Keratin network modifications lead to the mechanical stiffening of the hair follicle fiber

Spectral histology of hair and hair follicle using infrared microspectroscopy

OBJECTIVE

Today, there is only limited knowledge of the spatial organization of hair chemistry. Infrared microspectroscopy is a well-established tool to provide such information and has significantly contributed to this field. In this study, we present new results combining multiple infrared microspectroscopy methods at different length scales to create a better chemical histology of human hair, including the hair follicle, hair shaft, hair medulla and hair cuticle.

METHODS

We used hyperspectral IR imaging & spectroscopy (HIRIS) and synchrotron-radiation FTIR microspectroscopy (SR-μFTIR) to measure transversal hair sections and SR-μFTIR to obtain high-resolution maps of longitudinal sections from the hair shaft and from the hair follicle. We used optical photothermal IR microspectroscopy (OPTIR) to analyse the cuticle surface of intact hairs.

RESULTS

By mapping longitudinal sections of the human hair follicle with confocal SR-μFTIR, we report the first demonstration of glycogen presence in the outer root sheath of the hair follicle by spectroscopy, and its quantification at the micron scale. Spectral maps, combined with machine learning-based analysis, enabled us to differentiate the various layers of the hair follicle and provided insights into the chemical changes that occur during hair formation in the follicle. Using HIRIS and SR-μFTIR to analyse the hair medulla in transversal sections of human hairs, we report here, for the first time by vibrational spectroscopy methods, the detection of unsaturated lipids at very low concentrations in the medulla. By analysing longitudinal sections of the hair shaft with SR-μFTIR, we found that calcium carboxylates are present in large regions of the hair cuticle, and not just in small focal areas as previously thought. We then use OPTIR to analyse the hair cuticle of intact hairs at submicron resolution without sectioning and report the distribution of calcium carboxylates at the surface of intact hair for the first time.

CONCLUSION

These new findings illustrate the potential of infrared microspectroscopy for imaging the chemical composition of human hair and may have implications for biomedical research or cosmetology.

A consumer cross-over trial suggests that there are significant seasonal changes of the tensile properties (wet) of human hair

OBJECTIVES

Through the cooperation with an industrial partner, we gained a set of data for the tensile properties (wet) of human hair. The hair samples originated from a cross-over study with two groups of individuals, using for a topical application sequentially two products (A and B). Each phase of the study lasted 6 months. The phases of the study by chance covered first largely the winter and then the summer period. Initially, tensile variables were chosen, which not only reflect the mechanical properties of hair (modulus, break strain, and break stress) but which are also considered to have a good connection to practice-relevant hair properties. The initial analysis of the data showed that changes were observed for the variables due to the treatment phases. However, these were either small or difficult to interpret.

METHODS

Against this background and using two-factor analysis of variance, we investigated the hypothesis that the tensile properties of hair (wet) may show significant seasonal changes. For this, we chose those two independent variables, which reflect the properties of the intermediate filaments (modulus) and the matrix (break strain) in the composite structure of the hair cortex.

RESULTS

The results support the 'seasonal' hypothesis and clearly show that the variables show significant changes from Winter to Summer (modulus: 10% increase; break strain: 3% decrease). The seasonal effect was thus a major reason, why the first stage of the analysis of the data was inconclusive.

CONCLUSIONS

The tensile properties of the main morphological components of the cortex show distinct seasonal changes. Towards the summer, the hair becomes stiffer and more brittle. Furthermore, the results suggest that seasonal effects may need to be taken into account when conducting studies on lengths of hair grown during different seasons.

Recent Progress in Hair Science and Trichology

Hair is important to our appearance as well as to protect our heads. Human hair mainly consists of proteins (80-85%), melanin pigments (0-5%), water (10-13%), and lipids (1-6%). The physicochemical properties of hair have been studied for over 100 years. However, they are not yet thoroughly understood. In this review, recent progress and the latest findings are summarized from the following three perspectives: structural characteristics, delivery and distribution of active ingredients, and hair as a template. The structural characteristics of hair have been mainly investigated by microscopic and/or spectroscopic techniques such as atomic force microscopy integrated with infrared spectroscopy (AFM-IR) and rheological measurements. The distribution of active ingredients has been generally evaluated through techniques such as nanoscale secondary ion mass spectrometry (NanoSIMS). And finally, attempts to explore the potential of hair to be used as a substrate for flexible device fabrication will be introduced.

Myc-dependent dedifferentiation of Gata6+ epidermal cells resembles reversal of terminal differentiation

Dedifferentiation is the process by which terminally differentiated cells acquire the properties of stem cells. During mouse skin wound healing, the differentiated Gata6-lineage positive cells of the sebaceous duct are able to dedifferentiate. Here we have integrated lineage tracing and single-cell mRNA sequencing to uncover the underlying mechanism. Gata6-lineage positive and negative epidermal stem cells in wounds are transcriptionally indistinguishable. Furthermore, in contrast to reprogramming of induced pluripotent stem cells, the same genes are expressed in the epidermal dedifferentiation and differentiation trajectories, indicating that dedifferentiation does not involve adoption of a new cell state. We demonstrate that dedifferentiation is not only induced by wounding, but also by retinoic acid treatment or mechanical expansion of the epidermis. In all three cases, dedifferentiation is dependent on the master transcription factor c-Myc. Mechanotransduction and actin-cytoskeleton remodelling are key features of dedifferentiation. Our study elucidates the molecular basis of epidermal dedifferentiation, which may be generally applicable to adult tissues.

The biophysical property of the limbal niche maintains stemness through YAP

The cell fate decisions of stem cells (SCs) largely depend on signals from their microenvironment (niche). However, very little is known about how biochemical niche cues control cell behavior in vivo. To address this question, we focused on the corneal epithelial SC model in which the SC niche, known as the limbus, is spatially segregated from the differentiation compartment. We report that the unique biomechanical property of the limbus supports the nuclear localization and function of Yes-associated protein (YAP), a putative mediator of the mechanotransduction pathway. Perturbation of tissue stiffness or YAP activity affects SC function as well as tissue integrity under homeostasis and significantly inhibited the regeneration of the SC population following SC depletion. In vitro experiments revealed that substrates with the rigidity of the corneal differentiation compartment inhibit nuclear YAP localization and induce differentiation, a mechanism that is mediated by the TGFβ-SMAD2/3 pathway. Taken together, these results indicate that SC sense biomechanical niche signals and that manipulation of mechano-sensory machinery or its downstream biochemical output may bear fruits in SC expansion for regenerative therapy.

Innovative Approaches and Advances for Hair Follicle Regeneration

Pathological hair loss (also known as alopecia) and shortage of hair follicle (HF) donors have posed an urgent requirement for HF regeneration. With the revelation of mechanisms in tissue engineering, the proliferation of HFs in vitro has achieved more promising trust for the treatments of alopecia and other skin impairments. Theoretically, HF organoids have great potential to develop into native HFs and attachments such as sweat glands after transplantation. However, since the rich extracellular matrix (ECM) deficiency, the induction characteristics of skin-derived cells gradually fade away along with their trichogenic capacity after continuous cell passaging in vitro. Therefore, ECM-mimicking support is an essential prelude before HF transplantation is implemented. This review summarizes the status of providing various epidermal and dermal cells with a three-dimensional (3D) scaffold to support the cell homeostasis and better mimic in vivo environments for the sake of HF regeneration. HF-relevant cells including dermal papilla cells (DPCs), hair follicle stem cells (HFSCs), and mesenchymal stem cells (MSCs) are able to be induced to form HF organoids in the vitro culture system. The niche microenvironment simulated by different forms of biomaterial scaffold can offer the cells a network of ordered growth environment to alleviate inductivity loss and promote the expression of functional proteins. The scaffolds often play the role of ECM substrates and bring about epithelial-mesenchymal interaction (EMI) through coculture to ensure the functional preservation of HF cells during in vitro passage. Functional HF organoids can be formed either before or after transplantation into the dermis layer. Here, we review and emphasize the importance of 3D culture in HF regeneration in vitro. Finally, the latest progress in treatment trials and critical analysis of the properties and benefits of different emerging biomaterials for HF regeneration along with the main challenges and prospects of HF regenerative approaches are discussed.

Age-associated thin hair displays molecular, structural and mechanical characteristic changes

Hair thinning occurs during normal chronological aging in women and in men leading to an increased level of thinner hair shafts alongside original thicker shafts. However, the characteristics of age-associated thin hairs remain largely unknown. Here we analyzed these characteristics by comparing at multiscale thin and thick hairs originated from Caucasian women older than 50 years. We observed that the cortex of thick hair contains many K35(+)/K38(-) keratinocytes that decrease in number with decreasing hair diameter. Accordingly, X-ray diffraction revealed differences supporting that thin and thick hairs are different with regards to the nature of the intermediate filaments making up their cortices. In addition, we observed a direct correlation between hair ellipticity and diameter with thin hairs having an unexpected round shape compared to the elliptic shape of thick hairs. We also observed fewer cuticle layers and a reduced frequency of a medullae in thin hairs. Regarding mechanical properties, thin hairs exhibited a surprising increased rigidity, a decrease of the viscosity and a decrease of the water diffusion coefficient. Hence, aged-associated thin hairs exhibit numerous modifications likely due to changes of hair differentiation program as evidenced by the modulations in the expression of hair keratins and keratin-associated proteins and by the X-ray diffraction specters. Hence, hair thinning with age does not consist simply of the production of a smaller hair. It is rather a more profound process likely relying on the implementation of an "aged hair program" that takes place within the hair follicle.

Division of force among layers constituting human hair during bending and tension

Human hair is a three-layered structure comprising the inner medulla, middle cortex, and outer cuticle layer. When a hair is subjected to bending or tensile load, each of these layers absorbs a certain amount of the force applied. However, the magnitude of the force absorbed by each layer is not easy to estimate. This is because, in addition to Young's modulus of each layer, the absorption depends on the area of each layer as seen in the cross-section. This study used a strategic way of combining experiment and theory and found that Young's modulus of the cuticle layer changes in the face of bending and tension. Considering this, the ratio of force sharing inside a human hair was estimated. Bending and tensile tests were conducted on single human hairs to determine the structural elasticity for both deformations which expresses the deformability of the hair independent of its external dimensions. Moreover, Young's modulus of each layer was determined by nanoindentation of hair cross-section. By comparing the structural elasticity determined experimentally with that determined theoretically, Young's modulus of the cuticle layer against tension was found to be 25% of that against bending. Based on this finding, it was found that the cuticle layer bears 35% of the force endured during bending, and the cortex bears the rest; when subjected to tension, the cortex bears more than 90% of the force.

The susceptibility of disulfide bonds to modification in keratin fibres undergoing tensile stress

Cysteine residues perform a dual role in mammalian hairs. The majority help stabilise the overall assembly of keratins and their associated proteins, but a proportion of inter-molecular disulfide bonds are assumed to be associated with hair mechanical flexibility. Hair cortical microstructure is hierarchical, with a complex macro-molecular organisation resulting in arrays of intermediate filaments at a scale of micrometres. Intermolecular disulfide bonds occur within filaments and between them and the surrounding matrix. Wool fibres provide a good model for studying various contributions of differently situated disulfide bonds to fibre mechanics. Within this context it is not known if all intermolecular disulfide bonds contribute equally, and, if not, then do the disproportionally involved cysteine residues occur at common locations on proteins. In this study, fibres from Romney sheep were subjected to stretching or to breaking point under wet or dry conditions to detect, through labelling, disulfide bonds that were broken more often than randomly. We found that some cysteines were labelled more often than randomly and that these vary with fibre water content (water disrupts protein-protein hydrogen bonds). Many of the identified cysteine residues were located close to the terminal ends of keratins (head or tail domains) and keratin-associated proteins (KAPs). Some cysteines in the head and tail domains of type II keratin K85 were labelled in all experimental conditions. When inter-protein hydrogen bonds were disrupted under wet conditions, disulfide labelling occurred in the head domains of type II keratins, likely affecting keratin-KAP interactions, and tail domains of the type I keratins, likely affecting keratin-keratin interactions. In contrast, in dry fibres (containing more protein-protein hydrogen bonding) disulfide labelling was also observed in the central domains of affected keratins. This central "rod" region is associated with keratin-keratin interactions between anti-parallel heterodimers in the tetramer of the intermediate filament.

Site-specific variations in surface structure and Young's modulus of human hair surfaces at the nanometer scale as induced through bleach treatment

The effect of bleach treatments on the morphology and mechanical properties of hair surfaces was measured at the nanometer scale using atomic force microscopy. We used an ultrahigh-precision relocation technique to observe the variations in these properties at precise locations on hair surfaces in their virgin state and then after each of the two bleach treatments, to rule out position-dependent fluctuations. We demonstrate that statistically significant variations in roughness and Young's modulus are observed as a result of exposure to bleach, which is known to disrupt the disulfide linkage network throughout the fiber. The rate at which surface roughness changes increased with the number of treatments, with very little effect seen after 10 min, and an increase of up to 65% was observed after a further 10 min. The Young's modulus decreased by up to 40% after each treatment. We also investigate micropores and show that they are subsurface, but revealed through bleaching, and oriented along the direction of the hair shaft with a characteristic aspect ratio. This work demonstrates the profound effect bleaching has on the molecular structure of hair, which manifests as changes in morphology and stiffness, and this should be taken into account in the formulation of future hair-care products.

Processing hair follicles for transmission electron microscopy

Transmission electron microscopy (TEM) has greatly advanced our knowledge of hair growth and follicle morphogenesis, but complex preparations such as fixation, dehydration and embedding compromise ultrastructure. While recent developments with cryofixation have been shown to preserve the ultrastructure of biological materials close to native state, they do have limitations. This review will focus on each stage of the TEM sample preparation process and their effects on the structural integrity of follicles.

Anisotropic hair keratin-dopamine composite scaffolds exhibit strain-stiffening properties

Human hair keratin (HHK) has been successfully explored as raw materials for three-dimensional scaffolds for soft tissue regeneration due to its excellent biocompatibility and bioactivity. However, none of the reported HHK based scaffolds is able to replicate the strain-stiffening capacity of living tissues when responding to large deformations. In the present study, strain-stiffening property was achieved in scaffolds fabricated from HHK via a synergistic effect of well-defined, aligned microstructure and chemical crosslinking. Directed ice-templating method was used to fabricate HHK-based scaffolds with highly aligned (anisotropic) microstructure while oxidized dopamine (ODA) was used to crosslink covalently to HHKs. The resultant HHK-ODA scaffolds exhibited strain-stiffening behavior characterized by the increased gradient of the stress-strain curve after the yield point. Both ultimate tensile strength and the elongation at break were enhanced significantly (~700 kPa, ~170%) in comparison to that of HHK scaffolds lacking of aligned microstructure or ODA crosslinking. In vitro cell culture studies indicated that HHK-ODA scaffolds successfully supported human dermal fibroblasts (HDFs) adhesion, spreading and proliferation. Moreover, anisotropic HHK-ODA scaffolds guided cell growth in alignment with the defined microstructure as shown by the highly organized cytoskeletal networks and nuclei distribution. The findings suggest that HHK-ODA scaffolds, with strain-stiffening properties, biocompatibility and bioactivity, have the potential to be applied as biomimetic matrices for soft tissue regeneration.

Characterizing viscoelastic properties of synthetic and natural fibers and their coatings with a torsional pendulum

Characterizing and understanding the viscoelastic mechanical properties of natural and synthetic fibers is of great importance in many biological and industrial applications. Microscopic techniques such as micro/nano indentation have been successfully employed in such efforts, yet these tests are often challenging to perform on fibers and come with certain limitations in the interpretation of the obtained results within the context of the macroscopic viscoelasticity in the fiber. Here we instead explore the properties of a series of natural and synthetic fibers, using a freely-oscillating torsional pendulum. The torsional oscillation of the damped mass-fiber system is precisely recorded with a simple HD video-camera and an image processing algorithm is used to analyze the resulting videos. Analysis of the processed images show a viscoelastic damped oscillatory response and a simple mechanical model describes the amplitude decay of the oscillation data very well. The natural frequency of the oscillation and the corresponding damping ratio can be extracted using a logarithmic decrement method and directly connected to the bulk viscoelastic properties of the fiber. We further study the sensitivity of these measurements to changes in the chemo-mechanical properties of the outer coating layers on one of the synthetic fibers. To quantify the accuracy of our measurements with the torsional pendulum, a complementary series of tests are also performed on a strain-controlled rheometer in both torsional and tensile deformation modes.

Role of Scalp Health in Achieving Optimal Hair Growth and Retention

We have conducted a thorough review of the literature to assess the evidence for supporting a cause-and-effect linkage between scalp condition and resultant hair condition. Over 20 epidemiological studies have been published covering a wide range of abnormal scalp conditions in which consequent impacts to the hair have been documented. A treatment study was conducted to demonstrate not only that impaired scalp condition led to impaired hair quality but that the impacts to hair are reversible upon normalization of the scalp condition. A proposed explanation involves the impact of scalp oxidative stress, which is part of the etiology of these scalp conditions as well as normal aging, in interfering with the normal keratinization of the pre-emergent hair cuticle. This perturbed cuticle impedes normal fiber anchorage and emerges more brittle and fragile than normal cuticle leading to accelerated physical degradation, mirroring the effects of chronological aging of the hair fiber. The consequences of the rapid cuticle degradation result in hair that is more vulnerable to mechanical insults and compromised overall quality.

Structural elasticity for tensile deformation of a single human hair and the comparison with it for the bending deformation

Human hair is a multi-layered structure, which consists of the inner medulla, middle cortex, and outer cuticle. Therefore, the mechanical properties of the hair are related not only to the Young's modulus of each layer but also to the internal structures. Although the tensile test of a human hair has been performed elsewhere, the deformability of the hair for the tensile deformation is determined as the Young's modulus of the hair structure, which is similar to that of metals. In this paper, the structural elasticity of a single human hair for the tensile deformation, which expresses the deformability of a hair by tension without being dependent on external dimensions and shape, is defined based on the theoretical model, and is measured by performing the tensile test under the digital microscope observation. The values of the structural elasticity for the tensile deformation of the hair samples collected from healthy persons are compared with the values obtained for bending deformation. The structural elasticity for the tensile deformation of the hair sample is found to be lesser than that of the bending deformation, and this is verified to be always valid provided the Young's modulus of the outer cuticle is greater than that of the middle cortex.

Porosity at Different Structural Levels in Human and Yak Belly Hair and Its Effect on Hair Dyeing

Yak belly hair was proposed as a cheap substitute for human hair for the development of hair dyes, as its chemical composition closely resembles human hair in Raman spectroscopy. The absence of melanin in yak belly hair also leads to a strong reduction of fluorescence in Raman measurements, which is advantageous for the investigation of the effectivity of hair dyes. To assess the suitability for replacing human hair, we analyzed similarities and differences of both hair types with a variety of methods: Raman spectroscopy, to obtain molecular information; small-angle X-ray scattering to determine the nanostructure, such as intermediate filament distance, distance of lipid layers and nanoporosity; optical and scanning electron microscopy of surfaces and cross sections to determine the porosity at the microstructural level; and density measurements and tensile tests to determine the macroscopic structure, macroporosity and mechanical properties. Both types of hair are similar on a molecular scale, but differ on other length scales: yak belly hair has a smaller intermediate filament distance on the nanoscale. Most striking is a higher porosity of yak belly hair on all hierarchical levels, and a lower Young’s modulus on the macroscale. In addition to the higher porosity, yak belly hair has fewer overlapping scales of keratin, which further eases the uptake of coloring. This makes, on the other hand, a comparison of coloring processes difficult, and limits the usefulness of yak belly hair as a substitute for human hair.

Novel Polymeric Organosilica Precursor and Emulsion Stabilizer: Toward Highly Elastic Hollow Organosilica Nanospheres

The fabrication of hollow organosilica nanoparticles with high elasticity is greatly desirable but still challenging. Herein, we present a new and simple strategy to prepare such nanoparticles by using hyperbranched polyvinylpolytrimethoxysilane (PVPMS) via a soap-free oil in water (O/W) emulsion system. PVPMS was synthesized through the radical polymerization of vinyltrimethoxysilane (VMS) followed by the acid-catalyzed hydrolytic polycondensation of trimethoxysilyl groups, which works not only as an organosilica precursor but also as a sole emulsion stabilizer due to its hydrolysis-induced amphiphilicity at the oil/water interface. When styrene was used as the oil phase and initiated to polymerize, hybrid polystyrene (PS) core-organosilica shell (PS@organosilica) nanoparticles were obtained by controlling the reaction conditions. Furthermore, highly elastic hollow organosilica nanospheres with low Young's modulus (∼220 MPa) were yielded through solvent etching of the core. This study expands the scope of organosilica precursor from small molecule organosilane to polymeric macromolecule and provides useful guidance for application in other polyorganosilsesquioxane related hybrid organosilica particles and functional hollow nanoparticles.

Human pollution exposure correlates with accelerated ultrastructural degradation of hair fibers

Air pollution via phototoxic polycyclic aromatic hydrocarbons (PAHs) is a major risk factor for human health. While in vitro observations and in vivo correlations suggest a detrimental effect of PAHs at physiological concentrations, in vivo observations of the structural impact of PAHs are scarce. Here, we use transmission electron microscopy on human hair fibers containing known concentrations of 25 biomarkers of PAH exposure. We show an increased structural degradation of the hair fiber over time, when increased PAH concentrations are present. Moreover, we show that exposure to UV radiation explains part of the increased damage in more contaminated fibers. Our results point toward possible detrimental effects in other human tissues at physiological concentrations of PAHs.

Exposure to pollution is a known risk factor for human health. While correlative studies between exposure to pollutants such as polycyclic aromatic hydrocarbons (PAHs) and human health exist, and while in vitro studies help to establish a causative connection, in vivo comparisons of exposed and nonexposed human tissue are scarce. Here, we use human hair as a model matrix to study the correlation of PAH pollution with microstructural changes over time. Two hundred four hair samples from 2 Chinese cities with distinct pollution exposure were collected, and chromatographic-mass spectrometry was used to quantify the PAH-exposure profiles of each individual sample. This allowed us to define a group of less contaminated hair samples as well as a more contaminated group. Using transmission electron microscopy (TEM) together with quantitative image analysis and blind scoring of 82 structural parameters, we find that the speed of naturally occurring hair-cortex degradation and cuticle delamination is increased in fibers with increased PAH concentrations. Treating nondamaged hair fibers with ultraviolet (UV) irradiation leads to a more pronounced cortical damage especially around melanosomes of samples with higher PAH concentrations. Our study shows the detrimental effect of physiological concentrations of PAH together with UV irradiation on the hair microstructure but likely can be applied to other human tissues.

Prevention of lipid loss from hair by surface and internal modification

Surfactants during routine washing have a tremendous effect on lipid loss from hair. This study aims to understand the loss of lipids from hair upon contact with surfactants and develop a way to prevent the lipid loss. The change in lipid levels depends on the relative hydrophobicity of the lipid. We herein propose that the change in lipid levels can be protected by two modifications. In the case of fatty acids and cholesterol (group A), the concentration difference between virgin hair versus surface modified hair with highly charged polymer was 22 to 32% higher after washing with surfactants while the loss of squalene and wax esters (group B) in response to surfactants still occurred even after the surface modification. In the hair treated by internal modification with the carbodiimide reaction, 52.0 to 81.3% more lipids in group B were prevented than in the untreated hair. Finally, different types of lipids were successfully protected by surface and internal modifications from the surfactant treatment. This study will be the basis for understanding the mechanisms by which surfactants damage the lipid barrier of tissues including hair and for establishing strategies to defend the barrier.

Physics of animal health: on the mechano-biology of hoof growth and form

Global inequalities in economic access and agriculture productivity imply that a large number of developing countries rely on working equids for transport/agriculture/mining. Therefore, the understanding of hoof conditions/shape variations affecting equids' ability to work is still a persistent concern. To bridge this gap, using a multi-scale interdisciplinary approach, we provide a bio-physical model predicting the shape of equids' hooves as a function of physical and biological parameters. In particular, we show (i) where the hoof growth stress originates from, (ii) why the hoof growth rate is one order of magnitude higher than the proliferation rate of epithelial cells and (iii) how the soft-to-hard transformation of the epithelium is possible allowing the hoof to fulfil its function as a weight-bearing element. Finally (iv), we demonstrate that the reason for hoof misshaping is linked to the asymmetrical design of equids' feet (shorter quarters/long toe) together with the inability of the biological growth stress to compensate for such an asymmetry. Consequently, the hoof can adopt a dorsal curvature and become 'dished' overtime, which is a function of the animal's mass and the hoof growth rate. This approach allows us to discuss the potential occurrence of this multifaceted pathology in equids.

A natural energy absorbent polymer composite: The equine hoof wall

The equine hoof has been considered as an efficient energy absorption layer that protects the skeletal elements from impact when galloping. In the present study, the hierarchical structure of a fresh equine hoof wall and the energy absorption mechanisms are investigated. Tubules are found embedded in the intertubular matrix forming the hoof wall at the microscale. Both tubules and intertubular areas consist of keratin cells, in which keratin crystalline intermediate filaments (IFs) and amorphous keratin fill the cytoskeletons. Cell sizes, shapes and IF fractions are different between tubular and intertubular regions. The structural differences between tubular and intertubular areas are correlated to the mechanical behavior of this material tested in dry, fresh and fully hydrated conditions. The stiffness and hardness in the tubule areas are higher than that in the intertubular areas in the dry and fresh samples when loaded along the hoof wall; however, once the samples are fully hydrated, the intertubular areas become stiffer than the tubular areas due to higher water absorption in these regions. The compression behavior of hoof in different loading speed and directions are also examined, with the isotropy and strain-rate dependence of mechanical properties documented. In the hoof walls, mechanistically the tubules serve as a reinforcement, which act to support the entire wall and prevent catastrophic failure under compression and impact loading. Elastic buckling and cracking of the tubules are observed after compression along the hoof wall, and no shear-banding or severe cracks are found in the intertubular areas even after 60% compression, indicating the highly efficient energy absorption properties, without failure, of the hoof wall structure. STATEMENT OF SIGNIFICANCE: The equine hoof wall is found to be an efficient energy absorbent natural polymer composite. Previous studies showed the microstructure and mechanical properties of the hoof wall in some perspective. However, the hierarchical structure of equine hoof wall from nano- to macro-scale as well as the energy absorption mechanisms at different strain rates and loading orientations remains unclear. The current study provides a thorough characterization of the hierarchical structure as well as the correlation between structure and mechanical behaviors. Energy dissipation mechanisms are also identified. The findings in the current research could provide inspirations on the designs of impact resistant and energy absorbent materials.

Microscopy and development of a remarkable pitted quill from the thin-spined porcupine,Chaetomyssubspinosus

Scanning electron microscopy shows that the quill surface from the thin-spined porcupine (Chaetomys subspinosus (Olfers, 1818)) has several regional cuticular patterns of which the most expansive is highly pitted with 4 μm wide pores leading to pits, which in turn communicate circumferentially via tunnels to neighboring pits. The cell unit of the pitted layer is a hexagonal or pentagonal prism, the “pitted fibrillous cuticular cell” (PFCC), which has a superficial pitted cuticular scale derived part with an underlying part packed with fine fibrils and the nucleus. The scaled part has about six scales, which swell and fuse except where raised cuticular scales from the inner root sheath intervene. This unique and complicated cell soon dies even while below the skin’s surface during its development. The raised scale-bearing cells of the inner root sheath leave their imprints on the PFCC’s surface, thus indicating that the quill’s pitted layer is at one time soft. Histochemistry, energy dispersive X-ray spectrometry, and Fourier transform infrared spectrometry show that the PFCC is keratinous.

Impact of substrate stiffness on dermal papilla aggregates in microgels

Interaction between cells and the extracellular environment plays a vital role in cellular development. The mechanical property of a 3-dimensional (3D) culture can be modified to mimic in vivo conditions. Dermal papilla (DP) cells are shown to gradually lose their inductivity in hair cycle development in a 2-dimensional culture. They are shown to partially restore their inductivity when transferred into a 3D microenvironment. In this study, a microarray fabricated from three different concentrations of poly-ethylene-glycol-diacrylate 3500, namely 5%, 10% and 15% w/v, yielded increasing substrate stiffness. The impact of varying substrate stiffness was tested for DP cell viability, attachment, and selected hair inductive markers. DP aggregates were shown to be viable and exhibited greater spreading with increasing substrate stiffness. Moreover, DP aggregates cultured on a softer substrate showed a greater fold change of gene and protein expressions than those cultured on a harder substrate.

Computation predicts rapidly adapting mechanotransduction currents cannot account for tactile encoding in Merkel cell-neurite complexes

Distinct firing properties among touch receptors are influenced by multiple, interworking anatomical structures. Our understanding of the functions and crosstalk of Merkel cells and their associated neurites—the end organs of slowly adapting type I (SAI) afferents—remains incomplete. Piezo2 mechanically activated channels are required both in Merkel cells and in sensory neurons for canonical SAI responses in rodents; however, a central unanswered question is how rapidly inactivating currents give rise to sustained action potential volleys in SAI afferents. The computational model herein synthesizes mechanotransduction currents originating from Merkel cells and neurites, in context of skin mechanics and neural dynamics. Its goal is to mimic distinct spike firing patterns from wildtype animals, as well as Atoh1 knockout animals that completely lack Merkel cells. The developed generator function includes a Merkel cell mechanism that represents its mechanotransduction currents and downstream voltage-activated conductances (slower decay of current) and a neurite mechanism that represents its mechanotransduction currents (faster decay of current). To mimic sustained firing in wildtype animals, a longer time constant was needed than the 200 ms observed for mechanically activated membrane depolarizations in rodent Merkel cells. One mechanism that suffices is to introduce an ultra-slowly inactivating current, with a time constant on the order of 1.7 s. This mechanism may drive the slow adaptation of the sustained response, for which the skin’s viscoelastic relaxation cannot account. Positioned within the sensory neuron, this source of current reconciles the physiology and anatomical characteristics of Atoh1 knockout animals.

Slowly-adapting type I (SAI) cutaneous afferents help us discriminate fine spatial details. Their physiology and anatomy are distinguished by their slow adaptation in firing to held stimuli and innervation of Merkel cells, respectively. How mechanotransduction currents in Merkel cells and sensory neurons combine to give rise to neural spike firing is unknown. In considering wildtype animals, as well as Atoh1 conditional knockout animals that lack Merkel cells, this effort employs a computational modeling approach constrained by biological measurements. For the developed generator function to recapitulate firing responses across genotype, a previously unsuspected current source is required. Thus, the model makes specific predictions for future experimental studies.

Development of Hair Fibres

The growth of hairs occurs during the anagen phase of the follicle cycle. Hair growth begins with basement membrane-bound stem cells (mother cells) around the dermal papilla neck which continuously bud off daughter cells which further divide as a transient amplifying population. Division ceases as cell line differentiation begins, which entails changes in cell junctions, cell shape and position, and cell-line specific cytoplasmic expression of keratin and trichohyalin. As the differentiating cells migrate up the bulb, nuclear function ceases in cortex, cuticle and inner root sheath (IRS) layers. Past the top of the bulb, cell shape/position changes cease, and there is a period of keratin and keratin-associated protein (KAP) synthesis in fibre cell lines, with increases, in particular of KAP species. A gradual keratinization process begins in the cortex at this point and then non-keratin cell components are increasingly broken down. Terminal cornification, or hardening, is associated with water loss and precipitation of keratin. In the upper follicle, the hair, now in its mature form, detaches from the IRS, which is then extracted of material and becomes fragmented to release the fibre. Finally, the sebaceous and sudoriferous (if present) glands coat the fibre in lipid-rich material and the fibre emerges from the skin. This chapter follows the origin of the hair growth in the lower bulb and traces the development of the various cell lines.

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.

Desmosomes and Intermediate Filaments: Their Consequences for Tissue Mechanics

Adherens junctions (AJs) and desmosomes connect the actin and keratin filament networks of adjacent cells into a mechanical unit. Whereas AJs function in mechanosensing and in transducing mechanical forces between the plasma membrane and the actomyosin cytoskeleton, desmosomes and intermediate filaments (IFs) provide mechanical stability required to maintain tissue architecture and integrity when the tissues are exposed to mechanical stress. Desmosomes are essential for stable intercellular cohesion, whereas keratins determine cell mechanics but are not involved in generating tension. Here, we summarize the current knowledge of the role of IFs and desmosomes in tissue mechanics and discuss whether the desmosome-keratin scaffold might be actively involved in mechanosensing and in the conversion of chemical signals into mechanical strength.

Silk-fibronectin protein alloy fibres support cell adhesion and viability as a high strength, matrix fibre analogue

Silk is a natural polymer with broad utility in biomedical applications because it exhibits general biocompatibility and high tensile material properties. While mechanical integrity is important for most biomaterial applications, proper function and integration also requires biomaterial incorporation into complex surrounding tissues for many physiologically relevant processes such as wound healing. In this study, we spin silk fibroin into a protein alloy fibre with whole fibronectin using wet spinning approaches in order to synergize their respective strength and cell interaction capabilities. Results demonstrate that silk fibroin alone is a poor adhesive surface for fibroblasts, endothelial cells, and vascular smooth muscle cells in the absence of serum. However, significantly improved cell attachment is observed to silk-fibronectin alloy fibres without serum present while not compromising the fibres' mechanical integrity. Additionally, cell viability is improved up to six fold on alloy fibres when serum is present while migration and spreading generally increase as well. These findings demonstrate the utility of composite protein alloys as inexpensive and effective means to create durable, biologically active biomaterials.

Keratin-associated protein 5-5 controls cytoskeletal function and cancer cell vascular invasion

Cancer cell vascular invasion is a crucial step in the malignant progression towards metastasis. Here we used a genome-wide RNAi screen with E0771 mammary cancer cells to uncover drivers of endothelial monolayer invasion. We identified keratin-associated protein 5-5 (Krtap5-5) as a candidate. Krtap5-5 belongs to a large protein family that is implicated in crosslinking keratin intermediate filaments during hair formation, yet these keratin-associated proteins have no reported role in cancer. Depletion of Krtap5-5 from cancer cells led to cell blebbing and a loss of keratins 14 and 18, in addition to the upregulation of vimentin intermediate filaments. This intermediate filament subtype switching induced dysregulation of the actin cytoskeleton and reduced the expression of hemidesmosomal α6/β4-integrins. We further demonstrate that knockdown of keratin 18 phenocopies the loss of Krtap5-5, suggesting that Krtap5-5 crosstalks with keratin 18 in E0771 cells. Disruption of the keratin cytoskeleton by perturbing Krtap5-5 function broadly altered the expression of cytoskeleton regulators and the localization of cell surface markers. Krtap5-5 depletion did not impact cell viability but reduced cell motility and extracellular matrix invasion, as well as extravasation of cancer cells into tissues in zebrafish and mice. We conclude that Krtap5-5 is a previously unknown regulator of cytoskeletal function in cancer cells that modulates motility and vascular invasion. Thus, in addition to its physiologic function, a keratin-associated protein can serve as a switch towards malignant progression.