Non-invasive assessment of stratum corneum structure and function

Methods for the study of the stratum corneum in situ are described and their application to clinical problems and cosmetic science are discussed. In vivo measurements of the thickness of skin and the components of skin can be made using high frequency pulsed ultrasound. Improved resolution of the device should allow measurement of epidermal thickness, and even that of the stratum corneum, which may vary with hydration. The protective ability of the stratum corneum can be estimated in a number of ways, the most familiar technique being transepidermal water loss measurements. Mechanical properties of the stratum corneum have been widely investigated in vivo by numerous researchers. A recent technique devised by us to deform the stratum corneum using an extendable metal frame has been used on patients with ichthyotic disorders. The deformation or compliance of the skin surface has been measured using profilometric methods, and compared to normal individuals. The results appear to be in agreement with the loss of flexural ability experienced by ichthyotic patients. Other mechanical methods more usually involve the measurement of the forces generated by skin while undergoing extension. Three techniques are described which have been used to investigate hydrational change with emollient application. Finally, the phenomenon of desquamation is discussed and methods presented for its evaluation. Passive collection of corneocytes using chambers is useful for estimation of the rates of desquamation, but are laborious and inconvenient. Forced desquamation using the desquamator enables rapid controlled harvesting of corneocytes. In addition to this, the dansyl chloride fluorescence technique has been adopted for estimation of stratum corneum kinetics using visual comparison and fluorimetric techniques. These techniques require further refinement and validation, but the tests available now are nonetheless important in understanding the biology and pharmacology of the stratum corneum.     

Chitin–glucan,a natural cell scaffold for skin moisturization and rejuvenation

Two clinical studies were conducted to evaluate the tolerance and effects of a copolymer of chitin and beta‐glucan, forming the exoskeleton of fungal cell walls, now supplied for cosmetic applications. A 6‐week randomized, double‐blind, placebo‐controlled study was conducted on 13 volunteers with sensitive skin to compare with 0.5–2% formulations chitin–glucan applied twice daily. Biometrological evaluations showed that erythema did not develop, the water retention capacity of the stratum corneum increased and the transepidermal water loss moderately decreased. Another 16‐week randomized, double‐blind, placebo‐controlled study was conducted on 20 men showing signs of ageing skin. A 1.5% chitin–glucan formulation was applied twice daily. Objective biometrological assessments showed a progressive increase in skin firmness and stratum corneum hydration when desquamation and skin roughness decreased. In conclusion, the chitin–glucan formulations appear safe. They significantly mitigate some signs of skin ageing and improve both stratum corneum hydration and skin barrier function.     

Dry skin, moisturization and corneodesmolysis

The process leading to the loss of corneocytes form the skin surface is termed desquamation. In healthy skin it is an orderly and essentially invisible process whereby individual or small groups of corneocytes detach from neighbouring cells to be lost to the environment and replaced by younger cells from the deeper layers. Desquamation is carefully controlled to ensure that corneum cohesion and integrity, and hence tissue thickness, is maintained. The most important components of the corneocytes contributing towards intercellular cohesion are the corneodesmosomes and lipids. Corneodesmosomes are proteinaceous complexes which effectively rivet corneocytes together. The intercellular lipids, primarily responsible for the water barrier, also provide part of the extracellular cement. In addition, the shape of the corneocyte itself plays a role in stratum corneum cohesion. Through interdigitation along their peripheral edges, adjacent corneocytes become physically locked together, a process which reinforces the integrity of the tissue. For effective desquamation to occur corneodesmosomes must be degraded: a process catalysed by serine proteases present within the intercellular space and facilitated by subtle changes in lipid composition and phase behaviour. Ultimately, it is the availability of free water which controls corneodesmolysis. In healthy skin this proteolytic process leaves relatively few corneodesmosomes intact in the most superficial layers. By contrast, in chronic and acute dry skin conditions, corneodesmosomal degradation and hence the final stages of desquamation are perturbed, leading to the characteristic formation of visible, powdery flakes on the skin surface. The inability to degrade these structures ultimately reflects a decreased hydrolytic activity of the desquamatory enzymes, either through reduced synthesis of the enzymes, inherent loss of activity, leaching from the surface layers of the corneum or changes in the surrounding lipid-rich microenvironment, which may indirectly reduce enzyme functionality. Increased understanding of the desquamation process is providing new insights into the mode of action of current moisturizing ingredients and is offering opportunities to develop novel therapies for preventing and correcting dry skin.     

Studies on isolated corneocytes

The function of the stratum corneum is dependent on the functions of its constituent parts. This paper focusses on corneocyte dimensions and the implications of change in dimension for function. A technique in which it is possible to obtain a value for corneocyte thickness is described. The changes in corneocyte dimensions with age and site and after topical applications are documented.
Techniques which measure the rate of desquamation are also described. The rate of passive corneocyte loss has been estimated by counting the numbers of corneocytes shed into chambers fixed to the skin surface. In addition the corneocyte loss after a standardized rotational stimulus to the skin surface has been measured. Scanning and transmission electron microscope examination of corneocytes obtained by a scrub procedure is also described. A preliminary report on an immunological approach to the study of the biochemistry of corneocytes and stratum corneum is also included.
Etude de cornéocytes isolés     

Skin lipid organization,composition and barrier function

Citation: IFSCC Magazine, 10 (2007) (4) 297–307 The primary function of the skin is to act as a barrier against unwanted influences from the environment and to protect the body from water loss. The barrier function of the skin is located in the superficial layer of the skin, the stratum corneum. The stratum corneum consists of dead cells filled with keratin and water, the corneocytes, embedded in lipid regions. As the lipid regions are the only continuous structure in the stratum corneum, they are considered to be very important for the skin barrier function. The main lipid classes are ceramides, cholesterol and free fatty acids. In this paper the lipid organization in human stratum corneum is reviewed. In addition, the role the various lipid classes play in lipid organization is discussed using mixtures prepared from either native human ceramides or synthetic ceramides. Finally, a model is described which allows study of the relation between lipid composition, organization and barrier function. This model is referred to as the stratum corneum substitute. Keywords: ceramides, FTIR, lipid organization, stratum corneum, X‐ray diffraction     

In vivo transepidermal water loss and skin surface hydration in assessment of moisturization and soap effects

Transepidermal water loss (TEWL), skin surface water loss (SSWL) and water content of the stratum corneum are utilized to assess the hydration effects of moisturizers and soaps. The relationship among these parameters may help differentiating hydration obtained via occlusion or by water-holding in the stratum corneum. Furthermore, skin function (hydration, dehydration, barrier damage) can be studied comparing the data obtained with these techniques. In this study, the effects of glycerol, petrolatum, soaps and commercial moisturizers on the skin are investigated and discussed.     

A clinical, biometrological and ultrastructural study of xerotic skin

The purpose of this in vivo study was to investigate, non-invasively on human subjects, xerotic skin and its physiological evolution over time, compared to normal skin. Two groups of 17 female subjects were studied during the winter season, one made up of subjects with normal skin and the other subjects with xerotic skin. A clinical assessment and biometrological measurements of hydration and transepidermal water loss (TEWL) were performed on the same area of the external antero-lateral surface of the leg at the start of the study then after three weeks. At the end of the study, the ultrastructure of stratum corneum samples taken from the same area was examined by transmission electron microscopy. Subjects with xerotic skin were selected according to their impaired cutaneous barrier function, reflected in a TEWL higher than 12 g/m ; 2/h. Compared to normal subjects, they presented a hydration level more than 25% lower. After an interval of 21 days, no significant change in the hydration level or clinical appearance of the xerotic skin was observed. In contrast, the TEWL had decreased significantly (D _ 21- D _ 0=-3.6 g/m ; 2/h; p < 0.001) but still stayed higher than normal values. Changes in the ultrastructure of the stratum corneum were also observed in the subjects with xerotic skin. Unlike normal skin, corneosomes could be detected right up to the surface layers, accompanied by intercellular lipids in an amorphous form. These observations confirm the important roles played by both corneosomes and lipid organization in the cohesion/desquamation processes. In the subjects with normal skin, the hydration level and barrier function remained unchanged during the three week study but an onset of skin dryness was observed, the mean clinical score increasing by +1.3 (p = 0.01). These results confirm that there is no direct relationship between TEWL and the severity of skin dryness. It appears that a clinical evaluation is more sensitive than biometrological measurement for describing early state of cutaneous dryness. This study highlights the importance of a regular cosmetic or dermopharmaceutical treatment during the winter to prevent xerosis apparition on legs.     

Age‐related changes in skin barrier function – Quantitative evaluation of 150 female subjects

The protection against water loss and the prevention of substances and bacteria penetrating into the body rank as the most important functions of the skin. This so‐called ‘skin barrier function’ is the natural frontier between the inner organism and the environment, and is primarily formed by the epidermis. An impairment of the skin barrier function is often found in diseased and damaged skin. An influence of ageing on skin barrier function is widely accepted, but has not been conclusively evaluated yet. Therefore, the aim of this clinical study was to assess the potential influence of ageing on skin barrier function, including transepidermal water loss (TEWL), stratum corneum hydration, sebum content and pH value. One hundred and fifty healthy women aged 18–80, divided into five age groups with 30 subjects each, were evaluated in this study. TEWL, hydration level, sebum secretion and pH value of hydro‐lipid acid film were measured with worldwide acknowledged biophysical measuring methods at cheek, neck, décolleté, volar forearm and dorsum of hand. Whereas TEWL and stratum corneum hydration showed only very low correlation with subject's age, the sebum production decreased significantly with age, resulting in the lowest skin surface lipids levels measured in subjects older than 70 years. The highest skin surface pH was measured in subjects between 50 and 60 years, whereas the eldest age group had the lowest mean pH. The dorsum of the hand was the location with the highest TEWL and lowest stratum corneum hydration in all age groups. The results show that only some parameters related to skin barrier function are influenced by ageing. Whereas sebum production decreases significantly over lifetime and skin surface pH is significantly increased in menopausal woman, TEWL and stratum corneum hydration show only minor variations with ageing.     

What are meters measuring?

There is increasing pressure on manufacturers of cosmetic products to provide data to support claims. Data are available from many sources including historical (published literature), laboratory data, cell culture experiments and human studies. Undoubtedly, human studies are the most reliable, and there are a wide range of tests available. Many meters have been developed for measuring different aspects of skin physiology but an understanding of these devices is essential, otherwise the data generated is of little value. There is some confusion as to what exactly some meters measure, an example of which is transepidermal water loss (TEWL) and water content (hydration) of the stratum corneum. Measurement of TEWL is used mainly to support claims that a product may, in the short or long term, improve or repair the barrier function of skin. It is not an indicator of hydration of the stratum corneum. One way to measure hydration is to look for the changes in electrical properties of the stratum corneum that the increased water content produces, i.e. measure capacitance or conductance. It is important that we do not loose sight of the fact that meters may measure something that is imperceptible to the consumer or has no meaning to them. Reliance only on devices that give numbers may lead to problems. An example of a study where three facial cosmetic products were subjected to perceptual tests and to a standard volunteer test for moisturization will be discussed. The relationship between any measured parameter, and what it means to the consumer, needs to be understood. A moisturization claim may be technically supported by a study using a device such as the Corneometer. However, a 20% increase in water content almost certainly does not represent a 20% better moisturization as far as the consumer is concerned. The way forward is to relate the two approaches to product testing during product development. Hopefully, this will allow the product development process to be more systematic.     

Abstracts: Natural moisturizing factors in the stratum corneum I. Effects of lipid extraction and soaking

pp. 13–22 Natural moisturizing factor (NMF) is essential for appropriate stratum corneum hydration, barrier homeostasis, desquamation, and plasticity. It is formed from filaggrin proteolysis to small, hygroscopic molecules including amino acids. We hypothesized that common lipid extraction and soaking in water would alter the level of NMF in the upper stratum corneum (SC) and its biophysical properties. A novel method of measuring and quantifying the amino acid components of NMF is presented. Adhesive tapes were used to collect samples of the SC and were extracted with 6 mM perchloric acid for analysis by reverse‐phase high‐performance liquid chromatography (HPLC).The HPLC results were standardized to the amount of protein removed by the tapes. An increase in NMF was found with increased SC depth. Also, the combination of extraction and soaking was found to increase NMF loss relative to control or to extraction or soaking alone. Our results indicate that common skin care practices significantly influence the water binding materials in the upper SC. The findings have implications for the evaluation and formulation of skin care products.     

Bioengineering and subjective approaches to the clinical evaluation of dry skin

Dry skin (also known as xerosis) is a cutaneous reaction pattern indicative of abnormal desquamation, which has not only cosmetic considerations, but can also lead to the penetration of irritants and allergens through the stratum corneum (SC). Over the last few decades, our understanding of the structure, composition, formation and function of the SC has advanced tremendously; however, despite these advancements, the occurrence of dry skin remains prevalent in the adult population. The clinical evaluation of dry skin is therefore of significant importance to the cosmetic industry not only for understanding the condition but also for measuring the effects of treatment. Traditionally, dry skin has been evaluated by visual inspection, however, recently a variety of bioengineering techniques have emerged enabling the investigator to objectively assess the extent of xerotic conditions. The most frequently employed methods for the evaluation of dry skin are discussed in this review, including regression testing, squametry, measurement of transepidermal water loss, epidermal hydration, profilometry, confocal Raman spectroscopy, optical coherence tomography, in vivo confocal microscopy and magnetic resonance imaging.     

How do the skin barrier and microbiome adapt to the extra-uterine environment after birth? Implications for the clinical practice

The multiple protective functions of the skin derive from the interactions between epithelial skin and immune cells as well as the commensal microbiota. Developed in the last trimester of intra-uterine life, the skin barrier adapts dynamically after birth. Specific differences in the structure and physiology have been disclosed between infant and adult skin. The stratum corneum of infants is thinner and structured by thicker corneocytes with a more anisotropic surface in comparison to adult skin. Lower levels of the natural moisturizing factor and its constituents, together with the increased protease activity in the epidermis result in dry baby skin and ongoing adaptation of the desquamation to the extra-uterine environment. Infant epidermis is characterized by an accelerated proliferation rate and clinically competent permeability barrier in term neonates, despite the higher baseline values of transepidermal water loss in infants. The skin surface of newborns is less acidic, which could increase susceptibility to diaper and atopic dermatitis. Immediately after birth, skin is colonized by commensal bacteria—a process dependent on the mode of delivery and of major importance for the maturation of the immune system. Skin bacterial diversity and dysbiosis have been related to different pathology such as atopic and seborrheic dermatitis. This paper focuses on the ongoing structural, functional and biochemical adaptation of the human skin barrier after birth. We discuss the interactions on the ‘skin barrier/ microbiota/ immune system’ axis and their role in the development of competent functional integrity of the epidermal barrier.     

The influence of a cream containing 20% glycerin and its vehicle on skin barrier properties

Glycerin is widely used in cosmetics and well as in pharmaceutical formulations, mainly as humectant. In vitro studies have shown glycerin to prevent crystallization of stratum corneum model lipid mixture at low room humidity. Whether this may affect the skin barrier function during repeated application of glycerin in a cream base to normal skin is not known. Therefore, the influence of a cream containing 20% glycerin was compared with its placebo cream in a bilateral, double-blind study on 17 healthy volunteers. The effect was evaluated as influence on hydration with a corneometer and on skin barrier function. Skin barrier function was assessed as permeability to water with an evaporimeter (transepidermal water loss; TEWL) and as sensitivity to an irritating surfactant by measuring the biological response (measured as TEWL and skin blood flow). Ten days treatment of normal skin with 20% glycerin significantly increased skin corneometer values, indicating an increased hydration. However, our study failed to show an influence of glycerin on human skin, in terms of TEWL and skin sensitivity to SLS-induced irritation.     

Trends in stratum corneum research and the management of dry skin conditions

The structure, composition, formation and function of the stratum corneum have been the subject of intense research over the last few decades. As has become apparent, stratum corneum barrier function is not only dependent on one single component but also on its total architecture. Recent developments in understanding lipid composition have led to a new ceramide nomenclature system, a new proposal for a molecular model of the interactions between ceramides, cholesterol and fatty acids, and the demonstration of the presence of crystalline orthorhombic and gel hexagonal lipid phases in the stratum corneum. Linoleate-containing ceramide one, now known as CER EOS, have been shown to be essential for the formation of the 13 nm long periodicity phase (LPP) observed by electron microscopy and X-ray diffraction studies, whereas long-chain fatty acids are important for the formation of the crystalline lipid phases essential for barrier function. The role of the corneocyte envelope, its constituent proteins and its transglutaminase-mediated maturation processes have been shown to be essential for good skin condition. Several proteases may have a role in corneodesmolysis, particularly serine and cathepsin-like enzymes. Novel filaggrin polymorphisms have been identified that may be involved in the expression of a dry skin phenotype. Disturbances in lipid packing states, reduction in ceramide levels (particularly the phytosphingosine-containing ceramides), reductions in the levels of long-chain fatty acids and loss of the LPP largely account for the perturbations in lipid structure that occur in dry skin. The reduced corneodesmolysis that occurs in this xerotic skin disorder is now well accepted and is caused by reductions in the levels and activities of stratum corneum proteases together with elevated levels of corneodesmosomal glycoproteins in the superficial layers of the stratum corneum. Additionally, increased levels of fragile corneocytes are associated with reduced transglutaminase activity and corneocyte envelope cross-linking events. However, in comparison with the advances in our understanding of the textural changes that occur in dry skin, the somatosensory changes are poorly understood and the itching associated with dry skin is still an under-researched area. The unique biosensor role of the stratum corneum essential for a competent natural moisturizing barrier may also have a role to play in the action of anti-ageing technologies by controlling the expression and secretion of epidermal cytokines and growth factors. Technologies to treat the surface textural skin problems, enhance the differentiation process, particularly lipid biosynthesis, and to control the somatosensory problems in dry skin have received much attention in the last decade. This paper will review the state of the art of stratum corneum biology and the trends in the management of dry skin.     

A new covalently bound ceramide from human stratum corneum -omega-hydroxyacylphytosphingosine

The outermost layer of the skin, the stratum corneum, consists of non-viable keratin-filled cells, or corneocytes, embedded in a matrix of lipids. The boundary of the cells consists of cross-linked proteins with covalently bound lipids on the outer surface. The spaces between cells are filled with a mixture of ceramides, cholesterol and fatty acids. The stratum corneum provides a protective barrier against water loss through the skin and limits the penetration of potentially harmful substances from the environment. Among the covalently bound lipids on the corneocyte surface are omega-hydroxyacylsphingosine and omega-hydroxyacyl-6-hydroxysphingosine. The previously suspected presence of omega-hydroxyacylphytosphingosine is confirmed in this report through its specific isolation and characterization based on chromatographic behaviour and proton magnetic resonance spectral data.     

Skin barrier structure, function and formation - learning from cryo-electron microscopy of vitreous, fully hydrated native human epidermis

The hydration level of the stratum corneum largely depends on the physical state and molecular organization of the stratum corneum intercellular lipid matrix. A better understanding of stratum corneum lipid organization may thus aid the development of more rational cosmetic formulations. Several experimental and theoretical problems of a fundamental character remain, however, unresolved. These are, e.g. that precise quantitative skin barrier compositional data are difficult to obtain and that in vitro experimental skin barrier models usually are of limited value because of the prevailing non-equilibrium conditions in vivo. However, new experimental methods have recently been developed, which may help to overcome some of these limitations. These are, e.g. direct electrospray ionization-mass spectrometry (ESI-MS) of intact long-chain skin ceramides and direct high-resolution cryo-transmission electron microscopy (cryo-TEM) of vitreous sections of native, fully hydrated epidermis. Here, we show that cryo-transmission electron micrographs of vitreous normal human skin often dramatically differ from those obtained by conventional electron microscopy of resin-embedded skin. Our epidermal cryo-TEM data are subsequently discussed in relation to central problems of present conceptions of skin barrier structure, function and formation.     

Location‐related differences in structure and function of the stratum corneum with special emphasis on those of the facial skin

Between the two different kinds of the skin covering the body, the glabrous skin is found only on the palmo‐plantar surface because of its rather simple function to protect the underlying living tissue with its remarkably thick stratum corneum (SC) from strong external force and friction. Thus, its barrier function is extremely poor. In contrast, the hair‐bearing skin covers almost all over the body surface regardless of the presence of long hair or vellus hair. In regard to its SC, many dermatologists and skin scientists think that it is too thin to show any site‐specific differences, because the SC is just present as an efficient barrier membrane to protect our body from desiccation as well as against the invasion by external injurious agents. However, there are remarkable regional differences not only in the living skin tissue but also even in such thin SC reflecting the function of each anatomical location. These differences in the SC have been mostly disclosed with the advent of non‐invasive biophysical instruments, particularly the one that enables us to measure transepidermal water loss (TEWL), the parameter of the SC barrier function, and the one that evaluates the hydration state of the skin surface, the parameter of the water‐holding capacity of the SC that brings about softness and smoothness to the skin surface. These in vivo instrumental measurements of the SC have disclosed the presence of remarkable differences in the functional properties of the SC particularly between the face and other portions of the body. The SC of the facial skin is thinner, being composed of smaller layers of corneocytes than that of the trunk and limbs. It shows unique functional characteristics to provide hydrated skin surface but relatively poor barrier function, which is similar to that observed in retinoid‐treated skin or to that of fresh scar or keloidal scars. Moreover, there even exist unexpected, site‐dependent differences in the SC of the facial skin such as the forehead, eyelid, cheek, nose and perioral regions, although each location occupies only a small area. Between these locations, the cheek shows the lowest TEWL in contrast to the perioral region that reveals the highest one. Moreover, these features are not static but change with age particularly between children and adults and maybe also between genders. Among various facial locations, the eyelid skin is distinct from others because its SC is associated with poor skin surface lipids and a thin SC cell layer composed of large corneocytes that brings about high surface hydration state but poor barrier function, whereas the vermillion borders of the lips that are covered by an exposed part of the oral mucosa exhibit remarkably poor barrier function and low hydration state. Future studies aiming at the establishment of the functional mapping in each facial region and in other body regions will shed light on more delicate site‐dependent differences, which will provide us important information in planning the strategy to start so called tailor‐made skin care for each location of the body.     

Skin lipids

Concepts regarding the structure and function of the stratum corneum (SC) have changed from that of a tough film of loosely adherent cells to that of a two-compartment system of protein-enriched cells embedded in intercellular lipids. The two-compartment arrangement enlarged the role of epidermal lipids from that of the plasticizing component of the SC to that of the epidermal barrier governing water-holding properties as well as take-up of water, the differences in permeability of topically applied lipophilic and hydrophilic agents as well as cohesion and desquamation of the SC. Barrier properties of SC are dependent largely on the intactness of the lipid lamellae that surround the corneocytes. The pliability of the SC depends on a correct balance of lipids, hygroscopic water-soluble substances and water.
Mammalian differentiation involves characteristic changes in lipid composition consistent with the requirements for waterproofing. There is a progressive depletion of phospholipids and glycosphingolipids with enrichment in ceramides, cholesterol, free fatty acids, and small amounts of other polar (e.g. cholesterol sulphate) and non-polar species (e.g. hydrocarbons, cholesterol esters, triglycerides). The sphingolipids account for the most lipid by weight and are presumed to be of major importance for the water-retaining properties of the epidermal barrier. Decrease of sphingolipid content occurs in aged skin. Deficiency of essential fatty acids leads to enhanced transdermal water transport in addition to dryness and scaliness of the skin. A mixture of both saturated and unsaturated fatty acids produces the optimal barrier to water loss from the SC. The balance between solid crystal and liquid crystalline phases in epidermal lipids is determined by the degree of fatty acid unsaturation and the amount of water.     

Hydration of the stratum corneum

Topically applied water, occlusion and topically applied glycerol were used to investigate and characterize some of the changes which occur in the hydrated stratum corneum. The effects of these treatments were monitored using non-invasive techniques under controlled conditions. The Servomed Evaporimeter was used to determine natural water flux from the skin surface before and after treatment. The performance of the Evaporimeter in this type of study had previously been improved by attaching a paper baffle to the detector. This eliminated the variance in output caused by atmospheric movement. Experiments were carried out at temperatures below the threshold of thermal sweating and emotional sweating was minimized. Skin surface topography was characterized by means of a new type of profilometer. The instrument's design allowed a diamond stylus to traverse the living skin surface without significantly altering its structure. Changes in skin surface roughness were further elucidated using scanning electron microscopy and macrophotography. In vivo penetration of glycerol was assessed by chemical analysis of stratum corneum layers of treated skin. Samples were obtained by sequential stripping of the stratum corneum using adhesive tape. Topically applied water produced only a transient benefit because of rapid evaporation. More prolonged hydration was achieved by suppressing transepidermal water loss with polyethylene film. This occlusive hyperhydration was characterized by a significant reduction in profile roughness and by a smoother macroscopic appearance. Glycerol achieved the same effects by reducing the magnitude of the natural water flux from the skin surface and by reducing the rate of evaporation of water from applied aqueous glycerol solution or cosmetic product. Both effects were seen as the result of lowered water activity in the proximity of glycerol. Smoothing effects of glycerol on the skin surface, and improved appearance, persisted for at least 24 h. This persistence was explained by evidence for diffusion of glycerol into the stratum corneum where it formed a reservoir. Hydration of the skin is known to affect its barrier function and thereby exert a profound effect on penetration of both lipophilic and hydrophilic molecules. Clinically, this effect may be achieved using liberal applications of occlusive petroleum jelly and ointments. The results presented in this paper suggest that the use of humectants could achieve useful hydration using cosmetically acceptable materials.     

Reduced barrier efficiency in axillary stratum corneum

The skin of the axilla is cosmetically important with millions of consumers daily applying antiperspirant/deodorant products. Despite this, we know virtually nothing about axillary skin or how antiperspirant (AP) use impacts upon it. To characterize the axillary stratum corneum and determine whether this is a unique skin type, we have looked at stratum corneum composition and function, particularly its barrier properties, and compared it with other body sites. Transepidermal water loss (TEWL) and corneosurfametry (CSM) revealed a reduced barrier function in the axilla. HPTLC analysis of the stratum corneum lipids demonstrated statistically elevated levels of fatty acids, ceramides, and particularly cholesterol in the axilla. Both ceramide and cholesterol did not appear to change with depth, indicating that they were predominantly of stratum corneum origin. On the other hand, at least some of the fatty acid had a sebaceous origin. We hypothesized that the reduced barrier function might be owing to the changes in the crucial ceramide : cholesterol ratio. To address this, we used a combination of attenuated total reflectance-Fourier-transformed infrared spectroscopy (ATR-FTIR) with cyanoacrylate sampling. These results demonstrated more ordered lipid-lamellae phase behaviour in the axilla, suggesting that the elevated cholesterol might form crystal microdomains within the lipid lamellae, allowing an increase in water flux. Since an exaggerated application of antiperspirant had no effect upon the axilla barrier properties, it is concluded that this region of skin physiologically has a reduced barrier function.