The human skin barrier is organized as stacked bilayers of fully extended ceramides with cholesterol molecules associated with the ceramide sphingoid moiety

Structural and dynamical aspects of skin studied by multiphoton excitation fluorescence microscopy-based methods

This mini-review reports on applications of particular multiphoton excitation microscopy-based methodologies employed in our laboratory to study skin. These approaches allow in-depth optical sectioning of the tissue, providing spatially resolved information on specific fluorescence probes' parameters. Specifically, by applying these methods, spatially resolved maps of water dipolar relaxation (generalized polarization function using the 6-lauroyl-2-(N,N-dimethylamino)naphthale probe), activity of protons (fluorescence lifetime imaging using a proton sensitive fluorescence probe--2,7-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein) and diffusion coefficients of distinct fluorescence probes (raster imaging correlation spectroscopy) can be obtained from different regions of the tissue. Comparative studies of different tissue strata, but also between equivalent regions of normal and abnormal excised skin, including applications of fluctuation correlation spectroscopy on transdermal penetration of liposomes are presented and discussed. The data from the different studies reported reveal the intrinsic heterogeneity of skin and also prove these strategies to be powerful noninvasive tools to explore structural and dynamical aspects of the tissue.

Impaired epidermal ceramide synthesis causes autosomal recessive congenital ichthyosis and reveals the importance of ceramide acyl chain length

The barrier function of the human epidermis is supposed to be governed by lipid composition and organization in the stratum corneum. Disorders of keratinization, namely ichthyoses, are typically associated with disturbed barrier activity. Using autozygosity mapping and exome sequencing, we have identified a homozygous missense mutation in CERS3 in patients with congenital ichthyosis characterized by collodion membranes at birth, generalized scaling of the skin, and mild erythroderma. We demonstrate that the mutation inactivates ceramide synthase 3 (CerS3), which is synthesized in skin and testis, in an assay of N-acylation with C26-CoA, both in patient keratinocytes and using recombinant mutant proteins. Moreover, we show a specific loss of ceramides with very long acyl chains from C26 up to C34 in terminally differentiating patient keratinocytes, which is in line with findings from a recent CerS3-deficient mouse model. Analysis of reconstructed patient skin reveals disturbance of epidermal differentiation with an earlier maturation and an impairment of epidermal barrier function. Our findings demonstrate that synthesis of very long chain ceramides by CerS3 is a crucial early step for the skin barrier formation and link disorders presenting with congenital ichthyosis to defects in sphingolipid metabolism and the epidermal lipid architecture.

Spatially resolved two-color diffusion measurements in human skin applied to transdermal liposome penetration

A multiphoton excitation-based fluorescence fluctuation spectroscopy method, Raster image correlation spectroscopy (RICS), was used to measure the local diffusion coefficients of distinct model fluorescent substances in excised human skin. In combination with structural information obtained by multiphoton excitation fluorescence microscopy imaging, the acquired diffusion information was processed to construct spatially resolved diffusion maps at different depths of the stratum corneum (SC). Experiments using amphiphilic and hydrophilic fluorescently labeled molecules show that their diffusion in SC is very heterogeneous on a microscopic scale. This diffusion-based strategy was further exploited to investigate the integrity of liposomes during transdermal penetration. Specifically, the diffusion of dual-color fluorescently labeled liposomes--containing an amphiphilic fluorophore in the lipid bilayer and a hydrophilic fluorophore encapsulated in the liposome lumen--was measured using cross-correlation RICS. This type of experiment allows discrimination between separate (uncorrelated) and joint (correlated) diffusion of the two different fluorescent probes, giving information about liposome integrity. Independent of the liposome composition (phospholipids or transfersomes), our results show a clear lack of cross-correlation below the skin surface, indicating that the penetration of intact liposomes is highly compromised by the skin barrier.

Structure and function of the stratum corneum extracellular matrix

The stratum corneum extracellular matrix (ECM) is enriched in lipids that are organized into lamellar bilayers, whose molecular architecture is now known. Although these bilayers are important for the permeability barrier, the ECM contains not only lipids but also enzymes, structural proteins, and antimicrobial peptides that impact barrier function. Yet, how such diverse components affect barrier function remains largely unknown. Static models of the epidermis may not do justice to the ECM, which is metabolically active, as it changes both structure and function as it transits to the surface.

Cleansing formulations that respect skin barrier integrity

Surfactants in skin cleansers interact with the skin in several manners. In addition to the desired benefit of providing skin hygiene, surfactants also extract skin components during cleansing and remain in the stratum corneum (SC) after rinsing. These side effects disrupt SC structure and degrade its barrier properties. Recent applications of vibrational spectroscopy and two-photon microscopy in skin research have provided molecular-level information to facilitate our understanding of the interaction between skin and surfactant. In the arena of commercial skin cleansers, technologies have been developed to produce cleansers that both cleanse and respect skin barrier. The main approach is to minimize surfactant interaction with skin through altering its solution properties. Recently, hydrophobically modified polymers (HMPs) have been introduced to create skin compatible cleansing systems. At the presence of HMP, surfactants assemble into larger, more stable structures. These structures are less likely to penetrate the skin, thereby resulting in less aggressive cleansers and the integrity of the skin barrier is maintained. In this paper, we reviewed our recent findings on surfactant and SC interactions at molecular level and provided an overview of the HM technology for developing cleansers that respect skin barrier.