We recently developed a pig skin model to determine the role of corneosomes (modified desmosomes in the stratum corneum) and extracellular lipids in desquamation. The present study provides control morphometric data on the morphological changes in desmosomes and corneosomes leading to desquamation in adult pig epidermis in vivo. The extracellular space within desmosomes gradually widened from the basal to the granular layer, and decreased slightly in the stratum corneum. Mid-dense line broadening, and increased electron density of the distal light layers, coincided with membrane-coating granule extrusion in the outer granular layer. Corneocyte attachment correlated with corneosome distribution. Compactum packing was relatively tight and corneosomes were numerous. Cohesion was mainly peripheral in the disjunctum, and corneosomes were restricted to corneocyte edges. Adhesion had a tongue-and-groove appearance with corneosomes riveting corneocyte peripheries into a lipped groove on adjoining cells. Cells shed by peeling radially towards the lipped groove, and corneosomes decreased from lower to upper disjunctum. Corneosome breakdown commenced with an electron lucent band forming between the plug and lipid envelope. The plug was then unzipped from the lipid envelope and degraded. Corneosomes did not form squamosomes.
Three factors were examined for their relative contribution to corneocyte cohesion in normal adult pig ear: (1) extracellular lipids derived from membrane-coating granules (MCG); (2) corneosomes (modified stratum corneum desmosomes); and (3) corneocyte covalently bound lipid envelopes. Cohesion strength of the outer stratum corneum was measured directly by cohesometry, then altered by removing MCG lipids with solvents of varying potency. Cohesion changes were related to degree of lipid removal and ultrastructural alterations. Trypsin was also used to see if proteolysis of corneosomes promoted squame shedding. Potent solvents increased cohesion in relation to the amount of MCG lipid extracted. Tighter cohesion was due to fusion of the outer leaflets from covalently bound lipid envelopes on adjacent corneocytes. However, lipid envelopes are unlikely to mediate normal stratum corneum cohesion since MCG lipids play a significant anti-cohesive role preventing their apposition. Mild solvents partially removed MCG lipids causing a slight decrease in cohesion compared with untreated samples. This suggests a minor cohesive role for MCG lipids, consistent with maintaining their barrier function. We believe that corneosomes are the major determinant of stratum corneum cohesiveness because, in untreated skin, both cohesion and the number of corneosomes increased from the surface towards the granular layer. Furthermore, corneosome digestion with trypsin induced superficial squame shedding.
Adhesional glycoproteins of desmosomes possess asparagine-linked, complex oligosaccharide side chains. We investigated the potential of these sugars to protect the core proteins of desmosomes and corneosomes (modified stratum corneum desmosomes) against proteolysis. Isolated pig ear epidermis was exposed sequentially to individual hydrolases, and their effect monitored ultrastructurally. Two major steps were employed: (1) glycosidases, to remove stepwise the sugars in a typical complex oligosaccharide chain; and (2) proteolysis using both endopeptidases and an exopeptidase. Controls were exposed to the same sequence of buffers, but without enzymes. Proteases alone induced no major changes in desmosomes or corneosomes compared with controls. Glycosidases alone, or proteases followed by glycosidases, caused mild fragmentation of the desmosomal interspace, but no widening. However, dramatic changes occurred when glycosidase treatment was followed by proteolysis. The interspace of both desmosomes and corneosomes was extensively digested, and consequently widened, causing loose packing of the epidermis. These findings indicate that sugars are potentially anti-proteolytic in both desmosomes and corneosomes. Sugars may, therefore, be a factor in preventing premature desquamation, by protecting desmosomes and corneosomes against extracellular proteases derived from membrane-coating granules.
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