The interfollicular epidermal stem cell saga: sensationalism versus reality check

Binary organization of epidermal basal domains highlights robustness to environmental exposure

Adulte interfollicular epidermis (IFE) renewal is likely orchestrated by physiological demands of its complex tissue architecture comprising spatial and cellular heterogeneity. Mouse tail and back skin display two kinds of basal IFE spatial domains that regenerate at different rates. Here, we elucidate the molecular and cellular states of basal IFE domains by marker expression and single-cell transcriptomics in mouse and human skin. We uncover two paths of basal cell differentiation that in part reflect the IFE spatial domain organization. We unravel previously unrecognized similarities between mouse tail IFE basal domains defined as scales and interscales versus human rete ridges and inter-ridges, respectively. Furthermore, our basal IFE transcriptomics and gene targeting in mice provide evidence supporting a physiological role of IFE domains in adaptation to differential UV exposure. We identify Sox6 as a novel UV-induced and interscale/inter-ridge preferred basal IFE-domain transcription factor, important for IFE proliferation and survival. The spatial, cellular, and molecular organization of IFE basal domains underscores skin adaptation to environmental exposure and its unusual robustness in adult homeostasis.

Maintaining the proliferative cell niche in multicellular models of epithelia

The maintenance of the proliferative cell niche is critical to epithelial tissue morphology and function. In this paper we investigate how current modelling methods can result in the erroneous loss of proliferative cells from the proliferative cell niche. Using an established model of the inter-follicular epidermis we find there is a limit to the proliferative cell densities that can be maintained in the basal layer (the niche) if we do not include additional mechanisms to stop the loss of proliferative cells from the niche. We suggest a new methodology that enables maintenance of a desired homeostatic population of proliferative cells in the niche: a rotational force is applied to the two daughter cells during the mitotic phase of division to enforce a particular division direction. We demonstrate that this new methodology achieves this goal. This methodology reflects the regulation of the orientation of cell division.

The steady state of epidermis: mathematical modeling and numerical simulations

We consider a model with age and space structure for the epidermis evolution. The model, previously presented and analyzed with respect to the suprabasal epidermis, includes different types of cells (proliferating cells, differentiated cells, corneous cells, and apoptotic cells) moving with the same velocity, under the constraint that the local volume fraction occupied by the cells is constant in space and time. Here, we complete the model proposing a mechanism regulating the cell production in the basal layer and we focus on the stationary case of the problem, i.e. on the case corresponding to the normal status of the skin. A numerical scheme to compute the solution of the model is proposed and its convergence is studied. Simulations are provided for realistic values of the parameters, showing the possibility of reproducing the structure of both "thin" and "thick" epidermis.

Cellular heterogeneity in the mouse esophagus implicates the presence of a nonquiescent epithelial stem cell population

Because the esophageal epithelium lacks a defined stem cell niche, it is unclear whether all basal epithelial cells in the adult esophagus are functionally equivalent. In this study, we showed that basal cells in the mouse esophagus contained a heterogeneous population of epithelial cells, similar to other rapidly cycling tissues such as the intestine or skin. Using a combination of cell-surface markers, we separated primary esophageal tissue into distinct cell populations that harbored differences in stem cell potential. We also used an in vitro 3D organoid assay to demonstrate that Sox2, Wnt, and bone morphogenetic protein signaling regulate esophageal self-renewal. Finally, we labeled proliferating basal epithelial cells in vivo to show differing cell-cycle profiles and proliferation kinetics. Based on our results, we propose that a nonquiescent stem cell population resides in the basal epithelium of the mouse esophagus.

Cycling up the epidermis: reconciling 100 years of debate

There is likely general consensus within the skin research community that cell cycle control is critical to epidermal homeostasis and disease. The current predominant model proposes that keratinocytes switch off DNA replication and undergo cell cycle and cell growth arrest as they initiate terminal differentiation. However, this model cannot explain key physiological features of the skin, mainly why squamous differentiation prevails over proliferation in benign hyperproliferative disorders. In recent years, we have proposed an alternative model that involves mitotic slippage and endoreplication. This new model is controversial and has encountered resistance within the field. However, looking back at history, the epidermal cell cycle has been a matter of controversy and debate for around 100 years now. The accumulated data are confusing and contradictory. Our present model can explain and reconcile both old and new paradoxical observations. Here, we explain and discuss the endoreplicative cell cycle, the evidence for and against its existence in human epidermis and the important implications for skin homeostasis and disease. We show that regardless of the strengths or weaknesses of the Endoreplication Model, the existing evidence in support of the Cell Cycle Arrest Model is very weak.

Skin stem cell hypotheses and long term clone survival--explored using agent-based modelling

Epithelial renewal in skin is achieved by the constant turnover and differentiation of keratinocytes. Three popular hypotheses have been proposed to explain basal keratinocyte regeneration and epidermal homeostasis: 1) asymmetric division (stem-transit amplifying cell); 2) populational asymmetry (progenitor cell with stochastic fate); and 3) populational asymmetry with stem cells. In this study, we investigated lineage dynamics using these hypotheses with a 3D agent-based model of the epidermis. The model simulated the growth and maintenance of the epidermis over three years. The offspring of each proliferative cell was traced. While all lineages were preserved in asymmetric division, the vast majority were lost when assuming populational asymmetry. The third hypothesis provided the most reliable mechanism for self-renewal by preserving genetic heterogeneity in quiescent stem cells, and also inherent mechanisms for skin ageing and the accumulation of genetic mutation.

EGFR-ras-raf signaling in epidermal stem cells: roles in hair follicle development, regeneration, tissue remodeling and epidermal cancers

The mammalian skin is the largest organ of the body and its outermost layer, the epidermis, undergoes dynamic lifetime renewal through the activity of somatic stem cell populations. The EGFR-Ras-Raf pathway has a well-described role in skin development and tumor formation. While research mainly focuses on its role in cutaneous tumor initiation and maintenance, much less is known about Ras signaling in the epidermal stem cells, which are the main targets of skin carcinogenesis. In this review, we briefly discuss the properties of the epidermal stem cells and review the role of EGFR-Ras-Raf signaling in keratinocyte stem cells during homeostatic and pathological conditions.

Monitoring the cycling activity of cultured human keratinocytes using a CFSE-based dye tracking approach

The development of methods and tools suitable for functional analysis of keratinocytes placed in an in vitro context is of great importance for characterizing properties associated with their normal state, for detecting abnormalities related to pathological states, or for studying the effects of extrinsic factors. In the present chapter, we describe the use of the intracellular fluorescent dye carboxyfluorescein succinimidyl ester (CFSE) to monitor cell division in mass cultures of normal human keratinocytes. We detail the preparation of CFSE-labeled keratinocyte samples and the identification by flow cytometry of cell subpopulations exhibiting different cycling rates in a mitogenic culture context. In addition, we show that the CFSE-based division-tracking approach enables the monitoring of keratinocyte responsiveness to growth modulators, which is here exemplified by the cell-cycling inhibition mediated by the growth factor TGF-β1. Finally, we show that keratinocyte subpopulations, separated according to their mitotic history using CFSE fluorescence tracking, can be sorted by flow cytometry and used for further functional characterization, including determination of clone-forming efficiency.

Epidermal stem cells in orthopaedic regenerative medicine

In the last decade, great advances have been made in epidermal stem cell studies at the cellular and molecular level. These studies reported various subpopulations and differentiations existing in the epidermal stem cell. Although controversies and unknown issues remain, epidermal stem cells possess an immune-privileged property in transplantation together with easy accessibility, which is favorable for future clinical application. In this review, we will summarize the biological characteristics of epidermal stem cells, and their potential in orthopedic regenerative medicine. Epidermal stem cells play a critical role via cell replacement, and demonstrate significant translational potential in the treatment of orthopedic injuries and diseases, including treatment for wound healing, peripheral nerve and spinal cord injury, and even muscle and bone remodeling.

Human skin cancer stem cells: a tale of mice and men

Carcinomas, cancers of epithelial tissues, are the commonest malignancies and cause the greatest cancer mortality worldwide. Among these, the incidence of keratinocyte-derived non-melanoma skin cancers (NMSC), by far the greatest, is increasing rapidly. Yet despite access to tumor tissue, acceptance of human NMSC as a model carcinoma has been hindered by the lack of a reliable xenograft model. Instead, we have relied on the murine two-step carcinogenesis protocol as a reproducible squamous cell carcinoma (SCC) model, but this differs from their human counterpart in cause, site, genetic basis and biological behaviour. By xeno-engraftment of primary human SCC, we were recently successful in demonstrating the presence of primary human SCC cancer stem cells or tumor-initiating cells. These findings once more align the study human SCC as the archetypal carcinoma model. In this review, we describe the evidence for the existence of tumor-initiating cells, with emphasis on skin cancer, limiting our discussions to primary human cancer studies where possible.

Interventional and intrinsic airway homeostasis and repair

Human airways are a paragon of intrinsic engineering. They experience 7,000-10,000 liters of airflow/day, have a 70-m(2) surface area, and undergo complete renewal every 100-400 days. Despite this, airways are susceptible to aging, injury, and diseases that are major causes of mortality. Current airway regeneration research is focused both on understanding the cells and strategies responsible for maintaining intrinsic tissue homeostasis as well as on establishing clinical interventions for improving repair.

Epithelial stem cells and implications for wound repair

Activation of epithelial stem cells and efficient recruitment of their proliferating progeny plays a critical role in cutaneous wound healing. The reepithelialized wound epidermis has a mosaic composition consisting of progeny that can be traced back both to epidermal and several types of hair follicle stem cells. The contribution of hair follicle stem cells to wound epidermis is particularly intriguing as it involves lineage identity change from follicular to epidermal. Studies from our laboratory show that hair follicle-fated bulge stem cells commit only transient amplifying epidermal progeny that participate in the initial wound re-epithelialization, but eventually are outcompeted by other epidermal clones and largely disappear after a few months. Conversely, recently described stem cell populations residing in the isthmus portion of hair follicle contribute long-lasting progeny toward wound epidermis and, arguably, give rise to new interfollicular epidermal stem cells. The role of epithelial stem cells during wound healing is not limited to regenerating stratified epidermis. By studying regenerative response in large cutaneous wounds, our laboratory uncovered that epithelial cells in the center of the wound can acquire greater morphogenetic plasticity and, together with the underlying wound dermis, can engage in an embryonic-like process of hair follicle neogenesis. Future studies should uncover the cellular and signaling basis of this remarkable adult wound regeneration phenomenon.

Cellular organization of the human epidermal basal layer: clues sustaining a hierarchical model

PURPOSE

The basal layer of adult interfollicular epidermis is a highly dynamic cellular system, ensuring the continuous physiological renewal of this tissue, as well as regenerative processes in the context of wound healing. In human skin, despite its major importance for the maintenance of epidermal homeostasis and regenerative processes, the functional organization of basal keratinocytes is still debated today. Progress in this understanding is closely linked to the development of research models enabling investigations of the different coexisting basal keratinocyte subpopulations, to address their specific functional and molecular characteristics, particularly through clonal analyses. We review here different strategies that have led to significant advances in the knowledge of human basal keratinocyte properties, at both phenotypic and functional levels.

CONCLUSIONS

Convincing clues supporting a hierarchical organization of the keratinocyte basal layer in humans have emerged from the different functional studies. In particular, the hierarchical model constitutes a straight forward interpretation of the clearly non-equivalent potentialities observed when basal keratinocytes were studied individually in a cell culture context.

Stem cells in ectodermal development

Tissue-specific stem cells sustain organs for a lifetime through self-renewal and generating differentiated progeny. Although tissue stem cells are established during organogenesis, the precise origin of most adult stem cells in the developing embryo is unclear. Mammalian skin is one of the best-studied epithelial systems containing stem cells to date, however the origin of most of the stem cell populations found in the adult epidermis is unknown. Here, we try to recapitulate the emergence and genesis of an ectodermal stem cell during development until the formation of an adult skin. We ask whether skin stem cells share key transcriptional regulators with their embryonic counterparts and discuss whether embryonic-like stem cells may persist through to adulthood in vivo.

25 years of epidermal stem cell research

This is a chronicle of concepts in the field of epidermal stem cell biology and a historic look at their development over time. The past 25 years have seen the evolution of epidermal stem cell science, from first fundamental studies to a sophisticated science. The study of epithelial stem cell biology was aided by the ability to visualize the distribution of stem cells and their progeny through lineage analysis studies. The excellent progress we have made in understanding epidermal stem cell biology is discussed in this article. The challenges we still face in understanding epidermal stem cells include defining molecular markers for stem and progenitor sub-populations, determining the locations and contributions of the different stem cell niches, and mapping regulatory pathways of epidermal stem cell proliferation and differentiation. However, our rapidly evolving understanding of epidermal stem cells has many potential uses that promise to translate into improved patient therapy.

Adherens junctions and stem cells

The specification, maintenance, division and differentiation of stem cells are integral to the development and homeostasis of many tissues. These stem cells often live in specialized anatomical areas, called niches. While niches can be complex, most involve cell-cell interactions that are mediated by adherens junctions. A diverse array of functions have been attributed to adherens junctions in stem cell biology. These include physical anchoring to the niche, control of proliferation and division orientation, regulation of signaling cascades and of differentiation. In this review, a number of model stem cell systems that highlight various functions of adherens junctions are discussed. In addition, a summary of the current understanding of adherens junction function in mammalian tissues and embryonic and induced pluripotent stem cells is provided. This analysis demonstrates that the roles of adherens junctions are surprisingly varied and integrated with both the anatomy and the physiology of the tissue.

Stem cells and corneal epithelial maintenance: insights from the mouse and other animal models

Maintenance of the corneal epithelium is essential for vision and is a dynamic process incorporating constant cell production, movement and loss. Although cell-based therapies involving the transplantation of putative stem cells are well advanced for the treatment of human corneal defects, the scientific understanding of these interventions is poor. No definitive marker that discriminates stem cells that maintain the corneal epithelium from the surrounding tissue has been discovered and the identity of these elusive cells is, therefore, hotly debated. The key elements of corneal epithelial maintenance have long been recognised but it is still not known how this dynamic balance is co-ordinated during normal homeostasis to ensure the corneal epithelium is maintained at a uniform thickness. Most indirect experimental evidence supports the limbal epithelial stem cell (LESC) hypothesis, which proposes that the adult corneal epithelium is maintained by stem cells located in the limbus at the corneal periphery. However, this has been challenged recently by the corneal epithelial stem cell (CESC) hypothesis, which proposes that during normal homeostasis the mouse corneal epithelium is maintained by stem cells located throughout the basal corneal epithelium with LESCs only contributing during wound healing. In this chapter we review experimental studies, mostly based on animal work, that provide insights into how stem cells maintain the normal corneal epithelium and consider the merits of the alternative LESC and CESC hypotheses. Finally, we highlight some recent research on other stem cell systems and consider how this could influence future research directions for identifying the stem cells that maintain the corneal epithelium.