Cellular Changes that Accompany Shedding of Human Corneocytes

Abstract

Corneocyte desquamation has been ascribed to either: 1) proteolytic degradation of corneodesmosomes (CD); 2) disorganization of extracellular lamellar bilayers; and/or 3) ‘swell-shrinkage-slough’ (SSS) from hydration/dehydration. To address the cellular basis for normal exfoliation, we compared changes in lamellar bilayer architecture and CD structure in DSquame® strips from the 1st vs. 5th stripping (‘outer’ vs. ‘mid’-stratum corneum [SC], respectively) from 9 normal adult forearms. Strippings were either processed for standard EM or for ruthenium (Ru-V)- or osmium-tetroxide (Os-V) vapor fixation, followed by immediate epoxy embedment, an artifact-free protocol that to our knowledge is previously unreported. CDs are largely intact in the mid-SC, but replaced by electron-dense (hydrophilic) clefts (lacunae) that expand laterally, splitting lamellar arrays in the outer SC. Some undegraded DSG1/DSC1 redistribute uniformly into corneocyte envelopes (CEs) in the outer SC (shown by proteomics, Z-stack confocal imaging and immunoEM). CEs then thicken, likely facilitating exfoliation by increasing corneocyte rigidity. In vapor-fixed images, hydration only altered the volume of the extracellular compartment, expanding lacunae further separating membrane arrays. During dehydration, air replaced water, maintaining the expanded extracellular compartment. Hydration also provoked degradation of membranes by activating contiguous acidic ceramidase activity. Together, these studies identify several parallel mechanisms that orchestrate exfoliation from the surface of normal human skin.

Distinguishing Ichthyoses by Protein Profiling

“…Most of the proteins did not change much in yield from exterior to interior of the SC, indicating relative insensitivity to endogenous proteolysis in the corneocytes. Several proteins exhibited maximal levels in the superficial layers, perhaps due to their continued incorporation into cross-linked material as the squames transit outward [27]. Some proteins displayed marked differences in yield among individuals not attributable to a gradient with depth.…”

Altered sphingoid base profiles predict compromised membrane structure and permeability in atopic dermatitis

“…Our prior studies demonstrated that ceramidases are enriched in SC [28]. Furthermore, we recently showed by in situ zymography that these ceramidases are functionally active in normal human SC [10]. It was noted in a prior study that a virulent microbial pathogen, Staphylococcus aureus that often colonizes in AD skin, as well as Pseudomonas , produce ceramidase activators ( i.e., glycerophospholipids) and/or ceramidases that could alter sphingoid base levels and ratios in AD epidermis [30].…”

“…Sphingoid bases are long-chain amino alcohols that are both immediate precursors, as well as catabolites of Cer, generated within the SC by one or more ceramidases [8–10]. While these molecules show amphiphilic, detergent-like properties that account for their potent in vitro antimicrobial activity [11, 12], they also inhibit both protein kinase C [13, 14] and phosphatidate phosphohydrolase [15] activities.…”

Mechanisms of abnormal lamellar body secretion and the dysfunctional skin barrier in patients with atopic dermatitis

“…These enzymes generate a family of ceramides (Cer), essential and non-essential free fatty acids (FFA), as well as much of the cholesterol that is required for the supramolecular organization of these non-polar lipid species into mature lamellar membrane structures 10 . In parallel, lamellar body-derived, desquamatory proteases and their inhibitors initiate the orderly digestion of corneodesmosomes (= transient intercellular rivets that connect adjacent corneocytes), a process that eventually leads to the desquamation of corneocytes from the skin surface 11, 12 (Fig. 1).…”