Transit-amplifying cells orchestrate stem cell activity and tissue regeneration

The integrated stress response fine-tunes stem cell fate decisions upon serine deprivation and tissue injury

Epidermal stem cells produce the skin's barrier that excludes pathogens and prevents dehydration. Hair follicle stem cells (HFSCs) are dedicated to bursts of hair regeneration, but upon injury, they can also reconstruct, and thereafter maintain, the overlying epidermis. How HFSCs balance these fate choices to restore physiologic function to damaged tissue remains poorly understood. Here, we uncover serine as an unconventional, non-essential amino acid that impacts this process. When dietary serine dips, endogenous biosynthesis in HFSCs fails to meet demands (and vice versa), slowing hair cycle entry. Serine deprivation also alters wound repair, further delaying hair regeneration while accelerating re-epithelialization kinetics. Mechanistically, we show that HFSCs sense each fitness challenge by triggering the integrated stress response, which acts as a rheostat of epidermal-HF identity. As stress levels rise, skin barrier restoration kinetics accelerate while hair growth is delayed. Our findings offer potential for dietary and pharmacological intervention to accelerate wound healing.

Cyclic renewal in three ectodermal appendage follicles: Hairs, feathers and teeth

Ectodermal appendages display a range of renewal mechanisms, with some undergoing continuous growth and others experiencing cyclic regeneration. The latter requires sustainable epithelial stem cells and mesenchymal niche essential for interacting with these stem cells. Furthermore, certain appendages dynamically adjust their mesenchymal niche in response to environmental factors, such as hormonal fluctuations, sex, and seasonal changes, enabling them to cyclically renew with different appendages phenotypes to adapt to different environments and to different life stages. Here we focus on amniotes, including reptiles, birds, and mammals, which exhibit integumentary adaptations that enable their survival across various ecological environments, from aquatic habitats and terrestrial landscapes to aerial domains. We highlight three representative integument appendage follicles: teeth, feathers, and hairs. Despite independent evolutionary origins, these structures share a fundamental architectural design characterized by the presence of stem cells and mesenchymal niches. They differ in the spatial arrangement and topology of these components. By examining the distinct architectural features of these follicles, we demonstrate the different strategies they use to orchestrate the physiological regenerative cycling, from growth initiation to cessation and molting, and regeneration after wounding. We delve into known molecular controls that govern these processes and unravel the evolutionary insights. We also identify new cell interactions that underlie the emergence of evolutionary novel follicle components. Various amniote scales have evolved independently with different configurations, but all lack follicle architecture and maintain homeostasis using a strategy similar to that of skin. The convergently evolved follicles in hairs, feathers, and teeth utilize different designs to achieve cyclic renewability, allowing them to produce spatially and temporally specific appendage phenotypes, thus enhancing the adaptability of the integumentary interface to external environmental pressures. This, in turn, enriches our understanding of evolutionary developmental biology (Evo-Devo) of the integument, shedding light on the intricate interplay between form and function across diverse taxa.

Apurinic/apyrimidinic endonuclease 1 (APE1) prevents alopecia by promoting regeneration of hair follicles

Hair follicle (HF) regeneration, which relies on the self-renewal and differentiation capacity of bulge cells, involves multiple molecular mechanisms. In this study, we found that Apurinic/apyrimidinic endonuclease 1 (APE1) acts as a positive regulator of spontaneous and depilation-induced HF regeneration. Loss of APE1 leads to hair thinning and delayed HF transition from telogen to anagen. As shown in our systematic conditional Apex1 knockout (Apex1flox/floxCre-ER+) mouse model, Apex1-/- mice gradually lost hair coat over time and eventually became hairless after 10 months. Histological analyses revealed that Apex1 knockout caused the retarded growth of HF and the reduction of hair density, as a result of repressed proliferation of bulge cells by downregulating the β-catenin pathway. Moreover, APE1 is indispensable in the depilation-induced HF regeneration, and its deficiency contributes to the depletion of bulge cells, which in turn causes failure of hair growth. These findings highlight the indispensable role of APE1 for HF activation, maintenance, and growth.

Circ 0020938 inhibits hair follicle stem cells proliferation via the miR-142-5p/DSG4 axis in cashmere goats

BACKGROUND

Shaanbei white cashmere goat is an excellent cashmere goat breed, and its market favored cashmere from the secondary hair follicles. Hair follicles mature around birth and each hair follicle repeatedly undergoes a growth cycle that comprises three distinct stages: anagen, catagen and telogen. Understanding the molecular mechanisms controlling cyclic hair follicle changes is essential for optimizing hair follicle function and improving cashmere production.

METHODS

The circRNA expression profile in the hair follicle cycle was constructed and differentially expressed circRNAs were identified, with particular focus on circ 0020938, which was highly expressed during anagen. The functional assays were performed to assess the effect of circ 0020938 on hair follicle stem cells (HFSCs) proliferation. Competing endogenous RNA (ceRNA) network was constructed to investigate the interaction between circ 0020938, miR-142-5p, and DSG4. Rescue experiment was conducted to validate the impact of circ 0020938 on HFSCs proliferation and DSG4 expression.

RESULTS

We found that circ 0020938 inhibited HFSCs proliferation. Further analysis revealed that circ 0020938 acted as a sponge for miR-142-5p, alleviating the repression of DSG4. Additionally, we confirmed that DSG4 inhibited HFSCs proliferation, suggesting that it play a key role in regulating the balance between proliferation and differentiation during the hair follicle cycle. Rescue experiments showed that the inhibition of HFSCs proliferation by circ 0020938 was partially reversed by miR-142-5p.

CONCLUSION

Our study provides novel insights into the regulatory role of circRNA in HFSCs proliferation during the hair follicle cycle. The results demonstrate that circ 0020938 acts as a miRNA sponge and inhibits HFSCs proliferation through the miR-142-5p/DSG4 axis, thereby contributing to the proper progression of the hair follicle cycle.

Hyaluronic Acid Microneedles Loaded with Chinese Herbal Extracts as an Intradermal Delivery System for Hair Regeneration

Androgenic alopecia is one of the most common chronic problems for dermatologists worldwide. Some Chinese herbal extracts have been shown to promote hair growth, but the active ingredients are difficult to enter the dermis. Therefore, delivering the active ingredients into the dermis becomes a key factor. Herein, Platycladus orientalis leaf extract (PO-ex) was obtained using ethanol as a solvent, and then hyaluronic acid methacrylate/hyaluronic acid (HAMA/HA) hydrogel was loaded with PO-ex to prepare hyaluronic acid microneedles (PO-ex MN). The double cross-linked HAMA/HA provides sufficient mechanical strength to pierce the stratum corneum and deliver PO-ex into the dermis; PO-ex can effectively improve the environment for hair follicle cell proliferation by removing reactive oxygen free radicals; in addition, the self-repair reaction caused by microneedle mechanical stimulation activates the Wnt/β-catenin pathway associated with trauma repair and promotes hair follicle growth. PO-ex MN is a potential therapeutic strategy for the treatment of androgenic alopecia.

Gas1-high quiescent neural stem cells are multipotent and produce oligodendrocytes during aging and after demyelinating injury

Quiescent neural stem cells (qNSCs) in the adult mouse subventricular zone (SVZ) normally have limited capacity to generate glia. Gliogenic domains are present in both dorsal and ventral SVZ, with the ventral region featuring a subpopulation of Gli1+ qNSCs. In dorsal SVZ, however, the molecular identity and developmental origin of oligodendrogenic qNSCs remains elusive. Here, through single-cell analysis and lineage tracing, we identify an undefined subpopulation of Gas1high qNSCs in dorsal SVZ, distinct from Gli1+ qNSCs. These cells originate from embryonic Gas1high dorsal radial glia, and persist into the aged SVZ. Remarkably, they are multipotent and more gliogenic than Gas1low/- qNSCs, continuously generating oligodendrocytes in the adult and aged brain, and can be mobilized for myelin repair upon demyelination. Together, our study uncovers a subpopulation of dorsally derived, multipotent long-term qNSCs in the adult and aged SVZ with enhanced gliogenic potential, shedding light on the heterogeneity and plasticity of NSCs in normal, aging, and disease conditions.

Developmental innovation of inferior ovaries and flower sex orchestrated by KNOX1 in cucurbits

In flowering plants, inferior ovaries are key morphological innovations that evolved multiple times from superior ovaries to protect female parts of the flower. However, the developmental mechanisms underlying inferior ovary formation remain largely unknown. Comparative spatial transcriptome mapping and cell lineage reconstructions in developing floral buds of cucumber and tomato, which have inferior and superior ovaries, respectively, revealed that inferior ovaries develop from accelerated receptacle growth resulting from the continuous activity of meristematic stems cells at the base of the cucumber floral organs. Genetic knockout of a receptacle-specific KNOX1 transcription factor in cucumber caused arrest in receptacle growth and yielded bisexual flowers with superior ovaries similar to those of tomato. Here we provide developmental and mechanistic insights into inferior ovary formation and sex determination in cucurbits.

Stem Cell Markers LGR5, LGR4 and Their Immediate Signalling Partners are Dysregulated in Preeclampsia

Leucine-rich repeat-containing G protein-coupled receptors 5/4 (LGR5/LGR4) are critical stem cell markers in epithelial tissues including intestine. They agonise wingless-related integration site (WNT) signalling. Until now, LGR5/LGR4 were uncharacterised in placenta, where analogous functions may exist. We characterised LGR5/LGR4, their ligands/targets in human placenta, with further assessments on dysregulation in preeclampsia/fetal growth restriction (FGR). LGR5 mRNA was unaltered in first trimester (n = 11), preterm (n = 9) and term (n = 11) placental lysate. LGR5 was enriched in human trophoblast stem cells (hTSCs) and downregulated with differentiation to extravillous trophoblasts (p < 0.0215) and syncytiotrophoblasts (p < 0.0350). In situ hybridisation localised LGR5 to unique, proliferative MKI67 + mononuclear trophoblasts underlying syncytium which concurred with proposed progenitor identities in single-cell transcriptomics. LGR5 expression was significantly reduced in placentas from early-onset preeclampsia (p < 0.0001, n = 81 versus n = 19 controls), late-onset preeclampsia (p = 0.0046, n = 20 versus n = 33 controls) and FGR (p = 0.0031, n = 34 versus n = 17 controls). LGR4 was elevated in first trimester versus preterm and term placentas (p = 0.0412), in placentas with early-onset preeclampsia (p = 0.0148) and in FGR (p = 0.0417). Transcriptomic analysis and in vitro hTSC differentiation to both trophoblast lineages suggested LGR4 increases with differentiation. Single-nucleus RNA sequencing of placental villous samples supported LGR5 and LGR4 localisation findings. Hypoxia/proinflammatory cytokine treatment modelling elements experienced by the placenta in placental insufficiency pathogenesis did not significantly alter LGR5/LGR4. Ligands R-spondins 1/3/4, and neutralising targets ring finger protein 43 (RNF43) and zinc and ring finger 3 (ZNRF3) were also reduced in placentas from preeclamptic pregnancies. This study is the first to describe LGR5/LGR4 and their signalling partner expression in human placenta. Their dysregulations in the preeclamptic placenta allude to disruptions to integral trophoblast stem cell function/differentiation that may occur during placental development related to WNT signalling.

Generation of the Krt24-CreERT2 Mouse Line Targeting Outer Bulge Hair Follicle Cells

Outer bulge (OB) hair follicle stem cells (HFSCs) play a crucial role in maintaining hair follicle structural stability and regulating the hair follicle cycle. Previous studies demonstrated that keratin 24 (Krt24) exhibits spatiotemporally restricted expression in OB HFSCs. Here, we report the generation of the Krt24-CreERT2 mouse line. When crossed with Rosa26LSL-tdTomato or Rosa26LSL-DTR reporter lines, offspring exhibited specific labeling (Krt24-CreERT2;Rosa26LSL-tdTomato) or ablation (Krt24-CreERT2;Rosa26LSL-DTR) of Krt24+ cells. In Krt24-CreERT2;Rosa26LSL-tdTomato mice, phase-specific tamoxifen (TAM) administration demonstrated spatiotemporal fidelity of Cre activity to endogenous Krt24 expression patterns. Lineage tracing revealed that tdTomato-labeled Krt24+ cells differentiated into the outer root sheath (ORS) during the anagen phase and persisted when hair follicles reentered telogen. Ablation of Krt24+ cells via diphtheria toxin (DT) administration significantly delayed anagen initiation. Mice under continuous depletion of Krt24+ HFSCs experienced substantial mortality after ionizing irradiation. Notably, ionizing radiation triggered a marked expansion of tdTomato-labeled Krt24+ cells, accompanied by maintained hair follicle homeostasis. Taken together, this study established a Krt24-CreERT2 mouse line targeting OB HFSCs, which are essential for hair follicle development and damage repair.

MCL‑1 safeguards activated hair follicle stem cells to enable adult hair regeneration

Hair follicles cycle through expansion, regression and quiescence. To investigate the role of MCL‑1, a BCL‑2 family protein with anti‑apoptotic and apoptosis‑unrelated functions, we delete Mcl‑1 within the skin epithelium using constitutive and inducible systems. Constitutive Mcl‑1 deletion does not impair hair follicle organogenesis but leads to gradual hair loss and elimination of hair follicle stem cells. Acute Mcl‑1 deletion rapidly depletes activated hair follicle stem cells and completely blocks depilation‑induced hair regeneration in adult mice, while quiescent hair follicle stem cells remain unaffected. Single‑cell RNA‑seq profiling reveals the engagement of P53 and DNA mismatch repair signaling in hair follicle stem cells upon depilation‑induced activation. Trp53 deletion rescues hair regeneration defects caused by acute Mcl‑1 deletion, highlighting a critical interplay between P53 and MCL‑1 in balancing proliferation and death. The ERBB pathway plays a central role in sustaining the survival of adult activated hair follicle stem cells by promoting MCL‑1 protein expression. Remarkably, the loss of a single Bak allele, a pro‑apoptotic Bcl‑2 effector gene, rescues Mcl‑1 deletion‑induced defects in both hair follicles and mammary glands. These findings demonstrate the pivotal role of MCL‑1 in inhibiting proliferation stress‑induced apoptosis when quiescent stem cells activate to fuel tissue regeneration.

Sustained NF-κB activation allows mutant alveolar stem cells to co-opt a regeneration program for tumor initiation

Disruptions to regulatory signals governing stem cell fate open the pathway to tumorigenesis. To determine how these programs become destabilized, we fate-map thousands of murine wild-type and KrasG12D-mutant alveolar type II (AT2) stem cells in vivo and find evidence for two independent AT2 subpopulations marked by distinct tumorigenic capacities. By combining clonal analyses with single-cell transcriptomics, we unveil striking parallels between lung regeneration and tumorigenesis that implicate Il1r1 as a common activator of AT2 reprogramming. We show that tumor evolution proceeds through the acquisition of lineage infidelity and reversible transitions between mutant states, which, in turn, modulate wild-type AT2 dynamics. Finally, we discover how sustained nuclear factor κB (NF-κB) activation sets tumorigenesis apart from regeneration, allowing mutant cells to subvert differentiation in favor of tumor growth.

Destabilisation of bam transcripts terminates the mitotic phase of Drosophila female germline differentiation

The tight control of the mitotic phase of differentiation is crucial to prevent tumourigenesis while securing tissue homeostasis. In the Drosophila female germline, differentiation involves precisely four mitotic divisions, and accumulating evidence suggests that bag of marbles (bam), the initiator of differentiation, is also involved in controlling the number of divisions. To test this hypothesis, we depleted Bam from differentiating cells and found a reduced number of mitotic divisions. We examined the regulation of Bam using RNA imaging methods and found that the bam 3' UTR conveys instability to the transcript in the eight-cell cyst and early 16-cell cyst. We show that the RNA-binding protein Rbp9 is responsible for timing bam mRNA decay. Rbp9 itself is part of a sequential cascade of RNA-binding proteins activated downstream of Bam, and we show that it is regulated through a change in transcription start site, driven by Rbfox1. Altogether, we propose a model in which Bam expression at the beginning of differentiation initiates a series of events that eventually terminates the Bam expression domain.

Unravelling neuroregenerative and neuroprotective roles of Wnt/β-catenin pathway in ischemic stroke: Insights into molecular mechanisms

Stroke is a serious condition often resulting in mortality or long-term disability, causing cognitive, memory, and motor impairments. A reduction in cerebral blood flow below critical levels defines the ischemic core and penumbra: the core undergoes irreversible damage, while the penumbra remains viable but functionally impaired. This functional impairment activates complex cell signaling pathways that determine cell survival or death, making the penumbra a key target for therapeutic interventions to prevent further damage. The Wnt/β-catenin (WβC) signaling pathway has emerged as a potential neuroprotective mechanism, promoting neurogenesis, angiogenesis, neuronal connectivity, and maintaining blood-brain barrier integrity after stroke. Activation of the WβC pathway also mitigates oxidative stress, inflammation, and apoptosis in ischemic regions, enhancing its neuroprotective effects. However, the overexpression of GSK3β and DKK1, or the presence of their agonists, can counteract these benefits. This review explores the therapeutic potential of WβC signaling, highlighting the effects of pharmacological modulation through antagonists, agonists, synthetic chemicals, natural products, stem cells, and macromolecules in preclinical models of ischemic stroke. While preclinical evidence supports the benefits of WβC activation, its role in human stroke requires further investigation. Additionally, the review discusses the potential adverse effects of prolonged WβC activation and suggests strategies to mitigate them. Overall, WβC signaling holds promise as a therapeutic target, offering insights into stroke pathophysiology and informing the development of novel treatment strategies.

Epigenetic Regulatory Processes Involved in the Establishment and Maintenance of Skin Homeostasis-The Role of Microbiota

Epigenetic mechanisms are central to the regulation of all biological processes. This manuscript reviews the current understanding of diverse epigenetic modifications and their role in the establishment and maintenance of normal skin functions. In healthy skin, these mechanisms allow for the precise control of gene expression, facilitating the dynamic balance between cell proliferation and differentiation necessary for effective barrier function. Furthermore, as the skin ages, alterations in epigenetic marks can lead to impaired regenerative capacity and increased susceptibility to environmental stressors. The interaction between skin microbiota and epigenetic regulation will also be explored, highlighting how microbial communities can influence skin health by modulating the host gene expression. Future research should focus on the development of targeted interventions to promote skin development, resilience, and longevity, even in an ever-changing environment. This underscores the need for integrative approaches to study these complex regulatory networks.

Biology and medicine on ocular surface restoration: Advancements and limits of limbal stem cell deficiency treatments

Ocular vision can be hampered by corneal damages, sensibly reducing patients' quality of life and having important social and economic consequences. Ocular surface diseases, which often lead to corneal opacities with visual impairment are the most severe forms of the Limbal Stem Cell Deficiency (LSCD). The present review provides an updated perspective on the available treatments for LSCD, focusing on clinical and biological features, as well as critical points to monitor during clinical translation. Recently developed surgical treatments for LSCD are described, along with their benefits and limitations, with the aim of addressing the issue of correct patient selection. Autologous surgical approaches have been attempted, such as conjunctival limbal autograft (CLAU), simple limbal epithelial transplantation (SLET), and others. Allogeneic limbal stem cell transplantation represents an alternative but carries risk of rejection and requires immunosuppression. Other potential treatments are based on induced pluripotent stem cells (iPSCs), but they require further investigation. The development of advanced therapy medicinal products (ATMPs) such as cultivated limbal epithelial transplantation (CLET), or the use of other epithelia as cultivated oral mucosal epithelial cell transplantation (COMET), has opened additional therapeutic possibilities. Some common critical issues in clinical translation are described, such as patient selection, biopsy procurement, or the use of human/animal derived components, which require rigorous validation to ensure safety and efficacy. Personalized medicine is a promising field for ocular surface restoration, where long-term follow-up studies and standardized criteria are crucial to evaluate the efficacy of these treatments and their cost-effectiveness in providing high-value healthcare.

Primary Cilia Regulate the Homeostasis and Regeneration of the Stem Cell Niche in the Tooth

Primary cilia, functioning as crucial hubs for signal sensing and transduction, are integral to the development and maintenance of homeostasis across various organs. However, their roles in tooth homeostasis and repair remain inadequately understood. In this study, we reveal an indispensable role for primary cilia in regulating the homeostasis and regeneration of teeth, primarily through the regulation of cell proliferation. Using cilium-deficient mice, we demonstrate that disruption of ciliary homeostasis leads to abnormal tooth morphology, stunted growth and notably impaired tooth repair. RNA sequencing reveals a dysregulation in genes associated with various biological processes such as cell proliferation, differentiation, and cycle regulation. Furthermore, we show that cilium-deficient mice display reduced cell proliferation. Our findings highlight a critical function for primary cilia in the regulation of tooth homeostasis and regeneration and have important implications for the development of tooth regeneration therapies.

Zinc Finger Proteins in Colorectal Cancer: Insights into Molecular Mechanisms and Therapeutic Implications

Colorectal cancer ranks as one of the most common malignancies globally, with mortality rates second only to lung cancer. Despite improvements in diagnostic and therapeutic approaches, long-term survival rates for colorectal cancer patients remain unsatisfactory. The onset and progression of this disease involve intricate molecular mechanisms, influenced by a range of biological factors. Zinc finger proteins play a critical role in these processes, impacting tumor development and patient prognosis. This review summarizes current research on zinc finger proteins in colorectal cancer, highlighting key findings and advancements in understanding their role in tumor biology.

Resilience of the replacing dentition in adult reptiles

The dentition is critical to animal survival and teeth are present in modern vertebrates including teleost fish, sharks, amphibians, mammals and reptiles. The developmental processes that give rise to teeth are not just preserved through evolution but also share high level of similarity with the embryogenesis of other ectodermal organs. In this review we go beyond the embryonic phase of tooth development to life-long tooth replacement. We will address the origins of successional teeth, the location of putative tissue-resident stem cells, how de novo tooth formation continues throughout life and how teeth are shed in a spatially and temporally controlled manner. We review the evidence that the dental epithelium, which is the earliest recognizable dental structure in the reptilian dentition, serves as a putative niche for tissue-resident epithelial stem cells and recent molecular findings from transcriptomics carried out in reptilian dentitions. We discuss how odontoclasts resorb the primary tooth allowing eruption of the successional tooth. The reptiles, particularly lizards, are emerging as some of the most accessible animals to study tooth replacement which has relevance to evolution of the dentition and human dental disorders.

Materials-based hair follicle engineering: Basic components and recent advances

The hair follicle (HF) is a significant skin appendage whose primary function is to produce the hair shaft. HFs are a non-renewable resource; skin damage or follicle closure may lead to permanent hair loss. Advances in biomaterials and biomedical engineering enable the feasibility of manipulating the HF-associated cell function for follicle reconstruction via rational design. The regeneration of bioengineered HF addresses the issue of limited resources and contributes to advancements in research and applications in hair loss treatment, HF development, and drug screening. Based on these requirements, this review summarizes the basic and recent advances in hair follicle regulation, including four components: acquisition of stem cells, signaling pathways, materials, and engineering methods. Recent studies have focused on efficiently combining these components and reproducing functionality, which would boost fabrication in HF rebuilding ex vivo, thereby eliminating the obstacles of transplantation into animals to promote mature development.

Dermal TRPV1 innervations engage a macrophage- and fibroblast-containing pathway to activate hair growth in mice

Pain, detected by nociceptors, is an integral part of injury, yet whether and how it can impact tissue physiology and recovery remain understudied. Here, we applied chemogenetics in mice to locally activate dermal TRPV1 innervations in naive skin and found that it triggered new regenerative cycling by dormant hair follicles (HFs). This was preceded by rapid apoptosis of dermal macrophages, mediated by the neuropeptide calcitonin gene-related peptide (CGRP). TRPV1 activation also triggered a macrophage-dependent induction of osteopontin (Spp1)-expressing dermal fibroblasts. The neuropeptide CGRP and the extracellular matrix protein Spp1 were required for the nociceptor-triggered hair growth. Finally, we showed that epidermal abrasion injury induced Spp1-expressing dermal fibroblasts and hair growth via a TRPV1 neuron and CGRP-dependent mechanism. Collectively, these data demonstrated a role for TRPV1 nociceptors in orchestrating a macrophage and fibroblast-supported mechanism to promote hair growth and enabling the efficient restoration of this mechano- and thermo-protective barrier after wounding.

The molecular anatomy of cashmere goat hair follicle during cytodifferentiation stage

BACKGROUND

Cashmere, named as "soft gold", derives from the secondary hair follicles (SHFs) of cashmere goat which is vital to Northwest China's economy. The cytodifferentiation stage (E120), mirroring the complete hair follicle (HF) structure of adult goats and marking a critical phase in SHF development. Therefore, this study aims to enhance the understanding of SHF development and its impact on fiber quality, informing breeding strategies.

RESULTS

From the scRNA-seq data analysis, the intricate processes and transcriptional dynamics of inner layer cell differentiation of HFs were unveiled in this study. we identified nine cell populations during cytodifferentiation and key structures such as the hair shaft and inner root sheath. And we discovered three main inner layer lineages and seven subpopulations, clarifying their roles in specialization and signaling. Pseudotime mapping analysis showed cell evolution from early stage to mature stages marked by unique gene expressions, and the intermediate stage on the differentiation of each lineage was revealed. The identification and spatial localization of specific transcription factors, such as GATA3, LEF1 and PRDM1, as well as keratin genes highlight regulatory pathways involved in HF development, which was further validated by immunofluorescence. These findings suggested the potential strategies to improve fiber quality, and the discovery of diverse cell types and their developmental molecular mechanisms, particularly in this species-specific context, offered a nuanced view of the regulatory mechanisms driving HF development in cashmere goats.

CONCLUSION

Overall, these findings provide a systematic molecular atlas of skin, defining three major branches and cell states of inner layer cells of HF, and determining how the branch-specific transcription factors, keratins, and signals coordinate HF morphogenesis during cytodifferentiation stage. This research not only advances skin tissue research in goats but also holds broader implications for the understanding of HF regeneration and development across various species.

Transcription factor KROX20 marks epithelial stem cells for hair follicle formation

Epidermal stem cells control homeostasis and regeneration of skin and hair. In the hair follicle (HF) bulge of mammals, populations of slow-cycling stem cells regenerate the HF during cyclical rounds of anagen (growth), telogen (quiescence), and catagen (regression). Multipotent epidermal cells are also present in the HF above the bulge area, contributing to the formation and maintenance of sebaceous gland and upper and middle portions of the HF. Here, we report that the transcription factor Krox20 is enriched in an epidermal stem cell population located in the upper/ middle HF. Expression analyses and lineage tracing using inducible Krox20-CreERT showed that Krox20-lineage cells migrate out of this HF region and contribute to the formation of bulge in the HF, serving as ancestors of bulge stem cells. In vivo depletion of these cells arrests HF morphogenesis. This study identifies a novel marker for an epidermal stem cell population that is indispensable for hair homeostasis.

Trophoblast Side-Population Markers are Dysregulated in Preeclampsia and Fetal Growth Restriction

Dysregulated progenitor cell populations may contribute to poor placental development and placental insufficiency pathogenesis. Side-population cells possess progenitor properties. Recent human trophoblast side-population isolation identified enrichment of 8 specific genes (CXCL8, ELL2, GATA6, HK2, HLA-DPB1, INTS6, SERPINE3 and UPP1) (Gamage et al. 2020, Stem Cell Rev Rep). We characterised these trophoblast side-population markers in human placenta and in placental insufficiency disorders: preeclampsia and fetal growth restriction (FGR). Trophoblast side-population markers localised to mononuclear trophoblasts lining the placental villous basement membrane in preterm control, preeclamptic and FGR placental sections (n = 3, panel of 3 markers/serial section). Analysis of single-cell transcriptomics of an organoid human trophoblast stem cell (hTSC) to extravillous trophoblast (EVT) differentiation model (Shannon et al. 2022, Development) identified that all side-population genes were enriched in mononuclear trophoblast and trophoblasts committed to differentiation under hTSC culture conditions. In vitro validation via 96 h time course hTSC differentiation to EVTs or syncytiotrophoblasts (n = 5) demonstrated ELL2 and HK2 increased with differentiation (p < 0.0024, p < 0.0039 respectively). CXCL8 and HLA-DPB1 were downregulated (p < 0.030, p < 0.011 respectively). GATA6 and INTS6 increased with EVT differentiation only, and UPP1 reduced with syncytialisation. SERPINE3 was undetectable. Trophoblast side-population marker mRNA was measured in human placentas (< 34-weeks' gestation; n = 78 preeclampsia, n = 30 FGR, and n = 18 gestation-matched controls). ELL2, HK2 and CXCL8 were elevated in preeclamptic (p = 0.0006, p < 0.0001, p = 0.0335 respectively) and FGR placentas (p = 0.0065, p < 0.0001, p = 0.0001 respectively) versus controls. Placental GATA6 was reduced in pregnancies with preeclampsia and FGR (p = 0.0014, p = 0.0146 respectively). Placental INTS6 was reduced with FGR only (p < 0.0001). This study identified the localisation of a unique trophoblast subset enriched for side-population markers. Aberrant expression of some side-population markers may indicate disruptions to unique trophoblast subtypes in placental insufficiency.

A feedback loop between plakophilin 4 and YAP signaling regulates keratinocyte differentiation

The Hippo signaling pathway is an important regulator of organ growth and differentiation, and its deregulation contributes to the development of cancer. The activity of its downstream targets YAP/TAZ depends on adherens junctions. Plakophilin 4 (PKP4) is a cell-type specific adherens junction protein expressed in the proliferating cells of the epidermis. Here, we show that PKP4 diminishes proliferation as well as differentiation. Depletion of PKP4 increased proliferation but at the same time induced premature epidermal differentiation. PKP4 interacted with several Hippo pathway components, including the transcriptional co-activators YAP/TAZ, and promoted nuclear YAP localization and target gene expression. In differentiated keratinocytes, PKP4 recruited LATS and YAP to cell junctions where YAP is transcriptionally inactive. YAP depletion, on the other hand, reduced PKP4 levels and keratinocyte adhesion indicative of a feedback mechanism controlling adhesion, proliferation, and differentiation by balancing YAP functions.

Induction of in vivo-like ciliation in confluent monolayers of re-differentiated equine oviduct epithelial cells†

We recently developed re-differentiated equine oviduct epithelial cell (REOEC) monolayers demonstrating various in vivo morphological characteristics, but lacking secondary ciliation. In this study, we evaluated the effects of fetal bovine serum, reproductive steroid hormones, Wnt- and Notch ligands and inhibitors, and different EOEC seeding densities, in both conventional wells and on microporous membranes, on EOEC morphology and, in particular, secondary ciliation. REOEC monolayers were assessed by confocal microscopy after combined staining of nuclei, cilia, and the cytoskeleton. Only Wnt ligands, Notch inhibitors and oviduct explant cell concentration affected EOEC morphology. Undesirable epithelial-mesenchymal transition was observed in REOEC monolayers exposed to Wnt3a containing medium and Wnt ligand CHIR 99021. With respect to secondary ciliation, only the combined effect of oviduct explant cell concentration and Notch inhibition steered REOEC monolayers to in vivo-like ciliation patterns. De-differentiated EOECs, formed 10 days after oviduct explant cell seeding, were reseeded on inserts; only at initial oviduct explant cell concentrations of 1 and 5 × 106 cells per well was the formation of REOEC monolayers with a high rate of diffuse ciliation supported. Within 1 month after air-liquid interface introduction, >40% and >20% of the REOECs showed secondary cilia, respectively. At higher oviduct explant cell seeding densities secondary ciliation was not supported after re-differentiation. Additionally, Notch inhibition helped boost secondary ciliation rates to >60% in REOEC monolayers with diffuse ciliation only. These monolayers demonstrated higher clathrin expression under follicular phase conditions. Overall, the ciliated REOEC monolayers better resemble in vivo oviduct epithelial cells than previous models.

Standardization and quality assessment for human intestinal organoids

To enhance the practical application of intestinal organoids, it is imperative to establish standardized guidelines. This proposed standardization outlines a comprehensive framework to ensure consistency and reliability in the development, characterization, and application of intestinal organoids. The recommended guidelines encompass crucial parameters, including culture conditions, critical quality attributes, quality control measures, and functional assessments, aimed at fostering a standardized approach across diverse research initiatives. The implementation of these guidelines is anticipated to significantly contribute to the reproducibility and comparability of results in the burgeoning field of intestinal organoid research.

Stem cells tightly regulate dead cell clearance to maintain tissue fitness

Billions of cells are eliminated daily from our bodies1-4. Although macrophages and dendritic cells are dedicated to migrating and engulfing dying cells and debris, many epithelial and mesenchymal tissue cells can digest nearby apoptotic corpses1-4. How these non-motile, non-professional phagocytes sense and eliminate dying cells while maintaining their normal tissue functions is unclear. Here we explore the mechanisms that underlie their multifunctionality by exploiting the cyclical bouts of tissue regeneration and degeneration during hair cycling. We show that hair follicle stem cells transiently unleash phagocytosis at the correct time and place through local molecular triggers that depend on both lipids released by neighbouring apoptotic corpses and retinoids released by healthy counterparts. We trace the heart of this dual ligand requirement to RARγ-RXRα, whose activation enables tight regulation of apoptotic cell clearance genes and provides an effective, tunable mechanism to offset phagocytic duties against the primary stem cell function of preserving tissue integrity during homeostasis. Finally, we provide functional evidence that hair follicle stem cell-mediated phagocytosis is not simply redundant with professional phagocytes but rather has clear benefits to tissue fitness. Our findings have broad implications for other non-motile tissue stem or progenitor cells that encounter cell death in an immune-privileged niche.

The development of hair follicles and nail

The hair follicle and nail unit develop and regenerate through epithelial-mesenchymal interactions. Here, we review some of the key signals and molecular interactions that regulate mammalian hair follicle and nail formation during embryonic development and how these interactions are reutilized to promote their regeneration during adult homeostasis and in response to skin wounding. Finally, we highlight the role of some of these signals in mediating human hair follicle and nail conditions.

Ontogeny of Skin Stem Cells and Molecular Underpinnings

Skin stem cells (SCs) play a pivotal role in supporting tissue homeostasis. Several types of SCs are responsible for maintaining and regenerating skin tissue. These include bulge SCs and others residing in the interfollicular epidermis, infundibulum, isthmus, sebaceous glands, and sweat glands. The emergence of skin SCs commences during embryogenesis, where multipotent SCs arise from various precursor populations. These early events set the foundation for the diverse pool of SCs that will reside in the adult skin, ready to respond to tissue repair and regeneration demands. A network of molecular cues regulates skin SC behavior, balancing quiescence, self-renewal, and differentiation. The disruption of this delicate equilibrium can lead to SC exhaustion, impaired wound healing, and pathological conditions such as skin cancer. The present review explores the intricate mechanisms governing the development, activation, and differentiation of skin SCs, shedding light on the molecular signaling pathways that drive their fate decisions and skin homeostasis. Unraveling the complexities of these molecular drivers not only enhances our fundamental knowledge of skin biology but also holds promise for developing novel strategies to modulate skin SC fate for regenerative medicine applications, ultimately benefiting patients with skin disorders and injuries.

Transcriptional responses consistent with perturbation in dermo-epidermal homeostasis in septic sole ulceration

The aim of this study was to evaluate transcriptional changes in sole epidermis and dermis of bovine claws with septic sole ulceration of the lateral claw. Assessment included changes in transcripts orchestrating epidermal homeostatic processes including epidermal proliferation, differentiation, inflammation, and cell signaling. Sole epidermis and dermis was removed from region 4 of lesion-bearing lateral and lesion-free medial claws of pelvic limbs in multiparous, lactating Holstein cows. Control sole epidermis and dermis was obtained from region 4 of lateral claws of normal pelvic limbs. Transcript abundances were evaluated by real-time QPCR and relative expression analyzed by ANOVA. Relative to normal lateral claws, sole epidermis and dermis in ulcer-bearing claws exhibited downregulation of genes associated with growth factors, growth factor receptors, activator protein 1 (AP-1) and proto-oncogene (CMYC) transcription components, cell cycle elements, lateral cell-to-cell signaling elements and structures of early and late keratinocyte differentiation. These changes were accompanied by upregulation of pro-inflammatory transcripts interleukin 1 α (IL1A), interleukin1 β (IL1B), interleukin 1 receptor 1 (IL1R1), inducible nitric oxide synthase (NOS2), the inflammasome components NOD like receptor protein 3 (NLRP3), pyrin and caspase recruitment domain (PYCARD), and caspase-1 interleukin converting enzyme (CASPASE), the matrix metalloproteinases (MMP2 and MMP9), and anti-inflammatory genes interleukin 1 receptor antagonist (IL1RN) and interleukin1 receptor 2 (IL1R2). Transcript abundance varied across epidermis and dermis from the ulcer center, margin and epidermis and dermis adjacent to the lesion. Sole epidermis and dermis of lesion-free medial claws exhibited changes paralleling those in the adjacent lateral claws in an environment lacking inflammatory transcripts and downregulated IL1A, interleukin 18 (IL18), tumor necrosis factor α (TNFA) and NOS2. These data imply perturbations in signal pathways driving epidermal proliferation and differentiation are associated with, but not inevitably linked to epidermis and dermis inflammation. Further work is warranted to better define the role of crushing tissue injury, sepsis, metalloproteinase activity, and inflammation in sole ulceration.

Characterization of Skin Interfollicular Stem Cells and Early Transit Amplifying Cells during the Transition from Infants to Young Children

In the interfollicular epidermis, keratinocyte stem cells (KSC) generate a short-lived population of transit amplifying (TA) cells that undergo terminal differentiation after several cell divisions. Recently, we isolated and characterized a highly proliferative keratinocyte cell population, named "early" TA (ETA) cell, representing the first KSC progenitor with exclusive features. This work aims to evaluate epidermis, with a focus on KSC and ETA cells, during transition from infancy to childhood. Reconstructed human epidermis (RHE) generated from infant keratinocytes is more damaged by UV irradiation, as compared to RHE from young children. Moreover, the expression of several differentiation and barrier genes increases with age, while the expression of genes related to stemness is reduced from infancy to childhood. The proliferation rate of KSC and ETA cells is higher in cells derived from infants' skin samples than of those derived from young children, as well as the capacity of forming colonies is more pronounced in KSC derived from infants than from young children's skin samples. Finally, infants-KSC show the greatest regenerative capacity in skin equivalents, while young children ETA cells express higher levels of differentiation markers, as compared to infants-ETA. KSC and ETA cells undergo substantial changes during transition from infancy to childhood. The study presents a novel insight into pediatric skin, and sheds light on the correlation between age and structural maturation of the skin.

[Research progress in regulation of hair growth by dermal adipose tissue]

Objective

To summarize the dynamic and synchronized changes between the hair cycle and dermal adipose tissue as well as the impact of dermal adipose tissue on hair growth, and to provide a new research idea for the clinical treatment of hair loss.

Methods

An extensive review of relevant literature both domestic and international was conducted, analyzing and summarizing the impact of dermal adipose precursor cells, mature dermal adipocytes, and the processes of adipogenesis in dermal adipose tissue on the transition of hair cycle phases.

Results

Dermal adipose tissue is anatomically adjacent to hair follicles and closely related to the changes in the hair cycle. The proliferation and differentiation of dermal adipose precursor cells promote the transition of hair cycle from telogen to anagen, while mature adipocytes can accelerate the transition from anagen to catagen of the hair cycle by expressing signaling molecules, with adipogenesis in dermal adipose tissue and hair cycle transition signaling coexistence.

Conclusion

Dermal adipose tissue affects the transition of the hair cycle and regulates hair growth by secreting various signaling molecules. However, the quantity and depth of existing literature are far from sufficient to fully elucidate its prominent role in regulating the hair cycle, and the specific regulatory mechanisms needs to be further studied.

Pilose antler extract promotes hair growth in androgenic alopecia mice by promoting the initial anagen phase

Androgenetic alopecia (AGA) is a prevalent disease in worldwide, local application or oral are often used to treat AGA, however, effective treatments for AGA are currently limited. In this work, we observed the promoting the initial anagen phase effect of pilose antler extract (PAE) on hair regeneration in AGA mice. We found that PAE accelerated hair growth and increased the degree of skin blackness by non-invasive in vivo methods including camera, optical coherence tomography and dermoscopy. Meanwhile, HE staining of sagittal and coronal skin sections revealed that PAE augmented the quantity and length of hair follicles, while also enhancing skin thickness and hair papilla diameter. Furthermore, PAE facilitated the shift of the growth cycle from the telogen to the anagen phase and expedited the proliferation of hair follicle stem cells and matrix cells in mice with AGA. This acceleration enabled the hair follicles to enter the growth phase at an earlier stage. PAE upregulated the expression of the sonic hedgehog (SHH), smoothened receptor, glioma-associated hemolog1 (GLI1), and downregulated the expression of bone morphogenetic protein 4 (BMP4), recombinant mothers against decapentaplegic homolog (Smad) 1 and 5 phosphorylation. This evidence suggests that PAE fosters hair growth and facilitates the transition of the growth cycle from the telogen to the anagen phase in AGA mice. This effect is achieved by enhancing the proliferation of follicle stem cells and matrix cells through the activation of the SHH/GLI pathway and suppression of the BMP/Smad pathway.

Modelling Human Hair Follicles-Lessons from Animal Models and Beyond

The hair follicle is a specialized appendage of the skin that is critical for multiple functions, including thermoregulation, immune surveillance, and sebum production. Mammals are born with a fixed number of hair follicles that develop embryonically. Postnatally, these hair follicles undergo regenerative cycles of regression and growth that recapitulate many of the embryonic signaling pathways. Furthermore, hair cycles have a direct impact on skin regeneration in homeostasis, cutaneous wound healing, and disease conditions such as alopecia. Here, we review the current knowledge of hair follicle formation during embryonic development and the post-natal hair cycle, with an emphasis on the molecular signaling pathways underlying these processes. We then discuss efforts to capitalize on the field's understanding of in vivo mechanisms to bioengineer hair follicles or hair-bearing skin in vitro and how such models may be further improved to develop strategies for hair regeneration.

The Potential Reversible Transition between Stem Cells and Transient-Amplifying Cells: The Limbal Epithelial Stem Cell Perspective

Stem cells (SCs) undergo asymmetric division, producing transit-amplifying cells (TACs) with increased proliferative potential that move into tissues and ultimately differentiate into a specialized cell type. Thus, TACs represent an intermediary state between stem cells and differentiated cells. In the cornea, a population of stem cells resides in the limbal region, named the limbal epithelial stem cells (LESCs). As LESCs proliferate, they generate TACs that move centripetally into the cornea and differentiate into corneal epithelial cells. Upon limbal injury, research suggests a population of progenitor-like cells that exists within the cornea can move centrifugally into the limbus, where they dedifferentiate into LESCs. Herein, we summarize recent advances made in understanding the mechanism that governs the differentiation of LESCs into TACs, and thereafter, into corneal epithelial cells. We also outline the evidence in support of the existence of progenitor-like cells in the cornea and whether TACs could represent a population of cells with progenitor-like capabilities within the cornea. Furthermore, to gain further insights into the dynamics of TACs in the cornea, we outline the most recent findings in other organ systems that support the hypothesis that TACs can dedifferentiate into SCs.

The promising prospect of human hair follicle regeneration in the shadow of new tissue engineering strategies

Hair loss disorder (alopecia) affects numerous people around the world. The low effectiveness and numerous side effects of common treatments have prompted researchers to investigate alternative and effective solutions. Hair follicle (HF) bioengineering is the knowledge of using hair-inductive (trichogenic) cells. Most bioengineering-based approaches focus on regenerating folliculogenesis through manipulation of regulators of physical/molecular properties in the HF niche. Despite the high potential of cell therapy, no cell product has been produced for effective treatment in the field of hair regeneration. This problem shows the challenges in the functionality of cultured human hair cells. To achieve this goal, research and development of new and practical approaches, technologies and biomaterials are needed. Based on recent advances in the field, this review evaluates emerging HF bioengineering strategies and the future prospects for the field of tissue engineering and successful HF regeneration.

Local and systemic mechanisms that control the hair follicle stem cell niche

Hair follicles are essential appendages of the mammalian skin, as hair performs vital functions of protection, thermoregulation and sensation. Hair follicles harbour exceptional regenerative abilities as they contain multiple somatic stem cell populations such as hair follicle stem cells (HFSCs) and melanocyte stem cells. Surrounding the stem cells and their progeny, diverse groups of cells and extracellular matrix proteins are organized to form a microenvironment (called 'niche') that serves to promote and maintain the optimal functioning of these stem cell populations. Recent studies have shed light on the intricate nature of the HFSC niche and its crucial role in regulating hair follicle regeneration. In this Review, we describe how the niche serves as a signalling hub, communicating, deciphering and integrating both local signals within the skin and systemic inputs from the body and environment to modulate HFSC activity. We delve into the recent advancements in identifying the cellular and molecular nature of the niche, providing a holistic perspective on its essential functions in hair follicle morphogenesis, regeneration and ageing.

Hair Follicle Transit-Amplifying Cells Phagocytose Dead Cells after Radiotherapeutic and Chemotherapeutic Injuries for Timely Regeneration

Efficient clearance of dead cells is critical for tissue regeneration after injuries. How dead cells are removed from the skin after radiotherapy and chemotherapy is unclear. In this study, we found that radiotherapeutic and chemotherapeutic damage induced extensive apoptosis of highly proliferative transit-amplifying cells in hair follicles. These apoptotic cells disappeared rapidly in the early stage of regenerative attempts, and the lost structures were regenerated with transient and low-level inflammation. Without the recruitment of macrophages as scavengers, the dying cells were engulfed directly by adjacent surviving transit-amplifying cells, which produced mature phagosomes through fusion with lysosomes in a manner similar to professional phagocytosis and remained active in proliferation. Autophagy did not contribute significantly to the clearance of engulfed cell debris. Perturbing phagocytosis in the transit-amplifying cells hindered apoptotic cell removal, delayed structural recovery, and aggravated hair loss. Therefore, transit-amplifying cells are capacitated with both proliferative and efferocytic functions that facilitate tissue regeneration after tissue injury.

Deciphering the molecular mechanisms of stem cell dynamics in hair follicle regeneration

Hair follicles, which are connected to sebaceous glands in the skin, undergo cyclic periods of regeneration, degeneration, and rest throughout adult life in mammals. The crucial function of hair follicle stem cells is to maintain these hair growth cycles. Another vital aspect is the activity of melanocyte stem cells, which differentiate into melanin-producing melanocytes, contributing to skin and hair pigmentation. Sebaceous gland stem cells also have a pivotal role in maintaining the skin barrier by regenerating mature sebocytes. These stem cells are maintained in a specialized microenvironment or niche and are regulated by internal and external signals, determining their dynamic behaviors in homeostasis and hair follicle regeneration. The activity of these stem cells is tightly controlled by various factors secreted by the niche components around the hair follicles, as well as immune-mediated damage signals, aging, metabolic status, and stress. In this study, we review these diverse stem cell regulatory and related molecular mechanisms of hair regeneration and disease conditions. Molecular insights would provide new perspectives on the disease mechanisms as well as hair and skin disorder treatment.

Single-cell sequencing analysis within biologically relevant dimensions

The currently predominant approach to transcriptomic and epigenomic single-cell analysis depends on a rigid perspective constrained by reduced dimensions and algorithmically derived and annotated clusters. Here, we developed Seqtometry (sequencing-to-measurement), a single-cell analytical strategy based on biologically relevant dimensions enabled by advanced scoring with multiple gene sets (signatures) for examination of gene expression and accessibility across various organ systems. By utilizing information only in the form of specific signatures, Seqtometry bypasses unsupervised clustering and individual annotations of clusters. Instead, Seqtometry combines qualitative and quantitative cell-type identification with specific characterization of diverse biological processes under experimental or disease conditions. Comprehensive analysis by Seqtometry of various immune cells as well as other cells from different organs and disease-induced states, including multiple myeloma and Alzheimer's disease, surpasses corresponding cluster-based analytical output. We propose Seqtometry as a single-cell sequencing analysis approach applicable for both basic and clinical research.

Hallmarks of stemness in mammalian tissues

All adult tissues experience wear and tear. Most tissues can compensate for cell loss through the activity of resident stem cells. Although the cellular maintenance strategies vary greatly between different adult (read: postnatal) tissues, the function of stem cells is best defined by their capacity to replace lost tissue through division. We discuss a set of six complementary hallmarks that are key enabling features of this basic function. These include longevity and self-renewal, multipotency, transplantability, plasticity, dependence on niche signals, and maintenance of genome integrity. We discuss these hallmarks in the context of some of the best-understood adult stem cell niches.

Stem cell heterogeneity, plasticity, and regulation

As a population of homogeneous cells with both self-renewal and differentiation potential, stem cell pools are highly compartmentalized and contain distinct subsets that exhibit stable but limited heterogeneity during homeostasis. However, their striking plasticity is showcased under natural or artificial stress, such as injury, transplantation, cancer, and aging, leading to changes in their phenotype, constitution, metabolism, and function. The complex and diverse network of cell-extrinsic niches and signaling pathways, together with cell-intrinsic genetic and epigenetic regulators, tightly regulate both the heterogeneity during homeostasis and the plasticity under perturbation. Manipulating these factors offers better control of stem cell behavior and a potential revolution in the current state of regenerative medicine. However, disruptions of normal regulation by genetic mutation or excessive plasticity acquisition may contribute to the formation of tumors. By harnessing innovative techniques that enhance our understanding of stem cell heterogeneity and employing novel approaches to maximize the utilization of stem cell plasticity, stem cell therapy holds immense promise for revolutionizing the future of medicine.

Acute irradiation causes a long-term disturbance in the heterogeneity and gene expression profile of medullary thymic epithelial cells

The thymus has the ability to regenerate from acute injury caused by radiation, infection, and stressors. In addition to thymocytes, thymic epithelial cells in the medulla (mTECs), which are crucial for T cell self-tolerance by ectopically expressing and presenting thousands of tissue-specific antigens (TSAs), are damaged by these insults and recover thereafter. However, given recent discoveries on the high heterogeneity of mTECs, it remains to be determined whether the frequency and properties of mTEC subsets are restored during thymic recovery from radiation damage. Here we demonstrate that acute total body irradiation with a sublethal dose induces aftereffects on heterogeneity and gene expression of mTECs. Single-cell RNA-sequencing (scRNA-seq) analysis showed that irradiation reduces the frequency of mTECs expressing AIRE, which is a critical regulator of TSA expression, 15 days after irradiation. In contrast, transit-amplifying mTECs (TA-mTECs), which are progenitors of AIRE-expressing mTECs, and Ccl21a-expressing mTECs, were less affected. Interestingly, a detailed analysis of scRNA-seq data suggested that the proportion of a unique mTEC cluster expressing Ccl25 and a high level of TSAs was severely decreased by irradiation. In sum, we propose that the effects of acute irradiation disrupt the heterogeneity and properties of mTECs over an extended period, which potentially leads to an impairment of thymic T cell selection.

Basic principles of hair follicle structure, morphogenesis, and regeneration

Diseases affecting the hair follicle are common in domestic animals, but despite the importance of an intact skin barrier and a fully functional hair coat, knowledge about the detailed morphological features and the diversity of these complex mini-organs are often limited, although mandatory to evaluate skin biopsies with a history of alopecia. The factors that regulate the innate hair follicle formation and the postnatal hair cycle are still not completely understood in rodents, only rudimentarily known in humans, and are poorly understood in our companion animals. This review aims to summarize the current knowledge about hair follicle and hair shaft anatomy, the arrangement of hair follicles, hair follicle morphogenesis in the embryo, and the lifelong regeneration during the postnatal hair cycle in domestic animals. The role of follicular stem cells and the need for a multitude of interacting signaling events during hair follicle morphogenesis and regeneration is unquestioned. Because of the lack of state of the art methods that can be applied in rodents but are not feasible in companion animals, most of the information in this review is based on rodent studies. However, the few data from domestic animals that are available will be discussed, and it can be assumed that at least the principal molecular mechanisms are similar in rodents and other species.

Gli1 marks a sentinel muscle stem cell population for muscle regeneration

Adult skeletal muscle regeneration is mainly driven by muscle stem cells (MuSCs), which are highly heterogeneous. Although recent studies have started to characterize the heterogeneity of MuSCs, whether a subset of cells with distinct exists within MuSCs remains unanswered. Here, we find that a population of MuSCs, marked by Gli1 expression, is required for muscle regeneration. The Gli1+ MuSC population displays advantages in proliferation and differentiation both in vitro and in vivo. Depletion of this population leads to delayed muscle regeneration, while transplanted Gli1+ MuSCs support muscle regeneration more effectively than Gli1- MuSCs. Further analysis reveals that even in the uninjured muscle, Gli1+ MuSCs have elevated mTOR signaling activity, increased cell size and mitochondrial numbers compared to Gli1- MuSCs, indicating Gli1+ MuSCs are displaying the features of primed MuSCs. Moreover, Gli1+ MuSCs greatly contribute to the formation of GAlert cells after muscle injury. Collectively, our findings demonstrate that Gli1+ MuSCs represents a distinct MuSC population which is more active in the homeostatic muscle and enters the cell cycle shortly after injury. This population functions as the tissue-resident sentinel that rapidly responds to injury and initiates muscle regeneration.

Stem Cells: Engines of Plant Growth and Development

The development of both animals and plants relies on populations of pluripotent stem cells that provide the cellular raw materials for organ and tissue formation. Plant stem cell reservoirs are housed at the shoot and root tips in structures called meristems, with the shoot apical meristem (SAM) continuously producing aerial leaf, stem, and flower organs throughout the life cycle. Thus, the SAM acts as the engine of plant development and has unique structural and molecular features that allow it to balance self-renewal with differentiation and act as a constant source of new cells for organogenesis while simultaneously maintaining a stem cell reservoir for future organ formation. Studies have identified key roles for intercellular regulatory networks that establish and maintain meristem activity, including the KNOX transcription factor pathway and the CLV-WUS stem cell feedback loop. In addition, the plant hormones cytokinin and auxin act through their downstream signaling pathways in the SAM to integrate stem cell activity and organ initiation. This review discusses how the various regulatory pathways collectively orchestrate SAM function and touches on how their manipulation can alter stem cell activity to improve crop yield.

Recent advances in age-related meibomian gland dysfunction (ARMGD)

Meibomian glands (MGs), located within the tarsal plate of the eyelid, secrete meibum which is the lipid-rich secretion necessary for stabilizing the tear film and preventing tear evaporation. Changes in the quality and quantity of meibum produced causes MG dysfunction (MGD), the leading cause of evaporative dry eye disease (EDED). MGD is an underdiagnosed disease and it is estimated that, in the US, approximately 70 % of the population over 60 have MGD. Three forms of MGD occur based on their meibum secretion: hyposecretory, obstructive, and hypersecretory MGD. The pathophysiology of MGD remains poorly understood, however aging is the primary risk factor. With age, MGs undergo various age-related changes, including decreased acinar basal cell proliferation, hyperkeratinization, MG atrophy, and eventual MG drop-out, leading to age-related MGD (ARMGD). Additionally, studies have suggested that MGs can suffer inflammatory cell infiltration and changes innervation patterns with aging, which could also contribute towards ARMGD. This review focuses on how the aging process affects the MG, and more importantly, how age-related changes to the MG can lead to MG atrophy and MG drop-out, ultimately leading to ARMGD. This review also highlights the most recent developments in potential therapeutic interventions for ARMGD.

The Rho GTPase exchange factor Vav2 promotes extensive age-dependent rewiring of the hair follicle stem cell transcriptome

Both the number and regenerative activity of hair follicle stem cells (HFSCs) are regulated by Vav2, a GDP/GTP exchange factor involved in the catalytic stimulation of the GTPases Rac1 and RhoA. However, whether Vav2 signaling changes in HFSCs over the mouse lifespan is not yet known. Using a mouse knock-in mouse model, we now show that the expression of a catalytically active version of Vav2 (Vav2Onc) promotes an extensive rewiring of the overall transcriptome of HFSCs, the generation of new transcription factor hubs, and the synchronization of many transcriptional programs associated with specific HFSC states and well-defined signaling pathways. Interestingly, this transcriptome rewiring is not fixed in time, as it involves the induction of 15 gene expression waves with diverse distribution patterns during the life of the animals. These expression waves are consistent with the promotion by Vav2Onc of several functional HFSC states that differ from those normally observed in wild-type HFSCs. These results further underscore the role of Vav2 in the regulation of the functional state of HFSCs. They also indicate that, unlike other Vav2-dependent biological processes, the signaling output of this exchange factor is highly contingent on age-dependent intrinsic and/or extrinsic HFSC factors that shape the final biological readouts triggered in this cell type.

Adult tissue-specific stem cell interaction: novel technologies and research advances

Adult tissue-specific stem cells play a dominant role in tissue homeostasis and regeneration. Various in vivo markers of adult tissue-specific stem cells have been increasingly reported by lineage tracing in genetic mouse models, indicating that marked cells differentiation is crucial during homeostasis and regeneration. How adult tissue-specific stem cells with indicated markers contact the adjacent lineage with indicated markers is of significance to be studied. Novel methods bring future findings. Recent advances in lineage tracing, synthetic receptor systems, proximity labeling, and transcriptomics have enabled easier and more accurate cell behavior visualization and qualitative and quantitative analysis of cell-cell interactions than ever before. These technological innovations have prompted researchers to re-evaluate previous experimental results, providing increasingly compelling experimental results for understanding the mechanisms of cell-cell interactions. This review aimed to describe the recent methodological advances of dual enzyme lineage tracing system, the synthetic receptor system, proximity labeling, single-cell RNA sequencing and spatial transcriptomics in the study of adult tissue-specific stem cells interactions. An enhanced understanding of the mechanisms of adult tissue-specific stem cells interaction is important for tissue regeneration and maintenance of homeostasis in organisms.

Hdac1 and Hdac2 regulate the quiescent state and survival of hair-follicle mesenchymal niche

While cell division is essential for self-renewal and differentiation of stem cells and progenitors, dormancy is required to maintain the structure and function of the stem-cell niche. Here we use the hair follicle to show that during growth, the mesenchymal niche of the hair follicle, the dermal papilla (DP), is maintained quiescent by the activity of Hdac1 and Hdac2 in the DP that suppresses the expression of cell-cycle genes. Furthermore, Hdac1 and Hdac2 in the DP promote the survival of DP cells throughout the hair cycle. While during growth and regression this includes downregulation of p53 activity and the control of p53-independent programs, during quiescence, this predominantly involves p53-independent mechanisms. Remarkably, Hdac1 and Hdac2 in the DP during the growth phase also participate in orchestrating the hair cycle clock by maintaining physiological levels of Wnt signaling in the vicinity of the DP. Our findings not only provide insight into the molecular mechanism that sustains the function of the stem-cell niche in a persistently changing microenvironment, but also unveil that the same mechanism provides a molecular toolbox allowing the DP to affect and fine tune the microenvironment.