Stratum corneum lipid lamellae

Figure 9 Freeze-fracture electron microscopy of control (a) or after treatment of alkyl-azones (b-f). (b) Hexyl-azone (c) octyl-azone (d) dodecyl-azone (e) myristyl-azone and (f) oleyl-azone. Bar represents 100 nm. represents rough structures indicating either separate domains of enhancer or more perturbation of lipid lamellae. Arrows indicate a clear presentation of the intact smooth regions of intact lamellae with steps (fracture across the lamellae). C, Corneocyte scl, stratum corneum lipid lamellae. 

Kuempel, D., Swartzendruber, D.C., Squier, C.A., and Wertz, P.W, In vitro reconstitution of stratum corneum lipid lamellae, Biochim. Biophys. Acta, 1372, 135, 1998. 

Warner, R.R. Boissy, Y.L. Lilly, N.A. Spears, M.J. McKillop, K. Marshall, J.L. Stone, K.J. Water disrupts stratum corneum lipid lamellae damage is similar to surfactants. J. Invest. Dermatol. 1999, 113, 960-966. 

The lipid lamellae within the stratum corneum are thought to consist of a complex mixture of compounds but to contain predominantly cholesteryl sulfate (5%), free fatty acids (15%), cholesterol (25%), and ceramides (50%) (Abraham and Downing, 1990). Unlike DPPC monolayers, those formed from stratum corneum lipids do not undergo any obvious phase transitions during compression) therefore the information available from the resulting isotherms is more limited. The behavior of monolayers consisting of these types of compounds has been investigated in the presence and absence of Azone. 

The membrane-coating granules in keratinized epithelia contain electron-dense lipid lamellae [68, 77], and therefore, the intercellular spaces of the stratum corneum are filled with short stacks of lipid lamellae [67, 132], Most of the membrane-coating granules in nonkeratinized epithelia consist of amorphous material [120] however, some studies have shown that a small number of these granules in nonkeratinized epithelia contain lamellae [151]. Therefore,... 

The lipid composition changes dramatically during terminal differentiation. After extrusion from the lamellar bodies, the polar lipid precursors are enzymatically converted into more hydrophobic lipids. As a result, phospholipids are almost absent in the stratum corneum. The lipid lamellae surrounding the corneocytes are predominantly composed of ceramides, cholesterol, and free fatty acids. It is generally assumed that these lipids are present in nearly equimolar ratios. However, inspection of literature data shows that there is a high interindividual variability in the lipid composition [37],... 

The composition of the free fatty acids is also unique. In both human and pig stratum cornea, the free fatty acid fraction consists mainly of long and saturated hydrocarbon chains [44,45], Oleic and linoleic acid are the only unsaturated free fatty acids detected in the stratum corneum. There are various sterols present in human stratum corneum, of which cholesterol predominates. Cholesterol is the only major lipid class that is present in both plasma membranes and the intercellular lipid lamellae. Cholesterol is synthesized in the epidermis and this synthesis is independent of the hepatic one. A minor fraction is sulfated to... 

Madison, K.C., et al. Presence of intact intercellular lipid lamellae in the upper layers of the stratum corneum. J Invest Dermatol 88 714. 

Figure 4 Nat 106 liposomes have a strong effect on the microstructure of the stratum corneum. The corneocytes (C) were swollen considerably, and the smooth ultrastructure of the intercellular lipid lamellae showed flattened spherical structures (see arrows). The linear arranged keratin filaments along the cell boundary of the corneocytes are absent. The scale bar indicates 0.1 pm. 

Figure 7 The possible mechanisms involved in the effect of penetration enhancers on the lipid organization of the intercellular domains in the stratum corneum. (A) Intercalation of the enhancer in the lipid lamellae. (B) Phase separation between enhancer and skin lipids in the lamellae. (C) Phase separation between lipid lamellae and an enhancer-rich phase. (D) Intercalation of the enhancer in the lipid lamellae and simultaneous phase separation between lipid lamellae and enhancer. (E) Phase separation within the lamellae and separation between an enahncer-rich phase and the lamellar phase. (F) Disappearance of the lamellar phases.

FIGURE 3.3 Intercellular lipid lamellae in the stratum corneum. Bar equals 30 nm. 

Interactions between water and the polar head groups of lipid molecules are necessary for the formation of lamellar phases however, it appears that there is no free water associated with the 13 nm trilaminar units. This is supported by the observation that this periodicity does not increase with increasing stratum corneum water content.22 There is likely water hydrogen bonded to the polar regions of the lamellae. In contrast, the minor short periodicity swells from 5.8 to 6.6 nm as the water content of stratum corneum increases from 12 to 50%.23 This suggests that the lipid lamellae are simple individual bilayers and free water molecules can exist between adjacent bilayers, thus causing the increase in the lamellar spacing. 

In the past decade a number of physical techniques have been used to evaluate the unique barrier properties of mammalian skin [1]. This chapter deals with the use of another physical technique, fluorescence spectroscopy, to study the barrier properties of the human stratum corneum (SC), specifically with respect to the transport of ions and water. The SC is the outermost layer of the human epidermis and consists of keratinized epithelial cells (comeo-cytes), physically isolated from one another by extracellular lipids arranged in multiple lamellae [2]. Due to a high diffusive resistance, this extracellular SC lipid matrix is believed to form the major barrier to the transport of ions and water through the human skin [3-5]. The objective of the fluorescence studies described here is to understand how such extraordinary barrier properties are achieved. First the phenomenon of fluorescence is described, followed by an evaluation of the use of anthroyloxy fatty acid fluorescent probes to study the physical properties of solvents and phospholipid membranes. Finally, the technique is applied to the SC to study its diffusional barrier to iodide ions and water. 

The cornified cell envelope is the outermost layer of a corneocyte, and mainly consists of tightly bundled keratin filaments aligned parallel to the main face of the corneocyte. The envelope consists of both protein and lipid components in that the lipid is attached covalently to the protein envelope. The envelope lies adjacent to the interior surface of the plasma membrane. " The corneocyte protein envelope appears to play an important role in the structural assembly of the intercellular lipid lamellae of the stratum corneum. The corneocyte possesses a chemically bound lipid envelope comprised of A-co-hydroxyceramides, which are ester linked to the numerous glutamate side chains provided possibly by both the ot-helical conformation and p-sheet conformation of involucrin in the envelope protein matrix. In the absence of A-oo-hydro-xyceramides, the stratum corneum intercellular lipid lamellae were abnormal and permeability barrier function was disrupted. 

The stratum corneum intercellular lipids exist as a continuous lipid phase occupying about 20% of the stratum corneum volume and arranged in multiple lamellar structures. They are composed of cholesterol (27 /o) and ceramides (41 /o), together with free fatty acids (9 /o), cholesteryl esters (10 /o) and cholesteryl sulfate (2 /o) (Table 1). Phospholipids, which dominate in the basal layer, are converted to glucosylceramides and subsequently to ceramides and free fatty acids, and are virtually absent in the outer layers of the stratum corneum. Eight classes of ceramides have been isolated and identified in human stratum corneum but the functions of the individual ceramide types are not fully understood. Similarly, the exact function of cholesterol esters within the stratum corneum lamellae is also elusive but it is theoretically possible that cholesterol esters may span adjacent bilayers and serve as additional stabilizing moieties. 

The remarkable barrier function of the skin is primarily located in the stratum corneum (SC), the thin, outermost layer of the epidermis. The SC consists of several layers of protein-filled corneocytes (i.e., terminally differentiated keratinocytes) embedded in an extracellular lipid matrix. Attached to the outer cor-neocyte envelope are long-chain covalently bound cer-amides that interact with the lipids of the extracellular space. These lipids are composed primarily of free fatty acids, ceramides, and cholesterol arranged in multiple lamellae.f Passive permeation across the SC is believed to occur primarily via the intercellular... 

Sheu, H.-M. Chao, S.-C. Wong, T.-W. Lee, Y.-Y. Tsai, J.-C. Human skin surface lipid film an ultrastructural study and interaction with corneocytes and intercellular lipid lamellae of the stratum corneum. Br. J. Dermatol. 1999, 140, 385-391. 

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