The ordered architecture of murine ear epidermis is maintained by progenitor cells with random fate

Computational modelling of epidermal stratification highlights the importance of asymmetric cell division for predictable and robust layer formation

Skin is a complex organ tasked with, among other functions, protecting the body from the outside world. Its outermost protective layer, the epidermis, is comprised of multiple cell layers that are derived from a single-layered ectoderm during development. Using a new stochastic, multi-scale computational modelling framework, the anisotropic subcellular element method, we investigate the role of cell morphology and biophysical cell-cell interactions in the formation of this layered structure. This three-dimensional framework describes interactions between collections of hundreds to thousands of cells and (i) accounts for intracellular structure and morphology, (ii) easily incorporates complex cell-cell interactions and (iii) can be efficiently implemented on parallel architectures. We use this approach to construct a model of the developing epidermis that accounts for the internal polarity of ectodermal cells and their columnar morphology. Using this model, we show that cell detachment, which has been previously suggested to have a role in this process, leads to unpredictable, randomized stratification and that this cannot be abrogated by adjustment of cell-cell adhesion interaction strength. Polarized distribution of cell adhesion proteins, motivated by epithelial polarization, can however eliminate this detachment, and in conjunction with asymmetric cell division lead to robust and predictable development.

Tracing the fate of limbal epithelial progenitor cells in the murine cornea

Stem cell (SC) division, deployment, and differentiation are processes that contribute to corneal epithelial renewal. Until now studying the destiny of these cells in a living mammal has not been possible. However, the advent of inducible multicolor genetic tagging and powerful imaging technologies has rendered this achievable in the translucent and readily accessible murine cornea. K14CreER(T2)-Confetti mice that harbor two copies of the Brainbow 2.1 cassette, yielding up to 10 colors from the stochastic recombination of fluorescent proteins, were used to monitor K-14(+) progenitor cell dynamics within the corneal epithelium in live animals. Multicolored columns of cells emerged from the basal limbal epithelium as they expanded and migrated linearly at a rate of 10.8 µm/day toward the central cornea. Moreover, the permanent expression of fluorophores, passed on from progenitor to progeny, assisted in discriminating individual clones as spectrally distinct streaks containing more than 1,000 cells within the illuminated area. The centripetal clonal expansion is suggestive that a single progenitor cell is responsible for maintaining a narrow corridor of corneal epithelial cells. Our data are in agreement with the limbus as the repository for SC as opposed to SC being distributed throughout the central cornea. This is the first report describing stem/progenitor cell fate determination in the murine cornea using multicolor genetic tracing. This model represents a powerful new resource to monitor SC kinetics and fate choice under homeostatic conditions, and may assist in assessing clonal evolution during corneal development, aging, wound-healing, disease, and following transplantation.

Towards understanding the roles of position and geometry on cell fate decisions during preimplantation development

The first lineage segregation event in mouse embryos produces two separate cell populations: inner cell mass and trophectoderm. This is understood to be brought about by cells sensing their position within the embryo and differentiating accordingly. The cellular and molecular underpinnings of this process remain under investigation and have variously been considered to be completely stochastic or alternately, subject to some predisposition set up at fertilisation or before. Here, we consider these views in light of recent publications, discuss the possible role of cell geometry and mechanical forces in this process and describe how modelling could contribute in addressing this issue.

Genetics of gonadal stem cell renewal

Stem cells are necessary for the maintenance of many adult tissues. Signals within the stem cell microenvironment, or niche, regulate the self-renewal and differentiation capability of these cells. Misregulation of these signals through mutation or damage can lead to overgrowth or depletion of different stem cell pools. In this review, we focus on the Drosophila testis and ovary, both of which contain well-defined niches, as well as the mouse testis, which has become a more approachable stem cell system with recent technical advances. We discuss the signals that regulate gonadal stem cells in their niches, how these signals mediate self-renewal and differentiation under homeostatic conditions, and how stress, whether from mutations or damage, can cause changes in cell fate and drive stem cell competition.

Modelling epidermis homoeostasis and psoriasis pathogenesis

We present a computational model to study the spatio-temporal dynamics of epidermis homoeostasis under normal and pathological conditions. The model consists of a population kinetics model of the central transition pathway of keratinocyte proliferation, differentiation and loss and an agent-based model that propagates cell movements and generates the stratified epidermis. The model recapitulates observed homoeostatic cell density distribution, the epidermal turnover time and the multilayered tissue structure. We extend the model to study the onset, recurrence and phototherapy-induced remission of psoriasis. The model considers psoriasis as a parallel homoeostasis of normal and psoriatic keratinocytes originated from a shared stem cell (SC) niche environment and predicts two homoeostatic modes of psoriasis: a disease mode and a quiescent mode. Interconversion between the two modes can be controlled by interactions between psoriatic SCs and the immune system and by normal and psoriatic SCs competing for growth niches. The prediction of a quiescent state potentially explains the efficacy of multi-episode UVB irradiation therapy and recurrence of psoriasis plaques, which can further guide designs of therapeutics that specifically target the immune system and/or the keratinocytes.

Unraveling the non-senescence phenomenon in Hydra

Unlike other metazoans, Hydra does not experience the distinctive rise in mortality with age known as senescence, which results from an increasing imbalance between cell damage and cell repair. We propose that the Hydra controls damage accumulation mainly through damage-dependent cell selection and cell sloughing. We examine our hypothesis with a model that combines cellular damage with stem cell renewal, differentiation, and elimination. The Hydra individual can be seen as a large single pool of three types of stem cells with some features of differentiated cells. This large stem cell community prevents "cellular damage drift," which is inevitable in complex conglomerate (differentiated) metazoans with numerous and generally isolated pools of stem cells. The process of cellular damage drift is based on changes in the distribution of damage among cells due to random events, and is thus similar to Muller's ratchet in asexual populations. Events in the model that are sources of randomness include budding, cellular death, and cellular damage and repair. Our results suggest that non-senescence is possible only in simple Hydra-like organisms which have a high proportion and number of stem cells, continuous cell divisions, an effective cell selection mechanism, and stem cells with the ability to undertake some roles of differentiated cells.

Switching roles: the functional plasticity of adult tissue stem cells

Adult organisms have to adapt to survive, and the same is true for their tissues. Rates and types of cell production must be rapidly and reversibly adjusted to meet tissue demands in response to both local and systemic challenges. Recent work reveals how stem cell (SC) populations meet these requirements by switching between functional states tuned to homoeostasis or regeneration. This plasticity extends to differentiating cells, which are capable of reverting to SCs after injury. The concept of the niche, the micro-environment that sustains and regulates stem cells, is broadening, with a new appreciation of the role of physical factors and hormonal signals. Here, we review different functions of SCs, the cellular mechanisms that underlie them and the signals that bias the fate of SCs as they switch between roles.

Cell cycle-dependent differentiation dynamics balances growth and endocrine differentiation in the pancreas

Organogenesis relies on the spatiotemporal balancing of differentiation and proliferation driven by an expanding pool of progenitor cells. In the mouse pancreas, lineage tracing at the population level has shown that the expanding pancreas progenitors can initially give rise to all endocrine, ductal, and acinar cells but become bipotent by embryonic day 13.5, giving rise to endocrine cells and ductal cells. However, the dynamics of individual progenitors balancing self-renewal and lineage-specific differentiation has never been described. Using three-dimensional live imaging and in vivo clonal analysis, we reveal the contribution of individual cells to the global behaviour and demonstrate three modes of progenitor divisions: symmetric renewing, symmetric endocrinogenic, and asymmetric generating a progenitor and an endocrine progenitor. Quantitative analysis shows that the endocrine differentiation process is consistent with a simple model of cell cycle-dependent stochastic priming of progenitors to endocrine fate. The findings provide insights to define control parameters to optimize the generation of β-cells in vitro.

MSCs seeded on bioengineered scaffolds improve skin wound healing in rats

Growing evidence has shown the promise of mesenchymal stromal cells (MSCs) for the treatment of cutaneous wound healing. We have previously demonstrated that MSCs seeded on an artificial dermal matrix, Integra (Integra Lifesciences Corp., Plainsboro, NJ) enriched with platelet-rich plasma (Ematrix) have enhanced proliferative potential in vitro as compared with those cultured on the scaffold alone. In this study, we extended the experimentation by evaluating the efficacy of the MSCs seeded scaffolds in the healing of skin wounds in an animal model in vivo. It was found that the presence of MSCs within the scaffolds greatly ameliorated the quality of regenerated skin, reduced collagen deposition, enhanced reepithelization, increased neo-angiogenesis, and promoted a greater return of hair follicles and sebaceous glands. The mechanisms involved in these beneficial effects were likely related to the ability of MSCs to release paracrine factors modulating the wound healing response. MSC-seeded scaffolds, in fact, up-regulated matrix metalloproteinase 9 expression in the extracellular matrix and enhanced the recruitment of endogenous progenitors during tissue repair. In conclusion, the results of this study provide evidence that the treatment with MSC-seeded scaffolds of cutaneous wounds contributes to the recreation of a suitable microenvironment for promoting tissue repair/regeneration at the implantation sites.

Towards a quantitative theory of epidermal calcium profile formation in unwounded skin

We propose and mathematically examine a theory of calcium profile formation in unwounded mammalian epidermis based on: changes in keratinocyte proliferation, fluid and calcium exchange with the extracellular fluid during these cells’ passage through the epidermal sublayers, and the barrier functions of both the stratum corneum and tight junctions localised in the stratum granulosum. Using this theory, we develop a mathematical model that predicts epidermal sublayer transit times, partitioning of the epidermal calcium gradient between intracellular and extracellular domains, and the permeability of the tight junction barrier to calcium ions. Comparison of our model’s predictions of epidermal transit times with experimental data indicates that keratinocytes lose at least 87% of their volume during their disintegration to become corneocytes. Intracellular calcium is suggested as the main contributor to the epidermal calcium gradient, with its distribution actively regulated by a phenotypic switch in calcium exchange between keratinocytes and extracellular fluid present at the boundary between the stratum spinosum and the stratum granulosum. Formation of the extracellular calcium distribution, which rises in concentration through the stratum granulosum towards the skin surface, is attributed to a tight junction barrier in this sublayer possessing permeability to calcium ions that is less than 15 nm s⁻¹ in human epidermis and less than 37 nm s⁻¹ in murine epidermis. Future experimental work may refine the presented theory and reduce the mathematical uncertainty present in the model predictions.

Cell competition: winning out by losing notch

Cell competition where 'loser' cells are eliminated by neighbors with higher fitness is a widespread phenomenon in development. However, a growing body of evidence argues cells with somatic mutations compete with their wild type counterparts in the earliest stages of cancer development. Recent studies have begun to shed light on the molecular and cellular mechanisms that alter the competitiveness of cells carrying somatic mutations in adult tissues. Cells with a 'winner' phenotype create clones which may expand into extensive fields of mutant cells within normal appearing epithelium, favoring the accumulation of further genetic alterations and the evolution of cancer. Here we focus on how mutations which disrupt the Notch signaling pathway confer a 'super competitor' status on cells in squamous epithelia and consider the broader implications for cancer evolution.

Mutation, clonal fitness and field change in epithelial carcinogenesis

Developments in lineage tracing in mouse models have revealed how stem cells maintain normal squamous and glandular epithelia. Here we review recent quantitative studies tracing the fate of individual mutant stem cells which have uncovered how common oncogenic mutations alter cell behaviour, creating clones with a growth advantage that may persist long term. In the intestine this occurs by a mutant clone colonizing an entire crypt, whilst in the squamous oesophagus blocking differentiation creates clones that expand to colonize large areas of epithelium, a phenomenon known as field change. We consider the implications of these findings for early cancer evolution and the cancer stem cell hypothesis, and the prospects of targeted cancer prevention by purging mutant clones from normal-appearing epithelia.

Exact, time-independent estimation of clone size distributions in normal and mutated cells

Biological tools such as genetic lineage tracing, three-dimensional confocal microscopy and next-generation DNA sequencing are providing new ways to quantify the distribution of clones of normal and mutated cells. Understanding population-wide clone size distributions in vivo is complicated by multiple cell types within observed tissues, and overlapping birth and death processes. This has led to the increased need for mathematically informed models to understand their biological significance. Standard approaches usually require knowledge of clonal age. We show that modelling on clone size independent of time is an alternative method that offers certain analytical advantages; it can help parametrize these models, and obtain distributions for counts of mutated or proliferating cells, for example. When applied to a general birth-death process common in epithelial progenitors, this takes the form of a gambler's ruin problem, the solution of which relates to counting Motzkin lattice paths. Applying this approach to mutational processes, alternative, exact, formulations of classic Luria-Delbrück-type problems emerge. This approach can be extended beyond neutral models of mutant clonal evolution. Applications of these approaches are twofold. First, we resolve the probability of progenitor cells generating proliferating or differentiating progeny in clonal lineage tracing experiments in vivo or cell culture assays where clone age is not known. Second, we model mutation frequency distributions that deep sequencing of subclonal samples produce.

MEK/ERK signaling directly and indirectly contributes to the cyclical self-renewal of spermatogonial stem cells

Coordination of stem cell fate is regulated by extrinsic niche signals and stem cell intrinsic factors. In mammalian testes, spermatogonial stem cells maintain constant production of abundant spermatozoa by alternating between self-renewal and differentiation at regular intervals according to a periodical program known as the seminiferous epithelial cycle. Although retinoic acid (RA) signaling has been suggested to direct the cyclical differentiation of spermatogonial stem cells, it remains largely unclear how their cycle-dependent self-renewal/proliferation is regulated. Here, we show that MEK/ERK signaling contributes to the cyclical activity of spermatogonial stem cells. We found that ERK1/2 is periodically activated in Sertoli cells during the stem cell self-renewal/proliferation phase, and that MEK/ERK signaling is required for the stage-related expression of the critical niche factor GDNF. In addition, ERK1/2 is activated in GFRα1-positive spermatogonial stem cells under the control of GDNF and prevent them from being differentiated. These results suggest that MEK/ERK signaling directly and indirectly maintains spermatogonial stem cells by mediating a signal that promotes their periodical self-renewal/proliferation. Conversely, RA signaling directly and indirectly induces differentiation of spermatogonial stem cells. We propose that temporally regulated activations of RA signaling and a signal regulating MEK/ERK antagonistically coordinates the cycle-related activity of spermatogonial stem cells.

Wnt-Notch signalling crosstalk in development and disease

The Notch and Wnt pathways are two of only a handful of highly conserved signalling pathways that control cell-fate decisions during animal development (Pires-daSilva and Sommer in Nat Rev Genet 4: 39-49, 2003). These two pathways are required together to regulate many aspects of metazoan development, ranging from germ layer patterning in sea urchins (Peter and Davidson in Nature 474: 635-639, 2011) to the formation and patterning of the fly wing (Axelrod et al in Science 271:1826-1832, 1996; Micchelli et al in Development 124:1485-1495, 1997; Rulifson et al in Nature 384:72-74, 1996), the spacing of the ciliated cells in the epidermis of frog embryos (Collu et al in Development 139:4405-4415, 2012) and the maintenance and turnover of the skin, gut lining and mammary gland in mammals (Clayton et al in Nature 446:185-189, 2007; Clevers in Cell 154:274-284, 2013; Doupe et al in Dev Cell 18:317-323, 2010; Lim et al in Science 342:1226-1230, 2013; Lowell et al in Curr Biol 10:491-500, 2000; van et al in Nature 435:959-963, 2005; Yin et al in Nat Methods 11:106-112, 2013). In addition, many diseases, including several cancers, are caused by aberrant signalling through the two pathways (Bolós et al in Endocr Rev 28: 339-363, 2007; Clevers in Cell 127: 469-480, 2006). In this review, we will outline the two signalling pathways, describe the different points of interaction between them, and cover how these interactions influence development and disease.

Heterogeneity and plasticity of epidermal stem cells

The epidermis is an integral part of our largest organ, the skin, and protects us against the hostile environment. It is a highly dynamic tissue that, during normal steady-state conditions, undergoes constant turnover. Multiple stem cell populations residing in autonomously maintained compartments facilitate this task. In this Review, we discuss stem cell behaviour during normal tissue homeostasis, regeneration and disease within the pilosebaceous unit, an integral structure of the epidermis that is responsible for hair growth and lubrication of the epithelium. We provide an up-to-date view of the pilosebaceous unit, encompassing the heterogeneity and plasticity of multiple discrete stem cell populations that are strongly influenced by external cues to maintain their identity and function.

The protective role of symmetric stem cell division on the accumulation of heritable damage

Stem cell divisions are either asymmetric—in which one daughter cell remains a stem cell and one does not—or symmetric, in which both daughter cells adopt the same fate, either stem or non-stem. Recent studies show that in many tissues operating under homeostatic conditions stem cell division patterns are strongly biased toward the symmetric outcome, raising the question of whether symmetry confers some benefit. Here, we show that symmetry, via extinction of damaged stem-cell clones, reduces the lifetime risk of accumulating phenotypically silent heritable damage (mutations or aberrant epigenetic changes) in individual stem cells. This effect is greatest in rapidly cycling tissues subject to accelerating rates of damage accumulation over time, a scenario that describes the progression of many cancers. A decrease in the rate of cellular damage accumulation may be an important factor favoring symmetric patterns of stem cell division.

Recently, highly symmetric patterns of stem cell division have been observed in a variety of adult mammalian somatic tissues. Here we identify conditions under which this behavior serves as a strategy to protect the organism against mutation accumulation. First, we find that a sufficient number of lifetime stem cell divisions must occur, potentially explaining why stem cell pools with the most symmetric divisions are rapidly cycling. Second, we find that late-occurring mutations must occur rapidly, a scenario known in cancer biology as genetic instability. These findings provide a potential explanation for the observation that cancer risks among large, long-lived organisms fail to rise as expected with lifespan and body size.

Emerging interactions between skin stem cells and their niches

The skin protects mammals from insults, infection and dehydration and enables thermoregulation and sensory perception. Various skin-resident cells carry out these diverse functions. Constant turnover of cells and healing upon injury necessitate multiple reservoirs of stem cells. Thus, the skin provides a model for studying interactions between stem cells and their microenvironments, or niches. Advances in genetic and imaging tools have brought new findings about the lineage relationships between skin stem cells and their progeny and about the mutual influences between skin stem cells and their niches. Such knowledge may offer novel avenues for therapeutics and regenerative medicine.

Understanding epithelial homeostasis in the intestine: An old battlefield of ideas, recent breakthroughs and remaining controversies

The intestinal epithelium constitutes the barrier between the gut lumen and the rest of the body and a very actively renewing cell population. The crypt/villus and crypt/cuff units of the mouse small intestine and colon are its basic functional units. The field is confronted with competing concepts with regard to the nature of the cells that are responsible for all the day-to day cell replacement and those that act to regenerate the tissue upon injury and with two diametrically opposed models for lineage specification. The review revisits groundbreaking pioneering studies to provide non expert readers and crypt watchers with a factual analysis of the origins of the current models deduced from the latest spectacular advances. It also discusses recent progress made by addressing these issues in the crypts of the colon, which need to be better understood, since they are the preferred sites of major pathologies.

Stem cell plasticity. Plasticity of epithelial stem cells in tissue regeneration

Tissues rely upon stem cells for homeostasis and repair. Recent studies show that the fate and multilineage potential of epithelial stem cells can change depending on whether a stem cell exists within its resident niche and responds to normal tissue homeostasis, whether it is mobilized to repair a wound, or whether it is taken from its niche and challenged to de novo tissue morphogenesis after transplantation. In this Review, we discuss how different populations of naturally lineage-restricted stem cells and committed progenitors can display remarkable plasticity and reversibility and reacquire long-term self-renewing capacities and multilineage differentiation potential during physiological and regenerative conditions. We also discuss the implications of cellular plasticity for regenerative medicine and for cancer.

Multiscale mathematical modeling and simulation of cellular dynamical process

Epidermal homeostasis is maintained by dynamic interactions among molecules and cells at different spatiotemporal scales. Mathematical modeling and simulation is expected to provide clear understanding and precise description of multiscaleness in tissue homeostasis under systems perspective. We introduce a stochastic process-based description of multiscale dynamics. Agent-based modeling as a framework of multiscale modeling to achieve consistent integration of definitive subsystems is proposed. A newly developed algorithm that particularly aims to perform stochastic simulations of cellular dynamical process is introduced. Finally we review applications of multiscale modeling and quantitative study to important aspects of epidermal and epithelial homeostasis.

Stochasticity in plant cellular growth and patterning

Plants, along with other multicellular organisms, have evolved specialized regulatory mechanisms to achieve proper tissue growth and morphogenesis. During development, growing tissues generate specialized cell types and complex patterns necessary for establishing the function of the organ. Tissue growth is a tightly regulated process that yields highly reproducible outcomes. Nevertheless, the underlying cellular and molecular behaviors are often stochastic. Thus, how does stochasticity, together with strict genetic regulation, give rise to reproducible tissue development? This review draws examples from plants as well as other systems to explore stochasticity in plant cell division, growth, and patterning. We conclude that stochasticity is often needed to create small differences between identical cells, which are amplified and stabilized by genetic and mechanical feedback loops to begin cell differentiation. These first few differentiating cells initiate traditional patterning mechanisms to ensure regular development.

Epithelial stem cells in adult skin

The skin is the first line of defense against dehydration and external environmental aggressions. It constantly renews itself throughout adult life mainly due to the activity of tissue-specific stem cells. In this review, we discuss fundamental characteristics of different stem cell populations within the skin and how they are able to contribute to normal skin homeostasis. We also examine the most recent results regarding the cell-intrinsic and -extrinsic components of the stem cell niche within the adult skin epithelium. Finally, we address the recent efforts to understand how abnormal regulation of stem cell activity contributes to the initiation and progression of skin-associated cancers.

Lineage analysis of epidermal stem cells

Lineage tracing involves labeling cells to track their subsequent behavior within the normal tissue environment. The advent of genetic lineage tracing and cell proliferation assays, together with high resolution three-dimensional (3D) imaging and quantitative methods to infer cell behavior from lineage-tracing data, has transformed our understanding of murine epidermal stem and progenitor cells. Here, we review recent insights that reveal how a progenitor cell population maintains interfollicular epidermis, whereas stem cells, quiescent under homeostatic conditions, are mobilized in response to wounding. We discuss progress in understanding how the various stem cell populations of the hair follicle sustain this complex and highly dynamic structure, and recent analysis of stem cells in sweat and sebaceous glands. The extent to which insights from mouse studies can be applied to human epidermis is also considered.

Interfollicular epidermal stem cells self-renew via autocrine Wnt signaling

The skin is a classical example of a tissue maintained by stem cells. However, the identity of the stem cells that maintain the interfollicular epidermis and the source of the signals that control their activity remain unclear. Using mouse lineage tracing and quantitative clonal analyses, we showed that the Wnt target gene Axin2 marks interfollicular epidermal stem cells. These Axin2-expressing cells constitute the majority of the basal epidermal layer, compete neutrally, and require Wnt/β-catenin signaling to proliferate. The same cells contribute robustly to wound healing, with no requirement for a quiescent stem cell subpopulation. By means of double-labeling RNA in situ hybridization in mice, we showed that the Axin2-expressing cells themselves produce Wnt signals as well as long-range secreted Wnt inhibitors, suggesting an autocrine mechanism of stem cell self-renewal.

Symmetric vs. asymmetric stem cell divisions: an adaptation against cancer?

Traditionally, it has been held that a central characteristic of stem cells is their ability to divide asymmetrically. Recent advances in inducible genetic labeling provided ample evidence that symmetric stem cell divisions play an important role in adult mammalian homeostasis. It is well understood that the two types of cell divisions differ in terms of the stem cells' flexibility to expand when needed. On the contrary, the implications of symmetric and asymmetric divisions for mutation accumulation are still poorly understood. In this paper we study a stochastic model of a renewing tissue, and address the optimization problem of tissue architecture in the context of mutant production. Specifically, we study the process of tumor suppressor gene inactivation which usually takes place as a consequence of two “hits”, and which is one of the most common patterns in carcinogenesis. We compare and contrast symmetric and asymmetric (and mixed) stem cell divisions, and focus on the rate at which double-hit mutants are generated. It turns out that symmetrically-dividing cells generate such mutants at a rate which is significantly lower than that of asymmetrically-dividing cells. This result holds whether single-hit (intermediate) mutants are disadvantageous, neutral, or advantageous. It is also independent on whether the carcinogenic double-hit mutants are produced only among the stem cells or also among more specialized cells. We argue that symmetric stem cell divisions in mammals could be an adaptation which helps delay the onset of cancers. We further investigate the question of the optimal fraction of stem cells in the tissue, and quantify the contribution of non-stem cells in mutant production. Our work provides a hypothesis to explain the observation that in mammalian cells, symmetric patterns of stem cell division seem to be very common.

Stochastic homeostasis in human airway epithelium is achieved by neutral competition of basal cell progenitors

Lineage tracing approaches have provided new insights into the cellular mechanisms that support tissue homeostasis in mice. However, the relevance of these discoveries to human epithelial homeostasis and its alterations in disease is unknown. By developing a novel quantitative approach for the analysis of somatic mitochondrial mutations that are accumulated over time, we demonstrate that the human upper airway epithelium is maintained by an equipotent basal progenitor cell population, in which the chance loss of cells due to lineage commitment is perfectly compensated by the duplication of neighbours, leading to "neutral drift" of the clone population. Further, we show that this process is accelerated in the airways of smokers, leading to intensified clonal consolidation and providing a background for tumorigenesis. This study provides a benchmark to show how somatic mutations provide quantitative information on homeostatic growth in human tissues, and a platform to explore factors leading to dysregulation and disease. DOI:http://dx.doi.org/10.7554/eLife.00966.001.

Stochastic homeostasis in human airway epithelium is achieved by neutral competition of basal cell progenitors

Lineage tracing approaches have provided new insights into the cellular mechanisms that support tissue homeostasis in mice. However, the relevance of these discoveries to human epithelial homeostasis and its alterations in disease is unknown. By developing a novel quantitative approach for the analysis of somatic mitochondrial mutations that are accumulated over time, we demonstrate that the human upper airway epithelium is maintained by an equipotent basal progenitor cell population, in which the chance loss of cells due to lineage commitment is perfectly compensated by the duplication of neighbours, leading to "neutral drift" of the clone population. Further, we show that this process is accelerated in the airways of smokers, leading to intensified clonal consolidation and providing a background for tumorigenesis. This study provides a benchmark to show how somatic mutations provide quantitative information on homeostatic growth in human tissues, and a platform to explore factors leading to dysregulation and disease. DOI:http://dx.doi.org/10.7554/eLife.00966.001.

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.

Spindle orientation and epidermal morphogenesis

Asymmetric cell divisions (ACDs) result in two unequal daughter cells and are a hallmark of stem cells. ACDs can be achieved either by asymmetric partitioning of proteins and organelles or by asymmetric cell fate acquisition due to the microenvironment in which the daughters are placed. Increasing evidence suggests that in the mammalian epidermis, both of these processes occur. During embryonic epidermal development, changes occur in the orientation of the mitotic spindle in relation to the underlying basement membrane. These changes are guided by conserved molecular machinery that is operative in lower eukaryotes and dictates asymmetric partitioning of proteins during cell divisions. That said, the shift in spindle alignment also determines whether a division will be parallel or perpendicular to the basement membrane, and this in turn provides a differential microenvironment for the resulting daughter cells. Here, we review how oriented divisions of progenitors contribute to the development and stratification of the epidermis.

Beyond the niche: tissue-level coordination of stem cell dynamics

Adult animals rely on populations of stem cells to ensure organ function throughout their lifetime. Stem cells are governed by signals from stem cell niches, and much is known about how single niches promote stemness and direct stem cell behavior. However, most organs contain a multitude of stem cell-niche units, which are often distributed across the entire expanse of the tissue. Beyond the biology of individual stem cell-niche interactions, the next challenge is to uncover the tissue-level processes that orchestrate spatial control of stem-based renewal, repair, and remodeling throughout a whole organ. Here we examine what is known about higher order mechanisms for interniche coordination in epithelial organs, whose simple geometry offers a promising entry point for understanding the regulation of niche number, distribution, and activity. We also consider the potential existence of stem cell territories and how tissue architecture may influence niche coordination.

Oral epithelial stem cells in tissue maintenance and disease: the first steps in a long journey

The identification and characterization of stem cells is a major focus of developmental biology and regenerative medicine. The advent of genetic inducible fate mapping techniques has made it possible to precisely label specific cell populations and to follow their progeny over time. When combined with advanced mathematical and statistical methods, stem cell division dynamics can be studied in new and exciting ways. Despite advances in a number of tissues, relatively little attention has been paid to stem cells in the oral epithelium. This review will focus on current knowledge about adult oral epithelial stem cells, paradigms in other epithelial stem cell systems that could facilitate new discoveries in this area and the potential roles of epithelial stem cells in oral disease.

Unravelling stem cell dynamics by lineage tracing

During embryonic and postnatal development, the different cells types that form adult tissues must be generated and specified in a precise temporal manner. During adult life, most tissues undergo constant renewal to maintain homeostasis. Lineage-tracing and genetic labelling technologies are beginning to shed light on the mechanisms and dynamics of stem and progenitor cell fate determination during development, tissue maintenance and repair, as well as their dysregulation in tumour formation. Statistical approaches, based on proliferation assays and clonal fate analyses, provide quantitative insights into cell kinetics and fate behaviour. These are powerful techniques to address new questions and paradigms in transgenic mouse models and other model systems.

Epidermal Stem Cells in Homeostasis and Wound Repair of the Skin

SIGNIFICANCE

The skin interfollicular epidermis (IFE) is an organism's first line of defense against a harmful environment and physical damage. During homeostasis and wound repair, the IFE is rejuvenated constantly by IFE stem cells (SCs) that are capable of both proliferation and differentiation. However, the identity and behavior of IFE SCs remain controversial.

RECENT ADVANCES

Two opposing theories exist regarding homeostasis of the IFE. On the basis of morphological and proliferative characteristics, one posits that the IFE is composed of a discrete epidermal proliferative unit comprised of ∼10 transit-amplifying (TA) cells and a centrally located SC in the basal layer. The other suggests that homeostasis of the IFE is maintained by a single progenitor population in the basal layer. A recent study has challenged these two apparently distinct models and demonstrated that the basal layer of the IFE contains both SCs and TA cells, which make distinct contributions to tissue homeostasis and repair. Moreover, phosphorylation levels of the transcription factor p63, the master regulator of the proliferative potential of epidermal SCs, can be used to distinguish self-renewing SCs from TA cells with more limited proliferative potential.

CRITICAL ISSUES

As technologies advance, IFE SCs can be identified at a single-cell level. Refinements of their identification and characterization are critical, not only for SC biology but also for the development of novel clinical applications.

FUTURE DIRECTIONS

Understanding the signaling pathways that control self-renewal and differentiation of IFE SCs will aid in developing novel cell-based therapeutics targeting degenerative epidermal diseases and wound repair.

A self-organization framework for symmetry breaking in the mammalian embryo

The mechanisms underlying the appearance of asymmetry between cells in the early embryo and consequently the specification of distinct cell lineages during mammalian development remain elusive. Recent experimental advances have revealed unexpected dynamics of and new complexity in this process. These findings can be integrated in a new unified framework that regards the early mammalian embryo as a self-organizing system.

The interfollicular epidermis of adult mouse tail comprises two distinct cell lineages that are differentially regulated by Wnt, Edaradd, and Lrig1

Current models of how mouse tail interfollicular epidermis (IFE) is maintained overlook the coexistence of two distinct terminal differentiation programs: parakeratotic (scale) and orthokeratotic (interscale). Lineage tracing and clonal analysis revealed that scale and interscale are maintained by unipotent cells in the underlying basal layer, with scale progenitors dividing more rapidly than interscale progenitors. Although scales are pigmented and precisely aligned with hair follicles, melanocytes and follicles were not necessary for scale differentiation. Epidermal Wnt signaling was required for scale enlargement during development and for postnatal maintenance of scale-interscale boundaries. Loss of Edaradd inhibited ventral scale formation, whereas loss of Lrig1 led to scale enlargement and fusion. In wild-type skin, Lrig1 was not expressed in IFE but was selectively upregulated in dermal fibroblasts underlying the interscale. We conclude that the different IFE differentiation compartments are maintained by distinct stem cell populations and are regulated by epidermal and dermal signals.

•

Tail interfollicular epidermis comprises two distinct differentiated lineages

•

The lineages are maintained by unipotent progenitors that differ in cell-cycle time

•

The lineages are independent of hair follicles and melanocytes

•

The lineages are regulated by Wnt, Edaradd, and Lrig1

Epidermal development in mammals: key regulators, signals from beneath, and stem cells

Epidermis is one of the best-studied tissues in mammals that contain types of stem cells. Outstanding works in recent years have shed great light on behaviors of different epidermal stem cell populations in the homeostasis and regeneration of the epidermis as well as hair follicles. Also, the molecular mechanisms governing these stem cells are being elucidated, from genetic to epigenetic levels. Compared with the explicit knowledge about adult skin, embryonic development of the epidermis, especially the early period, still needs exploration. Furthermore, stem cells in the embryonic epidermis are largely unstudied or ambiguously depicted. In this review, we will summarize and discuss the process of embryonic epidermal development, with focuses on some key molecular regulators and the role of the sub-epidermal mesenchyme. We will also try to trace adult epidermal stem cell populations back to embryonic development. In addition, we will comment on in vitro derivation of epidermal lineages from ES cells and iPS cells.

Stem cell competition: finding balance in the niche

Adult stem cells reside in local microenvironments (niches) that produce signals regulating the outcome of stem cell divisions and stem cell-niche interactions. Limited space and signals in the niche often force stem cells to compete with one another. Although previous studies have uncovered several examples of genetically distinct stem cells competing for niche access, recent studies demonstrate that genetically equivalent stem cells compete under normal conditions, resulting in dynamic stem cell behavior in the niche. New work in multiple vertebrate and invertebrate tissues shows that stem cell competition occurs continuously and mutations disrupting the balance between competing stem cells can cause diseases and defects in the niche. This review discusses recent insights into stem cell competition in mammals and Drosophila.

Cycling progenitors maintain epithelia while diverse cell types contribute to repair

It has recently been shown that stem and progenitor cells undergo population self-renewal to maintain epithelial homeostasis. The fate of individual cells is stochastic but the production of proliferating and differentiating cells is balanced across the population. This new paradigm, originating in mouse epidermis and since extended to mouse oesophagus and mouse and Drosophila intestine, is in contrast to the long held model of epithelial maintenance by exclusively asymmetric division of stem cells. Recent lineage tracing studies have now shown that wound responses vary between tissues, and that a stem cell reserve is not essential as cycling progenitors and even differentiating cells contribute to regeneration.

Tracking cells in their native habitat: lineage tracing in epithelial neoplasia

For tumours to develop, mutations must disrupt tissue homeostasis in favour of deregulated proliferation. Genetic lineage tracing has uncovered the behaviour of proliferating cells that underpins the maintenance of epithelial tissues and the barriers that are broken in neoplastic transformation. In this Review, we focus on new insights revealed by quantifying the behaviour of normal, preneoplastic and tumour cells in epithelia in transgenic mice and consider their potential importance in humans.

Lineage analysis of basal epithelial cells reveals their unexpected plasticity and supports a cell-of-origin model for prostate cancer heterogeneity

A key issue in cancer biology is whether oncogenic transformation of different cell types of origin within an adult tissue gives rise to distinct tumour subtypes that differ in their prognosis and/or treatment response. We now show that initiation of prostate tumours in basal or luminal epithelial cells in mouse models results in tumours with distinct molecular signatures that are predictive of human patient outcomes. Furthermore, our analysis of untransformed basal cells reveals an unexpected assay dependence of their stem cell properties in sphere formation and transplantation assays versus genetic lineage tracing during prostate regeneration and adult tissue homeostasis. Although oncogenic transformation of basal cells gives rise to tumours with luminal phenotypes, cross-species bioinformatic analyses indicate that tumours of luminal origin are more aggressive than tumours of basal origin, and identify a molecular signature associated with patient outcome. Our results reveal the inherent plasticity of basal cells, and support a model in which different cells of origin generate distinct molecular subtypes of prostate cancer.

Basal stem cells contribute to squamous cell carcinomas in the oral cavity

The cells of origin of oral cavity squamous cell carcinoma (OCSCC) are unknown. We used a cell lineage tracing approach (adult K14-CreER(TAM); ROSA26 mice transiently treated with tamoxifen) to identify and track normal epithelial stem cells (SCs) in mouse tongues by X-gal staining and to determine if these cells become neoplastically transformed by treatment with a carcinogen, 4-nitroquinoline 1-oxide (4-NQO). Here, we show that in normal tongue epithelia, X-gal(+) cells formed thin columns throughout the entire epithelium 12 weeks after tamoxifen treatment, indicating that the basal layer contains long-lived SCs that produce progeny by asymmetric division to maintain homeostasis. Carcinogen treatment results in a ~10-fold reduction in the total number of X-gal(+) clonal cell populations and horizontal expansion of X-gal(+) clonal cell columns, a pattern consistent with symmetric division of some SCs. Finally, X-gal(+) SCs are present in papillomas and invasive OCSCCs, and these long-lived X-gal(+) SCs are the cells of origin of these tumors. Moreover, the resulting 4-NQO-induced tumors are multiclonal. These findings provide insights into the identity of the initiating cells of oral cancer.

Transcriptional control of epidermal stem cells

Transcriptional regulation is fundamentally important for the progression of tissue stem cells through different stages of development and differentiation. Mammalian skin epidermis is an excellent model system to study such regulatory mechanisms due to its easy accessibility, stereotypic spatial arrangement, and availability of well-established cell type/lineage differentiation markers. Moreover, epidermis is one of the few mammalian tissues the stem cells of which can be maintained and propagated in culture to generate mature cell types and a functional tissue (reviewed in [1]), offering in vitro and ex vivo platforms to probe deep into the underlying cell and molecular mechanisms of biological functions.

New insights into mechanisms of stem cell daughter fate determination in regenerative tissues

Stem cells can self-renew and differentiate over extended periods of time. Understanding how stem cells acquire their fates is a central question in stem cell biology. Early work in Drosophila germ line and neuroblast showed that fate choice is achieved by strict asymmetric divisions that can generate each time one stem and one differentiated cell. More recent work suggests that during homeostasis, some stem cells can divide symmetrically to generate two differentiated cells or two identical stem cells to compensate for stem cell loss that occurred by direct differentiation or apoptosis. The interplay of all these factors ensures constant tissue regeneration and the maintenance of stem cell pool size. This interplay can be modeled as a population-deterministic dynamics that, at least in some systems, may be described as stochastic behavior. Here, we overview recent progress made on the characterization of stem cell dynamics in regenerative tissues.