Intravital imaging of hair follicle regeneration in the mouse

Low-level laser activates Wnt/β-catenin signaling pathway-promoting hair follicle stem cell regeneration and wound healing: Upregulate the expression of key downstream gene Lef 1

BACKGROUND

The objective of this study is to investigate the mechanism by which low-level laser stimulation promotes the proliferation of intraepithelial hair follicle stem cells (HFSCs) in wounds. This research aims to expand the applications of laser treatment, enhance wound repair methods, and establish a theoretical and experimental foundation for achieving accelerated wound healing.

METHODS

The experimental approach involved irradiating a cell model with low-level laser to assess the proliferation of HFSCs and examine alterations in the expression of proteins related to the Wnt/β-catenin signaling pathway. A mouse back wound model was established to investigate the effects of low-level laser irradiation on wound healing rate, wound microenvironment, and the proliferation of HFSCs in relation to the Wnt/β-catenin signaling pathway.

RESULTS

The research findings indicate that low-level laser light effectively activates the Wnt signaling pathway, leading to the increased accumulation of core protein β-catenin and the upregulation of key downstream gene Lef 1. Consequently, this regulatory mechanism facilitates various downstream biological effects, including the notable promotion of HFSC proliferation and differentiation into skin appendages and epithelial tissues. As a result, the process of wound healing is significantly accelerated.

CONCLUSION

Low levels of laser activates the Wnt signalling pathway, promotes the regeneration of hair follicle stem cells and accelerates wound healing.

The G1/S transition in mammalian stem cells in vivo is autonomously regulated by cell size

Cell growth and division must be coordinated to maintain a stable cell size, but how this coordination is implemented in multicellular tissues remains unclear. In unicellular eukaryotes, autonomous cell size control mechanisms couple cell growth and division with little extracellular input. However, in multicellular tissues we do not know if autonomous cell size control mechanisms operate the same way or whether cell growth and cell cycle progression are separately controlled by cell-extrinsic signals. Here, we address this question by tracking single epidermal stem cells growing in adult mice. We find that a cell-autonomous size control mechanism, dependent on the RB pathway, sets the timing of S phase entry based on the cell's current size. Cell-extrinsic variations in the cellular microenvironment affect cell growth rates but not this autonomous coupling. Our work reassesses long-standing models of cell cycle regulation within complex metazoan tissues and identifies cell-autonomous size control as a critical mechanism regulating cell divisions in vivo and thereby a major contributor to stem cell heterogeneity.

The extracellular matrix dictates regional competence for tumour initiation

The skin epidermis is constantly renewed throughout life1,2. Disruption of the balance between renewal and differentiation can lead to uncontrolled growth and tumour initiation3. However, the ways in which oncogenic mutations affect the balance between renewal and differentiation and lead to clonal expansion, cell competition, tissue colonization and tumour development are unknown. Here, through multidisciplinary approaches that combine in vivo clonal analysis using intravital microscopy, single-cell analysis and functional analysis, we show how SmoM2-a constitutively active oncogenic mutant version of Smoothened (SMO) that induces the development of basal cell carcinoma-affects clonal competition and tumour initiation in real time. We found that expressing SmoM2 in the ear epidermis of mice induced clonal expansion together with tumour initiation and invasion. By contrast, expressing SmoM2 in the back-skin epidermis led to a clonal expansion that induced lateral cell competition without dermal invasion and tumour formation. Single-cell analysis showed that oncogene expression was associated with a cellular reprogramming of adult interfollicular cells into an embryonic hair follicle progenitor (EHFP) state in the ear but not in the back skin. Comparisons between the ear and the back skin revealed that the dermis has a very different composition in these two skin types, with increased stiffness and a denser collagen I network in the back skin. Decreasing the expression of collagen I in the back skin through treatment with collagenase, chronic UV exposure or natural ageing overcame the natural resistance of back-skin basal cells to undergoing EHFP reprogramming and tumour initiation after SmoM2 expression. Altogether, our study shows that the composition of the extracellular matrix regulates how susceptible different regions of the body are to tumour initiation and invasion.

Non-invasive assessment of hair regeneration in androgenetic alopecia mice in vivo using two-photon and second harmonic generation imaging

The identification of crucial targets for hair regrowth in androgenetic alopecia (AGA) involves determining important characteristics and different stages during the process of hair follicle regeneration. Traditional methods for assessing key features and different stages of hair follicle primarily involve taking skin tissue samples and determining them through various staining or other methods. However, non-invasive assessment methods have been long sought. Therefore, in this study, endogenous fluorescence signals from skin keratin and second harmonic signals from skin collagen fibers were utilized as probes, two-photon excited fluorescence (TPEF) and second harmonic generation (SHG) imaging techniques were employed to non-invasively assess hair shafts and collagen fibers in AGA mice in vivo. The TPEF imaging technique revealed that the alternation of new and old hair shafts and the different stages of the growth period in AGA mice were delayed. In addition, SHG imaging found testosterone reduced hair follicle area and miniaturized hair follicles. The non-invasive TPEF and SHG imaging techniques provided important methodologies for determining significant characteristics and different stages of the growth cycle in AGA mice, which will facilitate future non-invasive assessments on human scalps in vivo and reduce the use of animal testing.

Apoptotic cells may drive cell death in hair follicles during their regression cycle

Intravital microscopy in live mice has shown that the elimination of epithelial cells during hair follicle regression involves supra-basal cell differentiation and basal cell apoptosis through synergistic action of TGF-β (transforming growth factor) and mesenchymal-epithelial interactions. In this process the basal epithelial cells are not internally committed to death and the mesenchymal dermal papilla (DP) plays essential role in death induction. Because the DP cells are not necessary for completion of the cycle but only for its initiation it is still an open question what is the mechanism leading to the propagation of apoptosis towards the regenerative stem cell population. Here, we use a quantitative analysis of the length of hair follicles during their regression cycle. The data are consistent with a propagation mechanism driven by apoptotic cells inducing apoptosis in their neighboring cells. The observation that the apoptosis slows down as the apoptotic front approaches the stem cells at the end of the follicle is consistent with a gradient of a pro-survival signal sent by these stem cells. An experiment that can falsify this mechanism is proposed.

Niche-specific macrophage loss promotes skin capillary aging

All mammalian organs depend upon resident macrophage populations to coordinate repair processes and facilitate tissue-specific functions1-3. Recent work has established that functionally distinct macrophage populations reside in discrete tissue niches and are replenished through some combination of local proliferation and monocyte recruitment4,5. Moreover, decline in macrophage abundance and function in tissues has been shown to contribute to many age-associated pathologies, such as atherosclerosis, cancer, and neurodegeneration6-8. Despite these advances, the cellular mechanisms that coordinate macrophage organization and replenishment within an aging tissue niche remain largely unknown. Here we show that capillary-associated macrophages (CAMs) are selectively lost over time, which contributes to impaired vascular repair and tissue perfusion in older mice. To investigate resident macrophage behavior in vivo, we have employed intravital two-photon microscopy to non-invasively image in live mice the skin capillary plexus, a spatially well-defined model of niche aging that undergoes rarefication and functional decline with age. We find that CAMs are lost with age at a rate that outpaces that of capillary loss, leading to the progressive accumulation of capillary niches without an associated macrophage in both mice and humans. Phagocytic activity of CAMs was locally required to repair obstructed capillary blood flow, leaving macrophage-less niches selectively vulnerable to both homeostatic and injury-induced loss in blood flow. Our work demonstrates that homeostatic renewal of resident macrophages is not as finely tuned as has been previously suggested9-11. Specifically, we found that neighboring macrophages do not proliferate or reorganize sufficiently to maintain an optimal population across the skin capillary niche in the absence of additional cues from acute tissue damage or increased abundance of growth factors, such as colony stimulating factor 1 (CSF1). Such limitations in homeostatic renewal and organization of various niche-resident cell types are potentially early contributors to tissue aging, which may provide novel opportunities for future therapeutic interventions.

Organ function is preserved despite reorganization of niche architecture in the hair follicle

The ability of stem cells to build and replenish tissues depends on support from their niche. Although niche architecture varies across organs, its functional importance is unclear. During hair follicle growth, multipotent epithelial progenitors build hair via crosstalk with their remodeling fibroblast niche, the dermal papilla, providing a powerful model to functionally interrogate niche architecture. Through mouse intravital imaging, we show that dermal papilla fibroblasts remodel individually and collectively to form a morphologically polarized, structurally robust niche. Asymmetric TGF-β signaling precedes morphological niche polarity, and loss of TGF-β signaling in dermal papilla fibroblasts leads them to progressively lose their stereotypic architecture, instead surrounding the epithelium. The reorganized niche induces the redistribution of multipotent progenitors but nevertheless supports their proliferation and differentiation. However, the differentiated lineages and hairs produced by progenitors are shorter. Overall, our results reveal that niche architecture optimizes organ efficiency but is not absolutely essential for organ function.

Cell cycle controls long-range calcium signaling in the regenerating epidermis

Skin homeostasis is maintained by stem cells, which must communicate to balance their regenerative behaviors. Yet, how adult stem cells signal across regenerative tissue remains unknown due to challenges in studying signaling dynamics in live mice. We combined live imaging in the mouse basal stem cell layer with machine learning tools to analyze patterns of Ca2+ signaling. We show that basal cells display dynamic intercellular Ca2+ signaling among local neighborhoods. We find that these Ca2+ signals are coordinated across thousands of cells and that this coordination is an emergent property of the stem cell layer. We demonstrate that G2 cells are required to initiate normal levels of Ca2+ signaling, while connexin43 connects basal cells to orchestrate tissue-wide coordination of Ca2+ signaling. Lastly, we find that Ca2+ signaling drives cell cycle progression, revealing a communication feedback loop. This work provides resolution into how stem cells at different cell cycle stages coordinate tissue-wide signaling during epidermal regeneration.

Injury prevents Ras mutant cell expansion in mosaic skin

Healthy skin is a mosaic of wild-type and mutant clones1,2. Although injury can cooperate with mutated Ras family proteins to promote tumorigenesis3-12, the consequences in genetically mosaic skin are unknown. Here we show that after injury, wild-type cells suppress aberrant growth induced by oncogenic Ras. HrasG12V/+ and KrasG12D/+ cells outcompete wild-type cells in uninjured, mosaic tissue but their expansion is prevented after injury owing to an increase in the fraction of proliferating wild-type cells. Mechanistically, we show that, unlike HrasG12V/+ cells, wild-type cells respond to autocrine and paracrine secretion of EGFR ligands, and this differential activation of the EGFR pathway explains the competitive switch during injury repair. Inhibition of EGFR signalling via drug or genetic approaches diminishes the proportion of dividing wild-type cells after injury, leading to the expansion of HrasG12V/+ cells. Increased proliferation of wild-type cells via constitutive loss of the cell cycle inhibitor p21 counteracts the expansion of HrasG12V/+ cells even in the absence of injury. Thus, injury has a role in switching the competitive balance between oncogenic and wild-type cells in genetically mosaic skin.

Streamlined Intravital Imaging Approach for Long-Term Monitoring of Epithelial Tissue Dynamics on an Inverted Confocal Microscope

Understanding normal and aberrant in vivo cell behaviors is necessary to develop clinical interventions to thwart disease initiation and progression. It is therefore critical to optimize imaging approaches that facilitate the observation of cell dynamics in situ, where tissue structure and composition remain unperturbed. The epidermis is the body's outermost barrier, as well as the source of the most prevalent human cancers, namely cutaneous skin carcinomas. The accessibility of skin tissue presents a unique opportunity to monitor epithelial and dermal cell behaviors in intact animals using noninvasive intravital microscopy. Nevertheless, this sophisticated imaging approach has primarily been achieved using upright multiphoton microscopes, which represent a significant barrier for entry for most investigators. This study presents a custom-designed, 3D-printed microscope stage insert suitable for use with inverted confocal microscopes, streamlining the long-term intravital imaging of ear skin in live transgenic mice. We believe this versatile invention, which may be customized to fit the inverted microscope brand and model of choice and adapted to image additional organ systems, will prove invaluable to the greater scientific research community by significantly enhancing the accessibility of intravital microscopy. This technological advancement is critical for bolstering our understanding of live cell dynamics in normal and disease contexts.

MicroRNA-205 promotes hair regeneration by modulating mechanical properties of hair follicle stem cells

Stiffness and actomyosin contractility are intrinsic mechanical properties of animal cells required for the shaping of tissues. However, whether tissue stem cells (SCs) and progenitors located within SC niche have different mechanical properties that modulate their size and function remains unclear. Here, we show that hair follicle SCs in the bulge are stiff with high actomyosin contractility and resistant to size change, whereas hair germ (HG) progenitors are soft and periodically enlarge and contract during quiescence. During activation of hair follicle growth, HGs reduce contraction and more frequently enlarge, a process that is associated with weakening of the actomyosin network, nuclear YAP accumulation, and cell cycle reentry. Induction of miR-205, a novel regulator of the actomyosin cytoskeleton, reduces actomyosin contractility and activates hair regeneration in young and old mice. This study reveals the control of tissue SC size and activities by spatiotemporally compartmentalized mechanical properties and demonstrates the possibility to stimulate tissue regeneration by fine-tuning cell mechanics.

Mechanisms of skin vascular maturation and maintenance captured by longitudinal imaging of live mice

A functional network of blood vessels is essential for organ growth and homeostasis, yet how the vasculature matures and maintains homeostasis remains elusive in live mice. By longitudinally tracking the same neonatal endothelial cells (ECs) over days to weeks, we found that capillary plexus expansion is driven by vessel regression to optimize network perfusion. Neonatal ECs rearrange positions to evenly distribute throughout the developing plexus and become positionally stable in adulthood. Upon local ablation, adult ECs survive through a plasmalemmal self-repair response, while neonatal ECs are predisposed to die. Furthermore, adult ECs reactivate migration to assist vessel repair. Global ablation reveals coordinated maintenance of the adult vascular architecture that allows for eventual network recovery. Lastly, neonatal remodeling and adult maintenance of the skin vascular plexus are orchestrated by temporally restricted, neonatal VEGFR2 signaling. Our work sheds light on fundamental mechanisms that underlie both vascular maturation and adult homeostasis in vivo.

Streamlined intravital imaging approach for long-term monitoring of epithelial tissue dynamics on an inverted confocal microscope

Understanding normal and aberrant in vivo cell behaviors is necessary to develop clinical interventions to thwart disease initiation and progression. It is therefore critical to optimize imaging approaches that facilitate the observation of cell dynamics in situ, where tissue structure and composition remain unperturbed. The epidermis is the body's outermost barrier as well as the source of the most prevalent human cancers, namely cutaneous skin carcinomas. The accessibility of skin tissue presents a unique opportunity to monitor epithelial and dermal cell behaviors in intact animals using noninvasive intravital microscopy. Nevertheless, this sophisticated imaging approach has primarily been achieved using upright multiphoton microscopes, which represents a significant barrier-for-entry for most investigators. In this study, we present a custom-designed 3D-printed microscope stage insert suitable for use with inverted confocal microscopes that streamlines long-term intravital imaging of ear skin in live transgenic mice. We believe this versatile invention, which may be customized to fit the inverted microscope brand and model of choice, as well as adapted to image additional organ systems, will prove invaluable to the greater scientific research community by significantly enhancing the accessibility of intravital microscopy. This technological advancement is critical to bolster our understanding of live cell dynamics in both normal and disease contexts.

SUMMARY

A new tool to simplify intravital imaging using inverted confocal microscopy.

Dedifferentiation maintains melanocyte stem cells in a dynamic niche

For unknow reasons, the melanocyte stem cell (McSC) system fails earlier than other adult stem cell populations1, which leads to hair greying in most humans and mice2,3. Current dogma states that McSCs are reserved in an undifferentiated state in the hair follicle niche, physically segregated from differentiated progeny that migrate away following cues of regenerative stimuli4-8. Here we show that most McSCs toggle between transit-amplifying and stem cell states for both self-renewal and generation of mature progeny, a mechanism fundamentally distinct from those of other self-renewing systems. Live imaging and single-cell RNA sequencing revealed that McSCs are mobile, translocating between hair follicle stem cell and transit-amplifying compartments where they reversibly enter distinct differentiation states governed by local microenvironmental cues (for example, WNT). Long-term lineage tracing demonstrated that the McSC system is maintained by reverted McSCs rather than by reserved stem cells inherently exempt from reversible changes. During ageing, there is accumulation of stranded McSCs that do not contribute to the regeneration of melanocyte progeny. These results identify a new model whereby dedifferentiation is integral to homeostatic stem cell maintenance and suggest that modulating McSC mobility may represent a new approach for the prevention of hair greying.

Live imaging reveals chromatin compaction transitions and dynamic transcriptional bursting during stem cell differentiation in vivo

Stem cell differentiation requires dramatic changes in gene expression and global remodeling of chromatin architecture. How and when chromatin remodels relative to the transcriptional, behavioral, and morphological changes during differentiation remain unclear, particularly in an intact tissue context. Here, we develop a quantitative pipeline which leverages fluorescently-tagged histones and longitudinal imaging to track large-scale chromatin compaction changes within individual cells in a live mouse. Applying this pipeline to epidermal stem cells, we reveal that cell-to-cell chromatin compaction heterogeneity within the stem cell compartment emerges independent of cell cycle status, and instead is reflective of differentiation status. Chromatin compaction state gradually transitions over days as differentiating cells exit the stem cell compartment. Moreover, establishing live imaging of Keratin-10 (K10) nascent RNA, which marks the onset of stem cell differentiation, we find that Keratin-10 transcription is highly dynamic and largely precedes the global chromatin compaction changes associated with differentiation. Together, these analyses reveal that stem cell differentiation involves dynamic transcriptional states and gradual chromatin rearrangement.

Photoablation at single cell resolution and its application in the Drosophila epidermis and peripheral nervous system

Tissues contain diverse cell populations that, together, make up physiologically functional units. A remarkable example is the animal epidermis, where neuronal and non-neuronal cells intermingle to allow somatosensory perception. In the peripheral nervous system (PNS), the tight association between heterogenous cell types poses challenges when the structural and physiological contributions of neuronal and surrounding cells need to be dissected with suitable precision. When genetic tools for cell-specific, spatiotemporally controlled gene expression are not available, targeted cell ablation represents a considerable obstacle. Here, we describe an efficient method to overcome this limitation and demonstrate its application to the study of the differentiating Drosophila epidermis and PNS. This methodology relies on the use of near infrared (NIR) femtosecond (fs) laser pulses for ablation of the desired cells at the desired time. We show how to confine the photodamage to the targeted cell to induce its death, without harming neighbouring tissues or structures. We validated our approach in the Drosophila PNS by studying the responses of photo-ablated neurons, non-neuronal cells, and the surrounding epidermis. Diverse cellular behaviours including cell extrusion, cell rearrangements and cell shape changes can be monitored in vivo immediately after damage, as well as for several hours post-ablation with high optical resolution using confocal microscopy. This methodology provides a flexible tool to ablate individual cells with high precision and study morphological responses to cell loss in targeted areas or neighbouring structures. We anticipate that this protocol can be easily adapted to other model systems and tissues.

Topological Distribution of Wound Stiffness Modulates Wound-Induced Hair Follicle Neogenesis

In the large full-thickness mouse skin regeneration model, wound-induced hair neogenesis (WIHN) occurs in the wound center. This implies a spatial regulation of hair regeneration. The role of mechanotransduction during tissue regeneration is poorly understood. Here, we created wounds with equal area but different shapes to understand if perturbing mechanical forces change the area and quantity of de novo hair regeneration. Atomic force microscopy of wound stiffness demonstrated a stiffness gradient across the wound with the wound center softer than the margin. Reducing mechanotransduction signals using FAK or myosin II inhibitors significantly increased WIHN and, conversely, enhancing these signals with an actin stabilizer reduced WIHN. Here, α-SMA was downregulated in FAK inhibitor-treated wounds and lowered wound stiffness. Wound center epithelial cells exhibited a spherical morphology relative to wound margin cells. Differential gene expression analysis of FAK inhibitor-treated wound RNAseq data showed that cytoskeleton-, integrin-, and matrix-associated genes were downregulated, while hair follicular neogenesis, cell proliferation, and cell signaling genes were upregulated. Immunohistochemistry staining showed that FAK inhibition increased pSTAT3 nuclear staining in the regenerative wound center, implying enhanced signaling for hair follicular neogenesis. These findings suggest that controlling wound stiffness modulates tissue regeneration encompassing epithelial competence, tissue patterning, and regeneration during wound healing.

Advances and challenges in intravital imaging of craniofacial and dental progenitor cells

Craniofacial and appendicular bone homeostasis is dynamically regulated by a balance between bone formation and resorption by osteoblasts and osteoclasts, respectively. Despite the developments in multiple imaging techniques in bone biology, there are still technical challenges and limitations in the investigation of spatial/anatomical location of rare stem/progenitor cells and their molecular regulation in tooth and craniofacial bones of living animals. Recent advances in live animal imaging techniques for the craniofacial and dental apparatus can provide new insights in real time into bone stem/progenitor cell dynamics and function in vivo. Here, we review the current inventions and applications of the noninvasive intravital imaging technique and its practical uses and limitations in the analysis of stem/progenitor cells in craniofacial and dental apparatus in vivo. Furthermore, we also explore the potential applications of intravital microscopy in the dental field.

Mouse tail skin wholemount staining and intravital calcium imaging

Mouse tail skin offers multiple benefits for skin research. Here, we present a protocol detailing high resolution tail skin whole mount staining and in vivo calcium imaging of hair follicle stem cells (HFSCs) using Sox9creERT2, GCaMP6s, and Ai14 mice. This approach enables the study of hair follicles and stem cells in different physiological and pathological conditions. The applications of the protocol include visualization of calcium signaling in other cell types with Cre/CreER lines or analyzing other cellular features with different reporter lines.

For complete details on the use and execution of this protocol, please refer to Xie et al. (2022).

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High resolution whole mount immunofluorescent analysis of mouse hair follicle

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Intravital calcium imaging of hair follicle

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Applicable to other cell types or featuress with different driver and reporter lines

Escape of hair follicle stem cells causes stem cell exhaustion during aging

Stem cell (SC) exhaustion is a hallmark of aging. However, the process of SC depletion during aging has not been observed in live animals, and the underlying mechanism contributing to tissue deterioration remains obscure. We find that, in aged mice, epithelial cells escape from the hair follicle (HF) SC compartment to the dermis, contributing to HF miniaturization. Single-cell RNA-seq and assay for transposase-accessible chromatin using sequencing (ATAC-seq) reveal reduced expression of cell adhesion and extracellular matrix genes in aged HF-SCs, many of which are regulated by Foxc1 and Nfatc1. Deletion of Foxc1 and Nfatc1 recapitulates HF miniaturization and causes hair loss. Live imaging captures individual epithelial cells migrating away from the SC compartment and HF disintegration. This study illuminates a hitherto unknown activity of epithelial cells escaping from their niche as a mechanism underlying SC reduction and tissue degeneration. Identification of homeless epithelial cells in aged tissues provides a new perspective for understanding aging-associated diseases.

Lgr6 marks epidermal stem cells with a nerve-dependent role in wound re-epithelialization

Stem cells support lifelong maintenance of adult organs, but their specific roles during injury are poorly understood. Here we demonstrate that Lgr6 marks a regionally restricted population of epidermal stem cells that interact with nerves and specialize in wound re-epithelialization. Diphtheria toxin-mediated ablation of Lgr6 stem cells delays wound healing, and skin denervation phenocopies this effect. Using intravital imaging to capture stem cell dynamics after injury, we show that wound re-epithelialization by Lgr6 stem cells is diminished following loss of nerves. This induces recruitment of other stem cell populations, including hair follicle stem cells, which partially compensate to mediate wound closure. Single-cell lineage tracing and gene expression analysis reveal that the fate of Lgr6 stem cells is shifted toward differentiation following loss of their niche. We conclude that Lgr6 epidermal stem cells are primed for injury response and interact with nerves to regulate their fate.

Skin-resident immune cells actively coordinate their distribution with epidermal cells during homeostasis

Organs consist of multiple cell types that ensure proper architecture and function. How different cell types coexist and interact to maintain their homeostasis in vivo remains elusive. The skin epidermis comprises mostly epithelial cells, but also harbours Langerhans cells (LCs) and dendritic epidermal T cells (DETCs). Whether and how distributions of LCs and DETCs are regulated during homeostasis is unclear. Here, by tracking individual cells in the skin of live adult mice over time, we show that LCs and DETCs actively maintain a non-random spatial distribution despite continuous turnover of neighbouring basal epithelial cells. Moreover, the density of epithelial cells regulates the composition of LCs and DETCs in the epidermis. Finally, LCs require the GTPase Rac1 to maintain their positional stability, density and tiling pattern reminiscent of neuronal self-avoidance. We propose that these cellular mechanisms provide the epidermis with an optimal response to environmental insults.

Intravital microscopy of dynamic single-cell behavior in mouse mammary tissue

Multiphoton intravital imaging is essential for understanding cellular behavior and function in vivo. The adipose-rich environment of the mammary gland poses a unique challenge to in vivo microscopy due to light scattering that impedes high-resolution imaging. Here we provide a protocol for high-quality, six-color 3D intravital imaging of regions across the entire mouse mammary gland and associated tissues for several hours while maintaining tissue access for microdissection and labeling. An incision at the ventral midline and along the right hind leg creates a skin flap that is then secured to a raised platform skin side down. This allows for fluorescence-guided microdissection of connective tissue to provide unimpeded imaging of mammary ducts. A sealed imaging chamber over the skin flap creates a stable environment while maintaining access to large tissue regions for imaging with an upright microscope. We provide a strategy for imaging single cells and the tissue microenvironment utilizing multicolor Confetti lineage-tracing and additional dyes using custom-designed filters and sequential excitation with dual multiphoton lasers. Furthermore, we describe a strategy for simultaneous imaging and photomanipulation of single cells using the Olympus SIM scanner and provide steps for 3D video processing, visualization and high-dimensional analysis of single-cell behavior. We then provide steps for multiplexing intravital imaging with fixation, immunostaining, tissue clearing and 3D confocal imaging to associate cell behavior with protein expression. The skin-flap surgery and chamber preparation take 1.5 h, followed by up to 12 h of imaging. Applications range from basic filming in 1 d to 5 d for multiplexing and complex analysis.

Dysfunction of Hair Follicle Mesenchymal Progenitors Contributes to Age-Associated Hair Loss

Skin aging is accompanied by hair loss due to impairments in hair follicle (HF) epithelial progenitor cells and their mesenchymal niche. This inductive mesenchyme, called dermal papilla (DP), undergoes progressive cell loss and eventual miniaturization that contributes to HF pathogenesis. Using laser ablation and fate mapping, we show that HF dermal stem cells (hfDSCs) reconstitute the damaged DP and maintain hair growth, suggesting that hfDSC dysfunction may trigger degeneration of the inductive niche. Fate mapping over 24 months revealed progressive hfDSC depletion, and in vivo clonal analysis of aged hfDSCs showed impaired self-renewal and biased differentiation. Single-cell RNA-seq confirmed hfDSCs as a central precursor, giving rise to divergent mesenchymal trajectories. In aged skin, hfDSCs exhibited senescent-like characteristics, and senescence-associated secretory phenotypes were identified in the aging HF mesenchyme. These results clarify fibroblast dynamics within the HF and suggest that progressive dysfunction within the mesenchymal progenitor pool contributes to age-related hair loss.

Roles for microtubules in the proliferative and differentiated cells of stratified epithelia

While microtubule dynamics and organization have been extensively studied invitro, both biochemically and in cultured cells, recent work has begun to extend this into tissues ex vivo and organisms in vivo. Advances in genetic tools and imaging technology have allowed studies on the dynamics, function, and organization of microtubules in the stratified epithelia of the epidermis. Here, we discuss recent work that highlights the varied roles that microtubules play in supporting epidermal function. These findings demonstrate that studying microtubules in tissues has revealed not only novel aspects of epidermal biology but also new principles of microtubule regulation.

Cell-Cycle-Dependent ERK Signaling Dynamics Direct Fate Specification in the Mammalian Preimplantation Embryo

Despite the noisy nature of single cells, multicellular organisms robustly generate different cell types from one zygote. This process involves dynamic cross regulation between signaling and gene expression that is difficult to capture with fixed-cell approaches. To study signaling dynamics and fate specification during preimplantation development, we generated a transgenic mouse expressing the ERK kinase translocation reporter and measured ERK activity in single cells of live embryos. Our results show primarily active ERK in both the inner cell mass and trophectoderm cells due to fibroblast growth factor (FGF) signaling. Strikingly, a subset of mitotic events results in a short pulse of ERK inactivity in both daughter cells that correlates with elevated endpoint NANOG levels. Moreover, endogenous tagging of Nanog in embryonic stem cells reveals that ERK inhibition promotes enhanced stabilization of NANOG protein after mitosis. Our data show that cell cycle, signaling, and differentiation are coordinated during preimplantation development.

Intravital Imaging Techniques for Biomedical and Clinical Research

Intravital imaging, the direct visualization of cells and tissues within a living animal, is a technique that has been employed for the better part of a century. The advent of confocal and multiphoton microscopy has dramatically improved the power of intravital imaging, making it possible to obtain optical sections of tissues non-destructively. This review discusses the various techniques used for intravital imaging, describes how intravital imaging provides information about cellular and tissue dynamics not possible to be garnered by other techniques, and details several ways in which intravital imaging is making a direct impact on the clinical care of patients. © 2019 International Society for Advancement of Cytometry.

Neuroendocrinology and neurobiology of sebaceous glands

The nervous system communicates with peripheral tissues through nerve fibres and the systemic release of hypothalamic and pituitary neurohormones. Communication between the nervous system and the largest human organ, skin, has traditionally received little attention. In particular, the neuro-regulation of sebaceous glands (SGs), a major skin appendage, is rarely considered. Yet, it is clear that the SG is under stringent pituitary control, and forms a fascinating, clinically relevant peripheral target organ in which to study the neuroendocrine and neural regulation of epithelia. Sebum, the major secretory product of the SG, is composed of a complex mixture of lipids resulting from the holocrine secretion of specialised epithelial cells (sebocytes). It is indicative of a role of the neuroendocrine system in SG function that excess circulating levels of growth hormone, thyroxine or prolactin result in increased sebum production (seborrhoea). Conversely, growth hormone deficiency, hypothyroidism, and adrenal insufficiency result in reduced sebum production and dry skin. Furthermore, the androgen sensitivity of SGs appears to be under neuroendocrine control, as hypophysectomy (removal of the pituitary) renders SGs largely insensitive to stimulation by testosterone, which is crucial for maintaining SG homeostasis. However, several neurohormones, such as adrenocorticotropic hormone and α-melanocyte-stimulating hormone, can stimulate sebum production independently of either the testes or the adrenal glands, further underscoring the importance of neuroendocrine control in SG biology. Moreover, sebocytes synthesise several neurohormones and express their receptors, suggestive of the presence of neuro-autocrine mechanisms of sebocyte modulation. Aside from the neuroendocrine system, it is conceivable that secretion of neuropeptides and neurotransmitters from cutaneous nerve endings may also act on sebocytes or their progenitors, given that the skin is richly innervated. However, to date, the neural controls of SG development and function remain poorly investigated and incompletely understood. Botulinum toxin-mediated or facial paresis-associated reduction of human sebum secretion suggests that cutaneous nerve-derived substances modulate lipid and inflammatory cytokine synthesis by sebocytes, possibly implicating the nervous system in acne pathogenesis. Additionally, evidence suggests that cutaneous denervation in mice alters the expression of key regulators of SG homeostasis. In this review, we examine the current evidence regarding neuroendocrine and neurobiological regulation of human SG function in physiology and pathology. We further call attention to this line of research as an instructive model for probing and therapeutically manipulating the mechanistic links between the nervous system and mammalian skin.

Dermal sheath contraction powers stem cell niche relocation during hair cycle regression

Tissue homeostasis requires the balance of growth by cell production and regression through cell loss. In the hair cycle, during follicle regression, the niche traverses the skin through an unknown mechanism to reach the stem cell reservoir and trigger new growth. Here, we identify the dermal sheath that lines the follicle as the key driver of tissue regression and niche relocation through the smooth muscle contractile machinery that generates centripetal constriction force. We reveal that the calcium-calmodulin-myosin light chain kinase pathway controls sheath contraction. When this pathway is blocked, sheath contraction is inhibited, impeding follicle regression and niche relocation. Thus, our study identifies the dermal sheath as smooth muscle that drives follicle regression for reuniting niche and stem cells in order to regenerate tissue structure during homeostasis.

The importance of the neuro-immuno-cutaneous system on human skin equivalent design

The skin is a highly complex organ, responsible for sensation, protection against the environment (pollutants, foreign proteins, infection) and thereby linked to the immune and sensory systems in the neuro-immuno-cutaneous (NIC) system. Cutaneous innervation is a key part of the peripheral nervous system; therefore, the skin should be considered a sensory organ and an important part of the central nervous system, an 'active interface' and the first connection of the body to the outside world. Peripheral nerves are a complex class of neurons within these systems, subsets of functions are conducted, including mechanoreception, nociception and thermoception. Epidermal and dermal cells produce signalling factors (such as cytokines or growth factors), neurites influence skin cells (such as via neuropeptides), and peripheral nerves have a role in both early and late stages of the inflammatory response. One way this is achieved, specifically in the cutaneous system, is through neuropeptide release and signalling, especially via substance P (SP), neuropeptide Y (NPY) and nerve growth factor (NGF). Cutaneous, neuronal and immune cells play a central role in many conditions, including psoriasis, atopic dermatitis, vitiligo, UV-induced immunosuppression, herpes and lymphomas. Therefore, it is critical to understand the connections and interplay between the peripheral nervous system and the skin and immune systems, the NIC system. Relevant in vitro tissue models based on human skin equivalents can be used to gain insight and to address impact across research and clinical needs.

Hair follicle regeneration suppresses Ras-driven oncogenic growth

Mutations associated with tumor development in certain tissues can be nontumorigenic in others, yet the mechanisms underlying these different outcomes remains poorly understood. To address this, we targeted an activating Hras mutation to hair follicle stem cells and discovered that Hras mutant cells outcompete wild-type neighbors yet are integrated into clinically normal skin hair follicles. In contrast, targeting the Hras mutation to the upper noncycling region of the skin epithelium leads to benign outgrowths. Follicular Hras mutant cells autonomously and nonautonomously enhance regeneration, which directs mutant cells into continuous tissue cycling to promote integration rather than aberrancy. This follicular tolerance is maintained under additional challenges that promote tumorigenesis in the epidermis, including aging, injury, and a secondary mutation. Thus, the hair follicle possesses a unique, enhanced capacity to integrate and contain Hras mutant cells within both homeostatic and perturbed tissue, demonstrating that in the skin, multiple, distinct mechanisms exist to suppress oncogenic growth.

The RpoS Gatekeeper in Borrelia burgdorferi: An Invariant Regulatory Scheme That Promotes Spirochete Persistence in Reservoir Hosts and Niche Diversity

Maintenance of Borrelia burgdorferi within its enzootic cycle requires a complex regulatory pathway involving the alternative σ factors RpoN and RpoS and two ancillary trans-acting factors, BosR and Rrp2. Activation of this pathway occurs within ticks during the nymphal blood meal when RpoS, the effector σ factor, transcribes genes required for tick transmission and mammalian infection. RpoS also exerts a 'gatekeeper' function by repressing σ⁷⁰-dependent tick phase genes (e.g., ospA, lp6.6). Herein, we undertook a broad examination of RpoS functionality throughout the enzootic cycle, beginning with modeling to confirm that this alternative σ factor is a 'genuine' RpoS homolog. Using a novel dual color reporter system, we established at the single spirochete level that ospA is expressed in nymphal midguts throughout transmission and is not downregulated until spirochetes have been transmitted to a naïve host. Although it is well established that rpoS/RpoS is expressed throughout infection, its requirement for persistent infection has not been demonstrated. Plasmid retention studies using a trans-complemented ΔrpoS mutant demonstrated that (i) RpoS is required for maximal fitness throughout the mammalian phase and (ii) RpoS represses tick phase genes until spirochetes are acquired by a naïve vector. By transposon mutant screening, we established that bba34/oppA5, the only OppA oligopeptide-binding protein controlled by RpoS, is a bona fide persistence gene. Lastly, comparison of the strain 297 and B31 RpoS DMC regulons identified two cohorts of RpoS-regulated genes. The first consists of highly conserved syntenic genes that are similarly regulated by RpoS in both strains and likely required for maintenance of B. burgdorferi sensu stricto strains in the wild. The second includes RpoS-regulated plasmid-encoded variable surface lipoproteins ospC, dbpA and members of the ospE/ospF/elp, mlp, revA, and Pfam54 paralogous gene families, all of which have evolved via inter- and intra-strain recombination. Thus, while the RpoN/RpoS pathway regulates a 'core' group of orthologous genes, diversity within RpoS regulons of different strains could be an important determinant of reservoir host range as well as spirochete virulence.

Intravital imaging of host-parasite interactions in skin and adipose tissues

Intravital microscopy allows the visualisation of how pathogens interact with host cells and tissues in living animals in real time. This method has enabled key advances in our understanding of host-parasite interactions under physiological conditions. A combination of genetics, microscopy techniques, and image analysis have recently facilitated the understanding of biological phenomena in living animals at cellular and subcellular resolution. In this review, we summarise findings achieved by intravital microscopy of the skin and adipose tissues upon infection with various parasites, and we present a view into possible future applications of this method.

Crawling wounded: molecular genetic insights into wound healing from Drosophila larvae

For animals, injury is inevitable. Because of this, organisms possess efficient wound healing mechanisms that can repair damaged tissues. However, the molecular and genetic mechanisms by which epidermal repair is accomplished remain poorly defined. Drosophila has become a valuable model to study epidermal wound healing because of the comprehensive genetic toolkit available in this organism and the similarities of wound healing processes between Drosophila and vertebrates. Other reviews in this Special Issue cover wound healing assays and pathways in Drosophila embryos, pupae and adults, as well as regenerative processes that occur in tissues such as imaginal discs and the gut. In this review, we will focus on the molecular/genetic control of wound-induced cellular processes such as inflammation, cell migration and epithelial cell-cell fusion in Drosophila larvae. We will give a brief overview of the three wounding assays, pinch, puncture, and laser ablation, and the cellular responses that ensue following wounding. We will highlight the actin regulators, signaling pathways and transcriptional mediators found so far to be involved in larval epidermal wound closure and what is known about how they act. We will also discuss wound-induced epidermal cell-cell fusion and possible directions for future research in this exciting system.

Single-Cell Live Imaging

Recent fluorescence microscopy allows for high-throughput acquisition of 5D (X, Y, Z, T, and Color) images in various targets such as cultured cells, 3D spheroid/organoid, and even living tissue with single-cell resolution. The technology is considered promising to augment insights on heterogeneous features of both physiological and pathological cell phenotypes, for instance, distinct responses of cancer cells to anticancer drug treatment. Here we overview microscopic applications to capture live cell events for different types of targets, together with a couple of proof of concepts. The 2D live imaging will be exemplified by a FRET-based time-lapse cultured cell imaging, and 3D tissue imaging protocol will be complemented with a method for mouse skin live imaging.

In Vivo Genetic Alteration and Lineage Tracing of Single Stem Cells by Live Imaging

Studies characterizing stem cell lineages in different organs aim to understand which cells particular progenitors can give rise to and how this process is controlled. Because the skin contains several resident stem cell populations and undergoes constant turnover, it is an ideal tissue in which to study this phenomenon. Furthermore, with the advent of two-photon microscopy techniques in combination with genetic tools for cell labeling, this question can be studied non-invasively by using live imaging. In this chapter, we describe an experimental approach that takes this technique one step further. We combine the Cre and Tet inducible genetic systems for single clone labeling and genetic manipulation in a specific stem cell population in the skin by using known drivers. Our system involves the use of gain- and loss-of-function alleles activated only in a differentially labeled population to distinguish single clones. The same region within a tissue is imaged repeatedly to document the fate and interactions of single clones with and without genetic modifications in the long term. Implementing this lineage tracing approach while documenting changes in cell behavior brought about by the genetic alterations allows both aspects to be linked. Because of the inherent flexibility of the approach, we expect it to have broad applications in studying stem cell function not only in the skin, but also in other tissues amenable to live imaging.

Long-Term Intravital Imaging of the Cornea, Skin, and Hair Follicle by Multiphoton Microscope

Multiphoton microscopy allows long-term direct visualization of cells in live animals due to its low photodamage. When coupled with fluorescence protein targeting and second harmonic generation signals from natural collagen as contrast, multiphoton microscopy enables intravital tracing of cells while providing structural information from the extracellular matrix. Compared with conventional histological analysis, it can bring new insight into the cell dynamics in stem cell research. Here, we demonstrate cell imaging and tracing at a single cell resolution in the cornea, skin, and hair follicles using multiphoton microscopy in transgenic mice of which specific cell populations are tagged with fluorescent proteins.

Positional Stability and Membrane Occupancy Define Skin Fibroblast Homeostasis In Vivo

Fibroblasts are an essential cellular and structural component of our organs. Despite several advances, the critical behaviors that fibroblasts utilize to maintain their homeostasis in vivo have remained unclear. Here, by tracking the same skin fibroblasts in live mice, we show that fibroblast position is stable over time and that this stability is maintained despite the loss of neighboring fibroblasts. In contrast, fibroblast membranes are dynamic during homeostasis and extend to fill the space of lost neighboring fibroblasts in a Rac1-dependent manner. Positional stability is sustained during aging despite a progressive accumulation of gaps in fibroblast nuclei organization, while membrane occupancy continues to be maintained. This work defines positional stability and cell occupancy as key principles of skin fibroblast homeostasis in vivo, throughout the lifespan of mice, and identifies membrane extension in the absence of migration as the core cellular mechanism to carry out these principles.

Homeostatic Epidermal Stem Cell Self-Renewal Is Driven by Local Differentiation

Maintenance of adult tissues depends on sustained activity of resident stem cell populations, but the mechanisms that regulate stem cell self-renewal during homeostasis remain largely unknown. Using an imaging and tracking approach that captures all epidermal stem cell activity in large regions of living mice, we show that self-renewal is locally coordinated with epidermal differentiation, with a lag time of 1 to 2 days. In both homeostasis and upon experimental perturbation, we find that differentiation of a single stem cell is followed by division of a direct neighbor, but not vice versa. Finally, we show that exit from the stem cell compartment is sufficient to drive neighboring stem cell self-renewal. Together, these findings establish that epidermal stem cell self-renewal is not the constitutive driver of homeostasis. Instead, it is precisely tuned to tissue demand and responds directly to neighbor cell differentiation.

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

Wound-induced hair follicle neogenesis (WIHN) is a regenerative phenomenon that occurs widely in the skin of adult mammalians. A fully functional follicle can regenerate in the center of a full-thickness wound with a large enough size. The cellular origin of this process is similar to embryonic process. Many growth and development-related pathways are involved in WIHN. Studying WIHN can deeply explore the mechanism of biological growth, development and regeneration, and can identify new treatments for hair-related disorders. Our review aims to enlighten future study by summarizing the clinical manifestation of WIHN, as well as the cellular and molecular mechanism of WIHN in recent studies.

Advancing insights into stem cell niche complexities with next-generation technologies

Adult tissue-specific stem cells are essential for homeostatic tissue maintenance and key to regeneration during injury repair or disease. Many critical stem cell functions rely on the presence of well-timed cues from the microenvironment or niche, which includes a diverse range of components, including neuronal, circulating and extracellular matrix inputs as well as an array of neighboring niche cells directly interacting with the stem cells. However, studies of stem cells and their niche have been challenging due to the complexity of adult stem cell functions, their intrinsic controls and the multiple regulatory niche components. Here, we review recent major advances in our understanding of the complex interplay between stem cells and their niche that were enabled by the tremendous technological leaps in single-cell transcriptome analyses, 3D in vitro cultures and 4D in vivo microscopy of stem cell niches.

Two Photon Intravital Microscopy of Lyme Borrelia in Mice

Two-photon intravital microscopy is a powerful tool that allows visualization of cells in intact tissues in a live animal in real time. In recent years, this advanced technology has been applied to understand pathogen-host interactions using fluorescently labeled bacteria. In particular, infectious fluorescent transformants of the Lyme disease spirochete Borrelia burgdorferi, an Ixodes tick-transmitted pathogen, have been imaged by two-photon intravital microscopy to study bacterial motility and interactions of the pathogen with feeding ticks and host tissues. Here, we describe the techniques and equipment used to image mammalian-adapted spirochetes in the skin of living mice in vivo and in joints ex vivo using two-photon intravital microscopy.

Current Understanding of the Pathways Involved in Adult Stem and Progenitor Cell Migration for Tissue Homeostasis and Repair

With the advancements in the field of adult stem and progenitor cells grows the recognition that the motility of primitive cells is a pivotal aspect of their functionality. There is accumulating evidence that the recruitment of tissue-resident and circulating cells is critical for organ homeostasis and effective injury responses, whereas the pathobiology of degenerative diseases, neoplasm and aging, might be rooted in the altered ability of immature cells to migrate. Furthermore, understanding the biological machinery determining the translocation patterns of tissue progenitors is of great relevance for the emerging methodologies for cell-based therapies and regenerative medicine. The present article provides an overview of studies addressing the physiological significance and diverse modes of stem and progenitor cell trafficking in adult mammalian organs, discusses the major microenvironmental cues regulating cell migration, and describes the implementation of live imaging approaches for the exploration of stem cell movement in tissues and the factors dictating the motility of endogenous and transplanted cells with regenerative potential.

Correction of aberrant growth preserves tissue homeostasis

Cells in healthy tissues acquire mutations with surprising frequency. Many of these mutations are associated with abnormal cellular behaviours such as differentiation defects and hyperproliferation, yet fail to produce macroscopically detectable phenotypes. It is currently unclear how the tissue remains phenotypically normal, despite the presence of these mutant cells. Here we use intravital imaging to track the fate of mouse skin epithelium burdened with varying numbers of activated Wnt/β-catenin stem cells. We show that all resulting growths that deform the skin tissue architecture regress, irrespective of their size. Wild-type cells are required for the active elimination of mutant cells from the tissue, while utilizing both endogenous and ectopic cellular behaviours to dismantle the aberrant structures. After regression, the remaining structures are either completely eliminated or converted into functional skin appendages in a niche-dependent manner. Furthermore, tissue aberrancies generated from oncogenic Hras, and even mutation-independent deformations to the tissue, can also be corrected, indicating that this tolerance phenomenon reflects a conserved principle in the skin. This study reveals an unanticipated plasticity of the adult skin epithelium when faced with mutational and non-mutational insult, and elucidates the dynamic cellular behaviours used for its return to a homeostatic state.

Compartmentation of Mitochondrial and Oxidative Metabolism in Growing Hair Follicles: A Ring of Fire

Little is known about the energetics of growing hair follicles, particularly in the mitochondrially abundant bulb. Here, mitochondrial and oxidative metabolism was visualized by multiphoton and light sheet microscopy in cultured bovine hair follicles and plucked human hairs. Mitochondrial membrane potential (ΔΨ), cell viability, reactive oxygen species (ROS), and secretory granules were assessed with parameter-indicating fluorophores. In growing follicles, lower bulb epithelial cells had high viability, and mitochondria were polarized. Most epithelially generated ROS co-localized with polarized mitochondria. As the imaging plane captured more central and distal cells, ΔΨ disappeared abruptly at a transition to a nonfluorescent core continuous with the hair shaft. Approaching the transition, ΔΨ and ROS increased, and secretory granules disappeared. ROS and ΔΨ were strongest in a circumferential paraxial ring at putative sites for formation of the outer cortex/cuticle of the hair shaft. By contrast, polarized mitochondria in dermal papillar fibroblasts produced minimal ROS. Plucked hairs showed a similar abrupt transition of degranulation/depolarization near sites of keratin deposition, as well as an ROS-generating paraxial ring of fire. Hair movement out of the follicle appeared to occur independently of follicular bulb bioenergetics by a tractor mechanism involving the inner and outer root sheaths.

Procedures and applications of long-term intravital microscopy

Intravital microscopy (IVM) is increasingly used in biomedical research to study dynamic processes at cellular and subcellular resolution in their natural environment. Long-term IVM especially can be applied to visualize migration and proliferation over days to months within the same animal without recurrent surgeries. Skin can be repetitively imaged without surgery. To intermittently visualize cells in other organs, such as liver, mammary gland and brain, different imaging windows including the abdominal imaging window (AIW), dermal imaging window (DIW) and cranial imaging window (CIW) have been developed. In this review, we describe the procedure of window implantation and pros and cons of each technique as well as methods to retrace a position of interest over time. In addition, different fluorescent biosensors to facilitate the tracking of cells for different purposes, such as monitoring cell migration and proliferation, are discussed. Finally, we consider new techniques and possibilities of how long-term IVM can be even further improved in the future.

Two-photon microscopy for intracutaneous imaging of stem cell activity in mice

The adult skin is a typical example of a highly regenerative tissue. Terminally differentiated keratinocytes are shed from the external layers of the epidermis or extruded from the skin as part of the growing hair shaft on a daily basis. These are effectively replenished through the activity of skin-resident stem cells. Precise regulation of stem cell activity is critical for normal skin homoeostasis or wound healing and irregular stem cell proliferation or differentiation can lead to skin disease. The scarcity and dynamic nature of stem cells presents a major challenge for elucidating their mechanism of action. To address this, we have recently established a system for visualizing stem cell activity, in real time or long term, in the intact skin of live mice using two-photon microscopy. The purpose of this review was to provide essential information to researchers who wish to incorporate two-photon microscopy and live imaging into their experimental toolbox for studying aspects of skin and stem biology in the mouse model. We discuss fundamental principles of the method, instrumentation and basic experimental approaches to interrogate stem cell activity in the interfollicular epidermis and hair follicle.

Tissue-scale coordination of cellular behaviour promotes epidermal wound repair in live mice

Tissue repair is fundamental to our survival as tissues are challenged by recurrent damage. During mammalian skin repair, cells respond by migrating and proliferating to close the wound. However, the coordination of cellular repair behaviours and their effects on homeostatic functions in a live mammal remains unclear. Here we capture the spatiotemporal dynamics of individual epithelial behaviours by imaging wound re-epithelialization in live mice. Differentiated cells migrate while the rate of differentiation changes depending on local rate of migration and tissue architecture. Cells depart from a highly proliferative zone by directionally dividing towards the wound while collectively migrating. This regional coexistence of proliferation and migration leads to local expansion and elongation of the repairing epithelium. Finally, proliferation functions to pattern and restrict the recruitment of undamaged cells. This study elucidates the interplay of cellular repair behaviours and consequent changes in homeostatic behaviours that support tissue-scale organization of wound re-epithelialization.

Two-Photon Intravital Microscopy Animal Preparation Protocol to Study Cellular Dynamics in Pathogenesis

Two-photon intravital microscopy (2P-IVM) is an advanced imaging platform that allows the visualization of dynamic processes at subcellular resolution in vivo. Dynamic processes like cell migration, cell proliferation, cell-cell interactions, and cell signaling have an interactive character and occur in complex environments. Hence, it is of pivotal importance to study these processes in living animals, using for example 2P-IVM. 2P-IVM can be performed on a variety of tissues, from the skin of the animal to internal organs, and a variety of methods can be utilized to perform 2P-IVM on these tissues. Here, we discuss the protocols and considerations for four of those 2P-IVM methods, namely tissue explant imaging, skin imaging, surgical exposure imaging, and multi-day window imaging. We carefully compare and explain in depth how to set up each method. Lastly, in the notes section we mention some alternative solutions for the 2P-IVM methods described. In conclusion, this protocol can be used as a guide towards deciding which 2P-IVM method to use and to enable the setup of this method.