The differentiation of the dermis in the laboratory mouse

Efficient Generation of Skin Organoids from Pluripotent Cells via Defined Extracellular Matrix Cues and Morphogen Gradients in a Spindle-Shaped Microfluidic Device

Pluripotent stem cell-derived skin organoids (PSOs) emerge as a developmental skin model that is self-organized into multiple components, such as hair follicles. Despite their impressive complexity, PSOs are currently generated in the absence of 3D extracellular matrix (ECM) signals and have several major limitations, including an inverted anatomy (e.g., epidermis inside/dermis outside). In this work, a method is established to generate PSOs effectively in a chemically-defined 3D ECM environment. After examining various dermal ECM molecules, it is found that PSOs generated in collagen -type I (COLI) supplemented with laminin 511 (LAM511) exhibit increased growth compared to conventional free-floating conditions, but fail to induce complete skin differentiation due in part to necrosis. This problem is addressed by generating the PSOs in a 3D bioprinted spindle-shaped hydrogel device, which constrains organoid growth longitudinally. This culture system significantly reduces organoid necrosis and leads to a twofold increase in keratinocyte differentiation and an eightfold increase in hair follicle formation. Finally, the system is adapted as a microfluidic device to create asymmetrical gradients of differentiation factors and improves the spatial organization of dermal and epidermal cells. This study highlights the pivotal role of ECM and morphogen gradients in promoting and spatially-controlling skin differentiation in the PSO framework.

Comprehensive Characterization of Tissues Derived from Animals at Different Regenerative Stages: A Comparative Analysis between Fetal and Adult Mouse Skin

Tissue regeneration capabilities vary significantly throughout an organism's lifespan. For example, mammals can fully regenerate until they reach specific developmental stages, after which they can only repair the tissue without restoring its original architecture and function. The high regenerative potential of fetal stages has been attributed to various factors, such as stem cells, the immune system, specific growth factors, and the presence of extracellular matrix molecules upon damage. To better understand the local differences between regenerative and reparative tissues, we conducted a comparative analysis of skin derived from mice at regenerative and reparative stages. Our findings show that both types of skin differ in their molecular composition, structure, and functionality. We observed a significant increase in cellular density, nucleic acid content, neutral lipid density, Collagen III, and glycosaminoglycans in regenerative skin compared with reparative skin. Additionally, regenerative skin had significantly higher porosity, metabolic activity, water absorption capacity, and elasticity than reparative skin. Finally, our results also revealed significant differences in lipid distribution, extracellular matrix pore size, and proteoglycans between the two groups. This study provides comprehensive data on the molecular and structural clues that enable full tissue regeneration in fetal stages, which could aid in developing new biomaterials and strategies for tissue engineering and regeneration.

Stem cell therapy for chronic skin wounds in the era of personalized medicine: From bench to bedside

With the significant financial burden of chronic cutaneous wounds on the healthcare system, not to the personal burden mention on those individuals afflicted, it has become increasingly essential to improve our clinical treatments. This requires the translation of the most recent benchtop approaches to clinical wound repair as our current treatment modalities have proven insufficient. The most promising potential treatment options rely on stem cell-based therapies. Stem cell proliferation and signaling play crucial roles in every phase of the wound healing process and chronic wounds are often associated with impaired stem cell function. Clinical approaches involving stem cells could thus be utilized in some cases to improve a body's inhibited healing capacity. We aim to present the laboratory research behind the mechanisms and effects of this technology as well as current clinical trials which showcase their therapeutic potential. Given the current problems and complications presented by chronic wounds, we hope to show that developing the clinical applications of stem cell therapies is the rational next step in improving wound care.

Learning from regeneration research organisms: The circuitous road to scar free wound healing

The skin is the largest organ in the body and plays multiple essential roles ranging from regulating temperature, preventing infection and ultimately defining who we are physically. It is a highly dynamic organ that constantly replaces the outermost cells throughout life. However, when faced with a major injury, human skin cannot restore a significant lesion to its original functionality, instead a reparative scar is formed. In contrast to this, many other species have the unique ability to regenerate full thickness skin without formation of scar tissue. Here we review recent advances in the field that shed light on how the skin cells in regenerative species react to injury to prevent scar formation versus scar forming humans.

Palatogenesis and cutaneous repair: A two-headed coin

BACKGROUND

The reparative mechanism that operates following post-natal cutaneous injury is a fundamental survival function that requires a well-orchestrated series of molecular and cellular events. At the end, the body will have closed the hole using processes like cellular proliferation, migration, differentiation and fusion.

RESULTS

These processes are similar to those occurring during embryogenesis and tissue morphogenesis. Palatogenesis, the formation of the palate from two independent palatal shelves growing towards each other and fusing, intuitively, shares many similarities with the closure of a cutaneous wound from the two migrating epithelial fronts.

CONCLUSIONS

In this review, we summarize the current information on cutaneous development, wound healing, palatogenesis and orofacial clefting and propose that orofacial clefting and wound healing are conserved processes that share common pathways and gene regulatory networks.

Understanding fibroblast heterogeneity in the skin

Fibroblasts are found in most tissues, yet they remain poorly characterised. Different fibroblast subpopulations with distinct functions have been identified in the skin. This functional heterogeneity reflects the varied fibroblast lineages that arise from a common embryonic precursor. In addition to autocrine signals, fibroblasts are highly responsive to Wnt-regulated signals from the overlying epidermis, which can act both locally, via extracellular matrix (ECM) deposition, and via secreted factors that impact the behaviour of fibroblasts in different dermal locations. These findings may explain some of the changes that occur in connective tissue during wound healing and cancer progression.

Distinct fibroblast lineages determine dermal architecture in skin development and repair

Fibroblasts are the major mesenchymal cell type in connective tissue and deposit the collagen and elastic fibres of the extracellular matrix (ECM). Even within a single tissue, fibroblasts exhibit considerable functional diversity, but it is not known whether this reflects the existence of a differentiation hierarchy or is a response to different environmental factors. Here we show, using transplantation assays and lineage tracing in mice, that the fibroblasts of skin connective tissue arise from two distinct lineages. One forms the upper dermis, including the dermal papilla that regulates hair growth and the arrector pili muscle, which controls piloerection. The other forms the lower dermis, including the reticular fibroblasts that synthesize the bulk of the fibrillar ECM, and the preadipocytes and adipocytes of the hypodermis. The upper lineage is required for hair follicle formation. In wounded adult skin, the initial wave of dermal repair is mediated by the lower lineage and upper dermal fibroblasts are recruited only during re-epithelialization. Epidermal β-catenin activation stimulates the expansion of the upper dermal lineage, rendering wounds permissive for hair follicle formation. Our findings explain why wounding is linked to formation of ECM-rich scar tissue that lacks hair follicles. They also form a platform for discovering fibroblast lineages in other tissues and for examining fibroblast changes in ageing and disease.

Genetic interactions between Pax9 and Msx1 regulate lip development and several stages of tooth morphogenesis

Developmental abnormalities of craniofacial structures and teeth often occur sporadically and the underlying genetic defects are not well understood, in part due to unknown gene-gene interactions. Pax9 and Msx1 are co-expressed during craniofacial development, and mice that are single homozygous mutant for either gene exhibit cleft palate and an early arrest of tooth formation. Whereas in vitro assays have demonstrated that protein-protein interactions between Pax9 and Msx1 can occur, it is unclear if Pax9 and Msx1 interact genetically in vivo during development. To address this question, we compounded the Pax9 and Msx1 mutations and observed that double homozygous mutants exhibit an incompletely penetrant cleft lip phenotype. Moreover, in double heterozygous mutants, the lower incisors were consistently missing and we find that transgenic BMP4 expression partly rescues this phenotype. Reduced expression of Shh and Bmp2 indicates that a smaller "incisor field" forms in Pax9(+/-);Msx1(+/-) mutants, and dental epithelial growth is substantially reduced after the bud to cap stage transition. This defect is preceded by drastically reduced mesenchymal expression of Fgf3 and Fgf10, two genes that encode known stimulators of epithelial growth during odontogenesis. Consistent with this result, cell proliferation is reduced in both the dental epithelium and mesenchyme of double heterozygous mutants. Furthermore, the developing incisors lack mesenchymal Notch1 expression at the bud stage and exhibit abnormal ameloblast differentiation on both labial and lingual surfaces. Thus, Msx1 and Pax9 interact synergistically throughout lower incisor development and affect multiple signaling pathways that influence incisor size and symmetry. The data also suggest that a combined reduction of PAX9 and MSX1 gene dosage in humans may increase the risk for orofacial clefting and oligodontia.

Overexpression of manganese superoxide dismutase in human dermal fibroblasts enhances the contraction of free floating collagen lattice: implications for ageing and hyperplastic scar formation

Cell-matrix interactions are of significant importance for tissue homeostasis of the skin and, if disturbed, may lead to ageing and hyperplastic scar formation. We have studied fibroblasts stably overexpressing manganese superoxide dismutase (MnSOD) with a defined capacity for the removal of superoxide anions and concomitant accumulation of hydrogen peroxide to evaluate the role of enhanced MnSOD activity on the dynamics of cell-matrix interactions in the three-dimensional collagen lattice contraction assay. MnSOD overexpressing fibroblast populated collagen lattices revealed a significantly enhanced contraction compared to collagen lattices populated with vector control cells. The enhanced collagen lattice contraction was in part due to an increase in active TGF-beta1 and the accumulation of H2O2 in MnSOD overexpressing fibroblasts populated collagen lattices. Inhibition of TGF-beta1 signalling by the ALK4,5,7 kinases' inhibitor SB431542 at least partly inhibited the enhanced collagen lattice contraction of MnSOD overexpressing fibroblasts populated lattices. In addition, supplementation of vector control fibroblast populated collagen lattices with recombinant TGF-beta1 concentration dependently enhanced the collagen lattice contraction. In the presence of the antioxidant Ebselen, a mimic of H2O2 and other hydroperoxides/peroxynitrite-detoxifying glutathione peroxidase, collagen lattice contraction and the activation of TGF-beta1 were significantly reduced in collagen lattices populated with MnSOD overexpressing fibroblasts. Collectively, these data suggest that H2O2 or other hydroperoxides or peroxynitrite or a combination thereof may function as important second messengers in collagen lattice contraction and act at least in part via TGF-beta1 activation.

Bone marrow cell transfer into fetal circulation can ameliorate genetic skin diseases by providing fibroblasts to the skin and inducing immune tolerance

Recent studies have shown that skin injury recruits bone marrow-derived fibroblasts (BMDFs) to the site of injury to accelerate tissue repair. However, whether uninjured skin can recruit BMDFs to maintain skin homeostasis remains uncertain. Here, we investigated the appearance of BMDFs in normal mouse skin after embryonic bone marrow cell transplantation (E-BMT) with green fluorescent protein-transgenic bone marrow cells (GFP-BMCs) via the vitelline vein, which traverses the uterine wall and is connected to the fetal circulation. At 12 weeks of age, mice treated with E-BMT were observed to have successful engraftment of GFP-BMCs in hematopoietic tissues accompanied by induction of immune tolerance against GFP. We then investigated BMDFs in the skin of the same mice without prior injury and found that a significant number of BMDFs, which generate matrix proteins both in vitro and in vivo, were recruited and maintained after birth. Next, we performed E-BMT in a dystrophic epidermolysis bullosa mouse model (col7a1(-/-)) lacking type VII collagen in the cutaneous basement membrane zone. E-BMT significantly ameliorated the severity of the dystrophic epidermolysis bullosa phenotype in neonatal mice. Type VII collagen was deposited primarily in the follicular basement membrane zone in the vicinity of the BMDFs. Thus, gene therapy using E-BMT into the fetal circulation may offer a potential treatment option to ameliorate genetic skin diseases that are characterized by fibroblast dysfunction through the introduction of immune-tolerated BMDFs.

Impaired skin and hair follicle development in Runx2 deficient mice

The transcription factor, Runx2, is known to play crucial roles in skeletal and tooth morphogenesis. Here we document that Runx2 has a regulatory role in skin and hair follicle development. The expression of Runx2 is restricted to hair follicles and is dynamic, pari passu with follicle development. Follicle maturation is delayed in the absence of Runx2 and overall skin and epidermal thickness of Runx2 null embryos is significantly reduced. The Runx2 null epidermis is hypoplastic, displaying reduced expression of Keratin 14, Keratin 1 and markers of proliferation. The expression pattern of Runx2 in the bulb epithelium of mature hair follicles is asymmetric and strikingly similar to that of Sonic hedgehog. This suggests that Runx2 may be a regulator of hedgehog signaling in skin as it is in bones and teeth. Supporting this possibility, we demonstrate that Sonic hedgehog, Patched1 and Gli1 transcripts are reduced in the skin of Runx2 null embryos. Moreover, we document Patched1 expression in epidermal basal cells and show that the skin of Sonic(+/-) embryos is thinner than that of wild-type littermates. These observations suggest that Runx2 and hedgehog signaling are involved in the well known, but unexplained, coupling of skin thickness to hair follicle development.

Dermal connective tissue development in mice: an essential role for tenascin-X

Deficiency of the extracellular matrix protein tenascin-X (TNX) causes a recessive form of Ehlers-Danlos syndrome (EDS) characterized by hyperextensible skin and hypermobile joints. It is not known whether the observed alterations of dermal collagen fibrils and elastic fibers in these patients are caused by disturbed assembly and deposition or by altered stability and turnover. We used biophysical measurements and immunofluorescence to study connective tissue properties in TNX knockout and wild-type mice. We found that TNX knockout mice, even at a young age, have greatly disturbed biomechanical properties of the skin. No joint abnormalities were noted at any age. The spatio-temporal expression of TNX during normal mouse skin development, during embryonic days 13-19 (E13-E19), was distinct from tropoelastin and the dermal fibrillar collagens type I, III, and V. Our data show that TNX is not involved in the earliest phase (E10-E14) of the deposition of collagen fibrils and elastic fibers during fetal development. From E15 to E19, TNX starts partially to colocalize with the dermal collagens and elastin, and in adult mice, TNX is present in the entire dermis. In adult TNX knockout mice, we observed an apparent increase of elastin. We conclude that TNX knockout mice only partially recapitulate the phenotype of TNX-deficient EDS patients, and that TNX could potentially be involved in maturation and/or maintenance of the dermal collagen and elastin network.

MT1-MMP-dependent neovessel formation within the confines of the three-dimensional extracellular matrix

During angiogenesis, endothelial cells initiate a tissue-invasive program within an interstitial matrix comprised largely of type I collagen. Extracellular matrix–degradative enzymes, including the matrix metalloproteinases (MMPs) MMP-2 and MMP-9, are thought to play key roles in angiogenesis by binding to docking sites on the cell surface after activation by plasmin- and/or membrane-type (MT) 1-MMP–dependent processes. To identify proteinases critical to neovessel formation, an ex vivo model of angiogenesis has been established wherein tissue explants from gene-targeted mice are embedded within a three-dimensional, type I collagen matrix. Unexpectedly, neither MMP-2, MMP-9, their cognate cell-surface receptors (i.e., β₃ integrin and CD44), nor plasminogen are essential for collagenolytic activity, endothelial cell invasion, or neovessel formation. Instead, the membrane-anchored MMP, MT1-MMP, confers endothelial cells with the ability to express invasive and tubulogenic activity in a collagen-rich milieu, in vitro or in vivo, where it plays an indispensable role in driving neovessel formation.

Fibroblast biology in three-dimensional collagen matrices

Research on fibroblast biology in three-dimensional collagen matrices offers new opportunities to understand the reciprocal and adaptive interactions that occur between cells and surrounding matrix in a tissue-like environment. Such interactions are integral to the regulation of connective tissue morphogenesis and dynamics that characterizes tissue homeostasis and wound repair. During fibroblast-collagen matrix remodeling, mechanical signals from the remodeled matrix feed back to modulate cell behavior in an iterative process. As mechanical loading (tension) within the matrix increases, the mechanisms used by cells to remodel the matrix change. Fibroblasts in matrices that are under tension or relaxed respond differently to growth factor stimulation, and switching between mechanically loaded and unloaded conditions influences whether cells acquire proliferative/biosynthetic active or quiescent/resting phenotypes.

Dendritic fibroblasts in three-dimensional collagen matrices

Cell motility determines form and function of multicellular organisms. Most studies on fibroblast motility have been carried out using cells on the surfaces of culture dishes. In situ, however, the environment for fibroblasts is the three-dimensional extracellular matrix. In the current research, we studied the morphology and motility of human fibroblasts embedded in floating collagen matrices at a cell density below that required for global matrix remodeling (i.e., contraction). Under these conditions, cells were observed to project and retract a dendritic network of extensions. These extensions contained microtubule cores with actin concentrated at the tips resembling growth cones. Platelet-derived growth factor promoted formation of the network; lysophosphatidic acid stimulated its retraction in a Rho and Rho kinase-dependent manner. The dendritic network also supported metabolic coupling between cells. We suggest that the dendritic network provides a mechanism by which fibroblasts explore and become interconnected to each other in three-dimensional space.

Ultrastructure of the embryonic snake skin and putative role of histidine in the differentiation of the shedding complex

The morphogenesis and ultrastructure of the epidermis of snake embryos were studied at progressive stages of development through hatching to determine the time and modality of differentiation of the shedding complex. Scales form as symmetric epidermal bumps that become slanted and eventually very overlapped. During the asymmetrization of the bumps, the basal cells of the forming outer surface of the scale become columnar, as in an epidermal placode, and accumulate glycogen. Small dermal condensations are sometimes seen and probably represent primordia of the axial dense dermis of the growing tip of scales. Deep, dense, and superficial loose dermal regions are formed when the epidermis is bilayered (periderm and basal epidermis) and undifferentiated. Glycogen and lipids decrease from basal cells to differentiating suprabasal cells. On the outer scale surface, beneath the peridermis, a layer containing dense granules and sparse 25-30-nm thick coarse filaments is formed. The underlying clear layer does not contain keratohyalin-like granules but has a rich cytoskeleton of intermediate filaments. Small denticles are formed and they interdigitate with the oberhautchen spinulae formed underneath. On the inner scale surface the clear layer contains dense granules, coarse filaments, and does not form denticles with the aspinulated oberhautchen. On the inner side surface the oberhautchen only forms occasional spinulae. The sloughing of the periderm and embryonic epidermis takes place in ovo 5-6 days before hatching. There follow beta-, mesos-, and alpha-layers, not yet mature before hatching. No resting period is present but a new generation is immediately produced so that at 6-10 h posthatching an inner generation and a new shedding complex are forming beneath the outer generation. The first shedding complex differentiates 10-11 days before hatching. In hatchlings 6-10 h old, tritiated histidine is taken up in the epidermis 4 h after injection and is found mainly in the shedding complex, especially in the apposed membranes of the clear layer and oberhautchen cells. This indicates that a histidine-rich protein is produced in preparation for shedding, as previously seen in lizard epidermis. The second shedding (first posthatching) takes place at 7-9 days posthatching. It is suggested that the shedding complex in lepidosaurian reptiles has evolved after the production of a histidine-rich protein and of a beta-keratin layer beneath the former alpha-layer.

Programming gene expression in developing epidermis

As the major proteins of adult keratinocytes, keratins provide biochemical markers for exploring mouse epidermal embryogenesis. Here, we used a modified method of whole-mount in situ hybridization to track skin-specific expression of endogenous keratin mRNAs throughout embryogenesis. To monitor transcriptional regulation, we coupled this with beta-galactosidase expression of a human epidermal keratin promoter-driven transgene. These studies have radically changed our perception of how the program of gene expression becomes established during epidermal development. Specifically, we have discovered that (1) basal keratin (K5 and K14) genes are first detected at E9.5 in a highly regional fashion, and surprisingly as early as the single layered ectodermal stage; (2) the early patterns do not correlate with morphogenesis per se, but rather with regional variations in the embryonic origin of underlying mesenchyme, supporting morphogenetic criteria that early inductive cues are mesenchymal; (3) epidermal keratin genes are expressed in periderm, supporting the notion that this layer arises from ectodermal stratification, even though it is simple epithelial-like in morphology and is subsequently sloughed during development; (4) later embryonic patterns of K5 and K14 gene expression parallel proliferative capacity and not stratification; and (5) K1 and K10 mRNAs are first detected as early as E13.5, and their patterns correlate with differentiation and not stratification. These patterns of epidermal gene expression led us to explore whether potential transcriptional regulators of these genes are expressed similarly. We show that AP2 (but not Sp1) cRNAs hybridize in a pattern similar to, but preceding that of basal keratin cRNAs. Finally, using gene expression in cultured cells, we demonstrate that AP2 has a strong inductive effect on basal keratin expression in a cellular environment that does not normally possess AP2 activity.

Either chick embryo dermis or retinoid-treated mouse dermis can initiate glandular morphogenesis from mammalian epidermal tissue

Excess retinoids can cause developing mouse vibrissa follicles to be transformed into mucous glands in organ culture. The objective was to test the hypothesis that retinoids act in this system by altering morphogenetic properties of the dermis. After inititation by retinoic acid (RA) in organ culture, glands were shown to develop further in embryonic skin grafted to the chick chorioallantoic membrane (CAM). Recombinants of 12.5 day mouse epidermis with untreated or RA-treated mouse or chick dermis were then grafted to CAM for 7 days. For homospecific recombinants, 13.5 day mouse dermis originated from 11.5 day skin cultured for 2 days, with or without 5.2 microgram/ml RA. For heterospecific recombinants, 12 day dermis came from chick embryos, previously injected with 250 microgram RA. Glands were absent from the homospecific recombinants including untreated mouse dermis, but appeared in 26% of those with RA-treated dermis. Among heterospecific recombinants, 75% of those with RA-treated chick dermis and 29% of those with untreated dermis had glands. Untreated 10-12 day chick skin contained two forms of endogenous vitamin A, retinol (4.5 microgram/g protein) and dehydroretinol (3.7 microgram/g protein), while 13-14 day mouse skin contained only retinol (1.8 microgram/g protein), as shown by high performance liquid chromatography. RA injection increased retinol and dehydroretinol in chick skin, while RA was undetectable. Thus RA can act through mouse dermis to form epithelial glands and through chick dermis to increase the incidence of glands. The glands in recombinants with untreated chick dermis may result from the higher levels of endogenous retinoids in chick skin, compared with mouse skin.