The stratum corneum is composed of cornified keratinocytes, joined tog
ether with corneodesmosomes and embedded in the relatively hydrophobic
intercellular substance. Formation of this horny layer and its consta
nt desquamation are fundamental processes leading to the establishment
of an efficient epidermal barrier. We examined structural changes occ
urring in the stratum corneum ex vivo, in order to better understand t
he mechanisms regulating corneocyte desquamation at the epidermal surf
ace and influencing percutaneous penetration of exogenous substances.
Morphologic modifications were induced by occlusion, increasing the ti
ssue hydration, or by application of propylene glycol, a hydrophilic s
olvent of small molecular size. Distribution of the major epidermal an
tigens, markers of terminal differentiation, was studied immunohistoch
emically and showed no modification related to the tissue alteration.
Skin samples were fixed in paraformaldehyde and either postfixed in Os
O4 and embedded in Epon or postfixed in RuO4 and embedded in epoxy and
acrylic resins. Structural composition and spatial organization of th
e elements present in the intercorneocyte spaces were evaluated ultras
tructurally, with special attention paid to the fate of corneodesmosom
es. Our results suggest that the spontaneous organization of lipids ex
creted into the intercellular spaces and constitution of the lamellar
multilayers in the lower stratum corneum are at the origin of partitio
n of the intercellular compartment to the hydrophobic and hydrophilic
domains. The latter, compatible with the proteinic contents, seem to b
e displaced by the hydrophobic components undergoing reorganization, t
owards the invariable hydrophilic foci represented by corneodesmosomes
. This mechanism may be involved in the delivery of proteolytic enzyme
s, thus contributing to the progressive degradation of corneodesmosoma
l proteins. (C) 1998 Wiley-Liss, Inc.