Cancer-associated fibroblasts (CAFs) are a key component of the tumour microenvironment with diverse functions, including matrix deposition and remodelling, extensive reciprocal signalling interactions with cancer cells and crosstalk with infiltrating leukocytes. As such, they are a potential target for optimizing therapeutic strategies against cancer. However, many challenges are present in ongoing attempts to modulate CAFs for therapeutic benefit. These include limitations in our understanding of the origin of CAFs and heterogeneity in CAF function, with it being desirable to retain some antitumorigenic functions. On the basis of a meeting of experts in the field of CAF biology, we summarize in this Consensus Statement our current knowledge and present a framework for advancing our understanding of this critical cell type within the tumour microenvironment. Extracellular matrix (ECM).The structural network of secreted proteins and glycosaminoglycans that provides structure to tissue. Angiogenesis The formation of new blood vessels. Mesenchyme A type of tissue composed of loosely associated cells surrounded by extracellular matrix. Mesoderm one of three fundamental layers of tissue formed early in development and the predominant source of fibroblastic lineages.

To investigate how substrate properties influence stem-cell fate, we cultured single human epidermal stem cells on polydimethylsiloxane (PDMS) and polyacrylamide (PAAm) hydrogel surfaces, 0.1 kPa-2.3 MPa in stiffness, with a covalently attached collagen coating. Cell spreading and differentiation were unaffected by polydimethylsiloxane stiffness. However, cells on polyacrylamide of low elastic modulus (0.5 kPa) could not form stable focal adhesions and differentiated as a result of decreased activation of the extracellular-signal-related kinase (ERK)/mitogen-activated protein kinase (MAPK) signalling pathway. The differentiation of human mesenchymal stem cells was also unaffected by PDMS stiffness but regulated by the elastic modulus of PAAm. Dextran penetration measurements indicated that polyacrylamide substrates of low elastic modulus were more porous than stiff substrates, suggesting that the collagen anchoring points would be further apart. We then changed collagen crosslink concentration and used hydrogel-nanoparticle substrates to vary anchoring distance at constant substrate stiffness. Lower collagen anchoring density resulted in increased differentiation. We conclude that stem cells exert a mechanical force on collagen fibres and gauge the feedback to make cell-fate decisions.

Fibroblasts are the major mesenchymal cell type in connective tissue and deposit the collagen and elastic fibers of the extracellular matrix (ECM)1. Even within a single tissue fibroblasts exhibit remarkable functional diversity, but it is not known whether this reflects the existence of a differentiation hierarchy or is a response to different environmental factors. Here we show, using transplantation assays and lineage tracing, that the fibroblasts of skin connective tissue arise from two distinct lineages. One forms the upper dermis, including the dermal papilla that regulates hair growth and the arrector pili muscle (APM), which controls piloerection. The other forms the lower dermis, including the reticular fibroblasts that synthesise the bulk of the fibrillar ECM, and the pre-adipocytes and adipocytes of the hypodermis. The upper lineage is required for hair follicle formation. In wounded adult skin, the initial wave of dermal repair is mediated by the lower lineage and upper dermal fibroblasts are recruited only during re-epithelialisation. Epidermal beta-catenin activation stimulates expansion of the upper dermal lineage, rendering wounds permissive for hair follicle formation. Our findings explain why wounding is linked to formation of ECM-rich scar tissue that lacks hair follicles2-4. They also form a platform for discovering fibroblast lineages in other tissues and for examining fibroblast changes in ageing and disease.

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Stem cells are currently in the news for two reasons: the successful cultivation of human embryonic stem cell lines and reports that adult stem cells can differentiate into developmentally unrelated cell types, such as nerve cells into blood cells. Both intrinsic and extrinsic signals regulate stem cell fate and some of these signals have now been identified. Certain aspects of the stem cell microenvironment, or niche, are conserved between tissues, and this can be exploited in the application of stem cells to tissue replacement therapy.

The recent advent of methods for high-throughput single-cell molecular profiling has catalyzed a growing sense in the scientific community that the time is ripe to complete the 150-year-old effort to identify all cell types in the human body. The Human Cell Atlas Project is an international collaborative effort that aims to define all human cell types in terms of distinctive molecular profiles (such as gene expression profiles) and to connect this information with classical cellular descriptions (such as location and morphology). An open comprehensive reference map of the molecular state of cells in healthy human tissues would propel the systematic study of physiological states, developmental trajectories, regulatory circuitry and interactions of cells, and also provide a framework for understanding cellular dysregulation in human disease. Here we describe the idea, its potential utility, early proofs-of-concept, and some design considerations for the Human Cell Atlas, including a commitment to open data, code, and community.

As a stress response, senescence is a dynamic process involving multiple effector mechanisms whose combination determines the phenotypic quality. Here we identify autophagy as a new effector mechanism of senescence. Autophagy is activated during senescence and its activation is correlated with negative feedback in the PI3K-mammalian target of rapamycin (mTOR) pathway. A subset of autophagy-related genes are up-regulated during senescence: Overexpression of one of those genes, ULK3, induces autophagy and senescence. Furthermore, inhibition of autophagy delays the senescence phenotype, including senescence-associated secretion. Our data suggest that autophagy, and its consequent protein turnover, mediate the acquisition of the senescence phenotype.Supplemental material is available at http://www.genesdev.org.Received December 24, 2008; revised version accepted February 11, 2009. Cellular senescence is a state of stable cell cycle arrest with active metabolism. Similar to apoptosis, senescence can be a failsafe program against a variety of cellular insults. In contrast to apoptosis, however, in which the cytotoxic signals converge to a common mechanism, senescence is typically a delayed stress response involving multiple effector mechanisms. These effector mechanisms include epigenetic regulation (Narita et al. 2006;Adams 2007), the DNA damage response (Bartkova et al. 2006;Di Micco et al. 2006;Mallette et al. 2007), and the senescence-associated secretion phenotype (Kortlever et al. 2006;Acosta et al. 2008;Coppé et al. 2008;Kuilman et al. 2008;Wajapeyee et al. 2008). The relative contribution of these effectors varies depending on the trigger and cell type.Oncogene-induced senescence (OIS) illustrates well the tumor-suppressive role of senescence (Collado and Serrano 2006). The initial phenotype of oncogene induction is a highly proliferative state, which mimics transformation. However, this mitotic burst is gradually replaced by senescence. Although it has been proposed that global and progressive epigenetic alterations play a crucial role in OIS (Narita et al. 2006), the precise mechanism by which cells can achieve such a dramatic change is still unclear.Autophagy is a genetically regulated program responsible for the turnover of cellular proteins and damaged or superfluous organelles. This evolutionarily conserved process is characterized by the formation of doublemembrane cytosolic vesicles, autophagosomes, which sequester cytoplasmic content and deliver it to lysosomes (Ohsumi 2001;Klionsky et al. 2007;Mizushima et al. 2008). Autophagy is often associated with acute metabolic changes and rapid protein replacement. For example, autophagy is required for preimplantation development, where maternal proteins are recycled by autophagy (Tsukamoto et al. 2008). Autophagy is also required for survival in the early neonatal starvation period (Kuma et al. 2004;Komatsu et al. 2005). In addition to these physiological conditions, cytotoxic stimuli can also activate autophagy, but its precise role as a stress response...

Within human epidermis there are two types of proliferating keratinocyte: stem cells, which have high proliferative potential, and transit-amplifying cells, which are destined to undergo terminal differentiation after a few rounds of division. We show that, in vivo, stem cells express higher levels of the alpha 2 beta 1 and alpha 3 beta 1 integrins than transit-amplifying cells and that this can be used both to determine the location of stem cells within the epidermis and to isolate them directly from the tissue. The distribution of stem cells and transit-amplifying cells is not random: patches of integrin-bright and integrin-dull cells have a specific location with respect to the epidermal-dermal junction that varies between body sites and that correlates with the distribution of S phase cells. Stem cell patterning can be recreated in culture, in the absence of dermis, and appears to be subject to autoregulation.