Measuring the permeability of thin solid layers of natural waxes

HYPOTHESIS

Previous experimental work has shown that microcapsule walls, made by solidification of a molten wax, are unexpectedly permeable. The hypothesis was that this was due more to the structure of the wall than the material itself.

EXPERIMENTS

The permeability of thin (sub and low micron thickness) natural waxes was measured where a membrane was placed between two cells and the diffusion of a dye (fluorescein) measured. A filter paper was used to support the membranes. Two methods were used to coat the filter paper; simple dipping and spin coating. The resulting surfaces were examined using SEM, XRD and contact angle.

FINDINGS

Results indicate that the permeability of very thin walled capsules can be investigated by forming a layer on a porous support and measuring diffusion rates. Both the composition of the wax and the sample preparation is extremely important to the structure and resulting permeability of the membranes. Spin coating was much more effective than dip coating in reducing permeability. Carnauba wax had a much lower permeability than beeswax. A difference in levels between the two cells was observed, indicating a potential Osmotic pressure difference at play which should be further investigated.

Naturally occurring ω-Hydroxyacids

ω-Hydroxyacids are fatty acids bearing a hydroxyl group on the terminal carbon. They are found in mammals and higher plants and are often involved in providing a permeability barrier, the primary purpose of which is to reduce water loss. Some ω-hydroxyacid derivatives may be involved in waterproofing and signalling. The purpose of this review was to survey the known natural sources of ω-hydroxyacids. ω-Hydroxyacids are produced by two different P450-dependent mechanisms. The longer (30-34 carbons) ω-hydroxyacids are produced by chain extension from palmitic acid until the chain extends across the membrane in which the extension is taking place, and then the terminal carbon is hydroxylated. Shorter fatty acids can be hydroxylated directly to produce C16 and C18 ω-hydroxyacids found in plants and 20-eicosatetraenoic acid (20-HETE) by a different P450. The C16 and C18 ω-hydroxyacids are components of polymers in plants. The long-chain ω-hydroxyacids are found in epidermal sphingolipids, in giant-ring lactones from the sebum of members of the equidae, as a component of meibum and in carnauba wax and wool wax.

Covalently bound lipids in keratinizing epithelia

Covalently bound lipids have been identified and compared in keratinizing porcine epithelia including epidermis and oral epithelium from palate and gingiva. Stratum corneum was isolated by tryptic digestion, and after extensive extraction of lipids using a series of chloroform-methanol mixtures, the residual tissue was subjected to alkaline hydrolysis to release covalently bound lipids. The lipids so released were analyzed by quantitative thin-layer chromatography. Stratum corneum from each of the three anatomical sites contained omega-hydroxyceramides, omega-hydroxyacids and fatty acids. In epidermal stratum corneum the total covalently bound lipids represented 2.4% of the dry weight of the tissue, but in the oral epithelia this figure was consistently lower: 0.24% in palatal stratum corneum and 0.20% in gingival stratum corneum. Transmission electron microscopy before and after lipid extraction confirms the presence of a lipid envelope in epidermal stratum corneum and demonstrates the absence of this structure in oral stratum corneum.

Partition of sodium dodecyl sulfate into stratum corneum lipid liposomes

Synthetic detergents produce deleterious effects on human skin as the result of being taken up by the stratum corneum (SC). The present study aimed to determine to what extent a typical detergent enters the SC lipid lamellae, and what effect this might have on the physical properties of the lipids. These effects were studied in large unilamellar liposomes prepared from SC lipids (50% by weight of epidermal ceramides, 28% cholesterol, 17% free fatty acids, and 5% cholesteryl sulfate) by extrusion through successive polycarbonate filters of decreasing pore size, finally 400 nm. Freeze-fracture electron microscopy and light-scattering particle size analysis indicated a uniform liposome diameter averaging 230 nm. Partitioning of sodium dodecyl sulfate (SDS) into the lipid phase from aqueous buffer solutions was measured using the SC lipid liposomes and [U-14C]SDS. The partition coefficient was 416, 450, and 588 at pH 8.5 and 524, 507, and 807 at pH 7 for three different concentrations (0.1%, 0.02%, and 0.004%) of SDS. This high degree of partitioning into the liposomes is consistent with the high level of SDS partitioning seen in full SC. At the maximum, the SDS represented 18% of the liposomal lipids. Preparation of stable liposomes from SC lipids to which 10% or 20% of SDS had been added confirmed the ability of the liposomes to survive these high concentrations of surfactant. The permeability of the liposomes was enhanced as a result of SDS partitioning into the bilayers, as measured by the increased release of trapped [U-14C]glucose from these vesicles, and by their increased permeability to water in osmotic shock experiments.(ABSTRACT TRUNCATED AT 250 WORDS)

Interaction between corneocytes and stratum corneum lipid liposomes in vitro

Small unilamellar vesicles were made from a mixture of epidermal ceramides (45%), cholesterol (35%), free fatty acids (15%) and cholesteryl sulfate (5%). Isolated corneocytes prepared from pig epidermis were added to the liposomes and the interaction between corneocytes and liposomes was studied by (1) thin-section electron microscopy and (2) monitoring the release of aqueous contents of the vesicles by following the fluorescence intensity of carboxyfluorescein entrapped in the vesicles. The vesicles adsorbed readily onto the corneocytes and slowly transformed into lamellar sheets. Enhanced fluorescence intensity indicated a corneocyte-induced membrane fusion process that resulted in the release of aqueous contents of the vesicles. The results suggest a cohesive role for the corneocyte cell envelope, which consists of a monomolecular layer of lipids covalently bound to the outside of a cross-linked protein envelope. This may be one of the major factors in the reassembly of extruded membranous disks into lamellar sheets which occurs during the final stages of epidermal differentiation.

Preparation of model membranes for skin permeability studies using stratum corneum lipids

Liposomes were prepared from stratum corneum lipids consisting of epidermal ceramides (55% by weight), cholesterol (25%), free fatty acids (15%), and cholesteryl sulfate (5%). Multiple lamellae were formed by air-drying the liposomal suspensions on hydrophilic filter disks, and water permeabilities through these filter-supported lamellae were measured using a diffusion cell. Ultrastructure of the lipid lamellae was characterized by scanning and thin-section electron microscopy. Water flux data and ultrastructure of the model membranes are discussed in relation to stratum corneum, the horny layer of the epidermis that constitutes the epidermal barrier.

Composition and morphology of epidermal cyst lipids

The contents of epidermal cysts were used as a source of desquamated human keratinocytes uncontaminated by sebaceous, subcutaneous, or bacterial lipids. Lipids extracted with chloroform:methanol mixtures included six series of ceramides (41% of the total extractable lipid), cholesterol (27%), cholesteryl esters (10%), fatty acids (9%), cholesteryl sulfate (1.9%), a novel class of ceramide esters (3.8%), and a sterol diester (0.9%). Electron microscopy revealed that the lipids in the cyst contents existed as multiple intercellular lamellae, as in stratum corneum. One lamella, adjacent to the horny cell protein envelope, was resistant to lipid extraction and is thought to represent covalently bound lipid on the outer surface of the keratinocyte. The results indicate that the degradation of intercellular lipid lamellae is not required for desquamation.

The time-course of lipid biosynthesis in horse skin

To observe the time-course of formation of sebaceous lipids in the horse, skin was pulse-labelled in vivo by intradermal injection of [1-14C]acetate and the injection sites were harvested at intervals for up to 12 days by skin punch biopsy. The distribution of radioactivity among the major neutral lipid classes and the phospholipids from these biopsies showed that, soon after pulse-labelling, the phospholipids were highly labelled followed by a long-term decrease in radioactivity. Over the same period, the low initial labelling of the dominant component, the equolides (giant ring omega-lactones, C32-C36), was followed by a long-term increase in radioactivity. This suggests a post-pulse transferance of radioactivity from the phospholipids to the equolides, presumably in the fatty acids. Of the phospholipid fatty acids from horse dermis, including sebaceous glands, 33% were found to contain iso-branched structures unique to horse sebaceous lipids. Of the iso-branched fatty acids, 40% were delta 9-18:1 and delta 9- and delta 11-20:1 acids, which are structurally appropriate to be precursors for the monounsaturated equolides. These findings strengthen the hypothesis that the sebaceous phospholipids of horse skin serve as long-term lipid intermediates in the biosynthesis of the equolides during sebaceous cell development.

The composition of the ceramides from human stratum corneum and from comedones

Human epidermal surface lipids were collected by an ethanol wash and the ceramides were quantified by thin-layer chromatography-photodensitometry. Six ceramide fractions were isolated and the structural components of each were analyzed in detail. The most unusual of the epidermal ceramides contained a sphingosine base with amide-linked 30- and 32-carbon omega-hydroxyacids and an ester-linked nonhydroxyacid, 41% of which was linoleic acid. The proportion of linoleic acid in the analogous ceramide from comedones was 6%. This supports the hypothesis that a localized insufficiency of linoleic acid in the follicular epithelium is an etiologic factor in comedogenesis.

Effect of essential fatty acid deficiency on the epidermal sphingolipids of the rat

The epidermal sphingolipids from rats maintained on either a rat stock diet or a fat-free diet have been analyzed. Thin-layer chromatographic analyses have revealed glucosylceramides, acylglucosylceramides and four fractions of ceramides, one of which proved to be an acylceramide. The relative amounts of the glucosylceramides, acylglucosylceramides and acylceramides were increased in the essential fatty acid-deficient epidermis while one ceramide fraction was diminished. The other two ceramide fractions remained unchanged. The acylceramides and acylglucosylceramides from normal rat epidermis both contained long-chain omega-hydroxy acids in amide linkage to sphingosine bases and high proportions of linoleic acid in ester linkage. The linoleate, which is known to be crucial for the formation and maintenance of the epidermal water barrier, was replaced by oleate in the essential fatty acid-deficient rats.

Comparison of the hydroxyacids from the epidermis and from the sebaceous glands of the horse

The acylglucosylceramides were isolated from the polar lipids of horse epidermis and examined to determine whether the component omega-hydroxyacids are straight chained as in the corresponding lipids from pig epidermis or branched as in horse sebum. The hydroxyacids from horse epidermis were found to be almost entirely straight chained compounds. The results indicate that sebaceous glands, although derived from epidermal cells, have evolved independent pathways of lipid metabolism.

Glycolipids in mammalian epidermis: structure and function in the water barrier

In the epidermis of terrestrial vertebrates, lipid lamellae between the horny cells are thought to form a barrier to water loss. The lipids are extruded from living cells after assembly in lamellar granules. This assembly might be promoted by recently identified 1-(3'-O-acyl)-beta-D-glucosyl-N-(omega-hydroxyacyl)sphingosines, which have 30- and 32-carbon hydroxy acids as amides and linoleic acid esterified to glucose. Such a role for these molecules could explain the effects of essential fatty acid deficiency, in which the lamellar granules fail to assemble and the barrier to water diffusion is lost.

Skin surface lipids of the horse

Skin surface lipids from the sides of male and female horses (Equus caballus) were collected in acetone and analyzed by thin layer chromatography and gas liquid chromatography. The sole components in both sexes were cholesterol, cholesteryl esters and the lactones of 32-, 32- and 36-carbon omega-hydroxy acids, each including a methyl group in the n-1 position. Most of the lactones were monounsaturated (either n-8 or n-10), but small amounts of saturated and dienoic species were present. A pooled sample of the skin surface lipids contained 14% cholesterol, 38% cholesteryl esters and 48% lactones.

Leaf epicuticular waxes

The external surface of the higher plants comprises a cuticular layer covered by a waxy deposit. This deposit is believed to play a major part in such phenomena as the water balance of plants and the behavior of agricultural sprays. The wax contains a wide range of organic compounds. These complex mixtures are amenable to modern microchromatographic and microspectrometric analytical procedures. The few surveys which have been made of the species distribution of certain classes of constituents indicate that such distribution may be of limited taxonomic value; however, the wax composition of a species may differ for different parts of the same plant and may vary with season, locale, and the age of the plant. This fascinating subject, in which the disciplines of botany, biochemistry, chemistry, and physics overlap and interact, is still in a very active state. Much remains to be learned about the composition and fine structure of the wax deposits, and, for this, experimental study of wax crystallization and permeation through artificial membranes will be required. Enzymic studies, radiolabeling, and electron microscopy will be needed to reveal the mode of biogenesis of the wax constituents and their site of formation and subsequent pathway through the cuticle to the leaf surface.