Electron probe analysis of human skin: element concentration profiles

Maintaining and Restoring Gradients of Ions in the Epidermis: The Role of Ion and Water Channels in Acute Cutaneous Wound Healing

Significance: Aquaporins and ion channels establish and regulate gradients of calcium, sodium, potassium, chloride, water, and protons in the epidermis. These elements have been found to play significant roles in skin biology and wound healing. In this study, we review our understanding of these channels and ion gradients, with a special emphasis on their role in acute wound healing. Recent Advances: Specifically, we assess the temporal and spatial arrangements of ions and their respective channels in the intact skin and during wound and healing to provide a novel perspective of the role of ionic gradients through the various stages of wound healing. Critical Issues: The roles of gradients of ions and channels in wound healing are currently not well understood. A collective analysis of their traits and arrangements in the skin during wound healing may provide a new perspective and understanding of the functionality of gradients of ions and channels in skin biology and wound healing. Future Directions: It is important to elucidate how the gradients of ions and ion channels regulate and facilitate wound healing. A better understanding of the ionic environments may identify novel therapeutic targets and improved strategies to promote wound healing and possibly treat other cutaneous diseases.

The role of glycosaminoglycans in blood pressure regulation

Essential hypertension (HT) is the global health problem and is a major risk factor for the development of cardiovascular and kidney disease. High salt intake has been associated with HT and impaired kidney sodium excretion is considered to be a major mechanism for the development of HT. Although kidney has a very important role in regulation of BP, this traditional view of BP regulation was challenged by recent findings suggesting that nonosmotic tissue sodium deposition is very important for BP regulation. This new paradigm indicates that sodium can be stored and deposited nonosmotically in the interstitium without water retention and without increased BP. One of the major determinants of this deposition is glycosaminoglycans (GAGs). By binding to GAGs found in the endothelial surface layer (ESL) which contains glycocalyx, sodium is osmotically inactivated and not induce concurrent water retention. Thus, GAGs has important function for homeostatic BP and sodium regulation. In the current review, we summarized the role of GAGs in ESL and BP regulation.

The ion balance of Shotokuseki extract promotes filaggrin fragmentation and increases amino acid production and pyrrolidone carboxylic acid content in three-dimensional cultured human epidermis

Natural moisturizing factor (NMF) in the stratum corneum contributes to the retention of moisture there. The purpose of this study was to determine the penetration of ions in Shotokuseki extract (SE) into the three-dimensional cultured epidermis and the effect of NMF on the biosynthesis of amino acids and pyrrolidone carboxylic acid formation. Various ions, amino acids and pyrrolidone carboxylic acid were quantified by inductively coupled plasma mass spectrometry, fully automatic amino acid analyzer or high-performance liquid chromatography (HPLC) in three-dimensional cultured epidermis after application of SE. Gene expression levels of profilaggrin, calpain1, caspase14, and bleomycin hydrolase, which are involved in NMF production, were determined by reverse-transcription qPCR and bleomycin hydrolase activity was determined by aminopeptidase assay. The application of SE increased Na, K, Mg, Ca, Al, and Fe levels in three-dimensional cultured epidermis. The mRNA levels of the starting material of amino acid synthesis profilaggrin, and calpain1 and bleomycin hydrolase, which are involved in its fragmentation, increased. The activity of bleomycin hydrolase also increased. Furthermore, the levels of amino acids and pyrrolidone carboxylic acid increased in the three-dimensional cultured epidermis. This suggests that the ionic composition of SE may be involved in its moisturizing effect on the stratum corneum.

Na+ is shifted from the extracellular to the intracellular compartment and is not inactivated by glycosaminoglycans during high salt conditions in rats

Recently, studies have emerged suggesting that the skin plays a role as major Na⁺ reservoir via regulation of the content of glycosaminoglycans and osmotic gradients. We investigated whether there were electrolyte gradients in skin and where Na⁺ could be stored to be inactivated from a fluid balance viewpoint. Na⁺ accumulation was induced in rats by a high salt diet (HSD) (8% NaCl and 1% saline to drink) or by implantation of a deoxycorticosterone acetate (DOCA) tablet (1% saline to drink) using rats on a low salt diet (LSD) (0.1% NaCl) on tap water as control. Na⁺ and K⁺ were assessed by ion chromatography in tissue eluates, and the extracellular volume by equilibration of ⁵¹ Cr-EDTA. By tangential sectioning of the skin, we found a low Na⁺ content and extracellular volume in epidermis, both parameters rising by ∼30% and 100%, respectively, in LSD and even more in HSD and DOCA when entering dermis. We found evidence for an extracellular Na⁺ gradient from epidermis to dermis shown by an estimated concentration in epidermis ∼2 and 4-5 times that of dermis in HSD and DOCA-salt. There was intracellular storage of Na⁺ in skin, muscle, and myocardium without a concomitant increase in hydration. Our data suggest that there is a hydration-dependent high interstitial fluid Na⁺ concentration that will contribute to the skin barrier and thus be a mechanism for limiting water loss. Salt stress results in intracellular storage of Na⁺ in exchange with K⁺ in skeletal muscle and myocardium that may have electromechanical consequences. KEY POINTS: Studies have suggested that Na⁺ can be retained or removed without commensurate water retention or loss, and that the skin plays a role as major Na⁺ reservoir via regulation of the content of glycosaminoglycans and osmotic gradients. In the present study, we investigated whether there were electrolyte gradients in skin and where Na⁺ could be stored to be inactivated from a fluid balance viewpoint. We used two common models for salt-sensitive hypertension: high salt and a deoxycorticosterone salt diet. We found a hydration-dependent high interstitial fluid Na⁺ concentration that will contribute to the skin barrier and thus be a mechanism for limiting water loss. There was intracellular Na⁺ storage in muscle and myocardium without a concomitant increase in hydration, comprising storage that may have electromechanical consequences in salt stress.

Electrical aspects of skin as a pathway to engineering skin devices

Skin is one of the indispensable organs for life. The epidermis at the outermost surface provides a permeability barrier to infectious agents, chemicals, and excessive loss of water, while the dermis and subcutaneous tissue mechanically support the structure of the skin and appendages, including hairs and secretory glands. The integrity of the integumentary system is a key for general health, and many techniques have been developed to measure and control this protective function. In contrast, the effective skin barrier is the major obstacle for transdermal delivery and detection. Changes in the electrical properties of skin, such as impedance and ionic activity, is a practical indicator that reflects the structures and functions of the skin. For example, the impedance that reflects the hydration of the skin is measured for quantitative assessment in skincare, and the current generated across a wound is used for the evaluation and control of wound healing. Furthermore, the electrically charged structure of the skin enables transdermal drug delivery and chemical extraction. This paper provides an overview of the electrical aspects of the skin and summarizes current advances in the development of devices based on these features.

Inorganic ions in the skin: Allies or enemies?

Skin constitutes a barrier protecting the organism against physical and chemical factors. Therefore, it is constantly exposed to the xenobiotics, including inorganic ions that are ubiquitous in the environment. Some of them play important roles in homeostasis and regulatory functions of the body, also in the skin, while others can be considered dangerous. Many authors have shown that inorganic ions could penetrate inside the skin and possibly induce local effects. In this review, we give an account of the current knowledge on the effects of skin exposure to inorganic ions. Beneficial effects on skin conditions related to the use of thermal spring waters are discussed together with the application of aluminium in underarm hygiene products and silver salts in treatment of difficult wounds. Finally, the potential consequences of dermal exposure to topical sensitizers and harmful heavy ions including radionuclides are discussed.

Clinical impact of tissue sodium storage

In recent times, the traditional nephrocentric, two-compartment model of body sodium has been challenged by long-term sodium balance studies and experimental work on the dermal interstitium and endothelial surface layer. In the new paradigm, sodium can be stored without commensurate water retention in the interstitium and endothelial surface layer, forming a dynamic third compartment for sodium. This has important implications for sodium homeostasis, osmoregulation and the hemodynamic response to salt intake. Sodium storage in the skin and endothelial surface layer may function as a buffer during periods of dietary depletion and excess, representing an extra-renal mechanism regulating body sodium and water. Interstitial sodium storage may also serve as a biomarker for sodium sensitivity and cardiovascular risk, as well as a target for hypertension treatment. Furthermore, sodium storage may explain the limitations of traditional techniques used to quantify sodium intake and determine infusion strategies for dysnatraemias. This review is aimed at outlining these new insights into sodium homeostasis, exploring their implications for clinical practice and potential areas for further research for paediatric and adult populations.

Abnormal sodium and water homeostasis in mice with defective heparan sulfate polymerization

Glycosaminoglycans in the skin interstitium and endothelial surface layer have been shown to be involved in local sodium accumulation without commensurate water retention. Dysfunction of heparan sulfate glycosaminoglycans may therefore disrupt sodium and water homeostasis. In this study, we investigated the effects of combined heterozygous loss of heparan sulfate polymerization genes (exostosin glycosyltransferase 1 and 2; Ext1^+/-Ext2^+/-) on sodium and water homeostasis. Sodium storage capacity was decreased in Ext1^+/-Ext2^+/- mice as reflected by a 77% reduction in endothelial surface layer thickness and a lower skin sodium-to-glycosaminoglycan ratio. Also, these mice were characterized by a higher heart rate, increased fluid intake, increased plasma osmolality and a decreased skin water and sodium content, suggesting volume depletion. Upon chronic high sodium intake, the initial volume depletion was restored but no blood pressure increase was observed. Acute hypertonic saline infusion resulted in a distinct blood pressure response: we observed a significant 15% decrease in control mice whereas blood pressure did not change in Ext1^+/-Ext2^+/- mice. This differential blood pressure response may be explained by the reduced capacity for sodium storage and/or the impaired vasodilation response, as measured by wire myography, which was observed in Ext1^+/-Ext2^+/- mice. Together, these data demonstrate that defective heparan sulfate glycosaminoglycan synthesis leads to abnormal sodium and water homeostasis and an abnormal response to sodium loading, most likely caused by inadequate capacity for local sodium storage.

High-Salt Diet Causes Osmotic Gradients and Hyperosmolality in Skin Without Affecting Interstitial Fluid and Lymph

The common notion is that the body Na + is maintained within narrow limits for fluid and blood pressure homeostasis. Several studies have, however, shown that considerable amounts of Na + can be retained or removed from the body without commensurate water loss and that the skin can serve as a major salt reservoir. Our own data from rats have suggested that the skin is hypertonic compared with plasma on salt storage and that this also applies to skin interstitial fluid. Even small electrolyte gradients between plasma and interstitial fluid would represent strong edema-generating forces. Because the water accumulation has been shown to be modest, we decided to reexamine with alternative methods in rats whether interstitial fluid is hypertonic during salt accumulation induced by high-salt diet (8% NaCl and 1% saline to drink) or deoxycorticosterone pellet implantation. These treatments resulted both in increased systemic blood pressure, skin salt, and water accumulation and in skin hyperosmolality. Interstitial fluid isolated from implanted wicks and lymph draining the skin was, however, isosmotic, and Na + concentration in fluid isolated by centrifugation and in lymph was not different from plasma. Interestingly, by eluting layers of the skin, we could show that there was an osmolality and urea gradient from epidermis to dermis. Collectively, our data suggest that fluid leaving the skin as lymph is isosmotic to plasma but also that the skin can differentially control its own electrolyte microenvironment by creating local gradients that may be functionally important.

The importance of hydration in wound healing: reinvigorating the clinical perspective

Balancing skin hydration levels is important as any disruption in skin integrity will result in disturbance of the dermal water balance. The discovery that a moist environment actively supports the healing response when compared with a dry environment highlights the importance of water and good hydration levels for optimal healing. The benefits of 'wet' or 'hyper-hydrated' wound healing appear similar to those offered by moist over a dry environment. This suggests that the presence of free water may not be detrimental to healing, but any adverse effects of wound fluid on tissues is more likely related to the biological components contained within chronic wound exudate, for example elevated protease levels. Appropriate dressings applied to wounds must not only be able to absorb the exudate, but also retain this excess fluid together with its protease solutes, while concurrently preventing desiccation. This is particularly important in the case of chronic wounds where peri-wound skin barrier properties are compromised and there is increased permeation across the injured skin. This review discusses the importance of appropriate levels of hydration in skin, with a particular focus on the need for optimal hydration levels for effective healing. Declaration of interest: This paper was supported by Paul Hartmann Ltd. The authors have provided consultative services to Paul Hartmann Ltd.

Tissue sodium storage: evidence for kidney-like extrarenal countercurrent systems?

Recent evidence from chemical analysis of tissue electrolyte and water composition has shown that body Na(+) content in experimental animals is not constant, does not always readily equilibrate with water, and cannot be exclusively controlled by the renal blood purification process. Instead, large amounts of Na(+) are stored in the skin and in skeletal muscle. Quantitative non-invasive detection of Na(+) reservoirs with sodium magnetic resonance imaging ((23)NaMRI) suggests that this mysterious Na(+) storage is not only an animal research curiosity but also exists in humans. In clinical studies, tissue Na(+) storage is closely associated with essential hypertension. In animal experiments, modulation of reservoir tissue Na(+) content leads to predictable blood pressure changes. The available evidence thus suggests that the patho(?)-physiological process of Na(+) storage might be of relevance for human health and disease.

The expression of proinflammatory genes in epidermal keratinocytes is regulated by hydration status

Mucosal wounds heal more rapidly, exhibit less inflammation, and are associated with minimal scarring when compared with equivalent cutaneous wounds. We previously demonstrated that cutaneous epithelium exhibits an exaggerated response to injury compared with mucosal epithelium. We hypothesized that treatment of injured skin with a semiocclusive dressing preserves the hydration of the skin and results in a wound healing phenotype that more closely resembles that of mucosa. Here we explored whether changes in hydration status alter epidermal gene expression patterns in rabbit partial-thickness incisional wounds. Using microarray studies on injured epidermis, we showed that global gene expression patterns in highly occluded versus non-occluded wounds are distinct. Many genes including IL-1β, IL-8, TNF-α (tumor necrosis factor-α), and COX-2 (cyclooxygenase 2) are upregulated in non-occluded wounds compared with highly occluded wounds. In addition, decreased levels of hydration resulted in an increased expression of proinflammatory genes in human ex vivo skin culture (HESC) and stratified keratinocytes. Hierarchical analysis of genes using RNA interference showed that both TNF-α and IL-1β regulate the expression of IL-8 through independent pathways in response to reduced hydration. Furthermore, both gene knockdown and pharmacological inhibition studies showed that COX-2 mediates the TNF-α/IL-8 pathway by increasing the production of prostaglandin E2 (PGE2). IL-8 in turn controls the production of matrix metalloproteinase-9 in keratinocytes. Our data show that hydration status directly affects the expression of inflammatory signaling in the epidermis. The identification of genes involved in the epithelial hydration pathway provides an opportunity to develop strategies to reduce scarring and optimize wound healing.

Infant epidermal skin physiology: adaptation after birth

BACKGROUND

Functional and structural skin adaptation is a dynamic process which starts immediately after birth in humans and in mammalian skin in general. This adjustment to the extrauterine dry environment is accomplished in the first year of postnatal life of humans.

OBJECTIVES

To assess the dynamic changes in vivo after birth in the molecular composition and skin physiology parameters compared with older children and adults.

METHODS

The molecular composition of the stratum corneum (SC) and the water profile were investigated noninvasively by in vivo Raman confocal microscopy as a function of depth. Functional parameters including transepidermal water loss (characterizing epidermal permeability barrier), capacitance (as an indirect parameter for SC hydration) and skin surface pH were assessed noninvasively. The measurements were performed in 108 subjects divided into six age groups: full-term newborns (1-15 days), babies aged 5-6 weeks, babies aged 6±1 months, children aged 1-2 years, children aged 4-5 years and adults aged 20-35 years.

RESULTS

We showed that skin acidification is still under development during the first weeks of life. While the basal epidermal barrier is competent immediately after birth, the SC is less hydrated in the first 2 weeks of postnatal life. Similar continuous decreasing water content towards the surface for all age groups was observed, whereas this gradient was lower for the newborns. Dynamic changes in the amounts of the natural moisturizing factor constituents were revealed in the period of infancy.

CONCLUSIONS

We demonstrated the relation of formation of an acidic pH as well as underlying mechanisms in the induction of a fully hydrated SC over the first weeks of human life as a dynamic functional adaptation.

Relationship Between NMF (Lactate and Potassium) Content and the Physical Properties of the Stratum Corneum in Healthy Subjects

Natural moisturizing factor (NMF) of the stratum corneum (SC) has been established to play important roles in the physical properties of the SC. Few studies, however, have investigated the specific influences of NMF components other than the amino acids. In this study, therefore, we focus on the relationship between the ion content and physical properties of the SC in 40 healthy subjects. Changes in the physical properties of the SC induced by the extraction of NMF were equivalent to the changes that took place from summer to winter, demonstrating the important role of NMF in the physical properties of the SC in healthy subjects. The seasonal changes in the physical properties of the SC from summer to winter were accompanied by significant decreases in the levels of lactate, potassium, sodium, and chloride in the SC. Lactate and potassium were the only components found to correlate significantly with the state of hydration, stiffness, and pH in the SC. Interestingly, the levels of lactate and potassium in the SC were also significantly correlated. Moreover, potassium lactate restored the SC hydration state decreased by extraction of NMF. These results suggest that lactate and potassium may play roles in maintaining the physical properties of the SC in healthy subjects.

Factors determining percutaneous metal absorption

Metals play a vital role in human, animal and plant physiology, and important research, past and ongoing, is directed towards exploring the interrelated mechanisms that govern their penetration through skin. Much insight has been gained through these efforts, but our understanding of the process is still incomplete, mainly due to the failure to allow for the effects of chemical speciation of metallic elements, especially the transition metals. Also, the skin as target organ presents imponderable and wide margins of variability. In vivo permeability is subject to homeostasis regulating the overall organism; in vitro, the sections of skin used for diffusion experiments are likely to present artifacts. Endeavors to define rules governing skin penetration to give predictive quantitative structure-diffusion relationships for metallic elements for risk assessment purposes have been unsuccessful, and penetration of the skin still needs to be determined separately for each metal species, either by in vitro or in vivo assays. Phenomena observed by us and other investigators, which appear to determine the process of skin permeation for a number of metals, are reviewed, separating the exogenous factors from the characteristics of the skin or other endogenous factors.

Influence of ion pairing on topical delivery of retinoic acid from microemulsions

The purpose of the present study was to determinate the significance of ion pairing on the topical permeation of retinoic acid (R.A) using microemulsions as delivery vehicles. Phenylalanine methyl ester, phenylalanine ethylester, histidine methyl ester, tryptophan methyl ester and valine methyl ester were used as counter ions. Results of diffusion studies through polydimethylsiloxane membrane (PDMS) indicate that retinoic acid permeation from ethanol-pH 6.4 buffer mixture significantly increased in the presence of counter ions. A linear relationship was found between apparent partition coefficients and permeation coefficients. The highest values were with valine methyl ester and phenylalanine ethyl ester. In order to develop alternative formulations for topical administration of R.A, microemulsions were evaluated as delivery vehicles. Oil-in-water (O/W) and water-in-oil (W/O) microemulsion formulations were prepared using water, isopropyl myristate, lecithin, caprylyl-capryl glucoside and ethanol or 1,2 hexanediol. Experiments with PDMS membranes showed decreasing permeabilities of R.A from microemulsions in the presence of counter ions. This was related to the increased lipophilicity and different vehicle membrane affinity of the ion pairs The ability of the systems to deliver R.A through the skin was evaluated in vitro using pig-skin. R.A permeabilities were much lower with microemulsions than with solution, while a large increase in R.A skin deposition was observed only from O/W microemulsions in the presence of counter ions. The depth of skin accumulation was below 100 microm after 24 h application. The results suggest that O/W microemulsions containing a counter ion can be used to optimise drug targeting without a concomitant increase in systemic absorption.

Impaired stratum corneum hydration in mice lacking epidermal water channel aquaporin-3

The water and solute transporting properties of the epidermis have been proposed to be important determinants of skin moisture content and barrier properties. The water/small solute-transporting protein aquaporin-3 (AQP3) was found by immunofluorescence and immunogold electron microscopy to be expressed at the plasma membrane of epidermal keratinocytes in mouse skin. We studied the role of AQP3 in stratum corneum (SC) hydration by comparative measurements in wild-type and AQP3 null mice generated in a hairless SKH1 genetic background. The hairless AQP3 null mice had normal perinatal survival, growth, and serum chemistries but were polyuric because of defective urinary concentrating ability. AQP3 deletion resulted in a > 4-fold reduced osmotic water permeability and > 2-fold reduced glycerol permeability in epidermis. Epidermal, dermal, and SC thickness and morphology were not grossly affected by AQP3 deletion. Surface conductance measurements showed remarkably reduced SC water content in AQP3 null mice in the hairless genetic background (165 +/- 10 versus 269 +/- 12 microsiemens (microS), p < 0.001), as well as in a CD1 genetic background (209 +/- 21 versus 469 +/- 11 microS). Reduced SC hydration was seen from 3 days after birth. SC hydration in hairless wild-type and AQP3 null mice was reduced to comparable levels (90-100 microS) after a 24-h exposure to a dry atmosphere, but the difference was increased when surface evaporation was prevented by occlusion or exposure to a humidified atmosphere (179 +/- 13 versus 441 +/- 34 microS). Conductance measurements after serial tape stripping suggested reduced water content throughout the SC in AQP3 null mice. Water sorption-desorption experiments indicated reduced water holding capacity in the SC of AQP3 null mice. The impaired skin hydration in AQP3 null mice provides the first functional evidence for the involvement of AQP3 in skin physiology. Modulation of AQP3 expression or function may thus alter epidermal moisture content and water loss in skin diseases.

Changes in tactile spatial discrimination and cutaneous coding properties by skin hydration in the elderly

Neurosensory tactile functions were investigated in human subjects by two different and complementary experimental approaches. First, a conventional psychophysical method (two-point gap discrimination) was used to determine the tactile discrimination threshold by analyzing the subjects' ability to detect a gap of variable width between two contact points when a series of stimuli was applied to the skin. Using this method we confirmed the marked degradation of tactile spatial acuity with age and showed that skin discriminative function was partially restored after hydration of the skin with a moisturizer. The second approach consisted of a microneurographic recording of tactile afferent fibers in response to two types of mechanical stimuli applied reproducibly to the corresponding receptive fields. With this method, we found that the afferent messages were depressed following hydration of the skin surface. Thus, partial restoration of tactile spatial acuity after hydration appears to be due to both a softening of the stratum corneum permitting better localization of the stimulus and a weaker transfer of the stimulus toward the sensory receptors.

Visual imaging of ion distribution in human epidermis

The distribution of calcium, magnesium, potassium, sodium, and hydrogen ions in the human epidermis was visualized by blotting to gel containing chemical indicators and the effects of skin barrier disruption were examined. In normal skin, both calcium and magnesium were localized with high concentration in the upper epidermis. EDTA blocked these imaging. The hydrogen ion was also high in the upper epidermis. Sodium did not show obvious gradation in the epidermis. The potassium concentration was the lowest in the upper epidermis. After the barrier disruption, the gradients of calcium, magnesium, and potassium disappeared while the pH gradation was not altered. Observation at a high magnification revealed lower calcium and sodium concentrations in the nucleus. The concentration of magnesium was slightly higher in the nucleus. The novel method of the present study could show the visual image of the ions in frozen tissue without further preparation.

Epithelial sodium channels are upregulated during epidermal differentiation

Terminal differentiation of epidermal keratinocytes is linked to transmembrane ion flux. Previously, we have shown that amiloride, an inhibitor of epithelial sodium channels, blocks synthesis of differentiation-specific proteins in normal human keratinocytes. Here, we have identified the specific subunits of amiloride-sensitive human epithelial sodium channels in relation to differentiation of cultured human keratinocytes, as well as to epidermal development. As assessed by northern hybridization, RNase protection assay, and reverse transcription-polymerase chain reaction, transcripts encoding functional alpha and regulatory beta subunits of human epithelial sodium channels were expressed both in cultured keratinocytes and in epidermis at levels comparable with the kidney. The mRNA expression of both human epithelial sodium channel-alpha and -beta increased during calcium-induced keratinocyte differentiation. Whereas the beta subunit of human epithelial sodium channel was induced by elevated concentrations of calcium, the alpha subunit increased with duration of culture. The regulatory gamma subunit was less abundant but also expressed in epidermis. Both human epithelial sodium channel-alpha and -beta were localized throughout the nucleated layers of human adult epidermis, but these channels were not detected in early stages of fetal epidermal development. This co-ordinated expression of subunits suggests that epithelial sodium channels may play an important part in both epidermal differentiation and skin development, presumably by modulating ion transport required for epidermal terminal differentiation.

Further optimization of culture method for rat keratinocytes: titration of glucose and sodium chloride

The present study further improved the serum-free method of culturing rat keratinocytes. To obtain the best growth of rat keratinocytes, we modified our previous serum-free medium (MCDB153 based medium), particularly the amounts of glucose and sodium chloride (NaCl). Titration experiments showed the optimal concentration to be 0.8 mM for glucose and 100 mM for NaCl. This modification eliminated the requirement for albumin, which had been essential for colony formation when our previous medium was used. Titration of glucose and NaCl, followed by adjustment of essential amino acids and growth factors, produced a new formulation. More satisfactory and better growth was achieved with the new medium than with the previous medium. Accumulation of monoalkyldiacylglycerol (MADAG) was consistently noted in this study, representing the unusual lipid profile. A tendency toward normalization was, however, noted with the neutral lipid profile of keratinocytes cultivated in the new medium: lower production of MADAG was obtained with the new formulation, rather than the previous one.

Acute barrier perturbation abolishes the Ca2+ and K+ gradients in murine epidermis: quantitative measurement using PIXE

Epidermal permeability barrier homeostasis requires the delivery of lipids and hydrolytic enzymes by lamellar body exocytosis from the uppermost granular cells, a process that is upregulated following barrier disruption. As lamellar body secretion is controlled by ionic concentrations, especially Ca2+ and K+, we used a quantitative technique, microbeam proton-induced X-ray emission, to measure Ca2+, K+, Cl-, and P concentrations before and after acute barrier perturbation by acetone applications. We found a steep gradient of Ca2+ in normal tissue, peaking in the outer stratum granulosum, which disappeared after barrier disruption, and partially reformed as the barrier recovered. A similar gradient, peaking somewhat lower in the epidermis (i.e., at the stratum granulosum-stratum corneum interface), was found for K+. Epidermal concentrations of K+ also decreased after barrier abrogation, although to a lesser extent than Ca2+. In contrast, P and Cl- demonstrated distribution gradients at baseline, which remained unchanged after barrier disruption. These studies quantitate the levels of Ca2+, K+, Cl-, and P within specific epidermal cell layers at baseline, and in relation to changes in permeability barrier integrity. Ca2+ and K+, but not Cl- or P, decrease after barrier disruption, consistent with these two ion's role in barrier repair.

Aspects on the physiology of human skin: studies using particle probe analysis

The cellular part of the skin, the epidermis, is a very thin structure, approximately 120 microns thick, a fact which has hindered the exploration of the physiology of the epidermis in normal and pathological conditions. An additional complication is the fact that the epidermis contains layers of cells at different stages of differentiation. Therefore, conventional physiological capillary probes cannot, with any satisfactory precision, be located within a specified cell of a specified layer of the skin in vivo. Hence, alternative ways for the exploration of skin physiology have been sought for. In the past, analysis of the elemental content of skin was done was done as bulk measurements, and surprisingly wide ranges of elemental content were recorded. The width of these ranges was most certainly due to the sampling methods used rather than the sensitivity of the chosen method of analysis. Also, these older measurements did not discriminate between the different strata, and therefore the information provided little if any substance for a functional analysis of processes involved in normal and pathological differentiation of the epidermis. Particle probes, however, have been able to overcome such methodological problems. Over a period of 15 years we have studied normal human skin, normal-looking, paralesional skin from psoriatics, and skin from persons suffering from atopic dermatitis using PIXE analysis. In recent years, trace elements have been shown to work as secondary messengers or regulatory substances. As an example, calcium (Ca2+) has proven to be a very important signalling substance in a great variety of cellular systems. Studies with the transmission electron microscope (TEM) as well as histochemical methods have allowed an understanding of the role of Ca2+ in the differentiation process of the epidermis. Ca2+ has also been shown to play an important role in apoptosis (programmed cell death), which is currently a hot subject for the obvious reason that the final differentiation step between the stratum granulosum level and the stratum corneum represents a particular aspect of programmed cell death. The importance of the balance between calcium and zinc in apoptosis has been clearly demonstrated in a number of cellular systems, but we have still to clarify the validity of topical treatment with Zn ointments in different skin conditions. Substantial iron (Fe) losses via psoriatic lesions were demonstrated more than two decades ago, and these data were given new meaning when we found that a more discrete loss occurs in clinically normal-looking psoriatic skin. Obviously, such findings stress the importance of understanding the relation between the elemental content and normal and abnormal physiology. The ultimate goal of particle probe studies is to provide an understanding of the formation of a mature stratum corneum with a functional barrier reflected in physiological/biochemical mechanisms behind the properties of changed skin in patients afflicted with skin disorders of genetic or constitutional origin. This paper aims to give an overview of the state of the art in skin physiology made possible through the use of particle probes.

Corneocytes undergo systematic changes in element concentrations across the human inner stratum corneum

Using analytical electron microscopy of freeze-dried cryosections, physiologic elements were visualized within individual cells across the human inner stratum corneum. Human corneocytes undergo systematic changes in element composition as they advance through this region. Phosphorus is largely excluded from the stratum corneum, undergoing a precipitous drop in concentration at the granular/stratum corneum interface. The cellular potassium concentration has a profile similar to that of phosphorus but with a slower decline, thus migrating further into the stratum corneum. In contrast, the cellular chloride concentration increases in the innermost corneocyte layer, increases further in the subsequent layer or two (as potassium declines), and then decreases to values comparable to those in the innermost corneocyte. The cellular sodium concentration (per unit volume of tissue) is relatively unaltered in transit across the inner stratum corneum. The initial potassium and chloride movements are oppositely directed and have the appearance of creating an electrical charge imbalance. The position-dependent alterations in corneocyte elemental composition may reflect sequential stages of chemical maturation occurring intracellularly during stratum corneum transit, an example of which is the breakdown of filaggrin that occurs over this same region of the inner stratum corneum.

Measurement of properties and function of skin

Dermatologists and skin biologists have traditionally approached the diagnosis of skin diseases by either a very gross measurement of its physical state, i.e. a skin pinch, or at the other extreme by being concerned with the cellular and sub-cellular organisation of the skin. Recently, using methods often borrowed from materials science, techniques have been developed and used to aid the investigation of the changes that occur with disease and ageing, for example. A review of these methods developed to study the structure and function of skin by the physical scientists working in close collaboration with dermatologists, pathologists and skin biologists is presented.

Magnesium and phosphate enrichment of culture medium stimulates the proliferation of epidermal cells from newborn and adult mice

The proliferation and differentiation of mouse epidermal cells can be sequentially analyzed by modification of extracellular calcium. Newborn cells cultured in low calcium medium (less than 0.1 mM) proliferate as a monolayer and maintain a typical basal cell phenotype in culture but have a limited proliferative capacity and short lifespan. Elevation of the magnesium content of the culture medium from 1 to 5 mM stimulated the proliferation of newborn mouse (1-3 days old) keratinocytes. Maximal DNA synthesis rates, as determined on day 5 of culture, were up to 2-3-fold higher in the magnesium-enriched cultures. Exposure to high magnesium caused 3-4-fold increases in the DNA content of newborn keratinocyte cultures, and extended the confluent phase of epidermal cell growth to over 10 days. Other divalent cations (strontium, copper, zinc, nickel, beryllium, and barium) did not improve keratinocyte growth in culture. Keratinocytes from the tail skin of adult (3 months old) mice displayed an absolute requirement for high phosphate in the culture medium. The medium containing an optimal (10 mM) phosphate concentration prevented the cell detachment caused by the standard low (1 mM) phosphate medium, and in combination with an elevated magnesium content (10-15 mM) it markedly increased both DNA synthesis rates and DNA content of the adult cell cultures. Optimally growing, newborn or adult cultures contained less cells in the G1 phase of the cell cycle and more cells in S and G2 +M. The addition of phosphate and magnesium per se did not induce keratinocyte differentiation and did not interfere with the high calcium (1 mM)-induced differentiation.

Electron probe analysis of human skin: determination of the water concentration profile

The water concentration profile across rapidly frozen human epidermis has been measured using electron probe analysis and analytical electron microscopy. Determinations were made within the cytoplasm of individual cells. From the basal layer, the water content remained relatively constant or decreased slightly across the viable tissue and decreased approximately linearly across the stratum corneum. A considerable discontinuity in water content occurred between these two regions over the stratum corneum-stratum granulosum junction and the last granular cell layer. The dominance of the water profile by a discontinuity suggests water loss is governed by a partitioning process, presumably a partitioning into the lipid domain. A water discontinuity offers important functional advantages in the conservation of substances within the body and in protection from intruding molecules.