Hair follicle stem cells

The increasing use of the hair follicle as a stem cell paradigm is due in part to the complex interplay between epithelial, dermal and other cell types, each with interesting differentiation potential and prospective therapeutic applications. This review focuses on research into the environmental niche, gene expression profiles and plasticity of hair follicle stem cell populations, where many recent advances have come about through novel technological and experimental approaches. We discuss major developmental pathways involved in the establishment and control of the epithelial stem cell niche, and evidence of plasticity between stem and transit amplifying cell populations.     

Epithelial stem cells: stepping out of their niche

In this issue of Cell, have shown that two subpopulations of cells exist within the hair follicle stem cell niche. Despite being partially differentiated, clonal populations of suprabasal bulge region cells can regenerate skin and hair follicles as well as a new stem cell niche. The findings suggest that early lineage commitments of epithelial cells in the hair follicle may be reversible.     

The bulge area is the origin of nestin-expressing pluripotent stem cells of the hair follicle

Nestin-expressing pluripotent stem cells have been found both in the bulge area (BA) as well as the dermal papilla (DP). Nestin-expressing stem cells of both the BA and DP have been previously shown to be able to form neurons and other non-follicle cell types. The nestin-expressing stem cells from the DP have been termed skin precursor or SKP cells. Both nestin-expressing DP and BA cells have been previously shown to effect repair of the injured spinal cord and peripheral nerve, with the BA being the greater and more constant source of the stem cells. The BA contains nestin-expressing stem cells throughout the hair cycle, whereas nestin-expressing dermal papillae stem cells were found in early and mid-anagen only. Our previous studies have shown that the nestin-expressing stem cells in the BA and DP have similar morphological features. The cells from both regions have a small body diameter of approximately 7 μm with long extrusions, as shown by 2-photon imaging. In the present study, using 2-photon imaging of whisker follicles from transgenic mice expressing nestin-driven green fluorescent protein (ND-GFP), we demonstrate that the BA is the source of the nestin-expressing stem cells of the hair follicle. The nestin-expressing stem cells migrate from the BA to the DP as well as into the surrounding skin tissues including the epidermis, and during wound healing, suggesting that the BA may be the source of the stem cells of the skin itself.     

Effects of Wnt-10b on hair shaft growth in hair follicle cultures

Wnts are deeply involved in the proliferation and differentiation of skin epithelial cells. We previously reported the differentiation of cultured primary skin epithelial cells toward hair shaft and inner root sheath (IRS) of the hair follicle via beta-catenin stabilization caused by Wnt-10b, however, the effects of Wnt-10b on cultured hair follicles have not been reported. In the present study, we examined the effects of Wnt-10b on shaft growth using organ cultures of whisker hair follicles in serum-free conditions. No hair shaft growth was observed in the absence of Wnt-10b, whereas its addition to the culture promoted elongation of the hair shaft, intensive incorporation of BrdU in matrix cells flanking the dermal papilla (DP), and beta-catenin stabilization in DP and IRS cells. These results suggest a promoting effect of Wnt-10b on hair shaft growth that is involved with stimulation of the DP via Wnt-10b/beta-catenin signalling, proliferation of matrix cells next to the DP, and differentiation of IRS cells by Wnt-10b.     

Hair multiplication with dermal papilla like tissue containing human dermal papilla cells

Alopecia is not a critical disease; however it is a disease that can affect the quality of life. Many remedies have been developed to cure alopecia, but only two have been approved by the FDA. Due to the steadily increasing number of young alopecia patients, the need for new therapies for curing alopecia is very high. Recent studies on cell therapy have reported using technique to treat various diseases. We introduce upgraded hair cell therapy which tested hair structure inducing activity with bioartificial dermal papilla tissue. Hair follicles contain two types of stem cells: Outer root sheath cells (ORSCs) derived epithelial cells, and dermal cells (DPCs). In this study, we reconstructed DP-like tissues (DPLTs) using cultured dermal papilla cells (DPCs) from human hair follicles. The DPLTs were produced special media (Dermal Papilla Forming Media: DPFM) conditions in vitro, which can induce epithelial stands from implanted healthy hair without DP. We tested in vivo hair-inducing with a modified hair sandwich model. Two to three weeks DPLT injection into the mouse scalp skin, we observed new hair in the injected site and detected injected human cells from DPLTs and Outer Root Sheath Cells (ORSCs) in the new hair via human Alu-DNA-specific probe. In the future, reconstructed DPLTs may be used in in vitro studies of hair development and the morphogenesis mechanism, as well as in vitro studies of the efficacy and toxicity of drugs for baldness. These tissues will be used as an alternative medicine product for hair transplantation     

Embryonic dermal condensation and adult dermal papilla induce hair follicles in adult glabrous epidermis through different mechanisms

Hair induction in the adult glabrous epidermis by the embryonic dermis was compared with that by the adult dermis. Recombinant skin, composed of the adult sole epidermis and the embryonic dermis containing dermal condensations (DC), was transplanted onto the back of nude mice. The epidermis of transplants formed hairs. Histology on the induction process demonstrated the formation of placode-like tissues, indicating that the transplant produces hair follicles through a mechanism similar to that underlying hair follicle development in the embryonic skin. An isolated adult rat sole skin piece, inserted with either an aggregate of cultured dermal papilla (DP) cells or an intact DP between its epidermis and dermis, was similarly transplanted. The transplant produced hair follicles. Histology showed that the epidermis in both cases surrounded the aggregates of DP cells. The epidermis never formed placode-like tissues. Thus, it was concluded that the adult epidermal cells recapitulate the embryonic process of hair follicle development when exposed to DC, whereas they get directly into the anagen of the hair cycle when exposed to DP. The expression pattern of Edar and Shh genes, and P-cadherin protein during the hair follicle development in the two types of transplants supported the above conclusion.     

Abnormal hair development and apparent follicular transformation to mammary gland in the absence of hedgehog signaling

We show that removing the Shh signal tranducer Smoothened from skin epithelium secondarily results in excess Shh levels in the mesenchyme. Moreover, the phenotypes we observe reflect decreased epithelial Shh signaling, yet increased mesenchymal Shh signaling. For example, the latter contributes to exuberant hair follicle (HF) induction, while the former depletes the resulting follicular stem cell niches. This disruption of the niche apparently also allows the remaining stem cells to initiate hair formation at inappropriate times. Thus, the temporal structure of the hair cycle may depend on the physical structure of the niche. Finally, we find that the ablation of epithelial Shh signaling results in unexpected transformations: the follicular outer root sheath takes on an epidermal character, and certain HFs disappear altogether, having adopted a strikingly mammary gland-like fate. Overall, our study uncovers a multifaceted function for Shh in sculpting and maintaining the integrity and identity of the developing HF.     

Sox9 is essential for outer root sheath differentiation and the formation of the hair stem cell compartment

BACKGROUND: The mammalian hair represents an unparalleled model system to understand both developmental processes and stem cell biology. The hair follicle consists of several concentric epithelial sheaths with the outer root sheath (ORS) forming the outermost layer. Functionally, the ORS has been implicated in the migration of hair stem cells from the stem cell niche toward the hair bulb. However, factors required for the differentiation of this critical cell lineage remain to be identified. Here, we describe an unexpected role of the HMG-box-containing gene Sox9 in hair development. RESULTS: Sox9 expression can be first detected in the epithelial component of the hair placode but then becomes restricted to the outer root sheath (ORS) and the hair stem cell compartment (bulge). Using tissue-specific inactivation of Sox9, we demonstrate that this gene serves a crucial role in hair differentiation and that skin deleted for Sox9 lacks external hair. Strikingly, the ORS acquires epidermal characteristics with ectopic expression of GATA3. Moreover, Sox9 knock hair show severe proliferative defects and the stem cell niche never forms. Finally, we show that Sox9 expression depends on sonic hedgehog (Shh) signaling and demonstrate overexpression in skin tumors in mouse and man. CONCLUSIONS: We conclude that although Sox9 is dispensable for hair induction, it directs differentiation of the ORS and is required for the formation of the hair stem cell compartment. Our genetic analysis places Sox9 in a molecular cascade downstream of sonic hedgehog and suggests that this gene is involved in basal cell carcinoma.     

The bulge area is the major hair follicle source of nestin-expressing pluripotent stem cells which can repair the spinal cord compared to the dermal papilla

Nestin has been shown to be expressed in the hair follicle, both in the bulge area (BA) as well as the dermal papilla (DP). Nestin-expressing stem cells of both the BA and DP have been previously shown to be pluripotent and be able to form neurons and other non-follicle cell types. The nestin-expressing pluripotent stem cells from the DP have been termed skin precursor or SKP cells. The objective of the present study was to determine the major source of nestin-expressing pluripotent stem cells in the hair follicle and to compare the ability of the nestin-expressing pluripotent stem cells from the BA and DP to repair spinal cord injury. Transgenic mice in which the nestin promoter drives GFP (ND-GFP) were used in order to observe nestin expression in the BA and DP. Nestin-expressing DP cells were found in early and middle anagen. The BA had nestin expression throughout the hair cycle and to a greater extent than the DP. The cells from both regions had very long processes extending from them as shown by two-photon confocal microscopy. Nestin-expressing stem cells from both areas differentiated into neuronal cells at high frequency in vitro. Both nestin-expressing DP and BA cells differentiated into neuronal and glial cells after transplantation to the injured spinal cord and enhanced injury repair and locomotor recovery within four weeks. Nestin-expressing pluripotent stem cells from both the BA and DP have potential for spinal cord regeneration, with the BA being the greater and more constant source.     

A dermal niche for multipotent adult skin-derived precursor cells

A fundamental question in stem cell research is whether cultured multipotent adult stem cells represent endogenous multipotent precursor cells. Here we address this question, focusing on SKPs, a cultured adult stem cell from the dermis that generates both neural and mesodermal progeny. We show that SKPs derive from endogenous adult dermal precursors that exhibit properties similar to embryonic neural-crest stem cells. We demonstrate that these endogenous SKPs can first be isolated from skin during embryogenesis and that they persist into adulthood, with a niche in the papillae of hair and whisker follicles. Furthermore, lineage analysis indicates that both hair and whisker follicle dermal papillae contain neural-crest-derived cells, and that SKPs from the whisker pad are of neural-crest origin. We propose that SKPs represent an endogenous embryonic precursor cell that arises in peripheral tissues such as skin during development and maintains multipotency into adulthood.     

Single-cell sequencing reveals the new existence form of dermal papilla cells in the hair follicle regeneration of cashmere goats

The problem of human hair loss has caused widespread concern, however, such research is difficult because the periodicity is not obvious and the deeper levels knowledge of dermal papilla (DP) stem cells' differentiation are limited. Here, cashmere goats which have obvious periodicity of hair follicles were used, based on unbiased scRNA sequencing, we constructed DP cell lineage differentiation trajectory and revealed the key genes, signals and functions involved in cell fate decisions. And then we revealed the molecular landscape of hair follicle on regeneration. Revealed that DP cells differentiate into four intermediate cell states at different periodicity: Intermediate-cell-10 showed important functions in the growth and maintenance of cashmere; intermediate-cell-1 acting on apoptosis and cashmere shedding; intermediate-cell-0 initiated new follicular cycles, the migration of hair follicles and the occurrence of cashmere; and intermediate-cell-15 are suggested to be DP progenitor cells. In general, we provide new insights for hair regrowth. At the same time, it provides a new research ideas, directions and molecular landscape for the mechanism of dermal papilla cells.     

Differential response of epithelial stem cell populations in hair follicles to TGF-β signaling

Epidermal stem cells residing in different locations in the skin continuously self-renew and differentiate into distinct cell lineages to maintain skin homeostasis during postnatal life. Murine epidermal stem cells located at the bulge region are responsible for replenishing the hair lineage, while the stem cells at the isthmus regenerate interfollicular epidermis and sebaceous glands. In vitro cell culture and in vivo animal studies have implicated TGF-β signaling in the maintenance of epidermal and hair cycle homeostasis. Here, we employed a triple transgenic animal model that utilizes a combined Cre/loxP and rtTA/TRE system to allow inducible and reversible inhibition of TGF-β signaling in hair follicle lineages and suprabasal layer of the epidermis. Using this animal model, we have analyzed the role of TGF-β signaling in distinct phases of the hair cycle. Transient abrogation of TGF-β signaling does not prevent catagen progression; however, it induces aberrant proliferation and differentiation of isthmus stem cells to epidermis and sebocyte lineages and a blockade in anagen re-entry as well as results in an incomplete hair shaft development. Moreover, ablation of TGF-β signaling during anagen leads to increased apoptosis in the secondary hair germ and bulb matrix cells. Blocking of TGF-β signaling in bulge stem cell cultures abolishes their colony-forming ability, suggesting that TGF-β signaling is required for the maintenance of bulge stem cells. At the molecular level, inhibition of TGF-β signaling results in a decrease in both Lrig1-expressing isthmus stem cells and Lrg5-expressing bulge stem cells, which may account for the phenotypes seen in our animal model. These data strongly suggest that TGF-β signaling plays an important role in regulating the proliferation, differentiation, and apoptosis of distinct epithelial stem cell populations in hair follicles.     

Bioengineering the hair follicle: fringe benefits of stem cell technology

Recent advances in epithelial stem cell biology have resulted in the isolation of hair follicle stem cells, which generate hair follicles when injected into immunodeficient mice. These isolated hair follicle epithelial stem cells must be combined with 'inductive' dermal cells to produce new hair follicles. The advent of techniques for cultivating inductive dermal cells and competent epithelial stem cells creates the opportunity to bioengineer hair follicles for the treatment of hair loss.     

Wingless signaling regulates the maintenance of ovarian somatic stem cells in Drosophila

Identifying the signals involved in maintaining stem cells is critical to understanding stem cell biology and to using stem cells in future regenerative medicine. In the Drosophila ovary, Hedgehog is the only known signal for maintaining somatic stem cells (SSCs). Here we report that Wingless (Wg) signaling is also essential for SSC maintenance in the Drosophila ovary. Wg is expressed in terminal filament and cap cells, a few cells away from SSCs. Downregulation of Wg signaling in SSCs through removal of positive regulators of Wg signaling, dishevelled and armadillo, results in rapid SSC loss. Constitutive Wg signaling in SSCs through the removal of its negative regulators, Axin and shaggy, also causes SSC loss. Also, constitutive wg signaling causes over-proliferation and abnormal differentiation of somatic follicle cells. This work demonstrates that wg signaling regulates SSC maintenance and that its constitutive signaling influences follicle cell proliferation and differentiation. In mammals, constitutive beta-catenin causes over-proliferation and abnormal differentiation of skin cells, resulting in skin cancer formation. Possibly, mechanisms regulating proliferation and differentiation of epithelial cells, including epithelial stem cells, is conserved from Drosophila to man.     

Comparison of hair dermal cells and skin fibroblasts in a collagen sponge for use in wound repair

The adult hair follicle has well-defined dermal and epithelial populations that display distinct developmental properties. The follicular dermal cells, namely the dermal papilla and dermal sheath, are derived from the same mesenchymal cells as dermal fibroblasts and therefore, we believed that follicular cells could be useful sources of interfollicular keratinocytes and fibroblast for skin wound repair. In this study, we evaluated the relative effect of various mesenchymal-derived cells on wound healing following skin injury. Human dermal cells, including two different follicular dermal cells and skin fibroblasts were cultured in collagen sponges and compared with respect to wound healing. Results indicated that there was no significant difference in wound contraction and angiogenesis among the cell types. Further, dermal sheath cells exhibited relatively poor results compared with other cells in new collagen synthesis. Finally, basement membrane reformation and new collagen synthesis for the dermal papilla cell grafts was superior to those of the dermal sheath cells or fibroblasts.