more extracellular matrix in progressive SSc. Immunohistochemistry studies demonstrate that nuclear translocation of MRTF-A in scleroderma tissues occurs in keratinocytes, endothelial cells, infiltrating inflammatory cells, and dermal fibroblasts, consistent with enhanced signaling in multiple cell lineages exposed to the stiff extracellular matrix. Inhibition of MRTF-A nuclear translocation or knockdown of MRTF-A synthesis abolishes the SSc myofibroblast enhanced basal contractility and synthesis of type I collagen and inhibits the matricellular profibrotic protein, connective tissue growth factor (CCN2/CTGF). In MRTF-A null mice, basal skin and lung stiffness was abnormally reduced and associated with altered fibrillar collagen. MRTF-A has a role in SSc fibrosis acting as a central regulator linking mechanical cues to adverse remodeling of the extracellular matrix.

Myofibroblasts in murine cutaneous fibrosis originate from adiponectin-positive intradermal progenitors

OBJECTIVE

Accumulation of myofibroblasts in fibrotic skin is a hallmark of systemic sclerosis (SSc; scleroderma), but the origins of these cells remain unknown. Because loss of intradermal adipose tissue is a consistent feature of cutaneous fibrosis, we sought to examine the hypothesis that myofibroblasts populating fibrotic dermis derive from adipocytic progenitors.

METHODS

We performed genetic fate mapping studies to investigate the loss of intradermal adipose tissue and its potential role in fibrosis in mice with bleomycin-induced scleroderma. Modulation of adipocytic phenotypes ex vivo was investigated in adipose tissue-derived cells in culture.

RESULTS

A striking loss of intradermal adipose tissue and its replacement with fibrous tissue were consistently observed in mice with bleomycin-induced fibrosis. Loss of adipose tissue and a decline in the expression of canonical adipogenic markers in lesional skin preceded the onset of dermal fibrosis and expression of fibrogenic markers. Ex vivo, subcutaneous adipocytes were driven by transforming growth factor β to preferentially undergo fibrogenic differentiation. Cell fate mapping studies in mice with the adiponectin promoter-driven Cre recombinase transgenic construct indicated that adiponectin-positive progenitors that are normally confined to the intradermal adipose tissue compartment were distributed throughout the lesional dermis over time, lost their adipocytic markers, and expressed myofibroblast markers in bleomycin-treated mice.

CONCLUSION

These observations establish a novel link between intradermal adipose tissue loss and dermal fibrosis and demonstrate that adiponectin-positive intradermal progenitors give rise to dermal myofibroblasts. Adipose tissue loss and adipocyte-myofibroblast transition might be primary events in the pathogenesis of cutaneous fibrosis that represent novel potential targets for therapeutic intervention.

Vital roles of stem cells and biomaterials in skin tissue engineering

Tissue engineering essentially refers to technology for growing new human tissue and is distinct from regenerative medicine. Currently, pieces of skin are already being fabricated for clinical use and many other tissue types may be fabricated in the future. Tissue engineering was first defined in 1987 by the United States National Science Foundation which critically discussed the future targets of bioengineering research and its consequences. The principles of tissue engineering are to initiate cell cultures in vitro, grow them on scaffolds in situ and transplant the composite into a recipient in vivo. From the beginning, scaffolds have been necessary in tissue engineering applications. Regardless, the latest technology has redirected established approaches by omitting scaffolds. Currently, scientists from diverse research institutes are engineering skin without scaffolds. Due to their advantageous properties, stem cells have robustly transformed the tissue engineering field as part of an engineered bilayered skin substitute that will later be discussed in detail. Additionally, utilizing biomaterials or skin replacement products in skin tissue engineering as strategy to successfully direct cell proliferation and differentiation as well as to optimize the safety of handling during grafting is beneficial. This approach has also led to the cells’ application in developing the novel skin substitute that will be briefly explained in this review.

Tissue engineering and regenerative medicine in basic research: a year in review of 2014

Tissue engineering and regenerative medicine (TERM) remains to be one of the fastest growing fields, which covers a wide scope of topics of both basic and applied biological researches. This overview article summarized the advancements in basic researches of TERM area, including stem cell biology, cell engineering, somatic nuclear transfer, genomic editing, discovery of new tissue progenitor/stem cells, and immunomodulation of stem cells and tissue regeneration. It reflects the cutting-edge achievements in basic researches, which will lay solid scientific foundation for future TERM translational researches.

Paracrine crosstalk between human hair follicle dermal papilla cells and microvascular endothelial cells

Human follicle dermal papilla cells (FDPC) are a specialized population of mesenchymal cells located in the skin. They regulate hair follicle (HF) development and growth, and represent a reservoir of multipotent stem cells. Growing evidence supports the hypothesis that HF cycling is associated with vascular remodeling. Follicular keratinocytes release vascular endothelial growth factor (VEGF) that sustains perifollicular angiogenesis leading to an increase of follicle and hair size. Furthermore, several human diseases characterized by hair loss, including Androgenetic Alopecia, exhibit alterations of skin vasculature. However, the molecular mechanisms underlying HF vascularization remain largely unknown. In vitro coculture approaches can be successfully employed to greatly improve our knowledge and shed more light on this issue. Here we used Transwell-based co-cultures to show that FDPC promote survival, proliferation and tubulogenesis of human microvascular endothelial cells (HMVEC) more efficiently than fibroblasts. Accordingly, FDPC enhance the endothelial release of VEGF and IGF-1, two well-known proangiogenic growth factors. Collectively, our data suggest a key role of papilla cells in vascular remodeling of the hair follicle.

Modulation of the tumor microenvironment for cancer treatment: a biomaterials approach

Tumors are complex tissues that consist of stromal cells, such as fibroblasts, immune cells and mesenchymal stem cells, as well as non-cellular components, in addition to neoplastic cells. Increasingly, there is evidence to suggest that these non-neoplastic cell components support cancer initiation, progression and metastasis and that their ablation or reprogramming can inhibit tumor growth. Our understanding of the activities of different parts of the tumor stroma in advancing cancer has been improved by the use of scaffold and matrix-based 3D systems originally developed for regenerative medicine. Additionally, drug delivery systems made from synthetic and natural biomaterials deliver drugs to kill stromal cells or reprogram the microenvironment for tumor inhibition. In this article, we review the impact of 3D tumor models in increasing our understanding of tumorigenesis. We also discuss how different drug delivery systems aid in the reprogramming of tumor stroma for cancer treatment.

Reduced granulation tissue and wound strength in the absence of α11β1 integrin

Previous wound healing studies have failed to define a role for either α1β1 or α2β1 integrin in fibroblast-mediated wound contraction, suggesting the involvement of another collagen receptor in this process. Our previous work demonstrated that the integrin subunit α11 is highly induced during wound healing both at the mRNA and protein level, prompting us to investigate and dissect the role of the integrin α11β1 during this process. Therefore, we used mice with a global ablation of either α2 or α11 or both integrin subunits and investigated the repair of excisional wounds. Analyses of wounds demonstrated that α11β1 deficiency results in reduced granulation tissue formation and impaired wound contraction, independently of the presence of α2β1. Our combined in vivo and in vitro data further demonstrate that dermal fibroblasts lacking α11β1 are unable to efficiently convert to myofibroblasts, resulting in scar tissue with compromised tensile strength. Moreover, we suggest that the reduced stability of the scar is a consequence of poor collagen remodeling in α11^−/− wounds associated with defective transforming growth factor-β–dependent JNK signaling.

Improved cutaneous wound healing after intraperitoneal injection of alpha-melanocyte-stimulating hormone

Skin wound healing is a complex process involving many types of cells and molecules and often results in scar tissue formation in adult mammals. However, scarless healing occurs in foetal skin and minimal scars may occur after cutaneous healing in the adult with reduced inflammation. Alpha-melanocyte-stimulating hormone (α-MSH) is widely distributed within the central nervous system and in other body regions, such as the skin, and has strong anti-inflammatory activity. The aim in the present experiments was to learn whether intraperitoneal (i.p) injection of α-MSH just before skin wounds antagonize inflammation and improves skin wound healing in adult mice. C57BL/6 young adult mice received an i.p. injection of 1 mg/kg of α-MSH and, 30 min later, two circular through-and-through holes (6.5 mm diameter) were made in their dorsal skin under anaesthesia. Control mice were wounded after vehicle injection. The wound healing process was analysed macroscopically and microscopically at 3, 7, 40 and 60 days. Skin samples were fixed in formalin, embedded in paraffin, sectioned at 5 μm, stained with H&E or toluidine blue for cell analysis or Gomori's trichrome for extracellular matrix (ECM) analysis. Other samples were fixed in DMSO+methanol, embedded in paraplast and incubated with anti-CD45, antismooth muscle actin, anticollagen-I and anticollagen-III for immunofluorescence analysis. Alpha-MSH significantly reduced the number of leucocytes, mast cells and fibroblasts at 3 and 7 days after injury. On days 40 and 60, α-MSH reduced scar area and improved the organization of the collagen fibres indicating that it may direct the healing into a more-regenerative/less-scarring pathway.

Sustained β-catenin activity in dermal fibroblasts promotes fibrosis by up-regulating expression of extracellular matrix protein-coding genes

Fibrosis is an end-stage response to tissue injury that is associated with loss of organ function as a result of excess extracellular matrix (ECM) production by fibroblasts. In skin, pathological fibrosis is evident during keloid scar formation, systemic sclerosis (SSc) and morphea. Dermal fibroblasts in these fibrotic diseases exhibit increased Wnt/β-catenin signalling, a pathway that is sufficient to cause fibrosis in mice. However, in the context of this complex pathology, the precise pro-fibrotic consequences of Wnt/β-catenin signalling are not known. We found that expression of stabilized β-catenin in mouse dermal fibroblasts resulted in spontaneous, progressive skin fibrosis with thickened collagen fibres and altered collagen fibril morphology. The fibrotic phenotype was predominated by resident dermal fibroblasts. Genome-wide profiling of the fibrotic mouse dermis revealed elevated expression of matrix-encoding genes, and the promoter regions of these genes were enriched for Tcf/Lef family transcription factor binding sites. Additionally, we identified 32 β-catenin-responsive genes in our mouse model that are also over-expressed in human fibrotic tissues and poised for regulation by Tcf/Lef family transcription factors. Therefore, we have uncovered a matrix-regulatory role for stabilized β-catenin in fibroblasts in vivo and have defined a set of β-catenin-responsive genes with relevance to fibrotic disease.

Wound healing and skin regeneration

The skin is a complex organ consisting of the epidermis, dermis, and skin appendages, including the hair follicle and sebaceous gland. Wound healing in adult mammals results in scar formation without any skin appendages. Studies have reported remarkable examples of scarless healing in fetal skin and appendage regeneration in adult skin following the infliction of large wounds. The models used in these studies have offered a new platform for investigations of the cellular and molecular mechanisms underlying wound healing and skin regeneration in mammals. In this article, we will focus on the contribution of skin appendages to wound healing and, conversely, skin appendage regeneration following injuries.

Innate immunity. Dermal adipocytes protect against invasive Staphylococcus aureus skin infection

Adipocytes have been suggested to be immunologically active, but their role in host defense is unclear. We observed rapid proliferation of preadipocytes and expansion of the dermal fat layer after infection of the skin by Staphylococcus aureus. Impaired adipogenesis resulted in increased infection as seen in Zfp423(nur12) mice or in mice given inhibitors of peroxisome proliferator-activated receptor γ. This host defense function was mediated through the production of cathelicidin antimicrobial peptide from adipocytes because cathelicidin expression was decreased by inhibition of adipogenesis, and adipocytes from Camp(-/-) mice lost the capacity to inhibit bacterial growth. Together, these findings show that the production of an antimicrobial peptide by adipocytes is an important element for protection against S. aureus infection of the skin.

Mast cells' integrated actions with eosinophils and fibroblasts in allergic inflammation: implications for therapy

Mast cells (MCs) and eosinophils (Eos) are the key players in the development of allergic inflammation (AI). Their cross-talk, named the Allergic Effector Unit (AEU), takes place through an array of soluble mediators and ligands/receptors interactions that enhance the functions of both the cells. One of the salient features of the AEU is the CD48/2B4 receptor/ligand binding complex. Furthermore, MCs and Eos have been demonstrated to play a role not only in AI but also in the modulation of its consequence, i.e., fibrosis/tissue remodeling, by directly influencing fibroblasts (FBs), the main target cells of these processes. In turn, FBs can regulate the survival, activity, and phenotype of both MCs and Eos. Therefore, a complex three players, MCs/Eos/FBs interaction, can take place in various stages of AI. The characterization of the soluble and physical mediated cross talk among these three cells might lead to the identification of both better and novel targets for the treatment of allergy and its tissue remodeling consequences.

Live fibroblast harvest reveals surface marker shift in vitro

Current methods for the isolation of fibroblasts require extended ex vivo manipulation in cell culture. As a consequence, prior studies investigating fibroblast biology may fail to adequately represent cellular phenotypes in vivo. To overcome this problem, we describe a detailed protocol for the isolation of fibroblasts from the dorsal dermis of adult mice that bypasses the need for cell culture, thereby preserving the physiological, transcriptional, and proteomic profiles of each cell. Using the described protocol we characterized the transcriptional programs and the surface expression of 176 CD markers in cultured versus uncultured fibroblasts. The differential expression patterns we observed highlight the importance of a live harvest for investigations of fibroblast biology.

Wound repair and regeneration: mechanisms, signaling, and translation

The cellular and molecular mechanisms underpinning tissue repair and its failure to heal are still poorly understood, and current therapies are limited. Poor wound healing after trauma, surgery, acute illness, or chronic disease conditions affects millions of people worldwide each year and is the consequence of poorly regulated elements of the healthy tissue repair response, including inflammation, angiogenesis, matrix deposition, and cell recruitment. Failure of one or several of these cellular processes is generally linked to an underlying clinical condition, such as vascular disease, diabetes, or aging, which are all frequently associated with healing pathologies. The search for clinical strategies that might improve the body's natural repair mechanisms will need to be based on a thorough understanding of the basic biology of repair and regeneration. In this review, we highlight emerging concepts in tissue regeneration and repair, and provide some perspectives on how to translate current knowledge into viable clinical approaches for treating patients with wound-healing pathologies.

Palatogenesis and cutaneous repair: A two-headed coin

BACKGROUND

The reparative mechanism that operates following post-natal cutaneous injury is a fundamental survival function that requires a well-orchestrated series of molecular and cellular events. At the end, the body will have closed the hole using processes like cellular proliferation, migration, differentiation and fusion.

RESULTS

These processes are similar to those occurring during embryogenesis and tissue morphogenesis. Palatogenesis, the formation of the palate from two independent palatal shelves growing towards each other and fusing, intuitively, shares many similarities with the closure of a cutaneous wound from the two migrating epithelial fronts.

CONCLUSIONS

In this review, we summarize the current information on cutaneous development, wound healing, palatogenesis and orofacial clefting and propose that orofacial clefting and wound healing are conserved processes that share common pathways and gene regulatory networks.

Mammalian skin cell biology: at the interface between laboratory and clinic

Mammalian skin research represents the convergence of three complementary disciplines: cell biology, mouse genetics, and dermatology. The skin provides a paradigm for current research in cell adhesion, inflammation, and tissue stem cells. Here, I discuss recent insights into the cell biology of skin. Single-cell analysis has revealed that human epidermal stem cells are heterogeneous and differentiate in response to multiple extrinsic signals. Live-cell imaging, optogenetics, and cell ablation experiments show skin cells to be remarkably dynamic. High-throughput, genome-wide approaches have yielded unprecedented insights into the circuitry that controls epidermal stem cell fate. Last, integrative biological analysis of human skin disorders has revealed unexpected functions for elements of the skin that were previously considered purely structural.

Epigenetic control of skin and hair regeneration after wounding

Skin wound healing is a complex regenerative phenomenon that can result in hair follicle neogenesis. Skin regeneration requires significant contribution from the immune system and involves substantial remodelling of both epidermal and dermal compartments. In this viewpoint, we consider epigenetic regulation of reepithelialization, dermal restructuring and hair neogenesis. Because little is known about the epigenetic control of these events, we have drawn upon recent epigenetic mapping and functional studies of homeostatic skin maintenance, epithelial-mesenchymal transition in cancer, and new works on regenerative dermal cell lineages and the epigenetic events that may shape their conversion into myofibroblasts. Finally, we speculate on how these various healing components might converge for wound-induced hair follicle neogenesis.

Understanding fibroblast heterogeneity in the skin

Fibroblasts are found in most tissues, yet they remain poorly characterised. Different fibroblast subpopulations with distinct functions have been identified in the skin. This functional heterogeneity reflects the varied fibroblast lineages that arise from a common embryonic precursor. In addition to autocrine signals, fibroblasts are highly responsive to Wnt-regulated signals from the overlying epidermis, which can act both locally, via extracellular matrix (ECM) deposition, and via secreted factors that impact the behaviour of fibroblasts in different dermal locations. These findings may explain some of the changes that occur in connective tissue during wound healing and cancer progression.

Fibroblasts and myofibroblasts in wound healing

(Myo)fibroblasts are key players for maintaining skin homeostasis and for orchestrating physiological tissue repair. (Myo)fibroblasts are embedded in a sophisticated extracellular matrix (ECM) that they secrete, and a complex and interactive dialogue exists between (myo)fibroblasts and their microenvironment. In addition to the secretion of the ECM, (myo)fibroblasts, by secreting matrix metalloproteinases and tissue inhibitors of metalloproteinases, are able to remodel this ECM. (Myo)fibroblasts and their microenvironment form an evolving network during tissue repair, with reciprocal actions leading to cell differentiation, proliferation, quiescence, or apoptosis, and actions on growth factor bioavailability by binding, sequestration, and activation. In addition, the (myo)fibroblast phenotype is regulated by mechanical stresses to which they are subjected and thus by mechanical signaling. In pathological situations (excessive scarring or fibrosis), or during aging, this dialogue between the (myo)fibroblasts and their microenvironment may be altered or disrupted, leading to repair defects or to injuries with damaged and/or cosmetic skin alterations such as wrinkle development. The intimate dialogue between the (myo)fibroblasts and their microenvironment therefore represents a fascinating domain that must be better understood in order not only to characterize new therapeutic targets and drugs able to prevent or treat pathological developments but also to interfere with skin alterations observed during normal aging or premature aging induced by a deleterious environment.

Developing stratified epithelia: lessons from the epidermis and thymus

Stratified squamous epithelial cells are found in a number of organs, including the skin epidermis and the thymus. The progenitor cells of the developing epidermis form a multi-layered epithelium and appendages, like the hair follicle, to generate an essential barrier to protect against water loss and invasion of foreign pathogens. In contrast, the thymic epithelium forms a three-dimensional mesh of keratinocytes that are essential for positive and negative selection of self-restricted T cells. While these distinct stratified epithelial tissues derive from distinct embryonic germ layers, both tissues instruct immunity, and the epithelial differentiation programs and molecular mechanisms that control their development are remarkably similar. In this review, we aim to highlight some of the similarities between the thymus and the skin epidermis and its appendages during developmental specification.

Neural-competent cells of adult human dermis belong to the Schwann lineage

Resident neural precursor cells (NPCs) have been reported for a number of adult tissues. Understanding their physiological function or, alternatively, their activation after tissue damage or in vitro manipulation remains an unsolved issue. Here, we investigated the source of human dermal NPCs in adult tissue. By following an unbiased, comprehensive approach employing cell-surface marker screening, cell separation, transcriptomic characterization, and in vivo fate analyses, we found that p75NTR⁺ precursors of human foreskin can be ascribed to the Schwann (CD56⁺) and perivascular (CD56⁻) cell lineages. Moreover, neural differentiation potential was restricted to the p75NTR⁺CD56⁺ Schwann cells and mediated by SOX2 expression levels. Double-positive NPCs were similarly obtained from human cardiospheres, indicating that this phenomenon might be widespread.

•

Human dermis-derived cultures show two types of SOX2⁺ cells: Schwann and perivascular

•

p75NTR⁺CD56⁺ Schwann cells are responsible for neural progeny

•

SOX2 expression levels regulate the neural competence of dermal precursors

•

p75NTR⁺CD56⁺ neural precursor cells similarly arise from human cardiospheres

Modulation of type II TGF-β receptor degradation by integrin-linked kinase

Cutaneous responses to injury, infection, and tumor formation involve the activation of resident dermal fibroblasts and subsequent transition to myofibroblasts. The key for induction of myofibroblast differentiation is the activation of transforming growth factor-β (TGF-β) receptors and stimulation of integrins and their associated proteins, including integrin-linked kinase (ILK). Cross-talk processes between TGF-β and ILK are crucial for myofibroblast formation, as ILK-deficient dermal fibroblasts exhibit impaired responses to TGF-β receptor stimulation. We now show that ILK associates with type II TGF-β receptors (TβRII) in ligand- and receptor kinase activity-independent manners. In cells with targeted Ilk gene inactivation, cellular levels of TβRII are decreased, through mechanisms that involve enhanced ubiquitination and proteasomal degradation. Partitioning of TGF-β receptors into membrane has been linked to proteasome-dependent receptor degradation. We found that interfering with membrane raft formation in ILK-deficient cells restored TβRII levels and signaling. These observations support a model whereby ILK functions in fibroblasts to direct TβRII away from degradative pathways during their differentiation into myofibroblasts.

BLIMP1 is required for postnatal epidermal homeostasis but does not define a sebaceous gland progenitor under steady-state conditions

B-lymphocyte-induced nuclear maturation protein 1 (BLIMP1) was previously reported to define a sebaceous gland (SG) progenitor population in the epidermis. However, the recent identification of multiple stem cell populations in the hair follicle junctional zone has led us to re-evaluate its function. We show, in agreement with previous studies, that BLIMP1 is expressed by postmitotic, terminally differentiated epidermal cells within the SG, interfollicular epidermis, and hair follicle. Epidermal overexpression of c-Myc results in loss of BLIMP1(+) cells, an effect modulated by androgen signaling. Epidermal-specific deletion of Blimp1 causes multiple differentiation defects in the epidermis in addition to SG enlargement. In culture, BLIMP1(+) sebocytes have no greater clonogenic potential than BLIMP1(-) sebocytes. Finally, lineage-tracing experiments reveal that, under steady-state conditions, BLIMP1-expressing cells do not divide. Thus, rather than defining a sebocyte progenitor population, BLIMP1 functions in terminally differentiated cells to maintain homeostasis in multiple epidermal compartments.

Defining dermal adipose tissue

Here, we explore the evolution and development of skin-associated adipose tissue with the goal of establishing nomenclature for this tissue. Underlying the reticular dermis, a thick layer of adipocytes exists that encases mature hair follicles in rodents and humans. The association of lipid-filled cells with the skin is found in many invertebrate and vertebrate species. Historically, this layer of adipocytes has been termed subcutaneous adipose, hypodermis and subcutis. Recent data have revealed a common precursor for dermal fibroblasts and intradermal adipocytes during development. Furthermore, the development of adipocytes in the skin is independent from that of subcutaneous adipose tissue development. Finally, the role of adipocytes has been shown to be relevant for epidermal homoeostasis during hair follicle regeneration and wound healing. Thus, we propose a refined nomenclature for the cells and adipose tissue underlying the reticular dermis as intradermal adipocytes and dermal white adipose tissue, respectively.