Skin: a promising reservoir for adult stem cell populations

Plasticity of adult cells has been identified in several post-natal tissues in the past few years and has attracted special attention in regenerative medicine. Skin is the biggest organ in the body. Adult skin consists of epidermis, dermis and appendages such as hairs and glands which are linked to the epidermis but project deep into the dermal layer. Skin stem cell biology has been a focus of increasing interest in current life science. Committed stem cells with limited differentiation potential for regeneration and repair of epidermis have been known for decades. Recent studies further report that adult skin tissues contain cell populations with pluripotent characteristics. Multipotent stem cells from hair follicle and non-follicular skin, both in epidermal and dermal tissues, are found to have the differentiation capacity to generate multiple cell lineages. Basing on the present data, our hypothesis is that skin may serve as a local reservoir of various adult stem cell populations, including committed stem cell populations and pluripotent stem cell populations both in epidermal and dermal tissues. Given its easy accessibility, stem cells in skin will not only provide an experimental model for skin biology, but also may provide an experimental model for studying the epithelial-mesenchymal interactions of several other organs outside of skin. The stem cell populations in skin tissues may also have extensive therapeutic implications in the replacement of skin and may serve as an alternative source of stem cells for several other organs outside of skin. The in situ activation and mobilization of stem cell populations in the skin is an ideal way to renew and repair epidermis and dermis, even appendages.     

Can hematopoietic stem cells be an alternative source for skin regeneration?

In the past few years, the plasticity of adult cells in several post-natal tissues has attracted special attention in regenerative medicine. Skin is the largest organ in the body. Adult skin consists of epidermis, dermis and appendages such as hair and glands that are linked to the epidermis but project deep into the dermal layer. Stem cell biology of skin has been a focus of increasing interest in current life science. Committed stem cells with a limited differentiation potential for regeneration and repair of epidermis have been known for decades. Recent studies further found that adult skin tissues contain cell populations with pluripotent characteristics. Multipotent stem cells from skin with and without hair follicles, both in epidermal and dermal tissues, can differentiate and generate multiple cell lineages. Especially, the hematopoietic system in epidermal and dermal tissue, like skin, may be a local, acceptable reservoir of various adult stem cell populations. Given their easy accessibility, such stem cells can provide an experimental model not only for skin biology but also for studying the epithelial–mesenchymal cell interactions of organs other than the skin. This review presents an overview of recent advances in research into skin repair and regeneration involving stem cells from epidermis, dermis, and bone marrow. In particular, we focus on the possible use of blood stem cells as an alternative resource for research advances in skin biology.     

Bone marrow cells engraft within the epidermis and proliferate in vivo with no evidence of cell fusion

In adults, bone marrow-derived cells (BMDC) can contribute to the structure of various non-haematopoietic tissues, including skin. However, the physiological importance of these cells is unclear. This study establishes that bone marrow-derived epidermal cells are proliferative and, moreover, demonstrates for the first time that BMDC can localize to a known stem cell niche: the CD34-positive bulge region of mouse hair follicles. In addition, engraftment of bone marrow cells into the epidermis is significantly increased in wounded skin, bone marrow-derived keratinocytes can form colonies in the regenerating epidermis in vivo, and the colony-forming capacity of these cells can be recapitulated in vitro. In some tissues this apparent plasticity is attributed to differentiation, and in others to cell fusion. Evidence is also provided that bone marrow cells form epidermal keratinocytes without undergoing cell fusion. These data suggest a functional role for bone marrow cells in epidermal regeneration, entering known epidermal stem cell niches without heterokaryon formation.     

表皮干细胞在组织工程创面修复中的应用

背景：随着细胞生物学、分子生物学技术、组织工程学的快速发展,探寻组织工程化皮肤创面覆盖物的“种子细胞”的研究逐渐增多。 目的：总结表皮干细胞的生物学特性,探讨其在皮肤创面修复过程中的再生作用与临床应用价值。 方法：应用计算机检索CNKI和PubMed数据库中2002-07/2011-12关于表皮干细胞修复皮肤损伤研究的文章,以“表皮干细胞,创面修复,组织工程,皮肤”或“epidermal stem cells,tissue engineering skin,wound surface”为检索词进行检索。选择文章内容与表皮干细胞修复皮肤损伤研究进展有关的文章,同一领域文献则选择近期发表或发表在权威杂志文章。初检得到129篇文献,根据纳入标准选择28篇文献进行综合分析。 结果与结论：表皮干细胞是表皮发生、分化和创面修复的基础,其正常增殖分化是维持皮肤正常组织结构和细胞内环境稳定的基本要求,也是皮肤组织工程理想的种子细胞。对烧伤、创伤等大面积皮肤缺损的治疗,对皮肤疾病的细胞疗法、基因治疗等方面都有很好的应用前景。表皮细胞的体外培养是复合人工皮肤组织工程学研究的先决条件。随着对表皮干细胞分离、纯化和培养技术的不断完善,可达到迅速构建表皮层的目的。但表皮干细胞的应用研究仍需要进一步的探索。     

Epithelial stem cells in corneal regeneration and epidermal gene therapy

Regenerative medicine refers to innovative therapies aimed at the permanent restoration of diseased tissues and organs. Regeneration of self-renewing tissues requires specific adult stem cells, which need to be genetically modified to correct inherited genetic diseases. Cultures of epithelial stem cells permanently restore severe skin and mucosal defects, and genetically corrected epidermal stem cells regenerate a normal epidermis in patients carrying junctional epidermolysis bullosa. The keratinocyte stem cell is therefore the only cultured stem cell used both in cell therapy and gene therapy clinical protocols. Epithelial stem cell identification, fate and molecular phenotype have been extensively reviewed, but not in relation to tissue regeneration. In this paper we focus on the localization and molecular characterization of human limbal stem cells in relation to corneal regeneration, and the gene therapy of genetic skin diseases by means of genetically modified epidermal stem cells.     

Plasticity of epidermal stem cells

The keratinocyte cell compartment in the continuously renewing epidermis of the skin is maintained by undifferentiated, self-renewing stem cells. We show that a small subpopulation of epidermal stem cells (EpiSCs) have the capacity to integrate into multiple tissues. These EpiSCs can change their phenotype in direct response of changes in cytokines in vitro, changes in cocultured cells, after injection into damaged environments in vivo. These changes appear to be unrelated to the age of the EpiSC. Even though we can isolate these cells and show that the age of thses cells appears to be irrelevant to this multipotent function, we still do not know how such cells are defined within a tissue or what the life span of a multipotent stem cell is.     

损伤皮肤修复和伤口愈合过程中的干细胞：问题与前景

BACKGROUND:Present treatments for chronic skin wounds have certain limitations, and adult stem cells play a potential part in cutaneous repair and regeneration. OBJECTIVE:To review effects of stem cells in skin regeneration and wound healing. METHODS:The first author retrieved CNKI and Medline databases by computer for relevant articles published from 2000 to 2010. The keywords were “epidermal stem cells, hair follicle stem cells, stem cells, transplantation, dermal stem cells” in Chinese and in English, respectively. Then totally 489 papers were obtained after initial survey, and according to the inclusion criteria, 30 articles were selected for review. RESULTS AND CONCLUSION:Epidermal stem cells and other adult stem cells have been applied to treat wounds and other skin diseases. Epidermal stem cells are the crucial cell source of skin development, repair and remodeling. Epidermal stem cells are always in a resting state in vivo. Unless, skin injure or culture in vitro, cell division and proliferation will be significantly fastened. The stability of the epidermis mainly depends on the asymmetric division of a subpopulation, in which two daughter cells are produced, including one with characteristics of stem cells, and the other differentiated into transient amplifying cells that will be differentiated into post mitotic cells after a series of cell divisions (3-5 times). Afterwards, those post mitotic cells are developed into terminal differentiation cells on the basal layer, finally detach from the epidermis as dander. In addition, it is unclear whether epidermal factors are related to apoptosis, migration and differentiation in the process of wound repair and even under physiological conditions. Therefore, application of stem cells in wound healing requires a further discussion.     

IFATS collection: Using human adipose-derived stem/stromal cells for the production of new skin substitutes

The ability to harvest and culture stem cell populations from various human postnatal tissues is central to regenerative medicine applications, including tissue engineering. The discovery of multipotent mesenchymal stem cells within the stromal fraction of adipose tissue prompted their use for the healing and reconstruction of many tissues. Here, we examined the influence of adipose-derived stem/stromal cells (ASCs) on skin's regenerative processes, from a tissue engineering perspective. Using a self-assembly approach, human skin substitutes were produced. They featured a stromal compartment containing human extracellular matrix endogenously produced from either dermal fibroblasts or adipose-derived stem/stromal cells differentiated or not toward the adipogenic lineage. Human keratinocytes were seeded on each stroma and cultured at the air-liquid interface to reconstruct a bilayered skin substitute. These new skin substitutes, containing an epidermis and a distinctive stroma devoid of synthetic biomaterial, displayed characteristics similar to human skin. The influence of the type of stromal compartment on epidermal morphogenesis was assessed by the evaluation of tissue histology, the expression of key protein markers of the epidermal differentiation program (keratin [K] 14, K10, transglutaminase), the expression of dermo-epidermal junction components (laminins, collagen VII), and the presence of basement membrane and hemidesmosomes. Our findings suggest that adipose-derived stem/stromal cells could usefully substitute dermal fibroblasts for skin reconstruction using the self-assembly method. Finally, by exploiting the adipogenic potential of ASCs, we also produced a more complete trilayered skin substitute consisting of the epidermis, the dermis, and the adipocyte-containing hypodermis, the skin's deepest layer. Disclosure of potential conflicts of interest is found at the end of this article.     

The Skin: A Home to Multiple Classes of Epithelial Progenitor Cells

To maintain homeostasis in the adult skin, epithelial keratinocyte stem cells are thought to divide infrequently giving rise to short-lived (transit amplifying) cells that undergo a limited number of cell divisions and ultimately terminal differentiation. This model for the epidermal stem cell niche has increased in complexity by the multiple putative progenitor keratinocyte populations that have recently been identified in distinct regions of the interfollicular epidermis and hair follicle appendages. Under normal conditions, these progenitor populations are long-lived and able to sustain the cellular input to certain epidermal structures including the interfollicular epidermis and sebaceous gland. Other putative epithelial progenitors derived from the hair follicle possess high in vitro proliferative capacity and are able to regenerate skin, hair and sebaceous lineages in transplantation studies. These new findings present the cutaneous epithelium as a highly compartmentalized structure potentially maintained by multiple classes of progenitor cells. In this review, we will discuss the implications of these new putative epithelial progenitor populations and their potential to be influenced by external stimuli for skin homeostasis and carcinogenesis.     

Enrichment and identification of human 'fetal' epidermal stem cells

BACKGROUND: Human epidermis, a continuously renewing tissue, is maintained throughout life by stem cells that proliferate and replenish worn-out or damaged cells in the tissue. Cultured human epidermal stem cells have great potential in clinical application. However, isolating and culturing a pure population of epidermal stem cells has proven to be challenging. METHODS AND RESULTS: We show that p63, a new marker for epidermal stem cells, is expressed in the basal layer of human fetal epidermis using immunohistochemistry, and that keratinocytes with the characteristics of stem cells can be isolated from the epidermis of aborted human fetuses aged >/=20 weeks based on high expression of beta(1) integrins by fluorescence-activated cell sorting. Furthermore, the enriched population showed the expression of molecular markers of putative human epidermal stem cells under a confocal microscope and a high colony formation efficiency when it was cultured at a clonal density. Under an electron microscope the sorted stem cells exhibited a high nuclear:cytoplasmic ratio and fewer organelles than the transit amplifying cells. The cultured epidermal stem cells can also be amplified and induced to terminal differentiation by suspension in vitro. CONCLUSIONS: Human 'fetal' epidermal stem cells have been successfully isolated and cultured in vitro. The cultured human epidermal stem cells could be used as a tool for studying stem cell biology and testing stem cell therapy.     

糖尿病模型皮肤创面中人真皮多能干细胞向表皮细胞的分化

背景：研究表明,干细胞分化为何种细胞与其所处的微环境密切相关。因此设想：人真皮多能干细胞处于皮肤创面的微环境中,可能具有向人皮肤细胞转化的潜能。 目的：探讨糖尿病皮肤创面微环境中的人真皮多能干细胞分化为表皮细胞的可能性。 方法：分离培养人真皮多能干细胞并制作糖尿病裸鼠模型皮肤创面,将标记5-BrdU的第3~5代人真皮多能干细胞以注射方式回植入糖尿病裸鼠创面组织周围,分别于注射后2,3周取材,常规石蜡包埋、连续切片,行BrdU和角蛋白免疫组织化学染色。 结果与结论：BrdU阳性细胞出现在表皮中,且连续切片中部分BrdU阳性细胞也同时表达角蛋白。说明在糖尿病创面愈合过程中,人真皮多能干细胞具有向表皮细胞分化的潜能。     

胚胎干细胞源性表皮干细胞对小鼠全层皮肤缺损的修复

目的 研究129小鼠胚胎干细胞(ES cell)源性表皮干细胞在同种小鼠全层皮肤缺损的生长和分化,探讨Es细胞源性表皮干细胞对全层皮肤缺损的修复作用.方法 以生物膜为载体,将羊膜诱导后带有核荧光标记的ES细胞源性表皮干细胞直接覆盖小鼠全层皮肤缺损创面.术后1～8周连续取材,HE染色,β1整合素、CK15、CK19、CK10、CEA免疫组织化学和荧光双标显色.结果 术后2周,创面完全长合,较厚的新生皮覆盖创面,基底层细胞增生,形成许多大小不一的细胞柱伸向真皮层,真皮层中可见管腔样结构,免疫荧光双标显示,1～3周新生表皮中可见核标记的细胞呈β1整合素、CK15阳性,真皮层中带有核标记的管状或泡状结构呈β1整合素、CK15阳性;4周后新生表皮开始变薄,基底层细胞呈CK19、CK10阳性,汗腺样结构呈CEA阳性,6～8周新生表皮下可见毛囊样、汗腺样、皮脂腺样等结构.结论 ES细胞源性表皮于细胞植入小鼠全层皮肤缺损创面,可修复缺损的表皮,并在其下真皮层内具有分化为汗腺样、毛囊样和皮脂腺样结构的潜能.     

Age-associated inflammation inhibits epidermal stem cell function

Altered stem cell homeostasis is linked to organismal aging. However, the mechanisms involved remain poorly understood. Here we report novel alterations in hair follicle stem cells during skin aging, including increased numbers, decreased function, and an inability to tolerate stress. Performing high-throughput RNA sequencing on aging stem cells, cytokine arrays, and functional assays, we identify an age-associated imbalance in epidermal Jak–Stat signaling that inhibits stem cell function. Collectively, this study reveals a role for the aging epidermis in the disruption of cytokine and stem cell homeostasis, suggesting that stem cell decline during aging may be part of broader tumor-suppressive mechanisms.     

表皮干细胞的生物学标记物

背景：大面积烧伤、严重创伤后皮肤缺损的修复问题一直以来是急待突破的临床技术瓶颈。随着组织工程时代的到来,表皮干细胞越来越多的被应用于组织工程、细胞替代治疗及基因工程等科技领域,因此其分离、鉴定成为备受关注的研究热点。 目的：综述国内外关于表皮干细胞生物学标记的研究进展。 方法：计算机检索中国生物医学文献数据库、中文科技期刊全文数据库、中文学术期刊全文数据库及PubMed、EMbase数据库有关表皮干细胞特异性标志物的文章,检索词为“表皮干细胞,整合素,角蛋白,P63,CD71,连接蛋白43,端粒酶,血管紧张素转化酶,烟酸己可碱,Epidermal stem cells,integrin,keratin,P63,CD71,telomerase,ACE,cx43,Hoechst”。排除较陈旧的理论观点以及重复性研究。 结果与结论：最后纳入40篇文献。结果显示,表皮干细胞在皮肤组织工程中的应用为皮肤缺损修复的组织来源带来了曙光,研究发现表皮干细胞分布在毛囊隆突部和表皮的基底层,然而整个基底细胞中只有约4%的细胞是干细胞,因此正确分离、培养、鉴定表皮肤干细胞成为关键问题,由此表皮干细胞特异性标记物的相关研究成为热点。目前在表皮干细胞的生物学标记物方面已取得多方面的进展,迄今为止,尚未能找到一种绝对、公认的表皮干细胞标记物。研究主要集中在整合素、角蛋白、P63、CD71、连接蛋白43、端粒酶等方面,另外近年来烟酸己可碱、CD90、CD98、CD200在表皮干细胞的生物学标记物研究领域有所报道。研究发现每一种标记物都有其缺陷及不足之处,目前在表皮干细胞研究领域里仍有很多问题急待解决。     

Tiers of Clonal Organization in the Epidermis: The Epidermal Proliferation Unit Revisited

As one of the most proliferative tissues in adult mammals, the epidermis is a good example of the precise regulation necessary between stem cell self-renewal and differentiation. The epidermis is derived from ectodermal progenitor cells and contains three distinct classes of cells: epidermal stem cells which are capable of infinite rounds of cell division; their immediate descendants, transient amplifying cells, which are capable of numerous but finite rounds of cell division; and finally, non-dividing, differentiating cells (Aberdam in Cell and Tissue Research 331:103–107, 2008). This proliferative hierarchy must be tightly regulated both temporally and spatially during epidermal development and homeostasis in order to prevent uncontrolled growth leading to hyperproliferative states and/or tumorigenesis. Historically, the most basic unit of epidermal proliferation has been described as the epidermal proliferation unit (EPU). The EPU, as originally characterized by Christophers, Potten and Mackenzie, is a proliferation unit consisting of approximately 10 basal cells with a clonogenic cell in the center and overlaid by the suprabasal and corneocyte progeny (reviewed in Potten, C. S. (1974). The epidermal proliferative unit: the possible role of the central basal cell. Cell and Tissue Kinetics, 7(1), 77–88). Numerous researchers have identified this classical EPU structure, consisting of approximately 20 cells, in a variety of mammalian skin sources. Recently however, lineage analyses have provided evidence for much larger clonal epidermal units consisting of hundreds to thousands of cells. Furthermore, cutaneous mosaicism as well as a variety of cutaneous pathologies indicate that clonal areas extend to whole patches of mammalian skin many centimeters across. In this review we revisit four decades of experimental evidence and put forward a model of clonal units derived from multiple classes of epidermal progenitors ranging from the largest and most primitive units, clonal ectodermal units, to epidermal stem cell units, and finally, to the most basic structural unit, the EPU.     

干细胞在皮肤抗衰老中的应用北大核心

背景:由于皮肤衰老的必然性,以及人们生活水平提高所致的皮肤衰老受到了更多的重视,因此带来抗衰老领域强烈的市场需求及研究需求。目的:综述皮肤衰老及干细胞用于皮肤抗衰老的研究进展,为干细胞相关皮肤抗衰老的应用及研究提供理论依据。方法:检索PubMed及中国知网数据库2016年11月至2021年11月关于皮肤衰老及干细胞相关皮肤抗衰的文献,排除陈旧以及重复的观点,将检索到的文献进行整理,共纳入91篇文献来进行分析。结果与结论:①皮肤衰老主要表现在皮肤出现皱纹、弹性降低、表皮变薄;皮肤衰老的因素包括内源性因素及外源性因素,以及皮肤衰老的机制包括表皮干细胞增殖及分化平衡的改变、氧化应激等,文章重点描述了衰老皮肤中表皮干细胞的变化,包括细胞内DNA损伤、活性氧增高;细胞增殖与分化减少,不对称及对称分裂平衡改变;生态微环境的改变;干细胞-免疫细胞信号传导异常;②文章介绍了间充质干细胞及干细胞衍生物在皮肤抗衰老中的研究应用,如在动物实验中皮下注射干细胞或其衍生物,通过观察真皮厚度、氧化应激及弹性纤维等,表明干细胞及其衍生物在皮肤抗衰老中的作用;③上述研究证明了干细胞相关皮肤抗衰老的有效性,具体抗衰老机制包括促进细胞增殖、降低活性氧水平及调节基因表达等。     

Covering the limb--formation of the integument

An organism's outermost covering, the integument, has evolved to fulfil a diverse range of functions. Skin provides a physical barrier, an environment for immunological surveillance, and also performs a range of sensory, thermoregulatory and biosynthetic functions. Examination of the skin of limb digits reveals a range of skin types including the thickened hairless epidermis of the toe pads (palmar or plantar epidermis) and thinner epidermis between the hair follicles (interfollicular epidermis) of hairy skin. An important developmental function of skin is to give rise to a diverse group of appendages including hair follicles, with associated sebaceous glands (or feathers and scales in chick), eccrine sweat glands and the nail. A key question is how does this morphological variety arise from the single‐layered epithelium covering embryonic limb buds? This review will attempt to address this question by linking the extensive morphological/anatomical data on maturation of epidermis and its appendages with (1) current research into the range, plasticity and location of the putative epidermal stems cells; (2) molecular/microenvironmental regulation of epidermal stem cell lineages and lineage choice; and (3) regulation of the differentiation pathways, focusing on differentiation of the interfollicular epidermis.     

Notch Signaling May Be Involved in the Abnormal Differentiation of Epidermal Keratinocytes in Psoriasis

Localization of each keratin isoform differs among epidermal layers. Proliferating basal cells synthesize keratin 14 (K14) and suprabasal cells express keratin 10 (K10) in normal skin. Notch signaling is essential for keratinocyte differentiation. Notch1 is expressed in all epidermal layers, Notch2 in the basal cell layer and Notch3 in basal cell and spinous cell layers in normal epidermis. It has been poorly elucidated how localization and expression levels of Notch molecules are related to epidermal molecular markers K10 and K14 in psoriatic skin with abnormal differentiation of epidermal tissue. This study aimed to investigate the relationship between abnormal differentiation of epidermal cells in psoriatic skin and expression of Notch molecules. We investigated keratins (K14 and K10) and Notches (1, 2, 3 and 4) using immunohistochemistry in psoriatic skin (n=30) and normal skin (n=10). In normal skin, K14 and K10 were discretely observed in the basal cell layer and suprabasal layer, respectively. In psoriatic skin, K14 was expressed in the pan epidermal layer while it and K10 were co-expressed in some middle suprabasal layer cells. Notch1, 2, 3, and 4 localized in all epidermal layers in normal skin. In psoriatic skin, Notch1, 2, and 4 mainly localized in suprabasilar layers and Notch3 is lacalized in pan epidermal, suprabasilar, and basilar layers. Protein and mRNA of Notch1, 2, and 3 isoforms decreased in psoriatic epidermis compared with normal epidermis. These data suggest that decrements in these Notch molecules might cause aberrant expression of K10 and K14 leading to anomalous differentiation of the epidermis in psoriatic lesions.