Embryonic-like regenerative phenomenon: wound-induced hair follicle neogenesis

Mechanical engineering of hair follicle regeneration by in situ bioprinting

Hair loss caused by various factors such as trauma, stress, and diseases hurts patient psychology and seriously affects patients' quality of life, but there is no effective method to control it. In situ bioprinting is a method for printing bioinks directly into defective sites according to the shape and characteristics of the defective tissue or organ to promote tissue or organ repair. In this study, we applied a 3D bioprinting machine in situ bioprinting of epidermal stem cells (Epi-SCs), skin-derived precursors (SKPs), and Matrigel into the wounds of nude mice to promote hair follicle regeneration based on their native microenvironment. The results showed successful regeneration of hair follicles and other skin appendages at 4 weeks after in situ bioprinting. Moreover, we confirmed that bioprinting only slightly decreased stem cell viability and maintained the stemness of the stem cells. These findings demonstrated a mechanical engineering method for hair follicle regeneration by in situ bioprinting which has potential in the clinic.

Measuring collagen injury depth for burn severity determination using polarization sensitive optical coherence tomography

Determining the optimal treatment course for a dermatologic burn wound requires knowledge of the wound’s severity, as quantified by the depth of thermal damage. In current clinical practice, burn depth is inferred based exclusively on superficial visual assessment, a method which is subject to substantial error rates in the classification of partial thickness (second degree) burns. Here, we present methods for direct, quantitative determination of the depth extent of injury to the dermal collagen matrix using polarization-sensitive optical coherence tomography (PS-OCT). By visualizing the depth-dependence of the degree of polarization of light in the tissue, rather than cumulative retardation, we enable direct and volumetric assessment of local collagen status. We further augment our PS-OCT measurements by visualizing adnexal structures such as hair follicles to relay overall dermal viability in the wounded region. Our methods, which we have validated ex vivo with matched histology, offer an information-rich tool for precise interrogation of burn wound severity and healing potential in both research and clinical settings.

Reprograming the immune niche for skin tissue regeneration - From cellular mechanisms to biomaterials applications

Despite the rapid development of therapeutic approaches for skin repair, chronic wounds such as diabetic foot ulcers remain an unaddressed problem that affects millions of people worldwide. Increasing evidence has revealed the crucial and diverse roles of the immune cells in the development and repair of the skin tissue, prompting new research to focus on further understanding and modulating the local immune niche for comprehensive, 'perfect' regeneration. In this review, we first introduce how different immunocytes and certain stromal cells involved in innate and adaptive immunity coordinate to maintain the immune niche and tissue homeostasis, with emphasis on their specific roles in normal and pathological wound healing. We then discuss novel engineering approaches - particularly biomaterials systems and cellular therapies - to target different players of the immune niche, with three major aims to i) overcome 'under-healing', ii) avoid 'over-healing', and iii) promote functional restoration, including appendage development. Finally, we highlight how these strategies strive to manage chronic wounds and achieve full structural and functional skin recovery by creating desirable 'soil' through modulating the immune microenvironment.

Parallels in signaling between development and regeneration in ectodermal organs

Ectodermal organs originate from the outermost germ layer of the developing embryo and include the skin, hair, tooth, nails, and exocrine glands. These organs develop through tightly regulated, sequential and reciprocal epithelial-mesenchymal crosstalk, and they eventually assume various morphologies and functions while retaining the ability to regenerate. As with many other tissues in the body, the development and morphogenesis of these organs are regulated by a set of common signaling pathways, such as Shh, Wnt, Bmp, Notch, Tgf-β, and Eda. However, subtle differences in the temporal activation, the multiple possible combinations of ligand-receptor activation, the various cofactors, as well as the underlying epigenetic modulation determine how each organ develops into its adult form. Although each organ has been studied separately in considerable detail, the mechanisms underlying the parallels and differences in signaling that regulate their development have rarely been investigated. First, we will use the tooth, the hair follicle, and the mammary gland as representative ectodermal organs to explore how the development of signaling centers and establishment of stem cell populations influence overall growth and morphogenesis. Then we will compare how some of the major signaling pathways (Shh, Wnt, Notch and Yap/Taz) differentially regulate developmental events. Finally, we will discuss how signaling regulates regenerative processes in all three.

STAT3 Partly Inhibits Cell Proliferation via Direct Negative Regulation of FST Gene Expression

Follistatin (FST) is a secretory glycoprotein and belongs to the TGF-β superfamily. Previously, we found that two single nucleotide polymorphisms (SNPs) of sheep FST gene were significantly associated with wool quality traits in Chinese Merino sheep (Junken type), indicating that FST is involved in the regulation of hair follicle development and hair trait formation. The transcription regulation of human and mouse FST genes has been widely investigated, and many transcription factors have been identified to regulate FST gene. However, to date, the transcriptional regulation of sheep FST is largely unknown. In the present study, genome walking was used to close the genomic gap upstream of the sheep genomic FST gene and to obtain the FST gene promoter sequence. Transcription factor binding site analysis showed sheep FST promoter region contained a conserved putative binding site for signal transducer and activator of transcription 3 (STAT3), located at nucleotides -423 to -416 relative to the first nucleotide (A, +1) of the initiation codon (ATG) of sheep FST gene. The dual-luciferase reporter assay demonstrated that STAT3 inhibited the FST promoter activity and that the mutation of the putative STAT3 binding site attenuated the inhibitory effect of STAT3 on the FST promoter activity. Additionally, chromatin immunoprecipitation assay (ChIP) exhibited that STAT3 is directly bound to the FST promoter. Cell proliferation assay displayed that FST and STAT3 played opposite roles in cell proliferation. Overexpression of sheep FST significantly promoted the proliferation of sheep fetal fibroblasts (SFFs) and human keratinocyte (HaCaT) cells, and overexpression of sheep STAT3 displayed opposite results, which was accompanied by a significantly reduced expression of FST gene (P < 0.05). Taken together, STAT3 directly negatively regulates sheep FST gene and depresses cell proliferation. Our findings may contribute to understanding molecular mechanisms that underlie hair follicle development and morphogenesis.

Multiple potential roles of thymosin β4 in the growth and development of hair follicles

The hair follicle (HF) is an important mini-organ of the skin, composed of many types of cells. Dermal papilla cells are important signalling components that guide the proliferation, upward migration and differentiation of HF stem cell progenitor cells to form other types of HF cells. Thymosin β4 (Tβ4), a major actin-sequestering protein, is involved in various cellular responses and has recently been shown to play key roles in HF growth and development. Endogenous Tβ4 can activate the mouse HF cycle transition and affect HF growth and development by promoting the migration and differentiation of HF stem cells and their progeny. In addition, exogenous Tβ4 increases the rate of hair growth in mice and promotes cashmere production by increasing the number of secondary HFs (hair follicles) in cashmere goats. However, the molecular mechanisms through which Tβ4 promotes HF growth and development have rarely been reported. Herein, we review the functions and mechanisms of Tβ4 in HF growth and development and describe the endogenous and exogenous actions of Tβ4 in HFs to provide insights into the roles of Tβ4 in HF growth and development.

Epidermal Stem Cells in Hair Follicle Cycling and Skin Regeneration: A View From the Perspective of Inflammation

There are many studies devoted to the role of hair follicle stem cells in wound healing as well as in follicle self-restoration. At the same time, the influence of the inflammatory cells on the hair follicle cycling in both injured and intact skin is well established. Immune cells of all wound healing stages, including macrophages, γδT cells, and T regs, may activate epidermal stem cells to provide re-epithelization and wound-induced hair follicle neogenesis. In addition to the ability of epidermal cells to maintain epidermal morphogenesis through differentiation program, they can undergo de-differentiation and acquire stem features under the influence of inflammatory milieu. Simultaneously, a stem cell compartment may undergo re-programming to adopt another fate. The proportion of skin resident immune cells and wound-attracted inflammatory cells (e.g., neutrophils and macrophages) in wound-induced hair follicle anagen and plucking-induced anagen is still under discussion to date. Experimental data suggesting the role of reactive oxygen species and prostaglandins, which are uncharacteristic of the intact skin, in the hair follicle cycling indicates the role of neutrophils in injury-induced conditions. In this review, we discuss some of the hair follicles stem cell activities, such as wound-induced hair follicle neogenesis, hair follicle cycling, and re-epithelization, through the prism of inflammation. The plasticity of epidermal stem cells under the influence of inflammatory microenvironment is considered. The relationship between inflammation, scarring, and follicle neogenesis as an indicator of complete wound healing is also highlighted. Taking into consideration the available data, we also conclude that there may exist a presumptive interlink between the stem cell activation, inflammation and the components of programmed cell death pathways.

IL-36α Promoted Wound Induced Hair Follicle Neogenesis via Hair Follicle Stem/Progenitor Cell Proliferation

Background

Wound-induced hair follicle neogenesis (WIHN) is a phenomenon of hair neogenesis that occurs at the center of a scar when the wound area is sufficiently large. Neogenic hair follicles are separated from the pre-existing follicles at the wound edge by a hairless circular region. This WIHN study provides a unique model for developing treatments for hair loss and deciphering the mechanisms underlying organogenesis in adult mammals.

Methods

The skin of a mouse was wounded by excising a 1.5 × 1.5 cm² square of full-thickness dorsal skin. iTRAQ technology was used to screen proteins differentially expressed between the inner and outer scar areas in a mouse model of WIHN, on post-wounding day 15, to identify the regulators of WIHN. Owing to the overexpression of interleukin-36α (IL-36α) in the de novo hair follicle growth area, the regulating effect of IL-36α overexpression in WIHN was investigated. Hair follicle stem/progenitor cells were counted by flow cytometry while the expression of hair follicle stem/progenitor cell markers (Lgr5, Lgr6, Lrig1, K15, and CD34) and that of Wnt/β-catenin and IL-6/STAT3 pathway intermediaries was detected by qPCR and western blotting.

Results

We found that wounding induced IL-36α expression. Incorporation of recombinant murine IL-36α (mrIL-36α) into murine skin wounds resulted in a greater number of regenerated hair follicles (p < 0.005) and a faster healing rate. The expression of hair follicle stem/progenitor cell markers was upregulated in the mrIL-36α-injected site (p < 0.05). Additionally, mrIL-36α upregulated the IL-6/STAT3 pathway intermediaries.

Conclusion

IL-36α is upregulated in de novo hair follicle growth areas and can promote wound epithelialization and WIHN.

A Mouse Model for Studying Stem Cell Effects on Regeneration of Hair Follicle Outer Root Sheaths

Objective

Stem cells hold promise for treating hair loss. Here an in vitro mouse model was developed using outer root sheaths (ORSs) isolated from hair follicles for studying stem cell-mediated dermal papillary regeneration.

Methods

Under sterile conditions, structurally intact ORSs were isolated from hair follicles of 3-day-old Kunming mice and incubated in growth medium. Samples were collected daily for 5 days. Stem cell distribution, proliferation, differentiation, and migration were monitored during regeneration.

Results

Cell proliferation began at the glass membrane periphery then spread gradually toward the membrane center, with the presence of CD34 and CD200 positive stem cells involved in repair initiation. Next, CD34 positive stem cells migrated down the glass membrane, where some participated in ORS formation, while other CD34 cells and CD200 positive cells migrated to hair follicle centers. Within the hair follicle matrix, stem cells divided, grew, differentiated and caused outward expansion of the glass membrane to form a dermal papillary structure containing alpha-smooth muscle actin. Neutrophils attracted to the wound site phagocytosed bacterial and cell debris to protect regenerating tissue from infection.

Conclusion

Isolated hair follicle ORSs can regenerate new dermal papillary structures in vitro. Stem cells and neutrophils play important roles in the regeneration process.

[Wound-induced hair follicle neogenesis: a new perspective on hair follicles regeneration in adult mammals]

OBJECTIVE

To explore the research progress of the cell sources and related signaling pathways of the wound-induced hair follicle neogenesis (WIHN) in recent years.

METHODS

The literature related to WIHN in recent years was reviewed, and the cell sources and molecular mechanism were summarized and discussed.

RESULTS

Current research shows that WIHN is a rare regeneration phenomenon in the skin of adult mammals, with multiple cell origins, both hair follicle stem cells and epithelial stem cells around the wound. Its molecular mechanism is complicated, which is regulated by many signaling pathways. Besides, the process is closely related to the immune response, the immunocytes and their related cytokines provide suitable conditions for this process.

CONCLUSION

There are still many unsolved problems on the cellular origins and molecular mechanisms of the WIHN. Further study on the mechanisms will enhance the understanding of adult mammals' hair follicle regeneration and may provide new strategy for functional healing of the human skin.