Rete ridges: Morphogenesis, function, regulation, and reconstruction

Numerical investigation of new rete ridge formation in a multi-layer model of skin subjected to tissue expansion

The skin is a multilayered organ with microstructural and antomical heterogeneities that contribute to its unique mechanophysiology. Between the epidermis layer at the top and the dermis layer below, the basal keratinocytes form an interface with sinusoidal-like geometry termed rete ridges. In previous computational work we showed that the rete ridges contribute to lower delamination risk by increasing surface area and reducing the stress jump across the interface. Experimentally, we and others have shown that upon repeated tissue expansion and growth, physiological rete ridge frequency is preserved. Here we implement a 2D multilayered skin model where each layer is able to grow in response to applied loading toward recovering the layer-specific homeostatic stretch. Our simulations support the hypothesis that mechanics of growing tissue can explain secondary buckling and new rete ridge formation in tissue expansion. The process is robust with respect to parameters such as homeostatic stretch, layer thicknesses, and shear moduli of the different layers. Thicker epidermis suppresses higher frequency features, and so does a stiffer epidermis with respect to the basal layer. Interestingly, new rete ridge valleys are formed at locations that were originally peaks of the sine wave, whereas original valleys remain valleys. This pattern might have a connection to the localization of stem cell and transient amplifying cells in the epidermis. This study does not discard the role of cell-cell signaling dynamics, but rather emphasizes the possibility of achieving robust geometric patterns with simple rules of growing tissue, even in the absence of complex regulatory networks.

Dermal Papilla Cells: From Basic Research to Translational Applications

As an appendage of the skin, hair protects against ultraviolet radiation and mechanical damage and regulates body temperature. It also reflects an individual's health status and serves as an important method of expressing personality. Hair loss and graying are significant psychosocial burdens for many people. Hair is produced from hair follicles, which are exclusively controlled by the dermal papilla (DP) at their base. The dermal papilla cells (DPCs) comprise a cluster of specialized mesenchymal cells that induce the formation of hair follicles during early embryonic development through interaction with epithelial precursor cells. They continue to regulate the growth cycle, color, size, and type of hair after the hair follicle matures by secreting various factors. DPCs possess stem cell characteristics and can be cultured and expanded in vitro. DPCs express numerous stemness-related factors, enabling them to be reprogrammed into induced pluripotent stem cells (iPSCs) using only two, or even one, Yamanaka factor. DPCs are an important source of skin-derived precursors (SKPs). When combined with epithelial stem cells, they can reconstitute skin and hair follicles, participating in the regeneration of the dermis, including the DP and dermal sheath. When implanted between the epidermis and dermis, DPCs can induce the formation of new hair follicles on hairless skin. Subcutaneous injection of DPCs and their exosomes can promote hair growth. This review summarizes the in vivo functions of the DP; highlights the potential of DPCs in cell therapy, particularly for the treatment of hair loss; and discusses the challenges and recent advances in the field, from basic research to translational applications.

Human lip vermilion: Physiology and age-related changes

BACKGROUND

The human lip vermilion, also known as the red lip, is important to the quality of life and has long attracted the attention of cosmetic researchers. However, there is limited existing literature on the physiological characteristics and age-related alterations in the human vermilion.

OBJECTIVE

We aim to provide an overview of the physiological characteristics and age-related alterations in the human vermilion.

METHODS

This article is a result of previous research. We conducted a literature search using various academic databases such as Google Scholar, Web of Science, and PubMed. Our findings provided a comprehensive understanding of the physiological characteristics and age-related changes of the human lip vermilion.

RESULTS

The human lip vermilion has a unique structure and physiological characteristics, and during the aging process, a few changes may occur in the human lip vermilion.

CONCLUSION

Understanding the human lip vermilion's physiological characteristics and age-related changes can provide key information for the future innovation of lip vermilion care products. Further investigations are necessary to reach a consensus on the physiological characteristics and age-related alterations in the human vermilion.

Evaluation of Porcine-Derived Collagen Membranes for Soft Tissue Augmentation in the Oral Cavity: An In Vivo Study

The use of porcine-derived collagen membranes (PDCM) to improve intraoral soft tissue rehabilitation remains under investigation. Different degrees of crosslinking have yielded differences in resorption time and inflammation surrounding collagen membranes. The aim of this study was to evaluate the in vivo performance of bilayered PDCMs with varying degrees of crosslinking for the regeneration of oral soft tissue defects. Bilateral split-thickness oral mucosa defects were created in mandibles of beagles (n=17) and assigned to one of the following: bilayer PDCM (high crosslinking porcine dermis in sheet form-H-xlink) and (low crosslinking porcine dermis in sheet form-L-xlink), bilayer PDCM (non-crosslinked predicate collagen membrane in spongy form-Ctrl), or negative control (Sham) and compared with positive control (unoperated). Animals were euthanized after 4-, 8-, or 12-weeks of healing to evaluate soft tissue regeneration and remodeling through histomorphometric analyses. H-xlink membranes presented delayed healing with a poorly developed epithelial layer (analogous to the sham group) across time points. Relative to Ctrl at 8 and 12 weeks, defects treated with H-xlink presented no difference in semiquantitative scores ( P > 0.05), while L-xlink exhibited greater healing ( P = 0.042, P = 0.043, at 8 and 12 weeks, respectively). Relative to positive control, L-xlink exhibited similar healing at 8 weeks and greater healing at 12 weeks ( P = 0.037) with a well-developed epithelial layer. Overall, groups treated with L-xlink presented with greater healing relative to the positive control after 12 weeks of healing and may serve as an alternative to autologous grafts for intraoral soft tissue regeneration.

The Role of PGC-1α in Aging Skin Barrier Function

Skin provides a physical and immune barrier to protect the body from foreign substances, microbial invasion, and desiccation. Aging reduces the barrier function of skin and its rate of repair. Aged skin exhibits decreased mitochondrial function and prolonged low-level inflammation that can be seen in other organs with aging. Peroxisome proliferator-activated receptor (PPAR)-γ coactivator-1α (PGC-1α), an important transcriptional coactivator, plays a central role in modulating mitochondrial function and antioxidant production. Mitochondrial function and inflammation have been linked to epidermal function, but the mechanisms are unclear. The aim of this review is to discuss the mechanisms by which PGC-1α might exert a positive effect on aged skin barrier function. Initially, we provide an overview of the function of skin under physiological and aging conditions, focusing on the epidermis. We then discuss mitochondrial function, oxidative stress, cellular senescence, and inflamm-aging, the chronic low-level inflammation observed in aging individuals. Finally, we discuss the effects of PGC-1α on mitochondrial function, as well as the regulation and role of PGC-1α in the aging epidermis.

T Cell Surveillance during Cutaneous Viral Infections

The skin is a complex tissue that provides a strong physical barrier against invading pathogens. Despite this, many viruses can access the skin and successfully replicate in either the epidermal keratinocytes or dermal immune cells. In this review, we provide an overview of the antiviral T cell biology responding to cutaneous viral infections and how these responses differ depending on the cellular targets of infection. Much of our mechanistic understanding of T cell surveillance of cutaneous infection has been gained from murine models of poxvirus and herpesvirus infection. However, we also discuss other viral infections, including flaviviruses and papillomaviruses, in which the cutaneous T cell response has been less extensively studied. In addition to the mechanisms of successful T cell control of cutaneous viral infection, we highlight knowledge gaps and future directions with possible impact on human health.

A Novel Method for Fabricating the Undulating Structures at Dermal-Epidermal Junction by Composite Molding Process

The dermal-epidermal junction (DEJ), located between the dermal-epidermal layers in human skin tissue, plays a significant role in its function. However, the limitations of biomaterial properties and microstructure fabrication methods mean that most current tissue engineered skin models do not consider the existence of DEJ. In this study, a nanofiber membrane that simulates the fluctuating structure of skin DEJ was prepared by the composite molding process. Electrospinning is a technique for the production of nanofibers, which can customize the physical and biological properties of biomaterials. At present, electrospinning technology is widely used in the simulation of customized natural skin DEJ. In this study, four different concentration ratios of poly (lactic-co-glycolic acid) (PLGA) and polycaprolactone (PCL) nanofiber membranes were prepared based on electrospinning technology. We selected a 15%PLGA + 5%PCL nanofiber membrane with mechanical properties, dimensional stability, hydrophilicity, and biocompatibility after physical properties and biological characterization. Then, the array-based microstructure model was prepared by three-dimensional (3D) printing. Subsequently, the microstructure was created on a 15%PLGA + 5%PCL membrane by the micro-imprinting process. Finally, the cell proliferation and live/dead tests of keratinocytes (HaCaTs) and fibroblasts (HSFs) were measured on the microstructural membrane and flat membrane. The results showed that 15%PLGA + 5%PCL microstructure membrane was more beneficial to promote the adhesion and proliferation of HaCaTs and HSFs than a flat membrane.

The role of interface geometry and appendages on the mesoscale mechanics of the skin

The skin is the largest organ in the human body and serves various functions, including mechanical protection and mechanosensation. Yet, even though skin's biomechanics are attributed to two main layers-epidermis and dermis-computational models have often treated this tissue as a thin homogeneous material or, when considering multiple layers, have ignored the most prominent heterogeneities of skin seen at the mesoscale. Here, we create finite element models of representative volume elements (RVEs) of skin, including the three-dimensional variation of the interface between the epidermis and dermis as well as considering the presence of hair follicles. The sinusoidal interface, which approximates the anatomical features known as Rete ridges, does not affect the homogenized mechanical response of the RVE but contributes to stress concentration, particularly at the valleys of the Rete ridges. The stress profile is three-dimensional due to the skin's anisotropy, leading to high-stress bands connecting the valleys of the Rete ridges through one type of saddle point. The peaks of the Rete ridges and the other class of saddle points of the sinusoidal surface form a second set of low-stress bands under equi-biaxial loading. Another prominent feature of the heterogeneous stress pattern is a switch in the stress jump across the interface, which becomes lower with respect to the flat interface at increasing deformations. These features are seen in both tension and shear loading. The RVE with the hair follicle showed strains concentrating at the epidermis adjacent to the hair follicle, the epithelial tissue surrounding the hair right below the epidermis, and the bulb or base region of the hair follicle. The regions of strain concentration near the hair follicle in equi-biaxial and shear loading align with the presence of distinct mechanoreceptors in the skin, except for the bulb or base region. This study highlights the importance of skin heterogeneities, particularly its potential mechanophysiological role in the sense of touch and the prevention of skin delamination.

Modeling the extracellular matrix in cell migration and morphogenesis: a guide for the curious biologist

The extracellular matrix (ECM) is a highly complex structure through which biochemical and mechanical signals are transmitted. In processes of cell migration, the ECM also acts as a scaffold, providing structural support to cells as well as points of potential attachment. Although the ECM is a well-studied structure, its role in many biological processes remains difficult to investigate comprehensively due to its complexity and structural variation within an organism. In tandem with experiments, mathematical models are helpful in refining and testing hypotheses, generating predictions, and exploring conditions outside the scope of experiments. Such models can be combined and calibrated with in vivo and in vitro data to identify critical cell-ECM interactions that drive developmental and homeostatic processes, or the progression of diseases. In this review, we focus on mathematical and computational models of the ECM in processes such as cell migration including cancer metastasis, and in tissue structure and morphogenesis. By highlighting the predictive power of these models, we aim to help bridge the gap between experimental and computational approaches to studying the ECM and to provide guidance on selecting an appropriate model framework to complement corresponding experimental studies.

Meibomian Gland Probing Stimulates a Proliferative Epithelial Response Resulting in Duct Regeneration

Purpose

To demonstrate that the meibomian gland ductal basement membrane and basal epithelial cell layer are in continuity with and may derive from lid margin orifice-associated rete ridge epithelial/basement membrane structures (OARREBS) and to characterize changes in the distal duct microanatomy after meibomian gland probing (MGP) using in vivo confocal microscopy (IVCM).

Patients and Methods

Pre/post-MGP IVCM examinations were performed on upper lids. Thirty-six identical glands from 20 lids of 16 patients (49.24 ±17.11 y/o with 13:3 F:M) were identified, analyzed, and compared to control cases. Statistical analyses were performed using ImageJ software and IBM SPSS version 27. All MGPs were performed within 12 weeks of the initial examination. Post-MGP follow-up exams occurred at 5.03 ±4.48 months.

Results

Post-MGP images showed more superficially organized OARREBS with accelerated and more superficial basement membrane formation, and an average increase of 32.2%, 25.4%, 32.04%, 77.7%, and 81.3% in duct wall epithelial cell layers (DWECL) (p < 0.001, compared to control (CTC) p < 0.001), distal duct wall thickness (DWT) (p < 0.001, CTC p < 0.001), proximal DWT (p < 0.001, CTC p < 0.001), distal lumen area (p < 0.001, CTC p = 0.037), and proximal lumen area (p < 0.001, CTC p = 0.007), respectively. The increase in the distal DWT and lumen area correlated with the months of follow-up (p = 0.004 and p = 0.010, respectively). Immediate post-MGP imaging revealed the probe track confined to the ductal epithelial compartment.

Conclusion

MGP appears to stimulate a proliferative epithelial response characterized by an accelerated more superficial formation of ductal basement membrane with increased DWECL as well as DWT and lumen area at two separate duct foci. These findings suggest activation of lid margin meibomian gland precursor cells and confirm that MGP stimulates an epithelial regenerative phenomenon, not a fibrotic one.

Recent trends and perspectives in reconstruction and regeneration of intra/extra-oral wounds using tissue-engineered oral mucosa equivalents

Many conditions, including cancer, trauma, and congenital anomalies, can damage the oral mucosa. Multiple cultures of oral mucosal cells have been used for biocompatibility tests and oral biology studies. In recent decades, the clinical translation of tissue-engineered products has progressed significantly in developing tangible therapies and inspiring advancements in medical science. However, the reconstruction of an intraoral mucosa defect remains a significant challenge. Despite the drawbacks of donor-site morbidity and limited tissue supply, the use of autologous oral mucosa remains the gold standard for oral mucosa reconstruction and repair. Tissue engineering offers a promising solution for repairing and reconstructing oral mucosa tissues. Cell- and scaffold-based tissue engineering approaches have been employed to treat various soft tissue defects, suggesting the potential clinical use of tissue-engineered oral mucosa (TEOMs). In this review, we first cover the recent trends in the reconstruction and regeneration of extra-/intra-oral wounds using TEOMs. Next, we describe the current status and challenges of TEOMs. Finally, future strategic approaches and potential technologies to support the advancement of TEOMs for clinical use are discussed.

Histological assessment of nanostructured fibrin-agarose skin substitutes grafted in burnt patients. A time-course study

A previously developed fibrin-agarose skin model-UGRSKIN-showed promising clinical results in severely burnt patients. To determine the histological parameters associated to the biocompatibility and therapeutic effects of this model, we carried out a comprehensive structural and ultrastructural study of UGRSKIN grafted in severely burnt patients after 3 months of follow-up. The grafted epidermis was analogue to native human skin from day 30th onward, revealing well-structured strata with well-differentiated keratinocytes expressing CK5, CK8, CK10, claudin, plakoglobin, filaggrin, and involucrin in a similar way to controls, suggesting that the epidermis was able to mature and differentiate very early. Melanocytes and Langerhans cells were found from day 30th onward, together with a basement membrane, abundant hemidesmosomes and lack of rete ridges. At the dermal layer, we found an interface between the grafted skin and the host tissue at day 30th, which tended to disappear with time. The grafted superficial dermis showed a progressive increase in properly-oriented collagen fibers, elastic fibers and proteoglycans, including decorin, similarly to control dermis at day 60-90th of in vivo follow-up. Blood vessels determined by CD31 and SMA expression were more abundant in grafted skin than controls, whereas lymphatic vessels were more abundant at day 90th. These results contribute to shed light on the histological parameters associated to biocompatibility and therapeutic effect of the UGRSKIN model grafted in patients and demonstrate that the bioengineered skin grafted in patients is able to mature and differentiate very early at the epithelial level and after 60-90 days at the dermal level.

Biomimetic human skin model patterned with rete ridges

Rete ridges consist of undulations between the epidermis and dermis that enhance the mechanical properties and biological function of human skin. However, most human skin models are fabricated with a flat interface between the epidermal and dermal layers. Here, we report a micro-stamping method for producing human skin models patterned with rete ridges of controlled geometry. To mitigate keratinocyte-induced matrix degradation, telocollagen-fibrin matrices with and without crosslinks enable these micropatterned features to persist during longitudinal culture. Our human skin model exhibits an epidermis that includes the following markers: cytokeratin 14, p63, and Ki67 in the basal layer, cytokeratin 10 in the suprabasal layer, and laminin and collagen IV in the basement membrane. We demonstrated that two keratinocyte cell lines, one from a neonatal donor and another from an adult diabetic donor, are compatible with this model. We tested this model using an irritation test and showed that the epidermis prevents rapid penetration of sodium dodecyl sulfate. Gene expression analysis revealed differences in keratinocytes obtained from the two donors as well as between 2D (control) and 3D culture conditions. Our human skin model may find potential application for drug and cosmetic testing, disease and wound healing modeling, and aging studies.

Constructing epidermal rete ridges using a composite hydrogel to enhance multiple signaling pathways for the maintenance of epidermal stem cell niche

The undulating microstructure rete ridge (RR) located at the junction between the dermis and epidermis plays a crucial role in improving skin mechanical properties and maintaining skin homeostasis. However, the investigation of RR microstructures is usually neglected in current tissue engineering for skin regeneration. Here, to create an epidermal model with RR microstructures, keratinocytes were cultured on a patterned GelMA-PEGDA hydrogel constructed using molding technology. Furthermore, grafting acryloylated Arg-Gly-Asp (RGD) peptides on the hydrogel surface significantly improved cell adhesion, fusion, and development. RT-PCR, Western blot, and immunofluorescence staining confirmed that cells on RR microstructures exhibited higher gene and protein expression associated with epidermal stem cells. RNA sequencing analysis of cells on RR microstructure showed higher gene expression profiles related to stem cell maintenance, basement membrane formation, and epidermal development. Furthermore, RT-PCR analysis of epidermal models of various dimensions demonstrated that smaller microstructures were more conducive to epidermal stem cell marker gene expression, which is analogous to human skin. Overall, we have successfully developed a method for integrating RR microstructures into an epidermal model that mimics natural skin to maintain epidermal stem cell niche, providing a valuable reference for researching skin regeneration within the fields of tissue engineering and regenerative medicine. STATEMENT OF SIGNIFICANCE: This study presents a method for precisely fabricating microstructures of skin rete ridges using composite hydrogels, thereby creating a skin model that mimics natural human skin. The findings reveal that this microstructure provides a stem cell niche that regulates the pathways and promotes the expression of proteins related to epidermal stem cells. This work advances the functional properties of tissue engineered skin and holds promise for improving the therapeutic efficacy of artificial skin grafts for the skin wounds.

3D bioprinting of heterogeneous tissue-engineered skin containing human dermal fibroblasts and keratinocytes

Three-dimensional (3D) printed skin substitutes have great potential for wound healing. However, current 3D printed skin models are limited in simulating heterogeneity and complexity of skin tissue due to the lack of customized bioinks optimized for different skin layers. Herein, different gelatin methacrylate (GelMA)/nano-cellulose (BNC) bioink formulations were used to develop heterogeneous tissue-engineered skin (HTS) containing layers of fibroblast networks with larger pores, basal layers with smaller pores, and multilayered keratinocytes. The results revealed that the 10%GelMA/0.3%BNC bioink was better to model bioprinted dermis due to its high printability and cell-friendly sparse microenvironment. Additionally, the 10%GelMA/1.5%BNC bioink as the basal layer presented a dense network and sufficient material stiffness to support the establishment of keratinocyte confluent monolayers. The HTS not only had the ability to remodel the extracellular matrix but also supported epidermis reconstruction and stratification in vitro, with the epidermal thickness growing to 80 μm after 14 days. Furthermore, the full-thickness wound healing experiments demonstrated that the HTS promoted granulation tissue regeneration and improved wound healing quality. The generated skin of the HTS group had hair follicles and early-stage rete ridge structures, which were similar to normal skin in vivo. The HTS may deliver effective skin grafts for future clinical treatments.