Cholesterol sulfate and calcium affect stratum corneum lipid organization over a wide temperature range

The main diffusion barrier for drugs penetrating through the skin is located in the intercellular lipid matrix in the upper layer of the skin, the stratum corneum (SC). The main lipid classes in the SC are ceramides (CER), free fatty acids (FFA) and cholesterol (CHOL). The lipids in SC are organized into two lamellar phases with periodicities of approximately 13 and 6 nm, respectively. Similar lipid organization has been found with equimolar CHOL:CER:FFA mixtures in SAXD studies performed at room temperature. However, one may conclude that the phase behavior of the mixtures is similar to that in SC only when the lipid organization of the lipid mixtures resembles that in SC over a wide temperature range. Therefore, in the present study, the organization of the lipid mixtures has been studied in a temperature range between 20 degrees and 95 degrees C. From these experiments it appeared that at elevated temperatures in equimolar CHOL:CER:FFA mixtures a new prominent 4.3 nm phase is formed between 35;-55 degrees C, which is absent or only weakly formed in intact human and pig SC, respectively. As it has been suggested that gradients of pH and cholesterol sulfate exist in the SC and that Ca(2+) is present only in the lowest SC layers, the effect of pH, cholesterol sulfate, and Ca(2+) on the lipid phase behavior has been investigated with lipid mixtures. Both an increase in pH from 5 (pH at the skin surface) to 7.4 (pH at the SC;-stratum granulosum interface) and the presence of cholesterol sulfate promote the formation of the 13 nm lamellar phase. Furthermore, cholesterol sulfate reduces the amount of CHOL that is present in crystalline domains, causes a shift in the formation of the 4.3 nm phase to higher temperatures, and makes this phase less prominent at higher temperatures. The finding that Ca(2+) counteracts the effects of cholesterol sulfate indicates the importance of a proper balance of minor SC components for appropriate SC lipid organization. In addition, when the findings are extrapolated to the in vivo situation, it seems that cholesterol sulfate is required to dissolve cholesterol in the lamellar phases and to stabilize SC lipid organization. Therefore, a drop in cholesterol sulfate content in the superficial layers of the SC is expected to destabilize the lipid lamellar phases, which might facilitate the desquamation process.     

Lipid mixtures prepared with well-defined synthetic ceramides closely mimic the unique stratum corneum lipid phase behavior

Lipid lamellae present in the outermost layer of the skin, the stratum corneum, form the main barrier for the diffusion of molecules through the skin. The presence of a unique 13 nm lamellar phase and its high crystallinity are characteristic for the stratum corneum lipid phase behavior. In the present study, small-angle and wide-angle X-ray diffraction were used to examine the organization in lipid mixtures prepared with a unique set of well-defined synthetic ceramides, varying from each other in head group architecture and acyl chain length. The results show that equimolar mixtures of cholesterol, free fatty acids, and synthetic ceramides (resembling the composition of pig ceramides) closely resemble the lamellar and lateral stratum corneum lipid organization, both at room and higher temperatures. Exclusion of several ceramide classes from the mixture does not affect the lipid organization. However, complete substitution of ceramide 1 (acylceramide with a sphingosine base) with ceramide 9 (acylceramide with a phytosphingosine base) reduces the formation of the long periodicity lamellar phase. This indicates that the head group architecture of acylceramides affects the lipid organization. In conclusion, lipid mixtures prepared with well-defined synthetic ceramides offer an attractive tool with which to unravel the importance of the molecular structure of individual ceramides for proper lipid organization.     

Preparation and characterization of a stratum corneum substitute for in vitro percutaneous penetration studies

The intercellular stratum corneum (SC) lipids form the main barrier for diffusion of substances through the skin. A porous substrate covered with synthetic SC lipids would be an attractive model to study percutaneous penetration, hereby replacing native human SC. Prerequisite is that this stratum corneum substitute (SCS) is prepared with a uniform lipid composition and layer thickness. Furthermore, the lipid organization and orientation should resemble that in SC. The objective of this study was to investigate the utility of an airbrush spraying device to prepare a SCS composed of cholesterol, ceramides and free fatty acids on a polycarbonate filter. The results demonstrate that a proper choice of solvent mixture and lipid concentration is crucial to achieve a uniform distribution of the applied lipids over the filter surface. A smooth and tightly packed lipid layer is only obtained when the equilibration conditions are appropriately chosen. The SCS possesses two crystalline lamellar phases with periodicities similar to those present in native SC. The orientation of these lamellae is mainly parallel to the surface of the polycarbonate filter, which resembles the orientation of the intercellular SC lipids. In conclusion, the airbrush technique enables generation of a homogeneous SCS, which ultimately may function as a predictive in vitro percutaneous penetration model.     

Preparation and characterization of a stratum corneum substitute for in vitro percutaneous penetration studies

The intercellular stratum corneum (SC) lipids form the main barrier for diffusion of substances through the skin. A porous substrate covered with synthetic SC lipids would be an attractive model to study percutaneous penetration, hereby replacing native human SC. Prerequisite is that this stratum corneum substitute (SCS) is prepared with a uniform lipid composition and layer thickness. Furthermore, the lipid organization and orientation should resemble that in SC. The objective of this study was to investigate the utility of an airbrush spraying device to prepare a SCS composed of cholesterol, ceramides and free fatty acids on a polycarbonate filter. The results demonstrate that a proper choice of solvent mixture and lipid concentration is crucial to achieve a uniform distribution of the applied lipids over the filter surface. A smooth and tightly packed lipid layer is only obtained when the equilibration conditions are appropriately chosen. The SCS possesses two crystalline lamellar phases with periodicities similar to those present in native SC. The orientation of these lamellae is mainly parallel to the surface of the polycarbonate filter, which resembles the orientation of the intercellular SC lipids. In conclusion, the airbrush technique enables generation of a homogeneous SCS, which ultimately may function as a predictive in vitro percutaneous penetration model.     

The Lipid Organisation of the Skin Barrier: Liquid and Crystalline Domains Coexist in Lamellar Phases

The superficial layer of the skin, the stratum corneum, is the main barrier for diffusion of substances across the skin. The stratum corneum is composed of corneocytes embedded in lipid lamellae. In previous studies two lamellar phases have been identified with periodicities of 6.4 and 13.4 nm of which the 13.4 nm phase (long periodicity phase = LPP) is considered to be very important for the skin banier function. The main lipid classes in stratumcorneum are ceramides, free fatty acids and cholesterol. Until now 8 subclassesof ceramides are identified in human stratum corneum referred to as ceramide 1 to 8. Studies with mixtures prepared with isolated human ceramides revealed that cholesterol and ceramides are very important for the formation of the lamellar phases. After addition of free fatty acids the lipids are organised in an orthorhombic packing with a small proportion of lipids in a liquid phase. Our most recent results show that the presence of ceramide 1 and the formation of a liquid phase are crucial elements for the formation of the LPP. These observations and the broad-narrowbroad sequence of lipid layers in the LPP led us to propose a molecular model for this phase. This consists of one narrow central lipid layer with fluid domains with on both sides a broad layer with a crystalline structure. This model is referred to as `the sandwich model'.     

The phase behaviour of skin lipid mixtures based on synthetic ceramides

The lipid lamellae present in the outermost layer of the skin, the stratum corneum (SC), form the main barrier for diffusion of molecules across the skin. The main lipid classes in SC are cholesterol (CHOL), free fatty acids (FFA) and at least nine classes of ceramides (CER), referred to as CER1 to CER9. In the present study the phase behaviour of four synthetic CER, either single or mixed with CHOL or CHOL and FFA, has been studied using small and wide angle X-ray diffraction. The lipid mixtures showed complex phase behaviour with coexistence of several phases. The results further revealed that the presence of synthetic CER1 as well as a proper composition of the other CER in the mixture were crucial for the formation of a phase with a long periodicity, characteristic for SC lipid phase behaviour. Only a mixture containing synthetic CER1 and CER3, CHOL and FFA showed similar phase behaviour to that of SC.     

The role of ceramide chain length distribution on the barrier properties of the skin lipid membranes

The skin barrier function is provided by the stratum corneum (SC). The lipids in the SC are composed of three lipid classes: ceramides (CERs), cholesterol (CHOL) and free fatty acids (FFAs) which form two crystalline lamellar structures. In the present study, we investigate the effect of CER chain length distribution on the barrier properties of model lipid membranes mimicking the lipid composition and organization of SC. The membranes were prepared with either isolated pig CERs (PCERs) or synthetic CERs. While PCERs have a wide chain length distribution, the synthetic CERs are quite uniform in chain length. The barrier properties were examined by means of permeation studies using hydrocortisone as a model drug. Our studies revealed a reduced barrier in lipid membranes prepared with PCERs compared to synthetic CERs. Additional studies revealed that a wider chain length distribution of PCERs results in an enhanced hexagonal packing and increased conformational disordering of the lipid tails compared to synthetic CERs, while the lamellar phases did not change. This demonstrates that the chain length distribution affects the lipid barrier by reducing the lipid ordering and density within the lipid lamellae. In subsequent studies, the effect of increased levels of FFAs or CERs with a long acyl chain in the PCERs membranes was also studied. These changes in lipid composition enhanced the level of orthorhombic packing, reduced the conformational disordering and increased the barrier of the lipid membranes. In conclusion, the CER chain length distribution is an important key factor for maintaining a proper barrier.     

Influence of different ceramides on the structure of in vitro model lipid systems of the stratum corneum lipid matrix

Human stratum corneum (SC) consists of several layers of keratinized corneocytes embedded in a lipid matrix of ordered lamellar structure which is considered to constitute the major barrier to percutaneous penetration. Artificial mixtures of SC lipids are often used as model systems to mimic the skin barrier or to investigate the effects of substances on the phase behaviour of the models. In the present study a SC lipid model composed of cholesterol, fatty acids and ceramides was used to investigate the effect of three different commercially available ceramide types on the microstructure and the physicochemical behaviour of the lipids. Polarized light microscopy, transmission electron microscopy, small-angle X-ray diffraction, wide-angle X-ray diffraction and differential scanning calorimetry (DSC) were used for physicochemical characterization. The results revealed a lamellar structure for all models but showed differences with regard to the thermal and optical behaviour depending obviously on the composition of the ceramide mixtures. A model containing a mixture of Cer[AS] was comparable to human SC lipids.     

Phase behaviour of skin barrier model membranes at pH 7.4.

The main function of the skin is to protect the body against exogenous substances. The skin barrier is located in the outermost layer of the skin, the stratum corneum (SC). This layer consists of keratin enriched cells embedded in lipid lamellae that form the main barrier for diffusion of substances through the skin. The main lipid classes in this barrier are ceramides, cholesterol and free fatty acids. Cholesterol sulfate and calcium are also present in SC. Furthermore it has been suggested that a pH gradient exists. In a previous paper the effect of cholesterol sulfate and calcium on the lipid phase behaviour of mixtures prepared from cholesterol, ceramides and free fatty acids at pH 5 was reported (approximate pH at the skin surface). In the present study the phase behaviour of mixtures prepared from cholesterol, ceramides and free fatty acids prepared at pH 7.4 (the pH of viable cells) has been examined between 25 and 95 degrees C. Our studies reveal that a reversed hexagonal phase has been formed at elevated temperatures. Addition of calcium inhibits the formation of the reversed hexagonal phase, while cholesterol sulfate promotes the presence of the reversed hexagonal phase at increased temperatures. From our results we can conclude that the lipid mixtures prepared at pH 5 resemble more closely the lipid phase behaviour in intact SC than the lipid mixtures prepared at pH 7.4.     

The role of ceramide composition in the lipid organisation of the skin barrier.

The lipid lamellae in the stratum corneum (SC) play a key role in the barrier function of the skin. The major lipids are ceramides (CER), cholesterol (CHOL) and free fatty acids (FFA). In pig SC at least six subclasses of ceramides (referred to as CER 1, 2-6) are present. Recently it was shown that in mixtures of isolated pig SC ceramides (referred to as CER(1-6)) and CHOL two lamellar phases are formed, which mimic SC lipid organisation very closely [J.A. Bouwstra et al., 1996, J. Lipid Res. 37, 999-1011] [1]. Since the CER composition in SC originating from different sources/donors often varies, information on the effect of variations in CER composition on the SC lipid organisation is important. The results of the present study with mixtures of CHOL including two different CER mixtures that lack CER 6 (CER(1-5) mixtures) revealed that at an equimolar molar ratio their lipid organisation was similar to that of the equimolar CHOL:CER(1-6) and CHOL:CER(1,2) mixtures, described previously. These observations suggest that at an equimolar CHOL:CER ratio the lipid organisation is remarkably insensitive toward a change in the CER composition. Similar observations have been made with equimolar CHOL:CER:FFA mixtures. The situation is different when the CHOL:CER molar ratio varies. While in the CHOL:CER(1-6) mixture the lamellar organisation hardly changed with varying molar ratio from 0.4 to 2, the lamellar organisation in the CHOL:CER(1-5) mixtures appeared to be more sensitive to a change in the relative CHOL content, especially concerning the changes in the periodicities of the lamellar phases. In summary, these findings clearly indicate that at an equimolar CHOL:CER molar ratio the lamellar organisation is least sensitive to a variation in CER composition, while at a reduced CHOL:CER molar ratio the CER composition plays a more prominent role in the lamellar phases. This observation may have an implication for the in vivo situation when both the CER composition and the CHOL:CER molar ratio change simultaneously.     

Phytosphingosine ceramide mainly localizes in the central layer of the unique lamellar phase of skin lipid model systems

Understanding the lipid arrangement within the skin’s outermost layer, the stratum corneum (SC), is important for advancing knowledge on the skin barrier function. The SC lipid matrix consists of ceramides (CERs), cholesterol, and free fatty acids, which form unique crystalline lamellar phases, referred to as the long periodicity phase (LPP) and short periodicity phases. As the SC lipid composition is complex, lipid model systems that mimic the properties of native SC are used to study the SC lipid organization and molecular arrangement. In previous studies, such lipid models were used to determine the molecular organization in the trilayer structure of the LPP unit cell. The aim of this study was to examine the location of CER N-(tetracosanoyl)-phytosphingosine (CER NP) in the unit cell of this lamellar phase and compare its position with CER N-(tetracosanoyl)-sphingosine (CER NS). We selected CER NP as it is the most prevalent CER subclass in the human SC, and its location in the LPP is not known. Our neutron diffraction results demonstrate that the acyl chain of CER NP was positioned in the central part of the trilayer structure, with a fraction also present in the outer layers, the same location as determined for the acyl chain of CER NS. In addition, our Fourier transformed infrared spectroscopy results are in agreement with this molecular arrangement, suggesting a linear arrangement for the CER NS and CER NP. These findings provide more detailed insight into the lipid organization in the SC lipid matrix.     

The skin barrier in healthy and diseased state

The primary function of the skin is to protect the body for unwanted influences from the environment. The main barrier of the skin is located in the outermost layer of the skin, the stratum corneum. The stratum corneum consists of corneocytes surrounded by lipid regions. As most drugs applied onto the skin permeate along the lipid domains, the lipid organization is considered to be very important for the skin barrier function. It is for this reason that the lipid organization has been investigated quite extensively. Due to the exceptional stratum corneum lipid composition, with long chain ceramides, free fatty acids and cholesterol as main lipid classes, the lipid organization is different from that of other biological membranes. In stratum corneum, two lamellar phases are present with repeat distances of approximately 6 and 13 nm. Moreover the lipids in the lamellar phases form predominantly crystalline lateral phases, but most probably a subpopulation of lipids forms a liquid phase. Diseased skin is often characterized by a reduced barrier function and an altered lipid composition and organization. In order to understand the aberrant lipid organization in diseased skin, information on the relation between lipid composition and organization is crucial. However, due to its complexity and inter-individual variability, the use of native stratum corneum does not allow detailed systematic studies. To circumvent this problem, mixtures prepared with stratum corneum lipids can be used. In this paper first the lipid organization in stratum corneum of normal and diseased skin is described. Then the role the various lipid classes play in stratum corneum lipid organization and barrier function has been discussed. Finally, the information on the role various lipid classes play in lipid phase behavior has been used to interpret the changes in lipid organization and barrier properties of diseased skin.     

Modelling the stratum corneum lipid organisation with synthetic lipid mixtures: the importance of synthetic ceramide composition

Cholesterol (CHOL), free fatty acids (FFA) and nine classes of ceramides (CER1-CER9) form the main constituents of the intercellular lipid lamellae in stratum corneum (SC), which regulate the skin barrier function. Both the presence of a unique 13-nm lamellar phase, of which the formation depends on the presence of CER1, and its dense lateral packing are characteristic for the SC lipid organisation. The present study focuses on the lipid organisation in mixtures prepared with CHOL, FFA and a limited number of synthetic CER, namely CER1, CER3 and bovine brain CER type IV (SigmaCERIV). The main objective is to determine the optimal molar ratio of CER3 to SigmaCERIV for the formation of the 13-nm lamellar phase. CER3 contains a uniform acyl chain length, whereas SigmaCERIV contains fatty acids with varying chain lengths. Using small angle X-ray diffraction (SAXD), it is demonstrated that the CER3 to SigmaCERIV ratio affects the formation of the 13-nm lamellar phase and that the optimal ratio depends on the presence of FFA. Furthermore, the formation of the 13-nm lamellar phase is not very sensitive to variations in the total CER level, which is similar to the in vivo situation.     

Structure of the skin barrier and its modulation by vesicular formulations

The natural function of the skin is to protect the body from unwanted influences from the environment. The main barrier of the skin is located in the outermost layer of the skin, the stratum corneum. Since the lipids regions in the stratum corneum form the only continuous structure, substances applied onto the skin always have to pass these regions. For this reason the organization in the lipid domains is considered to be very important for the skin barrier function. Due to the exceptional stratum corneum lipid composition, with long chain ceramides, free fatty acids and cholesterol as main lipid classes, the lipid phase behavior is different from that of other biological membranes. In stratum corneum crystalline phases are predominantly present, but most probably a subpopulation of lipids forms a liquid phase. Both the crystalline nature and the presence of a 13 nm lamellar phase are considered to be crucial for the skin barrier function. Since it is impossible to selectively extract individual lipid classes from the stratum corneum, the lipid organization has been studied in vitro using isolated lipid mixtures. These studies revealed that mixtures prepared with isolated stratum corneum lipids mimic to a high extent stratum corneum lipid phase behavior. This indicates that proteins do not play an important role in the stratum corneum lipid phase behavior. Furthermore, it was noticed that mixtures prepared only with ceramides and cholesterol already form the 13 nm lamellar phase. In the presence of free fatty acids the lattice density of the structure increases. In stratum corneum the ceramide fraction consists of various ceramide subclasses and the formation of the 13 nm lamellar phase is also affected by the ceramide composition. Particularly the presence of ceramide 1 is crucial. Based on these findings a molecular model has recently been proposed for the organization of the 13 nm lamellar phase, referred to as "the sandwich model", in which crystalline and liquid domains coexist. The major problem for topical drug delivery is the low diffusion rate of drugs across the stratum corneum. Therefore, several methods have been assessed to increase the permeation rate of drugs temporarily and locally. One of the approaches is the application of drugs in formulations containing vesicles. In order to unravel the mechanisms involved in increasing the drug transport across the skin, information on the effect of vesicles on drug permeation rate, the permeation pathway and perturbations of the skin ultrastructure is of importance. In the second part of this paper the possible interactions between vesicles and skin are described, focusing on differences between the effects of gel-state vesicles, liquid-state vesicles and elastic vesicles.     

Interactions of dipalmitoylphosphatidylcholine with ceramide-based mixtures

The outermost layer of the skin, the stratum corneum (SC), acts as the natural physical barrier. The SC consists of corneocytes embedded in a crystalline lipid matrix consisting of ceramides, free fatty acids and cholesterol.Although phospholipids are frequently present in topical formulations, no detailed information is reported on the interactions between phospholipids and SC lipids. The aim of this study was to examine the interactions between a model phospholipid, dipalmitoylphosphatidylcholine (DPPC) and synthetic ceramide-based mixtures (referred to as SC lipids).(Perdeuterated) DPPC was mixed with SC lipids and the lipid organization and mixing properties were examined. The studies revealed that DPPC participates in the same lattice as SC lipids thereby enhancing a hexagonal packing. Even at a high DPPC level, no phase separated pure DPPC was observed.When a DPPC containing formulation is applied to the skin surface it must partition into the SC lipid matrix prior to any mixing with the SC lipids. To mimic this, DPPC was applied on top of a SC lipid membrane. DPPC applied in a liquid crystalline state was able to mix with the SC lipids and participated in the same lattice as the SC lipids. However, when DPPC was applied in a rippled gel-state very limited partitioning of DPPC into the SC lipid matrix occurred. Thus, when applied to the skin, liquid crystalline DPPC will have very different interactions with SC lipids than DPPC in a (rippled-)gel phase.     

The skin barrier in healthy and diseased state

The primary function of the skin is to protect the body for unwanted influences from the environment. The main barrier of the skin is located in the outermost layer of the skin, the stratum corneum. The stratum corneum consists of corneocytes surrounded by lipid regions. As most drugs applied onto the skin permeate along the lipid domains, the lipid organization is considered to be very important for the skin barrier function. It is for this reason that the lipid organization has been investigated quite extensively. Due to the exceptional stratum corneum lipid composition, with long chain ceramides, free fatty acids and cholesterol as main lipid classes, the lipid organization is different from that of other biological membranes. In stratum corneum, two lamellar phases are present with repeat distances of approximately 6 and 13 nm. Moreover the lipids in the lamellar phases form predominantly crystalline lateral phases, but most probably a subpopulation of lipids forms a liquid phase. Diseased skin is often characterized by a reduced barrier function and an altered lipid composition and organization. In order to understand the aberrant lipid organization in diseased skin, information on the relation between lipid composition and organization is crucial. However, due to its complexity and inter-individual variability, the use of native stratum corneum does not allow detailed systematic studies. To circumvent this problem, mixtures prepared with stratum corneum lipids can be used. In this paper first the lipid organization in stratum corneum of normal and diseased skin is described. Then the role the various lipid classes play in stratum corneum lipid organization and barrier function has been discussed. Finally, the information on the role various lipid classes play in lipid phase behavior has been used to interpret the changes in lipid organization and barrier properties of diseased skin.     

New insight into phase behavior and permeability of skin lipid models based on sphingosine and phytosphingosine ceramides

The intercellular lipid matrix of the stratum corneum (SC), which consist mainly of ceramides (CERs), free fatty acids and cholesterol, is fundamental to the skin barrier function. These lipids assemble into two lamellar phases, known as the long and short periodicity phases (LPP and SPP respectively). The LPP is unique in the SC and is considered important for the skin barrier function. Alterations in CER composition, as well as impaired skin barrier function, are commonly observed in diseased skin, yet the understanding of this relationship remains insufficient. In this study, we have investigated the influence of non-hydroxy and α-hydroxy sphingosine-based CERs and their phytosphingosine counterparts on the permeability and lipid organization of model membranes, which were adjusted in composition to enhance formation of the LPP. The permeability was compared by diffusion studies using ethyl-p-aminobenzoate as a model drug, and the lipid organization was characterized by X-ray diffraction and infrared spectroscopy. Both the sphingosine- and phytosphingosine-based CER models formed the LPP, while the latter exhibited a longer LPP repeat distance. The ethyl-p-aminobenzoate flux across the sphingosine-based CER models was higher when compared to the phytosphingosine counterparts, contrary to the fact that the α-hydroxy phytosphingosine-based CER model had the lowest chain packing density. The unanticipated low permeability of the α-hydroxy phytosphingosine-based model is probably associated with a stronger headgroup hydrogen bonding network. Our findings indicate that the increased level of sphingosine-based CERs at the expense of phytosphingosine-based CERs, as observed in the diseased skin, may contribute to the barrier function impairment.     

Infrared spectroscopy studies of mixtures prepared with synthetic ceramides varying in head group architecture: Coexistence of liquid and crystalline phases

The barrier function of the skin is provided by the stratum corneum (SC), the outermost layer of the skin. Ceramides (CERs), cholesterol (CHOL) and free fatty acids (FFAs) are present in SC and form highly ordered crystalline lipid lamellae. These lamellae are crucial for a proper skin barrier function. In the present study, Fourier transform infrared spectroscopy was used to examine the lipid organization of mixtures prepared from synthetic CERs with CHOL and FFAs. The conformational ordering and lateral packing of these mixtures showed great similarities to the lipid organization in SC and lipid mixtures prepared with native CERs. Therefore, mixtures with synthetic CERs serve as an excellent tool for studying the effect of molecular architecture of CER subclasses on the lipid phase behavior. In SC the number of OH-groups in the head groups of CER subclasses varies. Furthermore, acylCERs with a linoleic acid chemically bound to a long acyl chain are also identified. The present study revealed that CER head group architecture affects the lateral packing and conformational ordering of the CER:CHOL:FFA mixtures. Furthermore, while the majority of the lipids form a crystalline packing, the linoleate moiety of the acylCERs participates in a “pseudo fluid” phase.     

Lanolin-derived lipid mixtures mimic closely the lipid composition and organization of vernix caseosa lipids

The aim of the present study was to use semi-synthetic lipid mixtures to mimic the complex lipid composition, organization and thermotropic behaviour of vernix caseosa (VC) lipids. As VC shows multiple protecting and barrier supporting properties before and after birth, it is suggested that a VC substitute could be an innovative barrier cream for barrier deficient skin. Lanolin was selected as the source of the branched chain sterol esters and wax esters — the main lipid classes of VC. Different lipid fractions were isolated from lanolin and subsequently mixed with squalene, triglycerides, cholesterol, ceramides and fatty acids to generate semi-synthetic lipid mixtures that mimic the lipid composition of VC, as established by high-performance thin-layer chromatography. Differential scanning calorimetry and Fourier transform infrared spectroscopy investigations revealed that triglycerides play an important role in the (lateral) lipid organization and thermotropic behaviour of the synthetic lipid mixtures. Excellent resemblance of VC lipids was obtained when adding unsaturated triglycerides. Moreover, these lipid mixtures showed similar long range ordering as VC. The optimal lipid mixture was evaluated on tape-stripped hairless mouse skin in vivo. The rate of barrier recovery was increased and comparable to VC lipid treatment.