Biological and physical factors influencing the successful engraftment of a cultured human skin substitute

Skin tissue may be engineered in a variety of ways. Our cultured skin substitute (Graftskin, living skin equivalent or G-LSE), Apligraftrade mark, is an organotypic culture of skin, containing both a "dermis" and "epidermis." The epidermis is an important functional component of skin, responsible for biologic wound closure. The epidermis possesses a stratum corneum which develops with time in culture. The stratum corneum provides barrier function properties and gives the LSE improved strength and handling characteristics. Clinical experience indicated that the stratum corneum might play an important role in improving the clinical utility of the LSE. Handling and physical characteristics improved with time in culture. We examined the LSE at different stages of epidermal maturation for barrier function and ability to persist as a graft. LSE grafted onto athymic mice before significant development of barrier function did not withstand bandage removal at 7 days postgraft. LSE grafted after barrier function had been established in vitro were able to withstand bandage removal at day 7. Corneum lipid composition and structure are critical components for barrier function. Media modifications were used in an attempt to improve the fatty acid composition of the stratum corneum. The barrier developed more rapidly and was improved in a serum-free, lipid-supplemented condition. Lipid lamellar structure was improved with 10% of the stratum corneum exhibiting broad-narrow-broad lipid lamellar arrangements similar to human skin. Fatty acid metabolism was not appreciably altered. Barrier function in vitro was 4- to 10-fold more permeable than human skin. Epidermal differentiation does not compromise engraftment or the wound healing ability of the epidermis. The stratum corneum provides features beneficial for engraftment and clinical use. (c) 1996 John Wiley & Sons, Inc.     

Human stratum corneum lipids: characterization and regional variations

The lipids of mammalian stratum corneum are known to be important regulators of skin permeability. Since the human stratum corneum displays remarkable regional variations in skin permeability, we assessed the total lipid concentration, the distribution of all major lipid species, and the fatty acid composition in Bligh-Dyer extracts from four skin sites (abdomen, leg, face, and sole) that are known to display widely disparate permeability. Statistically significant differences in lipid weight were found at the four sites that were inversely proportional to their known permeability. In all four sites, among the polar lipids, the stratum corneum contained negligible phospholipids, but substantially more cholesterol sulfate (1-7%) than previously appreciated. As in the stratum corneum from other mammals, the bulk of the lipids consisted of neutral (60-80%) and sphingolipids (15-35%). Of the neutral lipids, free sterols (4- to 5-times greater than esterified sterols), free fatty acids, triglycerides, and highly nonpolar species (n-alkanes and squalene) predominated. n-Alkanes, which were present in greater quantities than previously appreciated, comprised a homologous series of odd- and even-chained compounds ranging from C19 to C34. The sphingolipids comprised over 80% ceramides vs. lesser quantities of glycosphingolipids. In all four sites, the sphingolipids were the major repository of long-chain, saturated fatty acids. The neutral lipid:sphingolipid ratio generally was proportional to the known permeability of each site: higher neutral lipids and lower sphingolipids generally were associated with superior barrier properties. These studies provide: 1) the first detailed, quantitative analysis of human stratum corneum lipids and 2) information about the variability in lipid composition at four skin sites with known differences in permeability. The latter results suggest that variations in neutral lipids, rather than sphingolipids, may underlie local variations in skin permeability.     

The use of skin grafts in postburn contracture release: a 10-year review

Postburn scarring and contracture affecting function remain the most frustrating late complications of burn injury. Various techniques are used to release contractures; the choice depends on their location and/or the availability of unaffected skin adjacent to the contracture or elsewhere. A retrospective review was carried out of the case notes of patients who had skin grafting for the release of postburn contracture at the Burns Unit, City Hospital, Nottingham between May of 1984 and August of 1994 to evaluate the experience over this period. Information was obtained about the burn injury, contracture site, interval between burn and release of contracture, indication, age at first release, intervals between releases, operative details (donor and graft sites), complications and nonoperative treatment, and follow-up to the end of the study period. A total of 129 patients underwent skin grafting for release of contractures as opposed to any other method of correction. Full-thickness skin grafts were used in 81 patients (63 percent) and split-thickness skin grafts in 26 (20 percent). Twenty-two patients (17 percent) had both types used on different occasions. Flame burns (41 percent) were the most common causes, followed by scalds (38 percent). Two hundred thirty-nine sites of contracture were released, with the axilla (59) and the hand/wrist (59) being the most common sites involved, followed by the head/neck region (42). It was found that for the same site, release with split-thickness skin grafts was associated with more rereleases of the contracture than with full-thickness skin grafts. Also, the interval between the initial release and first rerelease was shorter than with full-thickness skin grafts (p < 0.048). It was also noted that children required more procedures during growth spurts, reflecting the differential effect of the growth of normal skin and contracture tissue. Patients reported more satisfaction with texture and color match with the full-thickness skin grafts. There was comparable donor-site and graft morbidity with both graft types. The use of skin grafts is simple, reliable, and safe. Whenever possible, the authors recommend the use of full-thickness skin grafts in preference to split-thickness skin grafts in postburn contracture release.     

Barnacle bonding: Morphology of attachment of Xenobalanus globicipitis to its host Tursiops truncatus

Xenobalanus globicipitis, a unique type of small pseudo‐stalked barnacle occurs on the appendages of cetaceans, including the common bottlenose dolphin Tursiops truncatus. In this study, we examined attachment structures of X. globicipitis and modifications to the skin of T. truncatus in areas of attachment compared to skin nearby an attachment site. Barnacles and their six calcareous footplates were measured for their length and width. There was a positive correlation of barnacle width and length to footplate width and length. The thickness of the stratum corneum increased significantly in areas of attachment compared to skin nearby a footplate. The mitotic stratum germinativum at the base of the dermal papillae did not change significantly in areas of attachment compared to skin nearby a footplate. The stratum germinativum lining the lateral walls of the dermal papillae was significantly thicker in areas of skin nearby a footplate compared to in areas of attachment. Skin of T. truncatus nearby a footplate, displayed dermal papillae extending from the dermis and pointing roughly perpendicular to the epidermal stratum corneum. At sites of X. globicipitis attachment, the dermal papillae were forced to extend laterally, parallel to the stratum corneum, and the dermal papillae length to width ratio at an attachment site was significantly higher than on skin near an attachment site. Our results show that attachment of X. globicipitis through production of footplates organized into calcareous rings, leads to a thickened stratum corneum of the epidermis, a thinner lateral mitotic stratum germinativum and displaced structures of the upper dermis. These resulting modifications to the epidermis and dermis of the host may add to securing barnacle attachment to its host. J. Morphol., 2012. © 2012 Wiley Periodicals, Inc.     

Clinical application of autologous cultured epithelia for the treatment of burn wounds and burn scars

This report presents our experience with autologous cultured human epithelia grafting on burn wounds, burn scars, and skin-graft donor sites in seven patients. Dispersed epidermal cells were cultured with 3T3 cells treated with mitomycin C. After 2 to 3 weeks, cultured epithelia (total 350 to 2250 cm2) were grafted to the wound. The results showed that cultured epithelia grafts did not take so completely compared to the meshed skin grafts used for the coverage of burn wounds. However, cultured grafts placed on aseptic wounds adhered well and showed good appearance. In the histologic findings, normal differentiation of epidermal cells was found. Cultured grafts were bordered from subepidermal granulative tissue with basement membrane. A rete ridge and the adnexal structures were absent in the specimens that adhered to the burn wounds. However, in the specimens that took on abraded wounds, a gently sloping rete ridge and elastic fibers were seen. The histologic findings showed structures resembling normal skin.     

Physical characterization of the stratum corneum of an in vitro psoriatic skin model by ATR-FTIR and Raman spectroscopies

The stratum corneum is an important permeability barrier for the skin. The disorganization of the skin protective barrier characterizes some skin diseases such as psoriasis. Indeed, psoriatic skin is known to be more permeable than normal human skin. An in vitro human skin substitute may be obtained by the auto-assembly method. This method was adapted to produce psoriatic substitutes. FTIR spectroscopy is a well-established method to evaluate the order of hydrocarbon chains in terms of population of trans and gauche conformers. Using ATR-FTIR, we have compared the physicochemical properties of the stratum corneum in skin models derived from uninvolved and involved psoriatic cells with those derived from normal cells. Our results suggest that the stratum corneum of involved psoriatic skin substitutes is less organized than that of normal skin substitutes. Also, it seems that the properties of uninvolved psoriatic skin may vary with seriousness of the disease. The development of a new psoriatic skin model would be helpful in the design of new treatments and to increase the understanding of the mechanisms of this pathology.     

Cultured epithelial autografts for giant congenital nevi

Eight pediatric patients with giant congenital nevi confluent over 21 to 51 percent body surface area were treated by excision and grafting. The nevus was excised to the muscle fascia, and the open wound was grafted with cultured epithelial autografts and split-thickness skin grafts. The patients have been followed from 17 to 56 months. Seventeen operations were performed in the eight patients, excising a mean of 6.9 percent body surface area at each procedure. The mean duration of anesthesia was 3.7 hours, and the mean operative blood loss was 12.3 percent estimated blood volume. The mean "take" for the cultured epithelial autografts was 68 percent, and for the split-thickness skin grafts, 84 percent. Epithelialization of open wound areas adjacent to the grafts was somewhat slower for the cultured epithelial autografts than for the split-thickness skin grafts, but it led to a healed wound in all patients except one. Ten of the 17 areas grafted with cultured epithelial autografts resulted in small open wounds that required regrafting. Wound contraction under the cultured epithelial autografts and under split-thickness skin grafts was similar and depended more on the anatomic site grafted than on the type of graft employed. in 16 of 17 operations, the cultured epithelium remained as a permanent, durable skin coverage. The use of cultured epithelial autografts allowed a larger area of excision than would have been possible with split-thickness skin grafts alone and, therefore, a more rapid removal of nevus. Cultured epithelial autograft are an important new technique in the care of patients with giant congenital nevi.     

Filaggrin breakdown to water binding compounds during development of the rat stratum corneum is controlled by the water activity of the environment

Filaggrin is a specific epidermal protein which is the precursor of the free amino acids, urocanic acid and pyrrolidone carboxylic acid which are largely responsible for the ability of the stratum corneum of the skin to remain hydrated at low environmental humidity. The distribution of filaggrin shown by immunofluorescence in the stratum corneum of the rat changed dramatically during the first hours of postnatal life. During late foetal development, filaggrin accumulated through the entire thickness of the stratum corneum, indicating that there was a block on the subsequent processing of the protein which normally would convert it to free amino acids. Immediately after birth this block was lifted and normal proteolysis of the filaggrin took place in the outer part of the stratum corneum, leaving the normal adult pattern of a thin zone of cells containing filaggrin at the bottom of the stratum corneum. This activation of filaggrin proteolysis was dependent on the drop in external water activity caused by the transition from an aqueous environment in utero to a dryer environment after birth and it could be blocked by maintaining a 100% humidity atmosphere around the newborn rat after birth. In isolated stratum corneum in vitro, filaggrin proteolysis took place only between 80 and 95% relative humidity, both higher and lower relative humidity blocked the proteolysis. Application of occlusive patches to adult rats prevented the normal proteolysis of filaggrin, indicating that this mechanism controls not only the massive filaggrin proteolysis occurring after birth but also the proteolysis occurring during normal stratum corneum maturation. The stratum corneum therefore has the ability to respond to changes in external humidity by altering the level of the stratum corneum where it converts its reserves of filaggrin into water binding amino acids, such that under humid conditions water binding components will be produced in only the most superficial stratum corneum, or even not produced at all.     

Interactions of inertial cavitation bubbles with stratum corneum lipid bilayers during low-frequency sonophoresis

Interactions of acoustic cavitation bubbles with biological tissues play an important role in biomedical applications of ultrasound. Acoustic cavitation plays a particularly important role in enhancing transdermal transport of macromolecules, thereby offering a noninvasive mode of drug delivery (sonophoresis). Ultrasound-enhanced transdermal transport is mediated by inertial cavitation, where collapses of cavitation bubbles microscopically disrupt the lipid bilayers of the stratum corneum. In this study, we describe a theoretical analysis of the interactions of cavitation bubbles with the stratum corneum lipid bilayers. Three modes of bubble-stratum corneum interactions including shock wave emission, microjet penetration into the stratum corneum, and impact of microjet on the stratum corneum are considered. By relating the mechanical effects of these events on the stratum corneum structure, the relationship between the number of cavitation events and collapse pressures with experimentally measured increase in skin permeability was established. Theoretical predictions were compared to experimentally measured parameters of cavitation events.     

Mons pubis as a donor site for split-thickness skin grafts

It is extremely important that a donor graft site be selected with attention to the type of graft needed as well as the specific patient's needs and lifestyle. While it is inevitable that any donor site heals with altered pigmentation and texture, we feel that by using a combination of several time-honored techniques in this well-concealed donor site, we have achieved long-term aesthetic results of the transfer of skin grafts from a hair-bearing area.     

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.     

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.     

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.     

Microbiome dynamics of human epidermis following skin barrier disruption

Background

Recent advances in sequencing technologies have enabled metagenomic analyses of many human body sites. Several studies have catalogued the composition of bacterial communities of the surface of human skin, mostly under static conditions in healthy volunteers. Skin injury will disturb the cutaneous homeostasis of the host tissue and its commensal microbiota, but the dynamics of this process have not been studied before. Here we analyzed the microbiota of the surface layer and the deeper layers of the stratum corneum of normal skin, and we investigated the dynamics of recolonization of skin microbiota following skin barrier disruption by tape stripping as a model of superficial injury.

Results

We observed gender differences in microbiota composition and showed that bacteria are not uniformly distributed in the stratum corneum. Phylogenetic distance analysis was employed to follow microbiota development during recolonization of injured skin. Surprisingly, the developing neo-microbiome at day 14 was more similar to that of the deeper stratum corneum layers than to the initial surface microbiome. In addition, we also observed variation in the host response towards superficial injury as assessed by the induction of antimicrobial protein expression in epidermal keratinocytes.

Conclusions

We suggest that the microbiome of the deeper layers, rather than that of the superficial skin layer, may be regarded as the host indigenous microbiome. Characterization of the skin microbiome under dynamic conditions, and the ensuing response of the microbial community and host tissue, will shed further light on the complex interaction between resident bacteria and epidermis.     

Leave the fat, skip the bolster: thinking outside the box in lower third nasal reconstruction

Defects of the lower third of the nose often present especially challenging reconstructive dilemmas. The surrounding skin to match is often thick, sebaceous, and sun damaged, none of which characterizes the historically ideal periauricular donor skin for grafting. The surrounding nasal skin is quite stiff, precluding very small local flaps. To avoid the "misplaced patch" appearance of most classic full-thickness grafts to this area or the depressed scar of an elliptical excision, many surgeons turn to larger local or regional flaps. These provide not only skin color and texture match but also the necessary several millimeters of subcutaneous fat necessary for proper tip aesthetics. Many defects of the lower third are small, making many surgeons reluctant to employ these larger flaps with their long scars and potential to twist or distort delicate tip or ala anatomy. The author has sought a means to transport skin and subcutaneous fat for lower third nasal defects outside of flaps. On the basis of the superiority of nasolabial fold scars and a vast positive experience in the literature utilizing skin and fat composite grafts with no bolsters, the author applied these techniques to 33 lower third nasal defects in 29 patients. Of 33 grafts varying in size from 4 mm circular to 17 mm x 16 mm and retaining 1 to 5 mm of fat, no grafts were lost. Four grafts developed a 30 percent area or less of central necrosis resulting in localized depression. Three of these four grafts were in active smokers and the fourth graft was in a former smoker. Aside from these four grafts and one with considerable excess fat early in the series, contour was good to excellent. Hypopigmentation is still common but improves with time. Easily performed composite grafts effectively carry the necessary fat for aesthetic reconstruction and do not risk long scars on the nose and twisting of the tip and ala that can result from flaps. Revisions are infrequent and extremely simple when indicated.     

Localization of prolactin receptor in the dorsal and ventral skin of the frog (Rana ridibunda)

The dorsal and ventral skin in amphibians plays an important role in osmoregulation. Prolactin hormone is involved in regulation of amphibian skin functions, such as water and electrolyte balance. Therefore, amphibians may be useful as a model for determining the sites of the prolactin receptor. In this study, prolactin receptor was detected in frog dorsal and ventral skin using immunohistochemical staining method. Prolactin receptor immunoreactivity was localized in all epidermal layers except stratum corneum of dorsal skin epidermis, stratum germinativum layer of ventral skin epidermis, myoepithelial cells, secretory epithelium and secretory channel cells of granular glands in both skin regions. The mucous glands and secretory granules of granular glands did not show immunoreactivity for the prolactin receptor. According to our immunohistochemical results, the more widespread detection of prolactin receptor in dorsal skin epidermis indicates that prolactin is more effective in dorsal skin. Presence of prolactin receptors in epidermis points out its possible osmoregulatory effect. Moreover, detection of receptor immunoreactivity in various elements of poison glands in the dermis of both dorsal and ventral skin regions suggests that prolactin has a regulatory effect in gland functions.     

Local temperature rises influence in vivo electroporation pore development: a numerical stratum corneum lipid phase transition model

Electroporation is an approach used to enhance transdermal transport of large molecules in which the skin is exposed to a series of electric pulses. Electroporation temporarily destabilizes the structure of the outer skin layer, the stratum corneum, by creating microscopic pores through which agents, ordinarily unable to pass into the skin, are able to pass through this outer barrier. Long duration electroporation pulses can cause localized temperature rises, which result in thermotropic phase transitions within the lipid bilayer matrix of the stratum corneum. This paper focuses on electroporation pore development resulting from localized Joule heating. This study presents a theoretical model of electroporation, which incorporates stratum corneum lipid melting with electrical and thermal energy equations. A transient finite volume model is developed representing electroporation of in vivo human skin, in which stratum corneum lipid phase transitions are modeled as a series of melting processes. The results confirm that applied voltage to the skin results in high current densities within the less resistive regions of the stratum corneum. The model captures highly localized Joule heating within the stratum corneum and subsequent temperature rises, which propagate radially outward. Electroporation pore development resulting from the decrease in resistance associated with lipid melting is captured by the lipid phase transition model. As the effective pore radius grows, current density and subsequent Joule heating values decrease.     

Recovery of Skin Barrier After Stratum Corneum Removal by Microdermabrasion

Microdermabrasion is widely used as a non-invasive cosmetic technique that has recently been adapted to selectively remove stratum corneum to increase skin permeability for transdermal drug delivery. This study measured the kinetics of skin barrier recovery after stratum corneum removal using microdermabrasion in hairless guinea pigs. The skin was abraded at two sites on each animal, one of which was allowed to recover under occlusion while the other remained non-occluded. Histological measurements showed that skin barrier properties to sulforhodamine B largely recovered within 12 h, and the stratum corneum appeared largely reformed within 24 h for both occluded and non-occluded skin. Skin electrical resistance measurements showed significant recovery of the skin barrier within 24 h. We conclude that transdermal drug delivery may occur for up to 12 h after microdermabrasion in guinea pigs; however, humans will probably have a longer recovery time due to expected slower skin healing rates.     

Glycoproteins modulate adhesion in terminally differentiated keratinocytes

Summary The stratum corneum can be dissociated into single squames by homogenization in ether. We have reaggregated the free corneocytes into a multilayered lamellar structure resembling an intact stratum corneum. The reconstituted stratum corneum reacts with fluorescein-conjugated lectins, unlike the intact tissue. We infer that the lack of binding in the intact tissue is due to masking of saccharide sites by lipids (which are extracted by the ether). In an extension of the procedure, the ether is removed and replaced by acetone. This system permits us to modulate corneocyte reaggregation by the addition of appropriate agents. We have used this system to corroborate our hypothesis that a 40 kD cell-surface glycoprotein (an endogenous lectin specific for amino sugars), which we have isolated from the stratum corneum, is instrumental in adhesion of corneocytes by cross-linking with amino sugar sites on adjacent cells. The reaggregation is inhibited by the antibody to the 40 kD glycoprotein. It is also inhibited by either the addition of amino sugars which bind to the endogenous lectin, or the addition of exogenous lectins specific for amino sugars which bind to the ligand.     

Using the method of homogenization to calculate the effective diffusivity of the stratum corneum with permeable corneocytes

The stratum corneum is the outermost layer of the skin, which acts as a barrier membrane against the penetration of molecules into and out of the body. It has a biphasic structure consisting of keratinized cells (corneocytes) that are embedded in a lipid matrix. The macroscopic transport properties of the stratum corneum are functions of its microstructure and the transport properties of the corneocytes and the lipid matrix, and are of considerable interest in the context of transdermal drug delivery and quantifying exposure to toxins, as well as for determining the relation of skin disorders to disruption of the stratum corneum barrier. Due to the complexity of the tissue and the difference in length scales involved in its microstructure, a direct analysis of the mass transport properties of the stratum corneum is not feasible. In this study, we undertake an approach where the macroscopic diffusion tensor of the stratum corneum is obtained through homogenization using the method of asymptotic expansions. The biphasic structure of the stratum corneum is fully accounted for by allowing the corneocytes to be permeable and considering the partitioning between the corneocytes and the lipid phases. By systematically exploring the effect of permeable corneocytes on the macroscopic transport properties of the stratum corneum, we show that solute properties such as lipophilicity and relative permeabilities in the two phases have large effects on its transdermal diffusion behavior.