Hair follicle stem cells as a skin‐organizing signaling center during adult homeostasis

Stem cells are the essential source of building blocks for tissue homeostasis and regeneration. Their behavior is dictated by both cell‐intrinsic cues and extrinsic cues from the microenvironment, known as the stem cell niche. Interestingly, recent work began to demonstrate that hair follicle stem cells (HFSCs) are not only passive recipients of signals from the surroundings, but also actively send out signals to modulate the organization and function of their own niches. Here, we discuss recent findings, and briefly refer to the old, on the interaction of HFSCs and their niches with the emphasis on the outwards signals from HFSCs toward their niches. We also highlight recent technology advancements that further promote our understanding of HFSC niches. Taken together, the HFSCs emerge as a skin‐organizing center rich in signaling output for niche remodeling during various stages of adult skin homeostasis. The intricate crosstalk between HFSCs and their niches adds important insight to skin biology that will inform clinical and bioengineering fields aiming to build complete and functional 3D organotypic cultures for skin replacement therapies.     

Hair follicle stem cell cultures reveal self‐organizing plasticity of stem cells and their progeny

Understanding how complex tissues are formed, maintained, and regenerated through local growth, differentiation, and remodeling requires knowledge on how single‐cell behaviors are coordinated on the population level. The self‐renewing hair follicle, maintained by a distinct stem cell population, represents an excellent paradigm to address this question. A major obstacle in mechanistic understanding of hair follicle stem cell (HFSC) regulation has been the lack of a culture system that recapitulates HFSC behavior while allowing their precise monitoring and manipulation. Here, we establish an in vitro culture system based on a 3D extracellular matrix environment and defined soluble factors, which for the first time allows expansion and long‐term maintenance of murine multipotent HFSCs in the absence of heterologous cell types. Strikingly, this scheme promotes de novo generation of HFSCs from non‐HFSCs and vice versa in a dynamic self‐organizing process. This bidirectional interconversion of HFSCs and their progeny drives the system into a population equilibrium state. Our study uncovers regulatory dynamics by which phenotypic plasticity of cells drives population‐level homeostasis within a niche, and provides a discovery tool for studies on adult stem cell fate.     

A simple culture method for epithelial stem cells derived from human hair follicle

The challenge arises among researchers when hair follicle stem cells (HFSCs) derived from a human hair follicle remain poorly expanded in defined culture medium. In this study, we isolated the HFSCs and they became confluent after 10 days of cultivation. Comparing the viability of HFSCs cultured in defined keratinocytes serum free medium (KSFM) in a coated plate and CnT07 medium in an uncoated plate, the number of HFSCs cultured in CnT07 was significantly higher at days 2, 4, 6 and 8 (P=0.004). The population doubling time of HFSCs was 21.48±0.44 hours in non-coated plates with CnT07 and 30.73±0.75 hours in coated plates with KSFM. Our primary HFSC cultures were positive for CD200 and K15 with brownish color. Flow cytometry analysis showed that the percentage of HFSCs expressing CD200 and K15 were 65.20±3.16 and 72.07±6.62 respectively. After reaching 100% confluence, the HFSCs were differentiated into an epidermal layer in vitro using CnT02-3D defined media. HFSCs were differentiated into an epidermal layer after 2 weeks of induction. Involucrin- and K6-positive cells were detected in the differentiated epidermal layer. This method is a simple technique for HFSC isolation and has a lower cost of processing and labor, and it represents a promising tool for skin tissue engineering.     

Cebpa is essential for the embryonic myeloid progenitor and neutrophil maintenance in zebrafish

In vertebrates, myeloid cells arise from multiple waves of development: the first or embryonic wave of myelopoiesis initiates early from non-hematopoietic stem cell(HSC) precursors and gives rise to myeloid cells transiently during early development; whereas the second or adult wave of myelopoiesis emerges later from HSCs and produces myeloid cells continually during fetal and adult life. In the past decades, a great deal has been learnt about the development of myeloid cells from adult myelopoiesis, yet the genetic network governing embryonic myelopoiesis remains poorly defined. In this report, we present an in vivo study to delineate the role of Cebpa during zebrafish embryonic myelopoiesis. We show that embryonic myelopoiesis in cebpa-deficient zebrafish mutants initiates properly but fails to produce macrophages and neutrophils. The lack of macrophages and neutrophils in the mutants is largely attributed to the cell cycle arrest of embryonic myeloid progenitors, resulting in the impairment of their maintenance and subsequent differentiation. We further show that Cebpa, perhaps acting cooperatively with Runx1, plays a critical role in embryonic neutrophil maintenance. Our findings reveal a new role of Cebpa in embryonic myelopoiesis.     

Prox1 Is Required for Granule Cell Maturation and Intermediate Progenitor Maintenance During Brain Neurogenesis

The dentate gyrus has an important role in learning and memory, and adult neurogenesis in the subgranular zone of the dentate gyrus may play a role in the acquisition of new memories. The homeobox gene Prox1 is expressed in the dentate gyrus during embryonic development and adult neurogenesis. Here we show that Prox1 is necessary for the maturation of granule cells in the dentate gyrus during development and for the maintenance of intermediate progenitors during adult neurogenesis. We also demonstrate that Prox1-expressing intermediate progenitors are required for adult neural stem cell self-maintenance in the subgranular zone; thus, we have identified a previously unknown non-cell autonomous regulatory feedback mechanism that controls adult neurogenesis in this region of the mammalian brain. Finally, we show that the ectopic expression of Prox1 induces premature differentiation of neural stem cells.     

Runx1 modulates developmental, but not injury-driven, hair follicle stem cell activation

Aml1/Runx1 controls developmental aspects of several tissues, is a master regulator of blood stem cells, and plays a role in leukemia. However, it is unclear whether it functions in tissue stem cells other than blood. Here, we have investigated the role of Runx1 in mouse hair follicle stem cells by conditional ablation in epithelial cells. Runx1 disruption affects hair follicle stem cell activation, but not their maintenance, proliferation or differentiation potential. Adult mutant mice exhibit impaired de novo production of hair shafts and all temporary hair cell lineages, owing to a prolonged quiescent phase of the first hair cycle. The lag of stem cell activity is reversed by skin injury. Our work suggests a degree of functional overlap in Runx1 regulation of blood and hair follicle stem cells at an equivalent time point in the development of these two tissues.     

Hair follicle stem cells provide a functional niche for melanocyte stem cells

In most stem cell systems, the organization of the stem cell niche and the anchoring matrix required for stem cell maintenance are largely unknown. We report here that collagen XVII (COL17A1/BP180/BPAG2), a hemidesmosomal transmembrane collagen, is highly expressed in hair follicle stem cells (HFSCs) and is required for the maintenance not only of HFSCs but also of melanocyte stem cells (MSCs), which do not express Col17a1 but directly adhere to HFSCs. Mice lacking Col17a1 show premature hair graying and hair loss. Analysis of Col17a1-null mice revealed that COL17A1 is critical for the self-renewal of HFSCs through maintaining their quiescence and immaturity, potentially explaining the mechanism underlying hair loss in human COL17A1 deficiency. Moreover, forced expression of COL17A1 in basal keratinocytes, including HFSCs, in Col17a1-null mice rescues MSCs from premature differentiation and restores TGF-β signaling, demonstrating that HFSCs function as a critical regulatory component of the MSC niche.     

Hair follicle stem cells: In vitro and in vivo neural differentiation

Hair follicle stem cells (HFSCs) normally give rise to keratinocytes, sebocytes, and transient amplifying progenitor cells. Along with the capacity to proliferate rapidly, HFSCs provide the basis for establishing a putative source of stem cells for cell therapy. HFSCs are multipotent stem cells originating from the bulge area. The importance of these cells arises from two important characteristics, distinguishing them from all other adult stem cells. First, they are accessible and proliferate for long periods. Second, they are multipotent, possessing the ability to differentiate into mesodermal and ectodermal cell types. In addition to a developmental capacity in vitro, HFSCs display an ability to form differentiated cells in vivo. During the last two decades, numerous studies have led to the development of an appropriate culture condition for producing various cell lineages from HFSCs. Therefore, these stem cells are considered as a novel source for cell therapy of a broad spectrum of neurodegenerative disorders. This review presents the current status of human, rat, and mouse HFSCs from both the cellular and molecular biology and cell therapy perspectives. The first section of this review highlights the importance of HFSCs and in vitro differentiation, while the final section emphasizes the significance of cell differentiation in vivo.     

Neural stem cells in the mammalian eye: types and regulation

Neural stem cells/progenitors that give rise to neurons and glia have been identified in different regions of the brain, including the embryonic retina. Recently, such cells have been reported to be present, in a mitotically quiescent state, in the ciliary epithelium of the adult mammalian eye. The retinal and ciliary epithelium stem cells/progenitors appear to share similar signaling pathways that are emerging as important regulators of stem cells in general. Yet, they are different in certain respects, such as in the potential to self-renew. These two neural stem cell/progenitor populations not only will serve as models for investigating stem cell biology but also will help explain the relationships between embryonic and adult neural stem cells/progenitors.     

Ersatzgewebe aus Bioreaktoren

Induced Stem Cells and Tissue Engineering Not only ethical reservations and restrictive laws direct medically oriented stem cell research towards adult stem cells, facilitated by new methods to reprogram differentiated cells back to proliferating progenitors. This allows new therapies with autologous, immunologically tolerant cells and tissues. In bioreactors autologous tissues are grown to replace, for instance, burned skin, cartilaginous elements, and heart valves. Even almost complete eyes, however in an embryonic state, can develop from stem cells in culture by self organization. Finally the German laws related to stem cell research are reviewed.