Epidermal stem cells of the skin

Abstract

The skin constantly renews itself throughout adult life, and the hair follicle undergoes a perpetual cycle of growth and degeneration. Stem cells (SCs) residing in the epidermis and hair follicle ensure the maintenance of adult skin homeostasis and hair regeneration, but they also participate in the repair of the epidermis after injuries. We summarize here the current knowledge of epidermal SCs of the adult skin. We discuss their fundamental characteristics, the methods recently designed to isolate these cells, the genes preferentially expressed in the multipotent SC niche, and the signaling pathways involved in SC niche formation, SC maintenance, and activation. Finally, we speculate on how the deregulation of these pathways may lead to cancer formation.

Figure 1

Epidermal development and hair follicle morphogenesis. The surface of the early embryo is covered by a single layer of ectodermal cells that adheres to an underlying basement membrane of extracellular matrix. As development proceeds, the epidermis progressively stratifies and acquires layers of terminally differentiating cells that are required to establish a functional barrier. During embryonic development, some of the undifferentiated basal cells are instructed by the underlying dermis (signal 1) to adopt a hair follicular fate. Subsequently, the epidermis sends a message to the dermis (signal 2) to make the dermal papilla (DP). Finally, the DP sends a message to the developing follicle (signal 3), allowing its growth and differentiation to form the discrete lineages of the hair follicle and its hair. Encased by a basement membrane, the basal layer of the follicle is referred to as the outer root sheath (ORS). At the base of the mature follicle is the highly proliferative compartment called the matrix (Mx). Matrix cells differentiate to form the concentric rings of differentiating cells that give rise to the hair shaft, its channel (the inner root sheath, IRS), and the companion layer. Hair follicles also contain sebaceous glands to ensure the water impermeability of the hair and lubricate the hair channel and skin surface.

Figure 2

The hair follicle cycle. When matrix cells exhaust their proliferative capacity or the stimulus required for it, hair growth stops. At this time, the follicle enters a destructive phase (catagen), leading to the degeneration of the lower two-thirds of the follicle. The upper third of the follicle remains intact as a pocket of cells surrounding the old hair shaft (club hair). The base of this pocket is known as the bulge, which is the natural reservoir of hair follicle stem cells (SCs) necessary to form a new hair follicle. After catagen, the bulge cells enter a quiescent stage (telogen), in which the DP is now in close contact with bulge SCs. In the mouse, the first telogen lasts approximately one day, after which all the hair follicles synchronously enter a new cycle of regeneration and hair growth (anagen stage). The bulge as a structure develops when the new hair must emerge from the original orifice, which is often shared by the old club hair. Subsequent hair cycles involve increasingly longer telogen phases, resulting in considerably less synchronous hair cycles.

Figure 3

The bulge stem cells (SCs). Bulge (Bu) SCs are more quiescent than are other keratinocytes with proliferative potential in the skin. Tumbar et al. (2004) developed a strategy for conducting fluorescent pulse-chase experiments in mice engineered to express a tetracycline-regulatable H2B-GFP transgene. After labeling all the skin epithelial cells with H2B-GFP, a four-week chase resulted in significant H2B-GFP-label retention only in the bulge (a). Label-retaining cells (LRCs) could be found along the basal layer of cells that express α6β4-integrins, as well as in a suprabasal location within the bulge (b). Bulge SCs express a high level of the cell surface protein CD34, which has been used with α6-integrin to isolate basal and suprabasal bulge cells, using flow cytometry (Blanpain et al. 2004, Trempus et al. 2003). [The approximate fluorescence of the outer root sheath (ORS) and interfollicular epidermis (IFE) cells is also indicated on the FACS profile.] Tissues were counterstained with Dapi (blue) to mark the nuclei. Abbreviations used: Cb, club hair; HF, hair follicle; SG, sebaceous gland.

Figure 4

The Wnt/β-catenin signaling pathway during hair follicle (HF) morphogenesis and regeneration. (a) Schematic of the canonical Wnt pathway (for more details, see http://www.stanford.edu/%7Ernusse/). In the absence of a Wnt signal, the excess of cytoplasmic β-catenin is targeted for degradation through its association with a multiprotein complex. Upon binding Wnt, its activated receptor complex recruits certain key components of the β-catenin degradation targeting machinery. Stabilized free cytoplasmic β-catenin is now translocated to the nucleus, where it can associate with transcription factors of the LEF/TCF family to transactivate the expression of their target genes. (b) Loss- and gain-of-function studies in mice have highlighted the different functions of Wnt/β-catenin signaling during morphogenesis and adult skin homeostasis. During HF morphogenesis, Wnt/β-catenin is required to specify the HF (placode) fate in the undifferentiated basal epidermis. During the adult hair cycle, Wnt/β-catenin is required to maintain HF stem cell (SC) identity. As judged by a Wnt reporter transgene, an increase in Wnt signaling promotes SC activation to initiate the growth of a new hair during the telogen-to-anagen transition. An even stronger signal appears to be involved later at the transition of matrix cells to commit to terminally differentiate specifically along the hair shaft lineage. (c) When a constitutively active form of β-catenin is expressed for sustained periods in skin epidermis, mice develop de novo HFs from the interfollicular epidermis (IFE), outer root sheath (ORS), and sebaceous glands (SGs). Eventually, these mice develop HF tumors called pilomatricoma, which consist of immortalized matrix-like cells at the periphery, and pure hair cells in the centers (no inner root sheath or companion layer cells). Visualization was enhanced by breeding the K14-ΔN mice on a background of K14-GFP mice. (d) The different signal strengths of Wnt reporter gene activity, combined with the β-catenin dosage dependency associated with these different outcomes in mice, can be explained by a model whereby the effective strength of Wnt signaling controls the behavior and fate of the follicle SC. Note: The so-called gradient of Wnt activity refers to the status of Tcf/Lef/β-catenin transcriptional activity within the cell, which in fact could be achieved as a gradient, without even involving a Wnt per se. DP, dermal papilla.

Figure 5

The sonic hedgehog (Shh) signaling pathway during hair follicle morphogenesis and adult hair cycle. (a) Schematic of the Shh pathway. In the absence of Shh, its receptor Patched (Ptch) inhibits Smoothened (Smo) activity. Upon Shh binding, Ptch can no longer repress Smo, which activates the translocation of Gli into the nucleus, allowing it to transactivate its target genes. (b) The role of Shh in the hair follicle. Loss-of-function studies in mice have revealed the importance of Shh in sustaining proliferation in the embryonic and adult hair germ. Gain-of-function studies underscore the striking relation between basal cell carcinomas and deregulation of the Shh pathway. (c) Shh is not expressed in the quiescent bulge stem cells. During hair regeneration, there is a lag before Shh is strongly activated in the developing hair germ. Sustained expression of Shh seems to rely on close association with the dermal papilla (DP). Both in embryonic development and the adult, Shh appears to act downstream of the Wnt/β-catenin signaling pathway. Bu, bulge; HG, hair germ.

Figure 6

Bone morphogenetic protein (BMP) signaling pathway during hair follicle morphogenesis and differentiation. (a) Schematic of the BMP pathway. The extracellular availability of BMP proteins is tightly regulated by soluble BMP inhibitors such as Noggin. BMP dimers bind a heterodimeric receptor complex (BMPR-I and BMPR-II) that phosphorylates and activates R-Smad (Smads 1, 5, and 8), which then associates with its co-Smad (Smad 4) partner. Once activated, the R-Smad/co-Smad complex is translocated into the nucleus, where it transactivates its target genes. (b) Role of BMPs in hair follicle morphogenesis. BMP signals are transmitted to and from the overlying epidermis to underlying dermal condensates. Although the role these BMP signals play is not fully understood, this exchange of signaling is thought to play a role in the early specification of sites of hair follicle morphogenesis. As dermal condensates form, they express the BMP-inhibitor Noggin, which is required for normal follicle development and permissive for Lef1 expression and Wnt signaling. Later, as follicle maturation has progressed, the activation of BMP receptor signaling is essential for the matrix cells to differentiate to form the hair shaft and its inner root sheath (IRS) channel. BMP signaling also regulates epidermal proliferation in the skin. DP, dermal papilla.

Figure 7

Notch signaling pathway during epidermal stratification and hair follicle differentiation. (a) Schematic of canonical Notch signaling. Upon ligand (Jagged or Delta) binding, the Notch transmembrane receptor is cleaved by proteases (ADAM protease and γ-secretase), releasing the Notch intracellular domain (NICD), which can then translocate into the nucleus and associate with the DNA-binding protein RBP-Jk to permit transcription of target genes. (b) Role of Notch1 in skin development. Notch1 is cleaved and generates its active form, NICD1, which controls epidermal stratification and differentiation. Early, NICD1 is present in basal cells but later it is found primarily in suprabasal cells. Loss-of-function studies suggest that Notch1 acts as a tumor suppressor in skin epidermis to restrict proliferation to the basal layer. Notch1 also plays a role in the hair follicle, where it has been demonstrated to play a critical role in the differentiation of the inner root sheath and the hair shaft.

Figure 8

Summary of the signaling pathways involved in hair follicle morphogenesis and regeneration. This schematic summarizes many of the signaling pathways involved in hair follicle morphogenesis and hair follicle regeneration. Wnt/β-catenin signaling acts early in hair follicle specification and quiescent bulge stem cell (SC) activation. The sonic hedgehog (Shh) signaling pathway acts in the second step to promote embryonic and adult hair germ proliferation. Bone morphogenetic protein (BMP), Notch, and Wnt/β-catenin signaling pathways act further downstream to allow the normal differentiation of matrix cells into the hair shaft (HS) and its inner root sheath (IRS) envelope.