Stratum corneum structural changes

All the above-mentioned changes in lipid composition and organization in diseased and dry skin likely contribute to an impaired stratum corneum barrier function and increased susceptibility to dry skin. However, as previously indicated, abnormalities in the process of envelope formation may also strongly influence the stratum corneum barrier integrity. Therefore, more information is required to elucidate the precise mechanisms by which stratum corneum structure and function are altered. 

An important aspect of enhanced transdermal transport is that it is often accompanied by irritation. This irritation is likely to be due to the disruption of the stratum corneum structure. It is important to note, however, that the composition of an applied multicomponent formulation, specifically that of a microemulsion, changes over time... 

Some data indicates that there is a change in composition and arrangement of the lipids during the transition through stratum corneum.30,31 This can in part depend on the presence of lipids from sebum secretion. Together with the decrease in water across stratum corneum it is possible that there is a rearrangement of the lipid structure, which in turn can be of importance for the desquamation process. 

In spite of the fact that stratum corneum cells are metabolically inert, changes in keratin structure and organization occur as each cell transits through the stratum corneum prior to desquamation (28). This suggests some asymmetry in physical and chemical properties through the thickness of the corneum. One demonstration of this is the swelling of fresh frozen transverse sections of corneum in dilute acid or base. The most mature surface cells swell considerably more slowly and to a lesser extent than the lower layers of the corneum (18). Such asymmetry is of particular importance in studying the diflFusion and mechanical properties of this membrane. 

NMR has provided the most useful information on the state and mobility of water in keratins (2, 3, 4). It was shown (4) that the water associated with the stratum corneum seems to exist in two distinct states a bound fraction and a free fraction. This type of information could be obtained conveniently by the new technique using desorption kinetics under a wide range of experimental conditions. Also, it would be of interest to examine the effect of various agents, which are known to produce structural changes in the corneum, on the ability of the corneum to hold water and to retain it under a variety of conditions. 

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