The skin as an immune organ.

As a protective interface between internal organs and the environment, the skin encounters a host of toxins, pathogenic organisms, and physical stresses. To combat these attacks on the cutaneous microenvironment, the skin functions as more than a physical barrier: it is an active immune organ. Immune responses in the skin involve an armamentarium of immune-competent cells and soluble biologic response modifiers including cytokines. Traversed by a network of lymphatic and blood vessels, the dermis contains most of the lymphocytes in the skin, other migrant leukocytes, mast cells, and tissue macrophages. Although the epidermis has no direct access to the blood or lymphatic circulation, it is equipped with immune-competent cells: Langerhans cells, the macrophage-like antigen-presenting cells of the epidermis; keratinocytes, epithelial cells with immune properties; dendritic epidermal T lymphocytes, resident cells that may serve as a primitive T-cell immune surveillance system; epidermotropic lymphocytes, migrants from vessels in the dermis; and melanocytes, epidermal pigment cells with immune properties. Although the components of the epidermis and dermis work in concert to execute immune responses in the skin, for purposes of this review, we focus on the cells and cytokines of the epidermal immunologic unit, the frontline of immune protection against environmental toxins and microbes.     

Regional specificity of anuran larval skin during metamorphosis: dermal specificity in development and histolysis of recombined skin grafts

Summary Skins from back and tail were dissected from tadpoles of Rana japonica prior to resorption of the tail and separated into epidermis and dermis by treatment with neutral protease. Homotypically and heterotypically recombined skins were constructed from the separated epidermis and dermis and transplanted into the tail of the original tadpole. Skin grafts using dermis from tail region degenerated simultaneously with resorption of the tail. However, skin grafts containing dermis from back region survived on the posterior part of the juvenile frog beyond metamorphosis. Furthermore, all epidermis underlaid with dermis from back region formed secretory glands and became flattened epithelia characteristic of adult back skin, regardless of region from which the epidermis came. Even when epidermis isolated from tail skin was cultured inside a back skin graft, the tail epidermis survived forming an epithelial cyst and developed secretory glands. These results suggest that regional specificities of anuran larval skin, i.e., development of back skin and even histolysis of tail skin, are determined by regionally specific dermis. The results also suggest that some of epidermal cells of tail skin are able to differentiate into epithelial cells similar to back skin of the adult under the influence of back dermis.     

Cell death in the skin

The skin is the largest organ of the body and protects the organism against external physical, chemical and biological insults, such as wounding, ultraviolet radiation and micro-organisms. The epidermis is the upper part of the skin that is continuously renewed. The keratinocytes are the major cell type in the epidermis and undergo a specialized form of programmed cell death, called cornification, which is different from classical apoptosis. In keep with this view, several lines of evidence indicate that NF-kB is an important factor providing protection against keratinocyte apoptosis in homeostatic and inflammatory conditions. In contrast, the hair follicle is an epidermal appendage that shows cyclic apoptosis-driven involution, as part of the normal hair cycle. The different cell death programs need to be well orchestrated to maintain skin homeostasis. One of the major environmental insults to the skin is UVB radiation, causing the occurrence of apoptotic sunburn cells. Deregulation of cell death mechanisms in the skin can lead to diseases such as cancer, necrolysis and graft-versus-host disease. Here we review the apoptotic and the anti-apoptotic mechanisms in skin homeostasis and disease.     

Niche interactions in epidermal stem cells

Within the epidermis and dermis of the skin, cells secrete and are surrounded by the extracellular matrix(ECM), which provides structural and biochemical support. The ECM of the epidermis is the basement membrane, and collagen and other dermal components constitute the ECM of the dermis. There is significant variation in the composition of the ECM of the epidermis and dermis, which can affect "cell to cell" and "cell to ECM" interactions. These interactions, in turn, can influence biological responses, aging, and wound healing; abnormal ECM signaling likely contributes toskin diseases. Thus, strategies for manipulating cellECM interactions are critical for treating wounds and a variety of skin diseases. Many of these strategies focus on epidermal stem cells, which reside in a unique niche in which the ECM is the most important component; interactions between the ECM and epidermal stem cells play a major role in regulating stem cell fate. As they constitute a major portion of the ECM, it is likely that integrins and type Ⅳ collagens are important in stem cell regulation and maintenance. In this review, we highlight recent research-including our previous work-exploring the role that the ECM and its associated components play in shaping the epidermal stem cell niche.     

Epidermal and hair follicle transglutaminases. Partial characterization of soluble enzymes in newborn mouse skin

Treatment of skins of newborn mice with the neutral protease Dispase in order to separate dermis and epidermis causes pronounced changes in the levels of transglutaminase activity in the epidermis. Two soluble transglutaminases, one anionic enzyme and one cationic enzyme, of Mr approximately 90,000 and approximately 50,000, respectively, are extracted from epidermis; and the activities of both enzymes increase as a function of the time of Dispase treatment of skin. When the anionic Mr approximately 90,000 enzyme is incubated with Dispase after its chromatographic isolation from epidermal extracts, it is converted to a lower molecular weight enzyme. Hair follicles isolated from dermis prepared by a 12-h Dispase treatment of the skin of newborn mice contain two soluble cationic transglutaminases, one of which is indistinguishable from that of epidermis and the other which is not seen in epidermis. Both of these hair follicle enzymes are of Mr approximately 50,000 and appear to exist in monomeric form. They have been partially purified. Based upon these findings, we suggest that transglutaminase processing and control occur during normal differentiation of keratinocytes in epidermis and of hair follicle epidermal cells in dermis and that production of the proper forms of the enzyme may be essential to the formation of mature cornified envelopes and hair shafts, respectively.     

Tgm2/Gh, Gbx1 and TGF-beta are involved in retinoic acid-induced transdifferentiation from epidermis to mucosal epithelium

We previously demonstrated that retinoic acid (RA) induces epidermis to transdifferentiate to mucosal epithelium with goblet cells in chick embryonic cultured skin. To characterize the molecular mechanism of this transdifferentiation process, we used rat embryonic cultured skin and immunohistochemistry to confirm that RA-induced epidermal transdifferentiation accompanies the expression of markers of esophagus epithelium. Because Gbx1, TG2/Gh (transglutaminase2) and TGF-beta2 are reported individually to be induced by RA in cultures of chick embryonic skin, mouse epidermal cells and human hair follicles respectively, here, we investigated whether cooperative interplay of Gbx1, TG2/Gh and TGF-beta2 is required for the transdifferentiation of epidermal cells to mucosal cells. We have shown that expression of Gbx1, TG2/Gh and TGF-beta proteins were all upregulated in RA-induced transdifferentiated skin and that the former two were expressed in the epidermis, while TGF-beta was expressed in the dermis. Inhibitors of the TGF-beta signal pathway partially inhibited transdifferentiation. Overexpression of both hTG2/Gh and mGbx1 together in the epidermis by electroporation resulted in cuboidal cells in the upper cell layers of the epidermis without keratinized layers, although epidermal keratinization was observed in skin by overexpression of either of them. Labeling DNA with BrdU indicated that RA directly transdifferentiated transient amplifying epidermal cells, not stem cells, to mucosal cells. This study showed that coexpression of TG/2 and Gbx1 in the epidermis was required for esophagus-like mucosal transdifferentiation, and that increase in TGF-beta2 expression by RA in the dermis was essential to induce transdifferentiation through epithelial-mesenchymal interaction.     

Embryonic dermal condensation and adult dermal papilla induce hair follicles in adult glabrous epidermis through different mechanisms

Hair induction in the adult glabrous epidermis by the embryonic dermis was compared with that by the adult dermis. Recombinant skin, composed of the adult sole epidermis and the embryonic dermis containing dermal condensations (DC), was transplanted onto the back of nude mice. The epidermis of transplants formed hairs. Histology on the induction process demonstrated the formation of placode-like tissues, indicating that the transplant produces hair follicles through a mechanism similar to that underlying hair follicle development in the embryonic skin. An isolated adult rat sole skin piece, inserted with either an aggregate of cultured dermal papilla (DP) cells or an intact DP between its epidermis and dermis, was similarly transplanted. The transplant produced hair follicles. Histology showed that the epidermis in both cases surrounded the aggregates of DP cells. The epidermis never formed placode-like tissues. Thus, it was concluded that the adult epidermal cells recapitulate the embryonic process of hair follicle development when exposed to DC, whereas they get directly into the anagen of the hair cycle when exposed to DP. The expression pattern of Edar and Shh genes, and P-cadherin protein during the hair follicle development in the two types of transplants supported the above conclusion.     

Dorsal skin responses to subchronic ultraviolet B (UVB)-irradiation in Wistar-derived hypotrichotic WBN/ILA-Ht rats

Dorsal skin responses to a subchronic UVB-irradiation (10kJ/m2/rat /day), were examined in Wistar-derived hypotrichotic WBN/ILA-Ht rats for up to 3 months. Hyperplasia of epidermal cells and hair follicle epithelial cells as well as parakeratosis developed at 1 month and progressed thereafter, resulting in a prominent epidermis thickening and formation of epidermal ingrowths projecting into the dermis. At the same time, the percentage of proliferating cell nuclear antigen (PCNA)-positive epidermal cells significantly increased after I month. In some portions of the hyperplastic epidermis, especially of the epidermal ingrowths, keratinocytes were somewhat pleomorphic and migrated into the dermis. In the upper dermis, edema with capillary congestion, mast cell infiltration and fibroblast proliferation developed at I month, and the intensity of edema and the number of dermal mast cells was most prominent at 3 months. Edema spread to the epidermis, resulting in intercellular edema and subsequent dissociation of epidermal cells. Degeneration of collagen fibers was also detected in the upper dermis, especially beneath the epidermis. In addition, although not significant because of a large individual difference, the serum IgE concentration, showed a tendency to increase after 2 months. The present study clarified the characteristics of the dorsal skin responses to a subchronic UVB-irradiation in rats.     

Reprogramming adult dermis to a neonatal state through epidermal activation of β-catenin

Hair follicle formation depends on reciprocal epidermal-dermal interactions and occurs during skin development, but not in adult life. This suggests that the properties of dermal fibroblasts change during postnatal development. To examine this, we used a PdgfraEGFP mouse line to isolate GFP-positive fibroblasts from neonatal skin, adult telogen and anagen skin and adult skin in which ectopic hair follicles had been induced by transgenic epidermal activation of β-catenin (EF skin). We also isolated epidermal cells from each mouse. The gene expression profile of EF epidermis was most similar to that of anagen epidermis, consistent with activation of β-catenin signalling. By contrast, adult dermis with ectopic hair follicles more closely resembled neonatal dermis than adult telogen or anagen dermis. In particular, genes associated with mitosis were upregulated and extracellular matrix-associated genes were downregulated in neonatal and EF fibroblasts. We confirmed that sustained epidermal β-catenin activation stimulated fibroblasts to proliferate to reach the high cell density of neonatal skin. In addition, the extracellular matrix was comprehensively remodelled, with mature collagen being replaced by collagen subtypes normally present only in developing skin. The changes in proliferation and extracellular matrix composition originated from a specific subpopulation of fibroblasts located beneath the sebaceous gland. Our results show that adult dermis is an unexpectedly plastic tissue that can be reprogrammed to acquire the molecular, cellular and structural characteristics of neonatal dermis in response to cues from the overlying epidermis.     

Induction of epidermal mucous metaplasia by culture of recombinants of undifferentiated epidermis and retinol-treated dermis in a chemically defined medium

Epidermal mucous metaplasia of cultured skin is known to be induced by excess retinol. Studies were made on whether retinol affects primarily the epidermis or the dermis during retinol-induced epidermal mucous metaplasia of 13-day-old chick embryonic skin in culture. When recombinants of 13-day-old normal epidermis and retinol-treated dermis were cultured for 7 days in chemically defined medium in the absence of retinol, hormones, and serum, they showed altered epidermal differentiation toward secretory epithelium (mucous metaplasia). Thus retinol acted primarily on dermal cells.     

Differentiation of embryonic chick feather-forming and scale-forming tissues in transfilter cultures

The dermal-epidermal tissue interaction in the chick embryo, leading to the formation of feathers and scales, provides a good experimental system to study the transfer between tissues of signals which specify cell type. At certain times in development, the dermis controls whether the epidermis forms feathers or scales, each of which are characterized by the synthesis of specific beta-keratins. In our culture system, a dermal effect on epidermal differentiation can still be observed, even when the tissues are separated by a Nuclepore filter, although development is abnormal. Epidermal morphological and histological differentiation in transfilter cultures are distinct and recognizable, more closely resembling feather or scale development, depending on the regional origin of the dermis. Differentiation is more advanced when epidermis is cultured transfilter from scale dermis than from feather dermis, as assessed by morphology and histology, as well as the expression of the tissue-specific gene products, the beta-keratins. Two-dimensional polyacrylamide gel analysis of the beta-keratins reveals that scale dermis cultured transfilter from either presumptive scale or feather epidermis induces the production of 7 of the 9 scale-specific beta-keratins that we have identified. Feather dermis, although less effective in activating the feather gene program when cultured transfilter from either presumptive feather or scale epidermis, is able to turn on the synthesis of 3 to 6 of the 18 feather-specific beta-keratins that we have identified. However, scale epidermis in transfilter recombinants with feather dermis also continues to synthesize many of the scale-specific beta-keratins. Using transmission and scanning electron microscopy, we detect no cell contact between tissues separated by a 0.2-micron pore diameter Nuclepore filter, while 0.4-micron filters readily permit cell processes to traverse the filter. We find that epidermal differentiation is the same with either pore size filter. Furthermore, we do not detect a basement membrane in transfilter cultures, implying that neither direct cell contact between dermis and epidermis, nor a basement membrane between the tissues is required for the extent of epidermal differentiation that we observe.     

Light and electron microscopy of fetal rabbit skin with special reference to the role of mesenchymal cells in epidermal differentiation

Fetal rabbit skin between 10 and 26 days of gestation was observed by light and electron microscopy. The present study indicates that rapid epidermal differentiation, including the epidermal downgrowths as primordia of the hair follicles, is induced by aggregation of mesenchymal cells associated with growing capillaries beneath the epidermis. In addition, the transformation of these mesenchymal cells to vasoformative cells for rapid capillary growth is further evidenced by this study. Glycogen-storing cells in the periderm are most numerous between 15 and 18 days of gestation, but disappear almost completely by 20 days when a capillary network develops beneath the epidermis. This may imply an active involvement of the periderm in glucose uptake from the amniotic fluid during early developmental stages of the skin.     

THE INTRAEPIDERMAL INNERVATION OF THE SNOUT SKIN OF THE OPOSSUM : A Light and Electron Microscope Study, with Observations on the Nature of Merkel's Tastzellen

The intraepidermal innervation of the snout skin of the opossum has been studied with the light and electron microscope. Numerous large nerve fibers loose their myelin sheath in the superficial dermis and pass into the epidermis. The basement membranes of the epidermis and Schwann cell become continuous at the point of entry of the neurite into the epidermis. Within the epidermis, the neurite is associated with a specialized secretory epidermal cell, termed a Merkel cell. This cell has many secretory granules apposed to the neurite. The Merkel cells are epidermal cells since they have desmosomes between them and adjacent epidermal cells. The neurite in the stratum spinosum is enveloped by Schwann cells in a manner analogous to the Schwann cell investment of unmyelinated neurites. In the upper stratum spinosum the nerve fiber evidences changes which can be interpreted as degenerative. The Merkel cell-neurite complex is interpreted as representing a sensory receptor unit.     

Adult epidermal Notch activity induces dermal accumulation of T cells and neural crest derivatives through upregulation of jagged 1

Notch signalling regulates epidermal differentiation and tumour formation via non-cell autonomous mechanisms that are incompletely understood. This study shows that epidermal Notch activation via a 4-hydroxy-tamoxifen-inducible transgene caused epidermal thickening, focal detachment from the underlying dermis and hair clumping. In addition, there was dermal accumulation of T lymphocytes and stromal cells, some of which localised to the blisters at the epidermal-dermal boundary. The T cell infiltrate was responsible for hair clumping but not for other Notch phenotypes. Notch-induced stromal cells were heterogeneous, expressing markers of neural crest, melanocytes, smooth muscle and peripheral nerve. Although Slug1 expression was expanded in the epidermis, the stromal cells did not arise through epithelial-mesenchymal transition. Epidermal Notch activation resulted in upregulation of jagged 1 in both epidermis and dermis. When Notch was activated in the absence of epidermal jagged 1, jagged 1 was not upregulated in the dermis, and epidermal thickening, blister formation, accumulation of T cells and stromal cells were inhibited. Gene expression profiling revealed that epidermal Notch activation resulted in upregulation of several growth factors and cytokines, including TNFα, the expression of which was dependent on epidermal jagged 1. We conclude that jagged 1 is a key mediator of non-cell autonomous Notch signalling in skin.     

Analysis of morphogenesis and keratinization in transfilter recombinants of feather-forming skin

The relationships between feather morphogenesis, histogenesis, and biochemical differentiation were examined by recombining backskin epidermis and dermis, from chick embryos (Hamburger-Hamilton stages 27-31), with an intervening Nucleopore filter (pore size of 0.4 micron). The filter inhibited normal feather morphogenesis and histogenesis of barb ridges, yet feather-like filaments, which were free of dermal cells, formed from the epidermal cells. Using indirect immunofluorescence, with antiserum against alpha- and beta-keratins, the biochemical differentiation of the feather-like filaments was compared to normal feathers. In the feather-like filaments resulting from tissues of stages 27-29, cells containing beta keratins were occasionally seen at the periphery of the filaments, yet cells containing alpha-keratins were inappropriately located throughout the filaments. In a few feather-like filaments on recombinants resulting from tissues of stages 29.5-31, cells positive for beta-keratins were found in the center of the filament, but again alpha-keratins were also found. Surrounding these cells there were several layers of cells, arranged circumferentially, resembling sheath cells. Some sheath-like cells contained beta-keratins. We conclude that although feather epidermal cells, which are separated from their dermis by a Nuclepore filter, can undergo limited morphogenesis and the production of alpha- and beta-keratins, normal feather morphogenesis, histogenesis, and biochemical differentiation require the intimate associations of epidermis and dermis.     

松江鲈鱼皮肤的显微和亚显微结构

采用光学显微镜、扫描电镜和透射电镜,对松江鲈鱼(Trachidermus fasciatus)成体皮肤的显微和亚显微结构进行了观察。结果表明,松江鲈鱼体表不同部位皮肤的厚薄不一,但基本结构相似。皮肤由表皮和真皮层构成。松江鲈鱼的皮肤裸露无鳞,表皮层较薄,由约4～8层细胞构成,主要由复层上皮细胞和黏液细胞及基底细胞组成。表层细胞呈扁平、多边形,细胞之间主要靠桥粒紧密连接,连接处形成增厚的边缘嵴状突起。表皮细胞游离面向内凹陷,表面形成指纹状微嵴。黏液细胞呈圆形或卵圆形,散布在上皮细胞之间。黏液细胞内的黏原颗粒具有椭圆颗粒状、均匀致密的块状和疏松丝状3种不同形态。真皮通过基膜与表皮相连,由稀疏层和致密层构成。真皮结缔组织在腹部较厚而在其他部位较薄。表皮与真皮连接处有色素层,头部、背部、尾柄和体侧皮肤色素细胞分布多,色素层明显,而腹部和颏部皮肤缺少色素。松江鲈鱼黄河群体真皮层中有角质棘状突起,而滦河群体则无。头部、体侧和尾柄处皮肤上还分布有侧线孔和表面神经丘等感觉器官。     

Abnormal keratinization in the pupoid fetus (pf/pf) mutant mouse epidermis

During its development the epidermis of the pf/pf mutant mouse is invaded by cells from the underlying dermis. These invading cells establish a network of cells including fibroblasts, endothelial cells, and nerve fibers, throughout the epidermis. Subsequent to these events the keratohyalin protein, filaggrin, is drastically reduced and keratinization fails to occur. Heterotypic tissue recombinations indicate that the pf gene is not expressed in the skin. After simply grafting whole mutant dorsal skin, filaggrin synthesis is initiated and an orderly process of epidermal differentiation is achieved. These results suggest that the pf gene acts systemically and that the failure of epidermal differentiation in the mutant occurs secondary to abnormal epidermal organization.     

A variation of pigmentation in the glabrous skin of dogs

The usual pigmentation pattern in mammalian skin consists of fixed melanocytes in the basal layer of the epidermis, supplying keratinocytes with melanosomes. We observed that the glabrous skin (rhinaria and footpads) of dogs deviates from this pattern. In dogs, melanocytes are found in both the dermis and epidermis. The epidermal melanocytes are situated in the intercellular spaces of the basal and spinous layers. They are characterized by a quantity of cytoplasm containing a centriole, also developing melanosomes, and in some cases annulate lamellae. There is a high frequency of closely apposed melanocytes in the epidermis. Melanosomes in different stages of formation are also abundant. The morphology of the glabrous skin of dogs suggests transport of melanocytes from the dermis into the epidermis and formation of melanosomes in the epidermis. A distributed and intense pigment formation may be necessary to achieve the black noses of many dog breeds and wild canids, as well as dark footpads despite heavy abrasion and rapid skin renewal.     

Ultrastructural and histochemical study on gills and skin of the Senegal sole, Solea senegalensis

This study was undertaken to identify the normal ultrastructural features of gills and skin of the Senegal sole, Solea senegalensis, for a comparative measure to morphological alterations caused by environmental stressors such as reduced water quality and diseases. In the Senegal sole skin, four morphologically distinct layers were identified: cuticle, epidermis, dermis and hypodermis. The epidermis was composed of stratified epithelium containing three cellular layers: the outermost or mucosa layer, the middle or fusiform layer and the stratum germinativum or the basal layer. In the mucosa, two mucous cell types were differentiated: type A cells containing several round vesicles of different electron density and type B cells containing mucosomes of uniform electron density. Senegal sole have five pairs of gill arches, each containing two rows of well‐developed and compactly organized primary filaments and secondary lamellae. Fingerprint‐like microridges were observed on the surface of epithelial cells. The branchial lamellae epithelium consisted of different cell types: pavement, mucous and chloride. Between the chloride cells and the larger pavement cells, accessory cells were observed. Complexes of tight junctions and desmosomes were frequently observed between adjacent chloride and epithelial cells. Neutral mucosubstances and/or glycoconjugates were observed in the epidermis, dermis and hypodermis of S. senegalensis skin. Proteins rich in different amino acids, such as arginine and cysteine, reacted negatively or weakly positive in the epidermis, dermis and hypodermis. In gills, some mucous cells responded weakly positive to periodic acid‐Schiff (PAS) reaction but were strongly stained with Alcian Blue at pH 0.5, 1 and 2.5. When Alcian Blue pH 2.5–PAS reaction was performed, most mucous cells were stained blue (carboxylated mucins) and some mucocytes stained purple, indicating a combination of neutral and acid mucins. Proteins rich in cysteine‐bound sulphydryl (‐SH‐) and cystine disulphide (‐S‐S‐) groups were strongly detected in branchial and epidermal mucous cells, whereas lysine, tyrosine and arginine containing proteins showed very weak staining in both epidermal and branchial mucous cells. Protein reactions were strongly positive in the pillar cells, except for those rich in tryptophan, whereas the branchial cartilaginous tissue did not show an important reaction. The performed lipid reactions were negative in goblet and chloride cells. It is concluded from this study that ultrastructural and cytohistochemical features of the Senegal sole skin and gills may serve as control structures in both natural and aquaculture systems to monitor or detect environmental stress responses at the histological level.