198 JOURNAL OF COSMETIC SCIENCE brane bilayer structures (97 ,98). The final membrane structural transformation correlates with changes in lipid composition, i.e., the polar lipid precursors (glycosphingolipids, phospholipids, and cholesterol sulfate) are metabolized to more nonpolar lipid products (91,96). The lipid bilayer maturation within the different depths of the stratum granu­ losum (SG) and the stratum corneum (SC) is mediated by different classes of pH­ dependant enyzmes, namely secretory phospholipase A2 , steroid sulfatase (both with a near-neutral pH optimum), and sphingomyelinase and beta-glucocerebrosidase (both with an acidic pH optimum). These enzymes are responsible for a coordinated matura­ tion of the extruded lipid sheets from lamellar bodies to lamellar bilayers (Figure 2). Topical application of physiologic lipids has distinct effects from those of nonphysiologic lipids like petrolatum. Studies have shown that topical application of only one or two of the three physiologic lipids to a disrupted hairless mouse skin impedes rather than facilitates barrier recovery, evidenced by changes in transepidermal water loss (95). However, if members of all three key lipid classes (i.e., cholesterol, ceramide, and free fatty acid or their precursors) are applied together to barrier-disrupted skin, normalized rates of barrier repair are observed (56,95). The topically applied physiologic lipids not only are concentrated in the stratum corneum membrane domains, but also are delivered to the nucleated layers of the epidermis (56,95). Depending on the composition of the lipid mixture, either normal or abnormal lamellar bodies are formed, ultimately result­ ing in either normal or abnormal lamellar membrane unit structures in the stratum corneum intercellular spaces (56,95). The process of passive lipid transport is across the stratum corneum as well as uptake into nucleated cells (stratum granulosum). The subsequent reorganization of lamellar unit structures takes about two hours after acute barrier disruption in murine epidermis (95 ,97). It appears that the incorporation of applied physiologic lipids into barrier lipids follows two pathways: First, there is direct incorporation into stratum corneum membrane domains second, lipids appear to tra­ verse the intercellular route in the stratum corneum and ultimately get incorporated into lower stratum granulosum cells (Figure 3). The intercellular lipids then appear able to enter the nucleated cells, incorporate into the appropriate lipid metabolic pathways, and ultimately utilize the lamellar body delivery system to re-enter the intercellular mem­ brane domains (56). Topically applied lipids to either intact or acetone treated skin did SG/SC INTERPHASE LOWER TO MID SC - - - .. - - - - - - - - - - - - - - - - .. - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Phospholipase �-GlcCer'ase Figure 2. Lamellar body exocytosis and end-to-end fusion oflamellar body-derived sheets to uninterrupted plasma membranes, and subsequent compaction of adjacent membrane sheets into lamellar bilayer unit structures. These changes correlate with extracellular lipid processing of polar lipids to non-polar lipids. These steps are required in order to form the intercellular bilayer structures. [From Elias and Menon, 1996 (ref. 99), with permission.]

ROLES OF VEHICLES FOR SKIN TREATMENT 199 not down-regulate the physiological lipid synthesis (56,100). These studies support the hypothesis that the epidermis can internalize and process physiologic lipids. In contrast, non-physiologic lipids like petrolatum appear to simply form a bulk hy­ drophobic phase in the stratum corneum intercellular spaces to restore the permeability barrier under similar conditions (56,83). The same studies showed further enhancement of barrier recovery if the proportion of one of the fatty acids (linoleic acid, palmitic acid, or stearic acid) or the other key species was augmented threefold in a four-component system, i.e., consisting of fatty acid, ceramide, cholesterol, and essential fatty acids in a 3:1:1:1 ratio (57). Interestingly, the structural requirements of this lipid mixture are not restricted to essential fatty acids, a finding that was confirmed in a similarly disrupted human barrier (5 7). Interestingly, acylceramides applied as a single agent delayed barrier recovery. However, acylceramides in a mixture with cholesterol (optimum ratio of 1.5:1 or 1:2, respectively) also revealed accelerated barrier recovery after acute barrier disrup­ tion (57). Moreover, in another study, stratum corneum hydration (measured by con­ ductance) was increased four hours after topical application of a cholesterol-, acylce­ ramide-, petrolatum-, and glycerol-containing vehicle (propylene glycol/ethanol) (101). These findings were confirmed, as accelerated barrier repair was noted using a similar Stratum corneum -- Stratum granulosum _ ........ Figure 3. Putative route of incorporation and processing of exogenous physiologic lipids within cells of the outermost granular layer. Exogenous lipids may be delivered by an endocytic pathway to the trans-Golgi complex. These lipids become available for the lamellar body formation. [From Mao-Qiang, Brown, et al., 1995 (ref. 56), with permission.}